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Help me get my 6G2 running

Che_Guitarra

Member
Messages
4,165
I finally finished the soldering in my 6G2 build today - but rather than a night of rocking out, it's a night of silence. I need help and advice to sort out where I went wrong.


My amp did turn on for half a minute (and I can confirm the volume and tone work), but the noise it made was a feedback/oscillating squeal before it finally gave up the spirit when I picked up the guitar for a strum. Now I have no visual or audible clues that the amp is working. No smoke, no smells, nothing to report.

Looking into the amp and the fuse hasn't blown, and my pilot light is also fine, but my power circuit is not completing. I figure something is up with my power transformer - a Classictone 40-18027. I have noticed it's missing a red/blue tap that can be found on the exact same trannie I bought for my 5F1 6 months earlier... but maybe that's just a coincidence.

Anyway - i've followed this schematic to the tee so unless there's something wrong with this circuit, I can't see what i've done wrong. But I have adapted the PT arrangement to accommodate my need for 240V, and based on Classictones 18027's tap/wiring chart (found here).

Can anybody offer any advice/suggestions?
 

trancedental

Member
Messages
229
Check the rectifier tube is working fine & the rectifier voltages are within specification with all the other tubes removed. Then check the connections to the board & voltage readings.

Pictures would help for when the real experts arrive on the scene!
 

Che_Guitarra

Member
Messages
4,165
Firstly - here's the schematic i've used. The bias circuit has been integrated into the main circuit board:




Here's the 'gut' shots. Still learning the fine art of lead dressing, but it's better than my last effort!











Oh yeah, I failed to mention that i'm building this into a tweed princeton chassis, so it's not a 6G2 per-se... maybe best to say it's 'based' on a 6G2.
 

Blue Strat

Member
Messages
30,243
First order of business before plugging in is checking all voltages against the schematic. They should be within about 10-20%. Also check idle current of power tubes.

The squealing could mean that the output transformer primaries are connected in reverse.
 

Che_Guitarra

Member
Messages
4,165
I can't get any readings as the power circuit is not completing.

Upon inspection the fuse is still fine, and a multimeter continuity test confirms the on/off switch still works. Solder has grafted perfectly on all points from the power plug/fuse/switch/PT... so for mine, something has to be up within the power transformer.
 

Blue Strat

Member
Messages
30,243
I only see 1 black power transformer wire, in the right transformer hole. Where's the other one?
 

Jerry Glass

Member
Messages
870
I only see 1 black power transformer wire, in the right transformer hole. Where's the other one?
Mike, it's hooked up correctly; he's in Australia (240vac). The black and brown/white are used and the other two leads are tied together.

OP: you need to verify your AC function; (1) that you have 240VAC at the brown/white and black leads and (2) that you have AC supplying the rectifier tube (550VAC total, 275VAC on each red/white lead) and the heaters (6.3VAC).
 

Che_Guitarra

Member
Messages
4,165
That's correct Jerry - and I have these taps snipped off short and neat so they're out of sight, out of mind as it were. Brown and black/white are trimmed twisted and soldered off under the heatshrink.

Anyway, an update... my bad here - I just got out the continuity tester and it seems the AC circuit is fine. It was just a blown fuse, but it's blown right at the base so there's no visual reference it's gone bad.

It's gotta be my heater wiring to the 6V6's, and perhaps the triode I left empty on my 12AX7. The factory 6G2 wiring says to group pins 7, 8 and 1 of the 6V6, but on a 5E3 (with same tube allocation) pin 1 is empty, 7 is heater, and 8 goes to ground via a cathode bias resistor. I think the problem lies within this... i'll have to do a bit of investigating.
 

Prattacaster

Member
Messages
1,369
Do you have a light bulb current limiter. Probably not since you are blowing fuses. I'd suggest building one; it could save your PT.


The factory 6G2 wiring says to group pins 7, 8 and 1 of the 6V6, but on a 5E3 (with same tube allocation) pin 1 is empty, 7 is heater, and 8 goes to ground via a cathode bias resistor. I think the problem lies within this... i'll have to do a bit of investigating.
I would not bother comparing to a 5e3. This is fixed bias so you need pin 8 of both 6v6's grounded or a bias resistor(1ohm 1%) in series between pin 8 and ground.
 

Che_Guitarra

Member
Messages
4,165
6V6's are now wired correctly. Tubes are glowing, it's making sound... unfortunately it's a high pitched slow oscillation - no guitar signal can be heard. Fuse seems to be holding out, and no smells or visual indicators something is wrong. Volume and tone controls having no impact.



One ohm resistors bridging pins 8 and 1, then running from 1 to chassis.


Update: another observation - when unplugged, as you pump up volume the high pitch oscillation drops in pitch.
 
Last edited:

Che_Guitarra

Member
Messages
4,165
Two steps forward, one step back. Oscillation was fixed by swapping OT primaries on the 6V6 plates, but now I think I have an input issue. Otherwise, volume and tone on the amp appear to be working fine, and if I tap the tip of a guitar lead I get the typical bump in hum... but plug in a guitar/s and still no glory.



Here's my inputs as they are, wired up as per the '62 schematic spec. I thought the switching jacks might ground out via the chassis, but now i'm not so sure. Can I get a confirm on this?
 

Che_Guitarra

Member
Messages
4,165
Yeah, I messed up the input wiring. The left input in the photo needed the soldering rotated around one lug, and to be sure I also ran a ground.

The amp is working like a charm now, although it's still quite early so I can't crank it yet to hear the 'fun' side of the volume dial :)
 

Che_Guitarra

Member
Messages
4,165
I better erase that last post. The amp's right input is working fine, but dime volume or tone control and the left input has a very unnatural note decay. My pedals don't want to work through the amp either, although when I first fired up the 6G2 my pedals were working fine (albeit there was a seemingly excessive AC hum). They're running off a one-spot and daisy chain, and am yet to see if it's the same story under battery power. But I can confirm the pedals still work fine with my other amp, and the static/hum isn't nearly as obvious.

Any ideas?
 

Che_Guitarra

Member
Messages
4,165
Here's a video to try and better explain my problems. Can anyone explain/indicate what the symptoms would be of a faulty grid resistor? I can't see why else one input is fine but the other is suspect.

 

Prattacaster

Member
Messages
1,369
Can anyone explain/indicate what the symptoms would be of a faulty grid resistor?
No way you have a faulty resistor, as in
"open" circuit.

Carbon comp resistors do drift upwards in value due to heat and age. Your grid resistor is a new carbon film and could not be the cause of your (symptoms).

If you have a multimeter you should resistance test your "tip" connection to ground. Insert your cable plug and see if you get the proper resistance readings.

Your problem is much more likely a bad solder joint or miswiring or some other human error. Just simple probability. I know how you feel; its a lot easier to think that you have a bad circuit drawing or a bad component rather than having made a mistake. NO big deal, we all have been there, done that. You'll find it.

Could you give us some updated pics of your input jack wiring?
 

Che_Guitarra

Member
Messages
4,165
Problem solved - it was a cold solder on one of the input lugs - a reheat and a bit of extra solder and the amp is working like a charm. Now my biggest problem is choosing between RCAs, JJs and Tung Sols :)
 






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