effects loop idea

[Chuck H]

Well now that you have an acceptable effects loop design for both function and tone, I'd handle it like this...

When you have time and have regained patience for such tedium (I get a little funny after a long stint of tricky soldering) you could:

Rip the whole thing out! Yep. That's what I said. Rip it out and return the amp to stock. Build the effects loop circuit anew on a separate rectangle of perf board so it can be connected with short leads. this gives you a lot more time for planning layout and will make servicing easier in the future. Locate the new board in the best place for amplifier stability. If you have to add spacers and just zip tie it on top of the existing board that's fine. Just so there aren't any long leads in sensitive locations. You know what that means by now and you'll do fine.

When it's done you'll be grumpy and proud all at once. A weird sensation

EDIT: P.S. I think the reason the amp is sounding "better then it did at any point" is most likely because it sounds just like it did before any loop experimentation. You loved it then, so you love it now. Aside from any tonal anomalies from additional lead wire routing, that loop should be almost invisible tonally. That was the goal. This isn't an issue for amps that are designed with a loop from the start because the loop is part of the tone during the entire voicing process. So your case was specific in that regard.

Also, the send level being right at the top of low level and right at the bottom of high level isn't a problem, but it's not ideal. It should be one or the other. No matter. I don't think you'll have any trouble with anything you plug into it. Sooo...

With a signal at the input and without changing any control setting, compare the signal voltage at the treble wiper to the input lug of the master volume. Hopefully it's pretty close. That's what you want.

[daz]

LOL! I plan on not being grumpy for a while so i don't think i'll go there. Seriously, theres just no room to even put a board of any size. I'd basically have to entirely rebuild it with a new main board to create more room other than on the tranny and mains side of the amp, and i surely don't think thats a good idea. In fact, theres a tube socket hole there that was for a tube recto when it was a 18 watt, but i drilled a new one on the other side just to avoid that area. But yeah, absolutely no room to mount a board no matter how small, even if i raised it up as high as i could.

If i DO try to make it more reliable I'd just use some terminals i could mount to the socket mount screws. But that will even be tedious. Besides....ya really wanna keep this thread going? It could break records if i run into more issues. What i'd love to do is just build a all new one but thats out of the question due to my financial state which is likely to remain as is till i'm in the ground. But maybe one day depending on how things go. If my financial situation changes for the good i believe i would, and from the ground up with a new cabinet too, possibly in head format this time.

[daz]

This is a bit strange, maybe. I have to put a switch on it to be sure. But i forgot to reverse the speaker leads and i think it may sound better reversed. When i set them back to + to +, the tone seemed lighter, more transparent in the highs, and the highs seemed a but higher in range. Plus richer harmonic content. I'll have to test it further and it's great both ways so not an issue, just a curiosity since the phase should now be back to how it originally was.

Anyways, question..........any reason not to use a non isolated speaker jack? Don't they just isolate them for noise? Mine''s a plastic one but it's shot, and i have several clean brand spankin' new non isolated switchcrafts just dying for a new home. The main ground point is only about 3-4" from the jack.

[Chuck H]

Well then consider this project prototyping. I have four well kludged up amps in various states of component theft and/or project abandonment. All could compete for "worlds most jaw dropping slop fest of electronic parts". Sometimes that's what prototyping circuits looks like, because you can't redesign the layout with a new board, tube socket location, different layout so unchanged circuits fit around new circuits, etc... every time you change something that isn't even a complete design yet!!! So it's all good. And yeah, I'm done with this thread

P.P.S. When you test for unity there should be nothing or just a cable in the loop. You can adjust gain by raising or lowering the value of the 220k resistor on the recovery stage. Remember, the treble wiper and the master pot input lug should be the same or very close.

Cheers.

[daz]

Tried that....cable in loop, VAC at treble wiper 10v, at master input 2v. So i guess i'll try paralleling another 220k with the one thats there now and see what that does IF i feel the need. I'd do it this moment but that thing is so far down the rabbit hole the last thing i wanna do is try and get a lead soldered to that pin. The amp is plenty loud and i have lot more output from the DSP if i want so it really isn't important unless it would improve tone, but i see not reason that would be the case.

Wait, thats the NFB resistor so i'd make it bigger right? I can just series one with it so that'll be easy, as accessible for a series resistor or trimmer.

[Chuck H]

Wait, thats the NFB resistor so i'd make it bigger right? I can just series one with it so that'll be easy, as accessible for a series resistor or trimmer.

Yep. Bigger. You're getting to know this stuff whether you like it or not.

The loop shouldn't depend on you being able to bump the output with your DSP. It should operate even with effects that don't have level adjustments. I'm actually surprised you only have two volts at the master. Be sure you're comparing the treble wiper to the INPUT LUG of the master and NOT the wiper of the master.

If you need that much more gain for unity we should check and adjust the send as well because it could be raised to about a volt and a half (if it's lower than that now). That would mean less negative feedback reduction at the return circuit and help maintain the existing fidelity. Whenever...

[daz]

If you need that much more gain for unity we should check and adjust the send as well because it could be raised to about a volt and a half (if it's lower than that now). That would mean less negative feedback reduction at the return circuit and help maintain the existing fidelity. Whenever...

What would you suggest? Cathode R? And yes, it was wiper of treble and input side of the master. I'll check voltages at the send.

EDIT: send is only showing .8Vac with gain on 10

[Chuck H]

send is only showing .8Vac with gain on 10

Considering you're only reading 10V at the treble wiper that is probably about right. One volt is about ideal and I suspect your 10V figure, while fine for establishing consistency, may be lower than what I might measure on my bench. No offence, it's just low for what I would expect there and I know your circuit is working correctly so I suspect something in the test conditions.

Did you measure signal at the master input lug (not the wiper)? Remember, all test conditions must be the same when you measure the treble wiper and the input to the master. The master volume setting shouldn't affect the reading at all because division is only at the wiper.

[daz]

Yes. Master.....looking from the from front of amp, right lug=ground, middle lug=output, left lug=where i clipped the lead. MP3 player into input with gain maxxed. Never touched anything between master and treble pot tests. I've done this a few times. Send is well under a volt. .8v was the most it showed.
But i just remembered one of the 2 sine wavs caused double the voltage so i just tried it again and tried both and while it didn't double, with one of them i now got 15 vac and 2.2 vac on the treble pot and master respectively. Send however still shows under a volt.

[Chuck H]

Ok. Replace the 22k feedback resistor on the send circuit with a 33k or a 47k (33k being my first choice, but I don't know what you have on hand). Replace the 220k feedback resistor on the return circuit with a 1M pot wired as a variable resistor. You get where this is going. When/if you achieve unity, take out the 1M pot and measure the resistance. Put in a fixed resistor of that value and Bob's your uncle.

EDIT: Lose the avatar. Funny, but not at all accurate. It's undue humility.

[daz]

I'll do that, thanks. I have resistors in all values....i save all my used ones. Makes for an even sloppier build.

[daz]

This is strange. I added 10k in series with the 22k at the 1st stage and put a 500k pot in series with the 220k at V2B. The voltage at the treble wiper is 15v and the master input lug reads 19v with the pot shorted, IE out of circuit. If there is resistance at the pot, the more i add the more voltage. So i assume that 220k actually needs to be lower and the 33k even higher? Or will making the feedback R at the input stage any bigger start to lose the advantage of low impedence? And the send still shows under a volt ! It never seems to change. I know what you;re gonna say, and i'll stare it down right now to be sure it's all configured right.

That said, i imagine 19 vs 15 v isn't far enough apart to matter? But even if thats the case, isn't the send being so low a bit of a problem?

EDIT: don't know how i did this but i used a 2.2k at the second grid instead of 33k !

[daz]

I changed the 2.2k to 33k and now i can balance them. I ended up with 900k total (pot read 670k) at the FB resistor on the return stage and was able to get the master to match the treble wiper at 15.8VAC on each. The problem however is the .8VAC send signal which didn't seem to change after i added 10k to the 22k Fb resistor.. Or is that ok?

[Dave H]

Lose the avatar. Funny, but not at all accurate. It's undue humility.

Says Mr Potato Head

[daz]

God, i just found another idiotic mistake. At this point i need to put that other avatar back. I used yet ANOTHER 2.2k where the 22k was supposed to be at the send side ! the FB resistor. Let me get these moronic mistakes rectified and i'll post back. good god.....

Ok, got it all straightened out and i have 15v on both the wiper and master and 2.2v at the send. How we doin' with that?

Showing 1.6vac now on the send. Don't know why, setting the same. Still double what it was at .8

Put 33k as u said now it's 2.3vac. Please don't ask why it was over 2v before with 22k....my head is spinning and i can't say what i did when. I'm bringing back the old avatar dammit ! So still with just under 16v at both pots and 2.3v at the send. I think we're done but i'll wait till you ok that send level before i wire it all up permanent. I know it sounds unbelievable, but i think this thread may almost be done !!!