el84 mystery amp

[razabri]

Not a stompbox, but it's something I wanted to share over here, as I did get so many good advices and notes before - no place else was ever so helpful.

So I scored this at a flea market at whopping price of 5$ - it looked to me as an amp head, seen the lamps inside, seller quoted 7$, I lowered it at 5, so here it is 

It is some kind of amp head, two DIN inputs, two old speaker outputs, 4 knobs, two switches and it works! I guessed it's a audio amp for a radio and/or turntable - works well with the guitar too - one input is with the preamp and it's mono (guess it's for turntable), and it's really loud, the other is cleaner. I'm getting some overdrive out of it, but strangely it's more audible from the valves, than out of the speaker. I assume it should be a stereo device, but I'm getting different sort of signal from the outputs.

Inside it's all very clean and I'm not knowledgeable about PTP, but it seems nicely put together to me. I've identified the valves - two in the front are ef86, middle ones, power tubes are el84, and in the back those are ecc83. Couldn't make anything from the transformers, the big caps are 50+50uF 350/385v.

The question is - what would you do with this one? I was thinking to build one for a guitar out of it's parts and place it in a nice bread box I have, with a little window to see the tubes and all, but is it awful to pull it apart? Someone did nice job of putting it together, and I'm guessing it's from the 80s and in very good condition... But I don't really need such amp, as I don't have any vinyl records or the turntable, I'd rather play guitar through it.

Any thoughts would be appreciated. Cheers!

[puretube]

Congrats!

[aron]

Really Nice!

[iainpunk]

looks like a home-stereo type of amp. i guess both channels are meant to be the same, for stereo use. some turn tables have 2 mono's out to act as stereo out.

if i were you, i'd put it on a shelf for now, take a bunch of time to learn how tube amps work, and when enough knowledge/time has been gained/passed you could mod this circuit into a guitar specific amp. cascade the preamp stages for more gain, add some sort of tone stack if its not already in there and put a new box around the original chassis.

cheers

[GibsonGM]

^  I second this. You seem to have a nice clean hi fi system there that is probably VERY transformable into a cool guitar amp.  That pentode offers some possibilities      Learn a bit, and then you can investigate modding it!  I've known more than one person who used an amp like that with a guitar cab and got some VERY nice tones for recording, without modification, tho most people would tinker to get nice overdrive.

There are many sources around to learn more.  https://www.valvewizard.co.uk/

[mozz]

Looks like stereo , you need to have a speaker or resistor load on the channel you are not using otherwise you may smoke that sides output transformer. 

[amptramp]

This looks like an ideal bedroom / practice amp.  The EL84 / 6BQ5 appear to be single-ended stages that can put out 6 watts per channel maximum (probably rated for 5 watts per channel), so you could run two separate speakers for 10 watts or use one channel for guitar and the other for voice.  Some venues (like a classroom) would not need more power than this.

If I had this amp, I would trace out the schematic to see what I had then do whatever modifications I wanted starting from a known baseline.  I have a similar amp with push-pull 6V6's (10 watts per channel) and an Armaco amp with push-pull 6BM8's (8 watts per channel).  These could be left as amplifiers for stereo source materials or voice/instrument amps or run from the same source into two speakers.  Biamplification has been popular at times where separate amp channels are used for bass and treble to run a woofer and tweeter.

There is no need to butcher the amp as it stands.  You may want to add 1/4 inch jacks but that can be done as inline jacks on a piece of shielded cable so the only holes you need to drill are the size of the cable.

[anotherjim]

They've already been suggested, but I would...
Fix any faults.
Use it with 2 speakers for guitar.
If you already have a 2x12" guitar cab, modify it so each speaker can be driven by each amp channel.

Edit. A serious concern, don't try to run a channel without a speaker load. It can damage the output stage. It may be ok if the balance control is set fully for the channel in use.

[razabri]

Thanks so much to all of you! I agree on all of your thoughts, and I wouldn't want to butcher it after all, even if I knew more about tube amps and how to build/mod them.

I'll probably just keep it as it is and make a adapter for guitar input with DIN and guitar plug.

I could maybe get a pair f 12" - just found one in local ads, for about 50$, old German made, ad says - Vintage 70's RFT L3401 12"/30 W/8 Ohm Pair, and build a cab for them, connect each to separate outs and see how it goes.

I'm grateful also for the link to https://www.valvewizard.co.uk/ - I do intend to learn more and try and build a simple, small valve amp in the near future. I've been watching this guy from time to time and he makes it looks simple and interesting - https://www.youtube.com/c/dlab500/videos

I do have this great bread box which I bought specifically to populate it with tubes - little window and all, but I'm still insecure in how to start, which amp to build, as I have only basic experience in building 5 or 6 stomboxes, vero and perf board based...

[GibsonGM]

Yes, the "D-Lab" guy on Youtube is great! He will chat with you via email about his projects and is a great source of information

I suggest you start by reading Merlin's web site, and perhaps getting his preamp book - that is full of answers and tips about tube circuits and construction.  I would save that GREAT breadbox until I was sure of what I want to do and know good techniques for making a solid, quiet amp     

I believe it is easiest to build a preamp first, and then maybe move on to a full amp, maybe from a kit like a Marshall 18 Watt.  Then to a custom project that you either design or modify, and source the components yourself.  Working with tubes is not particularly 'hard', it is just different from using the pedal parts we're used to.   I find it very rewarding - many people do!

[iainpunk]

Those L3401 speakers are made in Leipzig, Germany, only in the year 1975, by communists. They are a way broader bandwidth than most instrument/guitar speakers, as they come from a PA building company.

Cool, im jealous.
Cheers

[razabri]

He's Merlin and also the Valve Wizard! Seems like a proper way to go, to dab in this tube sorcery 
Perhaps I could search for a pdf edition, as I'm not sure they'll ship it to my region... Until then, I'll try and find out more, perhaps there is a simple mod that can get this amp to respond better with the guitar. But first, I've ordered those speakers - yes, I think they are wide range, and I don't know if it's good or bad thing, but as I just might use this amp for audio too, should be good. I'll probably build two separate cabs for these - figured I could also use amps stereo input for the effects that do stereo...

[razabri]

Maybe it wouldn't hurt just to source the schematic of this amp, as I might want to tweak it in (probably distant) future. Apart from identifying those tubes and caps, there's not much to go on...

[GibsonGM]

If there is a brand name, perhaps you CAN find a schematic. Or, if it is custom, you may not be able to.

The valve wizard site has LOTS of free information you can read to get you started!!   Especially how to read load lines.

I think you may find that your guitar sounds pretty good through that amp with guitar speakers. They often sound like crap if you play them using hi-fi speakers.  We do not want hi fi, ha ha.

Perhaps a dual-use idea, for audio too, is a good idea, instead of trying to mod this very much.  It is likely designed for clean, warm reproduction of music, not really intended to overdrive and cut lows here, remove treble there.   

With a guitar cab and an EQ pedal, and an OD/distortion, you may get some very nice tones from it      Just my feeling about the subject.

[Rob Strand]

If there is a brand name, perhaps you CAN find a schematic. Or, if it is custom, you may not be able to.

The transformers might be a good lead.  The transformers have cardboard bobbins and eyelet terminals.  I think some very early Grundig amps used this construction.    There's some text on the power transformer.  I tried to search for it but got some unsavory looking results  .

However, the build looks DIY so it's possible they re-used parts.  If it's DIY there's no easy way to trace down a schematic.  Just trace it out by hand perhaps using some similar schematics as a guide.

Something alarming is the spring on the top of the power transformer.  I can't make out the wiring details but it looks very worrying like the Neutral is strapped to ground via a spring - that would be very dangerous if it's connected to a power point with swapped wiring.

The wiring mains needs to be checked.   It should have a mains earth.

Good idea to mount the power transformer with screws with nuts not self tappers.

[mozz]

"the build looks DIY"
 I do agree on that. Also they do look like Euro transformers, Grundig, Telefunken, etc. Even the resistors. Might not be too easy to draw the schematic, he's got things crammed in there.

[Rob Strand]

The EL84 part is probably something *like* this (ignore R9).  Unfortunately there's many minor variations on the theme.  That's where it helps to ponder over a few schematics.



The easy way is to find the inputs and outputs and just use it as is.

As far as documenting it accurately the power supply details can take some effort.   But again you can dumb it down by identifying just a few key DC rails.    It's not obvious from the pics where the rectifiers are!

I guess it comes down to what you want to do with it.

An unknown is the correct speaker impedance.   That depends on the output transformer.   There are ways to work it out with some measurements.

[Nasse]

I have 50's tube radio with mono EL84 power amp and one possible mod to try might be to cut off negative feedback for guitar use. or so I believe.

[razabri]

Haven't been around for a few days - thanks for all your thoughts! Yes, it's VERY DIY, even it's cover box (haven't posted a picture, but I will, once I box it again), it's handmade, cut out of tin and covered with self-adhesive wood printed paper.

Most of the parts are from EiNis manufacturer, which was in Yugoslavia. NAPON is just voltage, to choose between 220V and 110V terminals. Some of the parts are Czech made. I've also had some concerns about the safety, since the mains power cable is just two wires, no earth (in my region it should be yellow/green one). I don't know what to make of that spring Rob mentioned - doesn't look very safe, but it doesn't look that it shouldn't be there - why, I don't know. Posting some more pictures of that area.

Good news is - speakers have arived! Now I'm at building cabs for them, they are both at 7.5Ohms. Not sure how to determine if these would fit the amp, or how to make them fit if they are not compatible. I also had concerns about these, being wide range, having not to be a good choice guitar wise, but we'll see, once I have it all set up. Only thing that concerns me a bit is that I had a slightly different output hooking up a test hifi speaker to one and to the other out, as it may suggest that one of them is already damaged in some way.

Perhaps the very first thing is to make sure that it's safe to use, maybe include the earth wire, connect it to the metal chassis, if that's correct. Need to understand more about what Rob was saying about swaped wiring and that spring...

[razabri]

Oh, and nuts on those transformer screws are actually present on the other side.