Low Volume only when played clean

[Pochie45566]

This is a marshall plexi/sunn amp that has been modified. the volume pot is clean up until 9-10 then the amp distorts like a regular plexi but only at 10, so that seems odd. I have repalced all tubes, cathode resistors, and capacitors, biased. where do i go now?

[SILVERGUN]

Jumper the channels?

[Pochie45566]

Jumper the channels?

tried it. Something is wrong with it i am sure. i can put the master and volume pot on 8, bedroom volumes almost

[shooter]

voltage readings, including bias would help, roll in new tubes one at a time might help.

[Tony Bones]

Same plugged into either channel?
What happens when you adjust the the tone controls and master?

[SILVERGUN]

That 1M "master" (GANG) looks tricky

How do you have the pot lugs wired?

[Pochie45566]

voltage readings, including bias would help, roll in new tubes one at a time might help.

el34s are at 35ma with 480 on the plate

[Pochie45566]

That 1M "master" (GANG) looks tricky

How do you have the pot lugs wired?

exactly like the schematic i think. negative feeback is disconnected though

[Pochie45566]

Same plugged into either channel?
What happens when you adjust the the tone controls and master?

same on both channels. tone does nothin to the volume and master acts as it should

[Pochie45566]

Basically here are the issues.

pre amp volume pot is low volume, until on ten then is very loud. the volume spike happens between 8-10

the volume is way too low. but only when played clean. or so it seems

I want the volume pot to have a smooth transition from clean to crazy distortion. right now its low volume clean or full out plexi disotrion.

[shooter]

pick a channel, put a 1M across the outer 2 lugs of that pot, any help?
on the same channel, put a 470k in parallel with the 470k already there coming off the wiper to G1, any help?

[Pochie45566]

pick a channel, put a 1M across the outer 2 lugs of that pot, any help?
on the same channel, put a 470k in parallel with the 470k already there coming off the wiper to G1, any help?

quick question, do you mean the 1M resistor goes across the 2 outer lugs of the volume pot for that channel? 

i assume that is what you meant and i tried it. No luck

[shooter]

Yup, ah well.  I'm thinking this is a "normal" problem, just trying ideas to get a little more signal downstream.

[Pochie45566]

Yup, ah well.  I'm thinking this is a "normal" problem, just trying ideas to get a little more signal downstream.

\

so what nexxt?

[shooter]

you have a boost/gain pedal? does that change the characteristics of the gain pot?
do different guitars change anything?
try changing the pot taper, I'm in the minority, but I like linier taper on gain n vol, and audio for TS

got a scope?

did you half the 470Ks leading to the grid?

[Pochie45566]

you have a boost/gain pedal? does that change the characteristics of the gain pot?
do different guitars change anything?
try changing the pot taper, I'm in the minority, but I like linier taper on gain n vol, and audio for TS

got a scope?

did you half the 470Ks leading to the grid?

i do have a boost. it does change it. the problem is really that it is too damn quiet on 0-9 on the pot and only is loud enough when cranked. There must be some issue. i have no scope sadly. i did half those 470k's and it did nothing.

[Tony Bones]

I was thinking the amp is broken, but you just want volume pots with a different taper. The problem is that they're too logarithmic. You could try linear pots. They put all the action at the low end, but that might be what you want. Otherwise try 25% taper.

There aren't currently resistors on the volume pots connected between ground and the wiper by any chance? It's a trick to turn a linear taper into log, but with too small a value they can have the effect you describe. See here for an explanation of the logifying resistor: http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

[sluckey]

Disconnect the wire between the two points labeled "F.B,I" on the schematic. What happens?

[shooter]

Disconnect the wire

ya beat me to it

[Pochie45566]

I was thinking the amp is broken, but you just want volume pots with a different taper. The problem is that they're too logarithmic. You could try linear pots. They put all the action at the low end, but that might be what you want. Otherwise try 25% taper.

There aren't currently resistors on the volume pots connected between ground and the wiper by any chance? It's a trick to turn a linear taper into log, but with too small a value they can have the effect you describe. See here for an explanation of the logifying resistor: http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

the issue is despite the pot, the thing is too quiet when clean, something seems wrong

[Pochie45566]

Disconnect the wire

ya beat me to it

not a whole lot! a bit less gain maybe

[Pochie45566]

Disconnect the wire between the two points labeled "F.B,I" on the schematic. What happens?

nothin really. a bit less gain. That is just feedback 1

[Pochie45566]

not a single resistor is out of spec either, i know this amplifier sat in a closet without a v1 for years and years. would some contact cleaner be worth a shot?

[shooter]

a bit less gain.

I woulda expected a little more

worth a shot?

If it reproduces sound without crackling, hiss, staticee sounds it's probably ok.
hook it up to a 4X12 cab with some good jbl's, that help?

[Pochie45566]

a bit less gain.

I woulda expected a little more

worth a shot?

If it reproduces sound without crackling, hiss, staticee sounds it's probably ok.
hook it up to a 4X12 cab with some good jbl's, that help?

i put it into my marshall 4x4 cab and it still sounds weak. my freinds plexi with the volume on 5 and master on 1 is so much louder than mine. same amp 2 el34's and 3 preamp tubes. just dont know where to go from here lol.

and honesty there could have been a bit more gain, it is so minuscule i cant tell

[shooter]

master on 1

what happens when they are both at 8?

I marked up your schematic, give us volts DC at all the red n blue dots, all knobs @5

measure the blue dots a second time with both gain pots at 10.

[Pochie45566]

master on 1

what happens when they are both at 8?

I marked up your schematic, give us volts DC at all the red n blue dots, all knobs @5

measure the blue dots a second time with both gain pots at 10.

when they are both at 8, it is bedroom volume lol. will do.

Red dots:

top left: 270
top right: 195
bottom left: 1.3
bottom left 2.6

blue dots on 5:

top left: 230
bottom left: 1.1
bottom right: 230

blue dots on 10:

no change in voltage

tell me if any of these readings are ridiculous i will check them again

[shooter]

when they are both at 8

I meant MV at 8, both amps,  you said MV was at 1

your volts look ok to me, what about PS rail volts?

[Pochie45566]

when they are both at 8

I meant MV at 8, both amps,  you said MV was at 1

your volts look ok to me, what about PS rail volts?

master on 8 and gain on 8, it is bedroom volume. oops one sec about the ps rail

a: 460
b: 444
c: 379
d: 355

[shooter]

it is bedroom volume

ok Houston, we might have a problem.

you said you changed tubes, still got them in?  are they known good?, what is your bias voltage?
she might just be running cold as Michigan door knob in january

[Pochie45566]

it is bedroom volume

ok Houston, we might have a problem.

you said you changed tubes, still got them in?  are they known good?, what is your bias voltage?
she might just be running cold as Michigan door knob in january

they are known good. i have -46v on my el34's about with a bias current of 35 ma. it is the v drop of that side of the output transformer divided by total resistance. total plate voltage 460

[Pochie45566]

I have a home made audio trace device thing but i dont think it would do a whole lot. I am not goint directly dead in signal anywhere. signal is just dropping off i beleive. I still think that volume pot is odd. doesnt feel like a normal audio pot to me.

[Pochie45566]

Big news! after some chopsticking around and re flowing some solder joints things have imporved. Channel A has not changed, still has a massive jump from clean to dirty. However channel b is much louder and brighter, however it will not clean up, volume on one has the same amount of gain as on 10. what have i gotten into.

[shooter]

what have i gotten into

the solution 
troubleshooting is a game of persistence, If you wanna win you have to keep at it til the system gives up and starts working!
change the pots, keep poking and proding, use the listening tool to find where the signal "doesn't" amplify

[Pochie45566]

what have i gotten into

the solution 
troubleshooting is a game of persistence, If you wanna win you have to keep at it til the system gives up and starts working!
change the pots, keep poking and proding, use the listening tool to find where the signal "doesn't" amplify

well god damn, I took a measurement of the pot. 7.5M on channel b and 12M on channel A. Thats an issue considering they should all be 1M