Biasing a Marshall VBA 400

Clammy · #1 11-11-2011, 01:38 PM

I got some new Tung-Sol reissue 6550s for my VBA 400, and I was wondering if anyone has biased one of these amps before, and if it's much different than biasing a 70s Marshall.

I understand there are is a bias pot for each quad, for a total of 2 bias pots, and a "blend" pot. I'm sure I can handle the individual quad biasing, but how does the "blend" thing work?

Thanks.

Cheers!

Fab Savage · #2 11-12-2011, 04:08 AM

Color me interested. Mine is coming up for a re-tube soon and I am deliberating whether to DIY. Especially as there isnt really anyone with much experience in my city with this amp.

Clammy please post how you go with this if thats cool.

Cheers

Mike

Clammy · #3 11-12-2011, 02:56 PM

Quote:

Originally Posted by Fab Savage

Color me interested. Mine is coming up for a re-tube soon and I am deliberating whether to DIY. Especially as there isnt really anyone with much experience in my city with this amp.

Clammy please post how you go with this if thats cool.

Cheers

Mike

Sure thing, once I figure it out for myself, hehehe...

I'm thinking it can't be that difficult, I just need to know how this "blend" pot works, and what to measure.

Cheers!

BassmanPaul · #4 11-12-2011, 03:18 PM

Take it to a tech and let him do the work. You'll be safer that way. If you don't know exactly what your are doing you have no business inside the chassis of a tube amp. There's going to be around 700V in there and that's more than lethal.

Clammy · #5 11-12-2011, 03:36 PM

Quote:

Originally Posted by BassmanPaul

Take it to a tech and let him do the work. You'll be safer that way. If you don't know exactly what your are doing you have no business inside the chassis of a tube amp. There's going to be around 700V in there and that's more than lethal.

I know how to work around the insides of high voltage amps. I've been doing the biasing and general maintenance (including having changed a few sets of filter caps) on my 1970s Marshall amps for years. Don't worry, I'm extremely careful, and mindful of the potentially lethal voltages in there.

I'm perfectly comfortable doing it myself. I have just never seen a "blend" pot, and have no idea how it operates or what to measure for setting it correctly.

Cheers!

**B-string** · #6 11-12-2011, 05:17 PM

If it like a normal balance pot, inject a 80 Hz signal attached to a load and adjust for as close to zero volts differential between the two quads.

Clammy · #7 11-12-2011, 05:41 PM

Quote:

Originally Posted by B-string

If it like a normal balance pot, inject a 80 Hz signal attached to a load and adjust for as close to zero volts differential between the two quads.

Thanks. Never dealt with a balance pot before.

Cheers!

**B-string** · #8 11-12-2011, 05:49 PM

No problem, be careful (as always

)

johnk_10 · #9 11-12-2011, 06:05 PM

if it's an output tube balance pot (like on some fenders) which it probably is, i just set them by ear for minimum hum after the tubes have warmed up and their bias is stabilized for at least 20 minutes. if it's a phase inverter balance like on an SVT then you run a signal thru the amp and set it to spec.

Clammy · #10 11-12-2011, 06:36 PM

Quote:

Originally Posted by johnk_10

if it's an output tube balance pot (like on some fenders) which it probably is, i just set them by ear for minimum hum after the tubes have warmed up and their bias is stabilized for at least 20 minutes. if it's a phase inverter balance like on an SVT then you run a signal thru the amp and set it to spec.

On the schematic, the pot is connected to the phase inverter, so I'm guessing it's that type.

Cheers!

Clammy · #11 11-14-2011, 07:37 PM

Got it done. New Tung-Sols are installed and biased/balanced. Here's the procedure I used:

Biasing instructions: Marshall VBA 400
--------------------------------------

--------------------------------------
Assumptions:

Plate voltage is known (if not, measure at pin 3 of a power tube with the amp on and off standby ("play mode")).

Power tube maximum dissipation (in Watts) is known (Get this from the tube manufacturer's data sheet).

Bias current calculated from above values (in mA. Suggested: around 70% of the max dissipation. Calculated thusly: (0.7 x max dissipation)/plate voltage )
---------------------------------------
---------------------------------------

Connect the amp to a 4 ohm load, and turn it on. Set the volume and gain controls to zero (0), with no signal and take it off standby (put it into "play" mode).

1. Connect Digital multitmeter (set to DC volts in the 100 - 200 mV range) to pins 1 and 2 of CON 2 on Power Supply Unit PCB

2. Adjust PR1 (located on PSU PCB) until the reading is 4x the calculated bias current (measured mV = calculated mA)

3. Connect digital multimeter (set to DC volts in the lowest available range) to pin 1 and pin 3 of CON 2

4. Adjust PR2 until the reading is 0 mV (+/- 2mV)

5. Feed a steady tone into the Passive input.

6. Raise the volume and gain controls a little.

7. Connect digital multimeter (set to DC volts in the lowest available range) to pin 1 and pin 3 of CON 2 <---editted to correct mistake!
8. Adjust PR3 (located on the "Front/pre-amp" PCB) until the reading is zero (+/- 2mV)

Hope that helps out.

Cheers!

Triad · #12 11-15-2011, 04:08 AM

Would you recommend using a bias tester? Would it make things easier?

Clammy · #13 11-15-2011, 08:49 AM

Quote:

Originally Posted by Triad

Would you recommend using a bias tester? Would it make things easier?

You don't need it. Pin 1 and 3 of CON 2 are connected to the cathode (each pin is connected to one quad) using a 1 ohm resistor, so when you're measuring the voltage drop across it, it translates directly into amps (milliamps really, cause you'll only measure millivolts). The only thing is that each pin (1 and 3; 2 is ground) will have the total of 4 tubes so, you need to do a tiny bit of math. Even if you did use a bias tester, you'd still have to do the step for balancing using the PR2 pot, and the PR3 adjustment as well.

It was a really simple procedure, and having a single measuring point saved a bunch of time.

Cheers!

Fab Savage · #14 11-16-2011, 08:52 PM

Legend...thanks for posting this!

Clammy · #15 11-19-2011, 10:41 PM

Quote:

Originally Posted by Fab Savage

Legend...thanks for posting this!

Happy to help out. I got a lot of help over at the Marshall Amp Forum.

Cheers!

Fab Savage · #16 01-28-2012, 02:17 AM

Quote:

Originally Posted by Clammy

I got some new Tung-Sol reissue 6550s for my VBA 400, and I was wondering if anyone has biased one of these amps before, and if it's much different than biasing a 70s Marshall.

I understand there are is a bias pot for each quad, for a total of 2 bias pots, and a "blend" pot. I'm sure I can handle the individual quad biasing, but how does the "blend" thing work?

Thanks.

Cheers!

o how are those Tung-Sol's working out Clammy? What made you pick them over a set of winged C's?

Clammy · #17 01-28-2012, 08:58 PM

Quote:

Originally Posted by Fab Savage

o how are those Tung-Sol's working out Clammy? What made you pick them over a set of winged C's?

They sound great! The main thing was price. It would have been over $120.00 more for the octet of SEDs. I read a lot of really glowing reviews of the Tung-Sols, and with their far more reasonable pricing, I decide to give them a shot, and I'm very happy with the results.

Cheers!

Fab Savage · #18 01-29-2012, 02:25 AM

Quote:

Originally Posted by Clammy

They sound great! The main thing was price. It would have been over $120.00 more for the octet of SEDs. I read a lot of really glowing reviews of the Tung-Sols, and with their far more reasonable pricing, I decide to give them a shot, and I'm very happy with the results.

Cheers!

Cool as thanks man , Im tryin to work out if I want SED's, Tung Sols or JJ's. I hear the Tung Sol's and JJ's tighten up and make for a more articulate low end, but not sure whether either would be better than a set of SED's for this amp!?

Will keep researching this a bit.

Cheers

Mike