Marshall blown PT cause

[Andrei_T]

Hi, I have a Marshall 100W model 1959 on the bench with a blown PT. This transformer and the OT were replaced by me about 3 months ago with the 100W replacements for Marshall from AES, because the owner wanted to upgrade his amp and the transformers that were in were refurbished.

This amp has a history of blown PT's and OT's. The customer is almost allways using the amp with a Marshall Powerbrake PB100. I seem to remember reading somewhere that using a powerbrake stresses the amp more than using a real speaker. Could this be the cause of the blown PT's and OT's?

Andrei

[Enzo]

The difference between using a power brake and not is like your moms car being used to drive to the store and back or to run in the racetrack. The whole point of a power brake is that you can run the amp wide open - driving it as hard as it goes - but being able to dial down the volume. Certainly the amp works harder, and at least the tubes will wear out sooner. That is the price you pay for that sound. Whether the cost is worth it is up to you.

While I am guilty of the same thing, when you use the word blown for the transformer, it tells us little about the nature of its failure. How are they failing? If the HV windings are failing, then the tubes are likely taking the thing down.

Does this amp also have a hostory of keeping fairly fresh power tubes in it?

ARE THE FUSES THE PROPER TYPE?

[R.G.]

Just to enlarge a bit on what Enzo said, if there has been a history of presumbly good PTs and OTs in the amp, then the amp is killing the transformers. There are several ways this happens, and how the transformer fails is the key to what is happening.

It is generally a power stage fault that kills PTs and OTs, the exception being a short on the heater wiring. The heater wire short will kill the PT but not the OT. Loss of bias voltage to the output tubes will kill both of them. Ultrasonic oscillation may kill either. And finally, just pure overloading with a power brake may cause high voltage spikes which eventually kill the transformers; moreover, running full out with a power brake causes as much or more heat as running full-bore on stage. It may be that just heating is killing things. In that case, a small fan in the box will fix it.

If it were mine to fix, I would remanufacture the power stage. The PT is already being replaced, and that's the most expensive part of the amp. So replace the other resistors and caps that make up the output stage, paying particular attention to the bias supply as that is one area that is dead certain to cause the problem.

And tell the owner that you will no longer warranty your work if he uses it on a power brake. You might gently suggest that if he wants full-out sound at low volume, he GET A SMALLER AMP. For the life of me I can't figure out why someone has to buy a 100W amp to run at full bore on a power brake all the time in his bedroom. There are a couple of all-tube small amps that will do this just fine. In fact, if he uses a smaller amp, he can use his power brake with the same sound output level and the only difference will be that the repairs are cheaper.

[Andrei_T]

Thank you for the reply Enzo,

I am aware of what a power brake does and how it functions, but I remember reading something about the impedance at higher frequency being to high in inductive/capacitive (not only resistive) power brakes/sokes and this generating some hi-freq oscillations that are bad for the OT and PT. I don't remember exactly.

About the amp:

3 months ago the guy brought it to me for a re-tube and the transformer replace. Before it had old Tesla tubes in that were worn out (probabily were in there for a very long time), refurbished transformers and the fuses were "repaired" with a wire.

I put in matched Svetlana EL34's, rebiased, changed the PT and OT and put in proper value fuses.

The PT broke in the middle of a gig and took the mains 2A fuse with it.

I dont think it's the HV winding because the 1A fuse is ok and the windings measure 9 and 11 ohms. The primary 220V winding measures 5 ohms which seems normal but the transformer fails the lightbulb test with all secondary windings open.

Andrei

[Valvehead]

I need to add---i got THREE bad 100w Marshall replacement PT's from A.E.S. Yes, three in a row.
They didnt put out the specified voltage and got extremely hot . The first one went up in a puff of smoke.

I put a Heyboer in it and the amp has been gigged for 6 months or so with 0 problems.

Not saying this is the case, but I personally will NEVER buy a tranny from AES again.

[dai h.]

a series of expensive repairs would seem catastrophic (esp. for the wallet), so if it were me, I'd check that amp over inch by inch from end to end for anything amiss (burnt, melted wire, intermittent contacts, whatever).

re: the PB, Randall Aiken points out that the impedance rise at high frequencies from the series 1mH inductor (which is part of the internal load) is not limited and recommends putting a R across it to do so. You can put one across the LC pair also. That should help with potential oscillation (which can cause overheating internally in the OT, and shorted/melted windings leading to damage). He also stated that if the OT is overrated (double the power IIRC) then no damage will result even with an oscillation occurring.

Also, there are other ways less stressful on the amp to reduce output such as putting a pot in series with the 470 ohm R in the PI to reduce cathode current and thus transconductance ("novosibir" or Larry of Larry Amps suggested a 50k reverse log), and also running the heaters separately and using a Variac to lower the B+ (more involved but still technically easy to accomplish).

[Enzo]

Here is a scenario to consider. The customer uses crappy worn out speaker cables, which intermit now and then leaving the poor OT loadless. Or just as bad, he uses guitar cords for the speaker. A cord's a cord, innit? No.

Or his 412 has a loose jack.

That one thing that is wrong with the amp could be its environment.


When you are measuring the resistance of transformer windings, you are not doing it at working voltage, you are instead doing it at a volt or two from your meter. The resistance might seem reasonable, but how can you tell from 11 ohms whether three turns are shorted together somewhere? SHort just one turn to its next door neighbor, and it serously stresses a transformer.

testing at low voltage will not disclose any arcs. An arc is a short circuit that disappears when power is removed, more or less.

Testing the resistance of a winding is one thing, but you would also need to test for resistance between windings, and also between windings and the frame. Two windings might be fine individually, but if they are shorted together somewhere, then trouble results. And again, it might only happen when full voltage is in the wires.

Speaking of one shorted turn, there is a caution for toroids. The toroid should be mounted with a nylon bolt, an insulated bolt, or at least a bolt that does not reach the top lid. if your bolt that holds the toroid to the bottom is long enough to touch the top lid of the rack chassis, that bolt and the walls of the chassis form a shorted turn through the toroid anad will cause serious current to flow.

In TV sets, some models used to get the CRT heater current from one 2-turn loop around the HV transformer core piece.

[Andrei_T]

Thanks for all the suggestions guys, RG I will take your advice and replace the power amp components and the bias supply comp.

Enzo I thought about crappy cables/jacks first thing he brought the amp but the customer said they were OK. You're right I will tell him to bring me the cable to check it also the jack on the cab and powerbrake.

Dai that's what I needed to know about the powerbrake, thanks!

Valvehead, where can I buy Heyboer PT's from? Weber seems to have only OTs.