Biasing Question RE: Vp & Ik relationship

[iggs]

Hi all,

don't think this was answered before, if it was, apologies for missing it while searching and/or noob-level question.

Just got the Bias Rite and decided to check out a new pair of SED EL34.

1) The Plate Voltage reads around 410V which seems kinda low since I was expecting around 475V as per builder's specs ... but, I assume the voltage is affected by the voltage in my house which I have no idea what it is or if it's even close to 120V. Is this correct?

2) If I still bias the amp to around 70% plate dissipation at 410V, which ends up being around 42mA is it correct to assume that if the plate voltage goes up to 475V (due to changes in line voltage) the amp will still operate within the 70% plate dissipation range?

3) OR ... will the increase in voltage increase the cathode current as well?

Thanks!

[Old Tele man]

1) maybe yes, maybe no. Plate voltage goes DOWN when plate current goes UP...could be just idling "hot."

2) Ipq = 0.7*25W/410V = 0.043A or 43mA...if plate voltage actually DOES go up to 475V but bias voltage stays approximately the same: Ppd = Vpq*Ipq = 475V*43mA = 20.4W which isn't too bad.

3) Since plate current is a function of SCREEN voltage far more than plate voltage, the cathode current would most likely only go up by a fraction (<10%?) of any changes in plate voltage.

...why? because the TOTAL "operating" voltage of a tube is the sum of: (a) control grid voltage (Vg); (b) screen grid voltage (Vs) divided by TRIODE amplification factor (u1); and (c) plate voltage (Vp) divided by PLATE amplificaiton factor (u2), like this:

Ip = K*(Vg + Vs/u1 + Vp/U2 )^(3/2)

...if Vg is -40V and Vs is 450/10.5 and Vp is 475/165, look at their respective "operating" proportions:

X = (-40Vg + 450/10.5 + 475/165 ) = (-40 + 42.9 + 2.9)

...notice how the "effective" SCREEN grid voltage (+42.9V) is many, many times greater than the "effective" plate voltage (+2.9V)

MORAL-of-this-STORY: worry about changes in SCREEN VOLTAGE more than changes in PLATE VOLTAGE, except when making power calculations and/or operating Class-A.

[electroid]

And, screen and plate voltage will increase nearly proportionately to AC line voltage. To answer your question more simply, in most amps, if line voltage goes up, plate voltage, screen voltage, heater voltage and plate current will increase. If you bias to 70%, and increase your line voltage, your plate current will most likely go over 70%. I would suggest adjusting your bias when the line is 120VAC.

[hasserl]

But as all those other voltages go up so will the bias voltage, compensating for the increase in plate voltage by shifting the bias point.

[Old Tele man]

But as all those other voltages go up so will the bias voltage, compensating for the increase in plate voltage by shifting the bias point.
+1, since power transformer output voltages are proportional to their primary-side input voltage, a change from 110VAC up to 120VAC will raise ALL secondary voltages by about 9%:

-70Vpk bias voltage will go down/up to -76Vpk
410Vdc plate voltage will go up to 447Vdc
475Vdc plate voltage will go up to 518Vdc

...about the ONLY time the PT secondary voltages won't follow the input primary voltage is when/if ZENER-diodes are being used to control voltages.

[iggs]

Thanks for the info!

I checked the plate voltage and cathode current in my Route 66 and it was around 450V (448V to be exact). That is to be expected in this amp so I have to assume that the reading in the other amp is correct as well and that it is wired for 410V on the plate and my expectation of 475V on the plate was not correct.
The amp sounds great regardless and I biased it to around 40mA for the hotter tube (mismatched by about 2mA).