Presence Control squeel and rumble issue.

[oscarecords]

My friend has a Metro JTM 45 Build. Recently, he had the preamp section changed to match a JCM 800 specs. The 16uf electrolytic on the board was changed to 30 uf and the 16uf cap can electrolytic inside the amp was bridged to change the spec to 32uf. Changes went well, no issues and he was very happy with the mod and overall sound of the amp. Last week, at his request, I changed the negative feedback resistor from the stock value to a Allen Bradley 47K ohm carbon comp. I also changed the negative feedback wire from the 16ohm tap on the selector switch to the 8 ohm tap. My friend wanted to match the plexi value negative feedback, and wanted to add more gain to the amp by moving the negative feedback wire from the 16ohm tap on the selector switch to the 8 ohm tap.

After the changes, when the amp is set to 16ohms on the selector switch and matched with a 16ohm cab the Presence control squeels at "1" as you turn the knob the squeel goes away, but the closer you get to max setting the amp begins to rumble. Amps seems to work fine when the ohm selector switch is on 8 or 4 ohms.

Any ideas as to what may be causing this? The amp is running KT66 tubes.

[PCollen]

My friend has a Metro JTM 45 Build. Recently, he had the preamp section changed to match a JCM 800 specs. The 16uf electrolytic on the board was changed to 30 uf and the 16uf cap can electrolytic inside the amp was bridged to change the spec to 32uf. Changes went well, no issues and he was very happy with the mod and overall sound of the amp. Last week, at his request, I changed the negative feedback resistor from the stock value to a Allen Bradley 47K ohm carbon comp. I also changed the negative feedback wire from the 16ohm tap on the selector switch to the 8 ohm tap. My friend wanted to match the plexi value negative feedback, and wanted to add more gain to the amp by moving the negative feedback wire from the 16ohm tap on the selector switch to the 8 ohm tap.

After the changes, when the amp is set to 16ohms on the selector switch and matched with a 16ohm cab the Presence control squeels at "1" as you turn the knob the squeel goes away, but the closer you get to max setting the amp begins to rumble. Amps seems to work fine when the ohm selector switch is on 8 or 4 ohms.

Any ideas as to what may be causing this? The amp is running KT66 tubes.

1) make sure you didn't cause a poor solder connection of the 16 ohm OT secondary tap to the impedance selector when moving the NFB wire to the 8 ohm tap.

2) recheck all solder connections which you tampered with during the mod.

Aside from that, I don't see how any of the mods you made could cause the problem you are describing.

[oscarecords]

Thanks for the response, but it goes with out saying, I am very meticulous when doing soldering work, especially when its for some one else. FWIW, I resolder all the connection at the ohm selector switch, because they were sloppy. I made sure to steel wool the leads of the Allen Bradley resistor before installing it and I cleaned up the connections at the presence pot because they were sloppy. I didn't really "tamper" with any other connections, other than what had been requested by my friend.

[PCollen]

Thanks for the response, but it goes with out saying, I am very meticulous when doing soldering work, especially when its for some one else. FWIW, I resolder all the connection at the ohm selector switch, because they were sloppy. I made sure to steel wool the leads of the Allen Bradley resistor before installing it and I cleaned up the connections at the presence pot because they were sloppy. I didn't really "tamper" with any other connections, other than what had been requested by my friend.

The JTM's and other 'old' Marshalls often tend to squeal like a pig when out of their cabinets and on the work bench, and particularly if you set the open chassis on top of a speaker cabinet. It's the presence circuitry that is usually the cause.

[bluessky]

I had a presence control squeal and changed V3 to a 12AT7 and that cured it.

[PCollen]

I had a presence control squeal and changed V3 to a 12AT7 and that cured it.

I lowered my B+ from 445 to 410 Vdc to cure mine. I like the lower voltages in the amp anyway, so I got double benefits from this.

I'd have to do some circuit analysis using DC load lines, and measurements, before I stuck a 12AT7 in place of the 12AX7 PI without changing some surrounding component values (re: BF Fenders). 12AT7's have a different characteristic curve chart than 12AX7. The 12AT7 will "work", but I'm just not sure if it will work just as well...that is, if it's optimized .

[oscarecords]

My friend has a Metro JTM 45 Build. Recently, he had the preamp section changed to match a JCM 800 specs. The 16uf electrolytic on the board was changed to 30 uf and the 16uf cap can electrolytic inside the amp was bridged to change the spec to 32uf. Changes went well, no issues and he was very happy with the mod and overall sound of the amp. Last week, at his request, I changed the negative feedback resistor from the stock value to a Allen Bradley 47K ohm carbon comp. I also changed the negative feedback wire from the 16ohm tap on the selector switch to the 8 ohm tap. My friend wanted to match the plexi value negative feedback, and wanted to add more gain to the amp by moving the negative feedback wire from the 16ohm tap on the selector switch to the 8 ohm tap.

After the changes, when the amp is set to 16ohms on the selector switch and matched with a 16ohm cab the Presence control squeels at "1" as you turn the knob the squeel goes away, but the closer you get to max setting the amp begins to rumble. Amps seems to work fine when the ohm selector switch is on 8 or 4 ohms.

Any ideas as to what may be causing this? The amp is running KT66 tubes.

1) make sure you didn't cause a poor solder connection of the 16 ohm OT secondary tap to the impedance selector when moving the NFB wire to the 8 ohm tap.

2) recheck all solder connections which you tampered with during the mod.

Aside from that, I don't see how any of the mods you made could cause the problem you are describing.

Finally got it working. Redid, everything and tidied up some of the wiring. Must have not been as meticulous as I thought I was first time around.