vta st 120 - no voltage on rectifier pin 2

[ronheadfi]

A week ego my vta st 120 (that fwiw was built by someone else) was working fine. It was obvious that one of the power tube was dying (crackled and was microphonic). Then it red plated and took out the bias resistor.

I replaced the bad tube and the resistor.

After that I had an intermittent problem with the other power tube on the same side. At times bias voltage was steady, other times it jump to 50V. Then I found that the bias resistor of that spot was not soldered properly - and I fixed that. Stand up bias on that spot drop back to zero. Great.

When I then fired up the amp, as soon as the delay board triggered (~36 seconds delay) the fuse blew. I replaced the fuse and the same story repeated -- as soon as the delay board is done, the fuse immediately blew.

I fired up the amp without the rectifier and the fuse didn't blew that time. When I put the rectifier in again I noticed he doesn't light up. I measured the voltage on the rectifier pin 2 - it's about -0.7 volt rather than the expected 5vac.

Sorry for the long story. I have a few questions:

1. is it common that the power transformer goes bad after the issues the amp has been through?
2. I ordered a SS rectifier (doesn't use the filament 5vac) - is that a reasonable solution to this problem?
3. Could I use the 6.3vac used for the power tube? Would that be disastrous?

Thanks,
Ron

 

[gadget73]

Where were you measuring from? If it was to ground, it won't read anything. Have to read 2 to 8 in order to see 5vac on the heater socket. There is no ground reference.

1) no, but if you are going to have a winding failure the 5v rectifier winding is not the one I'd expect to die.
2) good, honestly there is no way in hell a 5AR4 is capable of supplying a 120 watt amplifier
3) definitely not. The 5vac line also gets 450-odd volts DC on it, which is why it doesn't have a ground reference. There is extra insulation to make this happy. If you tried to share the 6v heater for the other tubes with the rectifier you'd have a whole lot of DC voltage riding on the heater which would grossly exceed the heater-cathode voltage limits on every tube, and there is a fair chance it would arc inside the transformer. Also the extra voltage applied to the rectifier heater would burn it out much more quickly. All in all, no joy to be had from this.

 

[ronheadfi]

Thank you, gadget73. I read 0 volt between pin 2 and 8 on the rectifier socket. (I read ~5.7 2 to 8 on a power tube socket, I mention this just to reassure myself that I measure correctly). I'll wait for the SS rectifier and take it from there.

I will refrain from (3).

Thanks again!

 

[eickmewg]

I believe that with the time delay circuit, the 5AR4 tube rectifier is not recommended. When I added the time delay to my VTA ST120 the rectifier arced. I went with a Coppercap WZ64 and all was well.

 

[gadget73]

I read 0 volt between pin 2 and 8 on the rectifier socket.

welp, sounds dead then. Not the one I'd have expected to quit. The high voltage winding is the one that would get abused from tubes failing like that. The 5v winding is good for 2-3 amps so its fairly fat wire.

 

[ronheadfi]

I'm embarrassed. Let me own my stupidity. The voltage between pin 2 and 8 reads 5 vac as expected. Yesterday I didn't set my multimeter correctly to AC. Sorry about that. I probably just blew another rectifier. I'll try my luck with the SS rectifier and then get another tube rectifier.

On the power tube socket, pins 2-8, I read ~6vac (as expected) but also ~5vdc (which contributed to my confusion yesterday). Is the dc voltage expected or a cause of concern? On the rectifier the vc voltage is 0.

 

[ConchBoy66]

I went with a Coppercap WZ64 and all was well.

+1 on the wz64

 

[gadget73]

Make sure the output tubes aren't shorted, and verify the negative grid supply is good before you put another rectifier of any flavor in there.

 

[Mr Boochie]

I'm embarrassed. Let me own my stupidity. The voltage between pin 2 and 8 reads 5 vac as expected. Yesterday I didn't set my multimeter correctly to AC. Sorry about that. I probably just blew another rectifier. I'll try my luck with the SS rectifier and then get another tube rectifier.

On the power tube socket, pins 2-8, I read ~6vac (as expected) but also ~5vdc (which contributed to my confusion yesterday). Is the dc voltage expected or a cause of concern? On the rectifier the vc voltage is 0.

What output tubes are you using? If they are 6550, you should read the heater's VAC between pins 2 and 7.

 

[eickmewg]

Here is the relevant information from the tubes4hifi regarding tube rectifiers and the TDR board for the VTA ST120:

This board is to be used ONLY for solid state rectifiers. If used with a TUBE rectifier, the TDR can cause problems with the normal slow warm up of the tube rectifier and may damage a tube rectifier. If you intend to use a TUBE rectifier ONLY with your amp, you do not need the TDR. If you do use a tube rectifier with the TDR, you should BYPASS the TDR. Instructions are included with the TDR on how to bypass the TDR.