Fender twin - evil twin - pulsating tube

[jbmdisco]

Evening,
I have taken in a Fender Twin to have a look at. I gave this amp a basic service a couple of months back i.e. sprayed the pots, vacuumed the inside checked for dry solder joints, checked and adjusted the bias + balance and tightened a couple of loose valve seat sockets.
The fault it has developed is that when on full power, after it has heated up to 'working' temperature is that there is a slight whoosh noise from the speaker which is accompanied by one of the power valves glowing bright - this lasts for a couple of seconds but recurs every 10 secs or so. I have been browsing through various posts both here and on other sites to no avail. I do not usually take many valve amps on and have only handled about half a dozen or so this year which have all been basic servicing and 1 with a failed transformer so any advice would be of great help. I have a schematic and will be checking the various test points to begin with. The customer is hoping for a relatively quick turnaround and as this is his main amp i am hoping to do so.

kind regards in advance.

JBM

[gbono]

What happens when you move the valve to another socket?

[jbmdisco]

I always overlook these finer details😐 I hope to post a picture of myself in a clown outfit tomorrow!!!

[TimmyP1955]

My guess would be a leaky coupling cap.

[drewl]

The tube that is glowing bright is probably losing bias voltage when it makes the noise.

Swap positions to see if it's the tube or the socket.
It could be a coupling cap bit most likely loose/intermittent socket pin connection .
Had one where the 1.5k bias feed resistor was cracked and making intermittent contact.

The pins on old Fenders often need re-tensioning.

[Jazz P Bass]

I would suggest that you monitor the bias pin itself.
Right on the tab.

If the bias voltage goes more positive when the tube acts up, then the problem is before the tube & the socket.

[g1]

My guess would be a leaky coupling cap.

This amp has 4 power tubes, if a coupling cap was that leaky, I'd expect to see the red-plating on 2 tubes, not just one? I suppose if it's mate had a bad screen resistor it might not red-plate though, so I won't rule it out.

[jbmdisco]

Thanks for the replies. Gives a lot of food for thought. I went to this amp today to tackle the issues with the intention of taking a video to post with the symptoms. Switched it on, left it on full power for nearly 2 hours without so much as a buzz 🤔 tried thrashing the 3 chords I know. Tapped it gently around the cab, slightly harder then sharp taps. Nothing! Customer is taking it to try out tomorrow. I was hoping to get 'into' it a bit to try out the various pieces of knowledge that has been shared.
Is there any books that could help me on my journey through valve amps? I would like to build 1 from scratch in order to learn how they do and don't work. I have an old Sound City previously written off by an engineer as 'beyond economical repair' but it has a host of spares including the transformers which are fully operational along with a bunch of components which I'm sure can be utilised.
Jbm

[Leo_Gnardo]

Screen grid resistors often found roasted in "evil" twins, it's worth checking them out carefully too.

I have an old Sound City previously written off by an engineer as 'beyond economical repair' but it has a host of spares including the transformers which are fully operational along with a bunch of components which I'm sure can be utilised.

Those engineers, so practical! Unless your Sound City is a total wreck I'm sure you can revive it without a total rebuild. Start another thread for this, I'm sure you'll get some help here and possibly surprise your engineer friend.

BTW in "engineerese", "beyond practical repair" means he knows now you can buy a brand new 100W transistor amp for less than what it would cost you to repair your old Sound City. That is true all right, if you buy a disposable MIC amp. I'd opt for the Sound City.

[jbmdisco]

Thanks LG. I have read in a couple of threads with regards to screen resistors being a bit temperamental in these amps and am glad that there has been a couple of replies to help back this up. I do expect the customer to bring it back early next week with the problem re-arisen.
As for the SS, I believe the transformers to be the most expensive part of these amps so with them working 100% and only the tubes missing I believe I have the basis for a really good 'learning' project.