Parasitic oscillation problem untraceable

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Bieworm
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Re: Parasitic oscillation problem untraceable

Post by 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 :cry:
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
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Re: Parasitic oscillation problem untraceable

Post by 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.
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

Bieworm wrote:
Thu 01/14/21 4:20 am

- 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.

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Re: Parasitic oscillation problem untraceable

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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
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

Bieworm wrote:
Thu 01/14/21 11:00 am
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.

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Re: Parasitic oscillation problem untraceable

Post by 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
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

The reverb tube bias is way off. Those plates should be around 200.
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

JMPGuitars wrote:
Thu 01/14/21 3:22 pm
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...
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

JMPGuitars wrote:
Thu 01/14/21 3:22 pm
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.

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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Thu 01/14/21 3:38 pm
JMPGuitars wrote:
Thu 01/14/21 3:22 pm
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. 🤷‍♂️
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Re: Parasitic oscillation problem untraceable

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It's arrived!!! hurray
16108038756216907152384156285122.jpg
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Re: Parasitic oscillation problem untraceable

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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
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Sounds good! Much better.

Did you test any capacitors out on your fancy new scope yet?
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

JMPGuitars wrote:
Sat 01/16/21 12:01 pm
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!!!
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Re: Parasitic oscillation problem untraceable

Post by 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.
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