stereomour PA conversion question

[balancedtriode]

hey guys! I converted my stereomour to a power amp and I have a few question about input resistors
1. should I use a 47k resistor as the input load resistor or is there a better value for use with bottlehead preamps?
2.what kind or resistor makes for a good  input resistor? (inductive? non inductive? magnetic? non magnetic?) I have heard that metal film or tants are the best?
any suggestions would be appreciated! also any subjective opinions would be appreciated as well!
thanks,
Coltrane

 

[Paul Birkeland]

Hello Coltrane,

The 47k resistor is in the circuit to keep the grid of the 12AT7 at DC ground potential.  If this resistor gets too high in value, there will be sufficient current flowing through this resistor to develop a negative grid voltage (the Tode does this with its 10M grid leak resistor). 

For a 12AT7, anything up to 500k, maybe even 750k is going to work just fine.

On the other end, if that resistor drops too low in value, it will load the stage before it, and you will end up with some frequency response issues.  Any of the Bottlehead preamps are going to work pretty well into a 15K load with relatively short cables, and 47k is plenty high.  For certain triodes prone to stability issues, having a low value grid leak resistor is a good idea (the old type 50 comes to mind).

 

[balancedtriode]

If I understand this correctly the input load resistor is the one that goes across the input jacks in an amp, and 15k is best for bottlehead preamps correct? or am I misunderstanding this?

 

[Doc B]

keep the value between 50K ohms and 250K ohms.

 

[InfernoSTi]

Here is what I used:

Just a quick update: I switched out my 100K resistors on the RCA input jacks to red bodied PRP resistors (Precsion Resistive Products 9372 audio series).  These are a little more "clear" and a little less "soft" but who can really say with this kind of tweak!  I just wanted to try out a second high quality resistor to see what I thought.  I can recommend the Kiwame "greens" if you like a smooth, groovy kind of sound and the PRP "reds" if you like a clean, precise kind of sound.  Both sound great, by the way...they just lean in different directions to my ear. 

John