Re: [Harp-L] Re: 5F6A Bassman mods for harmonica from Mark and Jim

[MARK BURNESS <markwjburness@xxxxxxxxxxxxxx>]

Hi Tom,

A very high input impedance is desirable for pretty well any kind of mic, except 
Shure's 520DX which has an increased tendency to feedback with loads over 1 meg. 
The reason has more to do with the dynamics of the harp signal as compared to 
guitar (any input load over 500K is technically "matched"), the very high load 
increases sensitivity and dynamics. This isn't so much of a problem for plucked 
string instuments like guitar, which have a very large initial signal followed 
by long decay (very hi loads can even make guitar sound hard/brittle at initial 
plucking of the string)...harp has no decay, all the note you hear is a blown 
note, with much less front to it (comparitively). 


The high input loads can also help reduce signal loss when using devices in-line 
between harp & mic, though obviously anything lost between mic and first device 
in the chain is most critical. Any delay/effect pedal should be tested to see 
how it affects signal loss, Ron Holmes & John Kinder have both offered mods to 
correct definciencies here, as will a well built Jayphat, which should be the 
first device after the mic.

Any volume control on the mic will be in parallel with the amp's input load 
resistor (if plugged straight in), so this must be considered too. So a 5 meg 
pot into a 1 meg amp input is the same as a 1meg pot into a 5meg amp input 
(833K, bridging channels also has an effect in this respect), 5Meg pot into a 
5meg load will give 2.5meg, which is adequate.

I wouldn't go over 10meg for the input resistor in a typical cathode biased 
Fender style input (late 50's onwards).

Regards, Mark.



________________________________
From: Tom Skailes <tom.skailes@xxxxxxxxxxxx>
To: harp-l@xxxxxxxxxx
Sent: Monday, 18 October, 2010 17:05:17
Subject: [Harp-L] Re: 5F6A Bassman mods for harmonica from Mark and Jim

Hey guys

New to this forum. I've just stumbled upon the Bassman Mods PDF and it looks 
very useful.  I'm using a Blues Blaster with MC-151 element with a 5Meg volume 
pot. I see lots of stuff on the forums about the crystal elements needing a 5Meg 
resistance in the amp - is that the purpose/effect of changing the R3/R6 grid 
load resistors to 5.6 or 8.2 Meg? I've also seen the Jayphat threads so will 
probably build one if that's needed as well.

Thanks
Tom

*Original message:*/
Mark Burness and I collaborated on a summary of modifications of the 5F6A 
Bassman circuit for harmonica. It is a PDF file that can be downloaded from the 
site below- http://groups.google.com/group/jdramps/files The file is called- 
5F6Aharpmod15feb2010.pdf
We welcome comments and suggestions.
Jim Rossen /