Bell 3710 tube amp project

[wrenchbender]

I picked up this tube amp a couple weekends ago for 25 bucks with an old astatic bullet mic for harp, and some old jensen speakers. It revived my interest in tube amp repair. I've replaced the electrolytics, and old paper caps with orange drops, put the old tubes back in, and tried it. At full volume, i got some sound very distorted. clearly I have more issues. I checked resistor values, and found some to be considerably out of the 10% tolerance. I don't have lots of test Equip. just a VOM, and I'm not an engineer but am enjoying trying to learn. Any suggestions?
Thanks.

[olddawg]

Right off the cuff from your picture, it looks like one of the output tubes has lost it's envelope. It really looks milky.

[wrenchbender]

Thanks for your reply. This amp came with 2 RCA 6V6 smoked glass??? power tubes. I think thats what they're called, have seen the term used before. Anyhow, both were identical grey colored and open on top. After changing caps, and trying the amp, right off the bat, one 6V6 was internally arcy sparky in conjunction with each strum on my guitar. I pulled it out and replaced it with a known working 6V6 hence the mismatched look of the power tubes.

[Marc]

you may have leaky coupling caps. If you're comfortable working with high voltage you can measure the caps at the plates for leakage. If they passing even a few tenths of a volt on the low side I'd replace them. You may also have some deformed or failing electrolytics (I'd be surprised if you didn't) Bell amps generally have good hifi iron. I'd think it's worth putting some work into....

Any way you look at it you got a great deal!
Marc

[wrenchbender]

Thanks Marc. I've made quite a bit of headway, replaced all the old paper caps with orange drops, replaced all the electrolytics. Replace a couple bad resistors that were way out of tolerance. removed a feedback loop resistor, 4.7M Installed a switch jack, removed the phono input, and removed a 10 megohm resistor on the input.
I checked the bias on the two 6v6's, its running at .04 amps @ 265 volts which gives me about 10 watts. What can I do to get a better tone out of this thing. I'd like to play a strat or tele through it and really want more of a 5E3 tweed deluxe tone.
Here is the schematic;
http://www.theused.com/manuals/bell/bell_sound_3710.pdf
Thanks again.

[JHow]

This looks more like a 5C5 than a 5E3, with grid leak on the input and parphase inverter. It's really cool circuit.

After looking at this for a while, it seems the plates are at 265V and the screens are at 275V, per schematic. I think you could try to move the plate supply to the 305V node to get a higher plate voltage.

[wrenchbender]

Thanks. I've had allot of fun tinkering on this unit and learned quite a bit. It really looks cool too with the grill on top. Totally retro. It sounds ok, except its a bit muddy, even tinny sounding at lower volumes. not really crisp and bright.

[capnjuan]

Hi wrenchbender; the front end of that amp is like an early '50s Gibson GA20 ... I have a later version ... the GA20T w/ a 12AY7 in the channel 1 preamp ... it too is pretty nondescript ... and, like your Bell, it also has capacitor input and 10meg grid leak. I haven't done the work yet but people smarter than I suggested losing the capacitor, replacing it with either a 56K or 68K R and 475K instead of the 10meg. I haven't done the change yet but you're going to need something along those lines to get something crisper and tighter than what you have. Good luck .. pretty cool amp. CJ

[wrenchbender]

Thanks Juan, I'm going to give that a try. I'll let you know how it sounds after doing the mods. Lately, I've been working on a different amp project that has been fun and a bit less techy. A new cab for my magnatone tonemaster custom 214 amp. Check out the pics. Pine wood box with finger jointed sides.

[Marc]

here's some additional tweaks to try.....

You can change the preamp plate and screen resistors to values you'd see in an early Vox... 220K/1M and then change the bias to cathode bias 1k5/25mfd (or smaller cap for more bite) and the input to 68k/1M the pentode is a different sound than the standard triode and can take some time to get used to


you could also change the preamp to a 6SL7 and cascade the two triodes.

If you want to bump the output up change the rectifier to SS or a 5AR4.



Marc