Andy,
If you are prepared to stack the output stage B supply on the driver B supply, you won't be needing the IT or a negative grid supply for that stage. Also, the whole thing moves in concert with line voltage so no issues there.
If you are designing with three stage DHT, you can DC couple them all and in fixed bias. No cathode bypass caps, one filament bias resistor and one high quality input trans (lundahl nickel core - the size of a matchbox and mu shielded, step up if you wish).
The best amp I've built, and that includes all three DHT tubes in the same line up, across SE to PP Class A, differential, and PSE.
Complexity moves to the PSU which can be isolated from the AC audio signal with some thought. Worth the trouble? IME yes, no contest.
All the best,
Shane
If you are prepared to stack the output stage B supply on the driver B supply, you won't be needing the IT or a negative grid supply for that stage. Also, the whole thing moves in concert with line voltage so no issues there.
If you are designing with three stage DHT, you can DC couple them all and in fixed bias. No cathode bypass caps, one filament bias resistor and one high quality input trans (lundahl nickel core - the size of a matchbox and mu shielded, step up if you wish).
The best amp I've built, and that includes all three DHT tubes in the same line up, across SE to PP Class A, differential, and PSE.
Complexity moves to the PSU which can be isolated from the AC audio signal with some thought. Worth the trouble? IME yes, no contest.
All the best,
Shane
Hi Rod,
A servo is a good idea, but I'm happy with manual bias control as can tweak and have the ability to test the sound impact of different bias points. With fixed-bias I learnt that the best is to have an ammeter in the cathode to ensure I can always check the anode current.
I like your suggestion to change the reference. What are you saying is that on the output of the cap multiplier I should connect a high voltage PNP follower with emitter resistance to ground and collector to +B? An MPSA92 will do for low HT supplies but otherwise I need to add a resistor divider from +B to the collector to ensure I don't exceed to VCEO correct?
Ale
A servo is a good idea, but I'm happy with manual bias control as can tweak and have the ability to test the sound impact of different bias points. With fixed-bias I learnt that the best is to have an ammeter in the cathode to ensure I can always check the anode current.
I like your suggestion to change the reference. What are you saying is that on the output of the cap multiplier I should connect a high voltage PNP follower with emitter resistance to ground and collector to +B? An MPSA92 will do for low HT supplies but otherwise I need to add a resistor divider from +B to the collector to ensure I don't exceed to VCEO correct?
Ale
Hi Shane,
Yes, I do agree but what a heavy investment in power supplies though! My current 4-65a SE amplifier has input transformer, 46 in filament bias (yes, crazy I know) and DC coupled to the 4-65a SE output stage which is floating on a second supply. Since it's A2 it has a third supply for A2 current thorough a gyrator on the 46 driver. As it's all DHT and am using Rod's regulators I have 7 power supplies in total! An absolute insanity. Still have 3 supplies left to build!
Ale
I did a quick simulation of varying the mains on the fixed bias raw input by +/- 10%. You get the same -/+ 10% output variation. Given that is an inverted supply it actually works fine as when the HT goes up +10% you get counter effect as the bias supply goes down. Problem is in practice that both raw supplies wont vary in the same way so you need one referenced to the other as Rod suggested...
...I started my amp build as an Flesh and Blood clone despite several people warning me about the limitations (Pieter, Thomas, Jeff). After a long time tweaking it sounds better than my Audio Note quest but as was to be expected I am running into the 6SN7 cascade driver limitations. THD measurements are really good but IM distortion is to high which I think explains why orchestral work could sound better.....
Isn't it because UNO's SS power amp used up to 500 hz (300 hz may be slightly optimistic from such small midrange horn in free air positioning ) is not up to the task and do not mesh with SET properly ? It is an interesting situation when specialized HI quality HF transformer would be the best instead of always compromised full range monster but the speaker design forces you to use full range amp to impare SET sound signature on SS Bass/midbass section . Did I get it right ?
Maybe it's better to invest time and resources in LF section ?
Isn't it because UNO's SS power amp used up to 500 hz (300 hz may be slightly optimistic from such small midrange horn in free air positioning ) is not up to the task and do not mesh with SET properly ? It is an interesting situation when specialized HI quality HF transformer would be the best instead of always compromised full range monster but the speaker design forces you to use full range amp to impare SET sound signature on SS Bass/midbass section . Did I get it right ?
Maybe it's better to invest time and resources in LF section ?
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The discussion about bias supplies is very interesting - let's keep it going! It would be nice to have some working schematics.
O, alright then!
Here is an example to show the B+ compensated grid bias driver.
I have omitted the filtering so that you can see the grid bias (magnitude) increase as the B+ increases (if you use the LTSPICE file).
How work?
B+ is 400V dc (in the SPICE file, it has a 40V sine imposed on it).
The voltage source "neg" is a loosely regulated -130 to -150V, derived from gas tubes, zener strings etc.
the voltage between R1 and R2 is set by the resistors to be -10 to -20V. In practice, this will be heavily filtered (eg MOSFET cap multiplier).
Q1 is a dc amplifier that multiplies the -10V up to (eg) -70V for your 300B. The useful thing about this is that R5 can be changed to vary the "gain" - ie how much the grid bias changes when the B+ drops by 10%.
Q2 filters the supply for the Q1 stage, and helps keep noise from the Neg supply out of the bias.
Q3 is a follower to present low dynamic impedance to the grid (strongly rejects the influence of any grid leakage currents, and gives a stable bias).
D1 protects Q1 against back bias of the base.
Simple & robust (use cheap 300V MPSA92 transistors - they only see < 150V).
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