My take on a venerable old classic

[Howlin' Mad Mac]

Tell me what you think

Mersey Beat Birmingham.pdf

[ThermionicScott]

Looks promising!

Interesting choice of a 12AT7 in the "plexi" side. Would it be anathema to give each EL84 a more substantial screen-stopper? I wonder about the 0.1uF coupling cap/470k grid-leak combo on the power tube grids -- seems like you could get some nasty blocking recovery from that. 18-watts use 0.01uF coupling caps, don't they?

- Scott

[Howlin' Mad Mac]

Thanks & good catch on the caps... oops on the decimal there... would tend to make things a little flabby eh? Will make value changes on my drawing ASAP! As for the AT7 the more obvious choice would be to put it in the 1st stage but then it would change the 1st input... might also work well as a PI but again its presence felt across both inputs... not sure about before the tone stack but I can always swap it out for an AX7. Was curious... do you think I should place a small load, maybe 470K from the wiper output of the "plexi" side as you put it to block any cross-talk? Once I've created my tagboard/chassis layout & build it might try playing around a bit with the other channel... thinking maybe something with an EF86

[ThermionicScott]

I don't think you'll have a problem with crosstalk. I'd definitely try the plexi side with a 12AX7 as well, maybe leaving off the cathode bypass cap for the second stage if there's too much gain. Or maybe a 12AY7 for the input stage of both sides? Just throwing out some ideas.

- Scott

[hasserl]

IME the AT7 doesn't work well with AX7 plate loads and bias resistors. I would consider dropping the plate load to about half of what you've got there. Gain will be decreased but audio quality should be better. I would just go with an AX7 there personally, leaving off the cathode bypass cap for the 2nd stage, as that channel will drive the EL84's into oblivion.

[jrfrond]

Luckily the cap thing was just a goof, because you'd be setting up one helluva time constant with .1 and 470K!

I agree about bringing up the value of the grid stoppers, more likely to 22K. I'd also drop the 470K to the standard 220K. You don't need tons of grid drive to push EL84's to max output.

I also agree with an AX7 for the second stage of the Plexi side and no bypass cap. You can also put that cap on a switch if your heart desires. Speaking of bypass caps, you don't REALLY need 47uF on the Deluxe side.

Once you get it breadboarded, you can tweak and work out the kinks. Watch the coupling cap values. If you get blocking distortion when driving the amp full, you'll need to bring the value down. However, it might just work with the 220K resistors. I like the quasi-Deluxe/Plexi concept. Why not make it THREE preamps and add an EF86 to round it out for the Matchless/AC-15 vibe?

[Hardtailed]

Essentially, this is the TMB version of the Marshall 18-watt, as seen elsewhere on the internet.

I'm not sure your tone control on the normal channel is drawn correctly, as it stands it would act as a volume control more than a tone control.

Beside that, I will echo what others have said: .1u coupling caps and 470k grid leak on the power tube, looks like you're asking the EL84 to do stuff they just can't.

Ever take a look at the TMB 18-watt or is it pure coincidence?

[J M Fahey]

Yes, the 100K to ground in the tone control does nothing.
When wiper fully up, the .1uF bypasses a lot of sound (not only highs and mids) to ground, when fully down, the 500k in series put it out of the picture.
As stated, will work more like a volume than a tone control.