Gibson Explorer GA-15RVT

[lowell]

So I got this amp in and the interstage audio transformer had an open primary. I got a 124a from hammond, threw it in, next thing I know one side of the secondary is open now. ACK!! How could this happen? I turned the amp on for literally 1 minute, and had the bulb limiter in place too. While it was on I noticed one EL84 was getting hot. I powered down and checked some things to find the open secondary. I'm shocked, frustrated, and oblivious as to how this could happen, and so quickly. Both EL84 tubes are brand new JJ.

Any ideas?

http://www.sophtamps.ca/images/ga15rvt/ga15rvt1.jpg

[lowell]

Actually after ordering a 2nd replacement PI driver transformer I realized this amp is NOT at all like the schematic. It has a reverb transformer. Also, the PI trans driver tube is V3 not V2. V3 is still a 12au7 like the schematic says. And V1/2 are 6EU7.

Anyone have the correct schematic for this amp? I've searched but didn't hit. All I can find is the schematic with the tube-driven capacitively coupled reverb circuit.

Oh and it has a choke as well...inside the chassis.

[DrGonz78]

You might have seen this one already but here it is anyway...
http://music-electronics-forum.com/a...5-rvt-1965.pdf

God those Gibson schematics are always a headache.

[lowell]

That's it! Thanks a bunch!

[Enzo]

Lowell, welcome to the world of Gibson/Epiphone amps. The schematic OFTEN is different from the model number on the amp.

In my collection, somewhere in the GA15-19 models I have one, might be the 15 in fact, where I have three different Gibson published schematics and THREE more I have hand drawn from the same model numbered chassis. That is six different circuits with the same model number.

Not only that, you will find sometimes a chassis that originally had 6EU7s has been rewired in one or more of those sockets to take 12AX7. The tubes are similar but the pinout is different. So in those cases we have to decide whether this is a different circuit or a rewire.

And sometimes you find a GAxxx model is really under the skin an EPiphoe EAxxx. Or vice versa.

This is one case I prefer the paper manual over stored computer files. I can flip through page after page of the book a lot faster than I can open 150 pdf files. If you ever see a copy of the Gibson MAster Book, grab it.

I have many times leafed through teh Gibson Master Guide book, and I find the tube chart to be most helpful. Most of their schematics include a small drawing of the chassis with the tube named. SO I can flip through pages looking for an amp with say two 6L6, a 12AU7, a couple 6EU7, and a 7025. or whatever. When I find a drawing with the same tubes inthe same order as my chassis, I look to see if it in fact agrees with my chassis.

[lowell]

Right on. Sounds like that book sure does come in handy. Think I found it, but in pdf CD format.

Is this it? CD Gibson Tube Amplifier Master Service Book - Manuals, Schematics and Parts pdf | eBay

[Tom Phillips]

Right on. Sounds like that book sure does come in handy. Think I found it, but in pdf CD format.

Is this it? CD Gibson Tube Amplifier Master Service Book - Manuals, Schematics and Parts pdf | eBay

That appears to be someone jerk selling a homemade CD with a copy of the file that is available for free download off the internet. It's the exact same scan right down to the splotches and page alignments. I downloaded mine from the old Schematic Heaven many years ago.
Edit: A Google search netted several hits such as https://elektrotanya.com/gibson_mast.../download.html
It's the same 177MB PDF file that is on the ebay CD.

Enjoy,
Tom

[Enzo]

Yes, the digital scan file can be found for free in several places. It is the same. But as I point out, flipping pages can happen way faster than opening 100 pdfs. Having said that, yes please do add a copy to your stored files. I like skimming to book for that one function - solving mystery circuits. But I do much prefer a schematic on my screen over that big clumsy book open on my bench.

[lowell]

I can have the local printer make it into a book too. I'll see if I can find the free file online somewhere.

Back to the amp: There is an insane amount of HUM. The dreaded hum. Some 60hz and 120hz. I've done quite a few things that I thought might fix it, but they haven't. One of 'em actually makes it MUCH worse. Here they are:

First I replaced all filter caps as they were original, not sure if this hum was there before because the driver transformer was bad
Checked that all grounds have continuity and all tube voltages look good and correct
Disconnect filament ground, added simulated center tap with 2 100ohms - no help
Pulling the 12au7 driver tube kills the hum, so it's not a PA (power tubes) issue, and the bias is good at 12watts each
Pulling only V1/2 doesn't help, actually makes it a bit worse
Grounding V1b reverb return grid makes it WAY WORSE
Cut out the death cap - no improvement
The high voltage secondary center tap is good and grounded
Ground the 12au7 driver tube grid doesn't help at all, so I'm thinking this must be the driver transformer or power supply cap or ground - but still not convinced because of the following:

I've been using my scope a bit to see if it's 60 or 120hz, and in doing so, it seems as though there is 44v p-p of 60hz riding on the chassis itself, and this is with the power switch OFF. If I unplug the power cord this goes away.
The hum seems to be there at the input of the amp as well on the scope, albeit much less, but again pulling V1/2 doesn't help the hum, makes it a bit worse, therefore I don't believe the source of this hum is from the input stage/s

I've tried to use my meter to measure AC voltage and it doesn't show anything... I think it's not sensitive enough to read lower AC voltage

At this point I'm a little confused - help needed!

[lowell]

Something tells me the chassis has voltage on it. If I measure AC voltage from earth ground (in my shop outlet) to the chassis, there is 6.6vac. Assuming this is because I added the 100ohm filament center tap resistors. Meter doesn't read the 44vac p-p like the scope did. What's baffling is that the power switch isn't on, the amp is simply plugged in.

I'm missing something obvious... I've thought multiple times about adding a 3-prong and seeing if earthing the chassis would fix it, but don't think so as grounding my scope at the chassis (therefore earthing the chassis) didn't help the hum level.

[Enzo]

Please do put a three wire power cord on it. Most Gibsons have two ON positions on the power switch. As in off/standby/on/on. Is that your case? If so, do both on positions hum equally? Also, in the old days of two prong plugs, we routinely had to turn plugs over to kill hum and shocks.. In other words pull the plug from the wall, and flip it over to reverse the polarity. Leakage to chassis is quite common, especially in amps with a "death cap". That cap after all is an AC connection from chassis to one side or the other of the power line.

When you have multiple sources of hum, they are often out of phase with each other and can cancel. When you pull a tube and hum INCREASES, first thing to my mind is you just removed one of the hums that was cancelling the other. Absent that cancellation, you hear more hum. So that input stage hum you think there isn't is probably there and was what was cancelling the hum from other sources.

Every source of hum has its own cure. heater leakage hum will not be helped by more filter caps. And power supply ripple will not be affected by a grounded heater center tap. Rather than just do everything you have heard reduces hum, find the sources of it and cure each one.

[lowell]

Forgot to mention that YES I did swap the 2-prong plug the other way, same result. Yes it hums in both ON positions.

I'm a bit at a loss as to what and where to attack. I feel like I don't have a good method for troubleshooting hum and it gets me almost every time. I've learned quite a bit about grounding schemes, and the heater center tap trick, and those have helped a lot. But in this case, I find this very odd: 12au7 grid grounded hum exists, pull the 12au7 hum is gone. And I mean LOUD, it's akin to the hum before a filter cap blows... that increasing intensely loud hum then BOOM...only, no boom.

And yes, tried 2 different brand new JJ 12au7 (ecc82)

[Enzo]

When you ground a grid, try tacking a very short wire from the grid pin on the socket right to the grounded end of the cathode resistor. Don't ground it to chassis.

[nosaj]

Forgot to mention that YES I did swap the 2-prong plug the other way, same result. Yes it hums in both ON positions.

I'm a bit at a loss as to what and where to attack. I feel like I don't have a good method for troubleshooting hum and it gets me almost every time. I've learned quite a bit about grounding schemes, and the heater center tap trick, and those have helped a lot. But in this case, I find this very odd: 12au7 grid grounded hum exists, pull the 12au7 hum is gone. And I mean LOUD, it's akin to the hum before a filter cap blows... that increasing intensely loud hum then BOOM...only, no boom.

And yes, tried 2 different brand new JJ 12au7 (ecc82)

That 12au7 has 2 grids one is Reverb input and the other half is (not really sure what to call it Tube that drives the driver transformer??)

Maybe grounding one or the other will put you in a direction?

nosaj

P.S. I read over thread several times after looking at the schematic and found no mention of the footswitch being available or not.( may not even be important though).
What about input jack washers? I don't remember how to tell by the schematic what they are supposed to be. My traynor has plastic washers(or a fibre washer) The junk box ones I've built toothed washer to get a good bite.

[Enzo]

"Driver" works for me.