What to do with Peavey VTB300 (200 watt tube head) with bad PT

[glebert]

Bought this as a project a while ago and finally got around to working on it. Unfortunately it seems like the PT has a short on the windings for the 610v plate supply. I measure 45 and 55 ohms from the windings to the transformer frame. The other windings seem OK. Since this transformer has separate windings for the 610V plate for the 6550s and the 350V screen/PI/preamp voltages it seems pretty unlikely to be able to get a new transformer that will work. These amps seem to be really rare but probably not that valuable, so I'm torn on what to do with it. These are supposedly designed by Hartley Peavey, and many were supposedly hand built by him (according to a Peavey employee who asked Hartley directly). I am assuming a rewind would be cost prohibitive if I even knew who to ask to do it. Only other thing I can think is to use the tube preamp and put a class D power amp in it. I could take out the OT and put a second PT there to give the class D rail voltages.

Thoughts?

No schematic available for this, but the VTA400 is similar for the power amp section.

[Enzo]

You could talk to Heyboer and ask what it might cost for a rewind.

[The Dude]

...... I measure 45 and 55 ohms from the windings to the transformer frame......

Just to be sure: When you measured, did you have the secondary disconnected from the amp to make sure you weren't reading a short from elsewhere- rectifier diodes, etc.

[gui_tarzan]

It will probably take some time but I agree with Enzo, Heyboer can likely do a rewind. It depends on how much time and money you want to invest. In looking up that model, it seems to have sort of a cult following so it may be worth it.

[glebert]

Just to be sure: When you measured, did you have the secondary disconnected from the amp to make sure you weren't reading a short from elsewhere- rectifier diodes, etc.

Yes.

It is measuring about 14 ohms across the winding. Windings to ground is actually 44 and 57 ohms, so almost like a resistance to ground at one end of the winding. I get about 400VAC across the windings when they are are disconnected from circuit.

[garytoosweet]

If a rewind is totally out of the question, what about getting a transformer with a ~250V winding and enough current capability to run both circuits, then using a voltage doubler (+ a dropping circuit, probably) to generate the 610V.

Just an idea. In theory I like it more than dropping in a class D amp. The tubes are worth saving!

[drewl]

I had one in not long ago.

Look around for a used trans or possibly another dead amp with a suitable PT.

[glebert]

If a rewind is totally out of the question, what about getting a transformer with a ~250V winding and enough current capability to run both circuits, then using a voltage doubler (+ a dropping circuit, probably) to generate the 610V.

The doubler is an interesting idea, but I wonder if I would be able to find a PT that can source enough current at those voltages for 4x6550. Wouldn't you need something like 800mA at 250V to get 200 watts? The PTs I see with 250V winding seem much lower current.

Wondering if a PT like this would work for the plate, screen, and bias voltages and then use a fillament transformer for the rest?
https://edcorusa.com/collections/tub...80ma-60v-100ma

[Helmholtz]

The doubler is an interesting idea, but I wonder if I would be able to find a PT that can source enough current at those voltages for 4x6550. Wouldn't you need something like 800mA at 250V to get 200 watts? The PTs I see with 250V winding seem much lower current.

A 200W tube amp consumes almost 400W DC power at full output.
AC power is even higher.

[garytoosweet]

If entertaining the idea of running a separate transformer for the filaments, it may be easier to find a toroidal for the HT. If you are able to mount it.

https://www.hammfg.com/electronics/t...ers/power/1182

If it were me I'd definitely want to know the rewind cost before going down this rabbit hole. I've heard pretty reasonable numbers for Heyboer rewinds. Could be worth it when you consider time putting all this together, making it all fit/work etc.

[glebert]

Guess it doesn't cost anything but a little time to call Heyboer and ask. They have been pleasant to deal with when I bought from them.

[J M Fahey]

Yes.

It is measuring about 14 ohms across the winding. Windings to ground is actually 44 and 57 ohms, so almost like a resistance to ground at one end of the winding. I get about 400VAC across the windings when they are are disconnected from circuit.

IF that transformer supplies 400VAC across that winding and normal voltages at other taps it is NOT shorted or shorted to ground.

Remeasure:

a) with amp OFF and unplugged from wall. 400V winding disconnected from anything else.

Just in case, with tubes removed, at least power tubes.

* DCR across 400V winding

* Resistance from one end to ground. Label it "A" (piece of tape)

* resistance from another end to ground. Label it "B"

b) With amp plugged and ON (be careful, tape wire ends to a piece of wood or something so they remain fixed).

* measure voltage from A to B

* Voltage from A to ground.

* Voltage from B to ground.

As an extra test: does fuse blow?

Does Transformer hum - buzz - overheat - burn?

Please post results but so far (unless measurements prove otherwise) I do not see it in need of rewinding.

[catalin gramada]

Get one from Europe: https://www.inmadout.com/T-power.asp. Is a classic replacement for Hiwatt. TADR201 and build a stacked style power supply.

[glebert]

Juan, I've done pretty much all of that except measure voltages to ground. DCR across winding is 14.8 ohms, DCR to ground from A and B is 45 and 58.5 ohms, give or take a tenth of an ohm or two. Running on a LBL there is no current draw (glow) when winding is floating. No current draw when hooked up to rectifier diodes if they are floating. When rectifier diodes are connected to ground it starts drawing current right away. It will blow fuses if not on an LBL.

Since the resistance to ground seems like it is at one of end of the winding I was wondering what would happen if I put that end directly to ground and did half wave rectification. Doing the VAC to ground measurement might help answer that. Even if was a temporary measure it would be nice to see what else is going on with the amp before I drop money into it.

[Helmholtz]

Since the resistance to ground seems like it is at one of end of the winding I was wondering what would happen if I put that end directly to ground and did half wave rectification. Doing the VAC to ground measurement might help answer that. Even if was a temporary measure it would be nice to see what else is going on with the amp before I drop money into it.

I don't like halfwave rectification in power supply (power as opposed to e.g. bias supply).
Reasons:
- Halved ripple frequency and increased ripple voltage, requiring considerably larger filter capacitance,
- Increased rectifier peak currents and increased filter cap ripple current,
- Most of all, it means an average DC current flowing in the PT secondary, magnetizing the PT core and increasing PT losses and buzz.