does this technique work to increase the voltage of a secondary winding?

[capacitorless]

I've got a tube guitar amp that I would like more B+ out of (to use a different power tube), and without changing the power transformer. I'm definitely not ruling that out, but want to use what I have if practical. The other secondary voltages should remain the same: 2 different filament supplies, and 5vac for a rectifier. I can't use the supplied alternate 100v primary winding (not shown) without side effects - grrr.

In a nutshell, I know that a Zener diode in the center tap circuit can be used to decrease B+, and a standby switch can be wired in the center tap like it is in the attached schematic.

My question is simply: could I add (say) 30v dc to the center tap, and thusly bring up the rectified voltage by the same amount? Basically the secondary would be 30vdc plus 265vac riding on top, if my thinking is correct.

I've not seen this done before. I would initially use a bench supply, then something like a SMPS (reliability concerns aside) if I wanted more permanence. Any takers?

 

[abbey road d enfer]

My question is simply: could I add (say) 30v dc to the center tap, and thusly bring up the rectified voltage by the same amount? Basically the secondary would be 30vdc plus 265vac riding on top, if my thinking is correct.

Why not? Just make sure the rectifiers and caps can handle the increased voltage.
BTW, tehre is no "attached schematic".

 

[capacitorless]

There is now (forgot to press Save earlier when adding it)!

In other words, given that the related components are specified properly, this is a valid technique to raise B+, then? It seemed that way but I'm in the learning curve.

 

[abbey road d enfer]

In other words, given that the related components are specified properly, this is a valid technique to raise B+, then? It seemed that way but I'm in the learning curve.

Please note that the voltage concerns also include the transformer windings.

 

[capacitorless]

And current draw but I've checked and I'm well within ratings in that regards. Thanks.

 

[JohnRoberts]

I've got a tube guitar amp that I would like more B+ out of (to use a different power tube), and without changing the power transformer. I'm definitely not ruling that out, but want to use what I have if practical. The other secondary voltages should remain the same: 2 different filament supplies, and 5vac for a rectifier. I can't use the supplied alternate 100v primary winding (not shown) without side effects - grrr.

In a nutshell, I know that a Zener diode in the center tap circuit can be used to decrease B+, and a standby switch can be wired in the center tap like it is in the attached schematic.

My question is simply: could I add (say) 30v dc to the center tap, and thusly bring up the rectified voltage by the same amount? Basically the secondary would be 30vdc plus 265vac riding on top, if my thinking is correct.

I've not seen this done before. I would initially use a bench supply, then something like a SMPS (reliability concerns aside) if I wanted more permanence. Any takers?

Yes in principle but the voltage source applied to the center tap may need to source and sink current.

JR

 

[capacitorless]

That's something I was wondering - whether it would need to sink current, as well as source it. I may be better off swapping out the power transformer for something more appropriate. I decided to not use a tube rectifier, so I don't need the 5vdc/2a winding, and if I'm going to provide the ability to run octal preamp tubes (via an adapter) I need to bring those filaments down to 6-6.3v. I did some planning along those lines last night and it's doable without a ton of grief.

The achilles heel of the Black Pearl amp could be the power transformer. Certain runs of them - mainly the chassis intended for the 1x12 model - had PT issues, notably with the HV/B+ winding. This one has been fine, but all it takes is the proverbial straw and poof, no more B+.

So that means this working power transformer (from a 2x12 model) has a certain amount of value. I think I'll not risk things with experimentation. Appreciate the feedback from both of you.

 

[abbey road d enfer]

That's something I was wondering - whether it would need to sink current, as well as source it.

No, current in the center tap is always of the same polarity. that would not be the case if the supply was bipolar (delivering both + and - voltages).

 

[capacitorless]

I figured that the technique of using a Zener to lower B+ implied that was the case. Thank you. I may experiment with the concept later, but for now I'm leaving this transformer alone, and planning on using another. I have a modest pile of old organ iron to play with and might adapt one of those. A cool thing about this chassis is that there's a good amount of room for both power and output transformer swaps.

The factory PT's leads terminate in multiple multi-pin plugs that connect to the power supply/output section board, so it's going to be valuable to the Black Pearl crowd, and I'll eventually get that to someone with a dead BP. I'll post a couple of photos.

 

[capacitorless]

Not all of these are from the power transformer; one goes to the choke, another to the fan switch, etc.

Very different from a PTP build, hahaha!

 

[Mr. Moscode]

You want another 30V on the B+? Put a 24 volt 1 amp power transformer with a bridge rectifier (no cap) in series with the standby switch. It will raise the B+ approx 33 volts to 400V. You will need to change C118 & 119 to 450 volts. Phase will not matter if the xfmer has 1 winding. Cheap and dirty yet clean.

Toroids are lighter and mount with 1 screw. A 24va with 2 primaries and 2 secondaries can keep your amp switchable to 220 or 110. Here primaries and secondaries need to be correctly phased.

You can get one with 2 24 volt 500ma windings and put them in parallel or 2 12v 1A windings and put them in series, properly phased. Xfmer Output into the bridge rectifier, rectifier output: (-) to the switch to ground, (+) to main PT center tap.

I would take the B+ to the output transformer off C118/119 and let the 5AR4 supply the rest of the amp. The 5AR4 will last a very long time and the amp will have more punch. Should be tuby-ish punchier. Unless you want the voltage drop of the 5AR4 in series with the output stage. You could put in a SPDT switch to pick off the power where you want it.

Have fun!

 

[capacitorless]

Appreciate all this from everyone. Upon further reflection, I think I'm going to simplify things for myself, and use the existing power transformer, B+ and voltages as they are for now.

I can use one of the old organ chassis to experiment with the power amp configuration, and I already have another one specifically set aside to do a "desktop" SLO preamp (only). I'll do a voltage multiplier with a VVR to dial in voltages on the preamp, and choose a power amp chassis separately.

 

[CJ]

getting rid of the tube rect would give you 30 volts

 

[capacitorless]

Thanks, CJ! I measured an actual 9v drop the way this circuit is configured. I'm guessing it's not drawing as much current as typical for what a GZ34/5AR4 supplies (meaning it's a quad of el84s, cathode biased, at 330v input). But that's just a semi-educated guess

I'll measure the actual current drawn without the rectifier in place here soon.