Volume increases as amp warms up

[nkg]

Hi All
I bought a Knight 93 SX 670 amp and when I received it the the out side of the chassis was ok for its age but when I took the cover off the plate that holds all of the components had been pushed in so I cut it out and rebuilt it . The opt is original but had made separate 5volt 6.3 volt and ht transformers made re did the lay out for the tubes as they were all very close to each other generally made it neater and easier to work on and all new components . The problem is as it warms up about 5 minutes the volume increases so does the distortion to the point where the volume pot is turned right down .It happens on both channels .
Have swapped all the tubes but no change . The transformers get barely warm and I am at a bit of a loss at what to do next .The only change I made was to swap the input tube from a 6SC7 to 6SL7 . Can someone please help.
Thanks
Nigel

 

[jane]

Did you rewire the tube socket after swapping from 6SC7 to 6SL7?

 

[nkg]

Hi Jane
Thanks for the reply . I have rewired the grid and cathode . The amp works and sounds good for a lunch box amp all except for when it warms up . All the components are new except the opt . When its cold the volume is quite low in comparison to when it warms up .
Nigel

 

[DF96]

DC voltages? Otherwise we are into wild guesswork.

 

[nkg]

Hi DF96
I have pulled out the input tubes no change pulled out the phase inverter seems to stop there will try and post the schematic .http://music-electronics-forum.com/attachments/3619d1227882411-picture.jpg

 

[DF96]

Sentences would be nice. Then I might know what it was that seemed to stop when you pulled out the phase inverter.

 

[nkg]

The distortion thats seems to coincide with increase in volume even though the volume pot is on zero .

 

[DF96]

If you are still getting sound through with volume pots at zero then you have a wiring error or faulty pots. My guess is that you forgot to ground the zero end of the pot, so the valves have no grid bias. This would do strange things as the valves warm up.

As I said, DC voltages will tell us what is really going on.

 

[nkg]

DC Voltages as requested .
V1 input
Cathode 0V
Plates 93V and 94V
V2 Cathode 1.3V
Plates 120 V each
V3 Driver
Cathode 0.7V
Plates 268V and 55V
V4 Power Valve 5881
Cathode 22V
Plate 355V
V5 also 5881
Cathode 22V
Plate 357V

 

[DF96]

V3 driver anode voltages are wrong. Pin numbers?

 

[nkg]

6SL7
Pin 2 268V
Pin 5 55V
I have checked both resistors and they are within their values re soldered the joints just to make sure but yes odd .

 

[DF96]

Check the driver grid voltages. Something is making one side of the valve take all the current.

 

[nkg]

Pin1 -.059
Pin4 -.0 something multi meter won't settle down to a fixed reading .
Nigel

 

[DF96]

Check the wiring around the phono control pot. There may be a ground connection missing, which would explain the wandering pin 4 voltage. The diagram is puzzling, as it seems to have that pot back to front.

Pin 1 seems a bit high at 59mV - could the valve be gassy?

 

[nkg]

During the rebuild I left the phono circuit out . As far as the tube being gassy there is no visible blue glow and I don't have a valve checker but do suspect it could be a bit of a problem , may have to change it back to a 6SC7 and see what happens .
Thanks
Nigel

 

[DF96]

If you left out the phono circuit then you also left out the grid bias for pin 4. You need a 470k to ground, to take the place of the missing pot.

 

[nkg]

Ok will do that tonight when I get back from work.
Thanks Nigel.

 

[nkg]

Thank you for your advice DF96 it's been running for about twenty minutes now and all is fine . I will put the phono circuit back in and have ordered some better quality pots , these one's don't even fully turn off . I have made two two amps and a active crossover from scratch all of them worked so have not been able to develop any trouble shooting skills . I now can put my nos 6L6G's to see how they sound .
Thanks Again
Nigel

 

[DF96]

Glad it is now fixed.