12AU7 where a 5751 was intended?

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Posted by jeffreybehr (A) on January 19, 2014 at 15:48:51

My WAVAC MD-805m monoamps...

photo 2014Jan13_RCon_1200w_zps253377f7-1.jpg

...use a 5751 in the voltage-gain stage, and I have no idea how the 2 triode sections are configured. I'd like the amp to have significantly less gain* and have tried and measured the relative gain of 5751s, 'X7s (which the distributor indicates is perfectly OK to use), 'T7s, and 'U7s**. I'd like to use a 'U7 since I'm cutting a bunch of amp gain by using its level control at 9 o'clock (on the normal 7- o'clock-to-5'o'clock full rotation) with 5751s.

While the amp sounds 'fine' with any of these frontend tubes in it, I have NO idea what performance attribute I'd be compromising if I used 'U7s (for lower gain).

What say you circuit designers? What will I lose (besides amp Voltage gain) using a 'U7?

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* so that I can use the preamp's level control in the gain range instead of the lots-of-cut range

** FWIW, and all referred to the output using the original 5751s, the 'X7 produced 2dB higher output, the 'T7 2-1/2dB less, and the 'U7 9-1/2dB less.

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Topic - 12AU7 where a 5751 was intended? - jeffreybehr 15:48:51 01/19/14 (15)

RE: 12AU7 where a 5751 was intended? - drtechno@rushmore.com 06:19:37 01/26/14 (0)

I thank each of you very much for your help. I have abandoned... - jeffreybehr 15:11:44 01/20/14 (0)

You have plugged a 12AU7 into it, how did it sound? - Chip647 06:44:31 01/20/14 (3)

I've had the amps for only 8 days, and all the while I've been rather congested. - jeffreybehr 10:44:01 01/20/14 (2)

RE: I've had the amps for only 8 days, and all the while I've been rather congested. - Chip647 11:19:13 01/20/14 (1)

"Some people really do not like 12AU7s." - jeffreybehr 15:01:07 01/20/14 (0)

RE: 12AU7 where a 5751 was intended? - Blackdog 04:04:50 01/20/14 (1)

TY Dan. A gain-reducing Voltage-divider sounds like an excellent idea. - jeffreybehr 10:50:36 01/20/14 (0)

RE: 12AU7 where a 5751 was intended? - Ray Moth 03:06:42 01/20/14 (0)

RE: 12AU7 where a 5751 was intended? - Michael Samra 20:58:06 01/19/14 (1)

TY Michael. I do NOT have a schematic. (NT) - jeffreybehr 10:33:03 01/20/14 (0)

RE: 12AU7 where a 5751 was intended? - Tre' 18:59:45 01/19/14 (1)

"BTW Nice looking amp!" - jeffreybehr 10:48:13 01/20/14 (0)

RE: 12AU7 where a 5751 was intended? - hennfarm 18:48:36 01/19/14 (0)

RE: 12AU7 where a 5751 was intended? - Caucasian Blackplate 16:15:39 01/19/14 (0)

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