Peavey VT Series Deuce - doesn't seem loud enough

[TomCarlos]

You can find YouTube videos on testing Output Transformers. Here is one...

How to test audio output transformers (OPT) : impedance, HV insulation, DC-R

[Enzo]

Go to geofex.com, and under Tech Tips, , then Tube Amp Tips, see "Transformer short tester". Very simple, needs a battery and a small neon bulb.

DO you have ANY output transformers in your shop? The Deuce is a 100 watt four 6L6 amp, and most any OT suited for that combination would work. And for just testing, even a "wrong" one would work. A few clip wires and you can have a space OT connected. If the amp wakes up, then the old one was the cause.

[Helmholtz]

I'm open to learning how to test the OT.

1) Take a picture of the OT wiring.
2) Completely disconnect and remove OT from amp. Remove power tubes.
3) Separate/isolate all wires and tape to a piece of wood etc.
4) Connect heater voltage to the 8 Ohm secondary using a current limiting series resistor of 10R/5W or larger between.
5) Measure actual voltage across the 8 Ohm secondary. (If significantly lower than 6V the OT is shorted and you can stop here.)
6) Measure primary voltages from each side to CT. Expect something like 2x60V with a good OT.

Post results.

[g1]

I don't want to de-rail here, but before getting into the OT make sure everything on the secondary side of it is ok. Like that funny 4 ohm switching jack is not messed up and shorting or something.

(edit: I see Helmholtz already touched on that in post #11)

[Delta362]

I supplied one lead of the 6.59VAC heater voltage to the black common lead of the OT and the other heater lead to one side of the 10 ohm 10W resistor. The other side of resistor went to the white lead (8 ohm tap) of the OT. Voltage drop across resistor (same as voltage between black and white OT leads) was 4.85VAC. Definitely much less than 6VAC. Just for shits and giggles, I also measured the primary side of the OT Red-Brown (10.8VAC) and Red-Blue (12.5VAC).

Given what you indicated above, it's a bad OT. I'll let them know. Very much appreciated!

[TomCarlos]

UPDATED - thank you Helmholtz. My edits are in red font. I added your comments just in case someone wants to copy and paste and make a checklist out of this.

For future use.... Enzo mentioned the tester circuit on the Geofex site. Here is photo of that circuit and description of the tester. See image below.

From this thread and other threads, I made the following notes:

DC Measurements

1. On the secondary, measure the dc resistance across the secondary leads. It should be around .5 ohms. Secondary DCR can be as low as 0.1R,
2. On the primary, measure between the center lead (typically red) to each of the ends. You should measure approx. 150 to 200 ohms. Primary DCR of a 100W OT is more like 15R per side.
3. Measure between each lead of the secondary to the leads of the primary. There should infinite resistance.
4. Test all leads of the secondary to the case of the transformer. There should infinite resistance.
5. Test all leads of the primary to the case of the transformer. There should infinite resistance.Measuring primary DCR can be tricky because of the large inductance involved. It helps to short the secondary while measuring primary DCR

Apply 6.3 volts ac to secondary leads

• Completely disconnect and remove OT from amp. Remove power tubes.
• Separate/isolate all wires and tape to a piece of wood etc.
• Connect heater voltage (approx. 6.3 vac) to the secondary winding pair using a current limiting series resistor of 10R/5W or larger between.
• Measure actual voltage across the secondary. If significantly lower than 6.3 vac the OT is shorted and you can stop here.
• Measure ac voltage on primary from each side to CT. Expect to see an elevated AC if you have a good OT.

[Helmholtz]

DC Measurements items 1. and 2.:

Secondary DCR can be as low as 0.1R,
Primary DCR of a 100W OT is more like 15R per side.
DCRs of primary halves can differ considerably and still be good.
Measuring primary DCR can be tricky because of the large inductance involved. It helps to short the secondary while measuring primary DCR.

Generally DCR measurements only show that the winding has no break (i.e. is not open).
Winding DCRs typically don't reveal partial shorts.

DC Measurements items 3. to 5.:

There should be no resistance.

No resistance is the same as zero resistance, aka a short.
What you want here is infinite resistance (open circuit).

[Delta362]

Very informative with the various methods to test and what to look for. Thank you everyone!

[dugdiamond]

ONE MORE THOUGHT ON OT testing.....

Magnetic inductive circuit theory has always seemed such a magical and mysterious thing to me.
With some heavy math and all those variables such as wave phase angles and other highly technical stuff.
Who the hell figures out that if I wrap so much of this gauge of wire this many times around such n such core, I will end up at such impedance and so forth. It's magical!

Ok, well as far as testing a transformer, it pretty easy to test for the obvious such as shorts and breaks using a simple DMM and measuring the DC resistance to get a pass or fail.
Yet there are many subtleties and malfunctions that may occur, especially given the magical like properties of alternating currents in real world environments.
A couple of the above dynamic testing procedures such as :
1. Measuring input voltage applied to the secondary and measuring the voltage back out the primary. This can help ascertain the ratio.
2. The neon light trick I suppose is good indicator of the mutual inductance happening within that magical electomagnetic force field being genarated within the core of the transformer.
I would like to add a third dynamic test that is rather simple and would like to hear anyone's input on the worthiness of what I propose.
3. How about if we input a small sinewave signal thru the primary and attach our scope to the secondaries and now we can now view and compare waveform from the input and the output. We can quantatively measure the amplitudes and also note frequency responses and such stuff....
This is a simple procedure and gives a visual representation of what you can see as far as a before and after signal going in and out or even reversed, you can go Out and in!
Yout know fairly quickly that if that magnetic component has lost its magic ....

[Enzo]

Your meter will detect open windings, but unless the ends of your winding are shorted together, your meter will not reliably find shorts. Shorts between windings or to frame yes, but if your 150 hm winding measures 149 ohms, will you interpret that as a short? Could be one turn shorted.

[Delta362]

I just realized I never resolved this thread. It was a failed OT. Replaced it with Hammond P-T1760W 100W, 2kΩ-4/8/16Ω and that solved it. Thank you!