5F2-A Question for a first build

[Scott S]

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.

- Scott

[limbe]

I don´t 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 doesn´t seem to bother it.)

[limbe]

Sorry!I just saw that Scott answered your question 20 minutes before me and he is absolutely right!That´s the way I built mine.

[PeteG45]

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]

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]

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?

[big-daddy-59]

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]

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]

your have some cap and resistor values that are different from hunters design? is that b/c you are using a different OT?

[PeteG45]

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]

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]

So I'd have to place two resistors on the board and a switch to go between them. Would something like this work?

[PeteG45]

Man, this thing has really earned the name "Scope Creep".

[big-daddy-59]

yes thats the way I would do it, just be sure the switch will handle the amount of current going thru it.

[Scott S]

Originally Posted by PeteG45

Man, this thing has really earned the name "Scope Creep".

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.

- Scott

[PeteG45]

How do I calculate current for a switch at that point?

[big-daddy-59]

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]

Originally Posted by PeteG45

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.



The same as this.



Which is just this?

[PeteG45]

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]

I have updated the layout (again) and here it is:

[PeteG45]

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]

Originally Posted by PeteG45

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.

[PeteG45]

Yup, I see it now. Doh! Thanks.

[big-daddy-59]

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]

nice take on the 5f2a!!!! wow the presence and resonance controls look promissing!

[PeteG45]

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!]

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 , and eliminated the parrallel output tubes. I had to drill out the chassis for the second output tube.. 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]

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!

[Docron10]

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/p...R_ID=287440673

[PeteG45]

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.

[Scott S]

Originally Posted by PeteG45

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.

- Scott

[PeteG45]

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.

[Scott S]

LOL, and that beaded fringe stuff they used to hang in cars.

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!

- Scott

[Juancete86]

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!]

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]

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.

[Scott S]

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]

I wish that I was doing that (taking sick days to work on something fun).

What's a 6AU6?

[limbe]

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]

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.