6v6 plexi-ish build questions

[Colossal]

I just wrote a really long post and decided to delete the whole thing.

I implore everyone, and me included, to stop asking questions or making suggestions until we have a documented schematic of what is actually already built.

Before tearing up the amp that's already built... let's confirm what is actually built.

Matt,

I've been helping Don for a while both on and offline. The amp that Don built is the first schematic that I posted on his behalf. It was originally a Loy Dockery 18W TMB amp. Don just reposted the schematic that I modified for him; that is what he currently has. He erroneously posted a different schematic in his discussion with you about the power amp and filtering which has an unrelated preamp. In order to reconcile the confusion about the PI inputs, phase issues, plate loads and the appropriate values, I just posted what is for all intents and purposes a Mark Huss 6V6 Plexi with one value changed, the PI tail resistor.

[matt h]

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[Colossal]

So if I'm keeping track of everything... he's already using both PI inputs, there is no NFB. Right? Let's keep that.

He is currently using both inputs, no NFB.

Bringing the Huss plexi into this is confusing.

Not really, it's one wire and one resistor. It solves a lot of problems and Don mostly uses the TMB channel anyway and originally wanted a Plexi variant with 6V6 tubes.

[matt h]

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[donzoid]

I will take some new gut shots and try to work through the schematic this weekend. Its gonna be a busy one, so this will take a little time.

I did get the last cap in the power string put in, and connected. Alas, my little girls (who ain't so little all of a sudden) wanted to go skating...I hope to test it tomorrow but since I have a gig to set up for prolly be Sunday.

Thanks both of you, for the input. The depth of knowledge here is an amazin' thing. If I can get Dave's 1st pre diagram grafted into my .sch file, it may actually be accurate, and I agree that's really needed. I blame Martin for suggesting a schematic in the first place!

[donzoid]

I am (finally) almost done with the CURRENT existing amp schematic - Matt I thought I owed you that at the very least. I am extremely grateful to both you and Dave, Statorvane, and many others here who have "endured" a very long thread and helped out a lot.



Then, we can talk about where to go from here. I know I have GOT to do what I do with computers every day - "OBSERVE THE EXISTING STATE BEFORE MAKING CHANGES"!!! I'll take a new gut shot or two, and measure the volts on the new node tonight so we can all make sure that I don't cause any more confusion. Or at least, less so...heheh!

[donzoid]

Well you may have expected it, but I am a bit surprised that:
adding the one cap raised the entire power string (albeit mostly the end of the string).

51pF cap on PI plates

22uF 450v cap at the last node (V1)

Certainly also sounding "better" in terms of punch and clarity, and volume. I won't go so far as to say the blanket is off, but maybe halfway off...

EDIT: Matt you were certainly right about that stabilizing the whole thing by adding that PI cap. I've had a chance to play it for a little while now and its much less fizzly. And I'm not hearing the oscillations - next step will be post the gut shots, updated schemo and look at some (prolly all) of the mods to combine channels by mixing instead of separate/jumping them.

[matt h]

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[donzoid]

Thank you sir.

Baby steps....baby steps....

Well to be honest Matt I dunno that the PI cap did much for the high end, but I think the reservoir cap did...proof (at least to me) that every valve should have it's own. My opinion (having no factual basis) is that raising the plate supply a few volts for V1 and V2 helped the most. OR, perhaps solder joints got repaired after another round of "updates". I only went over them 3 times before! Still, an area I need to improve.

I'll post the schemo tomorrow - note the spot I picked for the extra reservoir cap since I had three open terminals there.

[donzoid]

Here is the "actual" schematic, I asked Dave to give it a quick look over before posting as this whole thing is my first attempt at one. His comment about the 82k/100k PI resistors made me realize the channels for TMB and normal should probably be reversed, but I have a note here to switch back to both being 100K. Its easy to get to, and requires less futzing, also one of Matt's first recommendations.

[donzoid]

Here are some of Colossal's additional suggested tweaks. I think I have all the parts to do (all of them) just going to think over the results for each, then discuss. First thing is, this evening I will return the 82k to 100k and check the sonic results/voltages and update the chart and post.

[Colossal]

Here is the "actual" schematic, I asked Dave to give it a quick look over before posting as this whole thing is my first attempt at one. His comment about the 82k/100k PI resistors made me realize the channels for TMB and normal should probably be reversed, but I have a note here to switch back to both being 100K. Its easy to get to, and requires less futzing, also one of Matt's first recommendations.

Don,

My thought about the 82k plate load is that it should stay if you plan to just mix the Normal channel into the TMB/Bright channel. This would result in just the inverting side of the PI being used. However, you show a switch allowing you to either mix the Normal channel into the TMB or route it to the other input of the PI. My comment was that if you plan to keep that switching optionality, then I would go with Matt's suggestion of increasing the 82k plate load back up to 100k.

My suggested value changes on your schematic are based around trying to make it more like a Plexi in which both channels are mixed with a global tonestack and only one input into the PI.

[donzoid]

Thanks Dave, for the sake of future options I think it best to have the 100k in there, so I can switch that and other things without balance issues.

Also, I believe this allows me to bring an NFB loop in and even switch that with fewer complications yeah? Because after all this and the opportunity to play on both channels, I am leaning toward:

Getting rid of the "normal channel" tone pot and replacing with a presence control, since this channel sounds best with that tone pot all the way up anyway
and/or
Running a dark/bright volume control (to mix the two that are there) or, an extra gain stage control (I believe that would be like a variable one wire mod correct? or similar to a cascaded control). Granted that may be easier to implement off of a switch to cascade, and one pot.

[matt h]

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[donzoid]

Thanks Matt, all of those suggestions sound more interesting than gNFB.

"...as drawn, it's sorta run like a really dull "Rocket" style preamp."

-- well you're spot on with that comment, now that you say it like that, it really does sound like a dull rocket...and your 2k7/1uF with a .0022 coupler is exactly what Dave is suggesting for the TMB (to make it more traditional plexi in character). I think I'll try this on the normal channel and see how it sounds.

EDIT: the .0022 cap replaces the current .0047? Just to make sure