Silverface 135 UL 'output balance pot'

[Leo_Gnardo]

Seems an 'un-molested' amp is a unicorn these days!

Unless you're going to enter your prize into the "Unmodified" class at the local Fender Judging Contest what's the worry about modified?

Balance & bias, you can have both, here's how I do it. Use your 10K chassis-mount trimpot to make a balance control such as you find in your amp, just don't sweat the missing mystery lug. Typically you have a 33K resistor from ground to one lug: replace that with a 20K trimpot and 15K fixed resistor in series: bingo there's your overall bias control. You can bias n balance til the pineapples are ripe, whoopee! FWIW I use Bourns 3386 series "blue cube" trimpots, Mouser has 'em.

While you're on the learning curve, study up on the 3 main ways of reading your bias current. You may want to add some 1 ohm precision resistors to your parts order, I recommend Dale/Vishay 3W 1% wirewound. Put one on each 6L6's cathode & you can read bias current directly with a cheap DMM. Now we'll hear from the people who think current sense resistors should double as fuses...... In case you don't want to put in those resistors read up on shunt method, also a safer version where you first read OT primary resistance, then V across those windings with amp on, calculate I = V/R and that works too, but you're measuring your tubes by pairs.

Your filament hum balance pot probably went up in smoke a long time ago when some output tube shorted out. Nice to see it's coming back.

[52 Bill]

I seem to recall that when the hum/bias balance was first introduced on the Bassman amps, the pot was the original non-tapped version and resistors were used to form the balancing network.

See if you can find a schematic for that circuit and try and adapt it for your Twin.

[g1]

I was trying to think of that example also. It's the AB165.
The 15K going to ground would be replaced by a pot in series with a resistor.

[oahudog]

I've done the 1 ohm to the blue cube on one of my other amps, so I'm comfortable with that. The bias/balance sounds like a good thing, and, luckily, I 'know a guy that does that'. As the 'end user', taking a modded amp to a tech understandably turns into more bench time, and that's what prompted my 'rant'. I work on a lot of old 'shovelhead' motors, and if someone can tell me what's been changed, it saves time and money over me finding it the 'hard' way.
Thanks again for the help..I appreciate it! dave

[oahudog]

Thanks! Sounds like I'm getting closer to the 'work-around'! dave

[oahudog]

Thanks! I have access to the AB165 schem., and I'll mull that one! dave

[g1]

Balance & bias, you can have both, here's how I do it. Use your 10K chassis-mount trimpot to make a balance control such as you find in your amp, just don't sweat the missing mystery lug. Typically you have a 33K resistor from ground to one lug: replace that with a 20K trimpot and 15K fixed resistor in series: bingo there's your overall bias control.

Leo, sorry I had missed this post, I'm not sure how the first part looks (balance with 3 lug pot). I know you have mentioned your version of bias & balance before, do you have a diagram?

[oahudog]

Thanks for the info! The hum varies with the Vol, both MV and channel. No hum with the power tubes out, hum with the PI out and the power tubes in. It's quietest with the MV at 3..slightly louder at 1, and louder above 5 and increases from there up. With the MV at 3 (quietest), the channel volume varies it some as you go up, but not much. The MV produces a higher freq sound, and the channel volume a lower freq sound. It still has the pull boost connected, but not engaged..that definitely makes it worse! All of the aforementioned was done with nothing plugged into the inputs and no foot switches attached.
I appreciate the help! Thank you! dave

[Jazz P Bass]

Check the power supply nodes for unwanted Vac ripple.

[oahudog]

Will do! Thanks! dave