Higher than expected B+ from Hammond PT

Kevin · Post by **Kevin** » Mon Apr 07, 2008 9:18 pm

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

RACKSYSTEMS · Post by **RACKSYSTEMS** » Tue Apr 08, 2008 1:49 am

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

Maschinenmann · Post by **Maschinenmann** » Tue Apr 08, 2008 7:06 am

Which rectifier tube are you using?

chubs · Post by **chubs** » Tue Apr 08, 2008 7:38 am

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....

Kevin · Post by **Kevin** » Tue Apr 08, 2008 10:56 am

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...

RACKSYSTEMS · Post by **RACKSYSTEMS** » Tue Apr 08, 2008 11:06 am

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

RACKSYSTEMS · Post by **RACKSYSTEMS** » Tue Apr 08, 2008 11:07 am

Oh yeah whats your wall voltage read?

Kevin · Post by **Kevin** » Tue Apr 08, 2008 11:14 am

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

Wall voltage is 120-121...

Thanks for the responses.

sebastiano · Post by **sebastiano** » Tue Apr 08, 2008 4:12 pm

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

Kevin · Post by **Kevin** » Tue Apr 08, 2008 7:32 pm

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.

sebastiano · Post by **sebastiano** » Wed Apr 09, 2008 4:59 am

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.....

Kevin · Post by **Kevin** » Wed Apr 09, 2008 3:00 pm

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.

mathd · Post by **mathd** » Wed Apr 09, 2008 5:26 pm

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

Kevin · Post by **Kevin** » Thu Apr 10, 2008 10:59 pm

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

razelfrax · Post by **razelfrax** » Fri May 02, 2008 7:37 pm

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