Biasing a '67 Marshall with no bias pot! Er... how?

[loverocker]

OK - so I'm stumped. Marshall recently serviced this amp wot I've just bought. Marshall changed the bias caps (pretty usual, I know). I thought they'd 'forgotten' to put the bias pot back, but I now I see that some amps of this period didn't have a bias pot at all...

So I guess Marshall is correct but how do I bias this puppy?

And is it considered very bad form to add a bias pot in the same position that later plexis have them?

 

[John Phillips]

#1:

Get a set of Mullards (it deserves them), put them in, check the bias.

If it's in the right range (it probably will be), leave it alone.


#2:

On the other hand, since the caps have been changed now, who cares about strict originality? So you may as well fit a proper bias control. Remove the 56K resistor in parallel with the second bias cap, and fit a 47K resistor and 22K trimmer in series here. Connect the CW tag (looking from the input jack end) and the rotor of the trimmer to the grounded post (nearest the knobs), and the CCW tag to the end of the resistor. That's the standard Marshall layout from the later models.

 

[Wizard of Ozz]

Originally posted by John Phillips


Get a set of Mullards (it deserves them), put them in, check the bias.

If it's in the right range (it probably will be), leave it alone.

+1

If it ain't broke don't fix it. I personally wouldn't add the pot.

If however you desperatley feel the need to, go ahead and install the 22K trim pot described above.

 

[loverocker]

Thanks guys. I'll try the first approach - with some Mullards

Is there any particular mA range that Marshall considered 'in spec' - or didn't they bother measuring? Or is there a general consensus? I've got a bias-measuring widget that fits between valve and socket that I can use.

 

[John Phillips]

Measure the plate voltage, then the tube current. Don't assume the plate voltage from a schematic or anything BTW - those old Marshalls are quite variable and sometimes rather on the high side...

Plate voltage x tube current = power dissipation. This should be between 50% and 70% of the maximum for the tube type (25W for EL34s) for normal Class AB operation - you can often get them to run OK below 50%, but they don't sound so good there.

So voltage (in volts) x current (in amps, 1000 x the mA reading) needs to be between 12.5W and 17.5W.

Check all the tubes and average it if they're different - they shouldn't be very different with a matched set, but it's a good idea to check.


(Yes, I know I've left out the screen current component. It doesn't make a huge difference, and the error it causes is on the 'conservative' side anyway. If the dissipation is just over the 70% limit, you can probably assume the screen current is 5mA and see if that brings you back into the right range.)

 

[loverocker]

<Ctrl>+<P> That's perfect - thanks once again, John