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Old 1st November 2007, 11:23 PM   #1
apehead is offline apehead  United States
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Default Voltage Gain Confusion!!

O.K. I have a Bogen M120 amp. It originally took 6EU7 tubes in the preamp section. I have a cheap source of 5751 tubes, so I want to retrofit it for them. Since the amplification factor is lower for the 5751's (70) as opposed to the 6EU7's (100), I wanted to adjust the grid and plate resistors to better suit the new tube type. I attempted a formula for voltage gain on the first stage, and the answer was like 42v! That can't be right... wouldn't the second stage (another half of a 6EU7) not be able to handle that? The circuit is only designed for a -4v control grid bias right now, would that swing the grid voltage far too positive? I am very confused.

-Jack
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Old 1st November 2007, 11:55 PM   #2
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I suggest you use 12AX7s, not 5751s. The triodes in the 6EU7 and the 12AX7 are "identical", while those in the 5751 have a lower mu. Equally important is the increased heater current draw of the 5751. It's possible the power trafo could be overstressed.

BTW, use Sovtek 12AX7LPS tubes, as they have a spiral wound hum bucking heater.
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Old 2nd November 2007, 01:57 AM   #3
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Hi,

Contrary to popular belief, 5751 isn't an exact 12AX7 replacement, it's more of a mini 6SL7.

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Old 2nd November 2007, 02:08 AM   #4
cerrem is offline cerrem  United States
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A circuit Gain of 42 sounds reasonable for the 5751 tube...
Gain is not a unit...it is not in volts....
This means if you have a input signal of 200 mvolts rms .....
The AC voltage leaving the 1st stage would be (.2 x 42) would be 8.4 volts rms....
And yes this 8.4 volts into a -4V biased tube grid implies you would overload the input of stage 2 with a 200mV signal entering 1st stage input...
Is it possible there may be a mixer/ voltage divider in the circuit between the 1st and second stage??? If so then figure the divider is halfing the signal going to stage 2 ...bringing it down to 4.2 volts when 200mV is applied at the input of stage 1..... Which would make things work out for a 200mV input sensitivity....
If you then have global feedback from the output to the input of the first stage cathode....this would reduce the input sensitivity of the 1st stage by the Feed Back amount ....This "may" allow up to a 500mV or more input sensitity before overloading stage 2...
Need to see the entire circuit...

Chris
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Old 2nd November 2007, 04:21 PM   #5
Sheldon is offline Sheldon  United States
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Default Re: Voltage Gain Confusion!!

Quote:
Originally posted by apehead
O.K. I have a Bogen M120 amp. It originally took 6EU7 tubes in the preamp section. I have a cheap source of 5751 tubes, so I want to retrofit it for them. Since the amplification factor is lower for the 5751's (70) as opposed to the 6EU7's (100), I wanted to adjust the grid and plate resistors to better suit the new tube type. I attempted a formula for voltage gain on the first stage, and the answer was like 42v! That can't be right... wouldn't the second stage (another half of a 6EU7) not be able to handle that? The circuit is only designed for a -4v control grid bias right now, would that swing the grid voltage far too positive? I am very confused.

-Jack
Don't get too anal about the difference in gain. It's not that much on paper and probably even less in practice. I guarantee that if you check gain on a variety of x7's and 5751's you'll find some that overlap in gain. If your power transformer is running hot, the extra filament current of the 5751 might be an issue - unlikely, but worth noting. You've already got them, try em. Don't waste time retrofitting, I doubt you need it.

Sheldon
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Old 2nd November 2007, 06:29 PM   #6
Sheldon is offline Sheldon  United States
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What I meant to say, was don't retrofit first. Try first, retrofit if you need to.

Sheldon
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Old 6th November 2007, 01:47 AM   #7
apehead is offline apehead  United States
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First, I would like to say thanks for the replies.

O.K. so the "hotter" the (first gain stage) tube is running in its quiescent state, the greater the input sensitivity... that would make sense considering the following. On this amp, the channel and master volumes are one right after another (1 Meg pots) between the first stage and the second. That's the voltage divider that cerrem (correctly) concluded was there. If I use my bass guitar as an input to the extremely similar Bogen MX-60A I am currently using as an amp, I can't turn up past 7 or so on Channel and Master volumes (simultaneously) without going into total saturation. This must be due to the high input sensitivity of these amps. Would it be practical to attempt to redesign the fist stage for a lower input sensitivity for a gain in clean power tube headroom, or would I lose clarity and definition as a result?

As a side note, either way you slice it, I would have to do some re-arranging of the circiut. The 6EU7 tubes have a different pinout than the 12AX7 at its kin. Also the amp is in need of new electrolytics and some of the ceramic caps have turned into dust, so it will need some serious bench-time regardless.

Also, I am in staunch opposition of 12AX7s. The 5751s can be simply placed in an amp designed for 12AX7s, and run fine. While not really appropriate for overdriving like the 12Ax7s are famous for, if you are looking for a sweeter, mellower, more defined, less harsh sound, the 5751 has the 12AX7 beat, hands down.

P.S. the filament currents between the 6EU7 and the 5751 are only a 50mA difference. This shouldn't affect my transformer adversely, should it?
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