Sunn Solarus PT substitute

[drewl]

So, here is the other power trans question I warned I'd be bugging you with.

A friend has a Sun Solarus with a bad PT.
Trying to rebuild it without putting a lot of $$$ into it.

He has a bunch of old Ampeg transformers, so I grabbed a big one that has proper HV, filament and 5v for the rectifier.

It powers up everything fine, but doesn't have the tap for bias and solid state stuff like the original.

Using a variac, 60 vac powers the bias circuit and solid state stuff and the amps sounds great.

I tried to run those from one of the high voltage secondaries of the PT like some amps do, like Marshall, but it loads down that side of the HV.

Marshall used like about a 150K resistor to power the bias in amps like the 1962 or 1963, but even if I uses a 47K feed, I'm only getting about -20v for the bias.

I'm guessing the feed to the solid state board is loading it down too much?

Or am I missing something?

here's the Solarus, and Marshall to see how they powered the bias circuit.

[Helmholtz]

I can't think of a good solution with this PT.
The LV supply with the zener draws a lot of current (maybe 25mADC) and if connected to a common dropping resistor with the bias supply will result in low bias voltage.
You may try to use separate dropping resistors for bias (say 150k) and the LV supply (maybe 5k to 10k/25W ) but I don't like this solution. Values are very tentative - be careful especially with zener dissipation and power resistor temperature. DCV after the LV rectifier diode should be like in the original amp (maybe 60VDC).
The asymmetric loading of the HV winding may introduce some 60Hz hum.

What is the tap voltage of the original PT?

[drewl]

Don't know tap voltage of original trans since it's bad.

Feeding 60vac to that point with a variac makes the amp work perfectly

[g1]

I think you are fine to take the bias that way but will have to find another source for the LV. Looks like it's 22V?
Maybe another small transformer, or could a tripler be made from the 5V winding?

[Helmholtz]

..or could a tripler be made from the 5V winding?

Certainly not as the 5V winding is floating on B+.

[drewl]

yeah, another small transformer is always a last resort.

just curious as to why it has difficulty even powering the bias by itself (I think I tried that)

I wonder if powering the bias from one side of the HVAC and the solid state board from the other side of the HVAC would work.

anyway, thanks all.

[Helmholtz]

just curious as to why it has difficulty even powering the bias by itself (I think I tried that)

Try individual/separate AC series dropping resistors for LV and bias as advised above. Start with 10k/25W for the LV supply and see if you get close to 60VAC before the rectifier diode. If higher, increase resistance. If lower vice versa.
Same with the "150k" bias supply dropping resistor. Adjust (lower) to get around 60VAC before the bias rectifier diode. Expect a resistor value considerably lower than 150k as the Sunn's bias circuit is lower impedance than the Marshall's. Calculate dropping resistor dissipation by AC voltage drop Vac squared divided by resistance.
Or use Marshall bias circuit values for lower losses (recommended).

[g1]

Certainly not as the 5V winding is floating on B+.

Well good thing I asked as a question rather than making a statement then.

How about a tripler off the heater winding? Not practical? (and I'm suggesting only in the case that deriving LV off the HV does not work out)

[drewl]

Using separate resistors, a 47K for bias gets me in the ballpark, if I raise the 10k to ground after the pot it should be fine.

Still trying to find the right value for the SS board.

I have a separate transformer I can power the SS board with if necessary, just need to figure out where to mount it.
Luckily there's plenty of room.

Thanks.

[Helmholtz]

If your separate transformer delivers ~60VAC, I would just use the original wiring for both bias and LV. This would be the best solution IMO.

[drewl]

If your separate transformer delivers ~60VAC, I would just use the original wiring for both bias and LV. This would be the best solution IMO.

Well I was feeding it the 60vac from a variac, don't think I have a 60vac transformer.

I do have a ~ 48vac trans which seems to power the solid state board up fine when I take the bias from one of the HV windings.

And this one is an older version with a slightly different power supply for the SS board.

[Helmholtz]

Ok, so just make sure the 47k series resistor is sufficiently rated. It will have to drop around 300VAC. Dissipation should be around 2W, I would use a 5W type.

To bring the SS DC supply voltage up with 48VAC input, it's best to increase the value of the 10k shunt resistor.

[drewl]

Thanks, that should ease up on the stress.