re Vibroverb RI Class A ?
Lets think about the output impedance problem that occurs when you run a
Vibroverb with just one of the 5881 output tubes. Normally with the two
5881's the plate to plate primary impedance is going to be 4200 ohms.
Assuming that the output transformer is wound for 2 10" 8 ohm speakers
wired in paralell for a load of 4 ohms nominal we find that the turns
ratio for the stock transformer would have to be 16.2 to 1. Running just
one of the 5881's in the output doubles the plate to plate impedance to
8400 ohms. The problem with this is that running the stock speaker
configuration the output tube is now running into a 4200 ohm load when
it wants to see 8400 ohms. How do you solve this problem? Change the
output transformer is one way to do it. If the reissue uses a dual tap
transformer you could use the 8 ohm tap or you could also replace the
speker with a single 15" 8 ohm speaker. This would make the amp more like
the coveted single 15" version of the original Vibroverb. The other no
money fix that you could do is to rewire the 10" 8 ohm speakers to run in
series instead of paralell so that the impedance would be 16 ohms instead
of 4 ohms. This would give the single output tube a load of 16,800 ohms
which would be better than the 4200 ohms it sees with just one tube and
everything else unchanged.
The real question is whether this would sound any better. Who knows.
I'd have to agree with Mike Curtis about the Deluxe Reverb. I never
could get a sound out of one of those that I liked but I've heard others
that sounded great through one. Be the first on your block to own a
Class A single 15" 15 watt Vibroverb. The neat thing about it would be
the spectacular WHOesque stage shows you could come up with. Igniting
your amplifier while onstage beats playing on your back any day. fjm
ps while I thought about the above post before i sent it the possibility
of errors exists. Corrections accepted with a smile. After all it's not
a contest. Is it?
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