Loud squealing noise from one channel when powering off.

A loud squealing noise occurred this morning from one channel / speaker when powering amplifier off. After some time I disconnected my speakers and turned back on. All tubes operated normally and upon shutoff I heard a noise coming from the amplifier itself (with speakers disconnected) that I imagine is what was amplified when speakers were connected. Any thoughts or direction here for next steps to diagnose would be greatly appreciated...!!

Kettnet wrote:A loud squealing noise occurred this morning from one channel / speaker when powering amplifier off. After some time I disconnected my speakers and turned back on. All tubes operated normally and upon shutoff I heard a noise coming from the amplifier itself (with speakers disconnected) that I imagine is what was amplified when speakers were connected. Any thoughts or direction here for next steps to diagnose would be greatly appreciated...!!

NEVER NEVER disconnect the load on a tube amp !

Problem could be oscillation during power changes. Are the caps increased from mfg values ?
ANy other chnges done ?

Thanks Peterh - thanks for your quick reply. It turned out to be a bad output tube was the culprit. I do appreciate your tip on not turning on the amp with speakers disconnected. One other thing you may be able to help me with - Intermittently get a various loudness audible pop through my speakers when I switch the amp off. (that's been happening for quite some time) Your thoughts??

Kettnet wrote:Thanks Peterh - thanks for your quick reply. It turned out to be a bad output tube was the culprit. I do appreciate your tip on not turning on the amp with speakers disconnected. One other thing you may be able to help me with - Intermittently get a various loudness audible pop through my speakers when I switch the amp off. (that's been happening for quite some time) Your thoughts??

Very hard to say without additional information.
What amp is it ? Any departures from factory configuration ?

Hi Peterh - yes, this ST-70 was redone 2X - originally before I purchased it (around 15-years ago) it was updated with the VTA board which has been great. I have since had it completely stripped down, re-wired and updated caps replaced. The quad cap has been replaced 3 times - twice before the last re-build, and once more after with higher values. Very unfortunately the gentleman who last rebuilt it sadly passed away suddenly last year. (see the attached images)

Pictures dont show screen stoppers, which could be a problem.
Pictures shows that this is a triode board ( not VTA) . Is there an accurate schematic available ?

Yes correct, my mistake. I have schematic for the Triode board somewhere. I will dig up and scan, thanks!

Kettnet wrote:Yes correct, my mistake. I have schematic for the Triode board somewhere. I will dig up and scan, thanks!

Waht is important is the wiring around the EL34 tubes and the schematics and values of the
capacitors.

Is anything changed before the turn-off sound started  ? The history of changes, even benign could be of interest.

You might want to add a small ceramic capacitor ( 0.01 mfd or so ) across the 2 legs of the power switch. This will shunt the power on /power off surge that sometimes creates an audible pop. I also put a CL-80 NTC on one of the power cord leads to slow down the initial current inrush, not necessary but it helps.

I would need a better, close up picture of the underside of the EL-34 tube sockets to see if you have a 100 ohm or higher resistor connecting the output transformer Ultralinear tap to the screens which is a highly recommended mod that can save your tubes from turn on surges as well.

Triode boards are a nice improvement, I use one in my ST-70.

-Ed

The view you showed of the connections to the can electrolytic in the power supply look to be poor. I suspect that the soldering iron used to do the soldering, especially the twist-lock ground lugs did not apply enough heat to melt the solder to high enough temperature to cause the solder to flow smoothly. One tab of the electrolytic where the 2200 ohm resistor is attached looks like it does not even have any solder on it. Sometimes people have the idea that they are going to damage components by applying too much heat. That is true sometimes when dealing with small signal transistors, signal diodes and integrated circuits. However in a ST-70, or a clone of one, you should be more concerned about getting good smooth solder flow when soldering all connections. If you only have a pencil iron to solder the can electrolytic connections, especially the ground connections, get a high wattage soldering iron or gun and go over the existing connections and apply heat and if necessary add some fresh rosin core solder or rosin flux (never acid or plumber's flux) to the connection until you see a bright smooth liquid flow in and over the connection tabs and wires. You may well solve several problems in the process.

Also, I could be wrong BUT... looking at the picture of the top of the amp, you see where is says Triode Electronics on the circuit board... if you look down to the second resistor below that on the board, it looks like it may have been overheated and possibly is cooked. Could be wrong, but you might want to check the value against the same resistor on the bottom half of the board, which does look somewhat different. I believe that is part of the feedback circuit, but I'd need to check my amp to be sure.

I agree with jeoztech, I would re-solder all the connections on the power supply capacitor. The connections don't look well soldered.

One other thing, and I may be picking nits here.... I would replace the (4) orange coupling caps with Sprague / Vishay 716P caps. They really sound great especially for the price.

-Ed

Solder Slinger wrote:Also, I could be wrong BUT... looking at the picture of the top of the amp, you see where is says Triode Electronics on the circuit board... if you look down to the second resistor below that on the board, it looks like it may have been overheated and possibly is cooked. Could be wrong, but you might want to check the value against the same resistor on the bottom half of the board, which does look somewhat different. I believe that is part of the feedback circuit, but I'd need to check my amp to be sure.

I agree with jeoztech, I would re-solder all the connections on the power supply capacitor. The connections don't look well soldered.

One other thing, and I may be picking nits here.... I would replace the (4) orange coupling caps with Sprague / Vishay 716P caps. They really sound great especially for the price.

-Ed

Good advise. But i would stay clear of additional cap replacements until original issue is resolved.

All great suggestions worth investigating. I hope to dedicate some time to the amp this weekend. I need to order those capacitors if someone can point me to the best source/quality in the USA would be very helpful.

Good afternoon: so I finally got to work on my amp last weekend and both replaced the power switch, as well as resoldered all of the Quad Cap connections with new rosin solder as recommended (see the attached images). Definitely not the cleanest soldering job but better than it was before. Replaced all the tubes, re-biased the amp, and now she's running just fine including no more loud pop when turning it off so I believe it was the original 60+ year's old on-off switch that was failing.

Just a comment for what it is worth...

The picture you show of the underside of the amp at the top of the post shows a clean C-354 Choke. The newly posted shot of the amp, in the last reply where you indicated the amp was repaired, shows a C-354 Choke with wax on it.

When you see wax on a Dynaco choke, it usually indicates too much current is flowing through the choke and it is overheating. This is usually because the 30/20/20/20 capacitor is bad with high internal current leakage.

You might want to replace that can capacitor. I would personally recommend the 40/80/30/20 mfd cap from Dynakit Parts, specifically the one rated for 550 volts operational, 600 volts surge. I would use the 30 mfd for the first section, then the 80 mfd for the B+ to the transformers, then the 30 mfd and finally the 20 mfd. Yes, it costs $75 but I recommend it over the cheaper 525 volt cap as I've personally had too many problems with the cheaper one. The other option is the Triode Electronics Cap Board, even more expensive but having even more capacitance.

And finally, since the amp is now repaired, I would again suggest replacing the (4) Orange coupling caps with Sprague / Vishay 716P caps. They really sound great especially for the price.

Good luck,

-Ed