(HIGH) HIGH VOLTAGE ON PT

[Marcomel79]

Hey all, ive got a set of transformers plus choke from an old PA. It was "modified" into a guitar amp, but i beg to differ. Since i couldnt find a schematic for it and the box had been painted black, with no model number or any other info, there was no way i could fix it. Anyway, it has two EL5 in the power section and a couple EBC3 plus an EF40 in the preamp. The rectifier is an EZ4.

I measured the HT and its 385-0-385. What can i build with a PT with such high HT? I welcome all suggestions, insults and sarcastic comments!

 

[PelliX]

A single ended KT150 monster Walking Disaster! A space heater!

 

[Marcomel79]

A single ended KT150 monster Walking Disaster! A space heater!

I would like to use the EL5s and rectifier if possible...

 

[Pete Farrington]

ive got a set of transformers plus choke from an old PA

The rectifier is an EZ4.

I measured the HT and its 385-0-385

With the EZ4, the HT looks like it should be quite saggy. I guess that the 770V is measured with the winding completely unloaded?
What’s the end to end resistance of the HT winding?
What’s the OT ratio?
Is the choke as big as the OT?

https://tubedata.altanatubes.com.br/sheets/046/e/EZ4.pdf

https://tubedata.altanatubes.com.br/sheets/046/e/EL5.pdf

 

[Marcomel79]

@Pete Farrington thank you for chiming in. Heres a picture of the OT and choke for comparison.

The OT has a winding ratio of 5:1. Very strange. The choke measured at 200ish Ohms DC resistance.
The only markings on the choke are these:

And yes, the 385-0-385 was measured unloaded

 

[Pete Farrington]

And the end to end resistance of the HT winding?

 

[Marcomel79]

And the end to end resistance of the HT winding?

And the end to end resistance of the HT winding?

Sorry that escaped me. Ill measure it tmw. I did test the B+ unloaded: 438VDC....

 

[Pete Farrington]

Sorry that escaped me. Ill measure it tmw. I did test the B+ unloaded: 438VDC....

That seems lower than I expected, is a reservoir cap connected?

 

[mickeydg5]

And if so what is the value of the first reservoir capacitor?

 

[Marcomel79]

That seems lower than I expected, is a reservoir cap connected?

Hey @Pete Farrington, no filter cap, i measured straight on the cathode of the ez4. All the eletrolytics were dead. Also the rectifier was totally out of circuit

 

[mickeydg5]

Hey @Pete Farrington, no filter cap, i measured straight on the cathode of the ez4. All the eletrolytics were dead. Also the rectifier was totally out of circuit

By out of circuit, did you mean the rest of the amplifier is not connected to it? Nothing, no capacitors or anything, nothing?

 

[Marcomel79]

By out of circuit, did you mean the rest of the amplifier is not connected to it? Nothing, no capacitors or anything, nothing?

Yes, nothing connected to the B+. It was a mess and all the ecaps were dead. And the other components were way out of spec...plus it had been modified by someone already, so no way of knowing what had been done...
This is how it looked when i got it

And a fun pic of one of the tubes

 

[Pete Farrington]

Nice!
It would be best for the HT caps to be 550V. The normal voltage on them will be much lower than that, but it’s just in case you’re ever in the scenario of powering up with some or all other valves missing, eg when testing, or otherwise not drawing HT current.
AskJanFirst sell 550V / 600V surge F&T axial etc ecaps http://www.askjanfirst.com/eindex.htm?/hello.htm

 

[Marcomel79]

Nice!
It would be best for the HT caps to be 550V. The normal voltage on them will be much lower than that, but it’s just in case you’re ever in the scenario of powering up with some or all other valves missing, eg when testing, or otherwise not drawing HT current.
AskJanFirst sell 550V / 600V surge F&T axial etc ecaps http://www.askjanfirst.com/eindex.htm?/hello.htm

Ok so the OT has three secondary wires. With the full winding i read 16.7Ohm, while with the other two combinations i get 9.3 and 7.6 respectivly. So the winding ratio is 2.7:1 for the full winding and 5.4 for each of the two half windings. This doesnt make sense for a guitar speaker... am i missing something?

 

[Pete Farrington]

That kinda ratio seems more appropriate as a 100V or 70V line driver, for a shop or factory etc PA / paging system.
Sorry but it looks like a more suitable OT will be needed for use with guitar speaker cabs.

 

[Chris-in-LA]

Is that an old Nazi tube?

 

[Marcomel79]

That kinda ratio seems more appropriate as a 100V or 70V line driver, for a shop or factory etc PA / paging system.
Sorry but it looks like a more suitable OT will be needed for use with guitar speaker cabs.

Thats what i thought.... so then what are my option for using that PT? The ht seems too high for a plexi style circuit....
The HT winding resistance is 353ohms

Is that an old Nazi tube?

It looks that way....

 

[PelliX]

Is that an old Nazi tube?

Nah, man - it's from one of the first computers they used to program Wolfenstein 3D.

 

[Marcomel79]

PROBLEM SOLVED!!:
The two thin wires that were joined into the CT snapped way inside the windings. I tried excavating to no avail. One of rhe wires has already been fixed by somebody else. Now ive got a cool vintage door stopper...

 

[Marcomel79]

You win some, lose some....