Huge b+ voltage drop after plugging in tubes.

[waynard]

Just installed a brand new Heyboer Marshall PT in my new 50 watt Plexi build. Before I install the tubes i have a b+ of 470 VDC on the OT center tap. After I plug in the power tubes it drops down to 330 vdc. It works and actually doesn't sound bad but I am not liking the numbers I see. There must be something wrong. In the 20 years I have been building I have never seen voltage drop that much.

[waynard]

Possibly a short in the OT?

[Paul1453]

Do you still have your - bias voltage readings on pins 5&6?

Your output tubes aren't red plating, are they? 

waynard, I'm thinking the OT is OK.
If it was partially short and responsible for the voltage drop,
I'd expect the B+ to be low with or without the tubes installed.
So, the next component is the output tubes.
If you somehow lost your - bias the plates would be flooded with electrons.
That could explain the voltage drop.

[Mike_J]

Possibly a short in the OT?

I had a Heyboer PT with what we were pretty sure was a short in it. What I recall about it was the PT being hot to touch when at idle for only about 10 minutes. Might try leaving it on for ten minutes and touching the PT to see how hot it gets. If it isn't real warm it probably doesn't have a short from my experience. Others would probably be able to help you more.

Thanks
Mike

[eleventeen]

A short in the PT will in almost all cases heat/smoke/spark/stink.


Is that 330 B+ the target voltage you were looking for?


This is where an assortment of power resistors (which can be junkers) comes in handy. An 8K resistor across 400 volts would pull 50 mils. But would dissipate 20 watts so would have to be a biggie.

[PRR]

330V DC is not right for a 50W amp.

As suggested, check tube current and temperature. If bias has failed and they are sucking hard, it *could* drop that much. (But I'd expect other problems also.)

Tube or silicon rectifier?

[Paul1453]

A short in the PT will in almost all cases heat/smoke/spark/stink.

I can confirm this statement.

On the one partially shorted PT I had.

It got HOT, smoked, and stank within a few minutes of use. 

[waynard]

With no power tubes in I am now getting 460 vdc at the first power cap. As soon as I plug in any power tubes the voltage drops to 380 now. It depends on which tubes I try. I started with 6550's and then tried 6CA7's, then 6L6's. The same voltage drop on all of them. Not sure what's going on here. Is it the brand new PT or maybe the new OT?   Until I plug tubes in everything looks fine. This is why I feel like it is maybe the ot. Could be wrong

[sluckey]

Give us all the info you have on the PT.

[waynard]

Target voltage is 480vdc. The tubes are not red plating and the current draw is in a safe range. There is no smoke or funny smells. The preamp tubes do not affect the voltage drop. I am afraid to go any further because a bad ot can short or overheat a good pt. Bam, down two new trannies.

[waynard]

It is the Heyboer 50 watt Mashall PT.

[Ritchie200]

You never said if you have silicon or tube rectifier?  Check you diodes or replace your tube.  Easy stuff first.

Disconnect your OP tranny and check input resistance to ground.

Jim

[Ritchie200]

Are any of your caps getting warm during operation?  Are they new?  Could still be leaking.

Jim

[waynard]

These are actual measurements. AC voltage no load 120vac on primaries/ 350-0-350 and 6.3 vac on secondaries./ no load silicone rectifier 400vdc/ stanbyswitch on with no power tubes 465 vdc at rectifier. With tubes 380 vdc at rectifier no redplating no high current draw. I have them running kind of cold to medium draw. Now the funny thing is that the amp actually sounds pretty good but I know the numbers are telling me something is wrong. I have a bad feeling about that OT.

[Paul1453]

Keep your output tubes out and check your - bias on pin 5.

If your grid does not have a large enough - voltage there,
all your output tubes will act like you have your accelerator pedal pushed to the floor.   

[Paul1453]

Are any of your caps getting warm during operation?  Are they new?  Could still be leaking.

Jim

Maybe C11 and or C12 are leaky?
Or the - bias is not getting to pin 5?   

It seems to me that the voltage has to be dropping through the output tubes somehow.

With the output tubes out all voltages are good, no?

If you test voltages/etc. with those tubes out, the chances of causing further damage to the other components seems to be slight to me.   

Do you have documentation of your voltages from before you made your changes?
Voltage readings with your output tubes both in and out of the circuit from before you made your changes would be helpful now. 

[Mike_J]

Had low voltages like you describe once. Did the yellow highlight procedure a couple of times couldn't find anything wrong. Sluckey asked which side of the choke the HT was connected to. Had it on the wrong side of the choke. Missed it a couple of times. If I remember correctly the choke in that amp could not handle the current the power tubes were drawing through it. Resulted in low voltages as you describe. Changed the connection point to the other side of the choke and voltages went into normal ranges.


Thanks
Mike