audio guild amp

[burt_toast]

howdy fellas,


all right - hypothetical question time (and probably a dumb one, since that's all i ever seem to ask).


my boss has an old audio guild 1x12 amp, in decent condition too, except that the two 7591s are broken - actually broken glass - not sure how. well, i've been trying to talk him outta this for years, and i might be close - and doesn't even play guitar! it's just been sitting there with broken tubes since i started at this company in '97! (he said it used to work fine before they broke, btw)


so, my dumb/hypothetical question is this: assuming i can buy this from him, i could get some EH or JJ 7591s, but would the amp possibly work with EL34s? i'm aware it's a different pinout altogether. would the output impedance be close? would it introduce a greater current draw somewhere? is the plate voltage compatible?


i've heard that 7591s sound cool and everything, but i do love el34s, and don't like the idea of having such a rare tube to replace later. i guess these things sound really cool and have maggie-style vibrato/reverb - been dying to get this!


thoughts?
thanks,
Scott Gorsuch

[sluckey]

EL34s will require much more filament current. If the PT has the extra juice they will work. 6L6s would be a better match to the amp. But, since new production 7591s are available at the same cost, why change? My old Ampeg Gemini II used 7591s and I thought it was a great sounding amp. I'd sure want to just fix it up first to evaluate. It's probably a decent sounding amp just as is.

Can you tell us more about the amp? What model? Tube line-up? Is it a split chassis? Can you post some pics? BTW, none of the Guild schematics I've seen have a Maggie style vibrato, just regular tremolo.

[burt_toast]

not Guild, Audio Guild. Apparently this fella, Don Bonham was the guy who designed Magnatone's vibrato, and left to form his own company - see: http://magnatoneamps.com/audioguild.html it looks just like the one in the link, except no added "grand prix" panel on the front.


*sigh* yeah Sluckey, i'm sure you're right and it's better to stick with the 7591s, just hadda ask. thank you. will post some pics next week from work. it's a 2 ch amp - 1 with vol/tone, and 1 with vol/treb/bass - spring reverb, and that maggie pitch-bending trem. haven't found a schematic for it yet. i'm gonna keep poking at my boss til he gives up.
thx again,
scott

[sluckey]

not Guild, Audio Guild

Duh! I completely missed that! 

That's a great find! Probably rare. I hope you get it. And I really hope you will fix it just like it is. And share everything with us. I'd love to learn more about this amp.

BTW, forum member "terminalgs" owns that magnatoneamps.com website. He's got a lot of knowledge about Maggie stuff and may be able to help locate more info.

[burt_toast]

thx Sluckey for the maggie-guy info! yeah, okay, i did promise my boss if he sold it to me i wouldn't modify it at all (rrr!). fingers crossed. will post pics soon.
scott

[terminalgs]

I don't know if I've ever seen an Audio Guild schematic, other than for Versatone amps.  Other Audio Guild amps: "Bonham", "Grand Prix", and the Deffner accordion amps "Titano" and "Panaramic" if that helps in your quest. 

The versatone schematics are out there.  If you look at them, you'll see they are very well engineered. Of course, Bonham's vibrato circuit needs no introduction around here. I would say the amp probably needs no tinkering or re-engineering.

[burt_toast]

oh how cool, thank you, sir!! :) yeah, really stoked about the Audio Guild! will look for a versatone schematic - great tip, thanks terminalgs!
scott

[PRR]

Another vote for keeping it 7591.

7591 is 6L6 plate structure with gain more like EL84/6BQ5. EL34 or 6L6 will run cold unless re-biased. Re-biased, gain is down, which may matter.

There was a 20 year period when you could NOT get a 7591, and many hacks were done.

The current production "7591" appear to be close-enough to the originals to work fine. While you might not find one in Lodi NJ on a rainy friday night, you can certainly order a quad (two to play, two spare) for not a lot of money. Most 7591 amps did not over-run the tubes, so the 4-pack could be a lifetime buy unless you have a busy playing career.

[burt_toast]

thanks PRR,
yeah, you're right, i guess 7591s aren't as scarce as they used to be. also, the more i research this amp, the more rare and special it seems. stock it is...
i'm really glad there's room on this forum for both actual techs, and meddlers like me :)
scott

[PRR]

> a different pinout altogether

Just for the record: there isn't a dime's difference in pinout across all the octal-base power tubes we would use for audio. There ARE a couple traps: EL34 has a G3 which 6L6 lacks, and "may" need to be strapped (to cathode). 7027 has a "G2" on a pin that is "N/C" on most others (and in some amps that used "N/C" as a tie-point for something else, this can be exciting!). Check, ask, but pinouts are a minor detail.

> EH or JJ 7591

What I'm reading is that JJ may fit better but may not be reliable. (Also I think the round top is ugly.) EH tubes are usually a good/reliable choice, but they cost more. (And the coin-base looks wrong, even if some old ones had this.) There is a "Tung-Sol" branded 7591 which surely is not made in the old Newark works, has the classic shape, but costs much more.

And:

me> 7591 is 6L6 plate structure

Should have said-- 7591 is 6L6 plate and cathode structure. The 0.9A heater sure is similar to 6L6 heater. The plate seems to be based-on the 6L6G (old) ratings and general size, though old and new 7591 may have the up-rated plate metal of 6L6GC. However they moved the grids around to get much higher gain.

7591 comes from when hi-fi was going stereo. Stereo makers needed to cram twice as much stuff in the chassis. They shaved-down the designs, squeezing more gain in every stage, including demanding "a cheap high-gain 6L6". It seemed like a good idea, it was readily available (1959-1970s), and I think one company price-warred the 7591 which is something amp-makers always love.

[VMS]

Here is a link to 7591 tube comparisons:

http://www.tronola.com/html/7591a_tubes.html

[octal]

I tried some JJ 7591As  (current production) in a custom Vox AC-30 build -w- 7591A outputs. Couldn't get the sound I wanted from them. After close inspection, it appears that the JJs might actually be beam power tubes rather than true pentodes. I guess I won't know for sure without cracking them open, but there seem to be beam forming plates rather than an open-wound screen grid.  So, on that sample of one, I'd say maybe the EH or Tungsols would be a better choice if you want true 7591A tone. NOS would probably be the best choice yet. My 2c.

Nathan

[Toxophilite]

Wow that sounds like a really neat amp
2 7591s can be almost 40 watts.
Did you manage to get it?