From: SRSN81–(at)–rodigy.com (Joseph Pampel)
Subject: Re: Info on Gibson amp
> There are only 4 tubes: a 12AU7 (which I replaced with a
>12AX7 for more gain so it will break up)
Not a good substitution. If you need dirt from this amp, pls use a pedal..
The 12AU7 specs are not always conducive to great tone from a 12AX7.
They pin out the same, but are very different tubes inside. Could be why
you’re having problems with oscilation.
> two 6BN8’s
Are you :sure: about those?? Maybe you mean a 6BN6? The 6BN8 is not a
power output tube, it’s an old TV tube (2 diodes and a hi mu triode..)
The 6BN6 is a beam power tube, it has a 7-pin base. Wierd choice for a
guitar amp in any case. The EL84 is a noval (9-pin) so you’d have to
change sockets at the very least to convert the amp to a more common
output tube. A more likely (if I had to guess) choice for the output tube
if I had to pick a 7-pin power tube would be a 6AQ5. The 6BN6 does not
pin out the same way though so it’s either one or the other. You can
tell by following the plate lead – which will go to the output
transformer primary. If the OT is connected to pin 5 of both tubes, it
might be a 6AQ5; if the OT is connected to pin 7 of both tubes, then it’s
probably a 6BN6.
>2) Are these truly the right tubes?
The Gibsonette’s I’ve seen were all Champ knock-offs with a 6V6 power
tube running SE. No trem in any of ’em. Unless I have my amp models
mistaken. The Gibson Master Service Manual lists 2 different Gibsonette
models, so if you get get hold of that book (avail at most guitar shows.
I don’t actually have one or I’d offer.. I do have a copy of the table of
contents..) you’ll have a schematic. I’m sure someone on the board here
has one. I keep meaning to get one myself.. 🙂
>3) It was too clean with the 12AU7 and the 12AX7’s additional gain
>it break up a bit but it oscillates when turned past about 8 1/2. Any
>ideas about how to help this a bit?
Put the AU7 back in.
>4) Also oscillates when the vibrato depth is turned up past 5.
Did it do that with the AU7 in the amp? The tube is doing double duty as
pre-amp stage and trem oscilator. This is another reason why the tube
swap is not a good idea in this case.
>5) Vibrato speed is too fast even at the lowest setting.
The 12AX7 could cause this. The oscilator is affected by the loading on
the feedback circuit, and the 12AU7 and 12AX7 have very different output
impedences (in English, the 12AX7 swap could very well change the trem
speed) If the trem is still too fast even with the 12AU7, then you would
have to modify the feedback loop on the trem oscilator, which can be
rather tricky. Best off either leaving it alone, or if you’re handy with
a soldering gun, change the osc circuit wholesale over to the osc circuit
from another amp whose trem speed range you do like.