Epi Electar M 1260

[lowell]

Cool lil very very old combo amp. No power transformer, runs off 3 rectifiers off the mains, into a field coil for B+. There is a whole slew of things that COULD be wrong, but, wanna make sure I understand the fuse value correctly before replacing it and powering on. It came in with a blown fuse. The schematic has a fuse looking thing that says 135 on it. My guess is 135ma. But, seems like, with 9 tubes total, this is rather low.

http://prewaramps.org/media/EEM1260Schematic.JPG

Or standby I'm being lazy... I'll calculate it and report back if 135ma seems correct.

[Enzo]

nononononononono

Look up the tube, it draws over twice that current, it needs 300ma.
It is a 135 ohm resistor. Note the rectangular symbol is the same as all the other resistors.

25L6 draws 0.3A at 25v. Three in series is 75v. SO we have to drop 45v from 120v in that resistor. R = V/i = 45/0.3 = 150 ohms. I am guessing they used something other than 120v, so, V = IxR = 0.3 x 135 = 40.5v. And then 75 + 40.5 = 115.5. 115 was a common mains voltage amount. In my lifetime, it has gone through 110v, 115v, 117v, 120v. At least on schematics. If you replace it, I recommend going to 150 ohms for those reasons


Also look at that part, it is only in series with the three tubes, not all nine. See the hot wire branches, one side through the resistor and three tubes, the other side through the other six tubes. Both strings return to ground, as does the neutral side of the mains plug. (shudder).

Those resistors get hot, I calculate at 120v mains, 45v across the resistor of 150 ohms, 13.5 watts is dissipated. I find a lot of those are the rough surfaced sand covered flat wirewounds. Usually have the value printed on them in big numbers. Others used the big ceramic ones.

There were some old radios that had a two prong cord, but a three wire cord. There were the normal two conductors from the two prongs, and a third wire in the cord was resistance wire. It was connected to the hot side of the plug. So from the wall plug would be two hot wires and one neutral wire. The resistance wire had resistance along the length. It was used for dropping voltage for series heater strings. Yes, the cable ran warm. Usually had that woven fabric cover like an old waffle iron or something.

Picture of one:
https://www.radiolaguy.com/info/resi...e_%20cords.htm


Schematic doesn;t show a fuse by the way.

[lowell]

Thanks. Yeah I thought that didn't make much sense. There is indeed a fuse in here between neutral and chassis/ground.

Those resistors get hot, I calculate at 120v mains, 45v across the resistor of 150 ohms, 13.5 watts is dissipated. I find a lot of those are the rough surfaced sand covered flat wirewounds. Usually have the value printed on them in big numbers. Others used the big ceramic ones.

Correct it's one of those wire wounds with the adjustable ring

25Z5 = .3Ax25v = 7.5w x 4 = 30watts
25L6 = "same" = 30watts
12SN7 = 13v @ .3A = 3.9watts
6SQ7 = 6.3v @ .3A = 2watts

TOTAL FILAMENT POWER = ~66watts

Then considering capacitors, preamp plate watts, and audio power output I'd say we could add another 10watts ish. Datasheet says the 25L6 are 10watts but I doubt that's converted to actual audio power. But, should I call it that nonetheless? 40watts? I know this amp is no way in hell 40watts output, but is that how I should calculate for the fuse?

So considering my 10watt approximation:

66 + 10 = 76watts
76w/120v = P/V = I = 630ma

[Enzo]

Fuse doesn't care about the output, fuse only cares what you use from the wall. If it never makes a sound, those heaters still use power.

If you use 1 amp, you don;t want a 1 amp fuse in it, it will blow all the time. How about a 1A slow for now, once you have it fixed up, measure the current draw while it is running full out. That will give you an idea what it uses. I picked that because it was half again what you calculated your use to be.

[lowell]

Ok so cool got the fuse issue fixed for now.

I am only getting about 80vDC on the first B+ cap. I have already replaced the main filters. I also am getting about 60ma per 25L6 when using t-shunt method. I get no audio output...but feel I should probably be getting about 165vDC at that first cap.

[Enzo]

Not through those rectifier tubes you won't. Think about how many volts a typical rectifier shaves off a B+. If you installed silicon diodes s=instead of those tubes, then you'd get your 165-170v.

Have you measured those power tube cathode resistors? And have you compared your shunt readings to current calculated on those resistances and the voltage drops there? 60ma per tube is 240ma total, which would be a lot to pull from that power supply I would think. Still that is only 4-5 watts per tube at 80v.

0.006? Now there is an odd value.

What have you replaced in this thing? Your power stage is cathode bias, meaning there ought to be zero volts DC on their grids. Is that the case? Those 0.05uf coupling caps from the phase inverter are ancient, and could be leaky. That would drive up idle current.

[lowell]

Right, I just didn't think it'd be that much of a voltage drop. Forgot to mention that the cathodes are grounded, the 80ohm isn't in here. Pretty sure at some point someone was fiddling around in here.

0.006? Now there is an odd value.

Where is this number from?

Only have replaced the main filters, that's it. I did check the grids, 0v. But we can guess that the large current can be from the lack of the 80ohm cathode resistance.

All this said, I should get some sound don't you think? I tried injecting signal post-PI coupling caps, but got nothing. It was capacitively coupled signal via my smartphone so I figured there'd be enough signal there to drive those power tubes but maybe not.

[lowell]

Also, what is the Field Coil, and what is its purpose?

In the amp I have here, I just realized that the B+ (rectifier cathodes) goes to a filter cap , thru a 2-leaded transformer (choke? field coil?) to OT center tap and filter cap, thru another one of those 2-leaded transformers, to screen supply, then on to preamp supply nodes.

So, should that first one be rewired from B+ to ground?

Finally, there is a giant battery in this cabinet that I had initially thought was the Field Coil, but it's 2 4.5v battery stacked in series. I'm just now realizing that it's the negative bias supply as it comes in with + grounded and - thru 2 resistors to the 25L6 grids. There is no negative voltage there, so I'm guessing those had to be charged somehow...??

Finally, the 1st preamp tube only has 6 pins. There is a shielded wire coming out of the chassis to the TOP of the tube with a metal sleeve that can be pulled off the top of the tube. Guessing that's the grid....but, still that only makes 7 pins. Confused about that tube and its wiring. I believe it's a 6C6 tube.

[Jazz P Bass]

I would forget about the Field coil for now.
It's usually attached to the speaker anyway.

I wouldn't 'ground' the choke!

As to the bias, treat it as any push pull/ grounded cathode/ biased amp.
Feed it a negative voltage as in any Fender.

And, yes, the cap of a 6C6 tube is the control grid..

[lowell]

Ahhhh.... so THAT'S why there are 5 terminals on the speaker. And also why I got a little tingle when I touched it.

So I've heard "pre-rola" before, maybe on Ted Weber's site. Does this Rola speaker relate somehow?

[J M Fahey]

Surprised nobody said yet that that amp will KILL you.

1) One pin of the mains connector is connected to ground.
Just plug it backwards or into reverse wired outlet and you are memory.

2) in this case the "keep one hand in your pocket" safety rule does not apply, touching any HV circuit inside can kill you even with just one hand because you are already touching the other terminal through your feet.

Almost forgot: fusing the neutral is the worst decision ever.

[olddawg]

Before doing anything I would look at that ancient field coil speaker and see if the cone is intact. Then.. Make sure the voice coil isn't open. Then make sure the field coil isn't open. If you don't know how a field coil speaker works, stop and google it. There is no permanent magnet. The magnet is an electromagnet energized by the field coil. Usually the field coil is also used as a choke in the power supply. Otherwise you may just be pissing in the wind. Consider using an isolation transformer while working on this thing.

[Enzo]

The batteries are charged by replacing them, just like the ones in your fuzz pedal or a flashlight. Since those grids draw no current, the battery life is long.

Well of course if there is no bias voltage from either the grid supply or a cathode resistor, the power tubes will conduct heavily.

[lowell]

Thanks all. Yes definitely putting my new iso transformer to use.

I grounded the grid leaks and added a 10watt 94ohm (2 5w 47ohm in series) cathode resistor. All 4 25L6 on one cathode resistor. At least there is a decent bias now, I may separate that into 2 cathode resistors after a few calculations in regards to bias and gain. I think there is probably less gain with this arrangement being that there's some negative feedback interaction on the cathodes.

I do get hum, and some guitar now, if I crank the volume and strum. There is a 400v capacitor between Neutral and Chassis. So the chassis isn't really neutral... but definitely COULD be if that cap failed. I removed it thinking it's like a death cap, but there was a massive hum, so I put it back. I now have about 90vDC on the B+. Now onto checking voltages in the preamp... pretty sure I got at least one open or extremely large value-drifted resistor. I'll report back on my progress. I will definitely encourage the customer to pay for unity ratio power transformer and add a 3-prong cord.

About that neutral/chassis capacitor. Why/how does that cure the hum? Is curing hum its intended purpose?

[Enzo]

It provides a signal ground path, when it is gone there is no longer a "ground".