Puzzling plate voltage drop when biasing

[Madjackhatter]

Cheers everyone, I have a JCM800-2203 from a Weber kit bought on the cheap build going now. I used Marhall's bias circuit with a Hammond PT and stock bias circuit. I experienced a huge voltage loss biasing. From about 470 vdc to 300 once the tubes were in a fair operating range.
Then set it aside a while and swapped the Hammond for the MM High Power Twin I got with the kit and changed the bias circuit to Weber's, which feeds the bias voltage from one leg of the secondary voltage. The bias control range maxed is -60, so that seems in line with the norms. But....same result. 463vdc goes down to just over 300 before it is at 70% in both cases. Anything I'm missing here ? The tubes are Apex matched JJ with the number being 21. Tubes test well with my ancient tester that lacks a few bells and whistles. Are the tubes posibbly causing this drastic shift ?
Thanks in advance, I don't have a ton of questions these days but this severe of a drop puzzles me.

[mresistor]

I had the same problem with a build once and I replaced the OT which solved the excessive V-drop / bias issue.  It could also be load related.

[sluckey]

Show us the data sheets for the PT and OT. Tell us how you are checking the bias.

BTW, that Weber bias circuit is dangerous because it will allow you to set the bias voltage to zero volts! Also C24 and R48 are unusual. You usually only see that on a PT that must use a bridge rectifier.

[_Steve]

Do you have a current limiter plugged in? That'll do it.

[pdf64]

I wonder if this is another case of have his meter set to measure current, rather than mV, when using 1ohm cathode current sensing resistors?
Such that the bias is being adjusted to load the HT supply very heavily, 100s of mA.

[Madjackhatter]

Well I will get that info to you once I'm home. I actually tailed a 1K resistor to ground, correct sir ! That is dangerous. And also a good point with it being a Weber OT, it is always a worthy suspect. I have not seen a bad from the box OT of their but PTs', I've seen a few. And yes...I usually have the ol' light bulb limiter plugged in. Last night I pulled the board and looked for "bleeding" and thanks for the zister and cap to check. That blew right by me, I changed the bias circuit and rectifier after I pulled the board, but missed them. Thanks guys, more tonight....

[Madjackhatter]

Also I have 1 ohm resistors,1w,- 1% to ground on the tubes for my readings. I have them as permanent fixtures to act as fuses in case of a tube short and to make it easier to service when that time is due.

[Soulfetish]

Double check those 1 ohm resistor values and make sure that 3 color band isn’t silver or something.
Is it only the plate voltage dropping 170V, or is the whole B+ sagging to 300V?

[pdf64]

…
Is it only the plate voltage dropping 170V, or is the whole B+ sagging to 300V?

Yes, check the resistance of the 1ohm resistors, and remember that you monitor current through them by measuring the voltage across them.

I think it’s regrettable that anode voltage has become a standard thing to measure, when assessing idle dissipation.
As doing so can easily invoke oscillation, which buggers up the reading, as the amp’s no longer idling. 
The only way to do so without risk of that occurring is to remove the valve in the phase splitter socket.
Hence unless there’s good reason to suspect the OT, my view is that it’s far better to measure the HT node voltage, eg at the OT CT.

[PRR]

Our grandfathers knew better.

A proper tube-man's voltmeter is a VTVM with a 1Meg resistor in the DC probe. So the whole length of lead is no longer spraying 300V of audio AC all over the preamp to tickle oscillation.

[mresistor]

I think it’s regrettable that anode voltage has become a standard thing to measure, when assessing idle dissipation.

So a better way might be the transformer shunt method to check idle dissipation?

[PRR]

So a better way might be the transformer shunt method to check idle dissipation?

No; where did he say that?

We need to know plate-cathode volts.

But this is a DC measurement. We do not need the meter lead spraying audio AC onto the preampery. There was a simple trick in nearly every VTVM. I have not seen it in DMMs.

Transformer-shunt has the same objection, plus deadly danger to technician and to meter.

[PRR]

https://www.industrial-electronics.com/vom-vtm_1975_6.html

[Madjackhatter]

A lot to digest, however it will be Thursday before I am home to check all the boxes for you. The voltage did sag to 302 vdc cathode to plate and I am truly sispecting the OT but until Thursday....I know nothing. I was pulled out of town for an emergency job.

[Madjackhatter]

It is awesome to be home again, a 3 day job turned into a month and change worth of travel. So I got back to the bench and checked the brand new OT and one leg of primary was toast. Upgraded to a Hammond that a friend sold me cheap and whatta you know, voltage holds under load....Thank you guys so much. OT was a spot on suggestion ! It was grating on my nerves to leave that chassis abandoned on the bench a month. I guess the moral of the story is to check my build transformer resistance the next time, lol

[mresistor]

So a better way might be the transformer shunt method to check idle dissipation?

No; where did he say that?
Transformer-shunt has the same objection, plus deadly danger to technician and to meter.

Sorry PRR   I used wrong terms and meant to say  Transformer Resistance Method - it's what I was thinking of.

[pdf64]

So a better way might be the transformer shunt method to check idle dissipation?

No; where did he say that?
Transformer-shunt has the same objection, plus deadly danger to technician and to meter.

Sorry PRR   I used wrong terms and meant to say  Transformer Resistance Method - it's what I was thinking of.

I suggested to avoid default probing of output valve anodes, rather to measure the voltage at the HT feed to the OT (eg at the CT) instead.