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Old 16th June 2011, 07:22 AM   #11
работник
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Quote:
Originally Posted by Matt.B.H. View Post

So, Rod, how did you get on with JMS? Are they any good?

Cheers Matt.
Here's a wide-open split-bobbin shot of the JMS 150VA. [Let's see what the smut filter makes of that ]

Secondary is 340V and 30V. 70pF C-winding as above.

Pros of JMS:
- Proper tough industrial quality split bobbin.
- Insulation standard, terminals and earth stud are rugged enough to satisfy a Victorian.
- Lowest cost I know of.
- Order on the web, specify the windings any voltage you like.
- email to Sean and he'll tell you what can be done, including accurate predictions of Rdc [if you are into critical damping, or just want to model it accurately before specifying the turns].

Cons of JMS:

- Only do courier shipping [10], so it's best to buy a stash for a few projects.
- Varnish finish is strikingly similar colour to the sauce around Heinz baked beans.
- 150VA is the smallest type that is definitely split-bobbin. ask Sean if you need something smaller, and check.
- No end covers for chassis-top showing off. Steampunks beware - These are best exiled to the hinterlands of the chassis set.

I have used them 3 or 4 times and rate the service highly - only a few days to deliver, even on full custom windings. I haven't used 6V heater secondaries, since I have a stash of heater/filament trafos, and don't like overwinding heater/filament supplies anyway, due to risk of coupling recharge pulses. Ask Sean if you need these, and see.
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Old 16th June 2011, 08:09 AM   #12
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Tom, here's the schematic for the regulator and the raw dc supply.

The production version is refined a little, to improve LF ripple rejection and render it adjustable. I don't want to post the as-manufactured schematic while they are still available, as there have already been a couple of clones seen out there [which are unsupported, BTW!]

The operation is unchanged though -

the top portion is a passive gyrator:
- reduces incoming ripple;
- provides high insertion loss (HF transmission path is <100pF only)

- bottom section is a Vbe-referred current limiter, using NPNs with low-noise capability, and low Cob. Again the insertion loss is good up to very high frequency, and permits operation of DHT amplifiers even in systems with noisy CD players nearby. [Try waving a scope probe in front of a CDP display, to see why this is important!]

Overall, the filament is treated to wideband isolation from outside noise, and includes no significant noise sources of its own.

I originally wanted to make the manufactured units with a voltage-servo, so that the regulator was a dc-servoed CCS. I tried adding an opamp voltage loop, with <1Hz LF rolloff - but this destroyed the 3D imaging and subtle resolution of orchestral music. Out with it!

There has been lots of good feedback for the outcome:

New DHT heater



.
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Old 16th June 2011, 08:37 AM   #13
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Quote:
Originally Posted by mogliaa View Post
It seems that the key point here is to design the cleanest HT and filament supplies as possible. Given the options thrown so far, I'm still confused as to whether a Salas Shunt HT supply or Tom's supply is the way forward? I'm keen to go for a simple design with components easy to get hold of (at least in the UK).
Tom has prepared some PCBs, and the parts should be easy to buy and fit - so you could do the comparison!

I don't know if the SSHV has been laid out by anyone yet, so some hacking would probably be needed.

In either case, the proper test of an audio regulator is by comparing the sound to LC or preferably LCLC unregulated supply.

Even when a regulator is used, an LC filter is a good idea, since the 1st-cap recharge pulses are lower in peak, and much smaller in bandwidth. These large peaks can find their way back into your circuit by EM coupling, even if a filter is used.
[/QUOTE]


Quote:
Rod, what about your filament regulator, do you have one suitable for this filament voltage/current?

Thanks
Ale

All models of the regulator are still available! please send me some PM. Maybe you have my email address from the 45 filament reg buy, or click on my name to send some email.

The standard 4P1L model can adjust from 260mA to 380mA [4P1L spec is 300 to 350mA]. It runs best from 8.5V to 12V of raw dc, so a 9Vrms 30VA split-bobbin trafo works best.

Use one trafo for each 4P1L for best results.
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Old 16th June 2011, 05:40 PM   #14
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Rod Coleman View Post
Tom, here's the schematic for the regulator and the raw dc supply.
Awesome. Thank you.

Quote:
Originally Posted by Rod Coleman View Post
The production version is refined a little, to improve LF ripple rejection and render it adjustable. I don't want to post the as-manufactured schematic while they are still available, as there have already been a couple of clones seen out there [which are unsupported, BTW!]
Clones... Damn. I don't mind if people build my circuits for themselves from the schematics published - that's part of DIY, but cloning the design for money is a completely different story. In a way that's sign of success, though.... But it is one reason I would never give out Gerber files (and I'm sure you don't either).

Quote:
Originally Posted by Rod Coleman View Post
I originally wanted to make the manufactured units with a voltage-servo, so that the regulator was a dc-servoed CCS. I tried adding an opamp voltage loop, with <1Hz LF rolloff - but this destroyed the 3D imaging and subtle resolution of orchestral music. Out with it!
I'm highly skeptical of that, but we've had that discussion before... I'll just have to try it out for myself. If I can tell a difference, count on orders from me.


Thanks,

~Tom
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Old 16th June 2011, 09:45 PM   #15
hihopes is offline hihopes  South Africa
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I have a 4p1l preamp playing in my system using Rod's filament supply boards. Power supply and filament supply are both in separate boxes and both use heavy, multistage crc filtering (HT supply also includes a choke). The preamp is even quieter than my //6cg7 one was and the overall sound is better too. Clarity is very good, but this is not a "plug it in and enjoy" type of tube.

Firstly, the sound tends towards the thin side. You need to be able to fiddle to get it sounding balanced. Even biased at 26mA, it still sounded thin, though also slightly aggressive. That having been said, they are driving a very unkind load - 1 solid state and 1 hybrid amp - but my previous pre was doing the same duty.

Secondly, microphonics are a something you have to be able to deal with or live with. Every touch on the volume control or box is greeted with an answering "ping" or "ting" - heck it even rings when my dog barks! Admittedly, I have them upside down at the moment and perhaps they will be better when I can turn them over, but it will be a while til that can happen.

HT supply at input is 233v and anode resistor is 4K. Anode voltage is 125v. I started out using 9v batteries for bias, but have since added a single red LED in the cathode circuit, bypassed with 330uF Oscon. I have a whole mess of input capacitors in place to "tune" the sound. It is very, very listenable at the moment and I wouldn't want to go back to my 6cg7's, but it is still a work in progress. I may try fiddling some more with caps and am considering adding a cathode follower to give it a kinder load, though I am dubious about adding more components into the signal path. If I can direct couple, then at least I won't have to add another capacitor in line.
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Old 16th June 2011, 09:58 PM   #16
kevinkr is offline kevinkr  United States
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In triode connection the rp is pretty low, (a few K) your 4P1L should be able to drive rather low impedances without much trouble. I'd run it at a higher plate voltage, and current. I think Anatoliy likes something around 200V, and I would say 30mA or more.. Choke loading or CCS would be icing on the cake...

You would need a very high current CF realized with a high transconductance triode or pentode to significantly better the performance of the 4P1L into low impedance loads.
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Old 16th June 2011, 10:21 PM   #17
mogliaa is offline mogliaa  United Kingdom
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Yes, the anode resistance is around 1.5K, see the plotted curves here:
One more 4P1L SE

Why run it at 30mA?
Thanks
Ale
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Old 17th June 2011, 03:34 AM   #18
tomchr is offline tomchr  United States
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One could also add a cathode follower or source follower... In the latter case, the output impedance would be measured in mOhm... Or at the worst a few ohms. In case of the cathode follower, it'd be about 1/gm or 150-ish ohm.

~Tom

Last edited by tomchr; 17th June 2011 at 03:36 AM.
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Old 17th June 2011, 04:25 PM   #19
hihopes is offline hihopes  South Africa
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Thanks for the replies guys.The preamp is actually doing a fairly good job as is, but I don't think I am alone on this forum in always wanting to find some way to improve things.

I don't know what the input impedences of the 2 solid state amps are. Both are without volume pots. One is a moscode (trioded EL34 / bjt cascode driving mosfet source followers). I have the original article about this design, but there is no concrete info on the input impedence to be found in it, except the following, (which I don't really understand):- "The shunt feedback reduces the input impedance seen at the base of Q1. As far as signals are concerned, it is virtually at earth potential, which is why it is called a virtual earth amplifier." It doesn't sound to me like an easy load for a tube of any sort to drive, particularly with no volume pot, but I am not very clear about this and could be wrong.
The other amp is a Hart Linsley Hood kit amp with passive RC low-pass filter at input. I can't imagine the combined impedences of these amps could be much, and the capacitance of the passive filter is probably also a bit of a bitch to drive, though no HF is required from this amp, as it drives my woofers.

Kevin, thanks for your input. I always value your suggestions. I am not about to throw money at this preamp. I am fiddling with it while I wait for my Edcor OPTs to arrive so I can get going with my //SE 4p1l power amp. I can raise B+ voltage a bit by reducing the values of the R's in my crc filtering, but the trafo has only a 200v secondary, so I will not be able to get the plate voltage up to that kind of level, especially at such high mA, without a new trafo, or resorting to solid state assistance (or significantly dropping the plate load). Choke loading would probably be good, but is not an option at this point. I tried loading preamp tubes with gyrators in the past, but was not overly enamoured of the sound, which I found a bit on the analytical side. Even worse was the very noticeable hiss - so I would be reluctant to go down that road with the inherently bright sonic signature of this tube (and the high dissipation of SS devices required at this kind of idling current.) I do not need more gain either, and could in fact live comfortably with less. Maybe some local feedback? What would you be inclined to do if you were in my shoes?

My thoughts are revolving around possibly using 6n6p's as White cathode follower or CF with active load (a la M.Jones) or possibly Mr. J.Broskie's Aikido CF. Other triodes I have in stock are 6CG7 and 5687. Pentodes include EL34's, 6CA7's and 6j4p's. Thoughts?
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Old 23rd June 2011, 12:29 AM   #20
Matt BH is offline Matt BH  United Kingdom
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Wow, thanks Rod, Sorry it took so long to reply (I have no excuse). Looks like JMS are the boys then. Been thinking about a circlotron and all the PS troubles that entails. Will give them a call.

Cheers Matt.
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