Volume increases as amp warms up

nkg · 30th September 2012, 07:04 AM

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 · 30th September 2012, 07:24 AM

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

nkg · 30th September 2012, 09:40 AM

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 · 30th September 2012, 01:27 PM

DC voltages? Otherwise we are into wild guesswork.

nkg · 30th September 2012, 01:51 PM

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/a...11-picture.jpg

DF96 · 30th September 2012, 02:05 PM

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

nkg · 30th September 2012, 02:17 PM

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

DF96 · 30th September 2012, 03:36 PM

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 · 1st October 2012, 11:39 AM

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 · 1st October 2012, 11:51 AM

V3 driver anode voltages are wrong. Pin numbers?

30th September 2012, 07:04 AM	#1
nkg diyAudio Member Join Date: Jul 2004 Location: perth	Volume increases as amp warms up 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

30th September 2012, 07:24 AM	#2
jane diyAudio Member Join Date: Jul 2004 Location: Norway	Did you rewire the tube socket after swapping from 6SC7 to 6SL7? __________________ Life is hard - Then you die.

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30th September 2012, 09:40 AM	#3
nkg diyAudio Member Join Date: Jul 2004 Location: perth	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

30th September 2012, 01:27 PM	#4
DF96 diyAudio Member Join Date: May 2007	DC voltages? Otherwise we are into wild guesswork.

30th September 2012, 01:51 PM	#5
nkg diyAudio Member Join Date: Jul 2004 Location: perth	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/a...11-picture.jpg

30th September 2012, 02:05 PM	#6
DF96 diyAudio Member Join Date: May 2007	Sentences would be nice. Then I might know what it was that seemed to stop when you pulled out the phase inverter.

30th September 2012, 02:17 PM	#7
nkg diyAudio Member Join Date: Jul 2004 Location: perth	The distortion thats seems to coincide with increase in volume even though the volume pot is on zero .

30th September 2012, 03:36 PM	#8
DF96 diyAudio Member Join Date: May 2007	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.

1st October 2012, 11:39 AM	#9
nkg diyAudio Member Join Date: Jul 2004 Location: perth	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

1st October 2012, 11:51 AM	#10
DF96 diyAudio Member Join Date: May 2007	V3 driver anode voltages are wrong. Pin numbers?