The article mentions the Fender Tweed Deluxe (specifically the 5E3) as
being a good amp to start with for harp. I built one from Fender
schematics, and my impression was that it just didn't have any mojo at
all for good harp tone. I modified it per Weber's recommendations, and
it didn't help at all. If you want a good harp amp, there is a lot
better stuff than a Deluxe. I have some opinions about the points made
in the article also:
Point #1 about voltage may be true. I have built a bunch of circuits
that seem to sound really good with low plate voltage on the preamp
tubes, but I find that biasing has far more to do with good tube tone
than plate voltage does.
Point #2 deals with coupling caps. It is generally accepted that .1 uF
is better for harp than .05 or whatever you generally find in a guitar
amp. Nothing new here.
Point #3 says that you don't want a lot of gain stages because that
creates feedback. Ask any harp player about the 59 Bassman or the 59
Reissue Bassman and they'll tell you otherwise. The 59 Bassman has 4
preamp gain stages rather than just two that Weber recommends, and it is
generally thought of as THE quintessential blues harp amp. This is
probably because it has more stages to create compression, which is
always a good thing for harmonica.
#4 is a true Weber innovation, which I incorporated in my 5D3 copy. In
my experience, the pot in the phase inverter ground did absolutely
nothing to the amp. Maybe I should have experimented with different pot
values... Who knows. But I wasn't impressed. I should try messing
around with it some more.
#5 proves that Weber really doesn't know much beyond his own
experimentation, which is OK. I'm not trying to insult anybody. Most
of what I know about building harp amps came from looking at what
everybody else was doing and then soldering a bunch of circuits together
myself. But I really figured out what was going on when I read some old
electronic textbooks. Anyway, he is right that a cathode biased output
section is a good idea for harp tone. But beware of anybody who tries
to tell you what Class A operation means and how it happens. Read a
textbook for that kind of information.
#6 ditto. He got the right answer, but he doesn't know why. The
cathode resistor is what sets the grid bias, and the grid is what
determines the current going through the tube. He seems to be implying
that the plate voltage is what determines the current, which it does
not. Read the diagrams in a tube manual and you'll figure that out.
Anyway, the grid bias is what determines the compression in the signal,
which is one thing that determines whether a harmonica sounds good or
not. It has nothing to do with idle current. Idle current is, however,
a side effect of biasing the tube for a compressed signal.
#7 ditto again. I wouldn't build a harp amp that didn't incorporate a
tube rectifier. Weber is right on this point, tube rectifiers
contribute a lot to the tone, but he doesn't tell you the right reason
why. I hate to sound disagreeable, but the tube rectifier doesn't have
much if anything to do with compression. It has a lot to do with sag
and response, which is very important in amplified harmonica playing.
It allows you to use the performance of the amp as an expressive tool.
This also tells you why Weber thinks that idle current and cathode
biasing create a spongy response. Tube rectifiers get maxed out when
you play a loud note, and they sag and give you a grungy sound. Very
cool. If you bias your tubes for more idle current, you get more sag
and a higher cool factor because the rectifier can't keep up. But you
also shorten the life of the tubes, depending on how far you push the
performance envelope. Ask a screaming guitar player about cooking
tubes. Anyway, you could get the same effect by using a smaller
rectifier to match the exact performance demands created by the rest of
your circuit and what kind of sound you want. Saying that the GZ34 is
the "best rectifier" is kind of silly. I would always use one that is
right at the edge of its performance capability, which would depend on
how many tubes are in the amp, what output tubes you are using, and how
you bias them. I accidentally improved the sound of my gigging amp
tremendously last week by replacing the 5U4 with a 5Z3 (with a lower
current capability). I had to do it because my 5U4 burned out and I had
a gig. It sounded great.
I also suspect that Weber's "waveform symmetry" comments are another
example of trial and error success without knowing what the hell is
really going on. Acoustic feedback occurs when you have a microphone
that picks up a signal that gets added to the signal in the amp, then
comes out the speaker, gets picked up again, added to the signal in the
amp, and so on. Your microphone/amp combo will have certain "hot
frequencies" that tend to feed back, depending on where you have your
tone and volume knobs set. Feedback can be decreased by standing in
just the right spot so that your hot frequency sound waves will be out
of phase with the amp signal after traveling through the air to where
you are standing. If you always test your amp while standing at the end
of the same microphone cord, you have a pretty good chance of altering
the feedback performance by reversing the speaker polarity. In other
words, these trouble frequencies will feed back with the speaker one way
but not the other. Move to a different location in the room and you'll
have to start over.
My main point is that Weber is obviously a guy who has put a lot of amps
together and has figured out what works in a lot of cases. I suspect
that he could improve your amp's harmonica performance quite a lot
because of his experience. Just don't try to learn electronics by
reading his articles. When I first started tinkering with this stuff a
couple of years ago I read his article and tried to understand it, and
it didn't make a lot of sense, because he really doesn't know what he's
talking about. But he does know how to put an amp together.
If you're looking for an amp with good harp tone, you would probably be
much better off asking other harp players, trying every amp in the
guitar store, shopping at junk shops or ebay, or learning how they work
(I mean how they REALLY work) and modifying it yourself. Works for me.
Post by JohnnyIn the article "Harp Amp Secrets and Tips" (see link below), Gerald Weber
discusses mods he and the others at Kendrick can do to improve tone,
increase volume and decrease feedback.
Do any of you have any experience with an amp that Gerald has modified for
harp? If so, what were the results?
http://www.harpamps.com/weber/Guitar51.htm
Regards,
Johnny
Fuquay-Varina, NC
US