Magma tone Troupador tone

[jamesedmunda]

I've replaced all the caps and any out of tolerance resistors
 The amp sounded rough and had low output. I replaced the 1 meg grid stoppers with 68k and have more volume but the tone is still rough. This is a grid leak bias should I try rewiring for cathode bias.
 The schematic shows a .01 cap before the grid but my amp never had this. I tried adding the input cap but no tone improvement.

[terminalgs]

There is no reason to redesign the circuit to make it function properly.   


How do the voltages of your amp compare to the ones on the schematic you posted?

[jamesedmunda]

Thanks for your interest. Here's the voltages. It almost sounds like a bad speaker but I tried a different one and still sounds terrible.

[sluckey]

You replaced a 1K field coil with a 25K resistor. Why? Use a 1K 10W resistor instead. Any better?

[jamesedmunda]

This is a latter model it came without the fieldcoil. Replaced the 25k resistor with a 2k. Here's the revised voltages. Also reinstalled the original meg ohm grid stoppers.
Sounds better but still bad. Tried another speaker, smoother but still bad.
There is a 1.5k on the cathode to ground on the 6SJ7 that not shown on the schematic also the coupling cap is a .05 on the schematic installed is a .022.
Here's a video;

[PRR]

You show 21V drop in 250r in the cathodes, 23V drop in '15k' in the screen+plates feed. I don't think that is a 15k resistor, or it isn't wired as shown.

57V drop between plates and screens is not "bad" but is very odd. We expect 5%-10% drop through OT to plates, 8V to 17V, not 57V. I dunno if an OT can be that bad (both sides?) or if it is a mis-reading.

[jamesedmunda]

Thanks PRR,
 I had the resistors on the first node of the power supply wrong. Now my plates have 325v and the screens 335v.
 Sounds much beter.

[Pietro]

could you send a detailed picture of wiring around 1st and 2nd filtercap?

[jamesedmunda]

Looks like the dc output from the rectifier tube goes back to the power transformer, choke winding?

[Pietro]

Personally...i find it difficult to see anything in this type of wiring. Maybe do some cleaning and crossreference with schematic while you are at it?

Does this chassis suppost to stand or hang ?  When hanging them resistors over center of the tube base will get fried because of rising heat.

[jamesedmunda]

Sideways should be good for another 75 years.

[sluckey]

Here's a basket case restoration by one of our forum members several years ago. A bit different from your but thought you might get a kick out of it...

     http://sluckeyamps.com/pila/pila.htm