Marshall Valvestate 8080 low gain on Boost Channel - diyAudio
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Old 28th May 2008, 03:04 AM   #1
mgjimbo is offline mgjimbo  Australia
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Join Date: May 2008
Default Marshall Valvestate 8080 low gain on Boost Channel

I am repairing my old 8080 Valvestate which initially had no output on the Boost channel. I tracked this down to a failing IN4007 diode D3 which was found to be passing ac. Replaced this and all seemed OK, except that the Boost channel volume level is now much the same as Channel 1.

When in use the normal settings for the amp are both Channel 1 and Boost channel Gain pots set to 4 (assume 5 to be half way), and the Boost channel Volume pot is set to 7. Switching to the Boost Channel used to give a significant increase in volume. But not now.

I suspected that the valve output may be low so I put in a new one but there was no difference. I have taken a few measurements on the Boost channel, these are as follows;

The + and 15 volts supplies are OK. Valve HT is 285v at the junction of R109 and R110 and 118 v at the bottom of R109 and the V1(a) anode.

The valve output at the junction of R71, R95, R36 & C34 is 118v dc.

With OD1 set and looking at the Boost channel path at the junction of the LEDs and R34 & R68 I see 3.2v pp. This is almost the maximum as increasing the input signal or gain only changes the waveform shape not the height.

Also at R68 and R69 junction (valve grid) I read 750mv pp and at the valve output at the junction of R71, R95, R36 & C34 I see 28v pp.

I would really appreciate if someone could confirm that these voltages are as they should be.
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