Metropoulos Forum

I just finished building a JTM45-type amp - inspired by the Traynor Dynabass, actually - and my B+ is a good deal higher than I expected.

The PT is a Hammond 273BX which is rated at 350-0-350. I measured the AC voltage on the standby switch/recto socket at 354V, but I'm getting almost 490VDC on the plates. Before I put in the output tubes, I was getting 530 volts on the output sockets.

I'm just a little curious because the original JTM45 PTs are supposed to have been 350-0-350, but I think that they put more like 450 on the plates. Any ideas as to the discrepancy? The plate voltage is higher that my 350VCT, diode rectified 100-watter! And my caps are only rated for 500V.

Thanks for any insight.

Kevin

What is the current rating of the hv secondary? jtm 45s are about 150ma Dave

Which rectifier tube are you using?

Hammonds are renouned to be under rated. I would check your heater voltages.... I think hammond base their numbers on 110V mains.... so you can see that at 120... you can have higher numbers....

Thanks for all the responses.

The current rating is 175mA; so, maybe a little high? Would this "underloading" amount for higher plate voltages?

I'm using a Sovtek tube right now; it was the only one I could find in town. Will switching to an old Philips brand X lower voltage? Or even a new Chinese tube?

It seems that the Hammond is based on a 115V primary, so there's a few extra for me. The heater voltages are fine; 3.0-3.1. Yeah, Hammonds are known to be underrated. I figured specs is specs, you know. I guess I'll go with the next one down next time.

Maybe I'll have to buy a variac. Or try a 5U4, for kicks...

What tubes are in it and what are they biased at?

Oh yeah whats your wall voltage read?

Winged =C= 6L6GC, at 42-43mA, at 488VDC last time I checked.

Wall voltage is 120-121...

Thanks for the responses.

Maybe your multimeter is not accurate.Check with other multimeters...

It is a cheap meter; it would be nice the meter were faulty. It's probably not the case, though, as my wall and heater voltages are good.

I bumped the bias up to 48mA and the B+ is down to 478 - still pretty high for this style of amp...

Thanks again for the responses.

I have some experiences with some cheap multimeter,and despite correct values at low voltage,some HIGH voltage were incorrect...

Is not a big problem if your HT voltage is a bit high,maybe you have an amp with a little more headroom.....

Actually, I've always found that the meter has a hard time with higher voltages; it jumps around for a while before settling down and giving me a proper reading. Maybe the high voltage readings are off. I'll have to get a better one soon...

No, it's not really a problem. With the preamp voltages adjusted accordingly and the appropriate filtering, it doesn't feel too stiff at all.

Thanks.

i got a cheap digital multimeter here, when batt are low give fake result(higher usually) when they are fine, it's fine.

replaced the battery - which was getting low - but i'm getting the same reading. maybe i'll look for an appropriate zener diode.

thanks for the responses.

kevin

You could put a 180 to 300 ohm 10 watt resistor right after the rec diodes to bring it down .