Nasty distortion in decaying notes, Fender Princeton clone, when vol is up high

[mikepukmel]

A guy on Facebook amp site built a Princeton clone. Ive been following the build for a long time, he's about the same knowledge level as I am. He's been fighting a nasty distortion that happens when notes decay, when the amp is cranked like 8 on the vol knob (this one doesn't go to 11 )

AFAIK he's replaced/added grid stops, changed values of signal capacitors between the PI and output tubes, put a bigger (ala Deluxe) output transformer, and i think he upgraded the PT as well, to no avail, still makes this awful harsh clipping. He's tried different speakers as well.

You can hear it at 13 seconds to 15 seconds on the following clip.

https://soundcloud.com/def_ada/clip


Curious because a Princeton 1 x 10, no reverb is my next project, but if this is more typical than not, Id switch to a different circuit. What's odd to me is that it seems to kick up, and be extremely noticeable pretty far into the decay, which makes it even more noticeable. its like at the boundary between heavy clipping, and not clipping, like the output tubes are going through a transition at that point.

[The Dude]

Boy, that could be any number of things. Poor filtering, bias too cold (crossover distortion), etc. It's not typical. It wouldn't be too hard to track down with a scope. If no scope is available, I'd check bias first, then ripple. Either/Both could be checked with a DVM.

[mikepukmel]

Boy, that could be any number of things. Poor filtering, bias too cold (crossover distortion), etc. It's not typical. It wouldn't be too hard to track down with a scope. If no scope is available, I'd check bias first, then ripple. Either/Both could be checked with a DVM.

Thanks Dude. I thought by a clip alone it wouldn't be much to go on. But your comment that its not typical is huge, thanks.

I have suggested to Moises to get an account here for a while. Great news that it shouldn't be hard to track down, I'll pass on the info, thanks.

[g1]

Having to be turned up to 8 makes me think of oscillation. He should disconnect the NFB wire to test for possible OT phasing issue.

[mikepukmel]

Having to be turned up to 8 makes me think of oscillation. He should disconnect the NFB wire to test for possible OT phasing issue.

Cool, thanks g1. I will pass it on (and suggest again that he get an account here!!!)

[R.G.]

Sometimes you just have to do the obvious instead of wandering around speculating. Put a scope on it and look at what the thing is actually putting out.

If it doesn't happen at lower volume settings, it is probably parasitic oscillation, though.

[Steve A.]

Sometimes you just have to do the obvious instead of wandering around speculating. Put a scope on it and look at what the thing is actually putting out.

If it doesn't happen at lower volume settings, it is probably parasitic oscillation, though.

For people who don't own a scope are there any cheap alternatives that you would recommend, perhaps something that would plug into a USB port? Just wondering...

Hmmm... any way to safely cut voltage down to 5vdc? Would something like this work?


http://www.piccircuit.com/shop/pic-d...RoCtz0QAvD_BwE

Steve A.

P.S. I wonder if builder experimented with lead dress which can often contribute to oscillations as Olive Oyl used to say...

P.P.S. Could you post link to thread or group at Facebook, Mike?

[Steve A.]

Having to be turned up to 8 makes me think of oscillation. He should disconnect the NFB wire to test for possible OT phasing issue.

Wouldn't that howl immediately?

Steve A.

[R.G.]

For people who don't own a scope are there any cheap alternatives that you would recommend, perhaps something that would plug into a USB port? Just wondering...

I do all my scope work with a Picoscope, which is a no-fooling USB oscilloscope. Cost me about $250 back when I got it, and I no longer even turn on the old Tektronix. But it will only go to 20V, so that doesn't cure the question.

If you look for USB stuff, mind the bandwidth. The Pico is only 20MHz, so it will cleanly miss a transistor howling its head off at 100MHz, in common with other low end "normal" o-scopes. Parasitic oscillation can easily be many MHz.

Hmmm... any way to safely cut voltage down to 5vdc?

Yes, but you have to be willing to concoct things. A 10:1 o-scope probe will make 50V down to 5V for you, and not lose frequency flatness in the process. But 10:1 probably isn't enough. You can make an external 10:1 in front of the 10:1 probe, although the frequency response flatness gets dicey. Or you can make your own frequency compensated 100:1 divider. That will make 500V be 5V, and you can even zener clamp the low voltage side so you don't take a chance on killing the USB on your motherboard if you don't have it isolated.

Dividers are simple: two resistors in series and bingo, bango, you have a divider!
... er, that is not very flat in response. Both resistors have some parasitic capacitance in parallel with them, and the capacitance is likely to be approximately fixed if both resistors have similar sized bodies. So you have to parallel the resistor divider with a capacitive divider to force the R||C on both top and bottom of the divider to be proportional. This usually means adding capacitance in parallel with the lower resistor in proportion to the division so that high frequencies are not unduly emphasized or lost in the resistors and parasitic caps. This is **exactly** what is going on in a 10:1 scope probe. But it's some work.

Would something like this work?

Sadly, no. It will be fine for things below 300Hz. But its maximum sampling rate is 1kHz. So the maximum frequency it can read and represent correctly will be half that, or 500Hz, and as a practical matter 1/3 or 333Hz will work well. Feed it anything higher frequency and it may read it OK, but it will heterodyne it down below half of the sampling frequency. Good for bass, though.

However, something like this:
https://www.amazon.com/Owon-VDS1022-.../dp/B00HC4KY2G
is amazing considering what I spent for the Picoscope.

But Pico is still trying. See
https://www.picotech.com/oscilloscop...-2000-overview
The Pico 2000 with 10Mhz is only $139 direct, maybe cheaper from a distributor, and it includes an arbitrary waveform generator that will make your test signals for you. In fact, you want to buy my older 20MHz Pico? I'm lusting for the newer models.

[g1]

Wouldn't that howl immediately?

Not always. Improper phase of NFB sometimes it just causes very slight oscillation or other strange issues. I was surprised to find this out as I was always taught it would be blatant as well. Now I probably suspect it too often, but it's easy enough to check, and has solved some fairly odd issues.

[SoulFetish]

A guy on Facebook amp site built a Princeton clone. Ive been following the build for a long time, he's about the same knowledge level as I am. He's been fighting a nasty distortion that happens when notes decay, when the amp is cranked like 8 on the vol knob (this one doesn't go to 11 )

AFAIK he's replaced/added grid stops, changed values of signal capacitors between the PI and output tubes, put a bigger (ala Deluxe) output transformer, and i think he upgraded the PT as well, to no avail, still makes this awful harsh clipping. He's tried different speakers as well.

You can hear it at 13 seconds to 15 seconds on the following clip.

https://soundcloud.com/def_ada/clip


Curious because a Princeton 1 x 10, no reverb is my next project, but if this is more typical than not, Id switch to a different circuit. What's odd to me is that it seems to kick up, and be extremely noticeable pretty far into the decay, which makes it even more noticeable. its like at the boundary between heavy clipping, and not clipping, like the output tubes are going through a transition at that point.

(At the risk of sounding like I think all shitty distortion can be traced back to this..)
Ask him if he wired the heater supply like the original princeton, with leg of the heater supply grounded to the chassis, or if he elevated his heater supply to the junction of the 6V6 cathode, 470Ω resistor, and Bypass Capacitor.

[John G]

Hi, Do you have reverb ?? if so remove the reverb driver tube or short the grid (s) to earth and try again.............
John

[mikepukmel]

Hi All, thanks for all the suggestions, and comments (and links to USB scope). Reading and re-reading, since a Princeton is the next "thing" for me to build I think.

I asked and his amp has reverb. he says it squeals when reverb is up, so ... we know what that is. I passed on your suggestion to pull the reverb tube and see what happens, I think that's in the works.

He said he set the bias by the adjusting the negative voltage into the output tubes, so I think the bias could be anywhere. ive seen a few photos, has a bias pot mod (I think the stock circuits for Princeton's didn't have a bias pot), but I don't see any helper resistors on the output tube cathodes. Passed on your suggestions that he does a careful bias calculation to see where those tubes are now.

Also sent Soulfetish's question about heater wiring. I will ask and I think he has some photos up as well.

[Chuck H]

I'm inclined to think it's parasitic oscillation. I'm also inclined to say it's somewhat normal. I've heard the same distortion in that model on other occasions (actual vintage PR's, not reissues), though that amp has a bad case of it. . All the typical stuff applies, short grid leads, twist OT plate leads together, twist OT secondary leads together and push them AWAY from the preamp, keep the NFB lead pushed away from the preamp, etc. Examine any circuit, layout, grounding or lead dress differences from stock as possible problems.

There really is a lot of LF through the that amp. Believe it or not mitigating LF may help more than mitigating HF for this sort of distortion. I don't know exactly why, but I think may have to do with what happens to the cathodyne PI when it clips too much LF causing phase errors with the NFB loop and oscillation. Lead dress is critical with these amps and copying the layout drawings is no guarantee since I've heard this on vintage amps only. Idealize the lead dress and grounding and turn the bass knob down a little when clipping the amp hard.

EDIT: Oh... And like The Dude says, check bias. Similar distortion occur on many of these amps when pushed when the trem is used. At the beginning of the bias recovery sweep (while the bias is cold) you can hear it. So it may be that the bias is too cold. The problem, as I've seen before with this model, is that it's near impossible to get the bias current up because the plate voltage is too high and that can really leave you stuck with a problem.

[mikepukmel]

You guys nailed it! He pulled the reverb tube and the problem went away. Looks like an oscillation problem. Dang the parasites! *** Chuck, the cathodyne works differently, of course, but something in particular about the cathodyne?