I haven’t read in detail, bu ti would consider this a PP EL34/6CA7 with 4 9 pin sockets, suitable for 8 triodes. Lots of space for potentiometers. 4 inputs. What ar ethe speake outputs?
2 EZ81 are going to be a bridge rectifier, if you want to use SS rectifiers that gives 2 7 pin sockets.
Gut it and build something around those bits.
dave
2 EZ81 are going to be a bridge rectifier, if you want to use SS rectifiers that gives 2 7 pin sockets.
Gut it and build something around those bits.
dave
It is a struggle with these old PA amplifiers. I have an 'affinity' for them too, with a few examples finding their way into my home via auctions. Often the chassis is a lovely piece of retro metal work, and the amps themselves were bespoke creations from fairly small scale electronic equipment suppliers. Here are two examples attached.
The one above I tried to draw the schematic, which looked deceptively simple, and removed a piece of paxolin with lots of dogs bone resistors and old capacitors, but somehow even with photos and labels I lost the ability to be sure how the tone control was connected, and also how the local feedback around the output tube had been working. It is easy to underestimate the creativity of the people who worked with this equipment in the old days. I did manage to get it working again and it looked quite nice on the oscilloscope. I have a plan that it will be a headphone amp one day, keeping the ECL11 output tubes.
Another one, untouched and probably going to stay that way.
In the case of this amp from the OP, it is in a bit of a forlorn state without its grill and the damaged controls, and also it is a fairly cluttered selection of controls, so that constrains the possibilities a bit. For the sake of two OPTs you have most the bits for an SE amplifier. On the negative side if you are going to invest a lot of time in a project, make-do-and-mend is not an option in my books, adding to the difficult decisions!
The output tubes are going to be high quality and valuable if they have not seen too much action. I restored a Dynacord guitar amp from the 60s, added JJ EL34s on the outputs, and kept the Telefunken EL34s that were in good nick for a Tubelab SSE.
It is not an easy decision!
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Well some good news... I have the grill, Hector! I also have replacement up nos Bulgin chicken head dials and matching escutcheons to go on.
I'll replace these and give it a clean but other than that I want it to retain its original aesthetic as much as.possible. I quite like old stuff to look aged.
I'll replace these and give it a clean but other than that I want it to retain its original aesthetic as much as.possible. I quite like old stuff to look aged.
Spent a bit more time drawing stuff out, really helping me to get my head around it. Got as far as the power supply and output valves. Will do the input stages and tone control next. I can't use LT spice so I'm trying to do it with pen and paper which is also my first time doing it so it's not easy. Trying to lay things out so that it doesn't look like spaghetti!
The 12v battery supple and it's switching are still confusing me slightly. I've drawn out how I think the power supply works. For the 240vAC it looks like live and neutral is fed into the 10 pin input connector to a double pole switch (marked mains 'mains on' (not on/off?). From there the live appears to go to pin 7 on the primary side MT (bottom pin on my drawing).
Then it seems to have different voltage taps (grey green yellow) which then feed into a screw post selector/switch. The other end of this is wired to the other side of the 'mains on switch'. That would appear to complete the circuit for the primary.
However there's also a blue and a brown wire on pins 1 and 2 if the primary. The brown wire comes straight off one of the 10 input socket pins onto the MT primary (pin 2).
I can trace the blue wire in pin 1 of the NT primary to a re-wire type fuse, it then goes to bottom of the other switch marked 'battery - standby'. The top of that switch goes to another of the 10 pin input socket.
So these would appear to be supplying the 12v which can power the amp via a battery. I'm not sure why these are connected to the mains transformer though? Or exactly how this part works to be honest.... something to do with standby?
There is another wire coming straight off the 10 pin input socket that connects straight to one of the 12v 'vibrator' pins. This also has a wire going to the MT 12v secondary pin.
What is the 'standby' for? That might help me understand better.
I also have a question about large a capacitor in the question which has 3 terminals. 2 top and 1 bottom (which is connected to ground). It can be seen bellow. Why does this have 3 terminals?
Thanks
The 12v battery supple and it's switching are still confusing me slightly. I've drawn out how I think the power supply works. For the 240vAC it looks like live and neutral is fed into the 10 pin input connector to a double pole switch (marked mains 'mains on' (not on/off?). From there the live appears to go to pin 7 on the primary side MT (bottom pin on my drawing).
Then it seems to have different voltage taps (grey green yellow) which then feed into a screw post selector/switch. The other end of this is wired to the other side of the 'mains on switch'. That would appear to complete the circuit for the primary.
However there's also a blue and a brown wire on pins 1 and 2 if the primary. The brown wire comes straight off one of the 10 input socket pins onto the MT primary (pin 2).
I can trace the blue wire in pin 1 of the NT primary to a re-wire type fuse, it then goes to bottom of the other switch marked 'battery - standby'. The top of that switch goes to another of the 10 pin input socket.
So these would appear to be supplying the 12v which can power the amp via a battery. I'm not sure why these are connected to the mains transformer though? Or exactly how this part works to be honest.... something to do with standby?
There is another wire coming straight off the 10 pin input socket that connects straight to one of the 12v 'vibrator' pins. This also has a wire going to the MT 12v secondary pin.
What is the 'standby' for? That might help me understand better.
I also have a question about large a capacitor in the question which has 3 terminals. 2 top and 1 bottom (which is connected to ground). It can be seen bellow. Why does this have 3 terminals?
Thanks
It's two capacitors in one package. The common terminal is the ground, as you said. Reasonably common in old gear. You can still get them if you hunt around.
Cheers, and regards,
Ant.
A also need info on these
I'm guessing grid resistors, but what type? The 2 mic input jacks positives feed into them before they go into the input stage valves control grids.
The 2 wires you see going in are actually 2 pairs of wire in one sheathing. The pos and ground from the jack input goes in and then comes back out in the other wire where the live goes to the grid. They're located away from the valve hence the wires.
I'm guessing grid resistors, but what type? The 2 mic input jacks positives feed into them before they go into the input stage valves control grids.
The 2 wires you see going in are actually 2 pairs of wire in one sheathing. The pos and ground from the jack input goes in and then comes back out in the other wire where the live goes to the grid. They're located away from the valve hence the wires.
Just two wires? I was thinking choke. Does it come apart?
Edit: 2 pairs of wires, then step-up transformers for the Mic input is most likely.
Edit: 2 pairs of wires, then step-up transformers for the Mic input is most likely.
Are those '2 wires' actually each a shielded cable? Be careful if you try and test that type of transformer to only apply low AC voltage - eg. for a turns ratio test, and perhaps best to only apply voltage to the 'high resistance' winding (say 0.5V), on the assumption that the high resistance winding connects to the input grid of a valve, and the lower resistance winding is for say a 50 to 200'ish type of microphone impedance and should only be energised with mV. For a high turns ratio transformer, the input voltage will be quite low and need a sensitive meter to read.
Vibrator circuits can be confusing. Links to indicate possible schematics:
https://dalmura.com.au/static/AWA Amp.pdf
https://dalmura.com.au/static/PA829 schematic.pdf
During restoration of amps with vibrator dc powering I have typically removed/disabled the vibrator related powering, partly due to electrical safety with mains ac on connector sockets, and partly due to not wanting to use that form of powering.
Vibrator circuits can be confusing. Links to indicate possible schematics:
https://dalmura.com.au/static/AWA Amp.pdf
https://dalmura.com.au/static/PA829 schematic.pdf
During restoration of amps with vibrator dc powering I have typically removed/disabled the vibrator related powering, partly due to electrical safety with mains ac on connector sockets, and partly due to not wanting to use that form of powering.
here's a shot in the dark-
Brimar 25P1 (this has 6br7, 12ax7)- British Company, like the amp in question.
5B/255 (like a 807)- place the el34 in it's place, maybe these schematics could help.
http://www.r-type.org/articles/art-101.htm
Brimar 25P1 (this has 6br7, 12ax7)- British Company, like the amp in question.
5B/255 (like a 807)- place the el34 in it's place, maybe these schematics could help.
http://www.r-type.org/articles/art-101.htm
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I think the limitation is the power transformer which as designed can supply just two power tubes. Maybe they are run at a very high level so there is some spare capacity. The only way to use these amps that keeps their provenance is for PA duty, e.g. guitar amplification, or if you are lucky enough to have two identical ones, then monoblocks in HiFi. Even then you might have to contend with 100V or 70V line out OPTs.
They can be a bit of a conundrum!
They can be a bit of a conundrum!
Sorry, but no. One never can build a bridge rectifier with only two EZ81's. But it isn't mandatory at all, 'cause the PT supposedly features a CT'ed secondary winding. I'd keep these two way diodes.
Best regards!
What is it? Philips EL6415 or EL6425? I've got two of the latter. Much more inconvenient output impedances, btw.
Best regards!
Best regards!
I thought 2x ECL34 in PP should give plenty of power though?
Strategy... I'm learning a lot atm just from drawing it out and making sense of it all.
I think I'll try and get it up and running as is ater replacing the capacitors. At this stage I have no idea if the valves are even useable so I think this is a good starting point.
If it gets to a stage where it's actually working, I'll take it from there. Maybe look at making some modifications. I don't need the 2 gramophone stages so that might give more room for adding stuff. I'd also have the 2 pots for other uses.
I'd like gain control, and I'd love to add a tremolo circuit. Reverb would be nice as well. There's plenty of space in there and I'd have the gram pots to use to control other stuff. I could feasibly add extra pots on the top half as I have the knobs and escutcheons.
I was thinking about rewiring one of the inputs like the classic fender high/low gain, maybe switched.
Next logical step after I've traced all the circuits out is to test the transformers are working.
Yes, only excite with a low voltage - and use at least 1k in series for safety. Use something that can’t accidentally drive it too hard, like a proper audio sig-gen or computer’s headphone out. Ruining it would be an expensive mistake - these things start at about $100 and only go UP from there. Most of us don’t have really good AC mV meters - but a dual trace scope works fine. Compare the voltage across both windings. Another method is to use a 2-channel amplifier with the “Gain“ controls at least loosely calibrated in dB. Even cheap PA amps have the front panel level controls in dB. The dB that you need to turn one side DOWN to get them to match will give you enough information to calculate the turns ratio. You can turn the level up to where your DMM will at least give a clear reading. Frequency response errors in the meter cancel, but use two identical if you have them so you don’t have to swap. You don’t even need speakers attached. A 1kHz tone at 100 dB can be loud. If you run out of range, make a 10:1 resistor divider at the higher input and add 20 dB…..
Are we talking about the microphone transformers here? In not sure I'll even need these, I won't be using mics with it