Need advice

[daz]

I can set that trimmer....no, better yet so theres no misunderstanding, i can remove it and put a 1M resistor between the treble and the 50k pot and the same thing happens with the treble pot. That said, if my posts irritate you this much i suggest you don't read them.

[J M Fahey]

They donīt irritate me at all, why would they? , itīs not my amp, but Iīm sincerely trying to help you and you seem to avoid all needed answers, yet on the contrary you keep asking for suggestions.
Weird.

Do you expect magical solutions?
It does not work that way.

Back to the electronics problem: *if* it worked well before, with the 1M pot and nothing else (no extra load added, just the tube grid) and now you use a fixed 1M resistor, in series with a 25 or 50K pot, treble pot action will not change, thereīs no reason for that, in that case treble pot always sees a 1M load.

Only effect is that now the 50K pot, when going from 0 to 10, will behave like if the former volume/master pot went from 0 to 1 or thereabout, so you'll have less *volume* . Period.
By the way, I think thatīs what you wanted.

If you insist that treble pot action changed, as in before action was smooth from 0 to 10 and now it seems to act only from 5 to 10, all I can tell you is that you probably made some wiring mistake and things are not like the schematic you posted.

Good luck

[cbarrow7625]

Daz, why did you wire the new master as just a variable resistor instead of as a pot anyway? Not that it likely has anything to do with the treble question (which, from what I have read, I am inclined to agree with JMF's last assessment above) but it would have been just as easy to wire it as a 50k pot with the 1M attached to lug 1, ground to lug 3 and signal to the next stage from the wiper. Then, there would be no question whether the tone stack is reacting to a changing impedance or not. At any setting of the 1M trimmer, at least the load seen by the tone stack would remain consistent at any MV level setting. Just curious.

Do you use Duncan's Tone stack calculator program? Go to it and keep adjusting the tone stack and MV load settings until you are able to approximately replicate what you are hearing. It may help you discover where an unknown wiring or soldering mistake may have been made.

Suggestion #2: Put the 1M MV back exactly as it was (and/or just unsolder the ground lug from the 50k pot and use the 1M trimmer as the temporary MV) and see if the problem goes away or remains.

Last but not least: Are you truly, truly, 1000% percent sure (like, you measured it with an ohm meter out of the circuit sure) that the trimmer is actually giving you 1M resistance? Just turning the pot all the way to one end doesn't assure that you are. Could be faulty, could be a misprinted resistance value; these things happen.

Good luck.

[daz]

No, i'm not and in fact he's right. But the weird thing is even after i tried the original 1M master circuit the treble STILL seems to have little to no effect in it's 1st 1/2 of travel. I took the treble pot out, checked it, tightened the lugs with a punch. Still nothing. So apparently this started before i changed the master, but from what is the question. I looked for any changes I've made that could remotely cause it and theres nothing aside from that new master circuit. I used to turn it to about 9:00 and it would really cut highs from noon. Now theres no noticeable difference. So F me....i have no idea what thats about.

As to why i did the master like this, you said to change the ground resistor in the voltage divider or pot to a small variable pot. Thats what i did, and the trimmer in front of it which at the time i had configured as a variable resistor was the series. So it was as you said but with the series also adjustable. Fahey says it's a incorrect circuit, but as long as the series is set less than 0 resistance the treble pot works the same as it did when i went back to the stock MV, and the sound is different and i think better. But i'm still playing with it along with several other things to try and get the thing to sound good cranked up a bit. It's very time consuming tho because i can't turn it up often due to social reasons if you catch my drift.

EDIT sorry, i misread. When you said "are you sure" i replied not realizing you meant about the trimmer being 1M. It definately is. And by the way, everyone has me thinking this is a "wrong" way to do it, so i went back to the original setup, a single 1M master. But every time I do the tone just isn't as good and gets much nastier loud. Not sure what i should do, look for other possible fixes of go back to the "wrong" master circuit.

[cbarrow7625]

No, it is not wrong! Done correctly it is exactly the same. You just get more detailed rotation in the useful range of the pot. The confusion came in because you didn't make it clear earlier that the trimmer was set st or close to 1M. If the treble pot issue hadn't come up, which now sounds to be unrelated, I don't think there would have been any confusion.

[daz]

Ok. Well, i guess i'll go back to it. But i will have to play with the trimmer because i have had it wired 2 ways, one as a simple variable resistor and also as a volume pot. Easy enough to switch back and fourth just by grounding/ungrounding the one lug. Each way has it's benefits. The problem with the volume configuration is too low and it DOES screw with the treble pot really badly. Much worse then just making the 1st 1/2 of rotation useless. But the advantage is it cuts the level down which is a benefit considering how hot it is. The advantage to the variable resistor method is that it needs to have a lot of resistance to do good things, and that reduces any treble pot wierdness further. So more experimentation is needed.

[daz]

I tried something today that seems too good to be true, and of course thats a bad sign usually. So tomorrow i may feverishly rip it back out faster than malmsteen can fire off 3 notes. But hopefully not. I'm not really too good at understanding the NFB aside from the results i get. But i put a .01 then a .022 cap across the 100k NFB resistor and now the 250k variable NFB acts like a smooth&rich to bright&cutting control. Before it was more like dull and less cutting to bright and cutting. It's really makes the overall tone of the amp incredibly fine tunable and i think it also does a lot for that high volume harshness i've been fighting. I've seen this used as a resonance to boost lows in the NFB and as a high cut, but it seems it's very dependent on cap value and resistor size as to just what it does. If anyone wants to explain it in layman's terms, i'm all ears. I understand how NFB works a bit, but it's one of those things that causes me to get dizzy thinking about it.

Chuck, i recall seeing you say in a post just today i think that you don't feel using EQ tricks in the power section is a good thing. Care to elaborate on that? You'll likely turn out to be right and i'll be removing this once i realize that. But i hope not.

[Leo_Gnardo]

I tried something today that seems too good to be true.

Heck I don't see anything wrong unless it causes some kind of HF oscillation or other instability. If it satisfies your ears and no electrical problems, terrific!

[daz]

Heck I don't see anything wrong unless it causes some kind of HF oscillation or other instability. If it satisfies your ears and no electrical problems, terrific!

Well, the point is that generally when something is not very common it's usually one of those things that you eventually come to realize isn't as good as you 1st thought. The u realize thats why it's not a common thing. Tonite it turns out after some jammin' with some A/B that the jury is still out. Gotta try different values too. I think it may be a good thing, but either it may require just the right cap value or it's one of those things that works some nites and not others. Gotta screw with it more...

[teemuk]

If anyone wants to explain it in layman's terms, i'm all ears. I understand how NFB works a bit, but it's one of those things that causes me to get dizzy thinking about it.

It's a variable RC filter. The filter controls the amount of feedback and therefore also affects entire open loop gain.

[daz]

Well, the point is that generally when something is not very common it's usually one of those things that you eventually come to realize isn't as good as you 1st thought.

Spoke too soon....the JCM 900 has this, tho it has 2 of them. The use 100k resistance split into 2 different filters....a 56k with a .01uf and a 47k with a 100pf.

It's a variable RC filter. The filter controls the amount of feedback and therefore also affects entire open loop gain.

Yes, but I meant i don't understand exactly what frequencies are affected and how. Being NEGATIVE feedback, does it do the opposite of what would happen elsewhere, IE: instead of attenuating frequencies the cap can't pass, does it attenuate the ones it DOES pass?

[Leo_Gnardo]

I meant i don't understand exactly what frequencies are affected and how. Being NEGATIVE feedback, does it do the opposite of what would happen elsewhere, IE: instead of attenuating frequencies the cap can't pass, does it attenuate the ones it DOES pass?

Yes. Well, it suppresses the amplification of frequencies the NFB network does pass, amounts to the same thing. It's all fair game unless the amp starts to oscillate or show any other instability. Tune it to your ears' satisfaction. As you discovered, it takes a bit more top end to cut thru the band than when you are practicing on your own. You could put in a switch to select optimum NFB for either situation.

[daz]

Yes. Well, it suppresses the amplification of frequencies the NFB network does pass, amounts to the same thing. It's all fair game unless the amp starts to oscillate or show any other instability. Tune it to your ears' satisfaction. As you discovered, it takes a bit more top end to cut thru the band than when you are practicing on your own. You could put in a switch to select optimum NFB for either situation.

Yes, and thats how i test these things...in a mix. I no longer do gigs (geez...been 10 years now !) aside from a rare sit in or such, so i test changes i make every saturday nite by cranking tunes loud (only time i can get away with it) and playing to see how the tone cuts. Seemed ok last nite but i ended up lifting it. I need to find just the right combo. I am going to try the method i saw in the JCM900, a picofarad value over a 47k in series with a .01uf over a 56k. Making it switchable is definitely on the menu if i can find a switch pot locally of a value of the controls near the components.

[Leo_Gnardo]

Making it switchable is definitely on the menu if i can find a switch pot locally of a value of the controls near the components.

I'd put such a control back panel, near the speaker jacks. Maybe a bit inconvenient but it does keep the speaker output and FB components away from the preamp circuitry, avoiding possible unintended HF feedback oscillation. OR you could get fancy and remote-control it via relay or LDR from the front panel if you wanted to go that far.

[daz]

I'd put such a control back panel, near the speaker jacks. Maybe a bit inconvenient but it does keep the speaker output and FB components away from the preamp circuitry, avoiding possible unintended HF feedback oscillation. OR you could get fancy and remote-control it via relay or LDR from the front panel if you wanted to go that far.

True, good idea. Also, what about wiring of the NFB as far as noise....would shielded cable be a good idea since it's pretty long run with the variable NFB pot and all?