Amp squeal when guitar too close.

[Silvertone Jockey]

One of the guys said that often he didn't even turn on his mono block amp hooked to the load, but instead just listened to his iron because it was almost bedroom volume loud. When that happens it's a very peaky speaker at higher frequencies. I have heard some radio antennas talk very loud before with an over modulated radio feeding a big amp. You can hear the speech very clearly . Tends to freak people out who've never heard it before

[Alan0354]

What are you guys talking about, how does that relays to the squealing?

I had the THD attenuator lowering the sound down to whispering level, I did not hear anything from the OT. The speaker was like 8' away and I was in front of the amp, never heard a bleep from the OT.

BTW, anyone can agree or disagree with my assertion that the Vibrolux OT is good enough to work with the EL34 with +B at 420V?

[R ski]

Having built a few marshal amp configurations, I had my share of nasty, hums, oscillations when gain cranked.

I gotta give you credit for being intuitive about your attempts to cure the problem....great effort and interesting reading.

Honestly, the OT transformer will not solve your dilemma, but looking at your picture in post #20, the pre amp wiring based on point to point,
that wiring routing is concerning. I noticed how the filter cap is nearby what looks to be an anode (1) close proximity to the cathode (3) AC bypass capacitor. I learned that layout does lend itself to problems, my first build was point to point, with identical to a marshal configuration squealed and diminished with a buzz.

Planning a layout with some guidelines to grounding (star grounding) and component placement will get better stability on your project, been there done that.

Work on the first triode stages V1 and V2 with the anode resistors and connection to the B+ filter caps placed on one side of the socket and orient the cathode and grid to be close to the jack and gain potentiometer and their resistors. The most gain happens in V1 & V2 &V3, the proxy of wiring and components does effect stray oscillations.

[Alan0354]

Honestly, the OT transformer will not solve your dilemma, but looking at your picture in post #20, the pre amp wiring based on point to point,
that wiring routing is concerning. I noticed how the filter cap is nearby what looks to be an anode (1) close proximity to the cathode (3) AC bypass capacitor. I learned that layout does lend itself to problems, my first build was point to point, with identical to a marshal configuration squealed and diminished with a buzz.

The OT change did solve everything. In both my other amp, it did not squeal. After I change the OT, I cannot make it oscillate. I did try to ground the first filter cap farther away, it did not make a difference. That was experimented very early on. I know what you mean. I was thinking to have the first stage totally self contained with no current injected into the chassis. So the anode current and the cathode current all at the same point. It is not critical because I have it both ways.

The amp is very quiet, no ground loop to talk about. I don't have a typical 33K resistor as grid leak on the first stage that is the dominant noise source of the amp. The hissing (thermal noise of the resistor) of the amp is very low( compare with using a 33K grid leak, big difference). The hum is still lower than the white noise when I have all 4 gain stages in. I am sure I don't have grounding problem.

This is the second time I battle this problem, the first time was in 1978 before I learn electronics. The problem started when I was stupid to change the original OT to the bigger Bassman 100 OT thinking bigger is better. I really feel like I won the final battle fought 37 year ago!!!!

[Alan0354]

Hi guys,

I am about to close up the amp. My final test is power output. I use a 1.25KHz sine wave and look at the output. The output clipped at 45Vpp. So to calculate power, I use W=(Vrms^2)/R=(45/2/1.4)^2/8ohm=31.65W

I use only 420V +B. I just want to hear from you guys that my Vibrolux OT is good enough for this build. This OT is for 40W 6L6GC. Even though I use EL34, but I keep the +B low. Also, it is a 4K primary instead of 3.4K, both factors lower the power to 32W that I measured. I think I am safe. What do you think? 32W is plenty for me.

BTW, I wrote to Amp Parts Direct where I bought the Classic Tone Marshall style OT about the squeal. They came back and said I switched the primary wires. I told them I check all those. They never wrote back. So be warned. I don't think I have a case as it's been 3 months since I bought it.

Thanks

[danman]

I like to purchase my Classictone transformers from Triode. I had an issue with a PT that I purchased and they quickly sent me a replacement and a prepaid shipping label to return the defective one. I have used their 50 watt Marshall OT in two different builds and both performed great but there is always the chance that you got a bad one.

[Alan0354]

I like to purchase my Classictone transformers from Triode. I had an issue with a PT that I purchased and they quickly sent me a replacement and a prepaid shipping label to return the defective one. I have used their 50 watt Marshall OT in two different builds and both performed great but there is always the chance that you got a bad one.

Problem is it's been 3 months. I am sure if I have the problem within the first month of purchase, they would do something about it.

Are your amps high gain? It squeal when turning up the volume and gain.

[danman]

I built a 1987 and a 2204 last year using the Classictone trannys in each. I do have a bit of squeal in the 2204 with a certain guitar, though it only happens when the pregain volume is above 6. I think it's the pickups in this guitar because others that I use do not produce this squeal. It does seem to be better when the chassis is installed in the head cab though because of the metal shielding plate. Other than that both amps do great and will only produce feedback squeal when I'm trying to make it happen.

[Alan0354]

I built a 1987 and a 2204 last year using the Classictone trannys in each. I do have a bit of squeal in the 2204 with a certain guitar, though it only happens when the pregain volume is above 6. I think it's the pickups in this guitar because others that I use do not produce this squeal. It does seem to be better when the chassis is installed in the head cab though because of the metal shielding plate. Other than that both amps do great and will only produce feedback squeal when I'm trying to make it happen.

I just finished putting the Classic Tone Marshall OT into my Bassman test bed. It squeal at high volume. So it is for sure the OT is the problem, nothing else.

1987 is normal gain amp, so it should never squeal. 2204 is rather low gain in the high gain arena. I actually have the 2204 front end built into the Bassman as one of the 3 preamp. I up the gain in order to get me decent OD with my strat. That's the channel I used to test and it squeal when volume to 4.

I don't think the OT is defective. I am not a tranny expert, JM Farhey is the one. But in my limited experiments as in the former posts on this thread, my suspicion is the mounting bracket that shield the Vibrolux OT. This OT does not have amounting bracket that wrap around the laminates, that's the only thing I can think of. I managed to use thick aluminum to shield the OT and stop the squeal before as described.

[g1]

In the bassman it squeals even if you reverse the primary connections?

[Alan0354]

In the bassman it squeals even if you reverse the primary connections?

I wired according to the schematic. I did not try reverse the primary. From my experience, if the primary is reversed, it becomes a fog horn the moment it is warmed up!!!! I had that happened me before and it is not fun. You think they might put the wrong color wire on the primary? It sounds fine, only squeal when I crank it up and guitar within 18" from the transformer.

I guess I can try it as it's easy AND the most important thing, I have the THD attenuator. I really don't want to try without that, it's loud!!!!

[Alan0354]

OK, I did two things

1) I open the feedback loop, this should stop the squeal if the polarity of the primary is reversed. Without the feedback, the wiring of the primary becomes irrelevant. It still squeal, but seem slightly harder to make it squeal.

2) I put back the NFB and reverse the wiring of the primary. To my big surprised, it did not become like a fog horn!!! It still work, but seems like the sound is brighter. BUT there were more squeal. I got the usually lower frequency ( say 3KHz) squeal when I move around the guitar to a certain position. On top, I got an even higher frequency ( say 7 to 8 KHz) squeal if I move the guitar to another position. The lower frequency squeal is slightly less sensitive, but I got two in different position.

So the squeal is not related to the polarity of the primary, it squealed either way. I am puzzled why it did not sound a fog horn as at low frequency, it is positive feed back.

Ha ha, I presented my crazy theory about the mounting bracket on the Vibrolux works as a shield to avoid the squeal. But people here with lot more knowledge in transformer might give better insight.

[pdf64]

Positive feedback does not necessarily result in oscillation, rather oscillation is a subset of positive feedback, and occurs when certain conditions are met.
Free running oscillation (ie foghorn) is a yet further subset, parasitic oscillation being another commonly encountered subset.
At lower levels, positive feedback can create peak/s in the frequency response etc (eg experiment with a delay / flanger etc pedal to get a feel for this).
My experience is that with regular tube guitar amps, there's about an even chance of (an unintended) positive feedback loop on the power amp resulting in free running oscillation.
So for a new build etc, even if there's no immediate 'foghorn', it's beneficial to always lift the feedback and check whether the power amp gain increases or decreases.

[Jazz P Bass]

Well now, to check that the OT primary is properly wired, with the feedback connected, run a signal through & measure the output voltage.
Lift the feedback wire & remeasure the output voltage.
If the signal goes higher, then the OT primary is wired correctly.
If it goes lower, then you had a positive feedback condition & the OT primaries where incorrectly wired.

[Alan0354]

Well now, to check that the OT primary is properly wired, with the feedback connected, run a signal through & measure the output voltage.
Lift the feedback wire & remeasure the output voltage.
If the signal goes higher, then the OT primary is wired correctly.
If it goes lower, then you had a positive feedback condition & the OT primaries where incorrectly wired.

I'll definitely try this later today after the dentist drilling!!!

Thanks