[HAM] Fire In the Hole!

[OF]

At 02:37 PM 11/5/2006, pete mazich wrote:
>scott wrote:
> >I was checking the high voltage at one of the 6550s. I touched the probe to
> >one of the pins, and suddenly.....a little fireball was born.

Hey Pete! Good to see you!

>Damn! A potential "burning man" sitch (I hope you had one hand in
>your pocket).

Nope, I had the other hand on the other probe to ground. This made me 
realize that it would be a good idea to have clip leads on my meter.

> >  What he had done was replace the 10K input resistors
> >at pins 1 and 6 with 6.6 Ks.
>
> >   * Makes me wonder if he even had a schematic- sounds like he was
> >thinking "guitar amp input" here-

Since he had to have removed the old resistors first, I guess he would have 
had a clue.

> >The four .005 MFD caps at the 6550s in both amps had been replaced with
> >round ceramic caps. Oy veh.
>
>Ceramic=cheap: they'll definitely work in that position but as far as
>the sound.....poop on a stick. When I rebuild guitar amps (and any
>other tube amps for that matter), I replace *all* of the ceramics w/
>polyprops (whether they need it or not) and it always improves their
>tone noticeably (the only exception tone-wise being AC suppressor
>caps on stand-by switches- here, ceramics work great).

OK, here's where I admit that I don't know nuttin'. I just assumed the 
discs were a bad idea because I haven't seen them used commonly in that 
position. They looked as if they had suffered some heat degradation. As to 
what came in a brand-new 122 amp, I have no idea.

But I have a hard time buying the idea that different cap types can change 
the tone. I mean, they all do the same thing: capacitate.

>    The beauty of all these Leslie amps is in their simplicity; most
>"hot-rod" mods I have seen are an attempt to get more *gain* out of
>the amp rather than headroom. No matter what you do, you're not going
>to get much more than 40 watts out of em' w/ the stock tranny's.

Yup. My 31H has extra power and headroom because it has a larger, non-stock 
power tranny, so a higher B+ actually works.

>   The
>best and most practical mods (IMHO) are the ones that try and achieve
>the latter (also less noise), and these could/should include: larger
>filter caps w/ a slightly higher voltage rating and/or distributed
>capacitance (two or more caps in parallel for the desired capacitance
>rather than one), flame-proof metal-oxide resistors in the power
>supply, tight-twisted wiring pairs in the power supply and audio
>section (also, layout of the wires makes a lot of difference in the
>noise floor),

Yes. I had a 122 go into RF oscillation, in part because the wiring wasn't 
dressed just right, close to the chassis.

I like Uncle Harv's mod advice best: these amps were well-designed to begin 
with and there really aren't any mods that improve them.

--Wet Blanket