Heater wiring question

[peacocksuit]

Greetings. I'm a relative newcomer to amp repair looking for a little help understanding the heater wiring arrangements for an old amplifier I'm working. It has 2 EL34's in pp, rectified by 2 EZ81's, input stages are ECC81, ECC82 and a pair of 6BR7's.

I've done the requisite safety upgrades and replaced old hunts electrolytic's etc.

Heaters for all tubes are powered from AC. Offload voltage checks and resistance measurements at the MT seem to confirm a CT 12v winding.

Each pair of tubes (expect the 6BR7's) appear to be wired in series, the MT feeding to the first power tube at pin 2 with a connection between pin 7 of this tube and pin 2 of the next. There are 3 lines also wired off pin 2 of the first power tube which supply the rectifiers and 2 pairs of input stage tubes, each pair in turn feeds back to a grounded star point on the chassis. The CT also connects to ground. The line which feeds the 6BR7's splits off into 2 dropper resistors, giving each tube its own supply. Thes outgoing pins feed back to the same star ground.

 The other 6v MT however is not connected to anything, and this is what I can't fathom.

How each tube gets 6v when the other 6v half of the CT MT heater winding is N/C.
For 2 tubes with 6v (6.3v) heaters to be wired series, don't they'd need 12v?

Hoping someone can help me understand this!

[sluckey]

Schematic?

[peacocksuit]

No schematics available anywhere unfortunately. It was made in relatively  small numbers by a company in the UK who are ling gone. I've had to attempt drawing my own sketch. This is how it appears to me. This is my first attempt at this so it's just rough, old school pencil and paper.



It's a PA amp which could optionally run off 12v battery power. For this it used a vibrator can. This is not included in the drawing. A lot of the wiring went to a really old fashioned 10 pin power socket, the different pins would have corresponded to the pins on the battery input or AC mains plugs.
Pin 7 of the first EL34 had a wire going to this socket as did one of the vibrator socket.

My plan is to get it up and running and then use it as a guitar amp, adapting and modifying as necessary.

Thanks.

[sluckey]

Just relabel the PT like this and everything works as it should.

[kagliostro]

Ciao peacocksuit

Some years ago I was asking if it will be possible to feed a pair of rectifiers tubes like EZ80 - EZ81 with in series filaments

Your drawing show the first example I see of a commercial amp that use that circuitry plan

Thanks for sharing

Franco

[peacocksuit]

Just relabel the PT like this and everything works as it should.

Here's the thing, I'm not sure I have a 12v CT winding as I first thought. I can't find any combination of secondary windings which gives me 12v. If each pair of valves is wired in series, it would need 12v? I'd initially assumed it would have a 12v CT winding 6v-0v-6v. However I did some offload measurements of both voltage and resistance on the second MT pins, I've added the results as attachments,  I'm not sure how they appear once I hit post as I'm not used to this forum.

It seems as though I'm getting 6.3v between MT secondary pin 5 and 7, but 6.3v would be be no use to this heater circuit with series wired pairs of valves?
Doesn't seem to make any sense to me.

[kagliostro]

Which is the voltage you measure between the point indicated by the red arrow and ground ?



Franco

[sluckey]

You have drawn that PT (you call it MT) wrong. There are two secondary windings, not a single winding as you have shown. The only reason the 12V winding shows resistance to the 350v winding is because each winding has one lead connected to ground.

Just measure the filament voltage ***BETWEEN*** pins 2 and 7 of each EL34. One probe on pin 2, the other probe on pin 7.

[JPK]

I'd be interested in answers to posts 6 and 7.

[peacocksuit]

Which is the voltage you measure between the point indicated by the red arrow and ground ?



Franco

4.2V AC

[peacocksuit]

You have drawn that PT (you call it MT) wrong. There are two secondary windings, not a single winding as you have shown. The only reason the 12V winding shows resistance to the 350v winding is because each winding has one lead connected to ground.

Just measure the filament voltage ***BETWEEN*** pins 2 and 7 of each EL34. One probe on pin 2, the other probe on pin 7.

Non of the pins on the mains transformer secondary give me 12v in any combination.

There is 6.3(ish) volts between pin 5 and 7.
Between 6 and 7 I get 5.9v, 5 and 6 I get 0.3.

If I connect 5 to the pin 2 of the first EL34 and pin 7 to ground and measure the voltage between pin 5 and ground, the I get around 4.2v.

Between EL34 socket pin 2 and 7 I'm getting around 2v on each valve.

   

[kagliostro]

If you measure between pin #2 and #7 you are measuring the voltage drop at the extremities of a resistor (the heater of the tube)

-----

At this point I'll disconnect all the tube heaters from the heater windings and also the heater winding from ground and measure the unloaded voltage you have at the heaters windings

Also, with the windings disconnected from ground,  verify if there is continuity between the windings and ground (if there will be continuity it means your PT is blown)

Franco