Harvard 5F10 debugged and running

[Edgar Sterling]

Here is a complete 5f10 chassis I've finished, been debugging about a week. All dumb mistakes that were easy to fix once discovered (when building an amp, do not carry on a conversation with someone and try to work at the same time!). I'm playing through several different speakers, including a Weber 10A 100T that I bought especially for this amp. The sound is quite beautiful, a bit more hi-fi than my cathode biased amps. If anyone wants to see pictures I just put them up here:

ipernity: Harvard 5F10 by Michael T Maxwell[view]=1

[Alby]

Nice job! , glad it worked out for you.

Nice to see the not so popular old circuits showing up more and more.

Alby

[mbratch]

That's a beautiful amp Edgar. Nice job on the build. I like the look of the chassis.

The Harvard is interesting. I never really looked at it. Interesting pre-amp tube. And it's funny that the 5f10 is not cathode biased, but the later 6g10 model is cathode biased. Fender historically went the other direction with their other amps.

Maybe you can do some sound clips if you have time.

[loudthud]

What did you use for the first preamp tube? The schematic I found calls for a 6AT6 which is a 7 pin minature with a mu of 70.

[Edgar Sterling]

Yes, some of these old Fenders are due for resurrection. I want to try to do a 5f4 super next.

[Edgar Sterling]

Thanks, you know, the fixed bias what one of the things that caught my attention. I had been wondering what a small amp would sound like with that.

I should mention something about the negative feedback on this amp, which was the last problem I resolved. I was playing it with the negative feedback disconnected because it howled like a banshee when hooked up. I was stumped for a day or two, wondering if I was reading the schematic wrong (it is a little cryptic) and checking out Alby's pictures...these appear to be the only resources out there on the Harvard. Then a couple of mornings ago I simply reversed the OT leads to pin 3 on the output tubes and that was all it took. I really didn't think it could be that simple but it was.

[mbratch]

Then a couple of mornings ago I simply reversed the OT leads to pin 3 on the output tubes and that was all it took. I really didn't think it could be that simple but it was.

Oh yeah. With the leads reversed, it's essentially a positive feedback.

[Edgar Sterling]

Oops, last post was meant for mbratch.

That tube is a 6sl7, which also has a mu of 70, so I used one triode and left the other unused. I have lots of these tubes and they sound good so I used it. I have also used a 6sl7 where a 12ax7 is called for in a split-type/concertina phase inverter (like several tweed amps use) and I actually like the sound better. If I ever do a Super, I might try using all 6sl7s in place of miniatures.

[Edgar Sterling]

Oh yeah. With the leads reversed, it's essentially a positive feedback.

The other thing that was reversed was the fixed bias supply diode. The rectifier was red plating within 45 seconds because I had positive voltage on the grids rather than negative. I immediately flipped that sucker around and added a pot for biasing.

With the negative feedback, I can see where a presence control might be desired. For now, I'm leaving it hooked up.

[PremiumPlus]

Nice build Edgar! One observation; and it's absolutely personal preference, but I never have trusted those little pc mount trimpots. They're small and cool but for me, for a bias control, I want it to be a little more robust, like a 1/2 watt pot. No electrical reason, only durability/accessibility and longevity I guess.
But a beautiful job. I hope my scratch build (a 5E3) comes out as sweet. How does it sound? What speaker are you using?

[Edgar Sterling]

I had to take my chances at Radio Shack, an assorted pack of potentiometers, and wouldn't you know, the one I needed was the least substantial of the lot (there were a few pretty decent ones in the bag). This was my first build with a fixed bias supply and I wasn't expecting to make it adjustable since the schematic didn't call for that, but it only makes sense to do it. Next parts order I'll stock up on some good quality ones. Speaker is a Weber 10A 100T and it sounds wonderful, and I can highly reccomended it for this amp.

[mbratch]

Heh -- yeah I've many times bought a small pack of stuff from RS just to extract one barely usable item.

BTW, I've used the small circuit board size pots for bias trims without any problems. They are commonly used in "after the fact" bias control mods.

[928Z06]

Nice looking build, and am glad you figured out the cross-wiring.

What power transformer are you using, and what are the plate voltages and current draw you ended up with?