Parasitic oscillation problem untraceable

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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Sun 01/10/21 8:25 am
Played it. Sounds pretty fine that way. Fizz, but yeah...
Doing some resoldering of the power supply right now. Who knows...
And oh yeah, wired a 250R cathode resistor which brings the bias to 89%. Seems more in line with the design...
Awesome. I'm thinking that getting the snubber circuit in there may help with the remaining carbonation. Certainly worth a shot.
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

JMPGuitars wrote:
Sun 01/10/21 8:30 am
Bieworm wrote:
Sun 01/10/21 8:25 am
Played it. Sounds pretty fine that way. Fizz, but yeah...
Doing some resoldering of the power supply right now. Who knows...
And oh yeah, wired a 250R cathode resistor which brings the bias to 89%. Seems more in line with the design...
Awesome. I'm thinking that getting the snubber circuit in there may help with the remaining carbonation. Certainly worth a shot.
Yeah, but I need some parts... have all but the 8M2 resistors.. + I used the 1k5 5W resistors for the screens.
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

Post by Bieworm »

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

Post by JMPGuitars »

Bieworm wrote:
Tue 01/12/21 11:20 am
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?
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.

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

Post by Bieworm »

JMPGuitars wrote:
Tue 01/12/21 12:06 pm
Bieworm wrote:
Tue 01/12/21 11:20 am
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?
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.

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.
Did the snubber yesterday. It only darkened the tone. I put the 1000pf back in because it didn't solve anything.
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

Post by JMPGuitars »

Bieworm wrote:
Tue 01/12/21 12:40 pm
But anyway.. I don't think I dig the OD on this amp.
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

Post by Bieworm »

JMPGuitars wrote:
Tue 01/12/21 2:15 pm
Bieworm wrote:
Tue 01/12/21 12:40 pm
But anyway.. I don't think I dig the OD on this amp.
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. ;)
Nice! With use of the deluxe reverb transformers I hope?
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Tue 01/12/21 2:37 pm
JMPGuitars wrote:
Tue 01/12/21 2:15 pm
Bieworm wrote:
Tue 01/12/21 12:40 pm
But anyway.. I don't think I dig the OD on this amp.
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. ;)
Nice! With use of the deluxe reverb transformers I hope?
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.
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

JMPGuitars wrote:
Tue 01/12/21 2:42 pm
Bieworm wrote:
Tue 01/12/21 2:37 pm
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. ;)
Nice! With use of the deluxe reverb transformers I hope?
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.
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
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Tue 01/12/21 4:39 pm
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?
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

deluxe_reverb_ab763_schem.gif
JMPGuitars wrote:
Tue 01/12/21 7:33 pm
Bieworm wrote:
Tue 01/12/21 4:39 pm
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
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Last edited by Bieworm on Wed 01/13/21 7:03 am, edited 1 time in total.
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Re: Parasitic oscillation problem untraceable

Post by 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 ;)
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Wed 01/13/21 4:58 am
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.
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

JMPGuitars wrote:
Wed 01/13/21 7:58 am
Bieworm wrote:
Wed 01/13/21 4:58 am
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...
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Wed 01/13/21 8:38 am
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.
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