Tentlabs Neg Bias Vs Machmat AFB

[jkeny]

I came accross Machmats Auto Fixed Bias here: http://www.machmat.com/sales/kits/afb.htm and then stumbled on the Tentlabs Negative Bias modules here: http://www.tentlabs.com/Products/Tub...24/page24.html

Is there any difference between the two? (apart from the Machmat product being less than half the price of the tentlabs version - (kit)€37.50*2 or (built)€60*2 compared to €179

How does CCS biasing compare to this option?

[jkeny]

OK then, no opinions! I might just try this - does anyone know a circuit for this auto neg fixed bias?

[SY]

It's tough without schematics, but it looks like the Tent circuit is based on a sort of window detector (something like the autobias stuff that Bill Chater published) whereas the cheaper one seems to just be a servo with a long time constant.

CCS loading is fine for class A amps that never overload.

[gingertube]

SY,
Am looking at bias servo designs quite seriously at the moment for a parallel push pull 300B amp (Class AB1). The Tentlabs/Menno Vandervenne circuit is quite similar to the Morgan Jones circuit but rather than clip at 2 x the idle current to define a window around the bias point from 0 (tube cut off) to 2 x the idle current it uses a much tighter window around the required bias current point. It seems to be a good idea to limit bias drift when driving hard (into Class B).

I'm not aware off the Bill Chater stuff - any links (meanwhile I'll set Google to work).

I'm also looking at some of the John Broskie auto bias ideas which are very clever but less flexible (need to optimise his design for each application).

Cheers,
Ian

[jkeny]

Funnily enough, Ian, it was a ECL86 Baby Huey I was thinking of trying this out on but I may well stick with a CCS - what do you think?

[SY]

Ian, the Chater articles were in Audio Amateur, maybe around 1988 or so? It was for solid state usage, but the basic circuit was an opamp version of what MJ showed in his book and easily adaptable for tube biasing.

[jkeny]

Ian,
Have you tried the Blumlein's Garter biasing? - I think Shoog has used it? It looks simple enough & no op-amps or SS in sight (not that I have anything against SS)

[TUBESMAN]

It's official, they don't sell the cheap bias board anymore.

"I am very sorry, we do not sell them anymore. We got far too busy and could not deliver support anymore on them. We have a new version, microcontroller based, tunes the tubes on power (even better than current alone)m but we only use these in our modification, and for OEM, nor DIY anymore.

Mattijs de Vries, Machmat"
This was from a reply to an email I sent to the company.

[Guido Tent]

Originally Posted by SY

It's tough without schematics, but it looks like the Tent circuit is based on a sort of window detector (something like the autobias stuff that Bill Chater published) whereas the cheaper one seems to just be a servo with a long time constant.

spot on !

you don't want to see the bias change with the music, hence we didn't go for traditional servo's.

best

[Guido Tent]

Originally Posted by SY

It's tough without schematics, but it looks like the Tent circuit is based on a sort of window detector (something like the autobias stuff that Bill Chater published) whereas the cheaper one seems to just be a servo with a long time constant.

spot on !

you don't want to see the bias change with the music, hence we didn't go for traditional servo's.

best