trobbins

Re: Heater Supply Circuits-AC and DC
« Reply #60 on: December 11, 2017, 06:43:53 PM »
The hi-fi Encel would be set up for class A, so bypassing is likely to have no major effect, and depends on how close the valves are pushed to grid conduction, and the valve's characteristic.

Amps set up for class AB operation need a bypass capacitor to try and maintain a fixed bias on the output stage during higher signals levels where one PP side stops conducting current (and hence a common cathode voltage would vary significantly if it was just resistance to ground).


alexc

Re: Heater Supply Circuits-AC and DC
« Reply #61 on: December 11, 2017, 07:35:42 PM »
Thanks for the clarification ....   that makes sense.

And PRR, that Fisher is similar to the Encel with the exception that it appears to be a full cathode bias, with bypass cap.

The thing I wonder, is what practical difference is made  with the 'combo' bias, with a fixed grid voltage providing about half of the cathode-grid dc voltage, and the cathode circuit resistance providing the rest.

My understanding is that

- 'fixed bias', sometimes also described as being 'Class AB' in the data sheets example setups, provides a less programme dependent bias (more stable)  and is generally of a higher efficiency and therefore power.

- 'self bias' or 'cathode bias' does vary the tube's operating conditions more with  the higher rms programme and usually results in a lower available power.

Fixed bias sometimes  is described as be more 'hifi' and cathode bias more 'compressed', leading some to favour one for hifi application and the other for instrument amplification.

Surely a simplification, but is 'combo' bias the 'best of both worlds'  ?

I generally use finals with fixed bias so I can closely achieve my desired operating conditions; cathode bias and combination of both is new to me. 

The only other time I've used 'combo' bias  it is with my vari-mu limiter builds, especially in the fc660 and it's local derivative, the poorman 660, where one is trying to set operation on a specific part of the remote cuttoff transconductance curve, where it has significant 'bendiness'.

----

Just found another popular 6BQ5/EL84 integrated amp using 'fixed + cathode' or 'combo' bias - the Scott 222 series.

 - it takes the lowest HV rail, neon bulb regulates it to +48V and uses that as fixed bias to the grids of the finals, which also have a +60V cathode bias circuit; giving a +12Vdc cathode-grid net bias voltage.

- that same +48Vdc neon regulated rail is also used for 4x series heaters of 12V each for the input stages (4x 12AX7)

(the remaining heaters use a 6.3Vac winding with one end connected to the ground line - I don't think I've seen that before; an ac winding seemingly 'ground referenced')

The Scott 222 is a most interesting circuit for anyone interested in an integrated amp design with AX7s and EL84s.  There is a write up in Vacuum Tube Valley  Issue 13   :)
« Last Edit: December 24, 2017, 06:00:57 PM by alexc »
I ping therefore I am

trobbins

Re: Heater Supply Circuits-AC and DC
« Reply #62 on: December 11, 2017, 09:00:19 PM »
A few have found combo bias of benefit for their particular application  Gingertube has had benefit with hifi use in reducing distortion.

Re: Heater Supply Circuits-AC and DC
« Reply #63 on: June 19, 2020, 09:19:20 AM »
Hi guys,

Trying to resurrect this topic for selfish reasons :) I know this has been discussed million times (AC vs DC heaters) but I have PCBs that expect DC voltages and I cannot change that. So to me this is not about the effectiveness / noise / efficiency debate.

I tried to "implement" some of the designs posted here in PSU designer but usually failed. The numbers just don't add up :) I'm very inexperienced with this obviously and I miss something to get the desired result - at least approximately.

The circuit visible on the attachment is the B+ from this post https://groupdiy.com/index.php?topic=57166.msg728679#msg728679

Thank you for being generous with your time!

trobbins

Re: Heater Supply Circuits-AC and DC
« Reply #64 on: June 19, 2020, 09:46:51 AM »
Turn off the soft start option.

Re: Heater Supply Circuits-AC and DC
« Reply #65 on: June 19, 2020, 10:15:30 AM »
Turn off the soft start option.

Thank you sir! Much closer :)