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Old 28th July 2004, 08:45 PM   #41
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Quote:
Originally posted by fdegrove
Hi,

Guido,

Regarding the preference (subjective thing, I know) , when the + side of the heater is grounded won't that change the bias with the respective heater voltage amount?

I mean, +5V side grounded means -5V added to negative bias or do I see this the wrong way?
If I am correct that may well account for the audible difference I suppose...

Cheers,
Hi Frank

Yes, the bias changes when reversing polarity. In the listening tests we corrected for this change. After all, the effective (negative) grid voltage had to be changed, which may change the ease of drive. I assume this effect is of minor importanve though.

I still think it is in the current distribution of the tubes, which is never symmetrical.

best regards
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Old 28th July 2004, 08:56 PM   #42
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Hi,

Guido,

Quote:
In the listening tests we corrected for this change
Ah...I wasn't sure you had compensated for that.

Quote:
I still think it is in the current distribution of the tubes, which is never symmetrical.
Exactly.
Current distribution may well vary from tube to tube though as I can't imagine a factory worker being able to tell the direction .
So some experimenting may be called for before deciding how hooking it up, I suppose.

Thanx.

Cheers,
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Old 29th July 2004, 11:44 AM   #43
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Quote:
Originally posted by Guido Tent

Yes, the bias changes when reversing polarity. In the listening tests we corrected for this change. After all, the effective (negative) grid voltage had to be changed, which may change the ease of drive. I assume this effect is of minor importanve though.
But the anode-cathode voltage also changes by 5V given a constant B+. This will change the load line. Did you compensate for that too?

Anyway the AC audio current has to flow through the DC gradient across the filament when the filament is only grounded at one side. But it is still AC. Whether you ground the + or side of the filament, that gradient stays the same.

It is known that with lo mu tubes (with wide grid spacing) magnetic fields influences also electron flow...? In this case the magnetic field generated by the filament current?

Up till now, just some observations but no explanation. Think it will be hard to find some explanation, although it stays intriguing

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Old 30th July 2004, 07:47 AM   #44
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hi Guido

Quote:
Originally posted by Guido Tent

Hi

This is an impressive setup

Thanks!

Quote:


Personally I am in favour of high impedance across the filaments, but as you can see, many ways lead to rome....(they are all long though :-)

I agree! The resistors on the cathode are the self-bias resistors, using different values according to Thorsten advice to balance the currents flowing from either end of the filament -

http://db.audioasylum.com/cgi/m.mpl?...31256&session=

The 100uF Cerafine is the cathode bypass capacitor.

Quote:

I guess balancing the sources is tricky, not ?

No need to balance the +/-.
On the positive side (T1 - Q1) Is only a ripple smoother - it drops about 2V or so and rejects noise. It's more like a voltage source (at dc) than a current source.

On the negative side T2 - T3 is the current source which is adjusted to make the right voltage appear across the filament.

Quote:

Oh, one last word, the Sovtek is not the best sounding tube, to be honest I iuse these for first time testing only......

The interesting thing about the Sovtek is the big difference between ac and current source heating. It's so noisy and bad sounding on ac - but sounds wonderful using the circuit in post #35.

The construction of the old Sovtek 300B uses a very thin filament, which at least partly accounts for the bad sound and microphony

If you're using Sovtek or another cheap DHT with ac heating, you owe it to yourself to try current-driven heat! Get Guido's heater supply, or, if you're transistor-competent, build your own design! It's much cheaper than upgrading to fancy 300Bs and probably just as good.
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Old 30th July 2004, 12:40 PM   #45
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Quote:
Originally posted by Rod Coleman


I agree! The resistors on the cathode are the self-bias resistors, using different values according to Thorsten advice to balance the currents flowing from either end of the filament -

http://db.audioasylum.com/cgi/m.mpl?...31256&session=

The 100uF Cerafine is the cathode bypass capacitor.



No need to balance the +/-.
On the positive side (T1 - Q1) Is only a ripple smoother - it drops about 2V or so and rejects noise. It's more like a voltage source (at dc) than a current source.

On the negative side T2 - T3 is the current source which is adjusted to make the right voltage appear across the filament.



The interesting thing about the Sovtek is the big difference between ac and current source heating. It's so noisy and bad sounding on ac - but sounds wonderful using the circuit in post #35.

The construction of the old Sovtek 300B uses a very thin filament, which at least partly accounts for the bad sound and microphony

If you're using Sovtek or another cheap DHT with ac heating, you owe it to yourself to try current-driven heat! Get Guido's heater supply, or, if you're transistor-competent, build your own design! It's much cheaper than upgrading to fancy 300Bs and probably just as good.

Hello Rod,

Yes, the balancing of DC current distribution (density) due to different values in the cathode is a nice one.

Still, the AC distribution can be defined by adding external cap(s) to ground.

On the current source: One follows the other, correct ?

I'll pay more attention to the Sovteks - thanks !

best regards
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Old 30th July 2004, 02:35 PM   #46
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How about another circuit idea for Push-Pull DHT amps like the Lynn Olson AMITY etc. (Idea ONLY so far - I don't have an AMITY yet, sadly):

Most of the circuit shown in post #35 of this thread can be kept as-is, but with the two push pull valves each having their own return circuit (current sink). That way the two valves can have different voltages (to cover production tolerances):

Click the image to open in full size.

2 x 2A filaments, like the KR300BXLS can be accommodated easily with this circuit. If the transistors are MJL1302A (PNP) and MJL3281 (NPN) - AND mounted on heatsinks of 2~4 deg C/W you can even have shorted filament protection! What's more you can have one filament shorted and the other will carry on working normally, avoiding the risk of cathode stripping in the good valve. Just be aware this means ONLY that the filament is shorted; I don't mean you can short the positive side to the raw supply return. Bypassing the current sink part of the circuit will vanish the safety feature. But if you properly insulate the return node you should be protected against all reasonably likely faults.

Be sure you can cope with the stability test and thermal ratings design/test as noted in post #35 if you attempt to build this circuit; you could burn up your DHTs if you don't check & burn in thoroughly.
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Old 5th January 2005, 07:07 PM   #47
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Konnichiwa,

This one has been among the dead for a while, time to voodoo it back among the living.

Over the recent times I have been thinking to and fro how one might achieve similar effects to Guido's circuit, preferably short circuit protected, simple and with as few parts and 3-legged fuses as possible. The following Circuit is what I came up with, it SHOULD work (P-Spice says it does) but I have not specifically build it (yet), so beware....

Click the image to open in full size.

The idea is to make a Circuit that is a high impedance at AC. We already know the "simple" LM1085 Curret Source seems to work fine sonically, but when supplying normal DHT's you need to re-adjust it as valves age and when you change Valves, far from ideal.

In my circuit the 1mH Choke (around 0.5R DCR) together with the 1,000uF Capacitor in effect establishes an AC Current Source where the transconductance of the 3-Pin Regulator in effect amplifies the impedance of the 1mH Choke. The 1,000uF Capacitor together with the 1K resistor between adj and out froms a highpass at 0.16Hz so at DC the whole thing will slowly turn bacvk to a low DCR resistance source.

In order to set the Output voltage accuratly a LM/TL431 is used, here the output voltage would be 5V. In order to stop the LM431 from "regulating away" our nice high output impedance the lower part of the voltage divider which stes the output voltage is bypassed (1st Order LPF at 0.03Hz), thus making sure that audio frequencies the impedance remains high. In the audio range the impedance is limited by the 10K unbypassed resistor in the voltage sensecircuit.

Added to this is a simple common mode filter and Schottky Rectifier Bridge, basically "best practice" for DHT Heater DC Supplies, lest you like to use Thyratron or high current mercury vapour rectifiers like this dude here:

300B SE Amp with 101D Driver, 300B Rectifier and 314A rectified heater supplies....

As usual comments etc welcome.

Sayonara
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Old 6th January 2005, 11:12 AM   #48
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Wang san, sumimasen!

Sure its worth raising this one again, its a dinner-money upgrade with far more impact than a mortgage-busting pile of boutique glassware, dielectricware or even old fashioned ironware!

I accept that you're going to have some adjusting to do when you only have a current loop controlling heater power. I did think about adding a voltage loop - with multiple-pole cutoff <1Hz - but decided I don't need it. The heater voltage with the gyrator/transistor current source shown above is plenty stable enough (over the few hundred hours I've run it), and can easily be made adjustable with a low power 20 ohm fixed in series with a 100 ohm/10T trimmer (all across the sense resistor).

I found it worthwhile to spend time finetuning the circuits in both heater terminals - I would certainly recommend testing your circuit with an effective ac break between the rectifier -ve and -F. In that position I like a PNP darlington gyrator formed with FZT1151A (Zetex, superbeta) and a PNP power transistor (smallest you can get which satisfies your safety margin). The specified Cob of the PNPs is what counts (35pF = good; >130pF = bad). The reason for the obsession is (as you know) the -ve current pulses that form the rectifier noise - with plenty of energy >100MHz.

General observation: With 3-terminal regs the multiple internal circuit paths from IN (& ADJ) to OUT risk degrading the insertion loss as frequency gets really high. So, in your circuit, watch out for the ac path from +F through the 1000uF into ADJ!

clic the pic - Easy to build circuit on plain copperclad board (hacksawed pattern) with SMD parts - (2512 cathode resistors, FZT1051A/2SD2061 gyrator, FZT1051A/2SC5511 current source with R47 R56 shunted sense resistors, SANYO 560/10 SVP (20mOhm ESR) caps.....

Click the image to open in full size.

Thanks to ImageShack for Free Image Hosting
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Old 6th January 2005, 11:50 AM   #49
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Konnichiwa,

Quote:
Originally posted by Rod Coleman
Sure its worth raising this one again, its a dinner-money upgrade with far more impact than a mortgage-busting pile of boutique glassware, dielectricware or even old fashioned ironware!
Absolutely.

Quote:
Originally posted by Rod Coleman
I accept that you're going to have some adjusting to do when you only have a current loop controlling heater power.
Yes, I had exactly that issue with my simple LM1085 CCS, which I had enhanced with the addition of a little choke (a few mH, small size) and a common mode choke.

In fact, this very simple supply:

Schottky Bridge
10,000uF
2.2mH/2A
LM1085 CCS
CMC Filter

actually sounded as good as ANY other heater supply I have been using so far, including serious choke filtered stuff, but it was too hasslesome when conducting for example comparison tests with 6 or 7 different 300B's.....

I have noticed your circuits previously (nice work actually), but they have no protection against overload. As I tend tend to tinker a lot I consider such mandatory. Any circuit that cannot survive the slip of a screwdriver or a 'scope probe is not to my liking.... I get to angry with myself if I have to swap out all the blown threelegged fuses each time....

Quote:
Originally posted by Rod Coleman
General observation: With 3-terminal regs the multiple internal circuit paths from IN (& ADJ) to OUT risk degrading the insertion loss as frequency gets really high. So, in your circuit, watch out for the ac path from +F through the 1000uF into ADJ!
Thanks, good one. I did wonder what it was (I showed up in the P-Spice simualtion) but I figured it would still "do". The solution is of course simple, add another resistor (1K) between the adj pin and the junction where the 1,000uF Cap, LM431 cathode and 1K resistor join.

My main idea was at any extend to come up with something that offered reasonable performance and mainly "aparent" simplicity (eg. simplicity in implementation, all the complexity is hidden inside the LM1085 & LM431).

I could have used a discrete pass transistor instead of the LM1085, but the 1085 makes a nice enough pass transistor with build in SOAR protection....

I might consider adding a second "electronic choke" in the negative Line as well, may be worth it....

Quote:
Originally posted by Rod Coleman
clic the pic - Easy to build circuit on plain copperclad board (hacksawed pattern) with SMD parts - (2512 cathode resistors, FZT1051A/2SD2061 gyrator, FZT1051A/2SC5511 current source with R47 R56 shunted sense resistors, SANYO 560/10 SVP (20mOhm ESR) caps.....
One might also build a more or less functional equivalent using LM1085's as current sinks and as Filters, with short circuit and overheat protection.... Performance at HF would be worse of course, all a matter of design goals, still, very nice job on this.

Ciao T
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Old 6th January 2005, 07:06 PM   #50
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Konban-wa Wang san

Quote:
but they have no protection against overload. As I tend tend to tinker a lot I consider such mandatory. Any circuit that cannot survive the slip of a screwdriver or a 'scope probe is not to my liking.... I get to angry with myself if I have to swap out all the blown threelegged fuses each time....
Don't let shortcircuit worries hold you back, if you're considering a discrete heater circuit. Provided the sense resistor is protected (use two series parts so the end-to-end gap exceeds a "pliers-span") you are OK. If you short across F+/F- the current stays at 1,3A - so the currentsource transistor's dissipation rises to ~10W (for ~9V raw input) - fine for TO220 types mounted on <10 deg. C/W. In the gyrator side nothing happens!

If that's still not tough enough, you could try some Raychem Polyswitches next time you're at Rapid - e.g. type RGE300

http://www.rapidelectronics.co.uk/rk...1870&XPAGENO=1

As for whether the gyrator contributes greatly to the improved sound, I suspect that varies with the leakage capacitance (through the filament transformer) back to the "B-" terminal of the rig in question. Big leakage = current path for diode recovery pulses.

Thanks for your kind words on my heater posts; they boost my enthusiasm for "converting" some of the ac-heated DHT designs out there. It's the only change I can remember getting unreserved plaudits from my "expert witness" listener - sings in the wonderful Armonico Consort http://www.armonico.org.uk/

Especially for those that have the lower cost/quality DHTs, if you're on ac-heat now, find out what you're missing!
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