Mark IIb bias mod ideas

[Tommy]

Hello guys! I'm here again :wink:

I am planning on adding a Bias pot in my old 100W mark IIb. I read that it does replace one 10kOhm resistor by 50kOhm linear trimpot in the bias area of circut board:

Originally 10kOhm and 6.8kOhm gives 4.047kOhm in parallel.
I've got three ideas:

Idea 1:

It's really simple. But, what if:
1) the trim pot is turned fully left so it's resistance value will be nearly 0hms and short circut will occuir ? Will it hurt my amp?
2) the trim pot is broken or dirty (open circut) so the whole resistance will be 6.8kOhm ?
Generally speaking circut resistance varies from 0Ohm to 5.9kOhm (6.8kOhm if pot is dirty/broken)


Idea 2:

Middle and one of outer legs are connected at one side of pot. If trim pot is broken or dirty (broken connection between the wiper contact and the surface of the potentiometer track) the whole resistance will be about 5.9kOhm which is is still far from the nominal 4.047kOhm but better than 6.8kOhm. Whole circut resistance varies from 0Ohm to 5.9kOhm


Idea 3:

2kOhm in series with pot. Whole circut resistance varies from 1.5 kOhm to 5.9kOhm
For me this is the safest. The question is will it give me enough range of biasing?
I don't want to fry my old boogie, especially by melting down OT.

 

[Boogiebabies]

In the MKIIA, the trim pot was where the 6.8K resistor was. It was 10K and adjusted to -52V.
It was the cause and reason for most bias supply failures and why Mesa never used a pot in the
MK series again. Now with a good pot, I don't see why a 25K would not get you a decent range.
The issue is in how to mount it. The IIA, IIB and IIC all used slightly different PCB's for the PSU's
with only the IIA's PS1 having been made for a pot. The pot's third lug was grounded and even at
very low to almost off settings it did not cause the circuit to short as it had the parallel resistance to the
10K resistor.

 

[stokes]

I have done the mod to many Boogies and other amps.If you use a cermet pot you can just glue it on its end to the PCB.I use a pot thats value is 2x the resistor that I am replacings value.If you dont turn it to the extreme maximum you wont have any problems.Since it is mounted inside the chassis no one can "accidentally"turn it to zero.The cermet pot is sealed so dirt is not a problem.Cermet pots also take dc voltage better than a standard volume type pot will,I've seen them actually burn the trace inside the pot from the dc voltage sitting across the same spot on the trace for years.A cermet will not do that.If you've ever had a leaky cap feed dc to a volume or tone pot,that scratchiness is the dc voltage slowly eroding the trace.

 

[acorkos]

are you changing power tubes that often, that you need to install a pot? and it's not like you can easily get to the pot from outside the chassis. since you have to pull the chassis to adjust the pot, why not just change the resistor when you need to adjust the bias? it's simple enough.

 

[stokes]

If you leave the pot exposed some idiot will eventually turn it,just to see.You dont think it is easier to turn a pot than to change a resistor each time you want to re-bias?Put a soldering iron to that PCB enough and the pads will surely lift,meaning you have to get a new PCB.The pot is the only way to go.

 

[Boogiebabies]

These amps sound very good when you get them out of the cold bias region.

 

[stokes]

These amps sound very good when you get them out of the cold bias region.

Always been my biggest gripe with Mesa amps,they are biased so cold.With an adjustable bias,even the crap tubes Mesa and GT sell you can sound so much better with a little tweaking.

 

[Tommy]

Thanks for replies. Most probably, I will mount 25k pot in the place of 10k resistor (i guess it's the one with red arrow) with middle and one of outer legs connected to ground.
Of course i don't want to put this pot somewhere outside the chassis. :wink:

I checked bias voltage and it's -41.8V at pin 5 on inner tube sockets (amp on standby, with STRs430 in outer sockets). Is it correct factory value? I've already ordered adapters that plug between tubes and sockets (they are simple tube sockets connected pin to pin with tube base with 1 ohm resistor from cathode to ground) to check bias current.

 

[Tommy]

Still waiting for package with bias probe.
Don't you guys think that -41.8V at pin 5 is a fairly low value? Mark IIc manual says that there should be -52V. Mains voltage form step-down transformer is 117V. Last year, i replaced filter caps: 220uF/285V -> 220uF/350V, 30uF/500V -> 33uF/500V and bias caps 50uF/75V -> 50uF/100V. I didn't replace caps on eq board, as I see on schematics something wrong with eq board can reduce bias supply voltage. Also, do this transistors on eq board look original ? Only one of them have heatsink (sorry about poor picture quality).

Eq is working ok except small amount of buzz at extreme settings (for example one of sliders maxed out) which I guess is normal.
Tell me guys, what else to check.

 

[Boogiebabies]

Did you get the circuit revision off the bottom of the cap board ? If it's a PS1 it's built to have a small cermet trim pot. It it's a PS1A, which I think you have, it's made for resistors only. The EQ looks stock. I just saw one from a IIA and it's transistors are identical to yours and it does hum at extreme settings, especially the 80Hz.

 

[stokes]

High gain amps with almost any of the controls "maxed" will increase the hum that is inherently there in high gain tube circuits.Its the nature of the beast.You can chase yourself forever trying to get rid of hum,but it will always be there to some degree.

 

[Murfman]

Tommy-

Please keep us posted on your progress with this progress. I have a Mark IIB boogie as well and would really like to do this mod as well.

-Murf