Take a look at this schematic

[JMT]

Take a look at this schemo. This is for an old Univox bass amp with a Weber 15F150. Would appreciate suggestions for biasing the power tubes FOR HARP as a starting place. Although a tech is doin' the work I don't have a tech in my area who knows harp amps. I'm also interested in any other suggestions you might have for optimizing the amp for harp. I've already had the electrolyticss updated. Suggestions for the other caps?
Thanks,
Jay

[MWJB]

Hi Jay,

In terms of bias, I'd start by looking to get the tubes idling around the 7mA mark with a 15" speaker (as a starting point). This amp uses very high power tube plate volltages, what bias & plate voltage do you have now? Establish this before changing anything. Be careful when biasing cooler that plate voltage does not continue to rise significantly (you migt already be arround the 700v area?).

Increasing the value of the 33K resistor to ground at the -32v supply will bias colder, as will reducing the value of the 6.2K resistor after the bias diode.

I don't like the look of the unattenuated stage right after the input, i would be thinking about moving the tone stack to between the 2 halves of V1 & replace V1 with a higher mu tube. You might find that you need to decrease the value of the 100K preamp dropping resistor, for example, I wouldn't recommend much below 150vdc on a 12AX7 plate minimum. Experiment with cathode bypass caps at V1a & b. Likewise I'd be looking to experiment with bypassing the additional 12AU7 gain stage that follows the PI tube, hook up the 0.1 caps feeding the 6L6 grids to the 12AX7 plate resistors (if you have time, doubling the plate resistor values here might be wrth a try). That 15K cathode resistor at the PI looks like it will really cut back on distortion, but a large value might be needed here to keep plate voltages at a decent level? Alternatively, you might be able to reduce the value of the 5K dropping resistor between the screens & PI to raise preamp voltage, then reduce the 15K PI cathode some? In short I don't know whether a few simple cap & bias changes will really transform this amp?

[Bruce / Mission Amps]

I typed this all in the other day and somehow it got deleted but since MWJB has addressed most of it anyhow, I'll just add:
Some of those old oil caps are crap... and or, if this one has the puke green coupling caps, remove and replace them all with any cheap polyester coupling cap and except for the .1uF caps at the power tube grids, use values that are bigger... like if it is a .01uF experiment with .022uF to .047uF. etc.
Replace the 33K resistor in the bias supply with a 25K pot wired as a variable resistor and hold the pot from ground with an 18K 1/2w resistor... now the bias supply will be adjustable.
With the screen node so low, you'll need a little higher idle current in the power tubes then normally seen.
I agree with the bias supply changes.
MWJB likes these power tubes colder to help with feedback issues, but at this high plate B+ and low screen B+, I think you'll like biasing the power tubes hotter, to maybe 18-22 ma each... With the bias supply now variable, you can try different combinations.
And, a bigger load on the B+ rail from higher power tube idle current, will pull the voltage doubler down.
12AU7 preamp, I'd also consider changing the bias resistors to 820 to 1K on the 12AU7s and reduce the 220k plate load resistors down to 100K-120K.
You'll have to experiment with this one.

[JMT]

Hi Jay,

In terms of bias, I'd start by looking to get the tubes idling around the 7mA mark with a 15" speaker (as a starting point). This amp uses very high power tube plate volltages, what bias & plate voltage do you have now? Establish this before changing anything. Be careful when biasing cooler that plate voltage does not continue to rise significantly (you migt already be arround the 700v area?).

Increasing the value of the 33K resistor to ground at the -32v supply will bias colder, as will reducing the value of the 6.2K resistor after the bias diode.

I don't like the look of the unattenuated stage right after the input, i would be thinking about moving the tone stack to between the 2 halves of V1 & replace V1 with a higher mu tube. You might find that you need to decrease the value of the 100K preamp dropping resistor, for example, I wouldn't recommend much below 150vdc on a 12AX7 plate minimum. Experiment with cathode bypass caps at V1a & b. Likewise I'd be looking to experiment with bypassing the additional 12AU7 gain stage that follows the PI tube, hook up the 0.1 caps feeding the 6L6 grids to the 12AX7 plate resistors (if you have time, doubling the plate resistor values here might be wrth a try). That 15K cathode resistor at the PI looks like it will really cut back on distortion, but a large value might be needed here to keep plate voltages at a decent level? Alternatively, you might be able to reduce the value of the 5K dropping resistor between the screens & PI to raise preamp voltage, then reduce the 15K PI cathode some? In short I don't know whether a few simple cap & bias changes will really transform this amp?

Thanks for the detailed ideas. Because I'm working with a tech rather than making these changes my self this project will take awhile. I report back when there's something to report.

[JMT]

I typed this all in the other day and somehow it got deleted but since MWJB has addressed most of it anyhow, I'll just add:
Some of those old oil caps are crap... and or, if this one has the puke green coupling caps, remove and replace them all with any cheap polyester coupling cap and except for the .1uF caps at the power tube grids, use values that are bigger... like if it is a .01uF experiment with .022uF to .047uF. etc.
Replace the 33K resistor in the bias supply with a 25K pot wired as a variable resistor and hold the pot from ground with an 18K 1/2w resistor... now the bias supply will be adjustable.
With the screen node so low, you'll need a little higher idle current in the power tubes then normally seen.
I agree with the bias supply changes.
MWJB likes these power tubes colder to help with feedback issues, but at this high plate B+ and low screen B+, I think you'll like biasing the power tubes hotter, to maybe 18-22 ma each... With the bias supply now variable, you can try different combinations.
And, a bigger load on the B+ rail from higher power tube idle current, will pull the voltage doubler down.
12AU7 preamp, I'd also consider changing the bias resistors to 820 to 1K on the 12AU7s and reduce the 220k plate load resistors down to 100K-120K.
You'll have to experiment with this one.

I follow up to let you guys know how this turned out. All the best, Jay

[JMT]

Thanks for your reply. So far I've done some of what you suggested. I've already had a cap job done and bumped up the values on the appropriate ones, modified the tone stack a bit for more sweep on the treble. Since the general consensus seemed to be that this amp is probably not the best platform for mods I decided to keep it simple. I've only had one gig in the right size room for this amp since the work was done and it sounded pretty good as it was. The new Weber and the cap job made a huge difference. I was wanting something for small/medium venues but now I'm playing with a real ROCKIN' blues band and need more volume/presence on stage so I'm working with my Brown Concert and Meteor. I may have to pass this on to someone who can get some good out of it.
All the best to everyone,
Jay

[arnenym]

Hi Jay,
I don't like the look of the unattenuated stage right after the input, i would be thinking about moving the tone stack to between the 2 halves of V1 & replace V1 with a higher mu tube.

Or you could do a voltage divider with two 220K resistors between the first two stages. The first 220k in serie, and after that a 220K to ground. That makes a fake pentode sounding first stage.