‘81 JCM 800 build help

[etneccas]

My secondary voltage is 445volts and the lowest the bias is 45ma with the pot at the lowest setting
The amp sounds amazing but noisy. When the volume pots are at near or 7 the high pitched squealing starts
The cabinet is unshielded too……
What is the best way to get the Voltage down and the bias range with in spec?

[pdf64]

What schematic have you used? Provide a link to it.
What output valve type have you used?
What mains voltage do you measure feeding the amp?
What heater voltage do you measure at the PT lead terminals?
What voltage do you measure at the PT’s HT winding terminals?

[etneccas]

1.) Stew-Mac ‘81 BRIT-800 50W I tried to upload a pic of the schematic but the file is to big……2.) 2-EL34’s3.) 122.4vac4.) 6.35vac5.) 724 vac with the standby switch open. 694vac with standby switch closed

[sluckey]

Replace that 220K resistor (between terminal 2 and 26) with a 180K or 150K to decrease the bias current.

[pdf64]

Please detail how you are measuring 45mA.
Some photos of the build may be beneficial.
Upload to a hosting site eg imgbb, and provide links.
https://www.stewmac.com/globalassets/video-and-ideas/online-resources/building-instruments/81-brit-800-50w-amp-kit-instructions/download-81-brit-800-50w-amp-kit------------------------instructions

[PRR]

...pots are at near or 7 the high pitched squealing starts   The cabinet is unshielded too……

These go together. Make neat logical wiring and close the metal chassis.

[zircontweezer]

Just a hunch (as I built one way back and had high frequency oscillation) maybe get a chopstick and move around the signal wires to the phase inverter.

[etneccas]

Replace that 220K resistor (between terminal 2 and 26) with a 180K or 150K to decrease the bias current.

N.G!  I replaced the 220K resistor with a 150K and the bias voltage went up at terminal 20 to 61v

[etneccas]

Please detail how you are measuring 45mA.

https://www.stewmac.com/globalassets/video-and-ideas/online-resources/building-instruments/81-brit-800-50w-amp-kit-instructions/download-81-brit-800-50w-amp-kit------------------------instructions

I measure terminal 20 to ground and was getting -45mV before I replaced the 220K with a 150K as Sluckey suggested and now I’m getting -68mV

[JPK]

Never mind, instructions already posted. Attached schematic jpeg.

Don't you want to increase that 68k resistor to get more negative voltage? I had to do that on a negative bias amp I built years ago. Was biased too hot and could not reduce it any more with the pot.

[etneccas]

Never mind, instructions already posted. Attached schematic jpeg.

“Don't you want to increase that 68k resistor to get more negative voltage?”


According to the manual I should get -38V at terminal 20

[sluckey]

Replace that 220K resistor (between terminal 2 and 26) with a 180K or 150K to decrease the bias current.

N.G!  I replaced the 220K resistor with a 150K and the bias voltage went up at terminal 20 to 61v

You said in your original post, "...the lowest the bias is 45ma with the pot at the lowest setting...", That was a red herring! You really meant that the lowest you could get the BIAS VOLTAGE was -45V as measured at turret 20. Big difference between bias current and bias voltage.

The info I gave you was correct for lowering the bias current, but now that I know what you really meant, I'll change that advice.

Replace that 220K resistor (between turrets 2 and 26) with a 240K or 270K to DECREASE the BIAS VOLTAGE as measured at turret #20. You should now be able to get the recommended -38V at turret #20 for your starting point.

[etneccas]

Replace that 220K resistor (between turrets 2 and 26) with a 240K or 270K to DECREASE the BIAS VOLTAGE as measured at turret #20. You should now be able to get the recommended -38V at turret #20 for your starting point.
[/color]
[/color]Sorry for the confusion….I don’t have any 240K or 270K metal oxides handy
[/color]I may be able to put something in series for now….
[/color]Will changing the 68K on the Bias Pot to a higher value accomplish the same thing?

[sluckey]

Will changing the 68K on the Bias Pot to a higher value accomplish the same thing?

No. You need a smaller resistor to lower the voltage at turret 20. Try 62K, 56K, 51K, 47K. I suggest removing the output tubes until you are happy with the voltage at turret 20. I would set the bias pot to midrange and keep lowering that 68K until you have approx -38v.

[JPK]

Will changing the 68K on the Bias Pot to a higher value accomplish the same thing?

No. You need a smaller resistor to lower the voltage at turret 20. Try 62K, 56K, 51K, 47K. I suggest removing the output tubes until you are happy with the voltage at turret 20. I would set the bias pot to midrange and keep lowering that 68K until you have approx -38v.

Agree. My advice was also backwards due to the red herring. Thanks for straightening this out Slucky.