Octal preamp 2204 build questions

[Yoda]

Hello all,

So I built a 2204 using 6SL7s instead of the usual 12AX7s and the idea was to use 6V6s and make it a 20-watt amp. The transformers came from another local amp builder on Craigslist, I was told they are Egnater transformers but I cannot find any model on them. Anyways the main thing is I’m not 100% sure of the OT impedance, I was told it was 6.6k but I’m sure there is an easy way to check this.

I just want to make sure I’m putting some compatible power tubes in this; I get 400v on the plates as it is. As I said I wanted this to be a 6V6 build but I can tube it differently if I have to.

Thanks in advance.

[martin manning]

You need two put some AC in at the primary or the secondary and measure voltages, primary plate-to-plate and secondary. Then find Zpri = (Vpri/Vsec)^2 x Zsec.

[dorrisant]

I built a 2203 with 6SL7s a while back. I could kick myself for ever letting it go. I have plans for another one very soon.

[Yoda]

I think using 6SL7s in some classic amps that didn’t originally use them is a good way to make something familiar but still unique. Since the JCM 800 is really gainy and the 6SL7 is supposed to be a noisy tube not suited for high gain, well…. challenge accepted! This amp didn’t turn out too noisy afterall. I will probably post some pics of it in here while I troubleshoot a couple things with it soon.

[Yoda]

Some pics:

[Yoda]

Initial testing: no sparks, smoke, or explosions…. always a plus.

Coin base GE 6V6GTs running at 403v, bias set to 24.5 mA. I also tried some 5881s but the 6V6s actually sound fuller in this amp. The goal is to make around 20 watts anyways.

The preamp is tubed with a vintage USA Tung-Sol VT-229 in V1 and Tung-Sol reissue 6SL7s in V2 and V3.

This thing is STUPID loud. It’s ridiculously loud for being a 2x6V6 amp. With the gain dimed, the master volume is unbearable above noon. I just finished a Princeton Reverb kit for a customer and was comfortably testing it at 8-9 on the volume. This amp is much more intimidating than a Princeton Reverb.

On the low gain input it sounds fine but I will say it lacks the chime, shimmer, and sparkle of the Fendery cleans I like.

The low input works fine with both volumes maxed out, but if I do the same on the high input there is oscillation above 1-2:00 on the master volume (with gain maxed out).

The biggest issue is the presence control; it works backwards it seems, getting brighter as you turn it counterclockwise; but worse it induces some bad oscillation when you turn it down below 3:00 on the knob. The presence control is wired per Mojotone’s British 800 Revision 9 schematic from last year which is no longer on their website (they’re on Rev 11 now) so I’d rather look at a different and proven way to wire so it works better for me.

This amp is still a work-in-progress and I believe I will still be messing with lead dress a lot as I’m still figuring out how to best lay everything out so that is one starting point for me, as well as rewiring the presence control.

[Yoda]

Well I confirmed the presence pot is wired correctly and not only the same as the Mojotone drawings but identical to the Ceriatone 2204 and the original Marshall 2204 Lead Mk.2 schematics and layouts. I think I will simply reverse the wires going to it so it’s not operating backwards anymore.

Oscillation persists however, so I will be installing shielded wiring in key places.

[stephenl]

Cool chassis design. Did you fabricate it yourself?

Bruce Clement, BC Audio, makes a lot of octal Marshall style amps. He typically does DC heaters on the 6SL7's to keep the noise down.

[Yoda]

Yes I fabricated the chassis myself, knobs recessed on top for a unique look. Just need to build a head shell for it.

[bepone]

nice chassis, but it is missing mounting holes from the bottom side to srew in the chassis

presence pot wire from the middle to the top viper..dont look at the layouts, just try oposite side until works.

for oscillations shielded cables will need to be used for sensitive points (gain pot, and grids V1 maybe V2a). your wires of all tubes are too buched together i would separate them little bit.

grounding of the all caps are on the 5 pin terminal? this is not correct, looks like grounding for the preamp section must be lifted from that terminal and brought to the preamp input

[Yoda]

Yes I have a habit of forgetting how I’m going to attach the chassis to a cabinet, doh!

Not sure what you’re seeing but there is no grounding attached to any of the pin 5s.

[danman]

Any chance that the oscillation is being caused by reversed primary wires on the OT? Have you tried swapping them? If they are reversed and causing positive feedback, that could be messing with the presence pot.

[Yoda]

It didn’t occur to me, I will try swapping the OT primary wires to see if there is any change.

[Yoda]

Yep, presence pot oscillation went away by reversing the OT primary wires. As a side-effect the presence control no longer operates backwards. Now I only get a real minor oscillation through a small portion of the gain pot’s sweep that wasn’t there before but it seems like it’s related to lead dress which is easy enough to address as this is still a prototype/experiment and I’m learning how best to route some of the wires before the layout is finalized.

[Mark]

How do the 6SL7’s sound different to the standard 12AX7’s.

I have assumed the differences wouldn’t be significant enough to go to the trouble of using 6SL7’s.