Greetings! Help identifying custom tube amp built from egnater Rebel 20

[dgy78]

Greetings everyone. I recently purchased this customized tube amp built in an egnater Rebel 20 head.

Looks like the printed circuit boards were removed and everything redone with point to point wiring. The previous owner said it was kind of a hot rodded Marshall type design. He had taken it in trade and didn't know much about it.

 2 6V6 tubes and five 12ax7? The amp works but the bias was set really high and sounded distorted. Now that I turned it down a little bit it sounds much better but I have no idea how to measure the bias or even what tubes should really be in there, which one is the phase inverter Etc.

 Figured I might be able to get some worthwhile information here from the experts.

Any idea what the circuit might be or how I can identify which preamp tubes are what? I'm more of a clean player so interested in maybe running a 5751 in place of one of the 12ax7s to tame it down a bit. 

Any advice or information is greatly appreciated! I can get better pictures at a later time.

[dgy78]

Some more photos if they help.  I know this is a shot in the dark.   Thanks again

[jordan86]

Not to be a Debbie downer, but that’s not a clean amp. It may be a fools errand trying to turn it into one. May I ask why you bought it?

The tonestack and LARMAR do seem to suggest Marshall topology. With the custom layout though, it’ll take some time to trace connections. You’re essentially needing to build a layout/schematic from scratch before being able to make the needed adjustments.

The two 12ax7s in the “back” (farthest from the input) seem like the first gain stage and the phase inverter from a quick glance.

[scstill]

Here is a video on biasing Pro Junior, but the concept is valid

Here is a thread on PJ biasing again should be able to apply the concept elsewhere
https://el34world.com/Forum/index.php?topic=29413.msg323749#msg323749

[punkykatt]

The 1st preamp tube should be the closest one to the input jack. Try your 5751 in that position and see if it tames the beast.

[JPK]

That's a bunch of pre-amp tubes. Also a third transformer. I was going to say reverb for the two rear pre-amp tubes but don't see any controls for it. The build is pretty scrappy with solder spattered on the leads and components. Also the components are crowded together and touching. Third turret from the left top row looks like it has very little solder on it, same as 4th bottom row, others as well. Soldering looks pretty bad. You're lucky it works. You might want to go over the entire amp with a hot iron and some solder.

[scstill]

Might also be worthwhile to draw the schematic from a circuit trace.
If you want to fix it this would be very handy.

[jordan86]

Img 170203 shows the three blue resistors in what looks like a long tail pair scenario, which makes me thing that’s the PI. The tube next to it looks like V1. The added tubes could very well sit in between those as additional gain stages. That’d be a ton of gain though. As others have said, you probably need to trace each wire and figure out what’s what with some rough schematic.

That fifth tube is puzzling.  I thought possibly reverb as well as it is next to a little transformer but there’s no control for it. It could be part of the original FX loop, and be non functioning now. You could always pull it and see. Maybe the seller popped a tube in just to sell it, feeling the amp needed it.

[pullshocks]

My theory is that the 3rd (smaller) transformer is a fender style choke.

[dgy78]

Thank you so much for all the input.  I only paid $100 for this amp so no major loss even if it didn't work i thought the chassis, tubes and other components could be handy as I have always wanted to attempt a small 5w build or something someday.    I do have the egnater head cabinet as well (not pictured)

 I played with the tube arrangement and put a known brand new preamp tube (jj ECC83S)  in what seems to be the first gain stage and holy moses sandals, the tone!!!! I know the pictures show the fender groove tubes but i just put them in for the photo , there was a mesa 12ax7a there before, and man what a difference, right now even with the gain on 10 its more of an acdc overdrive but when i put a distortion pedal in front of it its like blazing full gain thrash metal .. but with the gain dialed down it sounds glassy and smooth more like a fender or a vox.  I am really happy with it. 

I also wondered if the tubes were all functioning, they do all glow but I was really confused why there would be so many on such a low wattage amp.  Maybe I just remove one at a time and see if the amp still functions? 

About the bad solder joints, can I attempt to repair them with a standard soldering iron?  I have some repair experience but have never worked on amps really.

I will try to post some audio clips, I have a fender super champ x2 right now and this thing blows it out of the water , no comparison, through the same speaker (celestion tube 30 10") this franken-amp has double the volume and much more tube flavor to the tone even at lower volumes.   Reminds me of a modded peavey delta blues I previously owned.

Thanks again everyone, I am super interested to find out more.   

[acheld]

Hah!   If you like it, don't mess with it -- whatever it might be, let it be.

[JPK]

Hah!   If you like it, don't mess with it -- whatever it might be, let it be.

Agree 100%. If you've never worked on amps leave it alone. It works now, it may not after you've mucked with it.

[punkykatt]

If it aint broke don`t fix it!!!

[tubeswell]

If you want to try a 5751 or 12AY7 in V1, just plug it in.


On the plus side, I think the layout looks tidy (despite the odd slap-dash bits of solder spatter). You should take some voltages and reverse engineer a schematic.


Stupidly, what looks to be the two output tube sockets are hidden under what looks to be an FX loop board

[pdf64]

Just need 2 regular common cathode preamp stages to get full output from even a (unity gain) cathodyne phase splitter and pair of fixed bias 6V6. eg see a black panel Princeton.
More preamp stages allow for more peamp gain, depth of overdrive, fx etc. They can’t increase power output.

[Gator]

I have the same questions as now I own this amp. Very JCM45 on the gain side but super dynamic on the clean side 5 the tube seems to dampen the over output by like 10db have no idea what it really does. Operates as should unpopulated.

[dogburn]

Looks like you have an unused triode in one tube - the one in the upper corner as seen in the first photo. But it looks like the other tube sockets are hidden under the board, so it's going to be hard to see how they are wired up and whether all the triodes are being used in those as well.

[Psychodog]

I might be able to help a bit.

This looks like a Marshall 18w clone, but modded with 6v6 tubes and additional features. I’m getting ready to build one myself, though with EL84s. With five 12AX7s onboard your amp probably has two distinct preamp channel paths, one for bright and one for dark (normal). The bright channel is usually a cascaded gain stage, and the dark is a parallel gain stage where you can blend the bright and dark channels.

Your amp appears to have three volume controls as well, which consist of a gain control for the first stage of the cascaded preamp channel (usually bright), a master volume control, and a LARMAR post-phase inverter master volume. The PPIMV lets you also hammer the phase inverter instead of just the preamp tubes like a normal MV, for more harmonics and tone that make the 18w Marshall sound distinct.

If you check out the 18watt.com forum it’s a site dedicated to these little Marshall amp clones. They’ve been building these things since the early 00’s so it’s a wealth of information, including schematics.

Good luck!

[tubeswell]

... looks like the other tube sockets are hidden under the board,

Yep - that's the main dumb thing about this amp.