Bias control PLUS Bias balance control for 1972 Bandmaster Reverb

[telerepairman]

Having just modded my BMR for Bias Control (it having had the dreaded Bias Balance control.) I'm now seeing suggestions telling me to revert and ADD a Bias control pot to the 1970'2 design for the "best of both worlds". Is this a good idea? I like the idea of getting perfect bias balance...and/or of being able to find my own bias offset- if that gave a pleasant edge to the sound.

[sluckey]

Most of the bias adjust PLUS balance circuits I've seen are kinda limited in the balance range. They work well if the tubes are not too far apart.

IMO, a better solution is a simple dual bias adjust system. This provides a separate bias pot for each tube for total independent control of each tube. You can perfectly match bias for the tubes at any bias point. You can also totally mismatch tube bias if you desire. It's very easy to implement. Here's a schematic showing my dual bias mod to a Sunn amp...

     http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf

And here are some pics of the mod...

     http://home.comcast.net/~seluckey/amps/sunn/sunn.htm

[g-man]

Deleted

[sluckey]

It's a 10K PEC pot. Part number RV6LAYSA103A

http://www.digikey.com/product-detail/en/RV6LAYSA103A/RV6L103C-ND/222821

http://www.newark.com/eti-systems/rv6laysa103a/pot-rotary-10kohm-10-500mw/dp/79M6945

http://www.mouser.com/Search/Refine.aspx?Keyword=RV6LAYSA103A

Here's some specs...   http://www.etisystems.com/RV6.asp

[telerepairman]

Thanks Sluckey
Looks good 'though I can't see extra pots on bottom circuit diagram.
Think I'll keep the old Bandmaster close to standard and just revert to Bias Balance and solder a 10k variable resistor with 10K ohm resistor in series with it in place of the 15K. I'm looking forward to hearing 2 valves in balance as I just discovered that one of the 470 ohm resistors on a valve base is burnt and gone high res. Then I can resume trying to eliminate the farty bass response which seems to have migrated to the Normal channel since I put the green twisted pair back in phase on the power valves...and so it goes.

[sluckey]

Looks good 'though I can't see extra pots on bottom circuit diagram.

It's on page 1 of the pdf. Look for anything purple.