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S1-50 Biasing

Posted: Mon Jan 18, 2016 11:09 am
by Seehorse
Hey there!

I am about to install 6L6 power tubes in my amp, instead of the EL34's stock ones - so i need to do a bias adjustment -
I heard somewhere that the power section should have approx 11,5 volts across, is this true? or am i completely off?

Re: S1-50 Biasing

Posted: Mon Jan 18, 2016 11:11 am
by Seehorse

Re: S1-50 Biasing

Posted: Tue Jan 19, 2016 2:05 am
by thephantum
No. Since the maximum plate dissipation of the 6L6 is different than that of a EL34 (30W vs. 25W), the bias setting will be completely different.

In addition, there is no such thing as a generic bias setting. In other words, you cannot just set bias to xyz. You need to measure plate voltage and calculate what the bias current should be based on the desired plate dissipation of the tube.

It also makes no difference what brand the tubes are. I can take two tubes that were manufactured right next to each other and they will each require a different bias current...which is why you need to measure, calculate and adjust. Then, since adjusting the bias will change the plate voltage, you need to do it over and over until the amp is dialed in.

Re: S1-50 Biasing

Posted: Fri Jan 22, 2016 12:44 am
by Seehorse
Thanks phantum!

I took the thing apart, the innards (PCB) standing upright, however i can't seem to find any measuring points, nor trimpots for the bias adjustment - nor does anyone on the interweb seem to know anything about these amps ...

anyone know the location of these?

Re: S1-50 Biasing

Posted: Fri Jan 22, 2016 1:40 am
by thephantum
I don't know the PCB of the Series One, but there are trim pots someplace. They will be small and you'll probably need a jewelers screwdriver to adjust them.

As far as "measuring points", most tube amps don't have them. You either use a probe or you install 1 ohm 1W resistors between the cathode and ground, then measure the drop across those to get your bias reading.

...either that or you shunt the OT and measure it that way...but you need extremely accurate meters ($$$) to do that.

Re: S1-50 Biasing

Posted: Fri Jan 22, 2016 1:53 am
by Seehorse
Just measured the point called "CON1" next to the power tubes, which reads 53mV (would this value be close to true on this amp?)

I also used an octal probe on the old EL34 power tube, which read 32mA, if memory serves me (which seems to be correct)

- i just figured there would be a trimpot for each power tube, so i could balance them myself ... guess there isn't - good thing i bought a matched pair-
but i seem to have located the bias-trimpot, called PR1, which sits in line with the DPR knob

Re: S1-50 Biasing

Posted: Fri Jan 22, 2016 8:10 pm
by thephantum
I can't tell you what bias would be for the amp without knowing what the actual plate voltage is. If I had to guess, the plate voltage is going to be somewhere in the 450V-490V range. Based on that theoretical value, the 32mA you measured using the probe is cold. It should be closer to the 35mA - 39mA range. That also means that what you measured at CON1 is definitely NOT bias. If you measured 32mA at one tube, that means overall bias is set somewhere in the 64mA range.

Keep in mind that for a 6L6 tube, you'll need to get the Bias much hotter: closer to the 45mA range, which is also going to drive your plate voltage up higher. So the question is, will the existing trimpot have the sweep to do dial it up that high.

Also, to be clear, most tube amps only have an overall bias adjustment for each push-pull pair (so for a 100W amp, which has two push-pull pairs, there are two bias pots). It's only recently that manufacturers are providing a balance adjustment in addition to the bias adjustment.

Re: S1-50 Biasing

Posted: Fri Mar 04, 2016 4:40 am
by aqa
Hope this threads help

Can you solve and change the tubes? I swapped a pair of KT77's wich is similar to EL34, but I didnĀ“t like it too much, I biased on the trimpots to 50mV same as EL34 and played some months withput any problem, then dropped the EL 34 again and rebiased.

I would like to know how the 6L6's work in that amp


just for the record, i have a tube bias probe and set the amp to 35ma +- each tube, sounds a little better but the dam thing was hot as hell, going back to the original 50mv at the "com1" port