Snubber caps in power supplies

[Fischerman]

The amp that was the original inspiration for the B+/2 screen supply was the Ceriatone JTM100 JH West Coast mod. There they switch the screens ON with the SB. But in another thread someone pointed out that V2 would have high voltage on it in SB and stress V2.

So I'm trying to avoid that issue. I do have a DPDT SB switch.

[Helmholtz]

It just occurred to me that taking the screen voltage from the point between the stacked reservoir caps won't work.
The caps alone can't supply any DC current.
Assuming 56k balancing resistors, even a screen current of 5mA would drop the voltage by 140V, as the source resistance at that point would be 28k.
At full output total screen current will be much larger than 5mA.

[Roe]

this high-voltage JTM100 amp I'm building will have the screen supply taken from the middle of that main filter stack (i.e. - B+/2).

are you planning 560v on the plates and 280v on the screens? You may need 300v on the screens for full effect but 280v may be close enough.

The .047uf works ok here but is hardly necessary. But the screen supply won't work (w/o PT CT like you see on later Marshalls) since the current draw of the screens will pull down the voltage considerably despite the 56ks.

As an alternative consider a FULL WAVE Choke Input Load for the screens (I'm using this on a jtm45), see https://www.hammfg.com/files/products/7 ... -guide.pdf

[Roe]

The amp that was the original inspiration for the B+/2 screen supply was the Ceriatone JTM100 JH West Coast mod. There they switch the screens ON with the SB. But in another thread someone pointed out that V2 would have high voltage on it in SB and stress V2.

So I'm trying to avoid that issue. I do have a DPDT SB switch.

yes but this requires an PT with CT and a FULL WAVE BRIDGE Capacitor Input Load

[flemingmras]

You could place the standby switch between the screen supply and the screen bus.

[flemingmras]

It just occurred to me that taking the screen voltage from the point between the stacked reservoir caps won't work.
The caps alone can't supply any DC current.

Legit question...what do you think is the current source in an amplifier?

[Roe]

You could place the standby switch between the screen supply and the screen bus.

this works, but the screen supply has other issues here

[Helmholtz]

Legit question...what do you think is the current source in an amplifier?

In circuit analysis a circuit part that delivers a current is called a current source.
Other circuit parts connected to the output port of the source constitute the corresponding load.

[flemingmras]

In circuit analysis a circuit part that delivers a current is called a current source.
Other circuit parts connected to the output port of the source constitute the corresponding load.

I was looking for a more specific answer based on a statement you made.

The caps alone can't supply any DC current.

They absolutely can, and do! The amp is actually drawing from the filter capacitors. The power transformer continuously replenishes the charge as the amp draws from the capacitors.

Now...I would put a choke and a 2nd filter stage proceeding the FWB doubler center tap. But you can absolutely do it the way you were wanting to do it.

A single EL34 draws about 20mA screen current at full power out. At 400V, that's right at the 8 Watt screen dissipation limit. If you're doing a 4 tube amp, that'll be a max load of 40mA at that node.

[jbrew73]

It is my understanding that the op is not using a full wave bridge in this amp.

[Fischerman]

The amp that was the original inspiration for the B+/2 screen supply was the Ceriatone JTM100 JH West Coast mod. There they switch the screens ON with the SB. But in another thread someone pointed out that V2 would have high voltage on it in SB and stress V2.

So I'm trying to avoid that issue. I do have a DPDT SB switch.

yes but this requires an PT with CT and a FULL WAVE BRIDGE Capacitor Input Load

So this would not work? Sorry for sloppy labeling/etc.

[Helmholtz]

So this would not work?

As said, that won't work.
The screen current will significantly pull down the voltage across the lower cap (while the voltage across the upper cap will increase by the same amount, likely exceeding its rated voltage).
It's way different from a FWB circuit with the CT connected between the caps.

I like Roe's solution.

[Fischerman]

So this would not work?

As said, that won't work.
The screen current will significantly pull down the voltage across the lower cap (while the voltage across the upper cap will increase by the same amount, likely exceeding its rated voltage).

It's way different from a FWB circuit with the CT connected between the caps.

Thanks! I was originally going to just add an extra 'node' in the power supply for lower screen voltage but was advised against that earlier itt. But that advice was probably in the context of a FW Bridge rectifier.

I'm really just trying to make a high voltage (560-ish vdc) JTM100 with 6550s that won't blow up or eat tubes. The transformers are built. The extra node seems like the easiest path now.

[jbrew73]

https://ampgarage.com/forum/viewtopic.php?t=38408

See this thread for ideas.

[flemingmras]

Yeah the FWB doubler would be a far better candidate for that sort of thing. That's what I was under the impression you were doing.