Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plumcrazyfx on May 21, 2018, 06:43:30 am

Title: Bias Tap or Not?
Post by: plumcrazyfx on May 21, 2018, 06:43:30 am

If I'm planning a fixed bias amp from the ground up, should I be looking for a power transformer with a bias winding?  Many of the classics just tap the bias off the high voltage line and use different values to lower into the bias range.  Is there a benefit to having a separate tap?

Title: Re: Bias Tap or Not?
Post by: sluckey on May 21, 2018, 07:03:14 am

For low power amps I would just use the HT winding. For 30 to 50 watts I'd prefer a bias tap on the HT winding. For a high power amp or any amp that will use a FWB I would want a completely separate bias winding. Even though the bias supply is a very simple circuit, a lot of expensive components are at stake if the bias supply fails. I want it to be as bulletproof as possible.

Title: Re: Bias Tap or Not?
Post by: alerich on May 21, 2018, 09:48:17 am

Another important consideration is how the fixed bias supply is wired. Many production amps (some of them very pricey and well regarded) use a bias circuit that relies on a positive wiper connection at the bias pot. If that pot fails, negative bias voltage disappears and bad things happen. It's possible to wire the bias circuit in such a manner that if the bias pot fails the circuit reverts to max negative voltage or at least some fixed voltage that spares the tubes, OT and other expensive components. It's not really possible to make the bias circuit completely fail safe but we can improve the odds over common practice.

Title: Re: Bias Tap or Not?
Post by: sluckey on May 21, 2018, 10:09:09 am

Quote

It's possible to wire the bias circuit in such a manner that if the bias pot fails the circuit reverts to max negative voltage

Good point! Look at this Fender AB763 schematic (http://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_aa763_schem.pdf). Compare to this Marshall schematic (http://el34world.com/charts/Schematics/files/Marshall/Marshall_jmp_mastervol_50w_2204u.pdf). Then wire the pot like the Marshall.

Title: Re: Bias Tap or Not?
Post by: plumcrazyfx on May 21, 2018, 06:46:17 pm

Anyone got any recommendations for a PT for a 100w bass amp.  I like Hammond Transformers and the 291FX but it has a tap not a dedicated bias winding.  Any other suggestions?