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

[MARK BURNESS <markwjburness@xxxxxxxxxxxxxx>]

Hi Tom,

There won't be a significant audible difference between 5.6Meg or 8.2Meg, either 
works fine. Use whatever is most easily availble, you might even find that 
4.7Meg are more common.

I would still utilize a larger value at R3/R6 even when using the Jayphat. The 
tube still sees 1Meg, even if the mic sees the larger impedance offered by the 
Jayphat's op amp. If you haven't built the Jayphat yet, I would maximise your 
options. No Jayphat & 1Meg, no Jayphat & 5Meg, Jayphat into 1 Meg, Jayphat into 
5Meg will all sound different.

Regards, Mark.


 



________________________________
From: Tom Skailes <tom.skailes@xxxxxxxxxxxx>
To: MARK BURNESS <markwjburness@xxxxxxxxxxxxxx>
Sent: Monday, 18 October, 2010 18:38:17
Subject: Re: [Harp-L] Re: 5F6A Bassman mods for harmonica from Mark and Jim

Mark

Thanks for your prompt response.

My reading of your reply is that the upshot is that for use with the Blues 
Blaster I should replace the R3 and 6 with 5.6Meg resistors or would the 
8.2Meg's be better? - I plug straight into the RI Bassman with no effects 
pedals. Or I could just use the Jayphat to match impedance before plugging into 
the amp - in which case , is the R3/R6 mod needed at all?

Regards
Tom

On 18/10/2010 18:25, MARK BURNESS wrote: 
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 /
>