Squealing Output Transformer

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americanjetset
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Squealing Output Transformer

Post by americanjetset »

Hey guys, at a loss here with my first build.

A few weeks ago I completed a single-channel 18w cascade (using the "Cascade Classic" schematic as a roadmap). After a few small hiccups, the amp was working flawlessly, sounding fantastic, and virtually no unwanted noise.

This week, I decided to throw a DPDT switch in an extraneous hole in the chassis and wire it up to switch between cascade and parallel gain stages. After getting everything wired, the parallel "channel" works fantastically, but whenever I switch to cascade, I get a very high-pitched squeal coming from the output transformer as well some nasty distortion in the output signal. If I turn the tone pot down below ~3o clock, the squealing stops.

I'm at a complete loss here. I've re-checked all my connections, rewired the switch with and without shielded wire, rewired the connections from the tubes; nothing seems to make it go away.

I attached a quick layout of how I wired the switch, as I wasn't able to follow ZPs mod schematic exactly. The idea here is to switch the output of the V1A plate and the input to the V1B grid.
switch.png
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JMPGuitars
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Re: Squealing Output Transformer

Post by JMPGuitars »

That squeal could be a few things. Post some high quality photos of your build. Also post your voltages if you can.

Did you follow the ground scheme from my layout, or did you go a different way?

Do you have any plate and grid wires running in parallel?

Thanks,
Josh
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americanjetset
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Re: Squealing Output Transformer

Post by americanjetset »

Hi Josh,

Thanks for the reply. I was able to nearly eliminate the squealing by carefully re-wiring a new switch using shielded wire. I also re-wired the connections from the board to the pots, and now there is only a squeal when one or more of the pots is wide open on the cascade channel. Oddly enough, the squeal also appeared while I was probing Pin 7 on V4 -- only while on the cascade setting, and only on V4.

I did follow your ground scheme, although I had to tinker with the layout quite a bit to get the footprint down to where I wanted it (and made a few mistakes along the way, of course).

Attached some shots of the rats nest around the pots/PI and a voltage sheet. The V1 voltages on Pins 1 and 6 are out-of-whack because I was experimenting with some different value load resistors and some smoothing caps and it looks like I overshot the P1 resistor a bit, ha.

This was my first big project soldering, so don't be too harsh :P
asvolts.jpg
as3.jpg
as1.jpg
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Bieworm
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Re: Squealing Output Transformer

Post by Bieworm »

Are those thick white and red wires the shielded ones? If they are, I think there is a big chance they are a problem. You know only 1 side of the shielding should be grounded and the other end sealed off with heatshrink? It's also better to solder the shield NOT to preamp or power amp ground , but to a separate ground point ( I gently solder them to the back of the pots nowadays, with succes. But remember to dial the wiper extreme right or left)
There's also molten wire tubing here and there. Try to avoid that, it degrades the build quality at the least. I know it's not easy.. but it will improve your skills
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colossal
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Re: Squealing Output Transformer

Post by colossal »

Looking at the lead dress, some of the wires and capacitors got nicked with the soldering iron. There may be hidden issues there as well.
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geoff 1965
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Re: Squealing Output Transformer

Post by geoff 1965 »

This sounds like you might need to attenuate the signal,I did a lot of experimenting with a cascade mod and made the mistake of not having the volume & tone pots on both the amp and guitar on full. A voltage divider of equal resistance will halve the signal but you might need to adjust that to stop the squeal,I had a 220K by 220K divider but had to drop the resistor to ground to 76K to stop the squeal with the pots on max.where is your 470K divider? also i’m struggling to see the wiring to the dpdt switch on those pics.
those 8K2’s in the power section look tiny,ideally you want 1W resistors or 1/2W minimum,what value bias resistor is on your el84’s cathodes?
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americanjetset
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Re: Squealing Output Transformer

Post by americanjetset »

geoff 1965 wrote:
Wed 07/21/21 2:18 pm
where is your 470K divider?
you can see it in each picture, just above the 2200uF cap (with the diodes in parallel).
also i’m struggling to see the wiring to the dpdt switch on those pics.
yeah it's quite a rat's nest because of poor PI placement in my layout. I'll try to get a clearer pic when I get home
those 8K2’s in the power section look tiny,ideally you want 1W resistors or 1/2W minimum,what value bias resistor is on your el84’s cathodes?
They are 1/2W. My bias resistor is 220r.
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geoff 1965
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Re: Squealing Output Transformer

Post by geoff 1965 »

You should have a 470K voltage divider between the triodes of V1 for the cascade like shown on the schematic,the ones you referred to are the 470K grid leaks for the power tubes.
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americanjetset
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Re: Squealing Output Transformer

Post by americanjetset »

geoff 1965 wrote:
Wed 07/21/21 5:41 pm
You should have a 470K voltage divider between the triodes of V1 for the cascade like shown on the schematic,the ones you referred to are the 470K grid leaks for the power tubes.
My mistake. Very top of the board (with the white wire coming out and going under the board to the DPDT switch)
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geoff 1965
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Re: Squealing Output Transformer

Post by geoff 1965 »

Okay,if you keep to the schematic values on the phase inverter and check over your lead dress etc like suggested you could remove the 470K to ground and patch in a 500K pot which will let you adjust the signal strength I.e. turn your pots up until the squeal starts then adjust the 500K pot until the squeal stops then measure the resistance of the pot for the resistor you need.
Just an idea which is fairly easy to do,if you make the leads on the pot long enough so it’s outside the chassis and not dangling inside near the high voltages.
B292E4B3-7141-4BF5-8A2B-790CCE6035CF.jpeg
Note; when you got the squeal probing pin7 of V4 did you try swapping the el84’s around to see if the same happened?
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americanjetset
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Re: Squealing Output Transformer

Post by americanjetset »

geoff 1965 wrote:
Wed 07/21/21 6:48 pm
Okay,if you keep to the schematic values on the phase inverter and check over your lead dress etc like suggested you could remove the 470K to ground and patch in a 500K pot which will let you adjust the signal strength I.e. turn your pots up until the squeal starts then adjust the 500K pot until the squeal stops then measure the resistance of the pot for the resistor you need.
Great little patch! I tried this, and wound up dropping the resistor to ground to 220k and the squeal is completely gone. I'm still going to spend some time this weekend cleaning up the lead dress, particularly are the preamp tube and the PI, to see if I can eliminate the squeal with the 470k
Note; when you got the squeal probing pin7 of V4 did you try swapping the el84’s around to see if the same happened?
Yes; doesn't matter which tube, doesn't matter what the pots are set at, etc. Seems to be an entirely separate issue from the other squealing.
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geoff 1965
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Re: Squealing Output Transformer

Post by geoff 1965 »

Good! When you attenuate the signal make sure your vol & tone pots on the guitar are on max as well,have a good listen and you will find a sweet spot for the tone.
Regarding the V4 issue have a close look at the 8K2 signal wire to the valve base it looks like it’s burned into the white cathode wire.i would up those 8K2’s to 1W myself.
Good luck
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