Amp squeal when guitar too close.

[Alan0354]

This morning, I tried wrapping the OT completely with aluminum foil, sadly it did not help.

I want to point out the amp is very quiet, none of the hum normally associate with grounding issue. The amp sounds right and very similar to the prototype Bassman. IMHO, this kind of show the grounding and wiring is not too bad already. Particularly the only position the amp stop squealing was when the guitar facing the circuit straight on top of the chassis.

[Alan0354]

I just pull the chassis out of the Bassman and put the chassis on the table exactly like the Marshall, I use the identical OD channel and same guitar, there is no hint of squealing even when I have the 4th stage in and both preamp volume to full.


Guys, do you think the OT for the Vibrolux/Bandmaster that is for 2 6L6 40W with 4K primary can handle 2 EL34 that can run 50W perminently? I want to make sure this will work before I butcher the Bassman to harvest the OT.

I only use 420V for the power tubes. My understanding is the primary of the OT governs the max current through the tube. With 4K primary vs 3.4K of the Marshall type OT, the max current will be lower. Yes, I will loss power. But I don't want 50W, even 35W is plenty. That's the reason I lower the voltage to 420V using the circuit of the power scaling. If it is safe to use vibrolux OT, if it stops the squealing, I don't mind buying another one.

BTW, in case anyone wonder how I can stand the squealing when testing the amp. I use the THD attenuator and put it at max attenuation, it must be at least -30dB. The squeal is like a faint whittle only. I can even work on it 2am in the morning. I tell you, that THD really comes in handy. Also, this prove the feedback is not from the speaker.

[pdf64]

'That's the reason I lower the voltage to 420V using the circuit of the power scaling'
Can the power scaling circuit lower the VB+ any further? eg 360.

[Alan0354]

'That's the reason I lower the voltage to 420V using the circuit of the power scaling'
Can the power scaling circuit lower the VB+ any further? eg 360.

Yes, easily, just change one of the resistor in the resistor divider.

[Alan0354]

I already took out the OT from the Bassman, mounted onto the Marshall, just need to wire it up to test. I am going to take a snooze first, I don't think I can take it well if this does not solve the problem!!!

[gui_tarzan]

There is still something hokey going on with grounding OR shielding if the wrench being near or touching the OT makes the squeal stop. I don't understand the unseen magnetic field properties of transformers enough to say whether that's it or not, but something is interrupting something when you do that. Sort of the opposite of a Tesla sphere effect.

Is it possible that the OT core is getting saturated and oscillating or causing oscillation in the circuit unless something metallic that is grounded is put near it?

[Alan0354]

There is still something hokey going on with grounding OR shielding if the wrench being near or touching the OT makes the squeal stop. I don't understand the unseen magnetic field properties of transformers enough to say whether that's it or not, but something is interrupting something when you do that. Sort of the opposite of a Tesla sphere effect.

Is it possible that the OT core is getting saturated and oscillating or causing oscillation in the circuit unless something metallic that is grounded is put near it?

The squeal did not stop, the frequency just lower a little bit. I could hear the pitch change a little. Nothing else make any difference. As I said in the early post, a piece of 12"X12", 1mm thick aluminum in front of the amp between the guitar and the amp almost stop the squealing unless the guitar getting unreasonable close. The plate did not touch anything, I just hold it in my hand and wave around to find the best spot.

I am still siting myself up in testing with the new OT that is proven to work in the Bassman.

Thanks

[tedmich]

FYI their are more effective magnetic shields than steel bowls, you could try permetal, numetal or other high permeability magnetic shield
some here:
Magnetic Field Shielding Materials

[pdf64]

I would look to get the VB+ below 400, in order to restrict the power output to being well within the OTs capability.

[Alan0354]

It is the OT!!! ( I am not yelling, just excited!!!)

Can transformer expert like JM Farhey explain any of this? I can only say I did open the OT and took some pictures to show here before. I remember I broke the glue in order to separate the bell housing.

And other more far out observation, the Vibrolux OT have a metal bracket around the E-I core to form the bracket to screw onto the chassis, the Marshall style use the bell housing to screw onto the chassis. So the Vibrolux OT has a shield around the E-I core. BUT Marshall makes great amps that don't squeal, this cannot be the reason!!!

I don't know, I just report back. Thank you guys in working with me. I remember I fought the same squealing back in the 70s before I learn electronics, I ended up replacing the first stage with a low impedance SS circuit to stop the squealing. The problem did not occure until I replace the OT with the Fender Bassman 100 big OT( I was ignorant and stupid thinking bigger is better!!!) The Bassman 100 OT was open like the Marshall style OT). Perhaps that's the reason? That stop me from experimenting with the same Bassman 100 type OT that I have now.

I really appreciate all your time to make suggestion.

Many thanks

Alan

[Alan0354]

Now, the last question is whether the Vibrolux OT is good enough for the EL34? As I said before, I am using the power scaling circuit to drop the +B to 420V. The primary of the Vibrolux OT is 4K instead of 3.4K of the Marshall style. By default, I lower the current through the OT and the tube ( from load line). I think it is robust enough to be reliable. It is a 40W OT for two 6L6, it's not as if it's that far off. I don't even think I can get 35W out of 420V and 4K primary. I just need to confirm before I order another one.

Thanks

[J M Fahey]

It is the OT!!! ( I am not yelling, just excited!!!)

Can transformer expert like JM Farhey explain any of this?

Alan

Not that expert by any means and no, no clue.

*Everything* has a reason behind it, of course, but can't imagine it here
Or to be more precise, imagined quite a few but the final word lies in actual testing/measuring.

But glad you solved it

[Alan0354]

Drilled and installed the Vibrolux OT in, tested it. I couldn't make it squeal with 3 stages. It's that big a difference. I am going to write to Classic Tone and see whether they can do anything for me. The Vibrolux OT seems like having a little more sizzle, I am not sure, I wasn't looking for any changes as I tested in detail before in the Bassman that the two OT sounded very close.

Should I go back to 6L6 to be safe? Calculation tells me that I am safe with the EL34, but the Vibrolux OT is just so much smaller than the Marshall style.

I have no reservation in recommending the Classic Tone Vibrolux/Bandmaster OT, I have two and they work perfect. I am actually surprised that both my amps do not squeal that easy even with 4 gain stages. I don't have a lot of experience with different OT, but I am very happy with the sound.......BUT the Classic Tone Plexi 50W..........consider being warned!!!

[Silvertone Jockey]

There's a long thread on another forum about this speaker emulating dummy load, that someone made and it has almost the exact characteristics as some magical speaker cabinet. Tons of testing and tweaking, some of you have probably participated in it even. To make a long story just a little bit longer, a few of the builders were making jokes about how much loud audio that their output iron made that they never heard before because the speaker drowned it out. That's the answer to the why part of this problem

[Dave H]

When I test an amp with a sinewave and a resistive load I can usually detect the onset of clipping without looking at the scope. I can hear the change in sound from the OT.