Help with a LAB SERIES L5 (2x12 100W SS amp)

[timrichter9]

Hey all-

I recently scored a $54.00 1979 Lab Series L5. A couple of problems.

1. Someone replaced the speakers and it has 2 Eminence 12" wired as 4 ohm. The back panel says 8 ohm minimum. I assume it would be better to wire them in series for 16 ohms according to that warning, right? (my experience is with tube amps, and I was always told that matching the load was super important in tube amps).

2. Would someone else who owns this amp be willing to help me. I need a picture/description of the reverb pan. Mine has been fitted with one of the smaller (shorter -- 9") pans. However, in looking at the amp, there are two screws on the bottom panel of the cabinet. I am presuming that they are to screw down a vinyl pouch that holds the reverb tank like on a Fender twin reverb or super reverb. I am assuming this amp may have had a longer reverb pan (16"). Can anyone confirm to me that this amp would have had such a vinyl pouch holding the reverb pan, and if so, would someone care to read me the code of their pan. Accutronics pans have a code on them that tells the input impedance, out put impedance and type. I am thinking it should be akin to 4AB2A1B (16" 2 spring unit, 8 ohm input impedance, 2250 ohm output impedance, medium decay, grounded connectors, no lock and horizontal mount). 9AB2A1B if it was 3 spring.

3. The compressor will not work. When I got the amp, there was a slight distortion and the notes would trail off occasionally. I pulled the preamp PCB out and cleaned all the pots with DeOxit and that solved that problem. However, the onboard compressor seems to have failed. When I got it, you could engage the compressor and the compressor LED would light when you dug in hard on a humbucker. Now, switching the switch, lights the LED for a second and then no effect on the compressor. Anyone with SS experience care to venture a diagnosis?

http://www.rru.com/~meo/Guitar/Amps/...11_scheme.jpeg is the schematic.

What a killer clean tone.

[52 Bill]

Great score there. BB's favorite amp.

I remember that they had a bag and long type 4 tank originally. Your tank guess would be right except for the grounded connectors. This should be an isolated input and grounded output tank to avoid a ground loop.

If the compressor was working before you pulled the preamp card, go back and check for loose or cold solder joints, wires, etc. around the compressor circuit. Did you readjust the compressor trimmer per the schematic instructions?

With the exception of those rear shafted pots and one ic chip, all of the small parts are either available or replaceable. Great deal.

[timrichter9]

Thanks Bill. The compressor was not really working per se. The light would come on and it appeared to be compressing the guitar signal, but not really sure. I think the Darlington is the non-obtainable IC, but I have heard that it is essentially two transistors in one package, so maybe I could create one with two discrete parts.
Interesting your thoughts on the isolated input. Most of the Fenders used shielded cables and the shielding was always grounded to the negative of the input RCA connector. I will check that.
I did not adjust the trim pot on R186, as I did not know what signal to feed into the amp
http://www.rru.com/~meo/Guitar/Amps/...5_notes-2.jpeg is the service note explaining it, but I don't know what a 1k2 100mu -20dB sine wave is. I have a HP204C sine generator, and I can set it to 1.2k but what is 100mu -20dB? (Is that a typo and it may be 100mV??)
Thanks for your help
Tim
I did not check that A112 (the 4558) was getting +/-15vDC, but I believe the supply rails are good. Will check that tonight.

[g1]

1KH2 sinewave 100mu should read 1Khz sinewave 100mV.
Also appears that steps 2 & 3 of compressor adjustment should be reversed.

[timrichter9]

Yes. It seemed odd. Thanks
Tim

[Enzo]

Does the existing "wrong"reverb pan work OK? I mean other than tone. If so, take the number off it and just change the first digit to 4 or 9.

[timrichter9]

Yep. It works fine. I am just a stickler for trying to return a pawn shop fine to original specs.

[g1]

4AB3C1B, same as older Fender amps.

GIBSON 1970'S CMI AMP 1FB2A1C
GIBSON FALCON AMP 4FB2A1B
GIBSON FALCON GA19 RVT 4FB2A1B
GIBSON G40 GUITAR AMP 56-1 GIBBS MODEL C - 4FB2A1B
GIBSON G60 - SOLIDE STATE 1BB2D1B
GIBSON G60T - TUBE AMP 8AB2D1B
GIBSON 1972 G80 4FB2A1B?
GIBSON 1974 G80 Gibbs model H - Accutronics model 4FB2E1G
GIBSON GA-15-RV MISTRO Gibbs model C - 4FB2A1C?
GIBSON GA-15-RVT EXPLORER 4FB2A1C OR LETTER C
GIBSON 1964 GA-17-RVT SCOUT AMP RV4C-1 - 4BB2A1C
GIBSON 1960 GA-19-RVT FALCON - Serial #774551 Gibbs model C - 4FB2A1C?
GIBSON GA-20-RVT Minuteman Gibbs model C - 4FB2A1B
GIBSON 1961 GA-30-RV INVADER Gibbs model C - 4FB2A1C?
GIBSON GA-30-RVH 8BB2C1A
GIBSON 1965 GA-35-RVT Lancer Gibbs model C - 4FB2A1C
GIBSON 1964 GA-3-RV Gibbs model c - 4FB2A1A
GIBSON GA-55-RVT Gibbs model C - 4FB2A1C
GIBSON GA-77-RET - SIMILAR TO A RVT GIBBS MODEL C - 4FB2A1C
GIBSON 1963 GA-77-RVT C - 4FB2A1C
GIBSON 1961 GIBSON AMP HAMMOND 1122-6051 - 4FB2A1F
GIBSON GOLD TONE GA 15 RV 8CA3B1B
GIBSON GOLD TONE GA 30 RVS 8CA3B1B
GIBSON GOLDTONE GA 30 9BC2C1B
GIBSON GSS-100 SOLID STATE AMP 100W 4C - 4FB2A1A - CHICAGO MUSICAL INST P/N 984-012419
GIBSON 1968 MEDALIST 4-10 AMP 4FB2A1C
GIBSON 1969 PREAMP 800 GIBBS MODEL L (122-6936) Lowrey p/n 984-003365 4FB2B1B
GIBSON SO-CAL 50 Comes w/Belton BL2AB1C1D - replace with 4AB1C1D
GIBSON SUPER GOLD TONE 9BC2C1B
GIBSON GIBSON DUAL MEDALIST - 984012419 GIBBS C - 4FB2A1A
GIBSON - LAB SERIES 30 WATT SINGLE 12 COMBO AMP 1BC2C1B
GIBSON - LAB SERIES L5 COMBO AMP p/n 935-040694-001 also Moog Music 4AB3C1B - also had letter F stamped inside
GIBSON - LAB SERIES LAB 5 SERIES - P/N 935-040694-001 4AB3C1B
GIBSON - LAB SERIES LAB SERIES 4AB3C1B - 935-040694-001
GIBSON - LAB SERIES 1978 LAB SERIES I5 GUITAR AMP 4AB3C1B
GIBSON - LAB SERIES LAB SERIES L5 13142 4AB3C1B
GIBSON - LAB SERIES LAN SERIES L3 COMBO - Moog Music #935-044070-001 1BC2C1B
GIBSON - LAB SERIES MODEL 316A - L3 GUITAR AMPLIFIER LAB SERIES P/N 993-042756-001
GIBSON - LANCER GA-35-RVY GUITAR AMP 4FB2A1C
GIBSON - TRACE ELLIOT 2004 GA-15-RV / GA30RVS 8CA3B1B
GIBSON - TRACE ELLIOT 2004 GA-30-RV / GA-60-RV 9BC2C1B
GIBSON - TRACE ELLIOT 2004 GA-30-RVH 8BB2C1A

[timrichter9]

Thanks!

It appears I need 4AB3C1B. Long (16", 2 spring, 8ohm to 2250 ohm long decay (2.75 to 4.0s) Input insulated, Output grounded, no lock, open side down)
What is in there is 8BB2A1B Short (9: 3-spring), 150ohm to 2250 ohm, medium decay (1.75sec to 3.0s), input and output grounded, no locl, same mount

So, it appears there are two things wrong (impedence IN is way off and decay is different, but also input is grounded and shouldn't be. Is this a problem?


Also. I got out my meter today and noticed the following.
The -15 rail seems fine (measures -15.12)
However, the +15 measures out at 16.2vDC?
Is this a problem? I say this because the amp sounds FANTASTIC and my only issue is that the compressor is not working.
Would that difference be a lead to discover why?

[52 Bill]

So, it appears there are two things wrong (impedence IN is way off and decay is different, but also input is grounded and shouldn't be. Is this a problem?

Also. I got out my meter today and noticed the following.
The -15 rail seems fine (measures -15.12)
However, the +15 measures out at 16.2vDC?
Is this a problem? I say this because the amp sounds FANTASTIC and my only issue is that the compressor is not working.
Would that difference be a lead to discover why?

The grounded input jack on the tank will create a ground loop in the reverb circuit. If you look at the schematic, you will notice that the drive circuit grounds at the power supply and the return circuit grounds at the preamp circuit. If both jacks on the tank are connected to the case, both grounds in the amp will be connected together at the tank creating a second ground path.

The one volt difference in the power suppy should not make enough difference to cause any real problems. I still suggest that you review what you did when you pulled the preamp board and check the compressor circuit for cold solder joints or broken wires, etc.

[timrichter9]

Bill-
I have reviewed it. I checked all the connections with VOM and other than touching all the joints with a soldering iron, I cannot see a bad connection.
I did get some odd readings off of the compressor potentiometer (R185). I was getting less than 16k from terminal to terminal (but I did not take it out of the circuit when testing).
I am now getting 50k across those same terminals and wiper to one reads 0.22 then 49k when I swipe it. Going to reinstall and test.

[timrichter9]

THANKS THANKS THANKS to everyone who helped.
I got the signal on the scope, followed the compressor adjustment and it worked. I could actually SEE it on the scope.
When I fed the 100mV signal into the amp and set the R186 potentiometer, the LED came on. When I saw the preamp output on the scope as a clean sine wave,
I increased the compressor POT and the LED lit and the sine wave got smaller.
I am so happy people are willing to help. This was the first time I actually saw my amp through the scope and with help, I knew what I was doing!!!
Thanks all
Tim
(now I hope I can reassemble without flexing a solder or two (the PCB is 24" by 3" so it flexes!)

What I didn't see was the OUTPUT of the amp on the scope. I was not sure how to do that.
Where do I put my probe leads. Am I looking for an AC signal (I presume).
Do I just jump across my dummy load?

[g1]

Just so there is no confusion, that 100mV would be RMS as measured with a multimeter, not 100mV peak to peak on the scope (100mV RMS would be 283mV peak to peak).
To see the amp output, connect probe to dummy load "hot" side ( or spkr. out yellow), scope ground connected to amp chassis.