Voltage Issues

[lpd]

Hey guys. Perhaps someone here can shed some light or share experience to determine a voltage issue on my amp. B+ drops on start up from 395 on plates (which is where it usually measures) to 372. Screens drop too 20 volts. Preamp b+ drops as well. Current on output tubes does not drop and bias stays stable. AC is fine on diodes 300 per side.

Can anyone assist...

[xtian]

This sounds normal. I mean, all amps have initially high B+ voltage, that drops into operating range as the power tubes start to conduct current.

Are you saying, yeah, dude, I know that, but this is unusual behavior for this amp?

[lpd]

This sounds normal. I mean, all amps have initially high B+ voltage, that drops into operating range as the power tubes start to conduct current.

Are you saying, yeah, dude, I know that, but this is unusual behavior for this amp?

This amp usually operates with 395 b+ and recently dropped 20 volts. Should have clarified.

Thanks

[Littlewyan]

So you're saying the AC doesn't change the whole time this is happening?

I'm assuming this is an Express?

[lpd]

So you're saying the AC doesn't change the whole time this is happening?

I'm assuming this is an Express?

I should try to be less vague

It's an Express documented here with a voltage chart.

https://tubeamparchive.com/viewtopic.php?t=2858

AC does not change on start up with exception to a flicker between 1 volt of 298-297vac on each diode. The b+ and screens on all tubes drops as it warms up over a period of 1-2 minutes. On the EL34s it drops 20v lower than it usually measures at. Pat has minor mechanical buzz (edcor). Heaters and bias are stable and measure where they should be.

[Littlewyan]

Ok so does the actual HT change? Measured at the diodes?

Also that link goes to a buffered effects loop page

[lpd]

Ok so does the actual HT change? Measured at the diodes?

Also that link goes to a buffered effects loop page

https://tubeamparchive.com/viewtopic.php?t=28589

HT? If this is heaters they don't change. Just b+ and screens.

[Littlewyan]

Yeah so you say the voltage is dropping on the EL34 plates, is it dropping on the end of the rectifier? Or at the first filter cap? Have you checked the bias of the EL34s recently?

[lpd]

Yeah so you say the voltage is dropping on the EL34 plates, is it dropping on the end of the rectifier? Or at the first filter cap? Have you checked the bias of the EL34s recently?

Yeah so you say the voltage is dropping on the EL34 plates, is it dropping on the end of the rectifier? Or at the first filter cap? Have you checked the bias of the EL34s recently?

Drops at end of diodes. What should it read at rectifier? Also at filter cap. Bias is fine

[Littlewyan]

When I say diodes I mean the diodes in the rectifier. They'll be connected to the first filter cap and should read roughly 395VDC.

I'm trying to determine if something is drawing excessive current and pulling the HT down.

[lpd]

When I say diodes I mean the diodes in the rectifier. They'll be connected to the first filter cap and should read roughly 395VDC.

I'm trying to determine if something is drawing excessive current and pulling the HT down.

395 at rectifier I believe. Should be a little higher because the tubes will drag it down a little or no?

[David Root]

Let me jump in here. I built this amp and this is stumping me too. The amp probably has 30-40 hours on it, maybe more, over a period of several months. No trouble until a few days ago.

We have tried changing the EL34s. Three sets, same thing.
Customer noted what seemed to be one of the EL34 sockets acting up, so he cleaned and retensioned the pins. Sockets are new Belton.

The EL34s don't seem to be drawing more current as the plate/screen voltages go down. The bleed resistors on the plate cap check out OK. All grounds we checked out are OK.

All the tubes are used vintage but tested out 100%. Since this problem appears to develop as the tubes warm up I have to suspect a tube problem, and maybe that EL34 socket has something to do with it, but so far we have not been able to get to the cause.

[Littlewyan]

Well a few things don't make sense here. You say the voltage at the rectifier stays at 395VDC but the voltages on the EL34 Anodes, EL34 Screens and the Pre amp nodes all drop?

[David Root]

No, everything drops including the diodes output.

[martin manning]

Reservoir cap failure?