BA115HP initial bias potentiometer resistance

[okwestern]

I want to replace the bias potentiometer with a multiturn potentiometer on my BA115HP.
I want to make sure the pot isn't set too low when powering the amp(driving the mosfets too hard - I've heard about fireworks).
But I read some time ago about ringing/oscillation if the potmeter is too high also, which could also blow the mosfets.

Any idea of what a good starting point for the potentiometer?

 

[rickdog]

I want to replace the bias potentiometer with a multiturn potentiometer on my BA115HP.
I want to make sure the pot isn't set too low when powering the amp(driving the mosfets too hard - I've heard about fireworks).
But I read some time ago about ringing/oscillation if the potmeter is too high also, which could also blow the mosfets.

Any idea of what a good starting point for the potentiometer?

I'm not familiar with the BA115, just offering my intuition based on general electronics knowledge and experience adjusting the (very sensitive) bias pot on my SVT-3Pro.

First - why do you think you need to replace the bias pot at all? The bias shouldn't need to be adjusted (unless something unfortunate has happened and you had to replace the output transistors). Unless there's a compelling reason to adjust the bias, your best solution is probably to leave everything as it is.

The reason there's a bias pot instead of some fixed resistors is to accommodate the variation in characteristics of the output transistors. There isn't likely to be a standard "good starting point" that's the same for every amplifier. Perfect for one amp could be catastrophic in another.

If you're determined to proceed - assuming the bias is about right with the current pot, you could remove it (being careful not to change its adjustment), measure the resistance at the current setting, and adjust your new multiturn pot to match. That should at least be close. Unfortunately, "should be close" is not the same thing as "close enough". So you should power up the amp using a "dim bulb" tester, which might save your output transistors if the bias isn't close enough.

Do you have a service manual for the amp? If not, how do you know what the bias should be set to?

 

[agedhorse]

First question... how much experience and TECHNICAL familiarity do you have with these amplifiers and with proper safety protocols?

 

[okwestern]

I'm not familiar with the BA115, just offering my intuition based on general electronics knowledge and experience adjusting the (very sensitive) bias pot on my SVT-3Pro.

First - why do you think you need to replace the bias pot at all? The bias shouldn't need to be adjusted (unless something unfortunate has happened and you had to replace the output transistors). Unless there's a compelling reason to adjust the bias, your best solution is probably to leave everything as it is.

The reason there's a bias pot instead of some fixed resistors is to accommodate the variation in characteristics of the output transistors. There isn't likely to be a standard "good starting point" that's the same for every amplifier. Perfect for one amp could be catastrophic in another.

If you're determined to proceed - assuming the bias is about right with the current pot, you could remove it (being careful not to change its adjustment), measure the resistance at the current setting, and adjust your new multiturn pot to match. That should at least be close. Unfortunately, "should be close" is not the same thing as "close enough". So you should power up the amp using a "dim bulb" tester, which might save your output transistors if the bias isn't close enough.

Do you have a service manual for the amp? If not, how do you know what the bias should be set to?

The amp was bought with blown mosfets, so I can't trust that the pot is in a reasonable position. Since I'm replacing the mosfets, I might as well save me a lot of headache and replace the stock pot with a multiturn (much easier to adjust biasing). Yes, I have a service manual, and have worked on these before. I also have a mosfet matching rig, to pick out a good set. I also have a bulb tester, but missing the bulb (it's hard to get big enough bulbs these days (most of what's sold here is either LED or fluorescent)).

 

[okwestern]

First question... how much experience and TECHNICAL familiarity do you have with these amplifiers and with proper safety protocols?

Yes, I'm an electrical engineer, so I'm quite confident working with this. I've done quite a lot of work with various Ampegs (SVT3/7/8 PRO and BA115HP/HPT). Whether it was proper analytical work or just pure luck I got them running again, I don't know.

 

[agedhorse]

Yes, I'm an electrical engineer, so I'm quite confident working with this. I've done quite a lot of work with various Ampegs (SVT3/7/8 PRO and BA115HP/HPT). Whether it was proper analytical work or just pure luck I got them running again, I don't know.

Ok, then analyze the circuit to determine which position results in minimum bias across the MOSFET gates.

 

[rickdog]

The amp was bought with blown mosfets, so I can't trust that the pot is in a reasonable position. Since I'm replacing the mosfets, I might as well save me a lot of headache and replace the stock pot with a multiturn (much easier to adjust biasing). Yes, I have a service manual, and have worked on these before. I also have a mosfet matching rig, to pick out a good set. I also have a bulb tester, but missing the bulb (it's hard to get big enough bulbs these days (most of what's sold here is either LED or fluorescent)).

You likely know more than I do about this amp! Sometimes people with near-zero technical knowledge come on here and ask for advice on doing things better left undone, hence my cautious reply

You can bet I've built up a stash of incandescent bulbs... I picked up a couple 150W long-life bulbs recently at a garage sale to add to my stash.