Silvertone OT bad?

[kka]

Hello folks,

I've got this 1484 Silvertone on the bench. Symptoms are:

• very low output
• fart-like distortion
• volume drops to 0 as guitar chord rings out and comes back to previous level when strumming strings again (albeit low and farting as before)

I tested all tubes (all good). I tested all filter caps (all good). All voltages on filter caps and all tube pins are normal. The OT is getting normal voltage supplied.

About 2 weeks ago I did a full retube of this amp and all was well for 2 weeks. What I noticed is that the new 6L6 pair test both much lower quality on my tube tester than when I installed them. Like from 80% to now 60%. Could that be an indication of stress on tubes due to malfunctioning OT?

With the OT all I have measured so far is resistance between each of the 2 primary B+ leads to primary center tap lead which came to 80 and 90 Ohms respectively. That looks about normal, but of course I need to continue testing this thing. Right now I don't have a spare OT lying around to hook up the amp to.

My customer is not very eager to spend too much money on this amp, so before I proceed with my tests I would like to ask you folks if the symptoms above could hint a broken OT.

Thanks very much for your help !

[soundguruman]

I guess the secondary or your speaker could be bad.
Sounds like a blown speaker too.
You have old power tubes
you installed new tubes
The bias is adjusted way off?
The bias is WAY too cold?
DO the output tubes get HOT when you run it?
or do they stay pretty cool, just warm?

The tubes should get HOT when you have it on for a while.

[Amp Kat]

Agree with SG ^^^ and check the signal going into the grid then check it at the speaker secondary. If that signal is larger and sitting there at the speaker out I would expect either a bad speaker or possible bad speaker cable. If the signal isn't present at the secondary output and it's at the grid then there is either a loose wire on the tube socket or possible bad socket or cold solder joint or the OT is bad. From the ohms reading it seems that the tranny appears good however without doing an AC reverse voltage test like on RG's Geofex site it's hard to tell.

http://www.geofex.com/ampdbug/outtrans.htm

[kka]

Speaker and sp.cable are 100% ok. I'm using my own cab and cable.
I also checked the 6L6 bias and all correct at about 40mA per tube..

I briefly started getting in with my scope. I saw some funny ripples going on on the 6L6s' grid (2 waves overlapping) and saw exact same on OT secondary. Same at PI already. Need to continue tomorrow, not enough time in 1 day ;-)

Thank you for your input !

[kka]

I found the problem and fixed it. One of the load Rs for the PI was fried and the anode did not get power. Only 1 side of PI was working and that made the weird sound. When I initially checked voltages I must have missed that tube. Now I found the problem with the scope as no signal showed.

[Jazz P Bass]

Nice find!