Audio Asylum Thread Printer

Get a view of an entire thread on one page

FAQ / News / Events

SET Asylum

RE: You need to make a jump away from conventional thinking!

92.2.76.153

In Reply to: RE: You need to make a jump away from conventional thinking! posted by Tre' on September 16, 2013 at 18:04:42

Plate resistance of the 4P1L in triode should be between 1.5K and 2K depending on operating point. The Russian data refers to its use as a pentode, but almost everybody in audio uses it as a triode. Gain is somewhere around 12 depending on operating point.
It's a lot like a 45 but with about 3 times the gain. Max voltage around 260v, current up to 40mA, dissipation about 8 watts or 9 max. Use two of them in parallel and you're in similar territory to a slightly beefed up 2a3.
The big difference is you can use it in filament bias. The output section of my amp uses the parallel 4P1Ls at a bias of -19v. Add 2v for the filaments and you need to input 21v from the DC current regs (Rod Coleman's). Allow 6-7v for the current regs to function and you have a supply of 28v at 1.3 amps. That's not difficult. The bias for a 2a3 is more like -45v at 2.5A and the 300b more like -70v at 1.3A so clearly filament bias isn't very practical. So with them you're usually back to cathode bypass caps on the resistor which will affect the sound. In filament bias the resistor is only 15 ohms.

Edits: 09/17/13

Follow Ups:

RE: You need to make a jump away from conventional thinking! - Caucasian Blackplate 08:03:24 09/18/13 (4)
You don't really need low bias voltage to use filament bias. You can directly couple the previous stage and run it from a negative voltage supply.
- Filament bias - designs? - andy evans 10:18:52 09/18/13 (3)
  You don't really need low bias voltage to use filament bias. You can directly couple the previous stage and run it from a negative voltage supply.> >
  
  How would you do that? The voltage on the cathode is determined by the filament supply going through the tube and down to ground. On a 2a3, if you put 2.5A through the tube and down to ground through the cathode resistor, the bias voltage appears on the top of the cathode resistor and you need about 10v DC more than the bias to feed the filament. Are you suggesting taking the cathode resistor down to a negative voltage or what? Could you give a quick diagram maybe? That would be most helpful.
  
  Andy
  - RE: Filament bias - designs? - Caucasian Blackplate 11:01:37 09/18/13 (2)
    This is a total exaggeration of using filament bias, but just barely.
    
    You could rebalance the voltages as you see fit.
    - RE: Filament bias - designs? - andy evans 14:42:48 09/18/13 (1)
      Thanks for that!
      
      Hmmmm.... I'm still trying to get my head round this. What's the voltage on the plate of the 2a3?
      - RE: Filament bias - designs? - Caucasian Blackplate 19:24:30 09/18/13 (0)
        
        In Reply to: RE: Filament bias - designs? posted by andy evans on September 18, 2013 at 14:42:48
        
        If you wanted the classic operating point, 255V.
look like we all search for different goal here - lovetube 01:56:25 09/17/13 (0)
thanks
LT

Post a Followup:

FAQ

Post a Message!

Forgot Password?

Moniker (Username):
Password (Optional):
	Remember my Moniker & Password (What's this?) Eat Me
E-Mail (Optional):
Subject:

Message:	(Posts are subject to Content Rules)

Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:	Automagically notify you when someone responds.

To view your new posting or follow-up, click on the RELOAD or REFRESH button on your browser.

This post is made possible by the generous support of people like you and our sponsors:

Parts Connexion