Bias Range For ‘60’s Plexi

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braintheory

Well-known member
Hey guys, so I’ve been going deep in the tube rabbit hole lately since the summer, exploring each type as thoroughly as I can and finally getting into trying lots of different vintage EL34’s and 6L6’s. So I’ve got some EL34’s and 6CA7’s coming in that I wanna put in my ‘67 Marshall Tremolo 50 watt and wanted to ask what range is good/safe for it to be biased in that amp? Please bare with me, I’m still a newb with biasing. That’s why I saved these bigger bottle power tubes for last in my tube journey
 
33-36 seems within the average range. But hotter is not always better. I’ve got a 50 watt EL-34 head that starts to sound a bit mushy at 36/37 mA and sounds much better - firmer/tighter at around 32 mA.
 
Depends on the Z of the tubes and your plate voltage not just some arbitrary number.
 
Use your ears, then check to make sure the bias is in the safe operating range.

Also, you might not want to bias at 70% if you are cranking your amp into an attenuator.
 
ZEN Amps said:
You need to know the plate voltage, and then aim for 60-70% dissipation. Here's a handy calculator:

Tube Bias Calculator

As a guide 65% dissipation could range from 32 to 40mA in your amp, and that's why plate voltage really matters.
Click to expand...

This is good info, but isn't screen voltage equally important to know whether the tubes you plan on using can withstand the screen voltage your particular amp is putting out? I recently grabbed a NOS quad of black base Tesla E34L with the intention of using them in my MCII and I was told they would handle 450'ish across the grid screens, but the MCII was putting out 485. At low to moderate volumes it would have likely been fine, but cranked up a bit may have been pushing the limits? I wound up returning them because of issues with the pins not fitting in the tube sockets of the Wizard, so will never know how that would have worked out.
 
MetalHeadMike said:
This is good info, but isn't screen voltage equally important to know whether the tubes you plan on using can withstand the screen voltage your particular amp is putting out? I recently grabbed a NOS quad of black base Tesla E34L with the intention of using them in my MCII and I was told they would handle 450'ish across the grid screens, but the MCII was putting out 485. At low to moderate volumes it would have likely been fine, but cranked up a bit may have been pushing the limits? I wound up returning them because of issues with the pins not fitting in the tube sockets of the Wizard, so will never know how that would have worked out.
Click to expand...
If I remember correctly ,Mesa has/had a stash of those and made it a point NOT to use with high Plate voltage. So I'm glad you got to return those.
I passed on those as well and bought a few Wing C quads off of them,
 
psychodave said:
Use your ears, then check to make sure the bias is in the safe operating range.

Also, you might not want to bias at 70% if you are cranking your amp into an attenuator.
Click to expand...

True this.

Also, don't play your guitar through a bias meter - unless it is designed to let you do that.

@braintheory - I have a very detailed biasing tutorial with a lot of safety tips in it if you want it.
 
Teslas have worked amazing for me in Bogner 100B amps with 520 Volts on the plates for the last 30 years without one failure.

The EL34 tube that always got the ‘don’t use above 450 pv’ is the RFT Siemens 34’s.
Used them also for many years at 485-500 in Wizards and Steavens also without one failure👍
Both of my original Bogner 100B amplifiers came brand new with Siemens EL34 tubes. Both of them around 520 V on the plates.

I have used these Tesla tubes in old plexi Amplifiers with close to 540 Plate voltage, again, without any issue at all…

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Cathode current which bias meters measure is always screen grid current plus plate current at idle, so measuring the cathode current of a fixed bias amp like a Marshall will always error on the colder side because the true plate current subtracts the screen grid current from the cathode current, which is usually in the ballpark of 4-5mA or so with 1K screen grids.

I don’t follow 70%. I bias mine just out of crossover around 55% to 60%. I like my power amps to hit hard and I prefer not to eat tubes like candy.
 
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