Ideas on "relaxing the gain" in stereo Marshall plexi 20W

[pjd3]

Hello,

I'd be curious to hear what you might first do to address this "problem" which isn't really a problem, just something that may open up room for improvement.

I've only every built amps with tube rectifiers. But, since this stereo combo is purely a pedal platform and real estate was more of an issue, I did solid state diode rectification (two diodes, one on each leg of the HT coil and using CT as ground).

What surprises the hell out of me is how LOUD the damn amps are. I barely have to approach 1-2 on the Normal and Bright preamp volumes with Master up full to achieve nominal stage volumes. It does have a very brisky sound to it, and I'm willing to give some of that up to approach a more clean, relaxed platform to keep my overdrive and "marshall in a box" type pedals sounding good.

A few of the options I'm thinking of are:

1. a 5751 in V1
2. a 5751 in the PI
3. a 12AY7 in V1
4. A 5751 in both V1 and PI
5. a 12AY in V1 and a 5751 in PI
Or, the most obvious one,
6. Turn your freegan MV's down and just play the thing! (which could very well be the best solution.

I suppose I could also decrease the NFB resistor gradually until the amp's reached the optimal place.

Thanks for coming by,
Best,
Phil D

[sluckey]

Another option is to relax your brain. Lots of options for that.

[pdf64]

Schematic (of one channel)?

[pjd3]

Funny Sluckey, so this is what I get for building an amp from your schematic? What did you do to this thing? Oh, Ok, you're off the hook. I changed it to solid state rectification.

This amp is actually two "Sluckey 6V6 Plexi's" with SS rectification. It sounds great, and I might not have to change anything, but I was digging for what folks have done to relax their amps if they were found to be too Hairy or brisky. I have increased NFB and played with tube types but always looking to hear from anyone that had similar situations. This is for more conversation on a topic, not so much looking to be "fixed". I'm sure I'll try a number of things and come to a sonic conclusion on my own. But cool to hear people's ventures.

All I need to do now is put on the handles and the corner protectors - Done. Then assess what I'm hearing at gigs and decide if theres anything I can do to make it more compatible for the particular situation. I'm just really surprised how loud the things are. Ha, maybe I just need to get over that. And who knows, that may end up being what I conclude is a good thing. Yeah, it may look like I'm making too much over nothing but, I'm working a 40 hour week plus doing 32 concerts in a 5 week stretch and get very over-sensitized to every little thing. Each concert sees anywhere from 300-1200 people and alot of them listen very closely to what we are doing. Ha, that surprises me too

But anyways, so, so close to going stereo and never turning back. Why the hell not. It can be done.

Thanks,
Best,
Phil D

[sluckey]

Just picking at you for thinking so much. Don't ever stop.

My 6V6 Plexi was inspired by Mark Huss’s Plexi 6V6 idea, but I copied the JMP 1987 circuit exactly until the 6V6 output circuit. I changed a couple resistor values to accommodate 6V6s. I chose to use a tube rectifier just because my donor PT supported one. I also removed the bright cap from the volume control to make the amp a little less brittle. And I increased the NFB to refine the raw sound a bit. I added a pre-PI MV (as found in the 2204 amps) to "relax the gain".

This amp is surprisingly loud. Same with all the 18W EL84 amps I've built. I find the volume controls are very good at reducing the volume.

[Fischerman]

The typ Marshall PI uses a 470R cathode resistor and 10K tail resistor. Notice how a Vox AC30 uses a 1.2K cathode resistor and 47k tail? That drives the power tubes a little less hard. It won't reduce much but just going to something like 820r or 1k cathode resistor and a 15k or 22k tail resistor might take a little off. The higher tail resistor will also tend to balance the PI which might sound a tiny bit different (and not necessarily worse). The smaller power tubes (especially EL84) don't need as much signal so keeping a Plexi preamp and PI 'stock' is going to really push them.

You could also just put a voltage divider at the PI input. This would essentially be a pre-PI MV that was already hard-wired in the amp.

Marshalls are loud. They were designed to be loud. That big ole 5000pF bright cap in the early 70s Superleads is a flashing red light signaling: "Not intended for low volume playing! This was designed to be a STADIUM amp!".

[statorvane]

Try a 12AU7 in V1 and work your way up to the 12AX7 (a 12AT7 in V1 doesn't sound very good). This is a very quick way to see how you like lower pre-amp gain in a 1987 based circuit.

[cdemike]

Sounds to me like it may be an ergonomic issue related to usability of the lower registers of the volume controls than an electrical issue related to the overall gain structure. Have you tried more aggressively tapered volume pots? If you're using the common 30% taper, 15% or 10% may be easier to dial in as a pedal platform volume-wise, especially if you're using a bright/bypass cap across the bright preamp volume pot. If you're using these as pedal platform amps, I'd try that ahead of reducing the overall gain of the first stage of the amp in order to maximize signal-to-noise ratio.

It does have a very brisky sound to it, and I'm willing to give some of that up to approach a more clean, relaxed platform to keep my overdrive and "marshall in a box" type pedals sounding good.

This makes me think you may want to consider going with a different value bright capacitor, eliminating the bright capacitor, and/or trying a revised resistive mixer. 270k mixer resistors offer a wider range of control over the peaking capacitor's contribution on the sound since the lower value increases the authority of the unused channel's volume pot viz the interactivity between the channels' volume controls.

[Reeltarded]

this is not intuitive

Place a trimmer across the treble pot, dime the pot and trim until it's sweet and slightly too much, then back out of the treble to like 7 and trin a little back only just up to sounding trashy.

That calms the shhhsh down but still allows aggressive voice at just under the NFB collapse.