From tremolu–(at)–ol.com Sat Mar 18 22:05:59 CST 1995
Article: 514 of alt.guitar.amps
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From: tremolu–(at)–ol.com (Tremolux)
Newsgroups: alt.guitar.amps
Subject: Re: Tinny sounding Fender Pro Reverb
Date: 18 Mar 1995 18:43:10 -0500
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If you want your Fender to sound RIGHT, use the proper types of tubes as
stated on the tube chart. I DO NOT recommend substituting a 12AX7 for the
12AT7s in this amp. I don’t think Fender “cheated”, but rather some
previous owner put in the wrong tubes.

Other posts have correctly described the functions of the 12AT7s in this
amp and I concur. Using a 12AX7 to drive the reverb pan will actually
overdrive the damn thing because the 12AX7 has more gain than a 12AT7.
The result is shitty sounding reverb. Bad suggestion. Moreover, in
Fenders, this tube is subjected to very high plate voltage (typically in
excess of 400 volts), and the circuit has the proper cathode bias resistor
for a 12AT7. A 12AX7 won’t cut it. the other 12AT7 is in the phase
inverter. Again, this stage is set-up for maximum linearity and drive for
a 12AT7. Subbing a 12AX7 will increase the gain and give more distortion.
If you want distortion, sell your Fender and buy a Marshall. There are
no “good” 12AT7s being made any longer. Only current production is
Chinese. However, there is still a good stock of NOS American 12AT7s
around. The industrial equivalent is called 6201, and I prefer these
because they’re built more solidly.

Regarding the differences between 7025 and 12AX7, previous posts had
incorrect information. They’re basically the same damn tube. In the old
days, the differences were the 7025 was a low-noise version, but otherwise
identical. In today’s market for newly manufactured tubes, there is no
difference. They’re both considered “commercial”, unless a JAN or other
military suffix is appended. The best 12AX7 type of tube being made today
is the Russian Sovtek. The Chinese stuff sucks. Basically, the Russkies
make a 12AX7WB and a 12AX7WXT. There is little difference between the
two. According to the hype, the WXT has higher gain. This is not
necessarily so, according to the actual comparative testing I’ve done.
They both have about the same gain, within tolerance limits. Both have
low noise and low microphonics. Both can be bad right out of the box!!
Get more of these than you actually need, so you can have spares in case
you get a dud.

For output tubes, the supply of excellent American NOS 6L6GCs is just
about totally dried-up. The best current production equivalent is the
Russian 5881. Be sure to have the amp properly biased when you install
new output tubes or it will sound shitty. Also, be sure to buy matched
pairs.

Groove Tubes are worth it ONLY if you know nothing about working on amps
and you need lots of hand-holding. Otherwise, they’re way over priced.
All they are is just Chinese and Russian tubes that have been well tested
to get rid of the duds, and with GT’s logo painted on. I NEVER buy GT, or
any other high-priced designer-boutique tubes.

Regards.