Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on July 12, 2010, 03:38:36 pm

Title: changing PI bias
Post by: phsyconoodler on July 12, 2010, 03:38:36 pm

What would the effect on tone or volume if I were to change the PI bias resistor in a long tailed pair from 820 ohm to 470 ohm?Would I need to change the tail resistor and feedback resistor to compensate?
  I want to get an amp I've built to have more bottom end and that seems like the best way to achieve it.The rest of the amp is very typical values for coupling caps and bypass caps.
  It just has less bottom than it should have.I'm thinking it's in the PI.The tone is killer but it definitely lacks some bottom.

Title: Re: changing PI bias
Post by: jjasilli on July 12, 2010, 05:56:31 pm

??? The LTP bias resistor usually is 470Ω.  Aiken is pretty good on this topic: http://www.aikenamps.com/LongTailPair.htm

Title: Re: changing PI bias
Post by: phsyconoodler on July 12, 2010, 06:18:36 pm

It's a Dumble based amp that uses an 820 ohm resistor,24k tail and 4.7k feedback resistor.
  If I were to change it,the tone would definitely be different,but all I want is more bottom end.

Title: Re: changing PI bias
Post by: jjasilli on July 12, 2010, 06:31:38 pm

Ok, but checkout the Aiken article.  He discusses tonal response aspects of the LTP components -- though not specifically the bias resistor value with repsect to tonal response.  He does suggest using pots in place of the plate resistor, bias resistor & tail resistor to experiment interactively with their values. 

Title: Re: changing PI bias
Post by: sluckey on July 13, 2010, 08:14:58 am

2 or 3 years ago, plexi50 was experimenting with a pot in place of that bias resistor. I think he ended up with a 50K pot. We were calling it power damping (dampening) and the idea was to vary the gain of the PI and still be able to get that overdriven tone. I tried something very similar in my Marshall 18W except I used a 6 position rotary switch and fixed resistors. It definitely varied the gain of the PI but I don't recall bass response being affected. Large resistance made the amp sound strangled and thin to me. It just lost all it's balls. I was never satisfied with the switch and removed it shortly after playing with it.

Here's my circuit. It ain't worth the effort...

Title: Re: changing PI bias
Post by: jjasilli on July 13, 2010, 08:35:53 am

Yes, Aiken too speaks of the bias resistor in terms of headroom, but not bass, or tonal response.

Title: Re: changing PI bias
Post by: phsyconoodler on July 13, 2010, 12:36:07 pm

Well it may be the entire PI/presence circuit that's bringing the bass up a bit shy.The coupling cap values are not that much different that your typical marshall or fender.
 However,the plate resistors are.They use some 220k and 1800k plate resistors in the preamp.
Hmmm...