Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: brett on January 30, 2012, 05:00:53 am
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Hi all…I have a peavey encore, a 65 watt all tube 2x 6l6 amp (ss recto)
It’s a single channel , but with 2 gain structures ,normal and “pump”
So, when “pump” is switched on it all works as expected , as described in the manual here , all 3 pots in the gain block work ( except the pull smooth switch doesn’t alter the sound)
Link to the manual http://peavey.com/assets/literature/manuals/80300051.pdf (http://peavey.com/assets/literature/manuals/80300051.pdf)
schematic attached (hopefully)
When the “pump” is off (footswitch) the imput gain alters the signal, but post gain doesn’t … it seems to be full on and pull smooth doesn’t alter the sound . I can't follow how the pump works with the LDRs, I guess the signal is bypassing the post gain pot somewhere that it shouldn't when the pump is switched off
After some vigorous knob twiddling I buggered the carbon track in the post gain / pull smooth pot . I was hoping the problem may have been the pot but after replacing it the amp is just the same . On the plus side it was a good excuse to do the caps.. at least it's quiet now!!
I could just use the pump gain I guess but I hate it when things don’t work right
and I would like a bit more clean control
any help would be greatly appreciated ,Thanks brett
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> When the “pump” is off (footswitch) the imput gain alters the signal, but post gain doesn’t … it seems to be full on and pull smooth doesn’t alter the sound.
That is the way I read the circuit. Pump LED off, LDR2 off, bottom of "Post" pot and "Smooth" switch are disconnected from ground. Load on Post pot is hi-Z, so with pot un-grounded the pot turns 100%-90% of signal, instead of 100%-0% when pot bottom is grounded.
I have not read the manual to see if it says some other thing. Manual writers sometimes get bad info.
If you still suspect fault:
Measure voltages at LEDs and resistances at LDR resistors.
All LDR resistor legs should be at zero voltage (else a cap leaks). One resistance leg is (should be) zero resistance to ground. You can measure resistance at the other terminal with amp powered but silent. Turn "Pump" knob CCW; still this pump pot shunts LDR1 resistances so you won't get a real high reading; but you can tell it is much more than 1K.
LDR LEDs can't be seen so the "bright/dark" is conceptual. By voltage, under 1.5V across the LED is dark, over 1.6V is bright.
Pedal open:
LDR1 dark
LDR1 resistors over 100K
LDR2 and "Pump" bright
LDR2 resistors under 1K
point A at 3.7V
Pedal shorted:
LDR1 bright
LDR1 resistors under 1K
LDR2 and "Pump" dark
LDR2 resistors over 100K
point A at 1.6V
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Thanks PRR Hmmm ,maybe that is the way its meant to be..although there is the matter of the smooth switch not working at all.. Guess I will have to pull it out and do some measuring . Its hard when you don't know what it should sound like !!! Thanks, Brett