need advice to spec power transformer

[fredcapo]

I am repairing a hand-built clone type of amplifier for a friend of a shop owner. The amp is based on a 5AR4 rectifier, with 2 - 5881's and 2-12AX7's. The power transformer is shorted. All new tubes were installed by owner in fruitless attempt to solve fuse blowing. I dont have enough knowledge to fully understand what voltages to spec for each winding.

The transformer is a vertical mounted unit with footprint roughly 3 x 2.5 OC. Faded yellow label looks like a stancor. Colored cloth leads are:

BK-BK pri

GN/GN/GNstripe filament where GNstripe is tied to ground

YE/YE filament 2 to 5AR4 pin 8/2

RD/RD/RDstripe HV where RDstripe goes to chassis via what looks like a power diode.


The ohm measurements of the fried transformer are, for the record and any other useful info:

BK-BK 1.6 ohm
GN all combinations ~.2 ohm
YE-YE .2 ohm
RD-RD 10.7 ohm
RD-RDstripe 27 ohm (same either RD to RDstripe)

I thought the stripe would be the CT but the ohm meas don't concur. (though it is defective using inductance kick test, and blows fuse disconnected)

I would appreciate advice on what to use as a replacement. 5881's seem to run around 300vdc on plate.

Also, what is with the diode on the hv center tap? Thanks.

[Jazz P Bass]

I would appreciate advice on what to use as a replacement. 5881's seem to run around 300vdc on plate.

Also, what is with the diode on the hv center tap? Thanks.

As to an 'exact' replacement, without knowing what the original B+ voltage was, you are pretty much swinging in the breeze.
Is there any information that you can glean from the capacitor voltage ratings?
Where did that 300Vdc spec come from? (the data sheet?)
The diode to chassis ground (is it huge?) is probably a zener diode to pull down the original B+ voltage.
So you are faced with an unknown B+ & the fact that the designer was not happy with that, so the zener diode was added.

[fredcapo]

Thanks. I wont pick nits since "exact" is not possible, just need a PT that will happily drive the boat. Where will the 5881's be running at a desirable point? Transformer voltage as well as impedances will have to be best-guesses, not something I am good at for now. The diode threw me. I'll get a monkey wrench and remove it to try to identify, but if possible would like to eliminate it.

[Jazz P Bass]

So what are the caps rated at?
Is the diode big?
How is the transformer mounted?
What is the dimensions of the mounting hole pattern?
As you can see, it is more than simply the B+.

[potatofarmer]

Is it possible the diode is a zener, added to drop the HT?

What's the primary impedance on the OT? Figure that out then grab the datasheet and ballpark your power tube plate voltage from that. Might not match exactly what was in there before, but if they added a zener to drop the plate voltage, the original HT was probably too high for whatever the builder wanted. (edit - Jazz P Bass already brought this up, sorry)

Do you know what it's a clone of?

And there's still the question of why the original PT blew.

[fredcapo]

Two men meet in a bar. The following conversation is overheard.

So what are the caps rated at?

500vdc

Is the diode big?

Yes. See above hint "monkey wrench". I did not yet get the part number.

How is the transformer mounted?

Vertical (above).

What is the dimensions of the mounting hole pattern?

3 x 2.5 but need to recheck.

As you can see, it is more than simply the B+

Yes that's why I am in a bar...

[olddawg]

Have you completely floated all of the secondaries and tried to power the transformer? Are you sure it is shorted? Do the bells come off? Very occasionally lead wires get pinched.

[Jazz P Bass]

Now you need to pick the wattage that you want (2) beam tubes to produce.
Project Style Transformers By ClassicTone

[fredcapo]

Yes. It is removed completely and is shorted. The tubes were all replaced by owner in attempt to fix... thus all new so it is possible bad tube caused it to fail. I expect that I will have to tweak some dropping res values on the B+ chain and do some other "design" work... I only took the job so I could perhaps learn something... Don't know at this point if it has other problems, gave them a quote that should at least net me enough for a Starbucks...

Wattage can be the typical for the tube type, prob go on the low end.

When I determine the Zener value, if that's what it is, should know better. If that is so, it may be a smaller transformer.

Also, why the center tap on the filament winding? When center tap winding is shown as 6.5 V CT, is it 13V end to end?

Guys thanks.

[olddawg]

Where is the short? Primary? If it you can get it apart and get to the windings there are several things you might try depending on the nature of the failure. In similar circumstances I was able to wash the windings with alcohol, let them dry over night, then soak them with lacquer and let it dry a few days. Sometimes it will clear the short and you can take voltage measurements without putting the transformer under load.

[Enzo]

Imagine you have a transformer winding with red-red/yellow-red wires. And imagine you know the thing has a 50 ohm winding resistance. so 25 ohms from the center tap to each end, 50 across it all. Now short the two red wires together. Red to red. Bow you get zero ohms red to red, of course. What do you get from center tap to either red?



I get 12.5 ohms. You have essentially put two 25 ohmers in parallel.

Now think about that when you see 10 ohms red to red, but 27 ohms CT to red. You have the winding ends shorted to each other somwhere a few ohms in.

6.3v center tapped means 3.15v from either end to center.

Unless you are using vintage OLD tubes, the 5881 is just a variant of 6L6.

You have a basic amp, 5881 in push-pull and a pair of dual triodes. For all the world that sounds about like a Fender Pro with only one preamp channel. Surely a power transformer suited to that fender would work here. I don;t see mention of a bias tap, so is there a bias supply, or is the amp cathode biased?

You need 6.3v for heaters at about 2.4A, call it 3A. You need 5v for a heater tube. Looks like somewhere in the area of 700v CT for B+.


You got a big zener in the CT line? Is there a number on the thing?

[fredcapo]

Oh well. Could have gone to see Johnny Winter tonight (saw him and spoke with him january anyway).
Instead got a refresher ona transformer.

Quick look seems like the 40-18066 will be good. Though dont need the bias tap. Thanks for the link JPB.

Still will look at the diode tomorrow. Thanks all.

[fredcapo]

The diode that is connected to the hv center tap is a nte5275ak, 4.3v 50 watt zener. What is the purpose of this diode? Thanks.

[Jazz P Bass]

It is a 4.3 volt /50 watt zener diode.
If it is also hooked up to the B+, then it was being used to lower the original B+ voltage by 4.3 volts.
Was this a combo amp?
If so, what was the ratings on the speaker.
Ohms & watts.
That may be a hint, as I suspect you are still mulling over which transformer to use.

[52 Bill]

It is a 4.3 volt /50 watt zener diode.
If it is also hooked up to the B+, then it was being used to lower the original B+ voltage by 4.3 volts.

I wonder what is so critical in this circuit that you would need to reduce the B+ by 4.3 volts?