Cathode voltage 6L6gc query

[Steve Blackdog]

Hi folks

With pairs of tubes, should the cathode voltages be equal?

see the attached schematic section (Proamp viper 60/kmd gv60), I have different voltages at the output of T3 and T4 which goes to my cathodes of my two 6L6s. One is 4v greater than the other (63.5v and 59v).

I am scratching my head over this and wondering whether it points to a failed component elsewhere (if not one of the TIP41c Ts).

any help here?

cheers

Steve

[Helmholtz]

The output stage uses transistors and tubes wired in a series arrangement as cascodes.

The actual tube bias voltage (Vgk) is given as grid bias (+21V) minus cathode voltage.

Different cathode voltages not necessarily mean different idle currents with this circuit, as the 2 tubes may need somewhat different bias voltages for same idle current.
What happens to cathode voltages when you swap power tubes?

You can find actual idle currents from the voltage drops across R69/70 divided by 3.9.

Apart from that, a difference of less than 10% is rather insignificant.

[J M Fahey]

No, completely normal.
Tubes are not matched within 1% by any means but show the normal spread; both are passing the exact same current, by definition, (drive transistors FORCE them to) and they develop needed grid to cathode voltage to accomplish that.

In fact I would be surprised to find them the same.

FWIW at that very low idle current (which is fine, that´s the way it works); they are getting:

* top one is getting +21V (grid) -63.5V (cathode)=-42V net bias

* bottom one is getting +21V (grid) -59V (cathode)=-38V net bias

You sure those are 6L6?
I mean, maybe somebody used them, but those Bias voltages are definitely in the EL34 field.

6L6 would demand around -52V .... unless those 6L6 are in coma and the Music Man style circuit is squeezing down to the last electron out of those worn cathodes.

[Helmholtz]

BTW, it should be possible to balance power tubes via VR9. Adjust for minimum output hum. This typically means equal tube idle currents.

[g1]

Also check that R77 and R78 are both in spec.

[Steve Blackdog]

Also check that R77 and R78 are both in spec.

yes, these are 470r as per spec. The tubes were new JJs.

Unfortunately I have just taken a stupidity pill. As I was testing plate voltage, I forgot to put the insulated cover on my DMM probe and touched pin 4. The (new!!) tube red plated so I switched off.

I think I have fried T4. Permission to say “cock”!

Is there anything else I am likely to have killed?

p.s, anyone want to buy a matched pair of 6L6 tubes - less than an hour’s use!

[pdf64]

“As I was testing plate voltage, I forgot to put the insulated cover on my DMM probe and touched pin 4. The (new!!) tube red plated so I switched off. ”

Oh dear, that’s unfortunate, with most amps that wouldn’t be a problem. The associated cascode transistor may have taken a hit.
I find it’s much better (less prone to issues and errors) to measure the HT, rather than the anode, voltage.

[Steve Blackdog]

What’s so stupid of me is that I had already tested it once!

[stephen_G1]

What anode and screen voltages were you getting on the 6L6's please?

[Steve Blackdog]

What anode and screen voltages were you getting on the 6L6's please?

Hi Stephen

this is quite an old thread and I don’t think I have a record. Do you have a Proamp? If so, John Cooper, who built them can be contacted at valvetech44[at] aol.com. He is very helpful.

Best

steve

[stephen_G1]

Hi Steve, thanks for your reply. I believe that John Cooper sadly passed away recently.
https://guitar.com/news/gear-news/am...-dies-aged-71/

I have a Proamp Viper in for repair, and I'm scratching my head a bit about the HT power supply for the output stage.
In the Hi setting, I'm getting 700 volts on the anodes of the output valves, which is way above their maximum spec. The supply is half wave rectified, so the HT shouldn't be anywhere near that level, but I ca't work out why it is so high. The AC from the transformer at the high setting is 255 volts.

[Enzo]

What voltage is on the screens? And is your HV supply a pair of stacked filter caps? Your 250vAC rectifies and filters to about 360v, so a stack gets you into 700v land easy enough. ANd a screen feed from the center would be about 350, so...yeah.

Perhaps if we knew what it was you were trying to fix about it? Power supplies don't generally add a couple hundred volts.

[Steve Blackdog]

Very sad to hear about JC. He was all revved up about his new Hotbottle project.

Stephen - I will pull the valves and take some readings for you.

steve

[stephen_G1]

Hi Enzo, thanks for your input. I was not familiar with the full wave voltge doubler circuit used in the amp,(I am now!).
Plates are 700 volts, and screens are half that voltage (at the "Hi" setting) so all correct.
700 volts seemed too high for the 6L6GC's..
I fixed the original fault in the amp which was a faulty transistor in one of the valve cathode circuits.

[Steve Blackdog]

Hi Enzo, thanks for your input. I was not familiar with the full wave voltge doubler circuit used in the amp,(I am now!).
Plates are 700 volts, and screens are half that voltage (at the "Hi" setting) so all correct.
700 volts seemed too high for the 6L6GC's..
I fixed the original fault in the amp which was a faulty transistor in one of the valve cathode circuits.

I have to say, it is one loud amp!