Re: [Harp-L] technical help

[mike reid <shrimpdaddymike@xxxxxxxxx>]

thank you so much.  My amp tech is the one doing this work he however did this mod without modding the schematics it is an old masco tube pa the 7C7 is the 1st preamp tube the second originally was a 7F7 and then it has 2X6L6 and a 5Y3  i've asked him to mod the 7F7 to a 6sn7 because a friend of mine has the same amp modded that way and it is Boss obvbiously because of the higher gain there is a resistance issuie this is causing the amp to be ridiculously high gain right now i use a crappy crystal and cant get past 2 withouth feedback.  so now he wants to know the resistance value he should use to finnish the mod.  I think he should also remove the phono input and render the 6SN7 as the frist stage of the amp.  what are your thoughs?

----- Original Message ----
From: Bob Blunn <bblunn@xxxxxxx>
To: harp-l <harp-l@xxxxxxxxxx>
Sent: Monday, February 11, 2008 11:33:09 PM
Subject: Re: [Harp-L] technical help


Hi 
Mike,

This 
type 
of 
tube 
swap 
will 
require 
more 
than 
a 
single 
resistor 
change.  
While 
the 
7F7 
and 
6SN7 
are 
both 
dual 
triode 
tubes 
they 
are 
not 
pin 
compatible.  
(I 
checked 
the 
specs 
via 
http://tdsl.duncanamps.com/tubesearch.php<http://tdsl.duncanamps.com/tubesearch.php>)  
Hopefully 
you 
haven't 
yet 
done 
the 
swap 
and 
powered 
up 
the 
amp.  
 
It 
may 
not 
pass 
the 
smoke 
test.

Since 
tube 
amps 
contain 
lethal 
voltages 
(EVEN 
WHEN 
UNPLUGGED) 
it 
is 
usually 
recommended 
to 
leave 
these 
types 
of 
mods 
to 
an 
amp 
tech.  
Many 
musicians 
also 
work 
on 
their 
own 
equipment, 
(I'm 
guilty 
as 
charged!), 
but 
I'm 
also 
an 
electronics 
engineer 
and 
kinda 
know 
what 
I'm 
doing. 
At 
least 
enough 
not 
to 
get 
killed.

If 
you 
want 
to 
adjust 
the 
gain 
of 
the 
preamp 
section 
of 
your 
amp 
that 
can 
be 
done 
by 
changing 
some 
component 
values.  
(Usually 
a 
capacitor/resistor 
pair.)  
If 
you 
are 
comfortable 
with 
tearing 
into 
your 
amp, 
ripping 
it's 
guts 
out, 
and 
replacing 
them 
in 
your 
quest 
for 
tone, 
then 
there 
are 
plenty 
of 
resources 
online 
to 
help 
you 
out!  
 
Here's 
a 
good 
starting 
point...

http://www.diycustomamps.com/diyamplinks.htm<http://www.diycustomamps.com/diyamplinks.htm>

Knowing 
more 
about 
what 
you 
have, 
and 
what 
you 
need 
to 
do, 
would 
help 
get 
a 
better 
answer.  
What 
type 
of 
amp 
is 
it? 
What 
year/make/model?  
Is 
this 
for 
harp?  
Guitar? 
Other?  
Do 
you 
desire 
- 
More 
distortion/breakup 
at 
lower 
or 
higher 
volumes?

Best 
Regards,

Bob 
Blunn



--- 
In 
harp-l-archives@xxxxxxxxxxxxxxx<mailto:harp-l-archives@xxxxxxxxxxxxxxx>, 
mike 
reid 
<shrimpdaddymike@...> 
wrote:
>
> 
i've 
just 
swapped 
a 
7F7 
pre 
amp 
for 
a 
6SN7 
because 
of 
the 
gain 
difference 
i 
need 
to 
adjust 
the 
value 
on 
the 
resistor 
would 
anyone 
know 
what 
value 
i 
should 
adjust 
this 
to?
> 
> 
> 
> 
>  
  
  
 
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http://www.spah.org
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