Re: [Harp-L] Low plate voltage on the preamp tubes?



Hello,
I am not an export of this, but also interested in such technical things.
First, I think Webers article is a good resource, and it is quite clear, to
start with.
I say againt, I am not an expert, just try to read a lot about this. (and
learn some electronics)
Someone may correct me.

The difference betweet swaping and reducing the plate voltage is, that tube
swap result less amplification factor(this means lower overall volume, but
maybe you can turn the amp higher without feedback, so it gets louder),
reducing plate voltage means lower bias, so the tube's frequency response
will be dfifferent. Have a look at a tube datasheet. (eg 12ax7)
On high plate voltage the characteristics have longer flat part, on lower
voltage they have more curves (in the datasheet it is on the left side)
Because of this curve, the amplification modifies the signal, it gets
distorted/fatter etc.
Guitar players usually want clean,bright tone, so they dont want the amp to
modify the signal such way.
In the "ideal" amplified harp tone, there are a lot of low freqencies. For
those low registers, it is a good thing, to modify the signal with more
lower overtones.
A guitar has brighter tone, with more middle freqs, so the needs are
different. (of course it depends on the guitar tone you are looking for) So
for a clean sound higher plate voltage is good. But it "kills" those low
overtones, so it results thiner tone (for harp)
One important thing, if you do not know much about electronics, do not
modify your amp. Those HUNDRED VOLTS ARE NOT A JOKE. Keep this in mind! But
you may ask some experts.

About Mr Webers article.
A few years ago I planed to buy something "new" old amp, I read this article
first. I found an old amp for cheap price, but it was not in my town, and I
wanted something sure, that it worth the time to check that out. I managed
to get the amp's schematic, and fortunatelly the voltages were signed. It
has simple design low plate voltage, etc. Really similar as Webers advices.
I tried it out, and there was no more question about it.
So Webers article is a good starting point, but maybe you have to learn more
about tubes, and electronics if you want to know more about amplification
basics.

Unfortunatelly there are few resources related to harmonica, maybe such
information are treated as industry secrets.
And if you really want to modify an amp for harp, you need some more tools:
function generators, variable power supply, oscillator, etc... Not a cheap
hobby...

Of course I reserve the right to be wrong :)
I hope my English can be understood.

Attila

On 5/15/06, Michael Fugazzi <mfugazzi67@xxxxxxxxx> wrote:

Would anybody who has tried this on an amp like to share the results? What changes in tone did you notice? How about feedback rejection? What's the difference between this and just switching preamp tubes?

Are there any good websites regarding modding amps for
harp?  I am know NOTHING about the inner workings of
amps, but would like to learn at least the basics of
what makes an amp good for harp vs. good for guitar.
The Gerald Weber article is one resource I've found,
but I don't totally understand all the lingo yet.

Thanks,

Mike Fugazzi
vocals/harmonica
NiteRail

http://www.niterail.com
http://www.myspace.com/niterailband

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