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previous: Anonymous Joe said,The 5150 had 6 stages ... -- 4/30/2003 1:20 PM View Thread

Re: 5150 plate resistors

5/2/2003 12:42 AM
Trace - Voodoo Amps
Re: 5150 plate resistors
To quickly add to what Kevin and KB have already stated; 5 gain stages is a serious take to under take. It's *hard* to keep background noise down and the layout becomes **SUPER** critical!! At this point grounding becomes an art form and using shielded wire to every grid and in the EQ section is a must for quite operation (assuming it's hand wired and not on a PCB).  
6 gain stages is really stepping in the eye of the storm!! (ha, ha) It's doable and it can sound cool. We've modded numerous 5150's with great success but you have to start thinking differently. Most techs think in terms of 3 gain stages and sometimes 4 but 6 changes things a great deal!! (ha, ha)  
Some quick advice when dealing with 5-6 gain stages:  
1.) Do not worry so much about having a large resistor before the coupling cap, try to lower the grid resistor instead as you'll get better results.  
2.) Don't use large plate resistors as you are asking for more troubles. I would stick with 100K's for the most part (perhaps a little lower on some stages).  
3.) What your cathode resistors later on down the chain. You will eventually have to raise some up more or you will need to employee more suppression caps across the plate resistors (you will need some to begin with if you are using 6 gain stages).  
4.) How you set up the NFB loop and the PI will also help you out as well. When it comes to the 5150 I think they are using 100ohm screen grids on the 6L6's (going from memory so forgive me). To change those you have to drill and remove the PC board. If you do not want to do this then you can convert the amp to adjustable bias which will also help the tone as well.  
5.) When you are dealing with 5-6 gain stages *every* value (not matter how minimal of a change you make) becomes super important. Small changes will yield big sonic difference.  
I hope this helps and have fun designing!! :)  

Joe N. Thank you for the advise. I'm assu... -- 5/2/2003 8:09 PM