|
VALVE Magazine Online!
Dinkin' Around
Tech tips and other unsolicited advice.
By Dan Schmalle From VALVE Issue 12, December 1994
Hercules Vs. The Transformer
Got an H.H. Scott LT-110 (the kit version of the 350) in the shop last week that was misbehaving badly, just a buzz would come out. After a couple tubes and an alignment of the RF and IF/limiter stages, mono sounded great, stereo wasn't stereo and had a bunch of growlies.
When I got to the 19kHz transformer on the multiplex board, there was no signal coming out, and the slugs seemed to spin freely. Ah hah! I said (actually I said Shit!). Opened up the can and realized I was witnessing the work of the Gods. Apparently a previous alignment had been performed by Hercules.
Herc had adjusted the transformer with little regard for the fact that kit type tuning coils were apparently set and sealed in place with wax by the mere mortals at H.H. Scott. In attempting to adjust the slugs he had ripped loose the four hairfine wires coming out of the coils, along with tearing the cardboard tube holding them together. Two of the wires coming from the bottom layer of the winding were torn off flush and unrepairable.
The moral? Don't Herc on a tuning slug! If you feel resistance, STOP. It is either sealed in place or jammed against something like the other slug. Either live with it the way it's set or carefully take apart enough covering to see what's going on before torquing it up.
Preamp Filament Supplies
One of the most satisfying mods I've
tried lately has been increasing the capacitance
of the DC filament supply in
various preamps. You go to Raydeau
Shaque and pick up some of the 4700
mfd @ 35V caps, say four for a PAS, and
some 50V 3A 'barrel' diodes. The diodes
produce much less voltage drop
than the toxic selenium 'rectum fryer'
and give a genuine 12+ VDC to each
filament, instead of the stock 11, and the
new filter caps will stack up under the
stock clamping arrangement nicely. The
increased voltage and energy storage
seem to give much more bass and punch
to the presentation. As a guy who has
hacked PAS's to unrecognition, I heartily
recommend this as about the only mod
worth doing to them. Quite dramatic. It
made a slight difference on a Citation I
as well.
I also tried putting Zener diodes across the supply along with the caps and diodes. Didn't notice much difference, so I guess the brute force filter does its job.
Cables Do Sound Different
Thought I'd share a new experiment with
you here. After being told again and
again that interconnect cables do sound
different, and admitting to myself that
big fat fire hoses connecting amps and
preamps do look way cool, I decided to
come up with an alternative to the hundreds
of dollars per meter tweak bait
the dealers carry.
The bright side of cleaning up my flooded basement with Eileen's help yesterday was (along with having her support to bolster me through the grossest of the muck) finding a coil of about 30' of RG-8 coax cable that I'd scored from some old ham's collection, sitting high and dry out of the muck.
Well, I been thinkin' about cobblin' me some big mother cables so I pulled it out and set it on the bench. A couple hours later I was back from Raydeau Shaque with 4 PL-259 male connectors (the only thing that will fit the 3/8" thick RG-8 at RS), four PL-259 to BNC male adapters (so I can still use the cables when I convert my own stuff to BNC, which is a much better connection than RCA), and four gold BNC to RCA male adapters so I get like metals connecting at the chassis/ cable interface.
After consulting the old ARRL Handbook for proper cable end preparation, I cobbled together one 15' cable. You guys who have been to my shop know that I use 12' Radio Shack interconnects with gold ends to connect amps to preamp, letting us use nice fat 6' short Monster Cable speaker leads, with the audition amps sitting between the speakers. Thus my rationale for 15' coble to replace my fine sounding but unacceptably branded RS cables.
I compared the new cable with the RS by means of a switch box with mono source material. I switched the lead connections to the preamp channels occasionally to eliminate sonic differences internal to the preamp (like shitty Dynaco pots).
Well the new cable sounded dull, compressed, and definitely not as loud. Compensating for gain didn't change the dull highs. Crop! I wasted 38 bucks on connectors!
This morning I had a revelation. The high buck cables have a rep for being capacitive and rolling off highs with some equipment. The PAS has a rep for being terribly picky about output loads. Put it all together and you get a horrifying picture of me running downstairs in my bathrobe, to try the cable between Citation Ill tuner and amp directly, bypassing the PAS altogether.
Bingo! No rolloff, and a little more bass and midrange presence to boot. So was it worth the money? Not if you use a PAS (keep your cables real short or use a low capacitance cable), but for $60, including the RG-8 cable, you can have a pair of 15' cables that work with load independent sources, look serious, adapt to different connectors, and don't break at the cheap terminations all the time like the RS cables do.
My solution to the PAS problem is a passive preamp I'm brewing from more ham parts. But that's for next time.
-- Dan
|
