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5F2-A Question for a first build

PeteG45
September 10th, 2009, 02:58 PM
I'm getting ready to start on a 5F2-A like build as a first build. I read here (http://www.tdpri.com/forum/amp-central-station/110158-gibsonette-wiring-diagram-layout.html) and elsewhere that a 5F2-A with a second 6V6 in parallel is an nice sounding, simple amp. I'm using a combination of Dave Hunter's book and the Weber layouts to develop my layout.

Hunter's Two-Stroke amp has the signal coming into the power tubes on pin 6 with a resistor going to pin 5 (I think it was 1.5K but I don't have the book in front of me) where Weber's layout goes straight in to the grid on pin 5. Which is better or why is this?

I also read that if I add a second 6V6 power tube that I should halve the value of the cathode follower resistor. Does this sound correct?

Here is the power portion of my version of the layout (changes from the 5F2-A in yellow) does this look correct? Am I even close?

Jelle
September 10th, 2009, 03:18 PM
Pin 6 has no connection on a 6V6, so it's just a clean way to wire the grid stopper on pin 5 of that tube. IMO it's a good idea, but some people say it's not necessary in a properly designed amp; this (http://www.aikenamps.com/InputRes.htm) is what Randall Aiken has to say about it, and I think he's one of the most knowledgeable people around.

PeteG45
September 10th, 2009, 05:33 PM
Thanks for the link. I knew pin six was used just as a solder point and now I'll definitely add in the 1.5K grid stoppers. Any info on the cathode follower resistor value?

Kbibs
September 10th, 2009, 09:41 PM
Yes, halving the cathode resistor sounds right.

Here's a schematic posted on the Angela Instruments sight.

http://angela.com/angelasupersingle-ended6v6guitarampproject.aspx

I built a similar amp from a kit made by STF Electronics. A very simple and nice sounding amp.

Good Luck!

Ken

ThermionicScott
September 10th, 2009, 09:51 PM
Looks good to me. What are you using for the output transformer?

PeteG45
September 10th, 2009, 11:59 PM
Ken thanks for the link.

Scott, good question about the output transformer. I was planning to use the transformer that Weber uses in their 5F2-A (https://taweber.powweb.com/store/5f2a_layout.jpg) kit which is a W022913 (https://taweber.powweb.com/store/022913sch.jpg) but then I was reading another thread on similar circuits. Bruce from Mission Amps was pretty insistent that this transformer is wrong for this amp. He said that this is a push-pull transformer and doesn't fit in this circuit. He strongly recommended the Hammond 125ESE. Weber sells another transformer called a WSE15 which is 15 watts and has a 5K primary impedance with 4, 8 and 16 Ohm output taps (like the Hammond).

I'm leaning towards the WSE15 (https://taweber.powweb.com/store/se15sch.jpg), what do you think?

Will I need to change the circuit if I change to an output transformer with a 5K primary instead of the 8K primary?

ThermionicScott
September 11th, 2009, 12:27 AM
Since you have two output tubes in parallel, you need to halve the primary impedance of the OT. The tweed Champ (running the output tube in around the same conditions) had a primary impedance of around 5K, so I think if you used the WSE15, and hooked up an 8-ohm speaker to the 16-ohm tap (bringing down the primary impedance to 2.5K) that ought to work pretty well. :cool:

- Scott

PeteG45
September 11th, 2009, 12:50 AM
How do you know all of this? I feel so unprepared.

Thanks for the info though. That seems like a simple enough solution. I'll probably wire up both the 16 and 8 ohm taps for 8 and 4 ohm cabinets. As long as I select the right wires and label them on the back of the chassis I should be good.

ThermionicScott
September 11th, 2009, 01:01 AM
I don't have a life, but I can stuff my head with a lot of info when the subject interests me. :lol:

There's a bunch of great info in the threads here, and on other tube audio forums. A lot of classic tube audio books are downloadable online...

PeteG45
September 11th, 2009, 10:08 AM
So here is what I have now. Would I be crazy to think about adding a presence control? I like the effect that it has on an amp like a Bassman but I don't know how much impact it would have on an amp like this. It seems like all I would need is a pot and a cap of the right value and a little bit of wire. What do you think? Do you know what kind of values would be appropriate? I was thinking about a 5K pot and a 0.1uF (250v) cap.

ThermionicScott
September 11th, 2009, 02:14 PM
That 5K and 0.1uF combo is a proven one for Presence. If you want to be really cool, you could change the feedback resistor to a pot and add a cap to get a Resonance (low end) control as well. :cool:

- Scott

PeteG45
September 11th, 2009, 02:24 PM
That sounds like a cool idea, what kinds of values would be required? I may have to rename this circuit the "Scope Creep". :o)

Do you know where I could find some recommendations for tube spacing and control pot and input spacing? Is it better to have the tubes closer together so that the wire runs are as short as possible or is it better to have a little space between certain components?

Here is the whole thing so far:

ThermionicScott
September 14th, 2009, 12:16 AM
The "Scope Creep" -- I like it! :lol:

For the "resonance" control, the capacitor value depends on the value of your feedback resistor (22K here). How about switching to a 25K pot, and try out different caps between 0.1uF and 100pF? The idea is that the whole 25K is always in the circuit, but you wire one end of the cap to the wiper, so it bypasses more or less of the resistance as you turn the knob. :idea:

- Scott

PeteG45
September 14th, 2009, 04:14 PM
I'm still having a problem figuring out the OT input and output impedances with parallel power tubes from your earlier post.

One 6V6 tube has a load resistance of 8500 Ohms (according to ampage.org) so wouldn't two in parallel produce a resistance of half that value or 4250 Ohms? The impedance on the primary of the OT is rated for 5000 so do I still halve the impedance on the output tap? Am I looking at the wrong values?

PeteG45
September 14th, 2009, 04:45 PM
Would a resonance control look something like this? This would be used in place of the 22K resistor on the board?

ThermionicScott
September 14th, 2009, 04:46 PM
Well, the *ideal* load resistance depends on the screen voltage, and the examples in the tube manual aren't applicable to all designs.

When designing for max power, you want the load line to intersect the "knee" of the characterist curves, and that knee goes up as you raise the screen voltage, and goes down as you lower the screen voltage. When Fender went to fixed bias and higher screen voltages in the tweed era, the primary impedances went down ("steeper" load line) to compensate. The lowest screen voltage I see for the 6V6 examples is 285V, and my 5F2A runs about 300-310V.

All that boring junk said :wink:, you could try hooking them up to the 5K primary and not bother halving the speaker load. Might even sound good!

- Scott

ThermionicScott
September 14th, 2009, 04:47 PM
Would a resonance control look something like this? This would be used in place of the 22K resistor on the board?

Break the connection between the left and middle lugs, and yeah, should do it.

PeteG45
September 15th, 2009, 12:39 AM
OK, getting ever closer. Here is my updated layout. Can you look at the NFB loop/resonance control and tell me if it looks right? I also changed the config of the grid stopper resistors on the power tubes, do these look correct? I think that I had them wrong in the previous layout that I reworked.

ThermionicScott
September 15th, 2009, 01:05 AM
Looks good! I totally missed the grid stopper thing in the previous diagram, but you've got it right, now.

I think it'll be fun to dial in those cap values. :grin:

- Scott

PeteG45
September 15th, 2009, 09:01 AM
Yeah but I'm trying to come up with a way to make it easier to switch out the caps. I might start out with two small leads terminated with alligator clips so I can quickly change back and forth between the caps. Then, once I've settled on a value, I can go back and solder the cap into place.

The wire returning from the volume pot to pin 7 on V1 looks unusually long. Would it be better to route it under the board between the input grid stopper resistors and the cathode bias resistor? I'm not sure how this is all going to line up because I don't have the board yet and because this will be a head type chassis (with the tubes and transformers on top and the jacks on the back).

Docron10
September 15th, 2009, 04:53 PM
have you read Dave hunters update on his myspace page? He calls for a change in the diagram to eliminate one of power tubes and replace the 6v6 with a 6L6. here is his quote "The Two Stroke is a single-ended, true class A tube combo with single 6L6 output tubes in its “standard format” (swappable for a single 6V6, EL34, KT66, 6K6, etc) and a 1x12” speaker configuration. Power output is between 4 watts (6V6 or 6K6) and 8 to 10 watts (6L6 and larger tubes), depending upon tube used. The project published in Guitar Amp Handbook was for a "dual-single-ended" amp with 2x6V6s that could be swapped for a single 6L6 like the Two Stroke uses now, but this proved too complicated to supply for the extra effort involved, and after repeated testing the single 6L6 sounded just as good (if not better) and makes it an easier amp for beginners to build, so it made sense to make the change. You can still swap the single 6L6 for a single 6V6, or for any of the other tubes mentioned above. The project still uses the same diagram, you just don't connect the extra wires to the second output tube socket to wire it in parallel, and you delete the extra bias resistor and bypass cap. "
does this limit the amp? has anyone built it this way and got the results dave has?
do you have to rebias the tubes in this setup if you change them?

PeteG45
September 15th, 2009, 10:21 PM
That's an interesting post. Do you have the link to his myspace page? I found the Victoria amp page but not Dave Hunter's. I do have his book but it has the two stroke with dual 6V6 tubes. Cutting the amp back to a single output tube would be easy enough and then I could play around with 6V6, 6L6 and KT66 tubes? That might be worth it. The only down side is that I love to see lots of glowing tubes.

I would think that you "should' re-bias the tubes when you switch them but the amp that I was going to build does not have a bias pot. Hmmmmmm.....

PeteG45
September 15th, 2009, 11:05 PM
Scott, will the resonance control have no effect at zero? Will the amp sound basically like it would with just the resistor or does it color the sound regardless of the position of the pot?

ThermionicScott
September 15th, 2009, 11:13 PM
Theoretically, it should do nothing when the knob is all the way down. It'd be like wiring the two leads of the capacitor to each other on one end of a resistor.

PeteG45
September 16th, 2009, 08:42 AM
Thanks. What do you think of the idea of using a single 6L6 vs. a pair of 6V6 tubes? I'm still leaning towards the pair of 6V6s, kind of a Tweed Princeton on steroids.

How critical is tube placement on top of the chassis?

Is there a minimum distance between the preamp tube and the OT?

Docron10
September 16th, 2009, 10:26 AM
here is the link http://www.myspace.com/twostrokeampkits
a quick question re: your previous post on the OT.
would the Doberman 20wattSE made by heyboer work in this application you are talking about?

MikeMurray
September 16th, 2009, 10:37 AM
Don't mean to threadjack but with a larger transformer, would it not be possible to run something like a 6550 and what would the effects of using one be? (On the topic of the output circut :) )

ThermionicScott
September 16th, 2009, 02:15 PM
How critical is tube placement on top of the chassis?

Is there a minimum distance between the preamp tube and the OT?

I think some of the bigger tubes (like KT88, 6550, EL34) had specs for how close they could be to each other, but you probably don't have to worry here. The chief concern is heat -- you don't want them close enough so that they are "sharing" heat.

There was a time when I dreamed about making a Champ-on-steroids with a KT88 and some big iron, but I never got around to it... it would be novel, but *heavy* for what it puts out!

- Scott

PeteG45
September 16th, 2009, 08:32 PM
All right. Tonight I'll start marking the holes for the chassis.

Docron10
September 17th, 2009, 12:13 PM
I have been looking for a setup just like this...glad I happened upon this thread..
pete are you getting this kit from Weber and then modding it?...also..in Hunters book his 2 stoke project called for 2 speakers - a 10' and an 8'. then in his revision on his myspace mentioned using a 12"??? what speaker config are you planning on?

PeteG45
September 17th, 2009, 02:51 PM
I'm not buying it as a kit. I bought a flat piece of aluminum and bent up a chassis. I'm buying parts from both Weber and Tubes and More plus a few locally. I'm going to start a thread soon with photos but right now there isn't much to look at.

I'll be using Weber transformers and board, Sprague and Mallory caps, a mix of carbon comp and carbon film resistors, Switchcraft jacks and Alpha pots. I don't know which power tubes yet (maybe JJ's) but I plan to use a Sovtek 12AX7LPS in V1. I don't know anything about rectifiers so I'll probably look for recommendations.

I'm building this as a head unit. I built a 1X12 extension cabinet a few months back and I'll be purchasing a new speaker just for this amp. I haven't decided what speaker yet so if you've got suggestions I'm all ears. I thought about an Eminence Texas Heat or Can. Rex but I don't know if they would work well with this amp.

Docron10
September 17th, 2009, 03:45 PM
you may want to go to the ax84.com website. the have some interesting info and some templates you can download for your chassis that may help your layout. I dont know enuf about different speakers to give any reccomendations...thats why I asked you..lol..i do like the idea of two speakers...more air to push

ThermionicScott
September 17th, 2009, 05:46 PM
The nice thing about the JJ's is that you won't have to worry about the dissipation being too high in your amp. :cool:

trip20
September 18th, 2009, 11:14 AM
PeteG45 - Sorry to hijack this "very cool" thread but what program are you using to draw the layouts?

PeteG45
September 18th, 2009, 02:59 PM
I did use some of the info from the AX84 site to help with the layout and the drilling. No problem about hijacking the thread. I used the layouts from the Weber site and then used good old "MS Paint" to make the modifications.

I'm getting ready to order the transformers, board, wire and the rest of the chassis mounted components (switch, lamp, fuse holder, power jack or cord).

Does anyone know approximately how much wire I'll need? Weber's BOM says:

(8 ft. Red, Yellow, Green, 3 ft. Black, White)

Can that possibly mean 8 feet each of red yellow and green plus 3 feet each of black and white? That's 30 feet of wire. Do they mean maybe 2' of red, 2' of yellow, 4' of green, 1.5' of black and 1.5' of white?

amped_esquire
September 18th, 2009, 08:02 PM
If you haven't ordered the transformers yet I highly recommend getting Mercury trannies. I bought them from Ted Weber for $150 for the pair of them (OT and PT). Well worth the bucks my friend. I also picked up his signature alnico 10" and a chassis from him. The parts I got from Antique Electronic supply including the orange drop caps, and large blue sprague capacitors for that old school Fender mojo. Great experience with both of them. As far as tubes go I went with a NOS JAN Philips 5751, JJ 6v6, and a NOS RCA 5Y3. Happy building!

JohnnyCrash
September 18th, 2009, 09:09 PM
PeteG45 - Sorry to hijack this "very cool" thread but what program are you using to draw the layouts?



If it helps: Serif DrawPlus SE is free and so far, just as good as my old copy of Adobe Illustrator...

PeteG45
September 18th, 2009, 11:14 PM
I'd like to go with the Mercury Magnetics transformers or even the Heyboers but I'm afraid that they aren't in my budget. For now the Webers will have to do. Here is what I have done so far.

PeteG45
September 20th, 2009, 01:27 AM
If it helps: Serif DrawPlus SE is free and so far, just as good as my old copy of Adobe Illustrator...

I downloaded a copy this morning and this is so much better than paint. It's a pain to convert to JPEG for posting here but I can export to PNG and get a really clear image.

THe best part about this is that it's so easy to change the wire color. When I finally get my wire I can make the diagram match the colors that I plan to use. That should make wiring a lot clearer.

Here is the converted jpeg. Like I said I lost a lot of sharpness in the compression.

PeteG45
September 21st, 2009, 11:06 AM
This may be crazy but... right now, given my latest revision, I can only run with both power tubes. What if I add a second cathode bias resistor/capacitor to ground? I could wire the tube sockets so that one of them has all of the primary connections and the other runs only off of the first tube (except for the extra cathode bias resistor/capacitor). Then could I remove one of the two power tubes and run at half power? I would have to bump bump the resistor value back up to the 470-500 Ohm range but would I have to make any other changes?

ThermionicScott
September 21st, 2009, 12:37 PM
If I were to do that, I'd probably disconnect the two cathodes from each other, and give each one its own 470-ohm resistor + 25uF capacitor. Then it's pretty idiot-proof when it comes to plugging and unplugging output tubes. :grin:

- Scott

limbe
September 21st, 2009, 12:57 PM
I donīt think so.My amp built that way with a 2,5 k transformer and 2 6V6 still works fine after years of use with 1 or 2 powertubes.(The mismatch of load impedance with just 1 tube in the socket doesnīt seem to bother it.)

limbe
September 21st, 2009, 01:10 PM
Sorry!I just saw that Scott answered your question 20 minutes before me and he is absolutely right!Thatīs the way I built mine.

PeteG45
September 21st, 2009, 01:20 PM
Cool. This is starting to look very similar to the Dave Hunter two stroke amp. This should also handle a single 6L6 if I separate out the cathodes, correct?

I had also asked earlier about how much wire I would need to complete the job. Weber's B.O.M. says "( 8 ft. Red, Yellow, Green, 3 ft. Black, White)". Do you think that means 8 feet of red 8 feet of yellow and 8 feet of green plus 3 feet of black and 3 feet of white? That seems like an awful lot of wire. What seems reasonable?

Juancete86
September 22nd, 2009, 09:51 AM
Im trying to build the same amp, so im taking notes on everything, have u planned on adding a trebble, middle and bass control? is there a way to get more than thoose 15 watts output? im trying to make mine as much as 50w or so, but no idea regarding the ot transf.

PS. Sorry, my english kind of sucks...

PeteG45
September 22nd, 2009, 10:03 AM
I'm not going to add a TMB tone stack. I'm going to stay with the original tone circuit plus the presence and resonance controls. I don't know how you'd get 50 watts out of a pair of 6V6s (I am really new at this).

Here is my updated layout with separate cathode bias circuits for both power tubes. Can someone take a look specifically at the section between the fourth filter cap (left to right) and the first coupling cap? Also, is it OK to run the return from the tone stack under those two 100K resistors?

http://home.roadrunner.com/~pgiulietti/sc11_02.png

big-daddy-59
September 22nd, 2009, 10:29 AM
just a quick comment. Having just completed a 5f2a set up for a 6l6gc, let me warn you, if you want to try running a single 6l6gc in this amp,use 50v or higher rated caps for your cathode bypass. Don't ask how I know this,just do it. just my .02

PeteG45
September 22nd, 2009, 02:10 PM
Thanks for the tip. I had already seen a similar comment in a couple of different places. What about the 25uF cap that on the 12AX7? The schematic shows 25v but I was thinking 50v there as well.

Docron10
September 22nd, 2009, 04:33 PM
your have some cap and resistor values that are different from hunters design? is that b/c you are using a different OT?

PeteG45
September 22nd, 2009, 07:13 PM
I didn't start with Hunter's design. I started with a 5F2A and split out parallel power tubes with a different OT. Then I added the presence and resonance controls. Then I decided to bias the power tube separately so that I could run with one or two 6V6s and maybe even a single 6L6 or a KT66.

big-daddy-59
September 22nd, 2009, 10:10 PM
Pete, don't worry about the preamp cathode bypass, there's only like .2 volts going thru that one. But the power tube bypass cap with a 6l6gc and 470r resistor will be seeing 30 volts and that figure has the tube biased slightly cold at 24 watts (80% max dissipation). pop a 360 or 330 ohm resistor in there to get the bias up around 90% and that cap has to pass 35 volts or more.

PeteG45
September 22nd, 2009, 11:26 PM
So I'd have to place two resistors on the board and a switch to go between them. Would something like this work?

PeteG45
September 22nd, 2009, 11:27 PM
Man, this thing has really earned the name "Scope Creep".:grin:

big-daddy-59
September 23rd, 2009, 12:19 AM
yes thats the way I would do it, just be sure the switch will handle the amount of current going thru it.

ThermionicScott
September 23rd, 2009, 01:01 AM
Man, this thing has really earned the name "Scope Creep".:grin:

LOL, it sure has. It's possible that the 6L6 will make enough extra power no matter where the bias ends up that you won't have to worry about it. Consider that Fender used 250-ohm cathode resistors for the 2x6V6 and 2x6L6 amps.

BTW, good tip on the caps, big-daddy-o! I use 50V bypass caps on output tubes even if I don't expect to pass 25V. It makes ordering a little simpler. :lol:

- Scott

PeteG45
September 23rd, 2009, 07:40 AM
How do I calculate current for a switch at that point?

big-daddy-59
September 23rd, 2009, 11:06 AM
i would just use one thats rated for 2-3 times the fuse/circuit breaker value.i.e 1 amp fuse=2 or 3 amp rated switch.

RichardM
September 23rd, 2009, 10:01 PM
So I'd have to place two resistors on the board and a switch to go between them. Would something like this work?

I'm confused.

Isn't this.

http://members.optusnet.com.au/richardmurray/Richard/Graphics/Guitars/bias_switch.png

The same as this.

http://members.optusnet.com.au/richardmurray/Richard/Graphics/Guitars/bias_switch1.png

Which is just this?

http://members.optusnet.com.au/richardmurray/Richard/Graphics/Guitars/bias_switch2.png

:confused:

PeteG45
September 23rd, 2009, 10:01 PM
Speaking of fuses and amps, is the value of the fuse used based on the power transformer, the number and type of tubes, the combination of both or something altogether different?

The Weber diagram and the Two Stroke show a 1 amp fuse which I assume is still the correct value even given these changes.

PeteG45
September 23rd, 2009, 10:03 PM
I have updated the layout (again) and here it is:

http://home.roadrunner.com/~pgiulietti/sc11_03.png

PeteG45
September 23rd, 2009, 10:09 PM
Richard, the switch is there so that I can use the same cap with two different bias resistors. A 470 Ohm resistor to bias a 6V6 and a 360 Ohm resistor to bias a 6L6. If I connect the cathode to the common lug and then the capacitor and each of the resistors to the switch lugs I should be able to combine R1 and C or R2 and C but not R1 and R2 and C. Right?

RichardM
September 23rd, 2009, 10:16 PM
Richard, the switch is there so that I can use the same cap with two different bias resistors. A 470 Ohm resistor to bias a 6V6 and a 360 Ohm resistor to bias a 6L6. If I connect the cathode to the common lug and then the capacitor and each of the resistors to the switch lugs I should be able to combine R1 and C or R2 and C but not R1 and R2 and C. Right?

OK. it makes more sense in the newest layout.

Double pole switch so the the resistors are totally isolated from each other.

It was just that if the physical wiring was as the first drawing, the three components were always connected. :oops:

PeteG45
September 23rd, 2009, 11:01 PM
Yup, I see it now. Doh! Thanks.

big-daddy-59
September 24th, 2009, 07:22 PM
as far as figuring out the value of the fuse I'm in the same boat as you are..i.e. just stick with the value on the schematic. Just make sure the fuse is the weakest link in the chain by using parts with a least twice the current capacity.

Oh yeah regarding wire.. 3 feet each red blue black and green was plenty for a 5f2a but with the extra stuff you are planning making it 5 feet except for green should do. I didn't use a kit. I also found the solid core wire was easier to work with because it would retain its shape better.

fernando
September 24th, 2009, 09:32 PM
nice take on the 5f2a!!!! wow the presence and resonance controls look promissing!

PeteG45
September 25th, 2009, 12:50 AM
I just got the rest of the chassis mounted parts tonight (pots, switch, power connector, lamp, fuse holder). I'm short the switch to change the resistors but that was added after I placed the order. I'll get the holes drilled or cut for these this weekend and get everything test fitted.

I somehow ended up with an IEC mains connector that has a fuse inside of it. Since it's a pain in the butt to change compared with the panel mounted fuse I'm going to use a 3 amp fuse in the connector and a 1 amp fuse in the panel mounted fuse. Hopefully I never have to change the one in the power connector that way.

emu!
September 25th, 2009, 04:07 PM
I built the Two Stroke a few years back.

For some reason, right when I ordered my kit from Hunter, he decided to make it a One Stroke :sad:, and eliminated the parrallel output tubes. I had to drill out the chassis for the second output tube.:sad::sad:. But, I'm glad I did because I now have the ability to use 2 6V6s in parrallel mode. BTW, I used seperate cathode resistors for each power tube...that way, I could still use the amp with a single 6L6 or EL34. The output tranny has a 4 and 8 ohm connection...so, when using a single 6L6 or EL34 or dual 6V6s, the 4 ohm speaker connection is used. When running a single 6V6, the 8 ohm connection is used. I've been gigging with it for a couple of years trouble free. The EL34 output is so touch sensitive, I doubt I'll ever use another amp until this one craps out.

PeteG45
September 25th, 2009, 07:09 PM
Hmmm... based on that I may want to wire up the 4, 8 and 16 ohm output connections. I plan to buy a new speaker for this amp but right now I have a 16 ohm speaker in the cab. And to think that I just picked up a pair of Switchcraft jacks today. Oh well, one more trip to the electronics shop.

Creepin' again!:smile:


http://home.roadrunner.com/~pgiulietti/sc11_03.png

Docron10
September 27th, 2009, 11:45 AM
instead of having mulitple output jacks you may consider a rotary ohm selector switch to just switch between which ohm output you want
http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=thispage&thispage=parts7.htm&ORDER_ID=287440673

PeteG45
September 27th, 2009, 03:48 PM
I've considered using a switch instead of multiple jacks. When I was going to use only two output jacks I figured it was unnecessary but now I may have to reconsider. It looks like tubesandmore.com carries a similar, possibly identical switch.

Now that I look at my latest diagram mine is wired incorrectly? I think. Because all of the tips are wired together so all of the output taps will be connected to all of the jacks all of the time. That can't be right. can it?

I should use an impedance selector and run it between the OT and the output jack and then wire the NFB loop off the jack like I normally would.

ThermionicScott
September 27th, 2009, 06:39 PM
I should use an impedance selector and run it between the OT and the output jack and then wire the NFB loop off the jack like I normally would.

That would get my vote! Now, you've freed up a space for the NFB switch. :razz:

- Scott

PeteG45
September 27th, 2009, 10:12 PM
I don't really know what effect having an NFB switch would have but for the price of one switch it almost seems silly not to include one. Some day soon I'll be done designing this thing. All it needs now is some fuzzy dice.

ThermionicScott
September 27th, 2009, 10:39 PM
LOL, and that beaded fringe stuff they used to hang in cars. :lol:

I recently disconnected the NFB in my 5F2A and it seems a little more "overtoney" when clean, and the transition into distortion is more gradual. A single pentode (or beam power tube) will build up a lot of harmonic distortion (the RCA book says 8-12% for a 6V6) before they even get to clipping, just because of their nature. That's why you'll see that the narrow-panel Champ and Princeton have a lot more negative feedback than the other Fenders of the time.

I'd *highly* recommend trying it both ways! :cool:

- Scott

Juancete86
September 29th, 2009, 09:07 AM
seeing this post make me want to build it! more than anything cause i've wanted a valve amp for a while now....but i have a 4x12 240watt 8ohm cabinet.... woult it run ok on it? or would you suggest building more of an 5E3 or 5X3...??

emu!
September 29th, 2009, 10:45 PM
I incorporated a NFB switch on my 5F2A. But it always stays "off". I wish there was a way to set it up in a foot-switch...that way I would probably use it more.

PeteG45
October 2nd, 2009, 02:00 PM
Hey all sorry to drop of the radar screen like that but I'm having some health issues plus both my wife and daughter have been diagnosed with Swine flu. It's been a fun week around here.

I did get a chance to update my layout last weekend I just haven't been able to post anything. Here is my latest update, tell me what you think. I decided to rotate the power tube sockets to keep the heater filament wires as close to the bend in the chassis as possible.

http://home.roadrunner.com/~pgiulietti/sc11_05.png

ThermionicScott
October 2nd, 2009, 04:44 PM
Swine flu, eh? Good luck with that!

I used a couple sick days to work on my 6AU6 amp, but do what you need to get everyone better!

- Scott

PeteG45
October 2nd, 2009, 06:50 PM
I wish that I was doing that (taking sick days to work on something fun).

What's a 6AU6?

limbe
October 3rd, 2009, 04:24 AM
6AU6 is a vacuum tube,actually a RF Pentode normally used in tuners but if you wire it as a triode it can be used to drive an output stage in an audio amplifier.I assume that Scott is building an amplifier using 6AU6 as a driver. Limbe

PeteG45
October 6th, 2009, 07:47 AM
Looks like my project may be on hold for a while. This little health issue that I ran into is costing me over $600 out of pocket (so far). That's going to cut into my 'just for fun' money for a bit. Maybe I can get some parts for Christmas.

ThermionicScott
October 6th, 2009, 11:46 AM
6AU6 is a vacuum tube,actually a RF Pentode normally used in tuners but if you wire it as a triode it can be used to drive an output stage in an audio amplifier.I assume that Scott is building an amplifier using 6AU6 as a driver. Limbe

Nope, I built a push-pull amp using two pentode-wired 6AU6's in the output stage. Kind of an outside-the-box project. :cool: http://www.tdpri.com/forum/shock-brothers-diy-amps/145252-push-pull-6au6-build-thread-warning-verbose.html

Get better, Pete!!

- Scott

big-daddy-59
October 7th, 2009, 01:51 AM
Pete , sorry to here that you've got to put your project on hold. Hoping you and your family can all get well soon. Sucks having to give up your fun money to pay doctors bills.

PeteG45
October 7th, 2009, 02:08 PM
Thanks big-daddy. It's only on hold and I'm not in any hurry. I am going to take this as a chance to learn more about what I'm about to do. I found another error in my layout, the left most lug of my volume pot was not connected to ground.

Speaking of grounds, I was planning to run a star grounding scheme splitting the amp into three stages. The first stage would be: input jacks, V1 bias, volume and tone. The second would be: OT-speaker jack, cathode bias V2 and V3 and presence control. The third would be the main filter caps. These would each connect to a common wire located near their respective components that would all lead back to one of the PT mounting nuts.

I have a few questions about this. How do I handle the grounds for the power tubes? Do I run a common wire from the tubes back to the PT or would it be better to run the OT and the power tube grounds together and then back to the PT (instead of running the OT ground back up under the board)? Am I correct that this would mean that I have to isolate my jacks from the chassis? Would the pots also need to be isolated from the chassis?

big-daddy-59
October 8th, 2009, 09:11 AM
Pete I have a question for you. Are you participating in any other amp building forums? I like to suggest that you check out the Ampage forum http://music-electronics-forum.com/f10/ and the Weber forums as well. You will have to register for that one. But there is an extensive thread on grounding there started by Lbusters that you should really check out. I didn't isolate any of my jacks or pots and used a two point grounding setup with PT center taps connected to a PT bolt along with the first three filter caps. the fourth one is the preamp supply and it was grounded to a buss along with everything else pots jacks preamp cathode etc. that buss was terminated by forming a loop in the end and bolting it down to the chassis behind the the input jacks AWAY from the transformers. my amp is very quiet, no hum or hiss at all until it's dimed and even then its very slight. The power and rectifier tube don't need grounded. the power tube is grounded at the cathode and the rectifier grounds at the filament windings center tap of the PT.

PeteG45
October 8th, 2009, 02:42 PM
I was thinking about doing something like that but I wasn't sure how to connect the end of the bus and where. Where did you ground your output jacks and OT?

I'll definitely check out the Ampage forum. I have read a little in the Weber forum I guess I'll do some more. Thanks.

limbe
October 10th, 2009, 07:58 AM
Nope, I built a push-pull amp using two pentode-wired 6AU6's in the output stage. Kind of an outside-the-box project. :cool: http://www.tdpri.com/forum/shock-brothers-diy-amps/145252-push-pull-6au6-build-thread-warning-verbose.html

Get better, Pete!!

- Scott

Calling your amplifier an outside-the-box project is really an understatement.I hope that you have an isolation transformer and/or a ground fault breaker.No,I didnīt think that you would use 6AU6,s in the output stage but it looks like an interesting experiment.Iīm following your thread to see where it ends up.Good luck!Limbe

PeteG45
October 12th, 2009, 01:28 AM
Well I spent a lot of time on the Ampage forum over the last couple of days and I think that I have a pretty good understanding of what I need to do to ground this thing. I may even be able to order some more parts in the not too distant future and maybe get this thing done before Christmas.

PeteG45
October 23rd, 2009, 11:13 PM
Believe it or not the circuit is changing again. I've added a voicing switch like the two stroke and changed the connection point for the NFB to the 8 ohm tap of the OT (suggested as something to try).

I'm waiting on the board and some wire, they should be here next week and I'll be able to start in again. The sooner I start the sooner I'll stop making changes. Here it is:

http://home.roadrunner.com/~pgiulietti/sc11_08.png

Cap'n Kier
October 25th, 2009, 10:01 AM
Are you going to be making your own board or modifying a stock 5F2-A board? or getting one custom made?

Looking good so far! I'm hoping to make a two-stroke soonish so will be good to see how your mods work out :)

Dr.Ow!
October 25th, 2009, 01:57 PM
This is a fun thread to follow. Like you I have been pondering,learning , and reading to gather up enough infos to do a scratch build. Either 5f2a if I can get a good SE OT @ Ebaycheap $, or a 5E3ish thing using a 8kpp ot I already have. I got a potted Stancor 70mA PT same as these use @Ebay recently.

Anyway something I have considered due to my own 'Scope Creep'(great term) malady/syndrome Is when I do my first build I'll do my darnedest to K.I.S. by building first true to a proven layout and spec, and then step by step add in the alternates. That way you can get fired up with minimal hassle, and then too really be able to hear any worthwhile effect/impact of your mod ideas. I think I heard this suggested in my early reads @ MEF or Hoffmans.

Hope you and yours are feeling well now, and warming the iron, look forward to hearing if you like the presence/reso circ.

John

limbe
October 27th, 2009, 05:05 AM
Yes!It is a good idea to build the basic amp,see that it is working properly and then add the other things one by one.I work that way myself even though Iīve built more than a few amps.Limbe.

PeteG45
October 27th, 2009, 09:05 AM
Yeah that would probably be a better way to build it but then I couldn't call it "Scope Creep". :O)

I started a new thread to document the slow build process here. (http://www.tdpri.com/forum/shock-brothers-diy-amps/183393-slow-scope-creep-amp-build.html)

Dr.Ow!
October 28th, 2009, 02:27 PM
That's a good point. Where I tend to get pulled is into complicated tone stack designs. I want to keep to a simple build, but a tweed T just aint enough(even though it is:sad:)...so i get sucked down into the D***le ts thing and lay in 3 switches, or tmb w/ selectable slope and fat, or or...

Nice work your doing on yours. The really cool thing about building these is once you have it and know what's happening you can change it up whenever you want to. or go back to the original.

K.E.B.
August 31st, 2010, 09:31 PM
Wow. It's been almost a year! Update?