Gibson Falcon Crestline - GA 19 RVT
« on: July 29, 2019, 08:12:37 PM »
Hi folks - again looking for your expertise. Thanks in advance for any help you can offer.

Gibson Falcon Crestline - GA 19 RVT (But using M-216 RVT schematic which matches closer)

Speaker is dated: September 1975

V1 - 6EU7
V2 - 6EU7
V3 - 6C4
V4 - 6EU7
V5 - 6V6
V6 - 6V6
V7 - 5Y3

Reverb Transformer
Gibson Inc
E-6400
9966510
I took these measurements with the reverb tank disconnected and the other leads disconnected from the circuit.
Primary - 0.8 Ω
Secondary - 1.424 kΩ

I am repairing and trying to bring back to life this amp for a friend of mine. I’m not entirely new to amp repair but I would consider myself at best a novice.

I have replaced all the electrolytic capacitors and installed new JJ 6V6s and a new JJ 5Y3.  I have replaced the extension cable with a decent grounded power cable. The amp now will turn on and sounds normal. For the most part all the voltages seem to be approximately correct. The amp now sounds quite good actually. Except there is no tremolo and no reverb. I have been trying to trace the problems but I just don’t feel advanced enough in my study to move ahead any further. I have traced the signal through with a scope to the best of my ability.

I have signal on the grid and coming off the plate of the reverb driver.
When I give the reverb tank a shake I can hear the return at the speaker.
I would perhaps suspect that the transformer is bad but for a few moments a few months back, both the tremolo and reverb functioned for a few moments, ever so briefly and somewhat unreliably, but they sure did sound nice.

I don’t really know where to start with the tremolo circuit. It is I think a bias shifting oscillator acting on the grid of the 6V6s. But I’m relatively unsure how to trouble shoot this.


V1 - 6EU7

PIN 1 - Heater
PIN 2 - Heater
PIN 3 - NC
PIN 4 - Cathode 2 1.14 VDC
PIN 5 - Grid 2
PIN 6 - Plate 2 114.5 VDC
PIN 7 - Plate 1 176.3 VDC
PIN 8 - Grid 1
PIN 9 - Cathode 1 1.54 VDC

V2 - 6EU7

PIN 1 - Heater
PIN 2 - Heater
PIN 3 - NC
PIN 4 -  Cathode 2 1.26 VDC
PIN 5 - Grid 2 0 VDC
PIN 6 - Plate 2 167.5 VDC
PIN 7 - Plate 1 123.6.3 VDC
PIN 8 - Grid 1 0 VDC
PIN 9 - Cathode 1 1.05 VDC

V3 -  6C4 - This should be/is the reverb driver

PIN 1 - Plate
PIN 2 - NC
PIN 3 - Heater
PIN 4 - Heater
PIN 5 - Plate - 220 VDC to transformer
PIN 6 - Grid
PIN 7 - Cathode - ~ 10VDC

V4 - 6EU7

PIN 1 - Heater
PIN 2 - Heater
PIN 3 - NC
PIN 4 - Cathode 2 - 39.75 VDC
PIN 5 - Grid 2 - 21.48 VDC
PIN 6 - Plate 2 - 190 VDC
PIN 7 - Plate 1 - 136.4 VDC
PIN 8 - Grid 1 - 0 VDC
PIN 9 - Cathode 11.01 VDC


pucho812

Re: Gibson Falcon Crestline - GA 19 RVT
« Reply #1 on: July 30, 2019, 12:18:41 PM »
have you confirmed v1 -v4 are known good/working tubes?
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

CJ

Re: Gibson Falcon Crestline - GA 19 RVT
« Reply #2 on: July 31, 2019, 01:02:08 AM »
output of rev pan is a few millivolts, so goo connections are needed in the cables/jacks,

chek V4 grid with foot switch engaged?

i see abnormal DC there.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

pucho812

Re: Gibson Falcon Crestline - GA 19 RVT
« Reply #3 on: July 31, 2019, 12:28:32 PM »
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

Re: Gibson Falcon Crestline - GA 19 RVT
« Reply #4 on: July 31, 2019, 01:51:27 PM »
Hi Folks -

Thanks for the responses.

have you confirmed v1 -v4 are known good/working tubes?


I am not 100% certain on the functioning of the tubes. I will order replacement tubes and verify.

output of rev pan is a few millivolts, so goo connections are needed in the cables/jacks,

chek V4 grid with foot switch engaged?

i see abnormal DC there.


When I place my thumb over the input plug on to the reverb tank I get some good ground hum through the speaker.

When I knock on the reverb tank with the cable in place I do get some signal back to the speaker.

Is this a good enough indication that the cables themselves work?

Perhaps the transducer or input  jack on the tank could be faulty?

Can I replace the tank with any known working tank to verify that? Or do I have to match tank impedances etc?

I love this place. It's so awesome to have folks to help out. I really appreciate it.

Gus

Gene Pink

Re: Gibson Falcon Crestline - GA 19 RVT
« Reply #5 on: August 01, 2019, 02:35:01 AM »
Hi Gus.

Any chance you got the primary and secondary mixed up?

Schematic shows V3 plate at 220 V on one side of that reverb driver transformer, 240 V feeding it, that means a 20V drop across the primary. You measured 0,8 ohms of primary, that can't be right, that would mean 25 amps of current through that primary,  enough to MIG weld thin sheet metal. ;D

If you got them backwards, a 10K - 15Kish load on the plate would make sense, and the 0.8 ohms could be right for the secondary, they make reverb drivers as low as 8 ohms.

Rule of thumb: DC resistance of coil times 10 gets you in the ballpark of actual impedance. A very rough rule of thumb, but it works for me. Someone will disagree with this in ...5...4...3...2...

Gene

PRR

Re: Gibson Falcon Crestline - GA 19 RVT
« Reply #6 on: August 01, 2019, 03:24:43 PM »
> Any chance you got the primary and secondary mixed up?

Clearly they are mixed-up. Whether for-real or just the notes is still a mystery.

It is probably a 15k:8r transformer, so 1.5k:0.8 DC Ohms is expected. I missed the note that the 0.8r was on the wrong side, good eye!


 

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