Metropoulos Forum

Hi,

I have a strange issue with the plate voltage on V4 and V5. Measuring from pin 3 of both tubes I read from the multimeter 465 volts. As far as I know this value should be too high (wasn't the standard rating between 400 and 425/430?).
Some additional info:
- the ampli is an almost stock Jtm 45 kit
- the PT is a Classictone 40-18033, on the secondary I should have 690V @150mA, while on the primary I have 220V
- the rectifier is a JJ GZ34
- the choke is a 20H and not the stock 3H, I've used this because I already had one and in the very first version I saw it was used
- I wired the PI cap (the big internal one) to 32uF and not 16 like the original design

I'm able to bias the amp using the trimmer without changing anything else, currently I biased at about 38.5 mA.

Do you think that there is something wrong with the high tension I'm measuring at pin 3? Could it be caused by the choke or the PI cap?

Any suggestion is appreciated, thanks in advance,
Andrea

What is your wall voltage?

If your wall voltage is around 230-235 and you have the primary wired for 220v... you got the answer there.

Measure wall voltage and heaters voltage. You have to wire the transformer to get 6.3v on heaters or closer value depending on the voltage tap you use.

The wall voltage is 220v and the heaters voltage is 6.3... Things are getting strange...

Perhaps this kinda voltage is normal for the Classic Tone 40-18033 PT....Or perhaps you just ended up with one that was wound slightly different than normal where it ends up producing a higher B+ voltage. 'Typical' 50w Marshall PT voltages after rectification are:
Drake 1202-133 was around 440 - 450vdc I believe.
Drake 1202-118 was typically around 425 - 435vdc.
The later ('72ish) Drake 1202-164 around 390 - 400vdc.

Thanks for the info Tazin, just one more thing: as far as I know the amp is workin within the parametrs of both tubes and caps (should be rated at 500V) and the amp sound good. I've just the fear that I could have wired something in the wrong way!

Well, if the amp appears to be working correctly then that's a good sign. All you really can do is double check all the connections at the PT to make sure everything is hooked up correctly. BTW, someone over in the 50w build forum used a Classictone PT (Ref.# 1202-55) which I believe is the same model you used and he ended up with 456vdc after rectification. So, his voltage is not that far off from yours. The spec sheet for the Classictone 40-18033 has the secondary coils at 345-0-345Vrms which is pretty high, so I would think a typical dc voltage after rectification would be around 455-465vdc.

I got almost the same voltages on my 45 w/classictone PT. I ended up using the 240v primary with a 230v wall voltage, giving 440+v on the plates of the kt66s

Good suggestion!

I'll try to wire it to 240V, thanks!

I prefer around 44-450 V on a jtm45 but others prefer everything from 400v to 470v