Bad mechanical hum after installing Chinese "KT88" PSvane tubes and blowing 4 rectifiers

I had posted on here how I was using Gold Lion KT88 tubes on an original ST70 with the VTA board and upgraded transformer installed.  I also installed the "yellow sheet" mod on the rectifier transformer and the Triode Ultralinear switches.  So after current world events I decided I needed a backup set of KT88s just in case my Gold Lions decided to die on me so I ordered a matched quad of Chinese PSvane KT88 tubes.  I want to mention that the amp was dead silent and sounded wonderful prior to installing the PSvanes.  When I put the PSvanes in the first time, I was starting to adjust the BIAS when lightning in a bottle happened on the rectifier and blew the fuse.  So I turned the bias pots on the VTA board all the way down and tries again.  Same thing, after the amp warmed up, lightning in a bottle on the rectifier and blown fuse.  

After all this I now have a loud mechanical hum coming from the amp.  I replaced the power transformer and the hum is still there.  I have new output transformers that I purchased from ( https://www.laboratoriomusicale.net/en/output-transformer-dynaco-a470-for-st-70-4300-ct-4-8-ohm-replica-new ) (I am in the US but the site said:

"The primary is wound the same as the original A470, same number of turns, sections and wire size, using the same patented Dynaco winding method, and made on the same core size and material. Only improved insulation material used (thus making the transformer more resistant to abuse) and nice long (10 inch/ 25 cm minimum) high temperature insulated leads have been added."

and they were almost half the price of transformers here in the US).

I am hesitant to replace the original output transformers because they sounded so good prior to all of this.  When I push down on the left output transformer, the hum gets quieter (not all the way gone by any stretch but quieter)  This does not happen on the right transformer or the power transformer.  I am also not getting any hum through the speakers only on the unit itself.  

Could this be an output transformer?  Is there a way to repair the original output transformer?  If I were to replace the output transformer, should I replace both right and left?  All caps are less than a year old.

Thank you!
Mark

Sounds like the transformer could be vibrating. Make sure all fasteners are tight.
Don't know if it could be a ground issue ..?

I have not had this experience with my VTA ST120 btw.

They are as tight as they can be without stripping them but thanks for the input!

Don't know if you've done a search on the forum but I recall vibration / hum coming up in the past. Good luck with this...

90% chance it's the rectifier tube is now bad, replace it and all should be good

I would test each of the two yellow paper mod solid state rectifiers to make sure they were not shorted or flash damaged. I have that mod on my ST70 and when the rectifier tube shorts out, I get a buzzy/clicky sound out of my speakers when the GZ34 rectifier tube shorts out. My fuse does not blow though since adding the yellow mod. If I replace my bad rectifier tube the buzzing and clicking then goes away. All is silent, except for the music.

So I would replace the rectifier tube as recommended above. Your power supply choke or the filter caps could also be damaged. I also have that mod that replaces the selenium rectifier and the can capacitor and increases the total capacitance on my old '64 Dynaco.

Next I would install the previous known good KT88 tubes and see if the hum stops and the Amp works normally. Then try subbing out one of the known good tubes one at a time with a Psvane and readjusting the bias for that tube. If installing one of the Psvane tubes causes the fuse to blow, or the power transformer to hum or run hot, then have that beam power tube tested. It may have an internal short, or just replace it.

I once fixed a noisy Sola voltage regulating transformer by first removing the covers, then soaking it in a high temperature polyurethane resin in a covered bucket for 24 hours. This allows time for the resin to penetrate between the plates. After 24 hours remove the transformer from the resin, dry the outside off with a rag and give it a week to thoroughly dry out, before reinstalling the covers and reconnecting it. The resin fills any gaps between the core plates and stops them from vibrating.

You could also use WAGO 221 connectors, or wire nuts to make temporary connections to the sealed transformer for testing. If it runs quiet, then re-solder the wires back together and insulate with heat shrink tubing.

I own a set of Psvane KT77's and found they made my short stack power transformer run hot. I used a 5-Amp rated Variac to reduce my mains voltage from 122VAC down to 110VAC, the original design voltage for my '64 Dynaco factory power transformer. I have found the cheap Tesla EL34b's sound just as good and are reliable for everyday and testing use. I call them my burner tubes. I order them in matched pairs or quad matched sets for my 1964 Dynaco factory wired ST70. Good luck!

Thanks for all the suggestions.  I sent back the PSvanes (a couple more users on here commented on another post that they had tried the Chinese PSvane "KT88s" and it did the exact same thing to them so I am almost positive that it is a manufacturing flaw) back to China and I ended up having to get the refund from eBay because they never refunded my purchase.  

I did try replacing the 5AR4 since I keep a hefty supply on hand after this happened and that didn't help.

My wife loves to listen to vinyl so I will have to find time to take my ST70 apart and check the diodes and can capacitor and get it back together before she gets home.

Where did you get the high temperature polyurethane resin?  How do you keep the resin warm/hot when it is in the bucket?  This is one of the answers I was looking for when I wrote this in the first place!  Thank you!  I really hate to use new output transformers because I am pretty sure that when it didn't hum and sounded good, they were 90% of the reason.  I am able to press down on the output transformers and almost make the hum go away so I am pretty sure that they are the culprit and need to be replaced or fixed.  

I am completely willing to try to repair them (and install the new ones with wire nuts so my wife can play records for a week) using your method when I find out the information above.

Have you ever listened to new output transformers on a Dynaco ST70?  The website says that they use the same methods and number of turns etc. as Dynaco did but I am leery.

I purchased these last year: https://www.laboratoriomusicale.net/en/output-transformer-dynaco-a470-for-st-70-4300-ct-4-8-ohm-replica-new If the website shows in Italian, you can change it to English at the top of the page.

I varnished my Sola transformer about 30 years ago, so don't recall where I ordered the varnish from. If you Google transformer varnish all kinds of results come up, including You Tube videos on how to do it. You do not heat the varnish, it just can withstand higher temperatures without degrading.

The transformer I fixed was very large at 2.5KVA and I recall having to thin the polyurethane resin varnish slightly so it could be drawn in by capillary action. But once it dried it was much quieter. It was used to power a computer server. It had two oil filled capacitors and was designed to resonate to regulate and smooth the line voltage to the server. In today's world they have electronic methods to condition and provide standby power for servers.

My ST70 factory wired Dynaco still has the original output transformers. Changing them out for newer ones would devalue my amp. Besides they work fine. The factory wired amps use chrome plated rivets instead of screws and nuts to hold the parts in place. If you see one for sale online that is how you can tell them apart visually from the kit builds. I left the can capacitor in place, but disconnected since it was worn out from old age to preserve the appearance of the amplifier.

Perhaps obvious, but worth stating. You may find that adding/replacing the rubber grommet on transformers and / or tighten mounting to be enough to eliminate the mechanical hum.

1st, forget you ever heard the name Psvane. I've owned several sets and they are so inconsistently made that they will make you go crazy thinking something is wrong with your amp. I rebuilt my ST70 II and thought I made several mistakes until I installed good tubes. No more problems. 2nd, transformers are almost always the last to fail, I'm willing to bet those cheap tubes took out more than the obvious. There's a cap or resistor bad somewhere. Start probing for voltages and opens.