Biasing 5e3 with Bias-Rite

[flatfive]

I've been trying to bias my first build, a 5e3 from parts. I'm getting 330v and 37ma (subtracting 5% for screen current) with the Bias_Rite. As I understand it, I also have to get a reading across the 250k resistor and subtract that number from the 330v, then multiply that by .037 to get my dissipation. Is this correct?

[Chuck H]

All tech aside (bias-rite? never heard of it) With 330Vp and a cathode bias I would say bias 'em hot. Around 85% diss for 6V6s without bothering to subtract the screen current. Measure the voltage across the cathode R and subtract it from the plate voltage. Then divide that figure by the cathode R resistance. Omit any decimal. The number you have left will be your current. Divide by two (you are using two tubes) and this is your idle current per tube.

Chuck

[MWJB]

That cathode resistor is 250ohms, not 250K.

Typically, your plate voltage (read to ground) on a 5E3 with a stock 250ohm cathode resistor needs to be well in excess of 380vdc before you should be concerned about rebiasing.

If your plate voltage was any less than it is, I'd try a 200ohm cathode resistor. You don't want less than 30mA/tube.

[flatfive]

That would be pin 3 to ground on the plate voltage?

[MWJB]

Yes, that's it.

[flatfive]

Thanks.

[flatfive]

I decided to measure the plate voltage from pin 3 to ground. I got a higher reading of 345v. The Bias-Rite gave me 330. I'm wondering now which is the more accurate reading. I notice that some people measure from pin 3 to pin 8.

With the 345v reading, my dissipation is over 12 watts. With the 330, it around 11. That would be better, no?

[keysersoze808]

..

hi..
http://tubedata.tigahost.com/tubedat.../127/6/6V6.pdf

based on datasheet max rating for plate is 315v and S.grid is 285v.. (plate current 70mA) im not sure if exceed this limit will shorten tube life.. dont have any exp. yet.. im just finish 5e3 with ez81 on rectifier.. getiing 340 on the first filter caps.. so i have to reduce the voltage before supply into Ot center tap.. afraid of damaging the tubes..im getting about 69.2mA at chatode..(chatode resistor 270R)

[flatfive]

Electro_Harmonix says no more than 450v on it's spec sheet, I believe.

[keysersoze808]

Electro_Harmonix says no more than 450v on it's spec sheet, I believe.

is there a diffrent between 6v6s and 6v6gt? im kinda confius.. most of other brand~tungsol, philips, JJ, indicates 285v max(pushpull) ..but EH 6v6s goes till 475v?

[MWJB]

There are precious few production amps that run a 6V6/6V6GT at 315vdc. The data sheets are somewhat over cautious. Most NOS 6V6s will take 425vdc on the plates & screens, sometimes a little more depending on brand, but 425v could be regarded as the "safe" limit.

Manufacturers of current production 6V6s no doubt realised that a 425v 6V6 was going to be somewhat limiting as many tolex Deluxes & Princetons run more than this, hence the higher voltage handling of new 6V6 type tubes.

[keysersoze808]

There are precious few production amps that run a 6V6/6V6GT at 315vdc. The data sheets are somewhat over cautious. Most NOS 6V6s will take 425vdc on the plates & screens, sometimes a little more depending on brand, but 425v could be regarded as the "safe" limit.

Manufacturers of current production 6V6s no doubt realised that a 425v 6V6 was going to be somewhat limiting as many tolex Deluxes & Princetons run more than this, hence the higher voltage handling of new 6V6 type tubes.

thanks MWJB.. but those deluxe & princeton runs on how much current?

[flatfive]

My SFPR runs at 23mA and 390vdc.

[MWJB]

"thanks MWJB.. but those deluxe & princeton runs on how much current?" Depends on the brand of tube and where you set the bias pot on the Deluxe. Usually if you bias to 30mA per tube plate voltage wil drop to 425v. This is biasing well over the accepted "70% rule" but good quality NOS tubes can take it. With tubes like the EH & Sovteks (Reflector) plate voltages will be higher than with hot biased NOS, Miles Rose suggested 19mA per tube for the GT6V6R, which means higher plate voltages, usually 440-450v.

Flatfive - does your SF Princeton have good filter caps & the original PT & rectifier? Voltage seems unusually low.

"I decided to measure the plate voltage from pin 3 to ground. I got a higher reading of 345v. The Bias-Rite gave me 330. I'm wondering now which is the more accurate reading. I notice that some people measure from pin 3 to pin 8. With the 345v reading, my dissipation is over 12 watts. With the 330, it around 11. That would be better, no?" Doesn't matter, neither 330 or 345v with a 250ohm cathode resistor is going to prematurely wear your tubes or damage your amp. I'd only consider going up to 330ohms if your plate current was over 40mA per tube.

[flatfive]

"Flatfive - does your SF Princeton have good filter caps & the original PT & rectifier? Voltage seems unusually low."

No, it's got new caps, original trannies and compares with other people on the FDP forum.

Thanks again for your help.