$vboptions[bbtitle]

Harvard (5F10)

MeanGreenBlues
June 3rd, 2009, 10:56 PM
Thought I'd share the latest pics on my current personal project. A Tweed Harvard 5F10 made from a Weber 5F2A chassis and cab.

Got a bunch of parts including the transformers recently. All that's really left is to make the circuit board (I have all the electronics), a couple of gromets, and the speaker (still shopping). Here's the latest round of pics:

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-08.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-09.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-10.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-11.jpg

A few more pics to follow...

MeanGreenBlues
June 3rd, 2009, 10:57 PM
http://www.tjadamowicz.com/amps/gallery/5F10/5F10-12.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-14.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-15.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-16.jpg

That Allen/Heyboer TP25 is TALL! There's, maybe, 1/8" clearance from the bell to the front of the cab. It fits though. The OT needed one hole drilled in the chassis, too.

I'm thinking about a pot with SPST switch in it for the volume pot to wire up as a standby. Not sure if I'm going that way yet.

I'll update this thread as the wiring gets going.

shadowfan
June 3rd, 2009, 11:19 PM
Hi

beware of a pot mounted switch passing B+ as it may not be stout enough insulationwise. Also, I'm not sure I'd want B+ so close to the signal path. Some may disagree.

JohnnyCrash
June 3rd, 2009, 11:40 PM
It looks like he'll be using a 3 way Off-Standby-On switch instead of the Volume pot On/Off switch.

I like Harvards. Primitive, but with a bit more oomph than a Champ.

This will be a fun thread to watch...

MeanGreenBlues
June 3rd, 2009, 11:46 PM
Hmmm...that's not something I thought of, Johnny. That's just an On-Off DPST that I planned on for all the PT leads.

Also good points, Shadow. Again, I'm not decided if I even need a standby, but still entertaining ideas.

Thanks, fellas.

FiddlinJim
June 5th, 2009, 03:54 PM
That's always looked like a great in-betweener to me. With a few lower-valued coupling caps than the Deluxe, it might be a little brighter. I've always wondered how that 6AT6 preamp sounds. Cool build. I'll be watching this thread!

MeanGreenBlues
June 5th, 2009, 11:53 PM
Hey, FiddlinJim!

I actually scored five NOS RCA 6AV6s for this for 99˘ I'm as anxious as you are to hear them. They're apparenly just one half of your standard 12AX7 type with a diode section that's unused. The 6AV6 is slightly higher gain than the 6AT6, from what I have read/seen. Kind of like a 12AT7 to 12AX7.

I have the tube retainer clips on and plan to make the circuit board this weekend!

JohnnyCrash
June 6th, 2009, 12:15 AM
Hmmm...that's not something I thought of, Johnny. That's just an On-Off DPST that I planned on for all the PT leads.

Also good points, Shadow. Again, I'm not decided if I even need a standby, but still entertaining ideas.

Thanks, fellas.



I don;t believe the cathode stripping thing. On-Off is plenty good :)

Unless you want a "mute" switch... er I mean a Standby :)

I really hope you make sound clips!

MeanGreenBlues
June 6th, 2009, 12:50 AM
I hope to get some sound clips...if you can tolerate my playing. I'm hoping to capture the Steve Cropper Stax type sounds, but it's really just that I love the split-load/cathodyne tweed PI, but wanted a low watt, grab'n'go in the stable.

I'm thinking this is goint to be a 10-12 watt deal. Not sure if it needs a standby, personally, but I always keep that option in the back of my mind just in case.

I'm with you, though, just turn the volume down all the way if needed, eh?

MeanGreenBlues
June 7th, 2009, 10:20 PM
Got the board built and populated. Mounted it in the chassis and grabbed a few pics:

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-18.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-19.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-20.jpg

Wiring begins next! I always forget how tight it is in the tweed chassis until I get inside one again.

JohnnyCrash
June 8th, 2009, 02:07 AM
Nice work.

Very cool going with a trimpot in the bias circuit - old fixed-bias always benefit from adjustable trimpots.

Weird how the new Mal's use white for the smaller caps these days, huh? I guess they're not called "Mallorys" anymore though, are they? I still use em all the time :)

Is the switched output jack just temporary, or are you planning on putting a big-watt resistor in there for protection?

MeanGreenBlues
June 8th, 2009, 10:33 AM
Yeah, Johnny, I noticed that Mallory switched to the white on the latest few batches of .022µF caps that I got. I wonder if the rest will follow? I hear the white film isn't as warm sounding as the yellow film. :mrgreen:

The switched jack is pretty much what I had on hand. Which resistor are you referring to? I assumed having the jack shorted was safer for the OT than it being open in the case that a speaker wasn't plugged in or something like that. I was going to wire it modern style with the switch for shorting.

Maybe I'm just confused though...happens often.

shadowfan
June 10th, 2009, 12:25 PM
You DON'T want to short the output! (probably worse than an open circuit for the OT); open circuit will blow the primary, short will blow both primary and secondary.
For safety you really need an 8ohm 20W resistor; problem is finding one (you could use 2 10W@16ohms OR 4 5W@32ohms --or closest values--resistors in parallel).
Another solution is to hardwire the speaker to the jack in such a way as to disconnect it when you plug in an external cab.

MeanGreenBlues
June 10th, 2009, 04:22 PM
Interesting, Shadow.

I just re-read some transformer FAQs written by a transformer designer R.G. Keen, and he states, "It's almost never low impedance that kills an OT, it's too high an impedance." Following this, wouldn't an open load be worse since it's a maximum impedance rather a shorted load being minimum impedance?

My understanding is that an OT can handle a shorted secondary but an open secondary (or too high and impedance) can blow the OT.

Of course, this is a combo amp, so it's less of an issue to mistakenly forget to plug in the speaker.

Cleeve
June 10th, 2009, 04:40 PM
Fender went with the shorting option.

shadowfan
June 10th, 2009, 05:18 PM
"Too low impedance, for instance a 2 ohm speaker cabinet hooked up to an amp expecting 8 ohms, will overload the output tubes and the output transformer. The lower impedance will force lots of amperage through the tubes and out through the transformer. You would get a very loud amp for a short period of time. Something would blow. The tubes probably would go first, but you could also blow the output transformer or the speakers. It is possible that a small power transformer would not take the high amperage and blow up.

Too much impedance, for instance an 8 ohm speaker cabinet hooked up to a 2 ohm Bassman head can be damaging. Mostly you get crappy sound, but there is less danger to the amp. The tubes will operate at higher voltage because the load is not letting the voltage bleed off into output and you might get shorting in the tubes or the output transformer. This is another way to fry an output transformer and it is why you shouldn’t run your amp without a speaker load".

This came from here: http://www.harpamps.com/transformers.html

A dead short is by definition too low an impedeance, in fact it's 0 ohms. It stands to reason that in a dead short in the secondary, the current x voltage (=watts) have nowhere to dissipate except as heat and therefore causing self destruction of the secondary. This is why Fender cautions about turning the amp without a speaker plugged in. If there is an open circuit, essentially an infinite impedance, then the primary does not dissipate its energy which can take its toll on the output tubes as well. All the above points to the fact that a suitable resistor is a better substitute to either an open circuit or a dead short.

As to why Fender use (and I know they do) a shorting jack is a mystery to me unless the feedback loop plays a greater role than I ever thought it could in dissipating the bottled up energy.

Cleeve
June 10th, 2009, 06:43 PM
I think Fender went with the shorting thing to be cheap!
I was always assuming that if you really stepped on a fender with no speaker connected and one had the proper fuse in, the fuse would go before anything else smoked- with luck.
If it were open, the fuse would never blow, or not until the insulation broke down somewhere in a winding.. Maybe the feedback would save the day then in an open load situation?
Some modern things seem to have an r-c load if left open, the little crate v5 for example.. something like a two watt ten ohm in series with a .1uf cap or similar.. I believe that is an artifact of solid state training though.

MeanGreenBlues
June 10th, 2009, 09:03 PM
Hmm...great discussion.

TO expand on my source from earlier:

If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

It's almost never low impedance that kills an OT, it's too high an impedance.

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike. This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punch through was one time, it wouldn't be a problem, but the burning residues inside the transformer make punch through easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer."

I'm far from an EE, but I've read a lot. Still, I'm always open to new information.

SamBooka
June 10th, 2009, 10:23 PM
My understanding (very very limited) is that open load on an OT is very bad very fast. This is very similar to the flyback effect in tv's
No load (dead short) is also very bad but you can get away with it longer. Shorting jacks are the lesser of the two evils.

MeanGreenBlues
June 10th, 2009, 11:01 PM
On a lighter note some updated shots of the wiring:

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-25.jpg


http://www.tjadamowicz.com/amps/gallery/5F10/5F10-27.jpg


http://www.tjadamowicz.com/amps/gallery/5F10/5F10-28.jpg


http://www.tjadamowicz.com/amps/gallery/5F10/5F10-31.jpg

:D

boozer
June 11th, 2009, 10:56 AM
meengreen, you do beautiful work,
i just finished a 5f2a for a member of the forum, and i was so impressed with the tone that now i have to go back and build myself one.
but now you got me thinking on a harvard
hahahhha

MeanGreenBlues
June 11th, 2009, 02:49 PM
As to why Fender use (and I know they do) a shorting jack is a mystery to me unless the feedback loop plays a greater role than I ever thought it could in dissipating the bottled up energy.

I've been digging on this some since we talked about it, and an EE on another electronics forum said:

"I investigated the missing speaker thing many years ago. I found that if an amp has NFB, and is stable with no speaker load, then it can stand being turned on with no speaker load, and even played. As long as the power amp isn't overdriven, the NFB acts to keep the speaker voltage down to a safe level, and hence the OT primary voltage too.

Once the power amp is overdriven, the NFB loses its grip and dangerous voltage spikes start to appear, but the measures I outlined in 2 and 3 above help to mitigate them. However, the mismatched load also causes the screen grids to overheat and melt when you crank it, and none of the measures helps with that. If the molten pieces of screen cause an internal short in the tube, this could finally blow the OT from overcurrent."

Verrrry eeenterestink...

JohnnyCrash
June 11th, 2009, 07:43 PM
Have you fired this thing up yet?

I've been waiting for this Frankenstein to light up that jewel lamp and roar...

:)

MeanGreenBlues
June 11th, 2009, 09:44 PM
Ha! It's not done yet! Still have the pots and inputs and circuit grounds, plus the AC cord and power switch.

Working on it tonight!

bluesfordan
June 11th, 2009, 11:07 PM
I happen to really like Steve Cropper's work that he's done with a Harvard, so when I tried one at our local GC, I had high hopes for it. At the time, I had a '62 tweed champ which I loved, and wanted to go a step up in the tweed world. Alas, that Harvard was not a healthy specimen, it didn't work when I first plugged it in at the store, it went away for 2 weeks. When it came back, it worked, but it was a dark, muddy beast, even with my tele. I know they are out there, but they come up for sale about as often as a tweed lo power twin it seems.

So I'm following your build with great interest. What speaker are you going with?

MeanGreenBlues
June 15th, 2009, 10:54 AM
I finished it this weekend. Well, still need a speaker and a couple of tubes (a 5Y3GT and JJ 6V6 to be exact), but the build is done!

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-32b.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-33.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-36.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-39.jpg

So close to getting it fired up!

Cleeve
June 15th, 2009, 11:05 AM
It looks great- I'm jealous..
If that black resistor to ground that's got one end unsupported bothers you, a nearby unused tab on the rectifier socket could be used.

MeanGreenBlues
June 15th, 2009, 11:34 AM
Thanks! I thought about that, Cleeve, but it doesn't bother me, and I'll be measuring at the cathodes of the power tubes, so it should be okay.

It's the 1ohm resistor for bias measurement.

JohnnyCrash
June 15th, 2009, 11:46 AM
What speaker are you planning on using?

That thing looks great!

MeanGreenBlues
June 15th, 2009, 11:54 AM
Thanks, Johnny!

I've got some good pals who really like the Weber Sig10S, and I trust their opinions. I haven't really tried a Weber for any length of time either, so I figure this is a good opportunity to try.

mchet
June 15th, 2009, 12:26 PM
I am jealous!!!! Where did you get your schematic and layout from? My brother wants a princeton and I am pulling parts to build it for him, but maybe I will do a couple harvards instead!

MeanGreenBlues
June 15th, 2009, 02:31 PM
Hey, Mchet, there are a lot of layouts and schematics here: http://www.cnjradio.net/fndramp2.html

Of course, the Harvard is a push-pull amp compared to a Tweed Princeton being single-ended, but I chose the Harvard because I wanted a step-down from a Deluxe 5E3. Even then, the fixed bias and negative feedback loop of the Harvard makes it even different than the 5E3.

The trickiest parts to find are the 7-pin socket shield, and even that was not too hard to find. It's even easy to find good NOS 6AT6 or 6AV6 preamp tubes since they're not in high demand like the 12AX7 types. It's pretty straightforward of a build, really.

FiddlinJim
June 15th, 2009, 03:56 PM
I love my Princeton build, but this is giving me the itch to Harvardize it. Did you just drill a third input between the Princeton's original 2? I've got a pal with some Greenlee punches that I could probably use for the preamp and second 6V6... Mine is already the 10" cutout in the big cab.

Of course, I'm waiting on the "how's it sound?" question to be answered.

MeanGreenBlues
June 15th, 2009, 04:15 PM
Indeed I just drilled the third input in between the two existing on the 5F2A chassis. It's really tight, so the new hole has to be right in the middle. I punched the power tube socket, but had to use a step bit on the smaller preamp tube socket.

I'm even more excited to hear it fire up, too! I've got a lead on a Weber Blue Pup now, so it's a toss up on the Blue Pup or the Sig10S now.

appar111
June 23rd, 2009, 08:56 PM
I'll be swapping some speakers on my 5F6A (two ceramic Sig10S, and one Emi Ramrod) if you're interested... I'll sell 'em for a great price.

MeanGreenBlues
June 24th, 2009, 11:36 AM
Oooo...I just ordered the Alnico Sig10 S yesterday, but may be interested in the Ramrod. Sent a PM!

SnidelyWhiplash
June 24th, 2009, 01:58 PM
That has to be one of the most beautiful layouts i have ever seen. Fantas-
tic! Harvard are great underated amps. I hope she sounds as good as she
looks.

:smile:

bmcmusic
June 25th, 2009, 12:42 AM
Great Job, I built a Harvard and donated it for auction to the TJ Martell Foundation of which Steve Cropper is a board member. I tried using a 6AT7 preamp tube but couldn't find one quiet enough so I ended up with a 12AX7 and then decided to cathode bias it at 10 watts. It was signed by the various artists that performed at the event. Now I miss it and need to build another.

It sounded awesome and I have a few sound samples on my page.
http://www.mcmusicsound.com/ampscabs/mvapro2ube.html
Bob

MeanGreenBlues
July 9th, 2009, 12:23 PM
Sonic update:

I finally played the Harvard extensively a few times...well extensively for me...a couple of hours straight at a time. Have to wait for the kids to be in bed, but when I could, I got my CV Tele out to play through it.

Here's the thing...it's REALLY clean compared to a 5E3. I never got the volume up all the way yet, but at 2 o'clock on the volume knob if I hit a chord hard enough with the guitar up all the way, I could get it to break up a little.

It's definitely very touch sensitive, though, and it picks up all the little finger noises on the strings...it feels great. Of course, it could be that the NOS RCA 6AV6 preamp tube is helping in that department. The 12AX7 is a Tung-Sol RI and I have an NOS Tung-Sol 5Y3GT for the rectifier. I have JJ 6V6s in the power slots and I got it settled in at 68.5% max plate dissipation (measured against 14W max).

Man, I'm really impressed with this guy. I love the grunge of the tweeds, don't get me wrong, but I prefer them cleaner overall, so that the wide-open grit is still tight.

I will check the bias again soon to see if it's shifted, and the speaker needs some break-in time, but so far, I really really like it! I also plugged my OD pedal in at one point (MI Audio Blues Pro) and it really sounded nice! The amp really liked the pedal and it did what it was supposed to do to a 'T'. More so than with my Tweed Pro clone.

I'm wondering if the speaker break-in will affect a lot or just a little. I settled on the Weber Sig 10S Alnico for this amp.

I have been hearing a lot about how underrated these amps are, and now I believe it. I think that the clean tone is right on par with the 5E3, as is the feel, but the clean headroom is amazing for such a little tweed. This is the perfect grab'n'go amp for recording and practice. Especially considering how well it responded to the OD pedal.

Here are some final shots:

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-41.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-43.jpg

http://www.tjadamowicz.com/amps/gallery/5F10/5F10-44.jpg

DrivinSouth
June 4th, 2010, 07:51 PM
Very nice build!

What plate voltages did you end up with at the power tubes, and at how many mA did you set the bias?

MeanGreenBlues
March 25th, 2011, 11:22 AM
Has it been that long since I checked in here?! Sheesh...

I still have this amp (even after a few offers to buy it) and will see about getting some readings for you.

ThermionicScott
March 25th, 2011, 01:14 PM
Wow, I missed this thread back in 2009. Must resist the urge to convert my Princeton! :lol:

- Scott

MeanGreenBlues
March 25th, 2011, 02:14 PM
This little amp rules! It's got to be one of the most under-rated, overlooked Tweed, IMO.

FiddlinJim
March 29th, 2011, 03:35 PM
Wow, I missed this thread back in 2009. Must resist the urge to convert my Princeton! :lol:

- Scott

Scott, resistance is both futile and pointless! Ok, well, not pointless, since Princetons are mighty fine, but I does love me converted fella. I used a vibrolux tag board and cut out the oscillator part of the board. Next time, I'd probably just punch my own eyelet board, but this did work fine. Nice punchy, 'tubey' little bugger.

TT2346
March 30th, 2011, 04:16 PM
I have looked at the Harvard also but I have a few questions
as to how you did it, Mean Green Blues...

What about the selenium rectifier? Is there still such an item to be found?
If you did not use one, how did you change the circuit?

What transformers did you use?

Is there any updates on the values in the circuit because of the difference in transformers and house voltages?

The schematic I have from the Fender Amp Field Guide has voltage marks at different points in the circuit...did your amp hit those marks?

As you might tell, I am a Newbie with more questions than understandings.

Thanks for all of your replies.
TC

TNO
March 30th, 2011, 07:59 PM
So I guess to use a 5F2 chassis you have to punch for another octal?

FiddlinJim
March 31st, 2011, 01:21 AM
... And a 7-pin for the first preamp, and a 3rd input.

MeanGreenBlues
March 31st, 2011, 11:17 AM
Right...you may not necessarily need the third input, but it will fit in there.

The chassis is laid out in a manner that when the holes are added, they don't really stand out as being a custom modification, which is pretty cool.

MeanGreenBlues
April 2nd, 2011, 10:32 AM
I have looked at the Harvard also but I have a few questions
as to how you did it, Mean Green Blues...

What about the selenium rectifier? Is there still such an item to be found?
If you did not use one, how did you change the circuit?

What transformers did you use?

Is there any updates on the values in the circuit because of the difference in transformers and house voltages?

The schematic I have from the Fender Amp Field Guide has voltage marks at different points in the circuit...did your amp hit those marks?

As you might tell, I am a Newbie with more questions than understandings.

Thanks for all of your replies.

I'm sorry I didn't answer this earlier! Let me give it a go now....

The selenium rectifier is replaced with a diode, really. Honestly, I just put in a standard adjustable-fixed bias circuit.

The transformers are Allen/Heyboer - TP25 for the power, and TO22 for the output. No choke used in this amp.

I can't remember what the exact voltages ended up being, but I wasn't terribly concerned because of the modern variables in place. I do remember it was close to original circuit drawings.

guitar1amp2000
April 2nd, 2011, 09:55 PM
I built mine from the stock Fender schematics. I used a vibrolux fiber board, just omitted the tremolo area on the board. The chassis was from Mojo, one that had been part of a special order for another customer but did not take full shipment. It had to be modified for the rectifier tube and the first preamp tube. I did not install the third input jack, but went with two. The transformers that I used were from Mercury. The speaker I used is a jensen alnico 10". One sweet little amp with a tele.

celeste
April 3rd, 2011, 12:12 AM
...
What about the selenium rectifier? Is there still such an item to be found?
If you did not use one, how did you change the circuit?
...

TC

Any time you see a selenium rectifier, just replace it with a junction or schottky diode, Selenium does not make a great diode, and there is a health hazard associated with them exploding when they fail

keithb7
April 3rd, 2011, 02:12 PM
Just out or curiosity, what was the reasoning for 3 or 4 instrument input jacks back in the day, as seen on the Harvard?

MeanGreenBlues
April 3rd, 2011, 09:24 PM
The 1 and 2 inputs were always set up for single and double coil pickups, or instruments and microphones, but the middle on on the Harvard is beyond me. I recall someone mentioning it may have had something to do with teachers being able to play along with the student.

Hopefully someone may have more information on this.

JohnnyCrash
April 4th, 2011, 02:16 AM
Just out or curiosity, what was the reasoning for 3 or 4 instrument input jacks back in the day, as seen on the Harvard?



Accordions, microphones, the new fangled electric guitar, you name it.

Back then seems strange to us, but the uses of "modern" electronics meant something very different. Today we couldn't play a gig with a "PA" around 15-20 watts (we need hundreds, maybe even a thousand watts)... back then a PA was brand new technology. In the radio age, TVs were even newer and stranger (ergo the "futuristic" television nomenclature of the new fangled "Telecaster" and its earlier cousin the radio-tastic "Broadcaster").

I imagine mics were more commonly plugged into Fender amps than guitars back then.