edgy high pitched fizz when pushed

[Janglin_Jack]

Not sure what happened to my other post so hopefully this doesn't post twice...

I used an old carbon comp resistor like the one is your last pic. It caused me all kinds of fits. I used my scope and thought I had everything from crossover distortion to parasitic distortion to bad tubes. Changed tons of stuff and finally got to that resistor. Could be a faulty component. Clean up that wiring and think about that resistor

Mike

[Doug H]

Glad you are getting there

There is a reason why good lead dress and neat connections make a difference.

Understood, but I have spent time chasing ghosts before. There's some obvious culprits, but at the same time you see a lot of amps with the wires practically bundled together in the preamp. This is why I post on a message board hoping someone might chime in and save me hours of time. Believe it or not I actually care about the quality of the connections. If anything I risk having too heavy a hand getting the solder flowing, which I suppose can be a problem as well. I'm sure it helped smooth things out. I can go over the amp end to end once I get the layout sorted out, untill then I'm just pulling chunks uyp and repositioning them over and over, it's a waste of time to get too pretty.

I think I'm home now though. I am gonna get another board with nice holes in the turrets and a little narrower so I can try and give the wires a little more breathing room on the pot side, I think that's been the main problem I've been up against with this guy. It's such a narrow channel they tend to bunch up.

I'm also planning on putting a 4k tranny in there which should be a little more strat friendly and knock off a few watts. It should only get better from here

Thanks

[Doug H]

Not sure what happened to my other post so hopefully this doesn't post twice...

I used an old carbon comp resistor like the one is your last pic. It caused me all kinds of fits. I used my scope and thought I had everything from crossover distortion to parasitic distortion to bad tubes. Changed tons of stuff and finally got to that resistor. Could be a faulty component. Clean up that wiring and think about that resistor

Mike

The giant ones in the b+ line, I was thinking the other day they aren't the best idea, but the only other 10ks I have are these takman ones I bought a million years ago and it just seems stupid to put them in the power rail lol.

I might have some ugly ass oxide 10ks which is what really beongs in there if I'm not mistaken. Probably won't hurt to swap all those out, and this way the amp won't catch fire quite as easilly if something goes wrong

thanks

[Janglin_Jack]

I got a cache of old stuff from a friend. It was his Dad's old stuff who was into electronics. I thought I scored with a bunch of resistors. I used my usual carbon film/metal film for the amp and needed a 3.3M reverb mix resistor in a Fender build. I used my scope and could see that the normal channel signal was fine, it was the reverb channel. So I was able to narrow it down to a specific area. I changed tube socket, rewired..changed all the resistors in that area and it came down to that 3.3M. Got a Metal Oxide for that spot and bam..done.

So I guess my little score may not be such a good batch of resistors. When I viewed it on the scope it looked like parasitic oscillation to my amateur eye. Anyhow, don't rule it out.

Mike

[demonufo]

Not sure what happened to my other post so hopefully this doesn't post twice...

I used an old carbon comp resistor like the one is your last pic. It caused me all kinds of fits. I used my scope and thought I had everything from crossover distortion to parasitic distortion to bad tubes. Changed tons of stuff and finally got to that resistor. Could be a faulty component. Clean up that wiring and think about that resistor

Mike

Good point, dodgy CC's in certain positions can cause all sorts, and I can at at least a couple on the board. I bought a load from Weber once, and they were mostly junk. So the rest got junked along with them.

[Doug H]

Well I've been soldering on, added some more shielded wire into the post PI MV, helped a bit but...

on a whim I decided to replace the silver mica I had gonig into the tone network with one from a bag of NOS stuff I grabbed a while back, are you f''ing kidding me?

I'm afraid to believe I've found it, but that high pitched distorted whistle seems to have taken a big turn for the better. Are all these stupid lieele micas microphonic? I tapped it at some point to check it out, and it seemed fine, and its not one of the black ones I've heard may have gotten a bad rap.

More to come...

[Doug H]

yep, thought I was good, it had come a long way, but I couldn't completely shake this high pitched squawk behind the notes, like permanent string rattle. I thought maybe cleaning up the amp would be the last of it, it was almost musical, but the more I played the more it just didn't sound right to me. So I started playing around again, I noticed it reacted more to the treble sweep than the presence which got me thinking. Sure enough that did it. I swapped out the other SM on the mixer resister first thing today and played for so long my coffee got cold lol.

I'm not sure if it got damaged during the debugging process or if it's been dogging me from the start and was just aggravated and underlined by the other problems I found, but I'm gonna be hard pressed to put one of these things in one of my amps again.

I'm gonna pick up some polystyrene caps since it's kind of become a "thing" to not use any NOS parts in this amp.

I'm still hoping cleaning things up a bit tightens it up a bit more, always on that quest, especially playing a strat with a lot of gain. Went to see a local band last night, all I could think about was how happy I was to get that cap out of my amp .

[Doug H]

so still not satisfied I tracked down some more oscillation to the little "T" of bus wire I have across the v1 plate resistors. The connection is fine, but I didn't trim them all the way back to the resistor leads and they had the effect of putting the leads a lot closer to the couplers than the turrets are, and the turrets are already fairly tightly spaced. When I moved the tops of the plate leads together and farther from the output I noticed the difference right away. I have spaced them out across another turret to keep the plates as far away as seems reasonable.

Then I finally rewired the v1 cathode around the split switch and hung the 68/2.7 directly off the switch to ground. I also Made some sensible changes to the lead dress. I really didn't expect to hear the difference but I did. The layout seems to be virtually squawkless now.

I've got a fresh t-board coming , I'm hoping a nice clean rebuild will make it as tight as I want it to be. It never fails though, everytime I surgically rewire this thing I either have some problem that takes ripping everything to pieces and making a big friggen mess to find, or I realize I have to move a handfull of components across the board etc...

I proably should have looked for a generic turret board layout for these things when I started, but I never thought the circuit would be so delicate.

It's also occurred to me that all those wires running along the chassis in vintage plexis are like 22pf capacitors to ground shunting off very high frequencies. I think this is one of the little secrets of the traditional layout.

I'm also really surprised at the amount of builds I have seen across the net that seem to group the tone and volume wires together. Maybe if you have a stable v1 that's not that big an issue, or maybe I was wrong thinking that made the difference it did.

[bobtec]

Yes, there are a few things in there that cause concern, like unsupported flying leads that aren't insulated, etc, but a lot of the joints look terrible.

If it were me I'd rip it up and start again. Sorry if that seems harsh, but it doesn't look good... Not talking about the aesthetic perspective either.

I would listen to these guys,They Do Know what they are talking about.... you could at least clean some of it uplike the fly'n leads Think you went too fast(something I would do) or drank too much coffee...

[Doug H]

I listened and applied as much as I could get from "looks terrible" and "start over". I have ultimately isolated the problems regardless.

What perhaps I shold have got quicker than I did was the general state of the turret board, being covered in solder dust and silt. I wasn't gonna do a post-athon on this thread, but I have done a quick clean of importand areas with surprising results. which was great cause I'm used to every change I make frustratingly making no difference. I didn't bother to clean the board cause it's being replaced anyhow.

The amp sounded really good at the last jam, but not as perfect as I want it. I have my fingers crossed on the rebuilt.

And Ya it was done very quick, hastilly even, I have been rearranged the layout trying to find the source of the oscillation, that why I started this thread. Going through and making sure every jiont was perfect wasn't solving my problem, the answers have born that out. At any rate I'll post pics when I'm done with the rebuid.

I do appreciate the input and have made many of the suggested changes. I'm not going to bother insulating the bare wire on the back end of the V1 grid resitor without a functionaly reason why though. At any rate I"m getting a stand off to secure that cause I don't like really the "flying" part even though it's the norm.

[neikeel]

I'll post pics when I'm done with the rebuid.

I do appreciate the input and have made many of the suggested changes.

We are all here to learn different things

[Doug H]

well in case anyone is interested, I replaced the (crusty) board and most if not all of the rest of the flaky BS went away. I've also got some healthy power tubes and it's sounding like an amp and doing things that amps do now, thank god.

It's still bright as all hell, and I don't trust anything as a mixer bypass cap. I've gone from ceramic to polystyrene, to mica, NOS, new etc...I'm not really sure the 470k/500pf thing is gonig to last long in this amp. To my ears the older 270/556 is a lot easier to dial in with a strat. I find myself using the normal channel a lot, maybe with the amp on 10 and big closed back 4x12s the led channel warms up a bit.

I ditched the post PI MV for a feedback control, just awesome. in all my mad scientist plexi hacking this might be the most usefull. My amp is from a 2205 combo donar, so it's got the master on the front, which puts the feedback control right beside the presence, it's a lot of fun. I had the amp setup with very little negative feedback, 100k on the 8ohm for a 50 watter. I'm thinking Running that setup wide open with only 32u on the PI wasn't a recipe for success either. I've gone to 64u on the inverter, thinking that's a reasonable compromise for a "frankenplexi".

I've almost got the new layout cleaned up. It still looks like an old Fender amp , but the only real part of the circuit that isn't proper is the bias. I'm working on a solution. Will post pics for the hell of it once it's cleaned up.

oh, and not all TS el34bs like being slammed with 500 volts on the screens...

[Doug H]

Finally took some reasonable pics, I don't know everyone gets such great gut shots, mine all turn out terrible. I guess the trick is good lighting and no flash?

Anyhow, I haven't taken any pics cause it's been sounding great so I haven't opened it up in ages. I had a couple things I wanted to try and I needed to check the bias so here's a couple pics. I still have a few things I want to change, but I'm in no hurry.

some notes,

390 b+
4.2k bass man OT.
50u on the plates
32u pre amp nodes
Just put screens at 32, if that goes well I will be dropping them to 16 soon
470k mixers,
bright tone stack IIRC
a shared cathode switch
.1 output couplers
v2 cathode switch, you get the .68 with the 330 or none at all,
front panel feedback adjustment
line out that didn't work the way I wanted it to the last time I tried it, I might ditch the idea anyhow

Sort of going for that black flag or Dickenson Jimi thing and something that will do high gain with a few adjustments, and all of that at levels that won't push my eyes back into my head before it breaks up.

[Doug H]

so it's not the bright TS after all, guess I finally decided to find out how the other half lives

[Doug H]

a piece of the bias circuit is still sitting on a little perf board on top of the caps