Replacement Transformers

[martin manning]

I think your filament voltage will be too high with 240V on a 220V primary, maybe even on a 230V primary. Data sheets say +/- 10% is acceptable, but with higher than nominal mains you could easily go over that.

[hywelg]

Thanks Martin, wont go that route then.

So, Bassman PT is 330-0-330. The Ceriatone is 345-0-345. A Fender Twin (a drop in for the aperture) is 320-0-320.

So I'm going to lose approx approx 25 HT volts with a Bassman and 30 volts approx with a Twin. Are either of these likely to be a significant change to the amp? If not should I rejig the dropping string to keep the pre-amp voltages the same as now and accept a lower B+ on the output valves?

[martin manning]

A Twin PT will not be quite as low as its spec since you will only be running two output tubes. I'd guess it will be more like 330. For the same reason the filament voltage will be a bit high (still within +10%), but you will have plenty of current for EL34's. Any new transformer might require a tweak to the dropping string for the preamp voltages. The power amp voltage is not too critical IMO.

[hywelg]

Excellent . Looks like Hammond 291/290FEX will do the job then . Nicely inexpensive compared to some of the options...

[Mr. dB]

http://www.inmadout.com/download/pdf/TA34M50.pdf

This looks suspiciously like a Vox AC50 transformer.

[pamaz67]

mr Savino Luce, owner of InmadOut, will do any custom transformer you will require once it has specs compatible with the required dimensions.
And the price are really reasonable also for non standard irons.
i've bought from him many pieces ( some 100 pcs lets' say) and have never been disappointed.
I've no connections with him, so I'm just an happy customer.

Paolo Mazza

[hywelg]

Shame they can't be bothered to reply to my email then!

[hywelg]

I went with a Hammond 291FEX in the end along with a Hammond 266JB6 for the relays. Tight fit for the 266JB6 inside the chassis but its in.

Hammonds dont have a filament center tap so I've to create an artificial one. Seen somewhere 100R x2 to ground is normal. Is this OK and what wattage would I want?

[mrdarwin]

It's ok
Use 1 watt resistor, carbon or metal film...
I used 47r 1w... because I had no more 100r!

[hywelg]

Update on this.

Only just got around to firing the amp up with the Filament centre tap R's installed. First thing I notice is an audible hum coming from the PT. But at least theres no smoke!

Checking the voltages first thing I notice is V1 and V2 are way low, like 145v but there is a 5751 in v2 so I resolve to change that back to ECC83 and remeasure, but I got to measuring the output valves and these are way low, 350v ish so investigating further I check the dropping string and I find 351 volts at the first 100uF cap. I knew it would drop a bit as the Hammond is 320-0-320, lower than the 345-0-345 original. But a drop of nearly 100volts? Something amiss.

Incoming was 241v bang on spec for the transformer (its a UK spec 240 not an EU 230v rated one). Output from both sides of the HT was 285 and 334. Clearly an imbalance. Measuring 28v AC on the CT before the fuse.

Any ideas anyone?

[martin manning]

Maybe a failing rectifier? Disconnect the secondaries and check the voltages for balance. You can test the diodes then too, but a soft failure can be difficult to see. If the transformer voltages are good, then I'd just replace the diodes.

[hywelg]

Thanks Martin, will pull the rectifier board tomorrow. I'm guessing the rectifiers on the low volts side would be the problematic one(s) . OR just change them all and be done with it?

[martin manning]

They're cheap, just replace them all.

[hywelg]

So, I pulled the HT and the bias taps leaving the CT connected to ground via the HT fuse, powered up and i get 256-0-315 with the bias tap at 23v.

So something is obviously wrong here.

I wondered if I had misread the wiring schematic getting the CT and the bias tap mixed up but no, I checked and all the resistances tally and whilst one half reads 10.2ohms and the other 11.2 that doesn't account for being 20% down on one side.

The CT-bias reads 2.2ohms which I would expect to give approx 20% (2.2/11.2) of 315v i.e 62v but I'm only seeing 23v with no load.

Going to call the supplier tomorrow to see what they say, but it doesn't seem right to me.

Anyone had this sort of problem with a new PT?

[JazzGuitarGimp]

Is it possible you've grounded the bias tap, rather than the center tap? Try remeasuring, but use the bias tap as center tap, so you will measure the chassis as bias. It's possible the tranny was built wrong, and the center tap and bias tap got each other's colors.