What should the bias be for the 5E3?

[Ringwraith]

Just wondering what a good bias range is for my 5E3?
I used the center tap method listed in Dan Torres book to check MA on each 6V6. Going through a few different sets of tubes, here's what I got...

EH 6V6
Plates 347VDC
40.2ma & 33.3ma

JJ 6V6
Plates 345VDC
41.8ma & 40.8ma

Old Ratheon 6V6
Plates 348VDC
38.3ma & 39.2

Old RCA blackplate 6V6
Plates 388VDC
24.5ma & 28ma

Isn't 38-40 too high for 6V6's??
Also looking at the RCA, does this mean they're worn out?

Thanks
Sean

[tubeswell]

With cathode-biased amps, it depends on your plate-to-cathode voltage (as well as your plate current).

If you have a lower plate-to-cathode voltage, you can have more plate current before a 6V6GT exceeds its recommended (12W) dissipation.

With around 340-360V at the plate and 18-20V at the cathode (through a 250R resistor), somewhere around about 36-38mA plate current is about right for Class A operation. JJ 6V6s will handle slightly higher dissipation (~14W) better than RCA 6V6GT

[Ringwraith]

How do I measure plate-to-cathode voltage?
The voltage on the cathode is 19V measured to ground.
This was measured with the Ratheons in place.

Sean

[tubeswell]

348-19= 329

329 x 0.0392A = 12.89W = about right

[Five_E35]

FWIW, the RCA and Sylvania tube data sheets list maximum dissipation for 6V6GT/GTA's at 14 watts. The fairly common JAN Phillips 6V6GT is the same tube as a blackplate Sylvania, made after Phillips ECG took over the plant.

I'm not sure what the JJ 6V6 actually is or what it can take, but some people are running them in 40 watt Fender Hotrods. I think it's really some 5881 variant that draws filament current like a 6V6. Many also say it sounds more like a 5881. Current Victoria 20112's now come with them and it's the basis for their 20 watt claim when used with a GZ34 rectifier, 14 watts with a 5Y3.

The new EH tube is supposed to be an improved version of the old Sovtek, which had a bad reputation of blowing up in Deluxe Reverbs at ~400+ volts. Screen failure was the most common failure mode, which took out a lot of screen resistors in the RI's. That shouldn't be a problem in a 5E3, which runs the screens quite a bit lower.

12 watts is a safer number to use with most other 6V6's, keeping in mind there may be some leeway.

[barfoden]

This is some nice info...

Goes to see that not all 6v6 behave the same with regards to bias voltage (bias resistor value).

My experience with EL84 in a fixed biased amp is, that some brands measure very different to others. Chineese rubys would need a lot more bias voltage than a pair of jj EL84 in the same amp. WIth the amp I could adjust teh bias so no problem,,

With a cathode biased amp like the 5e3, the 250 ohm resistor might fit some brands of 6v6 tubes where other would require a 330 ohm resistor to be biased correctly.. I have seen a neat adjustable cathode biased system that used a 350 ohm resistor together with a 100 ohm 5W pot. With the pot the resistance could be adjusted from 350 to 450 ohm.

The difference between tubes are also why tube vendors grade their tubes. A german supplier would write a number on the supplied EL84 i bought there which would tell me if the tubes wants to draw a lot of current (needs more bias voltage) or it was a colder tube that doed not draw so much current and would need a slightly lower bias voltage or a smaller bias resistor.

I the case seen here,, some tubes would be biased at ~ 13 watt and another tube would be biased at 9-10 watt. I multiplied with the plate voltage minus ~ 21V which is think is reasonable to assume with the 250 ohm bias reisistor.