WAY high plate voltage in Fender clone

[deke99]

Hi all,

I have a Winfield Thomas Elizabeth Reverb head. The amp is clone of a Fender Deluxe (with only 1 channel and a mid control). I pulled the 6V6 tubes in favor of 6L6s and put in a SS rectifier (I was told this was safe by the builder). I biased the amp up and it worked great for about 6 months. Recently it shut down on me at a gig. My first suspicion was bad power tubes, since I didn't see them glowing when it wasn't working (the amp worked intermittently).

So I put in new power tubes, biased it up and everything was great for about 3 hours of playing time. Then it cut out on me, no sound whatsoever. Tubes not glowing. I get home from rehearsal and start doing some voltage readings. I get the tubes glowing and read the plate voltage on pin 3 and its at 875 volts!!! Needless to say the tubes stopped glowing shortly thereafter and I'm afraid to turn it on again.

What could make the plate voltage so high?? Obviously something is very wrong. What can I safely check to determine if this is a transformer issue, filter cap issue, etc?

I figured I could safely pull all the tubes and do some voltage measuring. Even with the tubes pulled I'm measuring over 700 volts on the plate pins. This leads me to believe that I have a bad power transformer (or perhaps output transformer). Would this make sense? I'm kind of a noob on this stuff and really just need a reliable gig amp that I so far lack due to these issues!

Thanks so much in advance for your help!

Derek
dsdjb2002 [at] hotmail [dot] com

[Chuck H]

Wow.!. Thats a huge jump.

FWIW the filament voltage is a seperate winding from the plate voltage. So just because the plate voltage is janked (a technical term), that shouldn't effect the filament voltage. Unless... Your PT primary is shorted. I have to suspect an unusual PT failure with these symptoms.

You should lift the filament and B+ lines from the PT and do additional voltage testing. If the primary IS shorted you will have higher than normal readings on both the filament and B+ windings.

Chuck

[Enzo]

OK, first, when you say thete is 700v coming from the transformer, may I assume you measured 700v between pins 4 and 6 of the rectifier tube socket? In other words each one was 350VAC from ground?

Power transformers can be rectified full wave with a pair of diodes and a center tapped winding - the way it is done with a tube rectifier. You can do it the same way with seniconductor diodes as in most Fender amps.

You can also full wave rectify the AC with a bridge - four diodes - but that is done without a center tap.

In the center tap, two diode version, the voltage across the transformer is twice what you need because the center tap splits it into two halves of the amount desired.

When you use a bridge, it rectifies the whole winding, not two halves. So the whole thing must be the voltage you need, not two halves of that amount.

Point I am trying to get to is that if you stick a bridge after a transformer intended to be used with the center tap, you will get twice th voltage you need.

I don;t know if you did this or not, but it sounds like it. Or you fooled your meter.

But all of this high voltage stuff should have nothing to do with wether the heaters in the tube are glowing or not.

[deke99]

Thanks Chuck, I'll check that out tomorrow ASAP!

[deke99]

Hi Enzo,

I'm measuring 312V from pin 4 to ground and from pin 6 to ground of the rectifier (5U4 rectifier tube per tube chart). I pulled the SS rectifier plug in case that was the issue.

Pin 2, is measuring 948V to ground. Pin 8 of the rectifier measures the same. That seems to be what is very wrong. Do you know what could cause that?

Thanks again for your help!

Derek

[RWood]

Derek,

Are the voltages that you posted Alternating Current or Direct Current? You didn't specify.

Richard

[Robert M. Martinelli]

Hi Enzo,

I'm measuring 312V from pin 4 to ground and from pin 6 to ground of the rectifier (5U4 rectifier tube per tube chart). I pulled the SS rectifier plug in case that was the issue.

Pin 2, is measuring 948V to ground. Pin 8 of the rectifier measures the same. That seems to be what is very wrong. Do you know what could cause that?

Thanks again for your help!

Derek

Pins 2 and 8 are the heater's pin ( with pin 8 internally tied to the cathode ), you have no heater to be supplied ( no current flowing=no voltage drop ), so whatever voltage you read on pin 2 will be the same on pin 8 in these conditions.

I'd rather think about what enzo said, because the original arrangement ( rectifier tube ) needs a center tap, and so does a two-diode SS rectifier, but if you used a Graetz bridge ( four diodes ) as a rectifier, you have to leave the PT's center tap disconnected, otherwise you're going to get twice the voltage you expect/need.

Can you post the schematics of the "original" and the "modded" version ?

HTH

Best regards

Bob

[soundmasterg]

Derek, rather than measure from each pin to ground, you should read closer at what Enzo said.....

may I assume you measured 700v between pins 4 and 6 of the rectifier tube socket? In other words each one was 350VAC from ground?

You should have one probe on pin 4 and the othe ron pin 6 of the rectifier tube. Or you can also measure from ground to pin 3 of the power tube and this will tell you what the tube is actually seeing. With the tubes out of the amp, the plate voltage will be higher than normal by 15-20%.

If the power transformer wasn't able to run the 6L6's safely then it would have overheated eventually and some of the winds would have internally shorted, thus causing higher than normal voltages. But you need to measure correctly first before we could assume something like that.

Set your meter for DC volts and measure from pin 4 to 6 on the rectifier tube socket, or measure from ground to pin 3 of a power tube socket , with the rectifier tube in the amp, and you should get a plate voltage number that is not as high as you're getting.......

I've never heard of that amp maker btw.....

Greg

[Robert M. Martinelli]

Derek, rather than measure from each pin to ground, you should read closer at what Enzo said.....

You should have one probe on pin 4 and the othe ron pin 6 of the rectifier tube. Or you can also measure from ground to pin 3 of the power tube and this will tell you what the tube is actually seeing. With the tubes out of the amp, the plate voltage will be higher than normal by 15-20%.

If the power transformer wasn't able to run the 6L6's safely then it would have overheated eventually and some of the winds would have internally shorted, thus causing higher than normal voltages. But you need to measure correctly first before we could assume something like that.

Set your meter for DC volts and measure from pin 4 to 6 on the rectifier tube socket, or measure from ground to pin 3 of a power tube socket , with the rectifier tube in the amp, and you should get a plate voltage number that is not as high as you're getting.......

I've never heard of that amp maker btw.....

Greg

Hi Greg,
While I pretty much agree with most of the above, I have some doubts about the effects of a shorted ( secondary ) HT winding on the PT. The HT requirements make for the need of PTs being elevating transformers, so the HT secondary winding has more turns than the primary, and it's wound using thinner wire, therefore, in case of overloads, the secondary is much more likely to get burnt, and, if some of the windings get shorted, the resulting lower transformer ratio should cause the voltage out of the HT winding to decrease IMHO.

OTOH, if some windings were shorted at the primary, the transformer ratios would increase, and so would do the voltages on ALL secondary windings, ( including the 5 VAC and 6,3 VAC ones ).

As you said, before we can assume something we need some correct measurements ( and, possibly, the schematics )

Cheers

Bob

[deke99]

Hi all,

Thanks so much for all of the replies . . . it's great to have a place with so many experts. Regarding my voltage readings, they have all been AC readings to this point.

I have put the 5U4 tube rectifier back in the amp and pulled the tubes so I have effectively 'un-modded' the amp. I made no circuit changes, I only switched power tube types and put the SS rectifier plug in and re-biased using the internal pot. I don't have the schematic for this amp unfortunately.

When I switch to VDC, I am measuring 420 volts on pin 2 of the rectifier. This seems to be more in line with what I measured when biasing (around 400 then with power tubes installed).

Between pin 4 and 6 on the rectifier I get a fluctuation between 610-610 VAC (I get .5VDC)

Thanks again for all of the help and insight!

[deke99]

Thanks to everyone for your help. I talked to the builder (Winnie Thomas) and he has agreed to repair the amp for the price of parts only. He had me disconnect the lead from pin 8 of the rectifier to the first filter cap and measure the VDC at pin 8. All other voltages at the rectifier seemed to check out reasonably well, but pin 8 is only putting out 295VDC when it should be closer to 425. This was the case with the SS rectifier plug along with 2 different 5U4 tubes. This leads him to believe that the issue is the PT, but he'll dive deeper once the amp is on his bench.

Derek

[Robert M. Martinelli]

....but pin 8 is only putting out 295VDC when it should be closer to 425. This was the case with the SS rectifier plug along with 2 different 5U4 tubes. This leads him to believe that the issue is the PT, but he'll dive deeper once the amp is on his bench.
Derek

Hi Derek,
that would confirm Greg's thoughts about a PT issue and my thoughts about the voltage getting lower due to some shorted windings on the PT HT sec.....
Hope you'll have your amp "up and running" soon!

Cheers

Bob

[deke99]

Thanks Bob. I definitely look forward to it being 100% operational, she's my #1