Blues Jr. Cathode Bias Mod question

[sgelectric]

I'm checking out a Blues Jr. for a friend.At one time the amp was converted to cathode bias and the NFB loop was removed.
The B+ is around 315VDC. The cathode resistor measures at 47 ohms while the cathode voltage at idle is about 6.4 VDC. The value of the cathode cap is 100uf/35v.
Does this value and voltage seem about right?Most other EL84 PP amps I've seen seem to use a higher value resistor. There is no big problem but I'm just curious and don't have a lot of experience with this type of mod.Any suggestions/opinions would be appreciated,TIA Steven

[RWood]

Plugging those numbers into the Weber Bias Calc shows the tubes to be dissipating 20 watts apiece......they should be redplating!

Those numbers don't sound right, though. Lowest Rk I've seen in a 2x6BQ5 circuit is 135Ω but most are in the 150 - 180Ω range depending on Va.
Make sure the amp is off and drained when you measure the cathode resistor. The with amp warmed up and at idle, cathode voltage is measured with respect to ground, and anode voltage is measured with respect to the cathode.

RWood

[Enzo]

Whatever it seems, if the tubes are not red plating, it is not running through tubes all the time, and it sounds good, then whatever was done was probably OK.

[jmaf]

There is no big problem but I'm just curious and don't have a lot of experience with this type of mod.

If your voltages are correct, the el84's are dropping 308.6 volts and there's 136 mA of current through that resistor. There is a big problem.

Did whoever mod it install grid leak resistors? There is no use in getting the cathode higher potential if the grids aren't clamped below.

[jmaf]

Whatever it seems, if the tubes are not red plating, it is not running through tubes all the time, and it sounds good, then whatever was done was probably OK.

I assumed he meant 42 watts quiescent. But if they're not red plating at that level, then there's probably something weird with the data provided ?

[sgelectric]

I checked the voltages again.There was some discrepancy. They are as follows:

B+ is 304 VDC measured to Ground
Cathode voltage is 6.4 VDC
Screens(g2) are at 275 VDC
Grids are at O VDC. The point where the bias voltage was disconnected has been grounded.

I checked the marking on the cathode resistor.It says 50 0hms 10w although it measures 47 ohms.Cathode cap is 100uf/35v
The amp is running V1(5751) V2(12ay7) V3(12ay7) V4+5(6bq5 s)

Thanks to all for your input. Any further comments/observations appreciated. Steven

[jmaf]

Increase cathode resistor to 100 ohms and you're fixed.

[sgelectric]

It turns out the amp *was* starting to redplate at idle.Although hard to see during the day, it was readily apparent last night with all the lights out.So I'll increase the resistor value.....
Thanks to all for your help. Steven

[jmaf]

It turns out the amp *was* starting to redplate at idle.Although hard to see during the day, it was readily apparent last night with all the lights out.So I'll increase the resistor value.....
Thanks to all for your help. Steven

If the amp has been played a lot like that, you've probably got two severely spent tubes. If that's the case, I'd consider getting a new pair of el84's to install when you apply the fix .

[jrfrond]

I'd run those power tubes on the cool side. From experience, I can tell you that the Blues Jr. power transformers pretty much operate balls-to-the-wall and WILL cook eventually if used for long periods of time even with a moderate increase in idle current.

[sgelectric]

When the friend gave me the amp to check out he commented that after playing for a while it would seem to lose power and dynamics.In hindsight,this all seems to square with improper bias,the resultant depleted output tubes and an overly-stressed power transformer.
jrfrond: What would you suggest as appropriate value cathode resistor?

Thanks again for all the great input. Steven