Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: telerepairman on March 04, 2014, 07:58:36 am

Title: Bias control PLUS Bias balance control for 1972 Bandmaster Reverb
Post by: telerepairman on March 04, 2014, 07:58:36 am

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.

Title: Re: Bias control PLUS Bias balance control for 1972 Bandmaster Reverb
Post by: sluckey on March 04, 2014, 08:49:21 am

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

Title: Re: Bias control PLUS Bias balance control for 1972 Bandmaster Reverb
Post by: g-man on March 04, 2014, 10:08:08 am

Deleted

Title: Re: Bias control PLUS Bias balance control for 1972 Bandmaster Reverb
Post by: sluckey on March 04, 2014, 10:49:49 am

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

http://www.digikey.com/product-detail/en/RV6LAYSA103A/RV6L103C-ND/222821 (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.newark.com/eti-systems/rv6laysa103a/pot-rotary-10kohm-10-500mw/dp/79M6945)

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

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

Title: Re: Bias control PLUS Bias balance control for 1972 Bandmaster Reverb
Post by: telerepairman on March 06, 2014, 04:59:24 pm

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.

Title: Re: Bias control PLUS Bias balance control for 1972 Bandmaster Reverb
Post by: sluckey on March 06, 2014, 05:27:41 pm

Quote

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.