Gibson GA 19RVT

[andyjazz]

I inherited this amp from my grandfather so it has more than just monetary value to me. I've been playing it for awhile and over the last couple months I've noticed it has been losing power/gain after being on for awhile. Because the OT is so precarious looking I always thought at one time I'd have to replace it and for whatever stupid-assed reason I thought "now is the time." So I order a OT from Mercury Magnetics. According to the chart on the website it seems to be the correct one for this model (The new MM OT has GA-19-0 printed on it)

In the course of installing it I inadvertantly banged the chassis around on my bench- kinda hard. After I install the new PT I turn the amp on and I get nothing but a high pitched squeal. I notice one of the 6V6's is all "lightning bolting" inside and I figure "Oh, I must have done that when I banged the thing on my bench, the tubes are old and need to be replaced anyway, no problem." Well I just tried it with the new 6V6's and I get the same thing, a high-pitched ear-piercing squeal.

Before I get into tearing this thing apart again I'm just wondering if it could possibly be that I got the wrong OT? It wouldn't be my first $150 mistake I guess. Does anyone have any experience with changing OT's? This is my first one. Could it be that I wired it in wrong? Is that what the squeal is all about? Did the bench banging incident screw something else up? Any help/thoughts would be most appreciated!

[Diablo]

You need to reverse the two wires on the primary side of the output transformer. These wires go to the plates of the two 6v6 output tubes. Typically they are a blue wire and a brown wire. The amp is oscillating because you have positive feedback. When you switch the wires around you will have the proper negative feedback. Save your old output transformer, there was likely nothing wrong with it.

[andyjazz]

Thanks Dialblo, I figured it was something simple that I over-looked. And I will save the old OT. I can tell no difference in the sound of the amp with the new OT. Another $100.00 lesson. I guess maybe you could say I pre-emted a failure down the line by putting in the new OT. The old one is in pretty scary shape-paper is all brown and crumbly.

[ivey]

Andyjazz:

I love old Gibbs amps. They have this sound that no amp has. I think its their use of 6EU7 tubes.

Transformers are hard for some people to understand. But I worked at Counter Point Electronics building and redesign transformers. They are simple items really.

Gibson had a transformer company in or around Detroit that wind their units, before they switched in 1967. I learned a good trade from them. Mason Wire and Inductor Co.

Winding transformers by hand is an art. It takes a good eye and hand to wind transformers using a machine by hand. I have done both. With and without a machine.

You do not need a lot of tools or special materials to rewind your transformer. In fact you need a block of wood, a plastic knife, clear nail polish, sand paper, wax paper (use for sandwiches) and vanilla folder, three 1\4" wooden dowels (wind the wire on). Other things your would need are screwdrivers, pliers, a solder iron, solder, and flat black paint.

If you ever decide to do it, let me know. I will assist you through it.

Take Care

Ivey

[Rick Erickson]

6EU7 tubes are just 12AX7's with a different pin configuration. There is no tonal magic in them. Gibson amps have different circuitry than most amps. It is the circuit that makes the tone happen for the most part. Consider that all houses are made of similar materials - so what makes one house nicer than the next? Same goes for cars, boats, amplifiers and women.

[ivey]

WOMEN!!!!!!!!!!!!!!!!!!!

Women are No where close to those wonderful,sweet things that you just stated.

You pay for a car once, a house once, a boat once, an amplifier.

But Women!

You work your butt off; and you pay and pay and pay some more.

I hate Walmart, Target, Sears, Macy, and all the damn stores that those WOMEN go to.

You never stop paying.

And they are made from the same materials. TOTAL CONFUSION.

But..., you are right about the tubes. Gibson used the 6EU7, because it made them different. All the others were using the 69 cent 12AX7 back in the 1950's. Gibson had to use 95 cent 6EU7. It was like a Macy vs Sears mental thing.

Take Care

Ivey

[capnjuan]

Hi andy and congratulations on your GA19; is it a tweed / 7199 / rear-facing controls model or a crest / 6C4 / front-facing controls? Yeah; you probably didn't need to change the OT but no harm done. If you got rid of the squeal and the 6V6 cathode resistor and its bypass cap are original, I'd change that bypass cap next.

Pic of my tweed version with the lower speaker baffle notched to accept a 12" Weber 12A125. The only knock on 6EU7s is that they were never manufactured in Europe so the makes/flavors are pretty much limited to Sylvania, RCA, and GE. Some of them NOS or used will sound better than others but that's tube-to-tube variation and not a 6EU7 v. 12AX7 thing. Good luck with you amp.

[andyjazz]

capnjuan.

Wow thanks for the photo. Sure looks nice. I was gonna post a pic of mine but the batteries ran out on my camera. I'll post one maybe tomorrow. Mine is the one with the 7199 and the front facing controls. It's a 1962 model. The 7199 is on the reverb circuit and i'm convinced it is why the reverb just has never sounded good. I'm pretty sure its the original 7199. I inherited it from my grandfather about 15 -16 years ago. Had the filter caps changed back in like '96 I think. It basically sat in the closet for a number of years thenI had it serviced about 5 years ago and the guy who did the work did such a crappy job - it's the reason I got into working on amps. Why pay a bunch of money to some guy that does half-assed work when you can get 3/4 assed for free!

Anyway, as you probably know these 7199's cost a fortune these days - if you can find one. There is a place called Tiode Electronics that sells an adapter so you can plug in a 6EU7 (Instead of rewiring pinout) and thereby get tubes that are still available. I haven't got around to getting one yet. I still play this amp. It has a real unique tone - nothing like it really. I also have a 64 Fender Pro Reverb but if it's a smaller room I really like the old Gibson. BTW which one is the bypass cap? Is that the 20mF that sits between the cathodes of the 6v6's?

[imaradiostar]

Do you perhaps mean a 6u8 or 6gh8? The 7199 contained a pentode and a triode, much like the 6u8 and 6gh8.

jamie

[andyjazz]

Jamie,

Yeah I just double checked the schematic - they are 6EU7's on this model for the preamp section. The 7199 drives the reverb circuit. I guess I look at the 7199 as kind of an exotic tube. Why Gibson chose the pentode/triode for the reverb is anyones guess. BTW the reason I'm convinced the 7199 is at fault for the "crappy" sound of the reverb is because it's old - like original equipment old. I'm willing to bet that a NOS 7199 that is up to snuff would drive the circuit fine. I just can't see laying out $100+ to get one. That is IF you can find one. Maybe for Xmas I'll finally get around to buying the adapter from Triode - It is like $35 or something.

[booj]

Angela Instruments Online Catalog - Tubes - Small Signal Tubes

7199 can be subbed by a 6U8. 6EU7's for $22

[ivey]

Tube Depot sales 7199's for $15.95 each. JAN 7199 are $50.00.

The Russian made 7199 is a good, no a very good tube. But yes, the 6U8 or 6GH8 are excellent subs, but require a rewiring. They sell for $6.95

So it is something to look into.

Take Care

Ivey

[capnjuan]

... Mine is the one with the 7199 and the front facing controls. It's a 1962 model. The 7199 is on the reverb circuit and i'm convinced it is why the reverb just has never sounded good. I'm pretty sure its the original 7199 ...
It has a real unique tone - nothing like it really ... BTW which one is the bypass cap? Is that the 20mF that sits between the cathodes of the 6v6's?

Hi Andy; sorry for the slow reply ... had forgotten that this BB software doesn't auto-notice. Anyway, the 'send' / pentode side of the 7199 also has a 20uf cathode bypass cap and yes, that 20uf sitting between the 6V6 cathodes is the bypass cap. Everybody feels a little differently but unless your amp is museum-grade, there's no point in hanging onto worn out electrolytics anywhere in the amp. Before I messed with either replacing or subbing in for the 7199, I'd want a fresh cathode resistor and bypass cap on the send side. If the others are original, I'd do those too but I'd spend a couple of bucks on parts before I walked away from the original 7199.

There's lots of reasons why tubes fail and just as many 'tube issues' that don't have anything to do with the tube. Besides, if either of the 7199 cathode resistor / bypass cap are out of tolerance or just worn out, you could go through all the trouble and expense of replacing/subbing in and still have a crappy result. Unless the amp has been abused hard, there's no reason to believe that 7199 is bad. It's cheaper to assume what you have is good than it is to assume that it's bad. Good luck with your amp.