So I've built an ac30ish amp (i have another thread about it in the vox section, but its died out) and im having some frustrating problems. Ive built it off a hoffman board, and it has no reverb, tremolo, or master volume. The amp sounds very nice while clean, but when i turn it up enough to get some power tube distortion, it gets pretty ugly. The distortion had this harsh, fizzy, crackly characteristic that trails off as the notes decay. Ive tried several things which have had no effect on the the ugly distortion: I've got a 220pf "enhance" cap on the pi, a 47pf cap across the plates of the pi, increased the power tubes' grid strippers by 10k, as well as added 10k grid strippers to the pi, so i think I've ruled out blocking distortion. Also, I've added a conductive filter across the ot primaries using values from the dr. Z z wreck.
Another symptom I've got is a sudden increase in hum at near full volume.
So my question is, does this sound like I've got some sort of oscillation going on that starts when the power tubes start to clip? Another problem I've got is that i dont have a scope, so im limited by that. But could this ugly distortion be caused by my lead dress? Im still having a difficult time getting things to be tidy, and i guess it just comes with practice.
I have tried tube swaps in all positions btw, and have tried several different brands with no improvement. Any help/suggestions are greatly appreciated!
does this sound like HF oscillation?
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JoshBernstein
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- Location: Key Largo, FL USA
does this sound like HF oscillation?
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Not really
Re: does this sound like HF oscillation?
Yes, I think oscillation is a likely suspect.
Lead dress is tricky as these amps have got a lot of gain when vol and treble are high.
I try to use lead dress to enhance local negative feedback (eg a twist in grid and plate wires), screen (eg a twist in grid and bypassed cathode wires) or roll off (eg a twist of conductors carrying balanced signals).
Stoppers on every grid terminal would be a good idea.
It would be helpful if the photos were a bit sharper.
The screen grid resistors act as grid stoppers too, so it's good practice to mount them as close to the tube socket terminal as practicable; mounting them remotely on a circuit board can lead to such issues.
It's a bad idea to use terminal #1 of el84 sockets; it's reported that some manufacturers make use of the pin, eg as an internal tie point.
What's an 'enhance' cap? Preferably link to a schematic of an amp that uses one.
Lead dress is tricky as these amps have got a lot of gain when vol and treble are high.
I try to use lead dress to enhance local negative feedback (eg a twist in grid and plate wires), screen (eg a twist in grid and bypassed cathode wires) or roll off (eg a twist of conductors carrying balanced signals).
Stoppers on every grid terminal would be a good idea.
It would be helpful if the photos were a bit sharper.
The screen grid resistors act as grid stoppers too, so it's good practice to mount them as close to the tube socket terminal as practicable; mounting them remotely on a circuit board can lead to such issues.
It's a bad idea to use terminal #1 of el84 sockets; it's reported that some manufacturers make use of the pin, eg as an internal tie point.
What's an 'enhance' cap? Preferably link to a schematic of an amp that uses one.
https://www.justgiving.com/page/5-in-5-for-charlie This is my step son and his family. He is running 5 marathons in 5 days to support the research into STXBP1, the genetic condition my grandson Charlie has. Please consider supporting him!
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JoshBernstein
- Posts: 289
- Joined: Tue Jan 27, 2015 1:31 am
- Location: Key Largo, FL USA
Re: does this sound like HF oscillation?
Okay.
ill do my best to take some better pictures. All I've got is my cell phone and the camera is a bit finicky.
So ill move the screen grid resistors closer to the socket and see if that helps, and ill pull the resistors off pin 1 just to be safe.. Is there any way to detect oscillation and/or its origin without a scope?
And in all honesty, in not exactly sure what an "enhance" cap is. It was recommended to me on my other thread... its a small cap across the plate resistor to the entrance of the pi. It seems to round/smooth out the high end. It seemed like it would cause some phasing issues to have it only on 1 side of the pi, but it actually sounds quite nice.
ill do my best to take some better pictures. All I've got is my cell phone and the camera is a bit finicky.
So ill move the screen grid resistors closer to the socket and see if that helps, and ill pull the resistors off pin 1 just to be safe.. Is there any way to detect oscillation and/or its origin without a scope?
And in all honesty, in not exactly sure what an "enhance" cap is. It was recommended to me on my other thread... its a small cap across the plate resistor to the entrance of the pi. It seems to round/smooth out the high end. It seemed like it would cause some phasing issues to have it only on 1 side of the pi, but it actually sounds quite nice.
Not really
Re: does this sound like HF oscillation?
It was mainly that last 'whole chassis' photo that was unclear, the others were ok.
In addition to the previous suggestions, consider lifting the cathode bypass cap off V2 to see if that helps; maybe even try that first.
I like to use 100k (or lower) grid reference resistors on the power tubes.
The benefits are threefold;-
1. almost all classic amps have way too much voltage swing available at the power tube grids, which leads to bias shift / blocking distortion when overdriven, and lowering those resistors acts to bring that down a bit.
2. the lower load resistance tends to lower the gain of the phase splitter a little, again reducing the tendency for bias shift under overdrive, and increasing the margin of stability.
3. it helps to prevent high levels of grid current (eg old tubes, hot running tubes) shifting the bias, which may end up in thermal runaway.
So it may help to mitigate the issues your build has.
In addition to the previous suggestions, consider lifting the cathode bypass cap off V2 to see if that helps; maybe even try that first.
I like to use 100k (or lower) grid reference resistors on the power tubes.
The benefits are threefold;-
1. almost all classic amps have way too much voltage swing available at the power tube grids, which leads to bias shift / blocking distortion when overdriven, and lowering those resistors acts to bring that down a bit.
2. the lower load resistance tends to lower the gain of the phase splitter a little, again reducing the tendency for bias shift under overdrive, and increasing the margin of stability.
3. it helps to prevent high levels of grid current (eg old tubes, hot running tubes) shifting the bias, which may end up in thermal runaway.
So it may help to mitigate the issues your build has.
https://www.justgiving.com/page/5-in-5-for-charlie This is my step son and his family. He is running 5 marathons in 5 days to support the research into STXBP1, the genetic condition my grandson Charlie has. Please consider supporting him!
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JoshBernstein
- Posts: 289
- Joined: Tue Jan 27, 2015 1:31 am
- Location: Key Largo, FL USA
Re: does this sound like HF oscillation?
Actually, i beleive that i currently have 100k's in there right now. I think i forgot to mention that the issue is in both the normal and the top boost channel, which makes me think that the issue around the pi or the power section. But ill lift the bypass cap anyway just to make sure. I also intended to post a layout/schematic:
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Not really