From mgarvi–(at)–anix.com Wed Oct 4 12:07:12 CDT 1995
Article: 4161 of alt.guitar.amps
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: vibrolux-to-vibroverb tremelo mod
Date: 4 Oct 1995 02:29:28 -0400
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In
>The tremelo on my 71 vibrolux went out, and it isn’t the tube. It’s
>..
>But I played a vibroverb reissue next to a vibrolux and I thought the
>sound of the vibroverbs tremelo was much better. It was less choppy,
>smoother overall. Plus I didn’t notice it ticking all the time like my
>vibrolux does.
>The Weber book makes a big deal about the circuit in the first vibroverbs
>modulating the power amp bias instead of grounding the phase inverter.
>Weber also indicates that the grounding tremelo circuit is never
>completely off, and sucks a little tone. I have the circuts for both
>styles. Has anyone ever converted a stanadard blackface tremelo (i.e
>super, Pro, vibrolux, twin etc) to the virbroverb style tremelo? Would
>you recommend it? is Lead dress especially critical? Does it get rid of
>the tick? Any recommendations appreciated.
Tricky question, Al. Kinda labor-intensive, and I’m not the one to
judge–I don’t use tremolo.
But I *can* answer:
The lead dress will not be as critical as the present version.
The tick comes from the neon light firing, so that would go away.
Fender knocked a bit of gain out of the circuit with their
LDR-neon vibrato. Not sure why they used a 50k pot, but it loads
the previous tube stage down drastically.
Chances are that your neon light popped or associated wiring broke.
So probably no major repair. You could try using a higher value pot
if you really need the gain from that channel. Make sure it’s a
reverse audio taper (hard to find). Values higher than 100k to 250k
will have dead space in the control’s rotation. Even the 250k will
take a bit to get started but it won’t load down the tube quite as
much as a 100k. The dynamic impedance (looking backward into the
plate circuit) is probably around 70k or so, depending on how the
tube is biased. So 100k will still lose a couple db.
Weber tends to equate volume/level with tone. The psychological
effect is similar, but can usually be had by cranking the volume
control. You’ll notice that more volume it *seems* to make the
amp sound fuller and richer. Weber consistently refers to this
as tone.
In other words, you can compensate for the 50k-ra pot loading
losses by cranking the volume a bit. Loading loss is usually
slightly asymmetrical, so it sometimes creates a bit of even-
harmonic distortion. The nice kind. In this case it won’t
be extreme.
About whether to do the bias-mod: use your own judgement.
Be careful though. Messing with the bias can toast the output
tubes which can then take the output transformer down with them.
No loose wires. And test thoroughly with the output tubes pulled.
But you knew this, right?
Regards,
Mark Garvin
