(cablespec@3web." bravely wrote to "All" (30 Dec 03
23:54:27)
--- on the heady topic of "Vintage R12R Ampeg - guitar amplifier - lack of
audio output gain - n"

c( From: )

c( I have a vintage amplifier Ampeg R12R reverberocket '62s. There are 2
c( X 6SL6, 2 X 6SN7, 2X 6V6 and 1X 5Y3GT. I would like to know what is
c( the power output of this amplifier? Not the reissue ... I mean the
c( vintage ... I know that the R12R reissue is a 50 W RMS and use others
c( vacuum tubes ( 6L6 instead of 6V6 and 3 X 12AX7).

A pair of 6V6 in push-pull won't output much more than about 15 watts.
Some amps use elevated plate voltage on them and may get 25 watts but
they don't last long when stressed that much.


c( Here is my problem. There is a lack of audio power at the output . The
c( volume position between 1 to 5 is very week (weak audio output) .
c( From 5To 10 (the maximun) the output is strong (guitar control at
c( max) and clean.I think that I have a lack of audio gain at the output
c( of my amp. I checked the bias resistor of cathodes 6V6 (275 ohms on
c( my meter) and I replaced the cathode bias capacitor by a 50 uf 50 V
c( located to the cathode of the 2x 6V6 output tubes.The problem is
c( always similar. I'm quite sure that I have a lack of audio power at
c( the output. I checked all voltages ... it seems all OK. I tested the 2
c( audio output tubes with a Hickok 600A and I read 4000 mho for the 2 X
c( 6V6 ... OK. The 6SL7 read normal. The 2 X 6SN7 read about 1700 mho the
c( normal value is 2600 mho).

c( I checked all the solders joint and I resolder each eyelets. Many
c( coupling capacitors have been replaced in the section V1, V2, and V3
c( with very good caps.I checked all power supply caps = OK. The 2 x 6V6
c( run a bit hot. There is no Hum or strange noise. My problem is ... I
c( have the feeling that ... my output have lack of power. Is it normal
c( with the vintage R12R? If not, what is the problem? Help me a bit.

c( Thanks a lot!

c( Mike

The gm on the 6SN7 is indeed low by about a half, obviously this means
it needs twice the input voltage to drive the 6V6 to maximum. If
anything this seems like the weak part. OTOH your 6SL7 may have too low
plate voltage to develop enough gain to begin with, perhaps due to a
leaky preamp supply filter electro. Maybe you have both problems.
Beware of high plate voltage

.... High voltage can give a dangerously uncomfortable discharge.