Hello and old Blues Jr.

[ArturoQ]

Hello Helmholz,
I do not know. I know the red plating followed the tube. Makes sense to find out . Will test it tomorrow. Thanks for the heads up!

[ArturoQ]

Hello Helmholtz,
The high plate dissipation followed the tubes. The higher dissipation is the old GT (believe Sovtek) and the lower one is the one from the pair of matched Sovtek el84s. I am going back to look for TP30 and get the voltage. The schematic shows it as 10.7 Vdc

[ArturoQ]

If I figured it out, I tested both sides of R37. TP30 was coming from one side of it. The top of the resistor to ground was -26.19 Vdc and the bottom of that resistor to ground was -10.65. I believe the bottom of the resistor is at the voltage divider for "C-"/
This to me seems to indicate that the GT tube is drawing too much amperage and is out of spec or even bad??

[Helmholtz]

If I figured it out, I tested both sides of R37. TP30 was coming from one side of it. The top of the resistor to ground was -26.19 Vdc and the bottom of that resistor to ground was -10.65. I believe the bottom of the resistor is at the voltage divider for "C-"/

Measured voltages almost perfectly match schematic values.
TP30 voltage is the grid bias.
Grid voltage (together with screen voltage) determines plate current. But there's considerable variation between individual tubes. That's why power tubes should be matched.
What matters in the end is actual plate current and balance.
A grid voltage around -11V is likely to cause excessive plate current and dissipation.
Go for a TP30 voltage around -15V and plate currents of 25mA or a bit lower.

This to me seems to indicate that the GT tube is drawing too much amperage and is out of spec or even bad?

No reason to suspect the GT tube. It might even be closer to typical spec than the other one.
It just needs a suitable "partner".

[Leo_Gnardo]

FWIW I find grid voltage typically around -12.0 to -12.5V when Blues Jr output tubes are in the happy bias zone 26-29 mA. YMMV.

[Helmholtz]

FWIW I find grid voltage typically around -12.0 to -12.5V when Blues Jr output tubes are in the happy bias zone 26-29 mA. YMMV.

Measured at the grids or at at TP 30?

...output tubes are in the happy bias zone 26-29 mA

Plate current or cathode current?

[Leo_Gnardo]

Measured at the grids or at at TP 30?



Plate current or cathode current?

Voltage measured at grids, current measured at plate using voltage between OT center tap and plate, divided by resistance of OT windings.

Other people may have other results, depending on qualities of output tubes. "YMMV" Your Mileage May Vary

Worked on many probably over 100 Blues Jr and Pro Jr by now, I watch output waveform driving R load and find crossover notch at lower bias current, notch goes away somewhere in 26-29 mA range, and plate power calculation often lands at 10W more or less. Some customers run their amps for years without complaint. Depending on hours used and how loud the amp is played as expected tubes will need replacing. BUT they last way longer than at Fender's typical 40 mA or so setting.

Yes I'm aware speaker is a dynamic load and its impedance varies with frequency but this is the best I can do with my meager experience and technical knowledge. I'm sure others can do better. Please let us know how. Always willing to learn better techniques.

[Helmholtz]

Voltage measured at grids, current measured at plate using voltage between OT center tap and plate, divided by resistance of OT windings.

Thanks for your reply.

I recommend to measure the grid bias at TP30 because this is a low impedance point. As there's no grid current, the voltages at the grids must be the same as at TP30.
When measuring at the grids, the meter's input resistance will cause the readings to be low depending on meter. Input current of a 1M meter drops 2V to 3V across the grid leaks.
But as always, grid bias is just for rough orientation and actual plate current varies with tubes.

I have no reason to doubt your findings regarding optimal plate current with this amp.

[ArturoQ]

Now you had to go and get all technical on me. Grid, plate jibberish. Guess I am going to have to learn more than I know and that is very little....... So could you give an elementary explanation of the grid and screen.The plate is the anode, I believe. Would the grid be the cathode? The screen is the modulator of the cathode electrons and the plates positive charge????
Thanks for the information!!

[Jazz P Bass]

Briefly:
The Cathode, when heated, boils off (emits) electrons
The Plate, being at a positive potential as compared to the Cathode, attracts the electrons.
The Grid, modulates the electron flow. (full on to full off and anything in between.) This is where the audio signal is applied. So the audio signal modulates the tube function.

[ArturoQ]

Aha and thank you Jazz P Bass! The Grid is also refered to as the Screen???

[Helmholtz]

The Grid is also refered to as the Screen???

No, the EL84 has a control grid (g1, pin 2) typically referred to as grid and (being a pentode) also a screen grid (g2, pin 9) typically referred to as screen.
G1 is at a negative bias voltage and the screen at a high positive voltage.

[ArturoQ]

Thanks Helmholtz, read up some on the pentode and I see the difference now!