# 5f11 bias resistor

Discussion in 'Shock Brother's DIY Amps' started by Linkjr, Aug 16, 2017.

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Age:
34
Sep 10, 2013
Georgia
Yes. If you go back up to the pic I posted, you can see a peach colored, tiny resistor coming off of pin 8 to the ground tab.

2. ### Commodore 64Friend of Leo'sAd Free Member

Age:
43
Mar 1, 2010
Kent, OH
Just to verify, what does fill CCW give you? Would like to know your span.

And I usually tack the 1-ohm directly to the chassis near the socket.

Age:
48
Aug 16, 2017
Uk
I have installed 1ohm resistor between pin 8 of power tube and ground and measured from 1 side of the resistor to the other and got 18.6 mv

however my 1ohm resistor is 5% tolerance

4. ### Commodore 64Friend of Leo'sAd Free Member

Age:
43
Mar 1, 2010
Kent, OH
383V*.0186A = 7.1W

You shouldn't be redplating even if those are 12W tubes.

Age:
48
Aug 16, 2017
Uk
I have measured 1 power tube with bias full ccw

pin3 422 v

pin 5 -45v

My 18.6 im not sure i am accurate with that figure please see my mm reading below for the mv across the 1 ohm resistor when placed between pin 8 and ground

Btw this is with the 68k resistor installed in place of the 100k which i took out perhaps i can go higher than that?

6. ### Commodore 64Friend of Leo'sAd Free Member

Age:
43
Mar 1, 2010
Kent, OH
The 1-ohm resistor means that your mV reading also reflects mA...which is why I moved a decimal 3 places for the following equation:

422V*.0186A= 7.8W you are still good.

Though I prefer to run my 6V6GT at 375V.

Ohms law V=IR (I is current or Amps) So setting the resistance to 1-ohm, means in the case of our cathode resistor V=I (hence your mV measurementbeing used to interpolate mA)

Age:
48
Aug 16, 2017
Uk
Thanks for the help jsnwhite and working that out Commodore - is that anywher near where i need to be for the magic 70% dissapation - have noticed that the amp is a lot quieter than before and tremolo has more span in the depth

8. ### Commodore 64Friend of Leo'sAd Free Member

Age:
43
Mar 1, 2010
Kent, OH
EH tubes are 14W and will be fine at 422V. That puts you at 55.7% bias. (7.8/14)*100= 55.7%

So now you can turn your bias pot CCW, thus reducing the bias voltage and increasing the cathode current. You can safely bring this up to 70% which is 0.7*14= 9.8W.

Note that as you increase cathode current, your Plate voltage will come down, so you will need to recalculate each time.

Edits:
(You can probably go to 10.3W or so since some of that current you are measuring is screen current).

(I don't expect that volume should be a LOT quieter at 55% bias vs 70%)

Age:
48
Aug 16, 2017
Uk
Thanks Commodore since bias pot is currently fully ccw when i took these measurements do you mean i turn cw And remeasure to get closer to correct bias?

10. ### Commodore 64Friend of Leo'sAd Free Member

Age:
43
Mar 1, 2010
Kent, OH
That depends. Does turning CW increase your readings? If it does, then yes. If it decreases your readings, we need to change your range resistor. I'm guessing you need to do that, and put in a 80k or 100k.

Age:
34
Sep 10, 2013
Georgia
Yeah, mine ended up with the 100k coming off the rectifier tap. 16-17k resistor on the 10k bias pot.
Here is where mine ended up at one point in the thread with a 315v-0-315v power transformer:
384v - plate
19.8mA @ #8
54.3% dissipation
-34 bias voltage

I ended up getting things closer to 70% later, and the plate voltage around 375v I think, but I can't remember the -bias voltage, but the tremolo still sounds great, but the speaker is a really efficient one, a Cannabis Rex, so I think that helps.

Check out Rob's Bias page with a calculator to help you get things set up right. https://robrobinette.com/Tube_Bias_Calculator.htm

Age:
34
Sep 10, 2013
Georgia

Age:
48
Aug 16, 2017
Uk
Thanks jsnwhite i think im aiming for that figure as well but run into a snag i fitted the higher value 82k resistor now and adjusted bias pot its almost fully cw but measurements i worked out give me only 375* .022 =8.2w
8.2/14*100=58.5%
bias pot fully ccw gives 407*0.12 =4.8w
im not sure i want to put another 100 in as i pulled out a 100k cos the tubes were red plating

I have 22k resistor on the bias pot and 82k resistor after it

Age:
48
Aug 16, 2017
Uk
Ok bias pot fully cw i can get 65% but the trem has really gotten weaker, im talking about from cleerdance clearwater to some cheap pedal trem and depth fully up on 12 it drops off to no trem

is 70% dissipation really what these should be run at?

Age:
34
Sep 10, 2013
Georgia
Be absolutely sure that the leads from the tremolo pots are going to the correct board terminals, multimeter can do it quick on continuity setting. I couldn't get mine right, really weak output, too fast -- I had the leads mixed up.

Age:
48
Aug 16, 2017
Uk
Weak output as in volume of amp? The trem isnt too fast either A professional amp maker built this amp so i dont think this could be the case if anything vol is too loud!

Edit : I checked continuity - fine as having read your thread jsnwhite my trem at 65% sounds similar to your trem before you got the wires sorted but wires are in correct position

Last edited: Aug 27, 2017

Age:
48
Aug 16, 2017
Uk
Thanks for all your help guys my original problem with red plating is sorted tubes are in the clear - now need to sort the tremolo out

Jsnwhite do you have a sound clip of your trem when your amp is at the 70% dissipation mine seems to sound like your final video clip of the tremolo in your thread when mine is close to 50% dissipation.

Age:
34
Sep 10, 2013
Georgia
If you saw the clip playing through the black tolex cabinet, I think I had it up to 68-70% by then. It looks like there was a couple weeks between posts, and I'm pretty sure I had it turned up by that clip.

May 5, 2015