1982 4010 Making a Weird Noise

[maico996]

Yesterday I got my 1982 4010 (crudely chopped into an head) out of storage and fired it up for the first time in about three years. I’d forgotten how good this amp sounds, and I’d also forgotten that I have a weird issue happening. When my guitar volume is rolled back I get what sounds like crinkling cellophane as the notes decay. At the first attack of a note it also sounds like the signal isn’t strong enough to “break through” the preamp section. I don’t know how else to describe that. I completely retubed it about five years ago and put KT88s in place of the 6550s. They’re all JJs purchased from Eurotubes and I was having the same issue prior to the new tubes. The only mod I’ve done is an added resistor to increase the bias range. The mains switch was also removed a long time ago when a mic stand fell over in the truck and broke the switch. Aside from that the guts are stock and all original.
Prior to firing it up I cleaned all the pots and tube sockets. I did not clean any of the input/output jacks.
Here are some videos of what it sounds like playing at normal volume, and with the guitar volume rolled back.





Any ideas what might be causing this? Filter caps look like they’re in good shape but I do have a replacement set that I haven’t put in yet.

 

[arthur.lowery]

Usually in any amplifier distortion reduces as a signal dies away, as any curve looks like a straight line over a small range. That's not happening. A guess might be that the power amplifier has cross-over distortion, which appears to be larger for smaller signals, as neither of the output tubes want's to amplify at its bottom end (low currents). This is common in transistor amps, but much less so in valve/tube amps, as they are Class A at low to medium levels (significant bias currents compared to the full power currents). Thus I suspect the bias arrangement. What did you change? I may be totally wrong about this cause, though. Can you put some pure music or a sinewave tone through it and look at the speaker output on an oscilloscope?

Many of the people with more practical experience should be able to help when they wake up!
Cheers
Arthur

 

[maico996]

Thank you. Unfortunately I don't have access to a scope. In terms of the bias circuit, I piggybacked a resistor onto the existing bias resistor to get an increased range of adjustment for the KT88s.

 

[anitoli]

Rule out the easy stuff. Clean every jack, pot, tube socket and tube pins, cable plugs. Then see what you get.

 

[dtier]

I sounds great with the guitar volume up but it sounds like the low guitar signal is having trouble getting through. There is a contact on the tip connector of the low sensitivity jack that is in the signal chain when plugged in to the high. It can get dirty. Just speculation of course. Do the tone controls affect the sound? That can help point you in the right direction.

 

[maico996]

Thank you. Last night I cleaned the input and output jacks with Deoxit and all is good. Glad it was something simple!