Epiphone Electar Century AC-DC 1940

[Apexelectric]

After picking this amp up as a fixer-upper on Reverb I’ve come to realize that there is not much readily available info on it and it took a while to even find the schematic that I eventually stumbled upon. The amp is in great shape for its age and a cool piece of history. Originally designed by Nat Daniels, founder of the future company, Danelectro.

Fortunately, the one I have is mostly original except for the filter capacitors and the three prong cord. One of the original filters is still present but only there for show. This amp is interesting in design since it was originally designed to run off either 110V AC or 110V DC wall voltage. Hard to envision having to even consider that when buying electric appliances. So the heaters are wired in series and the wall voltage goes directly to the rectifiers. Tubes are (2) 25Z5 rectifiers, (2) 25L6 output tubes and (1) 6SJ7 preamp tube. The only control is the volume. There is a separate switch and a 2A fuse.

This amp will need a little work to get it safe and operational. New caps all around as well as a handful of resistors and an isolation transformer to keep it from being a total widow maker. Shouldn’t be too tough to get it up and running again but due to the fact that there is almost no real tech info on this amp, I thought I’d take the time to post some info on it.

The schematic page does show the AC-DC version of this amp, along with a couple others,  but it doesn’t directly correlate to what I have in front of me. The schematic does not show the output transformer which in my amp is a double primary winding, two separate leads to the B+ and two separate leads to the plates of the individual output tubes, with a single secondary winding to the speaker. Essentially this makes it a double, single ended output as there is no phase inversion of the signal. The Rola field coil speaker is present and the output transformer resides in the chassis and not on the speaker frame. There are no screen grid resistors on my amp, unlike the schematic, and I am a bit uncertain on the original configuration of the filter caps in the power supply but it appears that there were only two stages, unlike three in the schematic, the first being two 40uf in parallel right before the choke and a single 40uf on the other side of the choke but no signs of there being a third stage in my amp.

It appears that someone added a 100K resistor a while ago to ground at the junction of the .05 coupling cap and the 400k resistor after the 6SJ7 preamp stage. Not sure how practical this is but I’ll most likely remove it as it seems like it might reduce the signal strength.

Any comments or suggestions are welcome as I have no experience in dealing with AC-DC designs like this so this is kind of new territory for me. Hope this can be of use or interest.

[sluckey]

I think that's the wrong schematic for the amp you have.

[Apexelectric]

Don’t think so, should be the right one. Top section matches pretty close shy of the output transformer not being shown. Some were mounted on the speaker. Doesn’t show the field coil on the schematic either, although there is mention. It’s not the best either way.

[sluckey]

Don’t think so, should be the right one. Top section matches pretty close shy of the output transformer not being shown. Some were mounted on the speaker. Doesn’t show the field coil on the schematic either, although there is mention. It’s not the best either way.

Well you called out several other differences besides the OT. Does your amp have two chokes? Does it have a 4-prong speaker plug? BTW, that schematic indicates the OT will be on the speaker frame. Just sayin'. Do you have the speaker? I'd keep looking for other differences.

As for the AC/DC, just forget about the DC part because when you put an isolation transformer on the power cord, DC operation will no longer work. You don't have a 110V battery anyhow, do you?

[Apexelectric]

It will obviously be used as an AC amp from here on in. Don’t really have a 110V battery floating around, lol. Yes it has the two chokes. Most everything seems to jive including the dates on the schematic page with the serial number of this amp. It wouldn’t be the first time that there were slight variations in the production from the schematic.

I have searched pretty hard for info and this is the only schematic that seems to be right. There are others floating around for different AC-DC models and other AC only models but this was the only thing I could find.

Putting it out there for others that might be looking for a schematic as well as any info someone might have to share.

I at least know that it worked for quite some time in the configuration it was in before something apparently smoked. No other tech wanted to take it on so the guy sold it.

[j_bruce]

Love the art deco motif.

good luck with it
Jerry

[Apexelectric]

Love the art deco motif.

good luck with it
Jerry

Thanks Jerry,

Should be interesting.

Just have to come up with something creative for the isolation transformer. Theres not much room inside for much of anything. Was considering a small matching maple cabinet with a similar handle to house the transformer with a small cord between the two. I found a 100W Hammond multi tap transformer that will allow a voltage drop as well as the isolation but its fairly large.

Will think on it some more

[PRR]

Note that the "90" choke is also the cathode bias for the power tubes.

And the 6SJ7 Screen bypass is not shown.

[Apexelectric]

I thought that too but then I looked into it a little more. The cathodes of the 25L6s are directly grounded to the chassis at the sockets and the data sheet shows that at only 110V to the plates they don’t require a cathode resistor.

[DummyLoad]

Note that the "90" choke is also the cathode bias for the power tubes.

And the 6SJ7 Screen bypass is not shown.

isn't the 90Ω coil source for ALL amp? 6SJ7 is ties to "common" as well.


--pete

[Apexelectric]

This is a little better representation of what the schematic should look like based on what I have and what I’m assuming was previously done to it. Still a bit unsure of the filter configuration but like I’ve stated before, I can’t find any evidence of a fourth cap as is shown in the Epiphone schematic.

[PRR]

> The cathodes of the 25L6s are directly grounded to the chassis at the sockets and the data sheet shows that at only 110V to the plates they don’t require a cathode resistor.
> isn't the 90Ω coil source for ALL amp? 6SJ7 is ties to "common" as well.

The 25L6 DOES need bias. The sheet shows two conditions, a resistor or a voltage. Since the current is quasi-steady it hardly matters which way we do it. But the 90r for two 25L6 is suspiciously like the 180r for one 25L6. The choke resistance is in the power supply return lead, >99% of that current is 25L6 current, and the grid bias taken from the other end through a filter. It looks funny but it IS cathode bias.

6SJ7 here sucks ~~0.1mA (compare to ~~90mA for power bottles) so we don't care where it sucks from.

[PRR]

Unconventional scheme and unhelpful drafting style. And I suspect it was "designed with a soldering iron" by throwing the parts from two 2Watt radios on the same chassis and re-re-working until it worked.

First: arty cleanup of the more recent plan which I do like a bit better. (Neither may be 100%.)

Second: I tried to re-draw the power for clarity, but I need my sleep too. (Happy New Year ALL!)

[Apexelectric]

> The cathodes of the 25L6s are directly grounded to the chassis at the sockets and the data sheet shows that at only 110V to the plates they don’t require a cathode resistor.
> isn't the 90Ω coil source for ALL amp? 6SJ7 is ties to "common" as well.

The 25L6 DOES need bias. The sheet shows two conditions, a resistor or a voltage. Since the current is quasi-steady it hardly matters which way we do it. But the 90r for two 25L6 is suspiciously like the 180r for one 25L6. The choke resistance is in the power supply return lead, >99% of that current is 25L6 current, and the grid bias taken from the other end through a filter. It looks funny but it IS cathode bias.

6SJ7 here sucks ~~0.1mA (compare to ~~90mA for power bottles) so we don't care where it sucks from.

Thanks for that info!

I agree as well that it seems to make sense that the choke be used as the bias resistance for the output. That would mean that the jumper to ground at the tube socket would need to be removed. It’s possible that it was added. What do you suspect that the voltage from the rectifiers would be? And is the series wired cathodes of the dual diode rectifiers doubling the voltage? Would that then bring us over 200 plate volts?

I guess I should go see about testing the tubes, replacing the filters and firing up the power supply today and see what’s what.

Happy new year all!

[PRR]

> the jumper to ground at the tube socket would need to be removed.

Why?

"Ground" is relative. You can float the isolation transformer winding (that's what it's for).

> is the series wired cathodes of the dual diode rectifiers doubling the voltage?

The rectifiers are not series.

[DummyLoad]

if it were mine, i'd get an external enclosed 100VA isolation transformer and hardwire AC cord of death trap amp to the secondary of it, and affix iso-trans in the cabinet somewhere convenient, if at all possible - that is a small cabinet.

it is yours however, and totally understandable that you may not want to poke holes in it. 

https://www.hammfg.com/electronics/transformers/line/171.pdf


--pete

EDIT: eye fixted myne scatmacticus...     2nd edit - miswired the bias filters.

[PRR]

Pete: I think you have added some clarity. But I doubt it should have FC and two 25L6 plates on one 25Z5, screens and 6SJ7 on another. The ratings imply one 25Z5 can carry the FC, or the 'L6 plate, but not both; OTOH screens and preamp is gross underload. Also the grid bias filter should terminate in a capacitor between 100k and 400k. And I don't see cathode return path.

Personally _I_ would be more comfortable with just lighting the heaters, then tucking a 12VDC wart and eBay chip-amp inside. (Hmmm, have to hack the FC.) We have no reason to think this way-cheap amp is the holy grail of tone, and it really looks eager to shock somebody.

[DummyLoad]

Pete: I think you have added some clarity. But I doubt it should have FC and two 25L6 plates on one 25Z5, screens and 6SJ7 on another. The ratings imply one 25Z5 can carry the FC, or the 'L6 plate, but not both; OTOH screens and preamp is gross underload. Also the grid bias filter should terminate in a capacitor between 100k and 400k. And I don't see cathode return path.

Personally _I_ would be more comfortable with just lighting the heaters, then tucking a 12VDC wart and eBay chip-amp inside. (Hmmm, have to hack the FC.) We have no reason to think this way-cheap amp is the holy grail of tone, and it really looks eager to shock somebody.

forgot a couple of very important wires. lemme fix...


--pete

EDIT:  fixed and attached updates to reply #15. apologies.

[Apexelectric]

Pete: I think you have added some clarity. But I doubt it should have FC and two 25L6 plates on one 25Z5, screens and 6SJ7 on another. The ratings imply one 25Z5 can carry the FC, or the 'L6 plate, but not both; OTOH screens and preamp is gross underload. Also the grid bias filter should terminate in a capacitor between 100k and 400k. And I don't see cathode return path.

Personally _I_ would be more comfortable with just lighting the heaters, then tucking a 12VDC wart and eBay chip-amp inside. (Hmmm, have to hack the FC.) We have no reason to think this way-cheap amp is the holy grail of tone, and it really looks eager to shock somebody.

My objective here is to document the original design as accurately as possible for reference then to redesign as minimally as possible but to make it safe. The isolation transformer is a given. Ground reference in the original design will be effected by a circuit ground that is bonded to the line neutral. It seems like we need to establish a separation from the cathodes of the output tubes and the chassis ground by inserting the choke in between. In my amp, currently, the cathode pin is tied directly to the chassis ground, and there is a circuit ground as well, which seems to eliminate the choke as resistance to the current flow.

Attached is a layout of what I’m dealing with to show where the chassis grounds are and how the wiring is laid out. There appears to be some non-original soldering at the chassis ground off the output tube socket other than that, all the wire runs and connections appear to be original.

[Apexelectric]

if it were mine, i'd get an external enclosed 100VA isolation transformer and hardwire AC cord of death trap amp to the secondary of it, and affix iso-trans in the cabinet somewhere convenient, if at all possible - that is a small cabinet.

it is yours however, and totally understandable that you may not want to poke holes in it. 

https://www.hammfg.com/electronics/transformers/line/171.pdf


--pete

EDIT: eye fixted myne scatmacticus... 

Thanks for doing that! The output transformer has two separate primary windings that are tied to a common B+ with a separate lead to each output tube.  Most definitely will be adding the isolation transformer

[PRR]

> lemme fix...

So you agree on 2 of my 3 points. I still think the grid-bias filter needs a mirror-flip.

And the as-built may be quite different again.

[DummyLoad]

> lemme fix...

So you agree on 2 of my 3 points. I still think the grid-bias filter needs a mirror-flip.

And the as-built may be quite different again.

duly noted and corrected - updated schema attached to reply #15. my apologies.

--pete

[Apexelectric]

> lemme fix...

So you agree on 2 of my 3 points. I still think the grid-bias filter needs a mirror-flip.

And the as-built may be quite different again.

So correct me if I’m wrong, this design will not work properly with a grounded cord? It seems as though an earth ground attached to the chassis would mess with the design and that the only way to make this work in its configuration is to either remove the earth ground or install the isolation transformer?

[PRR]

...So correct me if I’m wrong, this design will not work properly with a grounded cord? It seems as though an earth ground attached to the chassis would mess with the design and that the only way to make this work in its configuration is to either remove the earth ground or install the isolation transformer?

It needs the Isolation transformer for HUMAN safety. It is not for-sure lethal but such things killed a lot of people.

And yes as-is grounding the chassis would also kill a few bucks worth of tubes.

A transformer has to be a a *permanent* part of this amp if you will even think of playing it today. I realize it is tight but so is a coffin.

[Apexelectric]

...So correct me if I’m wrong, this design will not work properly with a grounded cord? It seems as though an earth ground attached to the chassis would mess with the design and that the only way to make this work in its configuration is to either remove the earth ground or install the isolation transformer?

It needs the Isolation transformer for HUMAN safety. It is not for-sure lethal but such things killed a lot of people.

And yes as-is grounding the chassis would also kill a few bucks worth of tubes.

A transformer has to be a a *permanent* part of this amp if you will even think of playing it today. I realize it is tight but so is a coffin.

I completely understand the safety aspect of the isolation transformer and the necessity. Was just theorizing on the functionality of the design when installing a grounded cord like the amp I’m dealing with has.

It seems like it was part of the problem that the previous owner had with this amp. There was an old filter cap that I thought was doing nothing that went from the line neutral to the chassis ground that must have provided some isolation to allow it to work with the earth ground.

I rewired the power supply according to the newly developed schematic and removed the ground, being sure to not put myself in the wrong path, and it’s working properly now.

I’ll get the isolation transformer permanently installed and all should be good.

Thanks PRR and Pete for your assistance in getting this info out. I’d probably make a note on the new schematic about the isolation transformer being a necessary part of dealing with one of these if anyone comes across the schematic.

Regarding the isolation transformer, it seems like a 1:1 would work fine if I swapped out one of the tubes to increase the filament string resistance to the extra 10V this would see compared to the designed voltage it wants to see. The extra voltage everywhere else would seem to be a non issue? That would allow me to use a smaller and cheaper isolation transformer.

Thanks again!

[PRR]

Don't "swap" odd tubes. Carefully(!!) measure the operating voltage on modern wall-juice and carefully compute a dropping resistor. No rush: These kitchen-radio tubes often worked at "wrong" voltages for decades.

[j_bruce]

A transformer has to be a *permanent* part of this amp if you will even think of playing it today. I realize it is tight but so is a coffin.

Ironically, both a transformer and a coffin will provide the user with isolation.

 

[TheAudioMechanic]

Hello all, I hope you don't mind me commenting on an old post but, given this item's rarity, I thought I would share for anyone curious or should they have a similar scenario. I have one of these on my bench right now in its original condition that I am going through. I can post pics and possibly help clear up the debate on the schematic. And yes, all 4 40uf capacitors are present.