Epiphone EA-12RVT pics and schematic wanted

[bepone]

Searching some original/scheme/pics of internals, maybe someone has amp and can tell me about the voltages, or send some pics.., i'm working on one canibalised unit, without transformers and strange power supply..
layout+ available schematic on the web is not following real condition in the amp..

SOme pics from orig. / untouched will help , or some schematic with 6 preamp tubes thx

Amp is like this one, but i see on the listing that has been worked out with some strange grounding (all the caps gnd in line)
https://reverb.com/item/4139105-vintage ... per-reverb

[Phil_S]

Here is a useful reference, which I hope you can see from your location: https://www.superiormusic.com/page199.htm Look about 75% down the page or better search for "Futura". The note says: "GA-30RVT Invader: 4-6EU7, 12AU7, 2-7591, 5AR4 (most likely corresponding to Epiphone EA-12RVT Futura)." The picture you show at Reverb is one of the Crestline series, so I'm feeling sure this is the correct reference.

I think this will be the schematic you want: https://schematicheaven.net/gibsonamps/ga30rvt.pdf

Finding photos of the board will be a serious challenge. For that, you will need to get lucky. I'm attaching a picture of my GA20-RVT, a simpler amp made slightly later and part of the White Panel series. I'm sorry for the poor resolution. This was taken many years ago before we had easy access to hi-res digital cameras built into our phones ;-/. These Gibson eyelet boards are typically a rat's nest and trouble to work on. Maybe one good thing was they used color coded wires, I think yellow for cathodes, green for grids, blue for plates/anodes, etc.

The GA20-RVT was the first amp I worked on. I should not have done it, but I did. I spent many, many hours making a hand draw layout with colored pencils and cross referenced to the schematic. That was a big help. Though I could to it much better and faster now, I feel it is fair to say that it would still be time consuming.

I have one tone related comment. You will see notch filters at R10-C5-R11 and at R44-C19-R45. These were a bad idea -- ice pick tone. Do as you want with them. Also, be very careful with LDR-1 if it still works. This LDR is no longer made and it is impossible to find a replacement. If not working, you will need to build your own with a light dependent resistor (LDR) and incandescent buib placed inside a tube with some heatshrink. You may need to guess at the appropriate LDR and bulb, so I hope yours is working.

I hope this helps.

[bepone]

Hi Phil thanks for the great page! First i will try just to rebuild the power supply , connect trafos and see what will be the voltages..This allready will be a victory

I the mean time i have opened every gibson schematic on the net, and i have narrow my problem to model GA-45RVT

GA-45RVT Saturn: 4-6EU7, 12AU7, 6CG7 (aka 6FQ7), 2-6L6, 0A2 (correspond to Epiphone EA-14RVT).

This is my amp:

IMG_20230309_143143.jpg

IMG_20230309_143145.jpg

On my backplate is written EA-12RVT... but now, i assume that someone mount EA12-RVT backplate which made the confusion, all are similar, and my amp should be GA-45RVT.

I have found a lot of similarities from GA-45RVT schematic and my amp.. so i will try to go this route

[Phil_S]

I assume you found this schematic: https://schematicheaven.net/gibsonamps/ga45rvt.pdf
No specs given for the transformers, but all the DC voltages are on the schematic, so I'm sure you'll figure it out. Glad to help. This will be journey.

[bepone]

it is going well..folowing GA-45RVT schematic..
power supply rebuilt on tag board..

IMG_20230314_184419__01__01.jpg

[bepone]

Added missing wires...connected almost everything..
Also this strange roto switch all in one, Power on, Standby, On, On.. rebuilt and rewired, for better functionality..

Output tubes will be EL34.. driver tube 6FQ7-6CG7 will be replaced and rewired for ECC82 for simplicity of tube replacement in the future..
All 6EU7 tubes will be replaced with ECC83, and sockets rewired.. I have those adaptors 6EU7 - ECC83 but they are disaster, clearance between the anode and heater wires are 0.2/0.5mm..

Grounding/ all the grounds in the amp going to one capacitor?? Very strange i need to see how to improve grounding, i will cut them all and make star or buss..

OA2 regulator tube doesn't have place in guitar amplifier.. will be replaced for zener diode ! Or simple resistor..

[Phil_S]

Yes, power supply needed something!

Added missing wires...connected almost everything..

Excellent! It can be difficult to identify parts on the board.

Also this strange roto switch all in one, Power on, Standby, On, On.. rebuilt and rewired, for better functionality..

On-on was to reverse polarity and kill hum. The switch wiring was common in 1963.

Output tubes will be EL34.. driver tube 6FQ7-6CG7 will be replaced and rewired for ECC82 for simplicity of tube replacement in the future..
All 6EU7 tubes will be replaced with ECC83, and sockets rewired.. I have those adaptors 6EU7 - ECC83 but they are disaster, clearance between the anode and heater wires are 0.2/0.5mm..

Good move. ECC83 and 6EU7 are the same inside the glass.

Grounding/ all the grounds in the amp going to one capacitor?? Very strange i need to see how to improve grounding, i will cut them all and make star or buss..

You probably started this. I'll tell you my experience in another Gibson amp. When I tried to improve the ground scheme, all I got was a lot of hum. Obviously, I didn't understand what Gibson had done. I returned it to stock.

OA2 regulator tube doesn't have place in guitar amplifier.. will be replaced for zener diode ! Or simple resistor..

In 1963, 0A2 was probably the only choice. Today we have inexpensive diodes and zeners. Anyway, another good change.

Thank you for the update.

[bepone]

Here is update on currents through ground and where must each cap be connected..

I was checking how to connect electrolytic caps, and on the pics i saw that the people using buss on all caps on terminal board.. the worst option..

Marked in colors where te main currents flow, and caps minus poles will be connected to those cathodes, schematic is not showing that, actually is easy to make error following only schematic.

Choke is quite small so i connect it only to g2 supply, originally also anodes /output trafo were powered through the choke, but this is maybe too big current- which can make saturation on the core, which is then cancelling the filtering effect, so i have removed 80mA of anode DC currents through the choke connecting it to g2 capacitor, like standard connection nowadays..

gnd2.png

[Phil_S]

If we can use my GA20-RVT as an example of what Gibson probably did in your Epi GA45 equivalent, The original would have had 2 multi-section can caps. Mine had 2x 20uf for the power tubes and 3x 20uf (or maybe 20-20-10) for the other nodes. Each can had one common ground. On the board, I didn't see it as a ground buss until I made the drawing. You can see the ground runs through the middle of the board (brown wire.) It may be helpful for you to see my drawings, so I'm posting them. The ground connections for the can caps are identified on the drawings - eyelet 36 and terminal 11 joined by a jumper.

I believe there was a terminal strip where you have the power supply board -- see the drawing of the GA20-RVT terminal strip. The GA45 would have been similar. The choke was inside the chassis between the terminal strip and the board. While that choke looks small, I believe it was OK where they put it as it is part of a C-L-C filter and not choke input power supply. I included a picture after the filter caps were replaced where you can see the placement of the choke and the terminal strip.

If you want to rearrange the circuit and bypass the choke, that is totally up to you. As for tone, this series of amps was nothing great, so maybe it will be an improvement.

For the ground scheme, whatever works is always the right thing to do. There is no reason to follow what you found or what was in the original. What I remember is that I only changed on thing about ground scheme. It was a floating ground. I think I added a jumper from terminal 11 to the unused terminal that was riveted to the chassis. After that, the amp was quiet.

I hope this helps you!

[bepone]

As for tone, this series of amps was nothing great, so maybe it will be an improvement.

Hi Phil thanks for all the help.. This sentence didnt make me feel good i was hoping to get something out.
I'm close to fire this thing up, rewired all the sockets to 12A/X/T/U/7's , PITA job.. all the pins were oxidized, wired wrapped around , very difficult to clean all and solder again..i have lost few days..Changed also one socket because one pin fell out

Heaters pulled "in the air"..Added on more 10uF 400V node on the first tube for filtration and current loops minimizing.. Grounds grouped in 2 main nodes.. We will see. FIrst i will reform el.caps because i have installed some used ones..

ga2.jpg

[Phil_S]

As for tone, this series of amps was nothing great, so maybe it will be an improvement.

...This sentence didnt make me feel good i was hoping to get something out.

Modify the tone stack. How? Not sure. I'd eliminate the notch filters. On the GA45-RVT, I think these are found at C8-C9 and C28-C29. I'm not sure what C13-C14 does These amps have excellent reverb and tremolo. You know more than I do about this. You can get a good sounding amp.

Set my tone comment c. 1965 when this amp was made. It was probably meant to be for jazz guitar, not rock and roll. I imagine, if you rolled off the treble on your guitar and didn't turn it up too loud, it may have been perfect with a hollow body, single humbucker in the neck position.

Heater wires in the air? Yes, this is that I do when I build an amp. I have two old Gibsons. Both have no twist wires on the chassis. Somehow it works. I saw no reason to change it.

Looks like you've made good progress. I'm looking forward to your next post!

[bepone]

I have built some voltages with the variac in the preamp, connections are ok, voltages present which is good thing..but.
I dont like what i'm seeing.. I have started preamp stage for clean only V1+V4+V5+V6= 3x ECC83 + ECC81.. but i have 430VDC+ on the plates on the first tube..schematic shows 265V, only 200V less

Then i have started to analise current draws through the stages...0A2 tube is main consumer...

ga-45rvtcurr.jpg

And on my point A voltage is higly dependent of the regulator tube 0A2 which i removed and will be dismissed permanently. This is very wrong approach/design.. The amp has many tubes and if you remove some, you will rebalance all other stages, specially if you remove reverb, or vibrato tube (12AU7-also major current draw)

If i put resistor instead this tube (cca 7k5=10W) then i will have permanent draw of current for all other stages.. But why to create all this heat for nothing..

It is easy to take 47k for the point A Ub+ and connect it to the choke 470VDC directly and create preamp voltage. But then i need to remove all the resistor string and find all the new values.. Not short job, this was not in the job plan I need to calculate now what is faster solution..If i put zeners there, they will draw 10mA so again i need to change resistor string. 3 options - more time wasted

[bepone]

first sounds and loud humm, but missing half of the tubes-- i will see tomorrow.. sound is not so, looks like too scooped, too much of deep bass and too much of treble........

[Phil_S]

Just checking on basics. At the PT, are you seeing something close to 460VDC before the choke? If you do, with power tubes in sockets, you should not be seeing those high voltages at preamp plates. I think you know this. Before rebuilding the dropping string, I'd look for the "real" problem. How is the bias voltage?

[bepone]

Just checking on basics. At the PT, are you seeing something close to 460VDC before the choke? If you do, with power tubes in sockets, you should not be seeing those high voltages at preamp plates. I think you know this. Before rebuilding the dropping string, I'd look for the "real" problem. How is the bias voltage?

i have converted bias and output stage for EL34's..470V on EL34's-.. PI and output section are not humming, very quiet..

instead of 0A2 i have put 7k5 10W to the ground and this pulled all the voltages in preamp to normal (250-300V), afternoon i will check for possible errors what has been done