diytube.com

Hi,

I did an ST-70 rebuild a few years ago, but haven't used it much due to a heat problem. After a few hours on (used or just idling), the Triode Electronics PA060S power transformer gets to 136℉.

Just did a little checking and I actually had the outputs running a bit cold at 18mA per tube rather than 50mA. I did just increase idle to 23mA per tube and I'll leave it at that for now.

Although I'm using ECC99s in place of the 12AU7s, I'm still within the filament specs at just under 4A per winding. B+ is 355V, but I do question the first stage of the PSU. Is this how the waveform is supposed to look on the first cap section before the inductor? scale is 100V/div

It's been running on the bench in my garage for 4.5 hours now. The PA060S is only at a mild 88.3℉. I think my problem is restricted space on the shelf in my living room.

Never had a PA-060 that didn't run between 55 and 60C. myself. Sounds ballpark to me.

Cheers!

The original, sure, that got roasty. The promise of the upgrade one was that it would run cooler.

And it will once I re-arrange the shelf where it sits

A nice "Whsiper" fan would work wonders. It would also
send that nice transformer/tube smell all over the room
to increase your listening pleasure!

davygrvy wrote:B+ is 355V, but I do question the first stage of the PSU. Is this how the waveform is supposed to look on the first cap section before the inductor? scale is 100V/div

Does Davy's scope trace look funny to anyone? I'm not sure I'd expect the voltage on the first cap to get pulled from 500VDC all the way down to zero on every cycle. I also think B+ of 355V sounds a tad low - shouldn't it be closer to 415V? It makes me wonder if a section of the cap is leaking, pulling down the whole supply.

Leave it on the bench, turn it up to 45-50ma a tube and see what the temperature and B+ is.

Ty_Bower wrote:Does Davy's scope trace look funny to anyone? I'm not sure I'd expect the voltage on the first cap to get pulled from 500VDC all the way down to zero on every cycle.

Come to think of it....

Is there a short in there somewhere?

I also think B+ of 355V sounds a tad low - shouldn't it be closer to 415V?

Mine has 420V with a 5U4G and 470V with a GZ34 (measured at the B+ lead on the OPT's).

Cheers!

I'm used to seeing 0.5 to 5 VAC on the first cap with a true RMS multimeter. Havn't checked any with a scope. 500 volts ripple would be excessive. Have you checked AC current at the fuse?