Hi,
Anyone try using regulator (i.e. LM337) to supply the bias voltage for 300b (fixed bias)? I would like to supply a better DC bias voltage.
Thanks in advanced
Leo from Hong Kong
Anyone try using regulator (i.e. LM337) to supply the bias voltage for 300b (fixed bias)? I would like to supply a better DC bias voltage.
Thanks in advanced
Leo from Hong Kong
Hi Leo
I have noticed the quality of the bias voltage to be very audible in fixed bias. Ideally, you should use vacuum rectification with lots of chokes in the filter. To use a 317 you'll obviously need to float it above ground reference and if this is to work well you'll need a lot of additional filtering at the output. I would imagine a 317-based regulator, feeding directly the grids to sound very dire.
regards
I have noticed the quality of the bias voltage to be very audible in fixed bias. Ideally, you should use vacuum rectification with lots of chokes in the filter. To use a 317 you'll obviously need to float it above ground reference and if this is to work well you'll need a lot of additional filtering at the output. I would imagine a 317-based regulator, feeding directly the grids to sound very dire.
regards
Most fixed bias designs seem to use a very simple power supply to supply the bias voltage (unregulated, half wave rectified etc). The reasoning being that only a tiny current is required and an RC filter is good enough.
I have tried this sort of arrangement and it sounds harsh and nasty to my ears - cathode bias seems much nicer.
A few possible reasons:
Cathode bias introduces local negative feedback which may reduce distortion and smooth out the sound a bit.
In fixed bias with capacitor coupling, the bias supply becomes the load for the driver stage. I guess its behaviour at ac then becomes important.
Similar sort of deal with interstage transformers except the bias supply is then in series with the grid circuit of the output tube. I suppose that here a low impedance supply might be better than the usual high impedance arrangement?
Don't know all that much about this stuff - just some thoughts to get the debate / argument started
I have tried this sort of arrangement and it sounds harsh and nasty to my ears - cathode bias seems much nicer.
A few possible reasons:
Cathode bias introduces local negative feedback which may reduce distortion and smooth out the sound a bit.
In fixed bias with capacitor coupling, the bias supply becomes the load for the driver stage. I guess its behaviour at ac then becomes important.
Similar sort of deal with interstage transformers except the bias supply is then in series with the grid circuit of the output tube. I suppose that here a low impedance supply might be better than the usual high impedance arrangement?
Don't know all that much about this stuff - just some thoughts to get the debate / argument started
Yes. That's exactly how poor fixed bias sounds. But even then the bass is very good, don't you think?
I never allow unbypassed cathode caps, so this can't be an issue in my case. Overloading behaviour, otoh, is much nicer with cathode bias.
FWIW, I've been considering a 300B SE amp that uses Dennis Boyle's combination bias. A smallish unbypassed cathode resistor (I'm thinking 330 Ohms) goes from the wiper of the the hum balance pot. to ground. The bulk of the bias voltage comes from a DECENT C- supply.
If you regulate the bias supply, you should also regulate the B+. I'd forego regulation. Even a 1/2 wave rectified bias supply is OK with adequate filtration.
5 KOhm O/P "iron" seems indicated, as the local current NFB from the unbypassed cathode resistor raises the O/P impedance.
If you regulate the bias supply, you should also regulate the B+. I'd forego regulation. Even a 1/2 wave rectified bias supply is OK with adequate filtration.
5 KOhm O/P "iron" seems indicated, as the local current NFB from the unbypassed cathode resistor raises the O/P impedance.
The bass is better when I use fixed bias. My running 300B SE is now using Fixed bias with simple RC circuit. I may try to built a small circuit board for the bias and see what happen. Actually, I need to built 4 individual bias supplies for two 6v6s and two 300Bs. May post the result and see if it sound good or bad.
Thanks your for all of the comment.
Leo
Thanks your for all of the comment.
Leo
I agree that bass is tighter with fixed bias, and on paper this arrangement should be far better than cathode bias.
In real circuits though, fixed bias won't always sound better.
In some of my own projects, I have found that bias current is a bit higher than expected. Try a meter across the grid resistor or a scope on the grid with no signal.
I found that bias voltage fluctuated a fair amount when the amp was on with no signal.
For the moment, the best compromise I've found is the use of grid chokes with cathode bias. Top and mid retain their 'sweetness' whilst the bass gets some 'bite' back
In real circuits though, fixed bias won't always sound better.
In some of my own projects, I have found that bias current is a bit higher than expected. Try a meter across the grid resistor or a scope on the grid with no signal.
I found that bias voltage fluctuated a fair amount when the amp was on with no signal.
For the moment, the best compromise I've found is the use of grid chokes with cathode bias. Top and mid retain their 'sweetness' whilst the bass gets some 'bite' back
Yes... the bias voltage will change a bit and it is the reason why I would like to try using voltage regulator in maintaining the bias voltage.
Regards,
Leo
Regards,
Leo
Yes, but as Eli Duttman points out you should also regualte B+ as otherwise your bias point won't be stable.
How is your bias voltage applied anyway? Resistor, grid choke, interstage?
For B+, I am using 5H choke and 5Uf x2. And for the bias, full bridge rectification plus one CRC (100uf 10K 100uf), then using 20K VR with 10uf in stabilize the bias output.
Regards,
Leo
Regards,
Leo
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