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
Moderators: JMPGuitars, Daviedawg, Graydon, CurtissRobin, colossal, zaphod_phil
- JMPGuitars
- Super Duper Admin
- Posts: 2512
- Joined: Tue 09/18/12 8:00 pm
- Location: South Central, MA
- Contact:
Re: Parasitic oscillation problem untraceable
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
- Bieworm
- Frequent poster
- Posts: 913
- Joined: Mon 02/10/20 8:24 am
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
1 x
"THIS should be played at high volume..preferably in a residential area"
- Bieworm
- Frequent poster
- Posts: 913
- Joined: Mon 02/10/20 8:24 am
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?
0 x
"THIS should be played at high volume..preferably in a residential area"
- JMPGuitars
- Super Duper Admin
- Posts: 2512
- Joined: Tue 09/18/12 8:00 pm
- Location: South Central, MA
- Contact:
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.
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
- Bieworm
- Frequent poster
- Posts: 913
- Joined: Mon 02/10/20 8:24 am
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.
0 x
"THIS should be played at high volume..preferably in a residential area"
- JMPGuitars
- Super Duper Admin
- Posts: 2512
- Joined: Tue 09/18/12 8:00 pm
- Location: South Central, MA
- Contact:
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.

1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
- Bieworm
- Frequent poster
- Posts: 913
- Joined: Mon 02/10/20 8:24 am
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.![]()
0 x
"THIS should be played at high volume..preferably in a residential area"
- JMPGuitars
- Super Duper Admin
- Posts: 2512
- Joined: Tue 09/18/12 8:00 pm
- Location: South Central, MA
- Contact:
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.![]()
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
- Bieworm
- Frequent poster
- Posts: 913
- Joined: Mon 02/10/20 8:24 am
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
0 x
"THIS should be played at high volume..preferably in a residential area"
- JMPGuitars
- Super Duper Admin
- Posts: 2512
- Joined: Tue 09/18/12 8:00 pm
- Location: South Central, MA
- Contact:
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?
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
- Bieworm
- Frequent poster
- Posts: 913
- Joined: Mon 02/10/20 8:24 am
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
You do not have the required permissions to view the files attached to this post.
Last edited by Bieworm on Wed 01/13/21 7:03 am, edited 1 time in total.
0 x
"THIS should be played at high volume..preferably in a residential area"
- Bieworm
- Frequent poster
- Posts: 913
- Joined: Mon 02/10/20 8:24 am
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

You do not have the required permissions to view the files attached to this post.
0 x
"THIS should be played at high volume..preferably in a residential area"
- JMPGuitars
- Super Duper Admin
- Posts: 2512
- Joined: Tue 09/18/12 8:00 pm
- Location: South Central, MA
- Contact:
Re: Parasitic oscillation problem untraceable
Change it to what you want, but that concept has...issues.
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
- Bieworm
- Frequent poster
- Posts: 913
- Joined: Mon 02/10/20 8:24 am
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

Is there maybe something else I have overlooked? You don't give much detail about your hunch on having issues...
0 x
"THIS should be played at high volume..preferably in a residential area"
- JMPGuitars
- Super Duper Admin
- Posts: 2512
- Joined: Tue 09/18/12 8:00 pm
- Location: South Central, MA
- Contact:
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.
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale
Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale