Bias tap PT-100

[M Fowler]

I've attached the schematic for Heyboer P-TM-P100 power transformer 100w Marshall type.

What does the bias board circuit look like when you have two wires for bias tap instead of one. I don't think I've ever had a PT with two bias wires before. Does one wire go to ground and the other to the bias board as usual?

Mark

[ToneMerc]

Does one wire go to ground and the other to the bias board as usual?

Mark

Yes that's correct.
TM

[M Fowler]

Thanks TM I appreciate the help.

Mark

[ToneMerc]

Thanks TM I appreciate the help.

Mark

10-4, your welcome.

TM

[Firestorm]

You can also rectify the bias full-wave instead of half. That approach is favored by some including KOC.

[M Fowler]

Firestorm

You can also rectify the bias full-wave instead of half. That approach is favored by some including KOC.

Okay I haven't rectified the bias source like that before.

[Structo]

You can also rectify the bias full-wave instead of half. That approach is favored by some including KOC.

Can you tell us the benefits of doing that?

[ToneMerc]

In my first HV D style build I used the FWB bias supply and IMHO it was the way long way around to a problem that didn't exit, any succeeding builds I grounded one side like Marshall and Dumble. As far as reducing ripple further, I followed HAD's lead by increasing the bias caps value to 220uf.

Now granted, in that HV situation you have 700V on the plates so you don't need nearly as much negative voltage on the grids, so the few volts you might gain by using a bridge is a moot point in my opinion. By the same token, the Marshall PT has a 98V winding, thus there's no advantage of chasing a few points of negative voltage in most guitar amp circuits.

My two zinc pennies

TM

[Firestorm]

You can also rectify the bias full-wave instead of half. That approach is favored by some including KOC.

Can you tell us the benefits of doing that?

It's easier to filter out 120Hz than 60Hz. But it doesn't really matter if both sides share the bias supply (common mode cancellation). But remember that O'Connor also likes individual bias for each tube, so there, if you're running tubes with different bias requirements, hum in the bias supply won't cancel as effectively.

[Structo]

I have two bias pots in my D'lite and 100w ODS clone.

One for each side of the power amp.

I have noticed before when adjusting that if I go too far one way the hum increases.
Probably the same principle as the hum balance on older Fenders.

But I like to balance the two sides for best idle bias.