Debugging a Gibson GA-75 amp

[sgmezei16]

Howdy folks,

I have been having a really tough time with this one. It is a Gibson GA-75 with a single 15" speaker.

I have been working off of this schematic:
http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-75_EARLY.pdf
It is the early version of the schematic if looking it up in the archive.

Here is the tube amp voltage page:
http://el34world.com/charts/valve/ValveData.php?e=view&f=17682

What I have done:
-Replaced all electrolytic caps
-Install 3 prong power cable
-Cleaned pots, tube sockets, and jacks
-Replaced capacitiors in tone stack

PROBLEMS
Excessive white noise and hiss still coming from the amp when running. I have been trying to figure out how to quiet down this amp as it is not useable with this much noise.

Has anybody confirmed this schematic? I know older schematics have a lot of errors and sometimes are revised. My big issue is that right now the Cathodes of the 6L6s are reading continuity to ground. This is not so in the schematic and I am confused as to how this is even working with that.

I checked the valve data index page and did not find anything.

[sgmezei16]

Here is the much more confusing, and less tidy, list of voltages. I can't believe I didn't know about this forums awesome voltage reading database before now!!! SO awesome.

V1 6SJ7
1 NC
2 H      27 (this was measured DC to ground, so ignore i guess)
3 G3    0.9
4 G1     -
5 K       0.9
6 G2     25
7 H       27
8 PL.     72

V2 6SC7
1 NC
2 PL.      79
3 G          0
4 G          0
5 PL.      79
6 K         1.2
7 H         6.64
8 H         6.64

V3 6SC7
1 NC
2 PL.      79
3 G          0
4 G          0
5 PL.      79
6 K         1.2
7 H         6.64
8 H         6.64

V4 6SC7
1 NC
2 PL.      151
3 G          0
4 G          77mV
5 PL.       153
6 K           1.8
7 H           6.64
8 H           6.64

V5 6L6GA
1 NC
2 H
3 PL.         408
4 G2          324
5 G1          68mV
6 NC
7 H
8 K             26.7

V6 6L6GA
1 NC
2 H
3 PL.         409
4 G2          324
5 G1          116mV
6 NC
7 H
8 K             26.7

V7 5V4G
1
2   414
3
4   352
5
6   352
7
8   414

[sluckey]

My big issue is that right now the Cathodes of the 6L6s are reading continuity to ground.

And they should.

What is continuity? Is it zero ohms? 10 ohms? 250 ohms? 1000 ohms? Is it just a beep?

My point is don't use a continuity checker inside a piece of electronic equipment. Set your meter to read actual resistance in ohms. The fact that you have 26.7v on the cathodes means there must be some resistance between cathode and ground.

[Willabe]

First off, Welcome!!!!!     

PROBLEMS
Excessive white noise and hiss still coming from the amp when running. I have been trying to figure out how to quiet down this amp as it is not useable with this much noise.

IF you are sure all your tubes are known to be good then try changing ALL the plate resistors (R's) to metal film (MF) R's. (Could be both, tubes and plate R's.) That amp was made from 50' to 54' so the R's were carbon composition (CC) which even when new were noisy. So after 50+ years........ (CC's drift with age, so when you change them for fun measure them for resistance and see how far they've drifted, maybe some maybe a lot.)

Some guys recommend using 2w or 3w for the MF R's replacement's as they will be a little more quietier then 1/2w's. Very common problem.

Our host Doug has them in stock and is very fast at getting them to your door.


                             Brad       

[HotBluePlates]

Stethoscope Listening Amp (near the bottom of the page)

[Willabe]

Take care of that amp! Looks like it a fairly rare amp.

Ok, from the Gibson Amplifiers 1933-2008 book, by Wallace Marx Jr;

Might be worth some $$ to a Gibby amp lover? (I really love the old Gibby 'leatherette' covering and on some, which yours should have, the slotted speaker baffle!) 

Gibson GA75;

4 knobs, Ch.1 vol., C.2 vol., bass, treble.

Inputs, 5(4 instrument, 1 microphone).

Amp's power; 25w's, came with a 1x15" Jensen speaker.

Production totals:

50', 410
51', 338
52', 274
53', 101
54', 23
 
Total = 1146.

The Gibby catalog says; "This amplifier has proven ideal for accordion and string bass amplification."

Yeah man!!!!!!! (I LOVE Cajon/Zydeco music.)     

If you can, PLEASE post the OEM#'s stamped on the transformer (iron) set and I might be able to give you some more information.


                              Brad     

[Willabe]

Oh and Gibson was notorious for changing an amps circuit and not releasing a schematic with the changes.

So, if you find different values or even differently wired up circuits don't be shocked.

I have made friends with the guy who owned/built Buffalo amps (no longer in business  ) who might be able to help if your amp still throws you a curve ball after the guys here lend their help and guidance.

Also is the choke on your amp bolted to the speaker frame like in the picture in the book? (Schematic shows it's NOT a field coil speaker amp.)


                        Brad     

[PRR]

> Cathodes of the 6L6s are reading continuity to ground.

I see 250 Ohms to ground.

For more fun, a ground symbol is omitted around the two 10u caps. Probably in the same can as the other 20u caps, so it is physically there, just not drawn. If you replaced all e-caps, I assume you got this right; without it, I don't think it would play at all.

27V DC on all heaters is correct-- they are biased-up by the 6L6 cathode voltage.

> Excessive white noise and hiss

Which knobs affect it? If "none", replace all old resistors from V4 to V5/6; however this is unlikely. The one Pentode input and the four Triode inputs have separate volumes, which should identify which area the hiss is from.

Pulling tubes until the hiss changes is another technique.

[sgmezei16]

Also is the choke on your amp bolted to the speaker frame like in the picture in the book? (Schematic shows it's NOT a field coil speaker amp.)
                       Brad     

The OT is mounted right on the speaker, which made for an annoying situation of working on the amp beside the cabinet on bench. This chassis was awful to work in! But I used some painters tape to keep it open while I worked on it.

Thanks to everyone for their help!!! Problem is fixed. Turned out to be a cooked 3w 10K CC resistor. I can't believe that the tiny resistor hiding underneathe the tag strips was the culprit. BLARG!

I did build the stethescope listening amp. I had one that I made for working on guitar pedals but built a nicer one which a higher rated cap. I used an old shielded guitar cable, connected the test probe to the middle wire, and used another piece of wire with a gator clip to connect the shield to ground on whatever I'm testing.

Thanks for the help and advice on this one! Sorry it took so long to reply.

Scott