Gibson GA19, 7199 ver, 6EU7 cathode cap

[capnjuan]

Hi folks; I'm new to the board and hoping maybe someone can help me with this. I have an early 60s Gibson GA19 3X6EU7, 1X7199, 2X6V6 plus 5Y3. This is a 'tweed' model with a 7199 at V2 as opposed to the later 'Crest' model with a 6C4 at V3. In both cases, these tubes operate the reverb.

Before going further, I have schematics for both; PM for copies. Besides the 7199/6C4 distinction, Gibson numbered the tubes from left to right in the schem for the 7199 model and right to left for the 6C4 version; this contributes to some misunderstanding about which tube goes where in these amps.

The 7199 version has a 6EU7 in V3 however only one of its two sections is wired; the other does nothing. It is downstream of the volume and tone controls. The two other 6EU7s in the amp have 20uf cathode caps bypassed by 2.7K resistors; both 'sides' of the two other 6EU7s are wired however the schematic shows a 02 cap - not a 20uf cap - in the conventional cathode cap position bypassed by a 2.7K resistor. In the 6C4/Crest schem, the single-sided 6EU7 is also downstream of the volume controls and its schem indicates a .1uf cathode cap bypassed by a 4.7K resistor.

The amp arrived with a tired 7199 and the amp hummed; I replaced the 7199 with a new Sovtek (don't razz me, seemed to make sense at the time), and the amp hummed. I have been completely through the amp with new passives, caps, everything; and the amp hums.

Is there anybody out there that thinks that the .02 cathode cap is really the right value for the application or is it possible that Gibson just made a drafting error transposing 20uf for '02uf'? Incidentally, the 02uf is not preceeded by a decimal point.

I've got a new JAN 7199 ready to go and intend to switch out the 02 (which I took to be a .02) for a 20uf/25V. Am I crazy here? Halloooo !!!

capnjuan

[Enzo]

Well, it would take mere seconds to either clip in a 20 or tack solder one in place and find out. What part is there now? Looking at the drawing, I'd have to say I'd expect a 20uf there. But if the original part is a .02, then so be it.

Either way, it is not the source of your hum.

If you think the 7199 and its circuits are the hum, then pull the tube.. If the hum stops - you were right. and if not, then the tube was not involved.

The old Gibsons had the volume control in the drive circuit rather than the recovery circuit. So there is no way to "turn down" the reverb. SInce more than one tube allows the hum, and asuming the reverb circuit is the hummer (pull the tube, remember) then try grounding the pentode grid - short across that 100k resistor. If that kills the hum, then the wiring, reverb pan itself, footswitch cable, etc. are at fault.

[capnjuan]

Thank you; the hum dissipates when the 7199 pulled. The reverb works; that is the pan, pot, and FS all function. I pulled the .02 orange drop, put in 20uf electrolytic, no change, will put back...maybe...I mean I don't undersand why it isn't a 20uf.

I'll start troubleshooting the rest of it; pan bought on ebay said to be 'working condition' but you never know. FS is new, will try the FS cable tie ins.

Thanks again.

[TD_Madden]

as an aside, the new 7199s hum like crazy..should be better with NOS. I have two of these amps and neither hum unless I try a new 7199 (sovtek).

[capnjuan]

10-4 on the noisy Sovtek 7199, also goes for their 6EU7s.

[capnjuan]

And the hummmm answer is....(drum roll)....bent contact arms on the input jacks. Thought it was the 7199 which, when pulled, dumped the hum. Nice try but the contact arms over the years had lost their tension and failed to keep sufficient pressure on the male plug.

My work on the amp had been limited to rebuilding the power supply and replacing passives. I mean I hadn't messed with anything else at all but had assumed it was electronic and not 'mechanical'.

There you have it; for all you that chase hum or might chase it in the future...one more possible souce for your checklist.

Thank you to those who offered assistance; have a hum-free day!!

[capnjuan]

Bumping my own post; apologies if considered bad form here. Further research into Gibson GA19RVT and zzz issues. Fixed input jack...thought I had it; like all fools chasing the end of zzz....Took back to shop and after a lot of effort; tests/bypass this/disconnect that, the tech looked up the 7199 in his copy of the RCA tube manual which sez that the 'median' noise for a 7199 is nearly 100 microvolts. The 'median' noise for a 12AX7 was on the order of 1 microvolt; that is, a 7199 can be expected to be nearly 100 times noisier than a 12AX7. The zzz is about what anyone would expect.


I note that the later Gibson GA19RVT 'Crest' model uses a 6C4 for reverb. Does anyone think a 6C4 would be less noisy? Off the top of your head(s), if quieter, how much effort to re-wire the socket for a 6C4? Worth the trouble?

While I'm on the subject, this GA19RVT has a center-tapped heater circuit; how much effort to re-wire the 3 6EU7s for 12AX7s? I have a sister GA18 tweed 2X12AX7/2X6V6 amp; did to it what I did to the GA19....the GA18 is sooooo quiet... Schematics for all amps mentioned here on request. Thank you. John

[Enzo]

Thing is, we use the 12AX7 as an input stage, where 100 microvolts would be a lot closer to the signal level than at say the phase splitter or the reverb driver. In those applications, the noise level is compared to a signal level of many volts. So 1 microvolt against 100 millivolts of guitar signal at the input stacks up against 100 microvolts against 20 volts of signal pretty well at the phase splitter. I don't think the noise figure is your hum problem. I bet if you ran the heater on DC, the hummy SOvteks might settle down.

You did not report if a good NOS 7199 made a difference.

Reverb circuits are notorious as hum producers. try grounding the grid of the humming stage. Does that kill the hum?

[capnjuan]

Thank you; I replaced the new Sovtek with a NOS Jan 7199, no difference. The heater wiring dress is decent; tightly 'twirled' and laying close to the chassis pretty much everywhere. The mid-60s interest in stand-alone reverb units is beginning to make more sense now.

I have a design for a DC heater circuit but I've tried to keep the amp as stock as possible; give it a chance to be as good as it can.

Assuming I found the offending section by grounding the grid and dumping noise, what then?

Any thoughts on the 6C4? Thanks again. J

[Enzo]

If grounding points in a circuit kill an unwanted noise, it means we have grounded the signal path of the noise. SO the noise is either entering right there or somewhere earlier in the circuit. if the noise is not affected, then the source of noise is after that point in the circuit.

[capnjuan]

Thank you Enzo; when my current project is off the bench, the GA19 is coming back up. Regards, John

[AlexK]

Bumping an old thread...capnjuan, did you ever fix this hum problem? I have the crestline GA19RVT and I have a hum problem in my reverb path also. I still haven't fixed it, but I did add a pot after the reverb recovery so I can adjust the amount of reverb (and hum) injected into the main signal path which helped a ton. There's a thread where Erik Euthymia mentioned he had put a high-pass filter before the reverb recovery tube on a Gibson with the same problem and that helped a lot. I haven't tried that yet.

Do you think you could email me a copy of the schematic with the 6C4 reverb tube? I lost my electronic copy a while ago. My email is a_kurylak AT hotmail DOT com. Thanks.

[capnjuan]

Hi Alex: email sent w/ 3 schematics; one of them has to match yours .....eh .... probably. Gibson made changes made on the fly back then , part of the amp's 'charm'.

Good luck with your project. j

[TD_Madden]

capnjuan, I sent you a PM about the Crestline schematic, in case you hadn't seen it :-)

[capnjuan]

Hi Tom; replied. J