From SRSN81–(at)–rodigy.com Sat Nov 25 00:18:26 CST 1995
Article: 5760 of alt.guitar.amps
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From: SRSN81–(at)–rodigy.com (Joseph Pampel)
Newsgroups: alt.guitar.amps
Subject: Re: Easy(?) Tube Rectifier Question
Date: 25 Nov 1995 04:16:38 GMT
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CXPN06–(at)–rodigy.com (Eric Erickson) wrote:
>
>I’ve a ’73 Vibrolux Silverface that uses a 5U4 rectifier. Is it
possible
>(without modification) to advantageously use a different tube? I’m not

>concerned with the overall RMS of the amp, JUST THE TONE…

You can use any of the 3 “popular” rectifier tubes by simply plugging
them in. The 5Y3, 5U4 and 5AR4/GZ34 all pin out the same (effectively).
Big difference is how much current each can deliver to the amp, and their
internal resistances which effect the voltage drop across them (and
thereby the overall B+ level) The 5Y3 is the “weakest”, and will make the
amp “saggy” or squishy.It will also be overtaxed in a circuit like this
and will not live as long as it should. The 5U4 and 5AR4 are both up to
the job and differ in two primary ways.
#1 the 5AR4 has a smooth turn-on characteristic which is good for your
filter caps and tubes since the high voltage ‘ramps up’ rather than
coming on all at once. The 5AR4 has a low internal resistance and your
B+ (plate) voltage will likely go up a bit, so you’ll want to re-bias the
amp to use it. I should say you should re-bias the amp whichever
rectifier tube you switch to. The 5U4 has a higher internal resistance
and will drop more voltage. If you have been using a 5AR4 and want to
drop your B+ voltage a little, a 5U4 is a good start.

Tone wise, make sure your amp has good output tubes, a good phase
inverter tube (try and find a NOS US made 12AT7) and make sure the amp is
biased correctly. Then and only then should you worry about which
rectifier tube it uses. The rectifier tube is only an incremental player
in the tone equation. Overrated really. The output tubes, phase inverter
and bias setting are the biggies, and they get overelooked far too often.
If you’ve never had the amp set up before, you’re in for a treat. It’s a
real eye opener to get an amp biased up correctly if it hasn’t been
before..

There is a BIG difference between working and working :just right:.

Joe

 

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