Marshall Valvestate 8080 low gain on Boost Channel - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 28th May 2008, 02:04 AM   #1
mgjimbo is offline mgjimbo  Australia
diyAudio Member
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.
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
marshall valvestate 8080 hiss raw/growl meta11icafreak Instruments and Amps 8 10th July 2013 02:20 PM
Marshall Valvestate 8080 kdawg Instruments and Amps 10 1st May 2012 12:08 AM
marshall valvestate 8100 trai1824 Instruments and Amps 4 27th March 2012 01:36 PM
Guitar amp problem (marshall 8080 valvestate) Palmu85 Instruments and Amps 3 26th September 2011 09:33 PM
marshall valvestate 8080 repair tips needed skid Instruments and Amps 4 25th April 2006 01:01 PM

New To Site? Need Help?

All times are GMT. The time now is 02:37 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2