6v6 plexi-ish build questions

[donzoid]

OK got 'er back together last night...haven't done the "latest version smoke test", that'll happen after tonight's rehearsal but here is what I dun:

"package deal" 82k / 100k plate loads and tail resistors to 470R/15k for the PI; PI null input cap to .047; output couplers to .022; output grid leak resistors from 470k to 220k, and reduced power tube cathode cap from 470uF to 47uF.

I removed the 200R in the screen supply so there are just the two 820R's at the screen pins. Brave, aren't I? If they are too close voltage wise I'll just increase that pair or the 2K5 before the choke, plus the 200 was in my way so out she went.

Put back the 1uF bypass on V1b (I was playing with that value for treble and it was down to 0.1...so back to the known starting point there).

Lifted the signal leads for both the "low" inputs just for now at their input jacks.

Elevated the heaters using 180K / 47K w/ 33uF 100v cap.

I went to take another look at /measure the values I had used on the pots. I think the treble issue was at least partly because I strapped a resistor across the treble pot outside lugs (I had a 500K and it calls for 250k which I gotta order some of).

And here's where the story gets a bit funny when using stuff "from the boneyard". The master vol pot, turns out is not a 1meg as I (could have sworn I know I DID...) measured before installing. Its a 0-250K center-0 pot (really odd) must have been a balance control or something but never seen that one before. I know that's what it is 'cause I measured it 5 times in disbelief. Replaced with a 1 meg.

Yet to do: Swap the bass pot for a 1 meg, and add the .68 cap to the V2a 820R since those are not "critical" ... yet. I have massive updates to do to my schemo now too and once this is done will edit the post to reflect the changes.

All proof that I should have asked FIRST, not rebuilt three times...ahem!

[donzoid]

Thanks to all for the suggestions.
We are making progress. Can't turn master up (at all right now) barely cracked open and it sounds better already.

One thing has me a bit puzzled...the vol/tone channel is WAY louder than the TMB channel now. Its almost like the MV pot is bypassed on that side. But it may be that it's because I can't even crack open the MV (it's on 1/4 of 1) to keep sleepin' kids where they are. I'll re-check the circuits, and re-connect the low inputs on the jacks. But I can tell there's way more power/headroom available.

I'll know more tomorrow but it's far closer now to "the" tone and the volts are:
365 plate
335 screen
65v DC heater elevation (and way less hum so far).

[Colossal]

Don,

Great job! Sounds like you are getting there. Yes, if you can, please update your schematic and/or layout with all of your changes. Then we can sort out that TMB channel. That is good to hear that heater elevation reduced the hum. The voltage is perfect. Nice plate and screen voltages too.

[Colossal]

Don,

I realized I misspoke in my earlier reply about the slave input on the PI. I keep thinking (without looking) about Mark Huss' amp in relation to yours. Your amp uses both inputs of the PI so the slave side is not a slave, it's the input for the Vol/Tone channel. I said, change that input coupling cap value to 0.047-0.1uF. That would be if the input was not being used. You said you have 0.047uF in there now. I would lower that to 0.022uF, same as the TMB channel. As I said, I keep focusing on the Plexi character of the amp but yours is a two channel amp.

I found a Dockery schematic (which I think you are basing your amp on). I marked up the changes we've been discussing. Please let me know if I have missed or misinterpreted anything there.

[donzoid]

Well perhaps this reveals other issues. The cap I changed to a .047 is tied to the wiper of the master volume. The cap which is tied to the "tone channel" vol wiper is .01.

EDIT: maybe I swapped the wrong cap???

Based on the comments from others about the mixing resistors and cap not being here (as they are on traditional plexi) perhaps those changes/additions are desireable as well? Heck I've gone this far, might as well discuss those too. Or, yanking the tone channel and putting in a cascade/parallel setup like many others do...was going to look at that after getting it running anyway.

Side question Dave, where on earth did you find that (or did you just make it) and what software are you using?

[Colossal]

Well perhaps this reveals other issues. The cap I changed to a .047 is tied to the wiper of the master volume. The cap which is tied to the "tone channel" vol wiper is .01.

EDIT: maybe I swapped the wrong cap???

Yes, I was actually referring to the Tone channel cap, on the other side of the PI. Sorry about that. I would put the TMB channel PI coupling cap back to 0.022uF and if you are happy with the Tone channel's sound, you could leave it at 0.01uF, or, bump its cap up to 0.022uF.

Based on the comments from others about the mixing resistors and cap not being here (as they are on traditional plexi) perhaps those changes/additions are desireable as well? Heck I've gone this far, might as well discuss those too. Or, yanking the tone channel and putting in a cascade/parallel setup like many others do...was going to look at that after getting it running anyway.

You could leave the amp as is if you like having the two channels and like their respective tones, or, you could easily mod the Tone channel into a Bright channel and then mix it with the traditional JMP 1959 Plexi circuit's 470k/470k mixing resistors. This would make it more like a Plexi and more like Mark Huss' 6V6 Plexi which is basically a 1959 circuit with a 6V6 engine. All depends on what you are after.

Side question Dave, where on earth did you find that (or did you just make it) and what software are you using

I googled Dockery 18W TMB layout and found it that way. There are a number of TMB/tone channel amps out there; Richie's, Trinity, Dockery, 18watt.com, etc. I picked the Dockery layout and just cut out all the other junk on it and added in the values we've all been discussing here. I use a vector drawing program for Mac called EazyDraw.

[donzoid]

Mac? Aw jeeze. Don't get me started...

actually I'm OK with them now that they are running linux core, I just don't use one. I fix enough of 'em though!

OK I had a chance to bring the MV up to 3/4 full, and the vol. diff between the two channels is much less noticeable, "however" there is still quite a bit of hum and there is some weird distortion coming out along with sustained notes and harmonics. It's "splatty" underneath the notes and far more noticeable when the pre's are up halfway or more.

I fired up sonar and grabbed a short clip so you "might" be able to hear it. Does it on both channels. But it may be a tube so I'll swap a couple and see.

EDIT: Just swapped v1 and v2 for known good ax7's and still does it.

[Colossal]

OK I had a chance to bring the MV up to 3/4 full, and the vol. diff between the two channels is much less noticeable, "however" there is still quite a bit of hum and there is some weird distortion coming out along with sustained notes and harmonics. It's "splatty" underneath the notes and far more noticeable when the pre's are up halfway or more.

I fired up sonar and grabbed a short clip so you "might" be able to hear it. Does it on both channels. But it may be a tube so I'll swap a couple and see.

EDIT: Just swapped v1 and v2 for known good ax7's and still does it.

Don,

My first thought is that this is likely oscillation harshing your tonal buzz. This may be related to layout issues such as lead dress, a grounding issue, etc. The hum is probably part of the greater problem too. Best thing to do is to take some high resolution shots of the interior standing about a foot or two over the chassis so that the entire guts are visible and post it. If any of your boneyard parts have spent time in service in other amps or elsewhere, that introduces risk of a failed or failing part being part of the problem, so something else to consider. It could be something simple like a bad ground but without a photo(s), it's only speculation. Don't give in to the temptation to randomly change parts or try this or that. Troubleshooting should always be done in a very linear manner, moving stepwise through the amp and eliminating possibilities one at a time.

If the schematic above is how your amp is built, I would measure the voltage on every single node, plate, grid, cathode, and cap in the amp and mark it up on the drawing and post. Also gut shots. You mentioned that you have rebuilt the amp three times so if many things have been tried in haste, it's possible that a wire could be cracked or maybe a bad solder joint somewhere.

[donzoid]

Well, that all makes sense. None of the major revisions were done in haste at least. I was chopsticking around and when I do that I use what I call a "spudge" I don't know if that's the correct term, its a fiberglass n plastic esd safe stick that's pointed on one end and flat (like a screwdriver) on one end.

Anyhow, I think the problem is either the jacks, or the cable I used, or resistors up to V1. I get the stick near the V1 pin 2 input and it gets majorly noisier. Moving these cables around reduced "some" hum but I also get that kind of response when I get near my 1 meg input jack resistor. In any case it seems to be grounding hum. I have two ground points for the power end and the pre end and I think I need to revisit the pre side.

I'm going to re-wire that whole part and yes I will take new readings and gut shots afterward. Thanks!

[donzoid]

ok, sorry for the delay... camera is only 10mp but I "tried" to hold her steady etc. here are some gut shots.

Before you tell me how badly dressed the leads are on the valve side, yeah I know. I had them much neater but after moving them so many times to get at the solder points on V1 and elsewhere, and knowing I'm prolly gonna have to do more, I just left 'em.

I'm about as graceful with my solder pencil as Barney Rubble so now some of the leads are getting scarred...ugh!

Let me know if you want other closeups.

[donzoid]

OK I actually made some progress here after MUCH chopsticking, re-dressing of the leads etc.

I replaced the plate load resistors with carbon films and one of the caps was swapped out with a like-kind from the same era that wasn't as noisy.

Now, I am up and it's working, with some oscillation at the very top end of the volumes.

But I have just come to the realization that the master vol. control, is working only on the TMB side. Is this as intended? I'm not smart enough at the schematics game to know. That may not be desireable. Still sounds a bit muffled and woofy too. Backing off the bass to 1/2 way, and the mids to abt 25% helps, but she ain't bright enough yet.

EDIT: Maybe 'cause she is a bit muffled, and not very bright I should name her Blondie???

[donzoid]

volts re-measured.

[matt h]

(deleted)

[Colossal]

Yeah, could be a grossly mismatched pair.

[donzoid]

Their values were checked on my Hickok 800 before I put them in they were pretty close but....yeah that discrepancy in readings was suspect I thought, especially considering I saw it in reverse (V4 and V5) but that was a few iterations ago. Mind you they are Chinese (although Ruby) its just a different set of startup tubes. Maybe they should get re-checked. Or I misread my meter but I checked it twice when I saw that.

Matt the values here are all relative to ground not across pins.

Cathode resistor is 250R with a 47uF cap.