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slowmotion
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#46

Post by slowmotion »

I haven't tried it that way myself, no.
I have seen historic schematics with biasing done that way in DC coupled amps.

edit:

I have tried fixed bias with batteries done the traditional way, of course, and liked the results, but I haven't tried it with the batteries in line like that.
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Paul Barker
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#47

Post by Paul Barker »

When you say traditional way I think you mean traditional for the most recent 15 years, not traditional prior to that. You are I expect saying you have tried the battery on the cathode.

The last time I a/b compared battery bias on the cathode with resistor capacitor combination the resistor capacitor sounded better. That doesn't mean it always will for everyone. It did on that occasion for me.

The last few amps I have built used fixed battery bias as drawn here and they sounded very good, though I didn't compare bias strategies.

Obviously you have to be sure you aren't going into grid current, on the two 5687 stages the -7v keeps the first one hundred's of miles from grid current and the second one almost 2v clear of it. there could be an issue on the px4 at clipping, and I may need to consider this issue. So let's work that through now.

If grid current flows it flows out from the grid to the negative terminal of the battery string. In a battery electrons from from neg to pos. So they would try to charge the battery. Not bad news. Stay clear of clipping if you can though. But battery would be charged by clipping. sound would be harsh more quickly as it would fight the positive peaks with more negative bias in response. Probably no worse than cap coupling where grid current produces blocking distortion.
Last edited by Paul Barker on Sun Apr 20, 2014 3:26 pm, edited 1 time in total.
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slowmotion
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#48

Post by slowmotion »

I have tried batteries from grid to ground with a resistor in between.
And liked the results.

My tries with battery from the cathode to ground has not been to my liking,
I always preferred the more traditional resistor/cap.
Last edited by slowmotion on Sun Apr 20, 2014 3:27 pm, edited 1 time in total.
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#49

Post by Paul Barker »

that's what this is but the resistor is replaced with an inductor. No other difference.

Did you like it? I did.

By the way it matters not if the battery is between ground and the transformer or between the transformer and the grid. It is just as much in the flow of electrons and therefore an influence on the sound in each position.
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#50

Post by Paul Barker »

On the data sheet for the bifilar wound 126 series it says maximum voltage 400v. but I am conservatively using 150v. I don't need any more for the voltage swing required, and 150v is a handy number for single VR tube regulation.
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#51

Post by Paul Barker »

Decided to use Autobias on the 5687's because they are all 4 operating at the same position, so can share a single unbypassed resistor. To find the actual gain we have to know the gm and the rp at this operating position.

gm 6.3 rp 2k7

As an aside the higher than expected rp puts the hi pass -0.5db at 12.5hz at each stage. this should be OK the combination should not degrade bass sufficiently for a re-design.

The actual gain will become 5.9 per stage without a bypass. I bought a pair of 270 ohm 1 Watt Tantalum's to parallel so my resistor is 135 ohms. and 4 times the required dissipation. I need 45v peak to drive the PX4 to full power, ÷ 5.9 = 7.63 ÷ 5.9 = 1.29 ÷ 1.414 = 0.91v RMS sensitivity. That is acceptable these days. No active preamp required.

Should I wish to bypass the resistor I would be in serious trouble because it would require 470µF to keep this extra hi pass out of the already risky additive situation. 470 gives us -3db at 2.8hz. IF we were to bypass we would be better using four separate resistors and capacitors. We would be going back to battery active bias instead of that.

there is an oddity which I don't know how to model. In many ways this emulates fixed bias, but there is that small interaction between all four stages. I think it will work to the good of the overall sound. I only used this approach once before when I had four 112's in an amplifier. It worked and there were no odd results. Yes clearly what happens in one valve affects the other three but also the behaviour of the other three limits the extravagance of the one and vice versa. All the effects are probably more or less limited and cancelled to the point of obscurity it should behave more like a constant current sink than an unbypassed cathode resistor, the truth will sit somewhere between those two. Both sound good.

you could say why didn't you put a CCS in there? Because, contrary to public opinion I believe in this scenario the ccs requires bypassing or all the gain is lost. If you can show me otherwise mathematically or by experiment to prove your case please do?
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#52

Post by Paul Barker »

Good news, Bluebell made me an offer I can't refuse so I have accepted a pair of AE interstage. This now gets really exciting.

Before I had this news I had already gone through some lateral thinking on the project, (as if there wasn't enough of that going on anway).

Seems a shame to not use the space occupied by the valve bases already punched for the 27's. At same time a great shame it isn't any longer a 10 sukuma'd to 10 Sukuma'd to PX4, just on the basis of measurements (bass response). Let's face it what I am after is the best midrange I have ever achieved.

So the thinking now is to change the 27 bases for 4 pin bases and punch two new holes for the 5687's.

So the plan is to put both sets of bases in parallel, filament starve the 10 to 6.3v with Andrews heater modules. Whichever set of valves is inserted is the one used.

I am fairly certain the 10 will kill the 5687 dead, but the 5687 will be pretty good and will give the better bass (which may not even be heard on my speakers.)
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#53

Post by Paul Barker »

So backtracking I am going to just build it for the 10 : 10 : PX4

and see if the midrange is sublime as hoped and the bass is sufficient.

Slight issues now unfolding are the 150/30 VR's nearly touch the 10's and the interconnect will come rather close to the 10's also.

Anticipated further issues : will the interstages fit on top? Will the DALUDHM's fit inside with the heater supply transformer?

Pretty unlikely.

And that is why I am crap at getting stuff looking nice. Should have just stuck to permanent breadboarding.

Image
Image

Meanwhile patiently waiting for delivery of considerable number of required components.
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Greg
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#54

Post by Greg »

Paul Barker wrote:And that is why I am crap at getting stuff looking nice. Should have just stuck to permanent breadboarding.
Yep, I think the answer is to make the enclosure/chassis the last purchase for the build. This is my approach for my seriously revamped 2A3PSE. Power supply is going to be in a separate chassis and I won't purchase that until I have everything else together so that size and proportions can be realistically assessed.

Keep at it Paul. I'm sure you'll get a good result in the end :D
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Paul Barker
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#55

Post by Paul Barker »

The B+ is to be external, in this form it just needs one B+.

I have changed direction, the chassis wasn't meant to house four interstage transformers. My bad.

Stupid to build in a final chassis something I haven't yet tried.

Tres idiot!
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#56

Post by simon »

Keep the faith Paul, 10-10-PX4 will be beautiful.
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#57

Post by Paul Barker »

thank you Simon. If I make a chassis big enough for my tribute transformers as used on the 212 and the 833a then it could be 10:10:801a.

the B+ supply shall start with a transformer I unveile at the first Eggborough which provided for the capacitorless PX25 power supply.

It is 625-0-625 at 500mA. I plan to put a 110v variac which I bought cheaply from Richard Mailer in series with the primary of the power transformer. Then by using passive feed forward shunt regulation I can simply alter the variac and at every output voltage have shunt regulated smoothness. So this is the plan in B+ to make an off board power supply which can be used for a variety of projects, this being the first.

It is a great transformer which I bought at what today would seem a bargain. £100 if I remember rightly. To have a double C core oil filled transformer of this capability today would cost £500 easily.

Obviously people know me well enough by now to know that there is no other way than choke input.
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#58

Post by Cressy Snr »

This I'm looking forward to seeing and hearing.
It'll be a fantastic sounding piece of kit; of that I have no doubt :)
Last edited by Cressy Snr on Fri Apr 25, 2014 8:24 am, edited 1 time in total.
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#59

Post by steve s »

Very good paul and and a good choice of valves too, irs worth the pain squeezing it all in.. Those old mazda's 35 vt idh would have done the trick,
But the 10 is a safe bet but with a littel more work
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Dave the bass
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#60

Post by Dave the bass »

Shoddy!

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