What is pulling voltages down in this amp?

[slidincharlie (Carlo P)]

Hello guys.
In a previous thread I described the main issue in my 1966 Silvertone 1482 amp: due to power tranny miswiring I had low heater voltages i.e., around 5.9 Vac.
I ended taking the tranny to a tech who rewound it following the specs that I drew from the schematic diagram (see attached image).
Out of the circuit the rewound tranny is right on specs, that is: 335 Vac from the HV winding and a bold 6.5 Vac from the heater winding.
Trouble comes when I hook it to the chassis with all the tubes in place: with 230 Vac supply provided by a variac, HV is about 30 Vac less (same for Vdc readings at the main three filter caps and for the plate voltage on each 6V6 tube) and the heater voltage is a mere 6.0 Vac. The only right readings are plate voltages on the two preamp tubes.
What is wrong with this amp? I know that the circuit 'eats' some voltage if compared with the unhooked PT readings, but is voltage supposed to get SO low?
Did I make a mistake giving the tech the readings that I read from the schematic? If so, how should I act the next time I need a tranny rewinding?
--Carlo

[tedmich]

its fairly common for a PT to be down ~0.5VAC under load, and the difference between 5.9VAC and 6.5VAC on the heaters is pretty minor, few would see this as requiring rewinding IMHO. Plug the measured numbers into Duncan Amps PSUD2 to see how the circuit would be approximated, I think it will be pretty close to what you see.

PSUD2

[Tom Phillips]

...Did I make a mistake giving the tech the readings that I read from the schematic? If so, how should I act the next time I need a tranny rewinding?

Power transformer secondary voltage requirements should be specified as volts at operating current draw. For example, 335-0-335 V @ 100ma, 6.3V @ 3A. There is more to it that that of course such as the total sag allowed, the operating temp etc. and there are limits to the current you can specify for a give size core. The main point is that the open circuit voltage of the transformer standing alone is always higher than the loaded voltage when it is connected to the circuit. The 335V shown on the schematic you posted is the reading you would expect under load.

[slidincharlie (Carlo P)]

Power transformer secondary voltage requirements should be specified as volts at operating current draw. For example, 335-0-335 V @ 100ma, 6.3V @ 3A.

This is what I asked to the tech who rewound the tranny. This is what the schematic says.

(...)The main point is that the open circuit voltage of the transformer standing alone is always higher than the loaded voltage when it is connected to the circuit. The 335V shown on the schematic you posted is the reading you would expect under load.

This is clear to me. My question is: if the requirements are as above (335-0-335 V @ 100ma, 6.3V @ 3A) what should I ask to a tech who rewinds my tranny? He knows the requirements but not the details of the circuit.

[Chuck H]

And, just to clarify what Tom said... It seems that the rewind resulted in the unloaded voltage matching what you wanted for a loaded voltage. And yes, an amp circuit will load the PT down that much IME. I don't know if this was a miscommunication. If you provided the schematic as part of the information there should have been no mistake. If this is the case you should take issue with the winder. Contrary to Tom's experience I have usually seen transformer specs such as "600V CT 150mA" and what you get is indeed an unloaded 600V. So Tom and I may disagree on how that spec is perceived (unless Tom is strictly speaking of how to order a unit). If you think you specified well enough that the higher voltage was what you wanted with the unit in circuit, that is, they had information about the expected current and your voltage goals for that circuit in operation, you should definitely try to put the problem on the winder. You won't get anything if you don't ask. That's for sure.

[Chuck H]

if the requirements are as above (335-0-335 V @ 100ma, 6.3V @ 3A) what should I ask to a tech who rewinds my tranny? He knows the requirements but not the details of the circuit.

There are two ways to go about it. You simply tell them the current that will be drawn through the PT and what you want the voltage/s to be. They know the math, it's not hard. Or... Do what I do. Use Duncan PSU to simulate your circumstances and just tell them the unloaded voltages you want and what current the winding should be able to handle. It will still be a little off because you won't know exactly what resistance the winding will have. But it will be really close. This is easier for them and it's always worked for me without pissing anyone off

[slidincharlie (Carlo P)]

There are two ways to go about it. You simply tell them the current that will be drawn through the PT and what you want the voltage/s to be. They know the math, it's not hard.

This is exactly what I did: I told him drawn current and voltages. What I did not specify is that those were the loaded voltages And I didn't provide the tech with a schematic
Anyway I guess I'll go to the man and discuss the thing, and see what he says.
As for the PSU simulator: I have just installed it but it goes definitely well beyond my technical ability and understanding, which are very very low.

[Chuck H]

It would be pretty easy to miscommunicate in that scenario. Short of saying flat out "I want V volts when I current is drawn". Just saying "I want V volts at I current" seems likely to get you a winding that delivers the specified voltage unloaded but is capable of handling the specified current. Not the same thing

[überfuzz]

Hmm... I'm a bit puzzled. You have the proper B+ after rectifier+smoothing filter? Then when you connect the circuit you lose roughly 30 VDC at B+? Check the circuit so there's no jumper from it to the board or any other short you shouldn't have.

[slidincharlie (Carlo P)]

Hmm... I'm a bit puzzled. You have the proper B+ after rectifier+smoothing filter? Then when you connect the circuit you lose roughly 30 VDC at B+? Check the circuit so there's no jumper from it to the board or any other short you shouldn't have.

With everything connected I have between 20 and 30 VDC less all over the place except on the plates of the two 12AX7 preamp tubes, where readings are pretty on specs.

[g1]

I'll have to disagree with Chuck as to the winder misunderstanding what you spec'd.
The voltages must hold up with the currents you specified. Any qualified winder should understand what you asked for.
6.3A@3A means exactly that. When delivering 3A it is 6.3V +/- whatever tolerance specified. Maybe it is within tolerance?
The only room for error is if you are not supplying the proper line voltage the transformer was spec'd for.
Is it supposed to be 240V input or something?

[Chuck H]

Maybe I'm being misunderstood. When I buy a PT with a 600V secondary rated at 200mA I don't get (or expect) 600V on that winding in a loaded circuit. It's never happened. So I naturally assume that the voltage must be an unloaded spec based strictly on ratio and not performance. The wind is designed to handle at least the specified current, regardless of voltage. Does anyone here buy PT's that deliver their spec'd voltage at their spec'd current? Because I've never seen it.

EDIT: I suppose some considerations are made for filament windings because the phenomenon is only really significant for the HV wind. Perhaps this has something to do with it's greater resistance and the higher voltage making it more apparent. A 30V drop on a 600V secondary is only 5%. A 5% drop in filament voltage under load is pretty normal.

[g1]

That's odd. I would expect them to be much closer when loaded than when unloaded. Were they close and maybe just within tolerance?
Here's a Hammond 272DX spec. It's the last one I did any major measurements on. It's rated at 600V@144mA, and they note that it is about 650V unloaded.
And here are my measurements: http://music-electronics-forum.com/t36264/#post343868

[Chuck H]

Well damn if Hammond doesn't show an over voltage to account for the loaded drop! I've used mostly Hammonds. When I plug the figures into Duncan PSU I account for the spec'd 115V primary since I often have 125V from the wall here. Which is roughly the same as the bump Hammond shows for unloaded voltage. My voltages and currents are nearly spot on every time.?. I should be 50V too high in the builds IF the Hammond primary is 115V and they have 50V more unloaded but I'm not. I've never measured the unloaded primary on a Hammond unit though so I must default to your research. Maybe Hammond isn't actually using a 115V primary but they never changed the lit? (except that my filament voltages are always a little high). The last Edcor I ordered went the same way And it wasn't THAT custom. I just had them add a 120V tap to an existing euro PT. I haven't had any problems or confusion. Until now everything has worked out just fine. Thanks a heap G. Now I'm confused

[slidincharlie (Carlo P)]

(...) The voltages must hold up with the currents you specified. Any qualified winder should understand what you asked for.
6.3A@3A means exactly that. When delivering 3A it is 6.3V +/- whatever tolerance specified.

This does make much sense to me. I guess that the winder should have understood that my request of voltage and current for each secondary was meant for a loaded tranny...

The only room for error is if you are not supplying the proper line voltage the transformer was spec'd for.
Is it supposed to be 240V input or something?

230V is the expected wall tension, more commonly it is 220 or lower. I don't know what the winder used. I made my raedings with both wall tension (around 220V) and with a variac (230V).
I definitely have to go and talk to him.

But... what about correct B+ on the preamp tubes and wrong B+ elsewhere?