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Old May 17th, 2010, 11:55 AM   #1 (permalink)
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Are 7591 tubes usable in a guitar amp?

I'm asking because I've got another junk organ with two 7591's a 5U4GB, some 12ax7a's and some 6x8's. I don't seem to find much info on thes 7591's other than they are octal power pentodes used in high quality audio.

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Old May 17th, 2010, 11:58 AM   #2 (permalink)
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Ampeg used them quite frequently.
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Old May 17th, 2010, 12:04 PM   #3 (permalink)
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I would say yes, though I've never played a 7591 based "pure" guitar amp. I have a Heathkit mono integrated amp model AA-181 that uses 7591s. It's 25 watts with 4 inputs - one of them for a mic. It sounds fantastic as a guitar amp with only minor modification (changed the loading on the mic input). Add a pedal to it and it's really awesome.
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Old May 17th, 2010, 12:37 PM   #4 (permalink)
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Thanks for the reply's guys! After looking this over it does actually work somewhat, altho some keys stick. I would like to try plugging a guitar into it but I don't see any inputs. Any Idea where to get a schematic for this? It's Lowrey model LSC
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Old May 17th, 2010, 01:45 PM   #5 (permalink)
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Quote:
Originally Posted by bdgregory View Post
I would say yes, though I've never played a 7591 based "pure" guitar amp. I have a Heathkit mono integrated amp model AA-181 that uses 7591s. It's 25 watts with 4 inputs - one of them for a mic. It sounds fantastic as a guitar amp with only minor modification (changed the loading on the mic input). Add a pedal to it and it's really awesome.
I've got a Heathkit AA-23. If you check over the schematics, they're identical between the AA-23 and AA-181.

You're right, they're killer guitar amps with just a few mods. I did one for a friend last year, transferred to a Mojo Princeton Reverb chassis. Just killer! Mine is still in the Heathkit chassis.
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Old May 17th, 2010, 01:46 PM   #6 (permalink)
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Maybe here:

http://www.combo-organ.com/spares_and_repairs.htm

Not sure how input section works on organs, but using tube data, you could find the input to the preamp tubes and try to 'insert' guitar signal.

There maybe issues with impedance, etc. I sure dunno.
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Old May 17th, 2010, 02:41 PM   #7 (permalink)
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I've got a Heathkit AA-23. If you check over the schematics, they're identical between the AA-23 and AA-181.

You're right, they're killer guitar amps with just a few mods. I did one for a friend last year, transferred to a Mojo Princeton Reverb chassis. Just killer! Mine is still in the Heathkit chassis.
you know except for the color scheme I really like the Heathkit case. It's compact, and solid.

If you don't mind, can you elaborate on the mods you did? I revised the loading on the mic input, and amp considering mod'ing the tone stack. Based on the sound I get with it, I'm not sure I want to do anything else with it. It has a fantastic crunch at higher volume.
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Old May 17th, 2010, 05:44 PM   #8 (permalink)
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Several of the Ampeg Reverberockets used them as power tubes.
Looks like some Jets, Geminis and Mercurys as well.
Check Schematic Heaven
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Old May 17th, 2010, 06:12 PM   #9 (permalink)
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Originally Posted by bdgregory View Post
you know except for the color scheme I really like the Heathkit case. It's compact, and solid.

If you don't mind, can you elaborate on the mods you did? I revised the loading on the mic input, and amp considering mod'ing the tone stack. Based on the sound I get with it, I'm not sure I want to do anything else with it. It has a fantastic crunch at higher volume.
Mine is still stock and has a bad cap can so it hums like it doesn't know the words.



On the conversion I did, it was basically moving the stock circuit into the PR chassis and doing a p-t-p layout for it with these changes:

1) Modded the magnetic phono input for guitar, took out the RIAA equalization circuit.
2) Changed the coupling caps from .047 mfd to .022 mfd.
3) Took out some of the frequency shaping stuff around the PI.
4) Per my friend's request, rewired the sockets for 12AX7 preamp tubes. Same function as the stock 6EU7s, but easier to get. I'm keeping mine with the old tubes
5) Paralleled the first triode pair. It had way too much gain with the stock 4 gain stages!

The most drastic change was using a 12DW7 in V2. The stock resistor sizes on that socket are perfect for that tube.

Other than that, it's the stock Heathkit circuit, complete with the little tonestack package wired to the tone pots

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Old May 17th, 2010, 07:04 PM   #10 (permalink)
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7591 are kind of a easy to drive JJ 6v6 (treat JJ's like a 18w tube). they take less voltage swing from the PI to get full clean power, so are easier to overdrive.
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Old May 18th, 2010, 02:13 PM   #11 (permalink)
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Quote:
Originally Posted by Jack Knife View Post
Mine is still stock and has a bad cap can so it hums like it doesn't know the words.

On the conversion I did, it was basically moving the stock circuit into the PR chassis and doing a p-t-p layout for it with these changes:

1) Modded the magnetic phono input for guitar, took out the RIAA equalization circuit.
2) Changed the coupling caps from .047 mfd to .022 mfd.
3) Took out some of the frequency shaping stuff around the PI.
4) Per my friend's request, rewired the sockets for 12AX7 preamp tubes. Same function as the stock 6EU7s, but easier to get. I'm keeping mine with the old tubes
5) Paralleled the first triode pair. It had way too much gain with the stock 4 gain stages!

The most drastic change was using a 12DW7 in V2. The stock resistor sizes on that socket are perfect for that tube.

Other than that, it's the stock Heathkit circuit, complete with the little tonestack package wired to the tone pots
thanks! I have a second AA-181 that needs refurb - I'll probably try some of these - including converting to 12ax7, although I have a good supply of 6EU7s, so the main goal there would be to be able to swap out to different versions of 12A*7.
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Old May 18th, 2010, 02:56 PM   #12 (permalink)
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My Reverberocket 2 has 7591's. I love 'em. Get lot's of compliments on my tone, which kinda bothers me 'cause I never get compliments on my playing! Go figure...
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Old May 18th, 2010, 04:01 PM   #13 (permalink)
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In answer to the OP; besides the Ampegs, Gibson used the 7591 tube in some of their mid '60's amps.
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Old May 18th, 2010, 04:15 PM   #14 (permalink)
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My 67 Ampeg ReverberRocket II uses 7591's. Kind like a heavy duty, hi-fi 6V6 with less mids. EH makes new ones. I think JJ may also but am not sure. They are very cool tubes. In Ampegs, they last forever.
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Old May 18th, 2010, 07:16 PM   #15 (permalink)
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Quote:
Originally Posted by bdgregory View Post
thanks! I have a second AA-181 that needs refurb - I'll probably try some of these - including converting to 12ax7, although I have a good supply of 6EU7s, so the main goal there would be to be able to swap out to different versions of 12A*7.
On mine I think I'm going to try a mod I found in a tweed builder's forum for a 5E3, internally jumpering two channels then using a blend pot to sway the balance between the two. I'd do this in place of the paralleled triodes in V1. Set one side to be dark and gainy, the other to be clean and bright, then mix at will
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Old May 26th, 2010, 11:47 AM   #16 (permalink)
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