Trouble with power transformer: uneven voltages

[slidincharlie (Carlo P)]

Again on the Weber power tranny replacement for my '77 Deluxe Reverb...
I mounted this new tranny, turned the amp on and made a few readings.
B+ is okay.
The trouble was upstream:
the two plate pins of the rectifier read respectively 339 VAC on pin 4 and 388 VAC on pin 6.
Their resistance - referred to their central tap - is respectively 70 and 77 ohm.
The above readings are the same with and without the rectifier tube.
The two heaters (?), pin 2 and 8, read the same (around 420 VDC).

Ted Weber told me yesterday that this is not an issue and I can use the tranny safely. I am a little dubious though... and wonder if such different voltages on the plates of the recto will harm my amp.
Should I put a resistance somewhere to drop the higher voltage?
Should I check something else?

[Bruce / Mission Amps]

Again on the Weber power tranny replacement for my '77 Deluxe Reverb...
I mounted this new tranny, turned the amp on and made a few readings.
B+ is okay.
The trouble was upstream:
the two plate pins of the rectifier read respectively 339 VAC on pin 4 and 388 VAC on pin 6.
Their resistance - referred to their central tap - is respectively 70 and 77 ohm.
The above readings are the same with and without the rectifier tube.
The two heaters (?), pin 2 and 8, read the same (around 420 VDC).

Ted Weber told me yesterday that this is not an issue and I can use the tranny safely. I am a little dubious though... and wonder if such different voltages on the plates of the recto will harm my amp.
Should I put a resistance somewhere to drop the higher voltage?
Should I check something else?

I wonder why he would say that is not an issue ...the 420vdc isn't an much of an issue but I've never seen a difference like that and I think one at 339vac and the other 388vac is wrong.

Are you sure you don't have a funny dual secondary PT with one red wire and one red/white wire or something odd like that?
Besides that, I'm surprised the first filter cap isn't trying to charge up to the higher voltage from the PT secondary winding at 388vac.
If so, your B+ rail will be higher then it should be.
With a good rectifier in this amp that would give you something like 450vdc or more...
Personally, if everything is wired up correctly, I think the PT is defective.
Anyone else?

[Enzo]

Any chance you have used a bias tap as a center tap?

[slidincharlie (Carlo P)]

Thorough readings

I have attached the schematic of the tranny, in case it is of any help.

I have read again all voltages with all wires in the air, to find where the bug is (use the schematic as a reference):

RED to RED: 698 VAC
1st RED to RED/YEL (ct): 326 VAC
2nd RED to RED/YEL (ct): 372 VAC

RED/WHT to RED/WHT: 555 VAC
1st RED/WHT to RED/YEL (ct): 300 VAC
2 nd RED/WHT to RED/YEL (ct): 255 VAC
(I don't use the red/wht secondary)

RED/BLU to RED/WHT (ct): 46 VAC.

The filament heaters (6.3V and 5V) read ok.

With he exception of the RED/BLU bias tap, the other HV secondaries are higher than specs and, most important, with one side unbalanced, correct?

May I use this transformer, or should I return it to Weber (for the second time!)?
Is there a way to correct this?

[Tom Phillips]

Carlo,
I think Enzo is on the path to the answer. However, I don't think that it's your mistake. I think the transformer manufacturer accidentally swapped where the RED/YEL and the BLU/RED are connected inside the transformer.

Try making the measurements again and pretend that the bias tap is the center tap. I'll bet that the voltages then come out correct.

I wouldn't worry about the slightly higher voltages. The open circuit voltage is always a little higher and, in any case, they are well within acceptable tolerance.

I would worry about the response you got from Webber.
Either there was a miscommunication or Ted has gone nutty. I hope it's the former.

Regards,
Tom

[slidincharlie (Carlo P)]

Try making the measurements again and pretend that the bias tap is the center tap. I'll bet that the voltages then come out correct.

Tom,
I did like you suggested. Pretended that the RED/BLU bias tap was the center tap.
Result (with the wires in air):
I have exactly the same readings but inverted: the wire that gave me the higher VAC reading now gives me the lower, and viceversa. Still an un-balance.
Then?

I would worry about the response you got from Webber.
Either there was a miscommunication or Ted has gone nutty. I hope it's the former

My question to Ted:
the two HV secondaries (red wires) read 339 VAC on pin 4 and 388 VAC on pin 6, with respect to ground.
(...)There resistance with respect to ground is 70 and 77 ohm respectively.
This does not sound normal to me, correct?(...)
Please tell me if I'm wrong somewhere, or if this new tranny is bad.
Ted's reply:
There is nothing wrong with the transformer. It will work fine, even though the voltages are not exactly the same.

So... may I use the tranny? Or do I risk to harm my amp?

[Tom Phillips]

Wow,
Risking "harm to the amp" is debatable. I don't think it will cause any real "harm". However, I personally would not want to use that transformer. The two halves of the windings are far out of balance compared to reasonable standards. As Bruce already stated, I too have not seen this much difference in any good transformers that I ever measured.
It may work "fine" for some people but the higher voltage half will be trying to do most of the work. (Charging the first stage filter cap) You will get higher than expected idle voltage and a higher percent sag when you play. You may like the sound but that transformer "ain't right". A real bummer considering the long distance purchase.
Tom

[Tom Phillips]

Tom,
I did like you suggested. Pretended that the RED/BLU bias tap was the center tap.
Result (with the wires in air): I have exactly the same readings but inverted: the wire that gave me the higher VAC reading now gives me the lower, and viceversa.

I'm still thinking about this. The results don't make sense to me. i.e "exactly the same readings but inverted in the other direction"

Might be worth more testing.

[Alex/Tubewonder]

A very small asymmetry in voltages on center tapped secondary windings is normal but almost 15% as in your case is totally unacceptable. Yes, the transformer will work but you will get only half wave rectification most of the time. Full wave rectification will occur only under heavy load condition and than the lower voltage winding will be providing only about 20% of the charge to filter cap, the rest will still come from higher voltage winding. This will give you ripple with high harmonic content above 3rd overtone of the mains frequency and may cause nasty buzz. My Spice simulations show this clearly.

In your place I would return the transformer and request my money back.

I mentioned Arie-Jan at Marble amps in Holland in your previous thread - just got a properly dimensioned PT for a Classic 30 from him (yes, original PT is heavily undersized and they do burn out). Solid work and up to CE safety norms. I'm recommending you get a proper PT from him.
Disclaimer: I have no connection with Marble Amps other than having bought a number of replacement transformers from them.

I'm afraid Ted, as service minded and a good guy as he is, got a bad shipment from his supplier.

[Enzo]

I think I see the problem now. Neither of those is the center tap. This transformer doesn't have a center tap. It is a high voltage transformer designed for bridge rectification. AT least that is what I see on the data sheet when I look at it.

Having a few volts difference would be OK, having 50 volts different is stupid. It won't "hurt" anything, but having the alternate peaks 50v apart will add ripple the poor filters have to fight.

[Tom Phillips]

I think I see the problem now. Neither of those is the center tap. This transformer doesn't have a center tap. It is a high voltage transformer designed for bridge rectification. AT least that is what I see on the data sheet when I look at it...

It looks like it's intended as a center tapped design to me. However, the drawing format sure doesn't give the right perspective. Attached is a diagram showing an alternate way to draw the schematic of the Weber transformer high voltage secondary winding.
I think this gives a much clearer perspective of the beast.
Tom

[Enzo]

I'll agree that seems more reasonable, And it is what we would normally expect, but it doesn't match Carlo's tranny.

I could put up with a few volts, but 50 volts either means something is very wrong in the thing or it is so poorly made that they lost count of the windings.

[slidincharlie (Carlo P)]

As you can read in my thread "New tranny bigger than old", this replacement is taller and a little heavier that the previous Weber tranny that it replaced. It also has got too large a top bell that does not fit inside the chassis hole.
I took photos under the hood: they look like different beasts, to a certain extent...
This probably would not explain the unbalance thing, but tells us that something odd is going on in the Chinese manufacturing plant, or at the quality control level...

[Satamax]

Huh guys, i might be daft, but what would happen if Carlo grounded the two wires labelled as 45v?

http://music-electronics-forum.com/a...0&d=1185562604

[Enzo]

Wel, if they are indeed two different taps, the same thing that happens when you short out the secondary of any other power transformer.