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Hello all,
Got stuff on the way for a new build. All Emission Labs tubes. Rectifier 5U4G or 274A, 20B V4 IT coupled to 2A3 V4. My question concerns the power supply.
Since the usual topology won't work here (output stage first, then further stages to lower the B+ and supply additional filtering ), and I'm pretty new at this, I've got questions. As the driver stage B+ is significantly higher than the output stage B+, I'm not sure how to configure it.
Can I use the normal topology of cascading sections, but pick off my driver tube first and then end with the output tubes. I realize I won't have the normal advantage of the additional filtering, but I plan to do a dbl. PI filter ( CLCLC ) to get started, so it should be mostly ripple free at that point. I'll also be using a star ground system and will take great pains to make sure the grounds from each section don't interact with each other.
But I'll be the first to admit that my understanding of this is nowhere at the level of most of the guys here. I have no idea if a topology like this would oscillate or cause other problems.So my question. Is this a stable and safe way to configure this supply, or should I go to Plan B. Two complete supplies, each with a separate transfo and associated components. One for the driver stage and one for the input stage.
as always, thanks in advance for any advice.
Cheers, Crazy Bill
Edits: 05/02/16Follow Ups:
Bill,
What's the end to end voltage of your rectifier winding? I want to do some "scheming and plotting".
Eli D.
Hello Eli,
Never heard it specced like that, but I'm assuming you're talking about the primary voltage. I haven't figured it yet, though I surely would love some help when that time comes.
If for some reason you're asking for something different and I'm too dumb to know the difference ( a distinct possibility ), let me know.
Cheers, Crazy Bill
There's another method that will work regardless of whether a CT is available. The output of a bridge rectifier can be connected in the usual fashion to a choke input filter. Then, also connect it through a forward-biased rectifier diode to a capacitor input filter. The much higher voltage created at the cap input filter will be isolated from the choke input section by the diode. I used this technique in my 211 SETs to achieve two voltages from the LV supply. Just like the method Eli outlined, it allows creating two very different voltages without resorting to resistors.
--------------------------
Buy Chinese. Bury freedom.
Hi Bill,
I don't know your design goals or operating points and I am only glancing at this, so please take this advice with caution.
Have you considered running the 20B at 250V plate to cathode at perhaps 16mA, for roughly -4V grid to cathode? This would provide sufficient voltage out to drive a 2A3 well beyond full power and allow a more conventional PS design. The bonus is that the 20B will be loafing and would likely outlive you; you could drop in the AM version if you tire of the B.
I know Jac in past has advised against low dissipation operating points, but he suggests the 20B can be run fine as low as 250V and 9mA.
You must have $$$ to burn if you are practising with EML tubes.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Some time ago, Pete Millett presented a scheme for obtaining 2 rails, 1 "short" and 1 "tall", from a single CT winding. The end to end AC voltage is bridge (either 100% SS or hybrid) rectified to obtain the "tall" rail and the "short" rail is taken off the CT, via an "extra" diode.
In fact, the "short" rail is FWCT rectified. In theory, the "5th" diode is unnecessary. Practice is definitely another matter. ;> )
Cree's C4D02120A, which is as noise free as vacuum rectifiers, is an obvious candidate for the bridge's ground legs.
You need a high current "short" rail. So, connect the paralleled plates of the 5U4 to the rectifier winding's CT. An inexpensive 70 mA. 7Y4 should be quite adequate for the bridge's "hot" legs, which feed a low current "tall" rail.
Set each rail's filter up, as you see fit, while respecting the vacuum rectifiers' published limits.
Eli D.
+1
I don't understand why the driver needs more voltage than the output. The use of an interstage transformer means the driver gets plenty of voltage at the plate.
But in any case the normal cascaded power supply provides less ripple to the driver, because any hum in the driver stage is amplified in the next stage - so the driver is more sensitive to the ripple than the output stage.
Some 15 or 20 years ago, Border Patrol (if I remember correctly) advocated independent power supplies for each stage and (again IIRC) many who experimented with this agreed. I have not seen much talk of this lately but it still makes sense to me. So I would expect Plan B to work best. You'll never know until you try both ... :^)
"Some 15 or 20 years ago, Border Patrol (if I remember correctly) advocated independent power supplies for each stage and (again IIRC) many who experimented with this agreed."
The driver stages for my 300b amps (left and right, just one triode each, on it's own chassis) are CCS loaded and operate from a VR tube shunt regulated, CCS feed power supply (the raw supply is tube rectified with a LCLC filter) on it's own chassis. (The DC heater supply (LCRC with one more C on the driver stage chassis) is also on that same PS chassis.
Each 300b output stage (each on their own chassis) has it's own LCLC B+ supply and it's own LCRC filament supply.
So it's a 4 chassis amp.
The driver stage and it's power supplies are also used with my stereo 6b4g amp (output stages only). So, when used, it's a 3 chassis amp.
The 6b4g amp has one LC B+ supply that splits to 2 LC filters (one for each 6b4g) and 2 LCRC filament supplies all on board.
Crazy? Yeah, probably.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Many years ago I built a 300B amp with interstage based on a design by SJS when they were making transformers. This project involved a separate winding for HT, driver, and bias supply, all tube rectified and choke input. It had it's pluses and minuses - one nice thing was that making changes to any one supply did not disturbed the others. A downside was the cost of the custom wound power transformers. Lessons I've learned over the years (which may only apply to my listening experience) is that the PS is very important but I find the driver to be more important, as is the output transformer quality. Today my driver of choice is based on Thorsten's EL84 driver for the 300b and OPTs are Magnequest. Overall, best arrangement I've heard.
Hello all,
First, many thanks to all who posted. Very much appreciated.
Eli, while I've learned a great deal from you and Eddie over the years, what you proposed looks daunting to me and over my head. I need simple.
Paul, the EML datasheets proposed operating points for the 20B start @ 380V and go up, hence my observation of the PS topology. It never occurred to me to hit the 20B w/ 250VDC @ 16ma as 91 proposed, but it sure looks like the way to go now. It would greatly simplify the PS design, as I could power both amp sections from the same well filtered PS section. I'm thinking CLCLC- split dual mono to a final RC section to dial my B+ in. The B+ ripple should be very low by then. Since I'm planning DC on the filaments, I should end up with a pretty quiet amp.
91, First, many thanks for pointing me in this direction.
Second, No, I don't have cash to burn, but my buddy does. we live about 25 miles from each other. I visit regularly to listen to his SOTA headphone based digital system, and he wanted to upgrade my vinyl based system for when he comes here. We decided on an all DHT amp to drive my Decware HDT speakers, hence the present project.
I'm sure, as this design unfolds, I will have more questions and be looking for more advice, as DHT's are an entirely new territory for me.
I'll be back.
Cheers, Crazy Bill
You might want to check this article where Jac suggests replacing 6SN7 types with the 20B. The point I suggested runs the current higher and therefore the the grid to cathode -V lower, but the plate to cathode V is roughly the same.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Hello 91,
sorry it took so long to get back. Been a little crazy.
Appreciate this. Saw it before when I first investigated the EML tubes, but didn't pay it much attention. I am now.
Clearly, it adds credence to the operating point you suggested. But what really stands out is the suggestion of a battery supply as noted in # 6.
Looking into this very seriously.
On another note to you or anyone else.
When I was playing around with small signal nine pin triodes ( 6922, 6n1p ), types, I used Shunt type gas regs in the PS to great effect. I could easily stack a couple of OD3 and then a RC section to bring it down to where I want to feed the 20B.
Anybody ever use gas regs to feed a DHT such as the 20B.
As always, ANY THOUGHTS are much appreciated.
Cheers, Crazy Bill
Hi Bill,"Anybody ever use gas regs to feed a DHT such as the 20B."
I'd say yes, someone will have. People's opinions will differ regarding whether shunt regulation will have any benefit or is worth the effort for these decent-sized DHTs performing line level amplification. I would first try a conventional approach... that provides low impedance, quick recovery, and isolation from the output stage node. With the voltages required, a LC PS node could be very nice.
I think low Rp tubes warrant some stage PS design if they are to perform well.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 05/09/16
"Eli, while I've learned a great deal from you and Eddie over the years, what you proposed looks daunting to me and over my head. I need simple."
Sometimes, words can be confusing. "A picture is worth a 1000 words." Some "hen scratches" are provided. The scheme really isn't terribly complex.
Eli D.
Here's Pete Millett's PSU schem. Notice how the B+ rail off of the center tap is about half of the B+ coming off the FWB arrangement.
No probs Bill, no criticism intended nor justification needed.
After my next build I will likely be building a similar amp, maybe a 30B directly coupled to a type 50 or a 20B directly coupled to 300B.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
"... the EML datasheets proposed operating points for the 20B start @ 380V and go up..."
That's what I suspected. But there is no need to restrict yourself to the published operating points - it's just a convenience and/or an illustration. 91derlust's suggestion seems quite reasonable to me, so I won't even offer anything different.
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