strange biasing behaviour (beware, long post)

[jezzbo]

ok, I just put a 220K resistor in place of the 470R resistor.
But that seems like way to much.
Now I get these readings (without tubes at the moment)

Bias voltage from PT = 63V AC
After the new 220K resistor (instead of the 470R resistor) = 37V AC
So that's a voltagedrop of 26V AC right? (63V - 37V)

After the diode = only 4,2V DC.

Is there any way I can calculate the right resistor or is it trial and error?

Thanks again, really learning here.

Jesse

[Leo_Gnardo]

ok, I just put a 220K resistor in place of the 470R resistor.
But that seems like way to much.
Now I get these readings (without tubes at the moment)

Bias voltage from PT = 63V AC
After the new 220K resistor (instead of the 470R resistor) = 37V AC
So that's a voltagedrop of 26V AC right? (63V - 37V)

After the diode = only 4,2V DC.

Is there any way I can calculate the right resistor or is it trial and error?

Thanks again, really learning here.

Jesse

-4.2V bias would definitely put you in deep meltdown. Put the 470R back where it was, and put your 220K in parallel with the 27K on the back of the bias pot. That will bring the bias range closer to what it should be.

[JazzGuitarGimp]

Here is how I would approach it:
- Restore the 220K resistor to 470R.
- Replace the 27K resistor with a 25K pot - use the wiper and one of the outside legs of the pot. Start with it at 25K
- Center the 10K pot.
- Use the 25K pot to adjust the bias where you want it (keeping the 10K pot centered).
- Remove the 25K pot, being careful not to disturb its setting, and measure its value.
- Replace the 25K pot with a fixed resistor of the nearest value to the measured value.

[Leo_Gnardo]

by the book these conditions yield a no-signal plate current of 116 mA. That's a plate dissipation of 52W in a tube that's rated for what, 25?

These values are for two tubes in P-P. This should be printed in block bold letters.
RCA biased them hot because they never imagined anyone sending heavily distorted power cords into these tubes. GC was supposed to be 30W plate dissipation tube.
One has to read the small print even ion the tube data sheets, we don't like it. /uttered with a fake nasal stiff upper lip Queens English/

VV, thanks, I thought 116W was way beyond belief. But 58 mA's still sky high. Maybe - in a class A hi fi amp, or a self-bias. Bring the marshmallows - roasty toasty.

The large print giveth and the small print taketh away. eth.

And those black plate RCA 6L6GC sure do sound extra good. It's not just hype.

[jezzbo]

Here is how I would approach it:
- Restore the 220K resistor to 470R.
- Replace the 27K resistor with a 25K pot - use the wiper and one of the outside legs of the pot. Start with it at 25K
- Center the 10K pot.
- Use the 25K pot to adjust the bias where you want it (keeping the 10K pot centered).
- Remove the 25K pot, being careful not to disturb its setting, and measure its value.
- Replace the 25K pot with a fixed resistor of the nearest value to the measured value.

great idea! I'm gonna try this tomorrow (I don't have a 25K at the moment)

All of you, thank you for helping.

[Leo_Gnardo]

Here is how I would approach it:
- Restore the 220K resistor to 470R.
- Replace the 27K resistor with a 25K pot - use the wiper and one of the outside legs of the pot. Start with it at 25K
- Center the 10K pot.
- Use the 25K pot to adjust the bias where you want it (keeping the 10K pot centered).
- Remove the 25K pot, being careful not to disturb its setting, and measure its value.
- Replace the 25K pot with a fixed resistor of the nearest value to the measured value.

great idea! I'm gonna try this tomorrow (I don't have a 25K at the moment)

All of you, thank you for helping.

If you haven't run out to the store yet for a 25k pot, first put that 470R back where it came from then once again,

You could parallel another resistor with the 27K, try 220K down to 100K. Dial the bias adust pot to the maximum voltage when starting each test session and adjust down until you get a sensible bias current. The "Twin-size" amps can run with as little as 25 milliamps per output tube, and you can increase that up to @ 40 mA if your ears detect an improvement. FWIW I usually set 'em at 30 mA.

- - - - -

I like JGG's idea too but if I can save you a trip & a couple euros, all to the better.

[pula58]

My experience is that the 10K blackface style bias pot with a 27K resistor is not right for todays wall-voltages and tubes. Try a 15K resistor with the 10K pot.

[Richie]

maybe I missed it, but also measure and be sure that the 1 ohm resistor you use, is 1 ohm. many times these go way off, and then you will get different readings. And also if it was off value, it could depending on what your using,bias socket gizmo.

[jezzbo]

Ok, the bias is working nice now. I've put a 47K resistor parallel with the 27K resistor. (total resistans of 17,14K)
Next I will try and replace it with a one 15K resistor.

Anyway, I fired up the amp and channel 1 is working nice.
It did have a groundloop, but that's already solved.

Channel 2 on the other hand does not work.
When I turn the tonecontrols I can hear differents in the hiss.
But no other sound is comming out.

here is a link to some pix and a layout drawing.

Any idea what I'm doing wrong here?

http://users.telenet.be/jezzbro/veine_amp/preamp/


Thanks!

[jezzbo]

the cathode resistor on the left side is 1K5, normally there is a 820R resistor but then there are two cathodes connected together. So is 1K5 right?

Also the last coupling cap (going to the PI) is a .1uF. (got that from a showman amp schematic)
On a Twin reverb I see that this is a .047uF cap.
Would that be a problem?

Thanks again

[martin manning]

If two cathodes were connected to an 820-ohm and you are now connecting one, the value should be doubled. I think that is what you said.

The PI coupling cap will work at 0.1 or 0.047. The smaller one will just start the bass roll-off one octave higher.

In your layout the volume pot feeding the second stage is not grounded. That is the ground reference for the second triode's grid, so you need that.

[jezzbo]

Indeed I made an error in my layout. Forgot the ground wire comming from the volume pot. But in the amp I did connect that wire to ground.

Now I'm going to resolder the tubesocket. Maybe there is a bad connection.
I don't know, there are no cold solderjoints.

thanks!

[jezzbo]

Ok, I found the problem.
The red wire that goes from the first 100K plate resistor to the second pin of the socket got a short circuit with the mounting screw of that socket!
(the screw punched through the isolation of that wire)
Strange is that the amp worked just fine. (except channel two that is).
anyway, I conneced a new wire and now everything is working perfect and the amp sounds really good (and very loud!)

Thank you people for helping me.

[Structo]

Cool!

Rock on!