[HAM] Fire In the Hole!

[pete mazich]

scott wrote:

>How do you know when you need new tube sockets in a 122 amp?
>
>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.

my sermon begins:

Damn! A potential "burning man" sitch (I hope you had one hand in 
your pocket). That would be a definite heads up that the sockets are 
tired (or the contacts have "sprung") :)
I've always replaced mine w/ ceramic ones (not the ones w/ "sleeve 
contacts but the mil-style straight ones). antique electronics has 
em' as well as several other vendors.

>
>Another thing worth talking about is how incorrect parts replacement and
>strange mods can affect both the sound and the safety of these amps. These
>  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- possibly didn't have a good 
>understanding of the mechanics of this balanced-line amp- the signal 
>off the g-g terminals is *way* hot so I would think that 10k is 
>absolute minimum. (even the unbalanced-line 147 goes thru a 10k pot 
>directly to the grid of the 12au7).  I usually put 1/2 watt metal 
>films in the 122  for lower noise (tolerance is moot).

>
>The four .005 MFD caps at the 6550s in both amps had been replaced with
>round ceramic caps. Oy veh. So I have 8 orange drops coming in the mail
>just to fix that little detail.

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).

   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.  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),  creating a "virtual center-tap" for the heater wiring 
(a 100 ohm 1W resistor is tied to each leg of the heater supply, the 
other ends of the resistors are tied to ground while the heater wires 
from the tubes are connected *before* the resistors), SS rectifier 
replacement and a good star-ground scheme.  Replacing the stock relay 
w/ a solid state one is also a must (The EIS one is a great choice 
and bullet-proof). Trying different brands of output tubes can alter 
the sound somewhat;  IMHO, The new production or NOS Tung-Sols are 
the holy grail while "Winged C"- (original svetlana factory) 6550's 
are real close runner-ups.   The tone of a totally rebuilt Leslie amp 
is truly amazing and you won't have any maintenance worries (save for 
oiling and cleaning the motors) for years.  At this point I would 
like to stress that this is nothing more than a glorified hobby for 
me (even tho' I've been doing it going on 10 years now), but it is 
one that I'm passionate about.  My interest (and subsequent side 
business) in tinkering w/ tube amps, hammonds and Leslies was 
cultivated by my membership on the original Hamtech list and the 
guitar amp thing was kind of an off-shoot of that. Hope some much 
more experienced techs chime in on this- I'm always learning.

>
>Hoping this post starts some discussion.

I'm way into it to say the least. Great thread Scott....



pete

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