Parasitic oscillation problem untraceable

[Bieworm]

deluxe_reverb_ab763_schem.gif

OT is a hammond 1750H
PT is a classictone deluxe reverb replacement type without heater center tap
I will look for the schematic and voltages

You could make that work.


The OT is only 8ohm output, but that's okay as long as you match your speaker.

The PT- did you do an artificial heater CT on this build?

Yup, 2 x 220R resistor joined at the cathode.
And the 8R output matches the 2x12 combo with the same value on 16 ohm speakers wired in parallel

[Bieworm]

Josh... I adapted the schematic so the gain sits in front of the 2nd gain stage and tone stack. I took the tone stack from the modern classic so I can use the 3 knobs on the OD channel. (VOL-TONE-GAIN)
unless I am missing things it looks to me it should work. I also marked some components that might be tweaked.

schematic pre design.pdf

If this is not satisfying I will change the entire amp over to a tremolo TMB reverb... but I would like to try it this way first. Less invasive

[JMPGuitars]

Josh... I adapted the schematic so the gain sits in front of the 2nd gain stage and tone stack. I took the tone stack from the modern classic so I can use the 3 knobs on the OD channel. (VOL-TONE-GAIN)

Change it to what you want, but that concept has...issues.

[Bieworm]

Josh... I adapted the schematic so the gain sits in front of the 2nd gain stage and tone stack. I took the tone stack from the modern classic so I can use the 3 knobs on the OD channel. (VOL-TONE-GAIN)

Change it to what you want, but that concept has...issues.

the shared V1 preamp? the imperial does that too.. but it too has issues. I will have to bring my plate and cathode voltages to tremolo TMB specs..I'm aware of that. The imperial has issues too
I guess conceptually I'm in line with what's happening in the imperial preamp, but the configuration of the OD side is changed. That makes me assume it has to operate to some level, but it could be a crapshot. Like I said... if it turns out a fluke (a thing I'm keeping into account) I will build exactly what I like very much. That's not really a disaster hey
You know me by now...I like to experiment. But who knows.. maybe it turns out great probably not, but now it isn't any better either. Let's call it the final chance for the imperial. The sequence of the tone shaping is not to my liking. You have to turn up the midbite knob to get overdrive, and when I do that the sound changes to raspy tones I prefer not to have.

Is there maybe something else I have overlooked? You don't give much detail about your hunch on having issues...

[JMPGuitars]

You don't give much detail about your hunch on having issues...

I don't have time to analyze and design another circuit right now. But for starters, think about how different tube stages work, how they should be isolated from each other, and how series capacitance works.

[Bieworm]

I had the chance to perform the changes yesterday evening. It works actually, but there are some (un)expected side effects . I haven't had the chance to fully test it.. but first findings here:

- I have the impression the overall volume is less than before
- still fizz I think, but clearly less.. like I said, I have to test it somewhat better
- looking at the voltages I noticed that V2B (2nd stage clean channel) have gone up. I will leave that for now and try to tweak the voltages of the OD channel
- V1B and V2A: raise the plate resistor from 100k to 120k and see where that puts them? Should I simultaneously lower the cathode resistors or do that as step 2?
- OR... should I Raise the plate resistor of V1B to start off with higher voltages and see where that puts the rest?
FIRST I have to put the 470k to ground between the .0047uf series caps between the plate of V1B and the input of the gain pot...THAT I SEEM TO HAVE OVERLOOKED
Although I can't understand the relation between channel A and B , apart from the shared 1st amplification stage and the grounding of one channel with the relays switching.. but.. seeing that V2B has risen voltages by altering the circuit of the OD side I kinda start to understand you mentioning issues.

schematic pre design 1st execution 20210113.pdf

voltages 20210113.pdf

[JMPGuitars]

I still don't have time...but I'll say:

Aren't those tubes cathode biased? What does that mean?

If you're copying the series capacitors with the grounded resistor in between, that's specifically for the tremolo. You might notice other parts of the circuit only use the .005µF caps between stages.

[TriodeLuvr]

- looking at the voltages I noticed that V2B (2nd stage clean channel) have gone up. I will leave that for now and try to tweak the voltages of the OD channel
- V1B and V2A: raise the plate resistor from 100k to 120k and see where that puts them? Should I simultaneously lower the cathode resistors or do that as step 2?
- OR... should I Raise the plate resistor of V1B to start off with higher voltages and see where that puts the rest?

You know, an hour or two spent learning to use a SPICE simulator could save you days of bench work. The public domain models for tubes like the 12AX7 are very accurate, and LTSpice is free. When it comes to predicting voltages and signal levels, this software can really make the most of your time.

Jack

[Bieworm]

Thx Jack. Thought about that too.. but my laptop is property of the company and it's secured by our administrator to prevent the installation of software

[TriodeLuvr]

Thx Jack. Thought about that too.. but my laptop is property of the company and it's secured by our administrator to prevent the installation of software

That's really a shame. LTSpice has always been virus-free. I know how admins are, though. Let me know if I can help. I wouldn't mind running a few simulations for you.

Jack

[Bieworm]

I'm glad I did the conversion to a marhallesque circuit... it's still a bit fizzy but when I keep the gain knob a little lower it's still plenty of OD, but it's useable now. I mean, now I can play the amp and enjoy the sound. I put it back together and leave ut for now. But I want to find the cause of the fizz some day!!!
Here's the voltage chart :

20210114_204829.jpg

[JMPGuitars]

The reverb tube bias is way off. Those plates should be around 200.

[Bieworm]

The reverb tube bias is way off. Those plates should be around 200.

Strange.. now that you mention it.. indeed.
It sounds great though.. weird.
The blackface reverb has a 410V plate voltage with 8.7V cathode. But that's with a 12at7.. I have a 12ax7 in there. I'll better look deeper into that...

[TriodeLuvr]

The reverb tube bias is way off. Those plates should be around 200.

I'm curious - what's the DCR of the reverb transformer primary? The schematic for the AA270 Twin Reverb indicates only a 5V drop across the winding.

Jack

[JMPGuitars]

The reverb tube bias is way off. Those plates should be around 200.

Strange.. now that you mention it.. indeed.
It sounds great though.. weird.
The blackface reverb has a 410V plate voltage with 8.7V cathode. But that's with a 12at7.. I have a 12ax7 in there. I'll better look deeper into that...

I never run them that high, and it is beyond the spec limit. But you're right, that's what the F*nder schematic shows. The voltage limits between the two tubes are the same. I think it's cuckoo to run it that high, but I guess it works.

[Bieworm]

It's arrived!!!

16108038756216907152384156285122.jpg

[Bieworm]

Here's the result of the conversion to 'sort of' tremolo TMB preamp with modern classic tone stack.

https://www.dropbox.com/s/rqt3a431o6eix ... 6.m4a?dl=0

I think all in all a very nice amp imho

[JMPGuitars]

Sounds good! Much better.

Did you test any capacitors out on your fancy new scope yet?

[Bieworm]

Sounds good! Much better.

Did you test any capacitors out on your fancy new scope yet?

It isn't that tremolo TMB thang.. but it's almost like a 5E3, modded for better OD.
The amp is much more logic on the controls. That midbite in the imperial hijacks everything .. spitting out mid crunch where there's hardly control over.
Much better now indeed.

The cap test is next on the list!!!

[Bieworm]

Did the testing. Great feature!!! It's really obvious.. and you were right about those striped vishays.. some are correct, while others are not.