Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on July 10, 2010, 11:05:17 am

Title: What about bias supply impedence ??? I need a tecnical explanation
Post by: kagliostro on July 10, 2010, 11:05:17 am

Bias supply impedence ?

I was looking in the London Power pages

and I find a strange (to me) Kit

the RBX - Raw Bias Auxiliary Supply  :huh:

http://www.londonpower.com/catalog/product_info.php?products_id=32

You know I have no study in the matter

please can you give help and make me understand about that ??

MANY THANKS

Kagliostro

Title: Re: What about bias supply impedence ??? I need a tecnical explanation
Post by: Rev D on July 10, 2010, 11:54:35 am

 See the first question of the FAQ section, this is one application of the product you were asking about. Its part of his building block amp scheme (not saying that's bad, just expensive):

http://www.londonpower.com/faq.htm

 I know this doesn't make you understand what you were asking about, but it gives you one reason for the kits existence.

Regards,

RD

Title: Re: What about bias supply impedence ??? I need a tecnical explanation
Post by: FYL on July 10, 2010, 12:15:38 pm

Fixed bias can either be derived from dedicated medium voltage taps, windings or a PT *or* from a high voltage source, such as raw B+ from a plate winding.

The former uses relatively low value resistors in the bias chain, typically around 1K in series from the PT and is a low impedance supply, with a lowish time constant even when relatively large caps are used. Bias settles quite fast during power up/down or B+ variations if you're using a bias tracker with a Power Scaling or similar circuit.

The latter uses high value R's - typically 150K to 220K - in series and is a high impedance supply, with higher time constants even when fitted with small caps. Bias takes at best a few seconds to settle either when power cycling or when B+ is modified thru Power Scaling and the tubes can be damaged or destroyed in the meantime.

That's why KOC offers a low Z dedicated bias supply kit with his Power Scaling solutions.

Title: Re: What about bias supply impedence ??? I need a tecnical explanation
Post by: Fresh_Start on July 10, 2010, 03:40:51 pm

I noticed recently that KOC shows a full-wave rectifier for his bias supply in the "super stock" circuits described in TUT 3, such as Bassman, Plexi, etc.  Never saw any reference to it in the text though.  He also uses 10uf caps in a pi filter instead of one big (100uf) cap.  Could that be addressing the same concern about the time for the bias supply to adjust or is it simply getting a "cleaner" (i.e. less ripple) bias voltage?

Cheers,

Chip

Title: Re: What about bias supply impedence ??? I need a tecnical explanation
Post by: FYL on July 10, 2010, 04:29:35 pm

Quote

I noticed recently that KOC shows a full-wave rectifier for his bias supply in the "super stock" circuits described in TUT 3, such as Bassman, Plexi, etc.  Never saw any reference to it in the text though.  He also uses 10uf caps in a pi filter instead of one big (100uf) cap.  Could that be addressing the same concern about the time for the bias supply to adjust or is it simply getting a "cleaner" (i.e. less ripple) bias voltage?

KOC uses a high Z supply derived from the HV with full-wave rectification, 10µ caps are required in order to obtain acceptable time constants, and yes, the Pi filter gives a much cleaner bias voltage - does it really matter as common mode ripple will be eliminated in the PP? The jury is still out.

Here's a typical KOC high Z supply, used in one form or another in the TUT series:

Title: Re: What about bias supply impedence ??? I need a tecnical explanation
Post by: kagliostro on July 10, 2010, 07:14:28 pm

Many Thanks

guitar amps are really very interesting

each day I discover something new

Thanks again for the explanation

Kagliostro