troubleshoot after recap

[hebaton]

THat one reads 1k2 but i'll lift one side and see

... With one side out it reads dead on at 1.04K

[martin manning]

So, as it is now how high can you get the idle current, and at what plate voltage?

[hebaton]

I lifted the two resistors on the pot. one reads 15K the other 45K so in parallel they shoul be 30K5 correct ? Why then was I getting a reading of 11K together before ? I assume that was taking the pot itself into account ?
THe schematics I find call for 33K there. 30K5 should work just fine ?

I am lost, everything seems to be to spec, or close enough...
Does this "one wire bias adjust" usually work ?, should I try moving the yellow wire to the opposite log on the pot ?
When I look at schematis I also see -37V before the pot. I am getting -82v ( no tubes in )

[hebaton]

So, as it is now how high can you get the idle current, and at what plate voltage?

THe most I got between CT and PLate was 2,6v. didn't check plate voltage but that was with the pot completely turned counterclok

With tubes in , the most I can get is 2,55v between ct and plate at 432v plate voltage

[hebaton]

Is there no way to mesure idle current directly ?

[martin manning]

15k in parallel with 45k is 11.25k.

The 2.6V across the OT primary is ok for now, I think, but it would be nice to know what the plate voltage is. Replace the caps, and If you want to increase idle current you can reduce the 45k (marked 47k) on the pot to make the bias voltage less negative. Maybe try a 27k.

You can measure idle current directly with a bias meter device, or if you ground the power tube cathodes through 1 ohm resistors you can measure voltage at the cathodes in mV, which will be numerically equal to mA.

[hebaton]

Thank you. Please correct my error, I that that resistors in parallel could be calculated as per : R1+R2+Rn/N or in thins case R1+R2 / 2
I changed that resistor to a 22K precision 1/2 watt I can now get 3.4v @ 329vplate voltage. How do I calculate dissipation from that ?

[martin manning]

For two resistors in parallel R = R1*R2/(R1+R2), which is a special case of R = 1/(1/R1 +1/R2 + ... + 1/Rn)

[hebaton]

For two resistors in parallel R = R1*R2/(R1+R2), which is a special case of R = 1/(1/R1 +1/R2 + ... + 1/Rn)

Thank you again. Loo up my last post, and, if you would care to suggest a reading on these things. Not wanting to go into to much detail but would like to know my way around this omewhat better

[martin manning]

I changed that resistor to a 22K prcision 1/2 watt I can now get 3.4v @ 329vplate voltage. HOw do I calculate dissipation from that ?

Wow, the plate voltage really came down! Are you sure that the 329 is correct?

[hebaton]

Misprint, more like like 339 when I wrote this.

But I let this warm up for a bit and now I am reading a stable 435v plate. Plate to CT kepeps jumping between 3.45 and 3.52v
Would this instablity have to do with the old caps ( not yet replaced ) or something else ?

The amp sounds nice, as far as I can tell with the cab abd speaker I have here at the moment. It does not sound Hyper clean like some Twinns do, but it could be the speaker I have on it that gives up some. I also expected it to be louder.
I can play it @ 5 in the house but as I said, an old Yamaha speaker I khow nothing about...

[martin manning]

Not likely to be the old caps. Not sure what might be causing that, but sometimes when you add your meter leads to the circuit you can cause an instability.

In any case,

Ia (anode) is (Va-VCT)/R = 3.5/39 = 0.090 A for two tubes, 0.045 for each one
The dissipation is now Va * Ia, 435 * 0.045 = 19.6 W
That is 19.6/30 = 65.3%.

You could leave it there or maybe dial it down a bit to ~60%.

[hebaton]

Again, so many thanks !
I would like recommendations for some reading and a very good meter .

Regards,

Claude

[martin manning]

I would like recommendations for some reading and a very good meter .

For reading material there is a sticky in the tech section of the forum. I would say Merlin Blencowe's Valve Wizard site https://www.valvewizard.co.uk is probably the most useful for amp related stuff. His books are very good and reasonably priced if you want to dig deeper. There are several basic electronic theory sites on the web, such as https://www.electronics-tutorials.ws and https://www.allaboutcircuits.com/

[pdf64]

Misprint, more like like 339 when I wrote this.

But I let this warm up for a bit and now I am reading a stable 435v plate. Plate to CT kepeps jumping between 3.45 and 3.52v
Would this instablity have to do with the old caps ( not yet replaced ) or something else ?
...

Keep the meter and probe leads well away from all preamp circuitry, ideally all signal circuits, eg put the meter over the power transformer side of the chassis.

But it’s still asking for trouble to probe output valve anodes. Yes I’m aware this is at odds with everything you may read on the net. 1ohm cathode current sensing resistors eliminate this nonsense.