Mis-connected Cathode Fried Something !??

[SingingCoil]

Greetings and much thanks for all of you building and nurturing this community.

First build: 5e3, with half dozen mods to tame the bass and git more headroom, the usual.
No real problems, sounds like The Voice of God !
Second build: JTM45, cath biased EL34s, with a few mods. Again, crazy amazing sonic alchemy!
Both McGuyvered from hand-me-down trannies, paperclips, rubber bands, no clue, and the internet.

Goin to bed seeing wires, gettin up every day and poppin the lid, tryin ideas out. Feelin a little bada55.
Started one morn before I had gotten half way through my cup o coffee, messin with the tone slope,
and ended up re-(mis)connecting 100K NOT to v2.8, but to v2.3(along with 820ohm v2.3 resi.

RESULT:
1) Much, much louder, in a good way, while sounding very sweet and clean
2) Feedback now needs 200K to hold down NFB, rather than 27K.
CHANGED:
1) tubes, 2) v1 & v2 plate resistors, 3)most of caps and resis in tonestack & NFB "area."
CHECKED:
1) all caps and resistors with multimeter, but not under load.
2) all voltages seem to be proper.

THE PUZZLE: What the heck did I fry !

My remaining suspects are the silver micas on vol pots, or my overpriced, behemoth 16uF filter cap.
Bettin it's obvious for those who have a firm grasp of the way of the electron,
but I'm just a dangerous newbie, and I'm stuck on this new chapter.
Maybe I shoulda made more mistakes earlier on to keep me in check.
I think I have properly attached the Bassman schem here.
Thank you all very much.

[Enzo]

Please state the problem. I can;t tell, it sounds like you miswired one socket pin and now the amp is louder. What is the issue you are actually trying to resolve? What makes you think anything is fried? Assuming the tubes are numbered in order, moving the 100k resistor over to the same cathode with the 820 simply puts the resistors in parallel, resulting in about 813 ohms. No real difference. It does leave the cathode follower stage unterminated, so it ought to just sit there. Unless it went along with the 100k, the send to the tone stack now lacks any drive signal.


Have you restored the 100k to where it belongs?

[SingingCoil]

Ahh, Enzo !
This is more exciting than a call from the President !

Apparent problem: Too much juice in my NFB line, and way more volume altogether.

NFB thing: Now need 200K to hold feedback down, rather than stock 27K. Doesn't seem like a "problem" with a 200K in there,
but it feels like a crazy pitbull ready to break the leash and devour everything. Kinda spooky.
Something changed after my blunder, and I'm trying to get back to that 27K.

Along with the NFB increase, was a volume increase ("1" now as loud as "5" was before). Also not a terrible "problem"
because it sounds clear, tight, and very right. It is however the other result of my f-up.

The signal seems to be doin its thing just fine...crisp, clear, responsive, and all there.

I was also thinking along your line that decreasing resistance a tad on one cath, and disconnecting the other cath is not catastophic
like a short.

I could just run this thing as is (w/200K NFB) if I was certain that I hadn't fried a resi or cap somewhere,
but I'm less interested in playing than figuring out why I need a 200K on my NFB now.

Thanks for your work and wisdom.

[g1]

Sounds like you flipped the phase on the OT primary, and now are getting positive rather than negative feedback.
Have you tried swapping the wires to the OT primary?

[Enzo]

You beat me to it, I had same thought. Now that it is positive feedback you need that very much larger resistor to kill the effect enough to make th amp stable. As to the amp being much louder, look up "regenerative radio" for well known examples of using positive feedback.

Try simply disconnecting the feedback line. How does that sound?


But I'd be willing to wager that swapping the power tube plate wires will put it straight.

[Mick Bailey]

Under normal conditions "Too much juice in my NFB line" would make the amp more quiet. The feedback signal taken from the output stage has a phase relationship with the signal in the preamp and is inserted at a point where the two signals are out of phase. The preamp signal is continually being opposed by the NFB signal - the amount of feedback being set by your resistor.

The amp designer factors in the gain reduction from NFB by increasing the underlying gain of the amp, so that the overall gain of the amp as a complete system is what is desired.

Now if you change the phasing of the NFB - either by flipping the OT leads, or by inserting the signal at a different point in the preamp, the signal now reinforces the preamp signal instead of opposing it. Usually the amp turns into an uncontrolled howling oscillator. By increasing the NFB resistor the oscillation may stop, but the amp will be unstable at higher volumes, with a tendency to feed back internally.

So you need to check the OT leads for reversal as suggested, but if you haven't touched these make double-sure that you didn't change the insertion point for the NFB signal at the PI/preamp end. If you have, flipping the OT leads may cure the problem but you'll still have a mis-wired amp.

[SingingCoil]

Rock and Roll Gentlemen !

The posfeedback as opposed to the negfeedback thing, riveting! Thanks for all the input on the subject, and can't wait to
read up on "regenerative radio thang. Thanks!

One question: It appears "flipping the leads" suggests mechanical wiring error, OR, is it a phenomenon inside the OT ?

That stuff was all untouched, and triple checked again today, but wiring is all in proper place, including point at which signal is
taken before sent to NFB resistor.

I did fire it up today without NFB hooked up, mmmm gorgeous! Then got the idea to voltage check two points I'd missed.
1) v3.7 meets 1M meets .1 cap: 32vdc (bassman shows +23vdc) ???
2) Other end of cap: .1 cap meets 10K meets 27K NFB resi(via underboard jumper), Point taking signal for NFB: 14vdc.

I'm into switchin the OT plate leads around just to see what happens, but thinkin maybe I don't want 32 volts off of v3.7.
Any thoughts on that ten volt difference before I plug the soldering iron in?

[SingingCoil]

Might I also ask, that instead of swithing the OT leads on powertube #3 pins, can I just switch lines from board to #5 pins ?

[SingingCoil]

Aaaah, Thank you so much Gentlemen,

For introducing me to The "inverter/OT primary phase issue" I've run into so many times in my readings.
Feels like I got to join the club on that problem, thinking that that kind of thing would never happen to me.

You all wrapped that up like the last few satisfying nails gettin sunk into the neighbors roofing job that's been
driving ya nuts for weeks.

I had scavenged the PT & OT from Crate V50, brown primary - tube - v3.7, blue primary - tube - v3.2.
Guess it was unstable from the begining, I just hadn't spent enough time test driving it to run into the instability.

Gotta build me a little regenerative radio now.

In closing, let me call you all American Heroes for Keeping tube amps alive and growing!
I believe part of the "dumbing-down" plan surely involes the removal of "Real Sound" and thus,
"Real Music" from the culture, true evil. Rock&Roll ain't headbanging, it's takin care of business!
Born '64, but know the Maharishi at Woodstock said America was the cup of Light for the World.
American ingenuity and it's Rock&/Roll child, all part of The Mighty Mighty Plan ! Thanks Again.

[g1]

Yes, swapping the leads from the board to the power tube grids (pin5) would accomplish the same thing as swapping the OT primary leads.

[Enzo]

Sometimes this is innocent. it is common for OT primary wires to be blue and brown with the red center tap. But don;t assume that all of them have the same phase relation within. Blue and brown let you know which wire is which, but you still need to determine which phase is which on your build.

[SingingCoil]

Excellent point on the wire colors !

I feel like an idiot, 'cause I'm always the one grumbling about the lack of standards with these humans.
Thought the amp biz was different...why the hey would I think that !

That is one tool that all of us newbies should have at the get-go!

Damn! Now I know what all that talk about Marshals being LOUD is about! Thanks again guys.