Conn Organ tube amp, 6L6G. Any recommendations for a new build?

[martin manning]

I would add grid stoppers to prevent oscillation, say 100-470Ω. The screen voltage can be a bit higher than the plate. It has to be much lower for the screen current to shoot up. You can see that on the data sheet curves.

[turbofeedus]

I would add grid stoppers to prevent oscillation, say 100-470Ω. The screen voltage can be a bit higher than the plate. It has to be much lower for the screen current to shoot up. You can see that on the data sheet curves.

Interesting, learning something there. I thought grid-stopper referred specifically to resistors on the control grid to prevent oscillation, but I see that it can be the screen grid as well. Thanks Martin.

[pompeiisneaks]

Yes the older amps omitted these, I think for 2 reasons.

1. the older tubes were more robust and handled it better
2. some of those amps were used when voltages were 110 or less at the wall, and nowadays they're 120 or more meaning B+ voltages are a lot higher.

Either way it's pretty common thinking now to definitely add droppers to the screens. anywhere from 1k to 5.6k is pretty common on most modern designs. Make sure they're 5w or they'll not do so well to the abuse.

~Phil

[turbofeedus]

Yes the older amps omitted these, I think for 2 reasons.

1. the older tubes were more robust and handled it better
2. some of those amps were used when voltages were 110 or less at the wall, and nowadays they're 120 or more meaning B+ voltages are a lot higher.

Either way it's pretty common thinking now to definitely add droppers to the screens. anywhere from 1k to 5.6k is pretty common on most modern designs. Make sure they're 5w or they'll not do so well to the abuse.

~Phil

One convenient thing about this PT, it has multiple primary taps. I'm not sure exactly where it's going to put me for the B+, but I measured all the primary taps vs the 6.3V heater supply, and am going to use the closest one. That should also compensate some for the B+. I'll know more when it's wired up. I'm probably going to use 1K grid stoppers for the screens, I have a bunch of metal oxide 1Ks

[turbofeedus]

Progress pic.
Added 194B choke from Hammond.
I dropped the coupling caps from the PI to 47n, thinking it might sound better for guitar.
The board is mostly copied from Fender's layout, but the bias circuit is tweaked to be adjustable, and to compensate for the lack of bias tap on this transformer.
Waiting for the dumb 10M resistor, I imagine anything 2.2M up would "work", but I guess I should just do it right.

progress-1.JPG

[turbofeedus]

Progress pic #2.
I'm modifying the circuit, just going for a more standard power amp design, with the OT CT and screens either side of the choke.
There's two 16uf/450V caps in parallel that I'm switching for a 22uf/500V. Also increased the reservoir cap to 22/500.
Added the DCCF diode protection mod ala Merlin. I'm going to skip the DC heaters this time. May reduce the cathode resistor value to drop the C-K voltage to within ratings.
This build is really cool, in that the chassis has a bunch of extra holes that could all kill you if you go pokin' fingies.

progress-2.JPG

[turbofeedus]

Build is complete and plays. HT ended up a bit lower than Fender (365 vs 400 on the plates), but the filament voltages are bang on.
Unfortunately I did see (and hear) an arc in one of the 6L6G. Might have been the reason for the jumped fuse, not sure. It didn't pop the 1A fast blow I have in the HT line now.
Going to keep tweaking and playing, thanks to all who lended advice.

progress-3.JPG

progress-4.JPG

progress-5.JPG

[pompeiisneaks]

Looks nice!

We'd love some sound clips.

I wish my lead dress was that nice looking, mine still look crappy

~Phil

[turbofeedus]

Looks nice!

We'd love some sound clips.

I wish my lead dress was that nice looking, mine still look crappy

~Phil

Thanks, I'm getting better with practice Who woulda guessed...
I will definitely try to make some samples, I just got a new 2x12 with V30's, will use that.
One thing I didn't consider with this build; that cathodyne phase inverter always messes with me. The PPIMV basically doesn't work. Well it does, but only the last 1/10th of the sweep. I suppose a 100K pot may work, but I may just rewire to just put the master after the tone stack? Not sure will have to think on it.
Also the cathode on the DCCF is at 189V, yikes! According to merlin, I can drop the cathode resistor to 47K to "sweeten" the compression, theoretically this should also reduce the H-K voltage right?

[sluckey]

The cathode voltage will be determined by the voltage on the grid. Changing the value of the cathode resistor should not change the cathode voltage.

[turbofeedus]

The cathode voltage will be determined by the voltage on the grid. Changing the value of the cathode resistor should not change the cathode voltage.

Hmm OK, so really the DC elevation on the heaters is the way to go. Maybe it's a bit of a solution in search of a problem, but I'm definitely way over the max H-K voltage.
I should mention I'm using 12AX7 for all three preamp tubes.

[sluckey]

All cathode followers and cathodynes will have a large voltage on the cathode. A lot of us, including some famous people named Leo and Jim just ignored this.

[turbofeedus]

All cathode followers and cathodynes will have a large voltage on the cathode. A lot of us, including some famous people named Leo and Jim just ignored this.

Fair enough, if it's good enough for y'all, then good enough for me. Not bad company to be in anyways.