Another Over-voltage problem...got any ideas?

[Amp Mad Scientist]

OK so we have these Leslie 22 power amplifiers (a crap load of them).
And the filter can caps are rated 475 volts...
and the output tubes are 6550 or KT 88, with the usual 6.3 volt filament.

So if I run these things on 110 VAC the filament voltage is 6.3 volts.
BUT If I run these things on 120 volts AC, the filament voltage is way over 7 volts.

And on 110 VAC, the turn-on Plate DC voltage is just within the filter cap rating of 475 DC. or a tiny bit over.
As the tubes warm up, the power supply is loaded, the DC voltage drops to about 400 volts.

BUT if the power is 120 Volts AC, the turn on voltage is about 550 volts DC, which over voltages the 475 volt filter caps...at least for a few seconds, before the tubes warm up and the voltage drops.

So this is the dilemma: power is 120 volts:
Every time the amp turns on the filter caps are 75 volts over voltage temporally...and will fail because of that sooner or later.
And every time the amp is turned on, the 6.3 volt filament is well over 7 volts continuously. And the tubes will fail because of that.

So what you gonna do to reduce the line voltage?
Looking for the most cost effective solution, that is church friendly. Yes these are church organs.

 

[67plexi]

Does the power transformer have a center tap if so you could use a 50 watt resistor from CT to ground
if no CT replace power transformer or if current draw is under 2.5 amps

Vintage Sound Workbench

Hand machined and assembled in USA

vintagesoundworkbench.com

 

[CoyotesGator]

Time delay the plate V.

 

[Mitch Pearrow SJMP]

I couldn’t tell ya brother

 

[Amp Mad Scientist]

Does the power transformer have a center tap if so you could use a 50 watt resistor from CT to ground
if no CT replace power transformer or if current draw is under 2.5 amps

Vintage Sound Workbench

Hand machined and assembled in USA

vintagesoundworkbench.com

View attachment 89062

Thanks for that.
Although this voltage control is "experimental" according to the vintage sound website...it handles 2.5 amps.
I'm thinking a B3 draws more power than 2.5 amps.
The power amp alone has a 2 amp fuse, and the rest of the organ power consumption is an unknown at this moment.

I'm thinking a small boost / buck transformer to reduce the voltage 8.5% or a 10 amp variac...

But one thing for sure, 120 volts is way too much.
I hate to run brand new KT88s with 7 volt (or higher) filament power seems like it will be destroyed.

 

[Amp Mad Scientist]

Time delay the plate V.

I never thought of that.

 

[Amp Mad Scientist]

I couldn’t tell ya brother

Would you be our state senator? We need a new one.

 

[syscokid]

In-rush current limiter for a soft start?
Like so:

 

[Don O]

2000w, $63 plus tax.

 

[syscokid]

Lowering heater voltage: I tried the following method and it works well:

Amp Mods

Generic Guitar Tube Amplifier Modifications

robrobinette.com

 

[Inspector #20]

Lowering heater voltage: I tried the following method and it works well:

Amp Mods

Generic Guitar Tube Amplifier Modifications

robrobinette.com

Interesting!!!!!

 

[67plexi]

Hammond does make filament transformers from 4 amps to 16 amps

https://www.amplifiedparts.com/products/transformer-hammond-low-voltage-filament-enclosed-63-vct

 

[CoyotesGator]

What is the unloaded heater V?

 

[Inspector #20]

@syscokid - What transformers did we use on the 2204 Ivanberg???

 

[SG John]

How about one of these?

AmpRx

 

[syscokid]

@syscokid - What transformers did we use on the 2204 Ivanberg???

Marstrans! Worth the wait.

 

[Amp Mad Scientist]

What is the unloaded heater V?

8 volts

 

[Amp Mad Scientist]

In-rush current limiter for a soft start?
Like so:
View attachment 89067

Good idea! Yes that was my first thought.
It solves the capacitor over voltage on start-up.
But it doesn't do anything for the 7.5 volt filament voltage.

 

[Amp Mad Scientist]

How about one of these?

AmpRx

That's not bad. But I need 8.5% reduction of 120V.

 

[Amp Mad Scientist]

I concluded that the most cost effective/ easily obtained voltage reduction was the cheap variac.
(It's a bucking adjustable autoformer) a one winding transformer.

Yes the filament can be lowered if the filament winding has a center tap...any resistor / zener is going to get hot
But the Leslie amp has no center tap and one leg of the filament is ground.

The Varistor drops voltage when the power first turns on. Then when the amp warms up voltage comes back to normal.
I think that good for protecting the filter caps from over voltage.
I have seen what happens when caps are temporarily over voltage repeatedly and they do short out I know it do.

The fancy voltage controllers are most impressive but the max is 5 amps. And it does cost more than a variac which is a subjective thing if there is money.
Another question (at the moment) is how much current the B3 and the Leslie are drawing? (the power of the organ and amplifier are combined.)
The organ and Leslie might draw less than 5 amps...