Awesome. I'm thinking that getting the snubber circuit in there may help with the remaining carbonation. Certainly worth a shot.
Parasitic oscillation problem untraceable
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Re: Parasitic oscillation problem untraceable
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Re: Parasitic oscillation problem untraceable
Yeah, but I need some parts... have all but the 8M2 resistors.. + I used the 1k5 5W resistors for the screens.JMPGuitars wrote: ↑Sun 01/10/21 8:30 amAwesome. I'm thinking that getting the snubber circuit in there may help with the remaining carbonation. Certainly worth a shot.
But will try for sure!!
I also think It's the nature of the amps circuit to be nasty fizzy midrangey
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Re: Parasitic oscillation problem untraceable
Josh.. if you look closely at the schematic it appears some pots have a different mark for grounding point. Is the series grounding of pots with 1 wire to ground near the input likely to cause oscillation? It looks to me like all preamp components and the midbite pot should be grounded on the ground buss?
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Re: Parasitic oscillation problem untraceable
That's not meaningful. One of them was done that way to not pass through the grid line (schematic clarity), and the other makes no sense not to just draw a line there, pretty weird, but still all of them mean the same thing. Good ground schemes are good ground schemes, if you followed what you know from the other builds, you should be fine.Bieworm wrote: ↑Tue 01/12/21 11:20 amJosh.. if you look closely at the schematic it appears some pots have a different mark for grounding point. Is the series grounding of pots with 1 wire to ground near the input likely to cause oscillation? It looks to me like all preamp components and the midbite pot should be grounded on the ground buss?
Either way, not likely to cause an oscillation, and also not likely that you actually have an oscillation. You've got a gremlin.
From this question I assume you're still having an issue. I think you should be patient until you get the snubber parts you need. Besides that, you could try leaving the 12AT7 in the PI, and try a lower gain tube in V1. 12AT7 would be a good starting point if you have another.
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Re: Parasitic oscillation problem untraceable
Did the snubber yesterday. It only darkened the tone. I put the 1000pf back in because it didn't solve anything.JMPGuitars wrote: ↑Tue 01/12/21 12:06 pmThat's not meaningful. One of them was done that way to not pass through the grid line (schematic clarity), and the other makes no sense not to just draw a line there, pretty weird, but still all of them mean the same thing. Good ground schemes are good ground schemes, if you followed what you know from the other builds, you should be fine.Bieworm wrote: ↑Tue 01/12/21 11:20 amJosh.. if you look closely at the schematic it appears some pots have a different mark for grounding point. Is the series grounding of pots with 1 wire to ground near the input likely to cause oscillation? It looks to me like all preamp components and the midbite pot should be grounded on the ground buss?
Either way, not likely to cause an oscillation, and also not likely that you actually have an oscillation. You've got a gremlin.
From this question I assume you're still having an issue. I think you should be patient until you get the snubber parts you need. Besides that, you could try leaving the 12AT7 in the PI, and try a lower gain tube in V1. 12AT7 would be a good starting point if you have another.
Even tried a 12au7 earlier on in V1. No change, just less gain.
I have a few other suspects OTOH. Looking for the common part of the circuit . V1 tube socket malfunctioning? I tightened the pin holders, but to no avail. What if the power supply caps are the origin of the problem? I changed nearly all electrolytic caps of the preamp yesterday too.
But anyway.. I don't think I dig the OD on this amp. I don't like the midbite circuit after the tone stack. It wipes the total tone control from the tablet. I was thinking of removing everything between the plate of V1 where the OD circuit branches off until the .047uf after the plate of V2b and insert the circuit of the tremolo TMB with the modern classic tone stack. (sans tremolo) I'm drawing a schematic that I will show you if it could work and what needs to be changed perhaps.
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Re: Parasitic oscillation problem untraceable
In that case, turn it into a Tremolo TMB Reverb 6V6 amp. I'll send you the docs in email if you want to make the first one.
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Re: Parasitic oscillation problem untraceable
Nice! With use of the deluxe reverb transformers I hope?JMPGuitars wrote: ↑Tue 01/12/21 2:15 pmIn that case, turn it into a Tremolo TMB Reverb 6V6 amp. I'll send you the docs in email if you want to make the first one.
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Re: Parasitic oscillation problem untraceable
Possibly. Email me the transformer info for what you have and I'll see if it will work. Also send a good deluxe reverb schematic including voltages.Bieworm wrote: ↑Tue 01/12/21 2:37 pmNice! With use of the deluxe reverb transformers I hope?JMPGuitars wrote: ↑Tue 01/12/21 2:15 pmIn that case, turn it into a Tremolo TMB Reverb 6V6 amp. I'll send you the docs in email if you want to make the first one.
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Re: Parasitic oscillation problem untraceable
OT is a hammond 1750HJMPGuitars wrote: ↑Tue 01/12/21 2:42 pmPossibly. Email me the transformer info for what you have and I'll see if it will work. Also send a good deluxe reverb schematic including voltages.Bieworm wrote: ↑Tue 01/12/21 2:37 pmNice! With use of the deluxe reverb transformers I hope?JMPGuitars wrote: ↑Tue 01/12/21 2:15 pm
In that case, turn it into a Tremolo TMB Reverb 6V6 amp. I'll send you the docs in email if you want to make the first one.
PT is a classictone deluxe reverb replacement type without heater center tap
I will look for the schematic and voltages
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Re: Parasitic oscillation problem untraceable
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?
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Re: Parasitic oscillation problem untraceable
Yup, 2 x 220R resistor joined at the cathode.JMPGuitars wrote: ↑Tue 01/12/21 7:33 pmYou 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?
And the 8R output matches the 2x12 combo with the same value on 16 ohm speakers wired in parallel
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Re: Parasitic oscillation problem untraceable
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. 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
unless I am missing things it looks to me it should work. I also marked some components that might be tweaked. 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
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Re: Parasitic oscillation problem untraceable
Change it to what you want, but that concept has...issues.
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Re: Parasitic oscillation problem untraceable
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...
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Re: Parasitic oscillation problem untraceable
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.
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Re: Parasitic oscillation problem untraceable
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.
- 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.
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Re: Parasitic oscillation problem untraceable
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.
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
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.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?
Jack
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Re: Parasitic oscillation problem untraceable
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
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
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