JMF Spectra

[ca7922303]

Got one of these, chassis only. Two(2) 6L6s, 3-12AX7s, replaced one of the screen grid resistors(burnt) with a 5w 470 ohm cement resistor. Works good now. Has lead channel only by footswitch. Has a mono cliff jack for a footswitch. Any thoughts on how to jump to get lead channel to engage? Thanks.

[J M Fahey]

Plug any simple footswitch there, it should work.

Or a bare plug and short/open its legs.

EDIT: extra cheesyness points for making a Tube amp look exactly like an SS Peavey or Roland amp.

Makes me think of those "Twin reverb inside" Plush amps made to look EXACTLY like SS Kustom amps, naugahyde cover and all.

[g1]

The cliff jack on the chassis bottom is the footswitch jack? Short the red to black and it should switch in the lead channel.
Chances are that the yellow wire tied off is for being able to kill reverb with a footswitch, but that would require a stereo cliff jack.

[ca7922303]

The cliff jack on the chassis bottom is the footswitch jack? Short the red to black and it should switch in the lead channel.
Chances are that the yellow wire tied off is for being able to kill reverb with a footswitch, but that would require a stereo cliff jack.

Used a two button ftsw from a traynor and have lead channel now. Thanks.

[ca7922303]

Also, was surprised that this thing produces 500+ volts DC

[g1]

Also, was surprised that this thing produces 500+ volts DC

Of those 5 big caps, 3 are rated for 450V. Make sure none of them have more than 450V on them. Other than that, the 500+V shouldn't be a problem.

[ca7922303]

Of those 5 big caps, 3 are rated for 450V. Make sure none of them have more than 450V on them. Other than that, the 500+V shouldn't be a problem.

Here are the readings for all 8 caps. 3-80v caps-(-0.3mv/60vdc)
(-61vdc/-0.7mv)
(-54vdv/-0.8mv)

3-450v
(0.3mv/410vdc)
(0.4mv/457vdc)
(0.5mv/494vdc)

2-350v(largest ones)
(-0.3mv/265vdc)
(262vdc/494vdc)

Both 6L6s pins 4&6(496vdc)

[g1]

The 450V cap that has 494V on it is not a good situation. If this is with all tubes installed and the amp is otherwise working normally I would suggest replacing it with a 500V type. Does that cap get hot?

[ca7922303]

The 450V cap that has 494V on it is not a good situation. If this is with all tubes installed and the amp is otherwise working normally I would suggest replacing it with a 500V type. Does that cap get hot?

All tubes installed and working normally, it does not get hot. I'm curious what you think about the two(2) rated at 350v, reading 265v & 494v?

[ca7922303]

Also, I replaced a burnt screen grid resistor on one 6L6 with a 470ohm 5w cement resistor, but left the other 6L6 with original resistor. Noticed that the tube I changed resistor on gets hotter than the one I didn't change. What's your thoughts on that? Thanks.

[Enzo]

I'm curious what you think about the two(2) rated at 350v, reading 265v & 494v?

SO you have 350v rated caps. One has 265v across it, and the other has only 229v across it. What is the concern?

[ca7922303]

SO you have 350v rated caps. One has 265v across it, and the other has only 229v across it. What is the concern?

(262/494vdc) on last one rated @ 350v.

[Helmholtz]

(262/494vdc) on last one rated @ 350v.

A cap only "sees" the voltage (difference) between its own terminals.

[Enzo]

Well step back and look at the numbers. The cap has 262v on one end and 494v on the other. SO how many volts are across it?


494 - 262 = 232v. Sorry, I was 3v off before. SO that is still WELL within the rating of 350v for the part.

[g1]

Also, I replaced a burnt screen grid resistor on one 6L6 with a 470ohm 5w cement resistor, but left the other 6L6 with original resistor. Noticed that the tube I changed resistor on gets hotter than the one I didn't change.

Does the one you didn't change measure 470 ohms?