80% Idle Bias on Vintage HiWatt Amp.... Too Risky?

[whalecalls]

Howdy fellow tube lickers.
      I recently acquired and successfully recapped and restored a nearly all original '69 Sound City L100 including its original Mullard tube set.
     Surprisingly,  two of the EL34s were actually pretty strong and perfectly matched according to my bias probe. However the other two were a little weak. Anyways....
     I put in a fresh quad of matched Electro Harmonix 6ca7s and they're all pushing around 40 to 41 ma with 488vdc on the plates when referenced to chassis ground.

I know the answer, but is it absolutely mad to pushing these tubes near 80% dissipation at idle? I get decent range out of the two bias trim pots, but they just don't swing low enough to bias the tubes down into the 60-70% range. What would be the best way to lower the bias in this amp? To change the trim pots or would it be to to change those 100ks coming off the pots which would be an easier task.

Any insights about the bias questions or any other insights about Vintage HiWatts and/or pre-active tone stack Sound Citys welcomed. Cheers y'all

[2deaf]

What would be the best way to lower the bias in this amp? To change the trim pots or would it be to to change those 100ks coming off the pots which would be an easier task.

The 100K's are the grid leaks and they don't play any part in the bias voltage. 

You could try raising the value of the 10K resistor going to ground.  Try 20K to 47K and see what happens.  The bias with that winding probably maxes out at -42V or so.  If that doesn't cool the tubes enough, you'll have to take more drastic measures like a voltage doubler.   

[mresistor]

Curious - what is your incoming ACv reading? And what ACv have you selected?

[PRR]

> 41 ma with 488vdc on the plates when referenced to chassis ground.

41mA plate or cathode?

If 41mA cathode, EL34/etc will pass 15%-20% to screen not plate. So 35mA Plate times 488V is 17W which is 68% of 25W rating.

[whalecalls]

2Deaf.
     Whoops! My mistake I will try and experiment with that 10k ground resistor.

MResistor
     This is part of the problem. I have 230vac wall voltage incoming (currently in Germany) and the voltage selector plug on the back is at 225. With the plug selected to 225 the heaters are about 6.6vac. With 245 plug selected heaters are at 5.7vac if I recall correctly. I feeling better running these tubes with the heaters at 6.6 to avoid cathode stripping.

PRR
    I am using a standard cheap bias probe in the link below that uses a precision 1ohm resistor to measure quiescent. Maybe you're onto something here as two of the original Mullard EL34 were pulling around 40ma, another 35ma and another pulling a weak 26ma. Both of the weaker tubes were paired if that means anything....

https://www.tube-town.net/ttstore/bias-probe-octal-assembled.html?language=en

Thank you all for the responses and the help.

[Latole]

"I know the answer, but is it absolutely mad to pushing these tubes near 80% dissipation at idle? "
-whalecalls

Don't do it ! You'll destroy tubes.

I won't go more than 60% power dissipation. And less if sound is good

[Latole]

 he 100K's are the grid leaks and they don't play any part in the bias voltage. 

You could try raising the value of the 10K resistor going to ground.  Try 20K to 47K and see what happens.  The bias with that winding probably maxes out at -42V or so.  If that doesn't cool the tubes enough, you'll have to take more drastic measures like a voltage doubler.

100%  right

wahlecalls you can feed bias circuit from main transformer output winding.
I do that few time on different amps.
It is this type of well done job that makes the difference between the amateur technician and the pro

[Latole]

You want more  ?  https://robrobinette.com/Generic_Tube_Amp_Mods.htm#High_Voltage_Tap_Adjustable_Bias


Scroll down to ;   High Voltage Tap Adjustable Bias for Center Tap PT

[mresistor]

Whalecalls - this is just FYI  (from Merlin Blencowe)
"You may have heard of 'cathode stripping', which is a specious argument wheeled out by standby-switch obsessives. In its purest form, cathode stripping occurs when particles of the oxide coating are physically torn from the surface of the cathode when it is exposed to a powerful electrostatic field from the anode. This would happen if the valve is operated at saturation, without a usual space-charge of electrons to protect it. Fortunately, this effect does not exist in receiving valves, even when operated at saturation, because it requires an electric field strength of at least 4MV/m (yes, 4 million volts per metre!). No guitar amp ever comes close to this.Another type of cathode stripping occurs when stray gas molecules in the valve become ionised by the electron stream. The positive ions will then be accelerated towards the more negative grid and cathode. If these manage to miss the grid then they may crash into the cathode, physically damaging its surface. The proper name for this process is cathode sputtering. Sputtering is a known problem in gas tubes and transmitting valves operating at kilovolt levels, near saturation. It doesn't occur to any significant degree in ordinary audio circuits. Note that even the RCA Transmitting Tubes Technical Manual No. 4, p65, states: “Voltage should not be applied to the plates or anodes of vacuum, mercury-vapor, or inert-gas rectifier tubes (except receiving types) until the filaments or cathodes have reached normal operating temperature” [My emphasis].Receiving valves are the small kind used in radio receivers, i.e audio valves like those in guitar amps, in case you were wondering."