Problem with biasing Twin reverb type amp

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pdf64
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Re: Problem with biasing Twin reverb type amp

Post by pdf64 »

tedsorvino wrote: Thu Sep 21, 2017 11:55 pm My bias probe is similar to this one: https://www.amazon.com/Eurotubes-Bias-P ... B019PAVXPG

My mistake. It juest measures current, no voltage. I measure plate voltage on pin 3.
So what about the 110mV mentioned? ie what are you measuring and how have you measured it.

Regarding the current measurements, are you sure that the meter is good? eg to verify, try measuring the current that a 220 ohm resistor pulls from a 9V battery.
When using the bias probe, are you keeping the leads and meter away from the pre-amp?
Do the readings change if the amp is operated with the tube removed from V6 LTP socket?

The issue with the diode is that as you have described things, the bias supply would created a positive voltage, rather than the negative that you have reported; so something is wonky.

Sketching out a schematic, rather than tedious, confusing and apparently erroneous circuit descriptions would save an awful lot of bandwidth.
tedsorvino
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Joined: Tue Aug 08, 2017 9:13 pm

Re: Problem with biasing Twin reverb type amp

Post by tedsorvino »

Thanks pdf64 for the help.
No change on readings without the P.I v6 (what LTP stands for?) . Very good idea to take out the whole NFB - P.I. circuit.
No change from a distance of about 1m. from the preamp. The meter is in excellent condition.
The probe must be defected. I may have destroyed it, by mistakenly measuring voltage as well as current with it.Obviously by placing the probe to the correct sockets. But still.....It's made just for current.
I 've made readings on a resistor across the cathode on pin 8 and everything seem properly adjustable. 45mA is what I've got.
I'm not really technical, so I'm trying to describe things as good as I can. Even if my descriptions sound the way you characterise them.
tubeswell
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Re: Problem with biasing Twin reverb type amp

Post by tubeswell »

tedsorvino wrote: Fri Sep 22, 2017 4:29 pm... check my PI - NFB capacitors for leakage and I know they are new, I decided to measure voltage around their pins.
The 0.1uF NFB Coupling cap takes about 67V and gives 0.4V and the 0.001uF P.I. Coupling cap takes about 61V and gives below 0.20mV (while the measurement doesn't stay steady). Do these measurements seem correct to you?
To test coupling caps for DC leakage, use the method in the attached Jack Darr book Ch 4, Fig 4-4, p57
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He who dies with the most tubes... wins
pdf64
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Re: Problem with biasing Twin reverb type amp

Post by pdf64 »

Just to note that in regard of terminology used for the cap test method noted noted by Darr, a modern digital multi meter DMM is equivalent to a vacuum tube volt meter VTVM.
A volt ohm meter VOM is a regular cheap old analog multi meter, with a moving needle type display.
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jjman
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Re: Problem with biasing Twin reverb type amp

Post by jjman »

Placing the meter in the current path, as THAT probe does, means the meter's internal resistance skews the measurement. Even a quality meter can have that issue. Transformer Shunt Method also has that issue. Using the transformer-voltage-drop method or the cathode-resistor-voltage-drop method avoids the issue. Some probes like THAT one are wired for the CRVD method instead of for the current-mode of a meter.

Using a cathode approach means you should adjust for the measured or guesstimated screen current. With screen resistors present, measuring screen current, via measuring the drop on that resistor, is easy.
If it says "Vintage" on it, -it isn't.
tedsorvino
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Joined: Tue Aug 08, 2017 9:13 pm

Re: Problem with biasing Twin reverb type amp

Post by tedsorvino »

Thanks Tubeswell for the attachment. Great insight man. Really valuable information. I'll read it carefully and I will test the coupling caps.
Thanks everybody for the advice. It's really helpful since I'm new to the world of amplifier servicing.
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