Can you create a fixed bias tap?

[M Fowler]

Your still taking the voltage off one leg of the secondary of the PT through a diode placed backwards to create negative voltage and then reduced to a -50 volts or so, within the bias circuit.

[talbany]

If your tranny is 340-0-340 then the Marshall 50W type (which was copied on the Trainwreck Express, so the schematic is available here) is probably the easiest to use.

Tony's example is capacitor-coupled as it's expecting you to have used a full-wave bridge rectifier on the entire secondary, and that's not the case in your build.

Here is a picture of the bias supply in my Non HRM KT-88 amp..
using the supply I posted..
This bias supply feeds separate bias pots for each tube..
If you don't think it works here is a clip of the amp..

http://www.soundclick.com/bands/page_so ... ID=9125192

This supply can also be implimented in a Music Man voltage doubler as praticed by myself and this schematic posted by Henry- Red Plate..
The circuit can also be used by the for-mentioned Full wave bridge as well.. Any other questions

Doc
Both supply's use one side of the HV..FWIW


Tony

[paulster]

If your tranny is 340-0-340 then the Marshall 50W type (which was copied on the Trainwreck Express, so the schematic is available here) is probably the easiest to use.

Tony's example is capacitor-coupled as it's expecting you to have used a full-wave bridge rectifier on the entire secondary, and that's not the case in your build.

Here is a picture of the bias supply in my Non HRM KT-88 amp..
using the supply I posted..
This bias supply feeds separate bias pots for each tube..
If you don't think it works here is a clip of the amp..

I never said anything about it not working, so I think you're reading way too much into things. I said that the schematic you posted was for a bridge rectified supply and since the OP was going to use a full wave rectifier the Marshall type was probably easier to implement.

[talbany]

Disclamer.. Bias supply's taken from the HV side of a power transformer called (parasitic) like the one I posted and the Marshall/ TW offers poor regulation (wider slower voltage swings) and in general a poor way to get a bias voltage..IMHO!! I HATE em!! Marshall and others who use them are going the cheap route by not having the extra tap.. I try to avoid them if at all possible..If you have a choice always pick the transformer that has the extra tap a far better method..This has been my experience in the past..Hope it helps!!

Tony

[paulster]

+1.

And if you choose the electrolytic cap value incorrectly (bigger is better, right?) you can easily get a ramp-up (down?) time of a minute or more, meaning you've got essentially no bias until this point.

Interestingly Marshall use a separate tap on the 100W amps (and a bridge rectifier) and no tap and a full wave rectifier on the 50W amps, and always have. I've never seen an explanation for this, but one can only guess that the transformers were cheaper from one supplier one day and it just stuck after that.

[talbany]

(bigger is better, right?)

Paul bigger is better (to a Point).. To big a cap (anything over 100uf generally speaking) and amp will lose some sustain bloom clamping the PI.. Especially low end..This info from Ken Fisher himself.. The problem here is the bias supply voltage is at the mercy of the HV tap which supply's current/voltage to the rest of the amp in constant flux.. So as you push the amp and the power supply(PT) begins to sag so goes the bias supply slowing the recovery time further.. (Much of this of coarse depends on the current handling of the PT and how hard you push it)..Long story short the larger cap will buy you only so much in the way of recovery time..RC constant..I know you know this thought I'd mention for those who don't..Again Hope it helps..
FWIW.. You will be better off helping the recovery time of the bias supply by lowering the bias feed resistors to say 100k which is what Marshall did when they went to 6550's (in some amps) .. Smaller PI couplers like .022 also help..If you don't really push the amp that hard it's really not that big a deal..

Code: Select all

nterestingly Marshall use a separate tap on the 100W amps (and a bridge rectifier) and no tap and a full wave rectifier on the 50W amps, and always have. I've never seen an explanation for this, but one can only guess that the transformers were cheaper from one supplier one day and it just stuck after that.

That's what I would guess!!

Tony

[Dr d]

Thanks for the clarification Paulster and Tony. It seems to me there is a strong element of compromise in setting up a bias supply using iether of these methods. Perhaps I should try to install an auxhiliary transformer or simply go for cathode biasing. Interesting discussion and I thank you all again for your input.

[M Fowler]

It clearly shows you how to build your bias section on this layout.

[Structo]

What do you guys think about the Dumble way with the 100uF cap and 220K bias feed resistors on the bias circuit?
Or is it OK to use those values when you have a separate bias tap on the PT?

[FUCHSAUDIO]

I have used some small filament style transformers to produce bias:. Take a 6 or 12 V transformer, and hang the low voltage side off the main filament line of the amp. The primary becomes your output (secondary), and you get enough voltage there to rectify (even with a bridge) and filter a nice little supply. If you use a 6V transformer you have 100 V ac, which is a little high. A 12 primary and a 120 V winding gives 50 volts raw, and about 75 volts rectified, which is perfect. NO issues about loading, time constants, etc. Cheap and easy.

[talbany]

What do you guys think about the Dumble way with the 100uF cap and 220K bias feed resistors on the bias circuit?
Or is it OK to use those values when you have a separate bias tap on the PT?

It's fine!!

Tony

[FYL]

A 12 primary and a 120 V winding gives 50 volts raw, and about 75 volts rectified, which is perfect.

Or 24V for those of us living in blessed countries with real 240V mains.


(Now could we get 240/60, best of both worlds)