Just another 6G2 build

smuc

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Seems that 6G2 are a popular build these days, and with a few Super Champ X2s lying around, I have decided to give a go with a build that will fit and use the existing iron. Took me a while to draw the whole thing in CAD and for the machine shop to build it, but the chassis is finally here...

On to the quick chassis test fit!
Capture.PNG IMG_6490.JPG IMG_6488.JPG IMG_6489.JPG
 

smuc

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Scubribde!


Btw, I've often wondered that- if I can just go to some machine shop and have them make a chassis. I just assumed it would be huge $$.
Well, I guess you just need to find a machine shop that does small volumes. I have found one not far from me and the labor is anyway north of 95% of the cost. I guess anything between 30 and 60 EUR for a single piece would be reasonable.
Cool. Are the holes threaded for mounting screws?
Yes. My previous builds all included cagenut mounts, and this is the first threaded mount. The Super Champ has the same mount on the chassis that‘s thinner than mine so I am guessing I will be fine.
 

smuc

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Turretboard done.

I needed to cut and drill this one by hand because the "vintage" board I ordered was not playing nice with the laser cutting machine and was leaving a nasty black ring around the holes.

All in all, happy with how it turned out!
 

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smuc

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Nice. What board are you using? I like it.
I believe that's some form of bakelite, cotton layers of cotton cloth formed with phenolic resin:

Which I guess explains why it did not play nice with laser...
 

smuc

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Made some progress in the past few days. The caps checked for outer foil, the main board populated and wired.

Next step - heater wiring & rectifier before the main board goes into the chassis.
 

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smuc

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Finally making some progress. Rectifier and bias circuit wired up, pilot light and the heater filaments for V2, V3 & V4 as well.

9V tap ready for the V1, just need to order the voltage dropping resistors.
IMG_6583.JPG

There is one thing that keeps me scratching my head - I wired up the bias circuit and I don't see any negative voltage on it. Both sides of the 100K resistor are at 150 VAC, the other side of the diode has only a few mV DC. I tested the diode and the diode seems to be working properly, however I don't see any voltage past it. I tested the resistance to ground and it is 27K + whatever is on the pot.

Now, did I mess something up or the vibrato circuit needs to be wired up completely before I will see a negative voltage on the bias?
IMG_6581.JPG Screenshot 2022-03-13 163628.png
 
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2L man

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Obviously the Diode has burn open! When there is the same voltage on both side of the 100k, it means there is no current flow! Although 150VAC seems low if Bias voltage is taken from HV and high if Bias has separate output?
 

D'tar

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150vac sure is low, Is your bus complete? Do you have return path to PT via Center tap or bridge connection to chassis.
 

smuc

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Obviously the Diode has burn open! When there is the same voltage on both side of the 100k, it means there is no current flow! Although 150VAC seems low if Bias voltage is taken from HV and high if Bias has separate output?
I checked the diode on the component checker and the multimeter and it works properly. I even tried an another one.
150vac sure is low, Is your bus complete? Do you have return path to PT via Center tap or bridge connection to chassis.
150 is what I should expect to see on one of the HV nodes from a 300V non centre tap transformer with a bridge rectifier, or? B+ is 410V unloaded.

I tested the bias circuit before I wired the whole thing up and reading your post I thought that was the problem so I wired the whole amp up. The issue persists, and reading through Merlin's pages on bias, it seems to me that the problem is that I am trying to use a bias supply designed for a centre-tap transformer and the one I have does not have the centre tap.

So, I can either use Merlin's workaround, or even simpler - I will just use the 34VAC transformer tap and put in a bias doubler circuit. (which is anyway the way the bias supply is made for the Super Champ X2 I took the iron from)
 

Uncle Daddy

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I've not used one of those trim pots before. When I built my 6G2 I used this bias layout and it works fine. Ignore the red hookups to the rectifier though!

bias 10.JPG


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2L man

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150 is what I should expect to see on one of the HV nodes from a 300V non centre tap transformer with a bridge rectifier, or? B+ is 410V unloaded.
If there is 300VAC on HV secondary and bridge rectifier is installed right and it work, there is 300VDC on B+ if there are no filter capacitors connected and about 420VDC when filter is connected.
 

smuc

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I've not used one of those trim pots before. When I built my 6G2 I used this bias layout and it works fine. Ignore the red hookups to the rectifier though!

View attachment 963230

View attachment 963231

If there is 300VAC on HV secondary and bridge rectifier is installed right and it work, there is 300VDC on B+ if there are no filter capacitors connected and about 420VDC when filter is connected.
Thanks for the suggestion guys. In the end, it was indeed the problem that the bias circuit was not playing nice with the bridge rectifier. I just took the 34VAC tap and plugged it onto the bias circuit and voila - bias works like a charm!

(Red and black leads are for the heater hookup from V2 to V1)

IMG_6613.JPG IMG_6616.JPG IMG_6617.JPG IMG_6618.JPG IMG_6619.JPG

There are two questions/issues:
1. The PT I used was the one from the Super Champ X2, that has 2x6V6 and 1x12AX7 which add up to the 1.2A. The replacement PT from Hammond (Datasheet) states [email protected], which should be just barely enough for one additional 12AX7. For the testing purposes, I connected all four tubes, which got me to 6.45V.

So, the question is - do you think it's fine to leave it like that or should I take the 9V tap and two voltage dropping resistors (which I originally intended, but would like to avoid if not needed). This will likely get the heater voltage to 6.6V.

2. The amp works, and when connected to the resistive load all pots noon no issues at all. Trem works, nice sine on the scope.

BUT (why would it work perfectly immediately if something can go wrong):

If I turn the volume and treble all the way up, I get parasitic oscillations which are unstable - they come and go and happen only with the volume & tone above 90%.

If I connect a speaker or an inductive load, I get oscillations (high pitched squeal) immediately even with the vol/tone at 0.

I'm scratching my head why is this happening because all of the grids have resistors on them (standard 68K input and 27K on the next two gain stages.

I tried moving the wiring around a bit but no significant success. Tomorrow I will try some other tubes, but any other ideas would be appreciated! Thanks
 

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D'tar

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If I connect a speaker or an inductive load, I get oscillations (high pitched squeal) immediately even with the vol/tone at 0.
output phase? disconnect nfb and recheck.
 

Uncle Daddy

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I had exactly the same issues with parasitic oscillation, which mainly was lead dress on V2 pins 1 and 2. I also added the diode to the trem intensity pot, and a snubber cap to the 220k resistor before the 0.1uF cap that feeds the intensity pot.

The thing that really threw me though was the copper tape I'd lined the inside of the cab. It all worked fine with the chassis on the bench, but once it was bolted up the noise would start again.

That heater voltage looks to be within the 10% margin. The Fender schematic specifies 6.3v.

I've never had an amp this quiet before- I'm thinking of calling it the Red Oktober!
P1130458.JPG
 




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