Marshall 1959SLPX Low Plate Voltage and Low BIAS Range

[brnsnd]

Hi,

I haven't posted on the forum in a few years, but I need some suggestions regarding my 1959SLPX from you.

I am in the process of retubing my Marshall. I have a bias probe and it showed the bias with my original Marshall EL34 tubes to be ~20mA.

The voltage out of my wall is ~118v.

I swapped out all the tubes in my head with brand new 12ax7's and TAD EL34B-STR:

Two issues I need help with:

- The plate voltage at pin3 to ground of my EL34s is ~414vDC.
- The maximum bias I get with VR1 dimed is ~26mA.

According to this formula I can set my bias somwhere around 39mA:

25 / 414 = 0.0605 x 6.5. = 39mA

Questions:

- Does my plate voltage need to increase or is it manageable? If it needs to be higher (I am expecting to read 450v), do you have any suggestions how to raise it?

- Any suggestion on the bias resistor value to try if the plate voltage is acceptable.

- what factors contribute to plate voltage being too low?

Info:
R28 = 47k ohms
R29 = 15k ohms
R30 = 27k ohms

I tried to upload a picture of my PC board but the board attachment quota has been reached.

Thanks you for looking at this.

brnsnd

[axeman]

The resistors are correct values. Measure on pin 5 V4 and V5 and record the reading. turn the bias pot all the way to the right then to the left. Do this without the power tubes in and make sure you are hooked up to a speaker or dummy load.

[brnsnd]

The resistors are correct values. Measure on pin 5 V4 and V5 and record the reading. turn the bias pot all the way to the right then to the left. Do this without the power tubes in and make sure you are hooked up to a speaker or dummy load.

Thank you axeman for requesting more info:

After removing the power tubes from the sockets:

Voltage reading of pin 5 from V4 and V5 to ground (pin 5 of V4 and V5 are tied together):

- bias pot turned all the way to the right = -44.6v DC

- bias pot turned all the way to the left = -35.8v DC

Please let me know if there are any additional readings I can collect.

--

Additional information: This head was modified by Racksystems with the kitchen sink mod several years ago -- it includes an added gain stage.

brnsnd

[bill bokey]

Have you checked the heater voltage ?

[brnsnd]

Have you checked the heater voltage ?

With power on and standby on, I read 5.9vAC accross pins 2 and 7 of a power tube -- does this seem ok?

[axeman]

Sorry I meant V5 and V6. Your bias is OK. Do you live in southern Cali?

[brnsnd]

Sorry I meant V5 and V6. Your bias is OK. Do you live in southern Cali?

No worries...

From the info I have collected, what tells you the bias is OK?

With the power tubes removed from their sockets:

Voltage readings of pin 5 from V5 and V6 to ground (results are the same for both):

- bias pot turned all the way to the right = -44.6v DC

- bias pot turned all the way to the left = -35.8v DC

Yes, South OC California...

[bill bokey]

Have you checked the heater voltage ?

With power on and standby on, I read 5.9vAC accross pins 2 and 7 of a power tube -- does this seem ok?

No, it should be 6.3V
Now we know why the B+ is too low

[brnsnd]

What's required to sort out my heater voltage issue?

Once the heater voltage is normal, I expect the b+ voltage to increase -- right?

brnsnd

[bill bokey]

I don't know if there is anything you can do... But if the heater is too low then all the other voltages are too low as well. I guess your amp is on set on the 120V tap ? If you try 110V all voltages will be too high. The SLP is never right either, mine has 6.5V on the heater and I checked a client's SLP not long ago and it was too high as well.
But what you can do is change the bias range resistor so you can at least bias your amp right. 5.9V is a bit low but better low than high

[neikeel]

Solution would be voltage conditioner/step up transformer?

[brnsnd]

Thanks.

I plan to parallel a 220k resistor over the 47k feeding the bias pot -- Would bring my bias pot within range?

All comments and suggestions appreciated as always.

[bill bokey]

Solution would be voltage conditioner/step up transformer?

Or Variac up until heater voltage reaches 6.3V ? (but this may be what you meant )

[brnsnd]

So, I put a 220k resistor in parallel with the 47k bias resistor and biased it successfully. It sounds awesome now even with the low plate voltage.

Are there any issues for me to expect?

[bill bokey]

Tube life is shortened by low or high heater voltage (5.9V isn't too far off so it shouldn't be that bad)