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Old 27th September 2004, 05:30 PM   #1
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Default Iron, glass, sand and lead.

I hadn't previously designed with tubes, not because I don't like them, but because I've this "thing" about high voltages after having a couple of rather nasty "shocking" experiences as a kid.

However I recently got the urge to try tubes in an existing design of mine after a friend clued me in on the ECC86 which operates at significantly lower voltages than most other tubes.

A few weeks ago I finally got 'round to ordering up a pair of ECC86s and some sockets.

Here's the result:

Click the image to open in full size.

This is essentially the front end circuit of my integrated amp. The ECC86 replaces a dual JFET I was using originally. Without the output stage, this circuit makes for a very nice unity gain buffered attenuator.

The use of PNP bipolar transistors makes this a hybrid circuit, but it's not like any of the hybrids I've seen before (which I'm sure is fewer than the more experienced bottleheads have seen).

The hybrids I have seen typically have a tube stage simply cascaded into a solid state stage, so that the tube stage does its thing and then passes the signal on to the solid state stage where it does its thing.

In this circuit, the tube and the transistor are mated in what's called a Sziklai pair and instead of being cascaded, have a master/slave relationship with the tube being the master, its grid-to-cathode voltage being the controlling element, imposing its transfer function onto the transistor and subsequently the output of the circuit.

There are no coupling capacitors in this circiut. Output offset voltage will depend on the matching of the devices and resistors that make up the follower/current source.

Because the tube behaves like a JFET, which is a depletion mode device, output offset voltage can be nulled by tweaking the relative values of the two resistors R2 and R3.

To make adjustment easier, you could use a 200 ohm trimpot instead of a fixed resistor for R3, tying one end of the pot to cathode/collector of the follower, the other end to the output, and the wiper to the emitter of Q2.

While the input transformer isn't reqiured, it contributes significantly to the overall character of this circuit. If you build this circuit without the input transformer, don't use it to judge this circuit.

Because the power supply voltages are relatively low compared to most tube circuits, it's a natural for battery power. I use four 7Ah SLAs for the job.

I'm using all carbon for the resistors in this circuit. Hot molded carbon comp pots from PEC and carbon comp fixed resistors from Stackpole which I prefer over Allen Bradleys.

R2 and R3 can be lower than 150 ohms. I just kept that value for convenience as that's what I use to drive the input of the step-up transformer for the output stage.

I've gone as low as 8 ohms for R2 and R3, but to me it didn't sound quite as good as higher values between 50 and 150 ohms.

The circuit is very simple and can be easily point wired. No heatsinks are required for the transistors if you don't opt for 8 ohm resistors for R2 and R3.

Well, that's about it.

Enjoy.

se
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Old 27th September 2004, 10:35 PM   #2
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You forgot to remove something.

Tim (played with HV for years without dying)

P.S. Just about anything under 200V is nonfatal. Yeah, it depends on how, but that's why you keep a hand behind your back.
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Old 27th September 2004, 11:57 PM   #3
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Quote:
Originally posted by Sch3mat1c


You forgot to remove something.
Hmmmm...

Ok, I'm stumped. What'd I forget to remove?

Quote:
P.S. Just about anything under 200V is nonfatal. Yeah, it depends on how, but that's why you keep a hand behind your back.
Lots of things are non-fatal. Just because something is non-fatal doesn't make it pleasant.

se
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Old 28th September 2004, 02:50 AM   #4
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Hi,

Quote:
Ok, I'm stumped. What'd I forget to remove?
The transissssssssstors?

Quote:
Just because something is non-fatal doesn't make it pleasant.
But it could....be pleasant, I mean.

Quote:
Because the tube behaves like a JFET, which is a depletion mode device, output offset voltage can be nulled by tweaking the relative values of the two resistors R2 and R3.
Not because of it behaving like a JFET really.
You could easily simplify the circuit, using just the tubes and still null the voltage at the output node just the same.
Naturally it won't be a Sziklai pair anymore but it would very likely do just as well.

Quote:
Because the power supply voltages are relatively low compared to most tube circuits, it's a natural for battery power.
True but a Sziklai pair has good PSRR already.
Maybe a good idea to run some listening tests between a voltage regulated mains run circuit and the battery set up...
I know that's what I'd do anyway.

Okay, now we'll also be waiting on the all balanced version of course.

Nice circuit...
Welcome to the wondrous world real audio devices.

Cheers,

TIP: Replace the ECC86 with a good Amperex Bugle Boy 6DJ8 or other good Q ECC88 and hear what the circuit really can sound like.....
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Old 28th September 2004, 03:40 AM   #5
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Quote:
Originally posted by fdegrove
The transissssssssstors?


Quote:
But it could....be pleasant, I mean.
You're a sick man, Frank.

Quote:
Not because of it behaving like a JFET really.
What I meant was that the offset voltage is of such a polarity that it can be nulled out by the voltage drop across the resistor, unlike a BJT or most MOSFETs where you'd have to resort to some sort of level shifting at the input.

Quote:
You could easily simplify the circuit, using just the tubes and still null the voltage at the output node just the same.
Naturally it won't be a Sziklai pair anymore but it would very likely do just as well.
Sure.

Though in my case, where I'm ultimately using this circuit to drive the step-up trasnformer for the output stage, I need the output impedance to be 150 ohms or less.

I haven't verified it for certain yet, but I'm pretty sure that I wouldn't be able to do that without exceeding the ECC86's maximum plate current of 20mA. At least not without using more ECC86es in parallel or something.

Quote:
True but a Sziklai pair has good PSRR already.
Yes. But of course the effects of AC supplies go beyond ripple on the supply rails. There's also parasitic coupling of the AC line into the local ground which can result in interchassis currents and subsequently hum, buzz and other noise when connected to other components.

The input transformer would largely address that with respect to the source component, but not with respect to the component the amplifier this circuit is feeding.

Quote:
Maybe a good idea to run some listening tests between a voltage regulated mains run circuit and the battery set up...
I know that's what I'd do anyway.
Been there. Done that. And I prefer battery supplies.

Even if I didn't perceive any differneces between the two, I'd still opt for batteries if I could on purely aesthetic grounds when I think of all the crap that needs to be involved in order to hammer out an AC waveform into something resembling DC.

Quote:
Okay, now we'll also be waiting on the all balanced version of course.
You'll be waiting an awful long time.

Not long after I first started using input transformers, I found that I preferred them driven from unbalanced sources.

'Bout the only reason for balanced interfaces is their common-mode noise rejection. But the transformer used here will give you as good or better common-mode rejection when fed from an unbalanced source as an electronically balanaced input will give you from a balanced source.

Quote:
Nice circuit...
Welcome to the wondrous world real audio devices.
T'anks.

Quote:
TIP: Replace the ECC86 with a good Amperex Bugle Boy 6DJ8 or other good Q ECC88 and hear what the circuit really can sound like.....
Yeah? Will those tubes work ok with just 24 volts on the plate?

se
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Old 28th September 2004, 04:19 AM   #6
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Unhappy HOLD YOUR HORSES....

Hi,

Quote:
I haven't verified it for certain yet, but I'm pretty sure that I wouldn't be able to do that without exceeding the ECC86's maximum plate current of 20mA.
20 mil plate current?
No way, Jose....not from a single ECC86 anyway.

Pa max is about 0.9mA per plate. Ik max. however is about 20mA.

Quote:
There's also parasitic coupling of the AC line into the local ground which can result in interchassis currents and subsequently hum, buzz and other noise when connected to other components.
Sure...That's where the real artists come in, right?

Quote:
Been there. Done that. And I prefer battery supplies.
Me too...but batteries are a hassle in daily use IME.
With a really well designed PS the difference (soundwise) isn't really worth it.
What I meant to make you see is that with a topology with good PSRR and good PS design it's not really worth the extra pain...
After I may be a sick man but not a masochistic one per se.

Quote:
Not long after I first started using input transformers, I found that I preferred them driven from unbalanced sources.
Understandable and you'll even like them less that way when they're driven from current sources....
The reason I was teasing you with that balanced version was simply because the drawing is showing a balanced input as a possible connection...
I know, I know...Sick man etc...

Quote:
Yeah? Will those tubes work ok with just 24 volts on the plate?
That's what I use for a DAC and a MC headamp but I would go much lower.
The ECC86 was designed for battery operation, either 6.3VDC or 12.6VDC on the plate. It works with 24VDC but not really optimally.
The ECC88s are also much quieter and more linear approaching cut off too.
It's actually the only tube that was ever designed to be used into pulsed positive grid oscillation AFAIK.

Cheers,
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Old 28th September 2004, 04:38 AM   #7
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Default Re: HOLD YOUR HORSES....

Quote:
Originally posted by fdegrove
20 mil plate current?
No way, Jose....not from a single ECC86 anyway.

Pa max is about 0.9mA per plate. Ik max. however is about 20mA.
Pa? Pa's a power rating not a current rating isn't it? How can you have more current flowing through the cathode than the plate?

Don't see any Pa on the Philips datasheet. They have an Na however which is 0.6 Watts and Ik is 20mA.

Quote:
Sure...That's where the real artists come in, right?
So they say.

Quote:
Me too...but batteries are a hassle in daily use IME.
Doesn't need to be a hassle. And it's a lot easier if you don't have to string a bunch of batteries in series to get the voltage you need.

Ultimately I plan to have it set up so that the "power" switch basically just switches the batteries between the amp and the charging circuit. In which case it would be no more hassle than an AC supply, other than having to replace the batteries every five years or so.

Quote:
With a really well designed PS the difference (soundwise) isn't really worth it.
What I meant to make you see is that with a topology with good PSRR and good PS design it's not really worth the extra pain...
After I may be a sick man but not a masochistic one per se.
Hehehe. Well like I said, I hope to have the end result be as simple to use as an amp with an AC supply.

Quote:
Understandable and you'll even like them less that way when they're driven from current sources....


Quote:
The reason I was teasing you with that balanced version was simply because the drawing is showing a balanced input as a possible connection...
I know, I know...Sick man etc...
Hehehe. Well, should at least let folks know what their options are. Just because I prefer them fed from unbalanced sources doesn't mean everyone will.

Quote:
That's what I use for a DAC and a MC headamp but I would go much lower.
Um, you WOULD go much lower or you WOULDN'T go much lower?

Quote:
The ECC86 was designed for battery operation, either 6.3VDC or 12.6VDC on the plate. It works with 24VDC but not really optimally.
The ECC88s are also much quieter and more linear approaching cut off too.
It's actually the only tube that was ever designed to be used into pulsed positive grid oscillation AFAIK.
Coolness!

That's good news. Means I can try a little tube rolling as well!

se
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Old 28th September 2004, 04:48 AM   #8
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Quote:
Originally posted by fdegrove
TIP: Replace the ECC86 with a good Amperex Bugle Boy 6DJ8 or other good Q ECC88 and hear what the circuit really can sound like.....
Daayyyamn! The Amperex 6DJ8s are expensive little *******s. $90 a pop at TubeDepot.

Geeez. The Nannyware here is so uptight than b a s t a r d is off limits? Hell, you can hear that word on prime time TV. There was even a miniseries called The B a s t a r d. C'mon. Pull that one out of the list.

se
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Old 28th September 2004, 05:18 AM   #9
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Hi,

Quote:
They have an Na however which is 0.6 Watts and Ik is 20mA.
Yep...Was looking at the wrong table...

Quote:
Ultimately I plan to have it set up so that the "power" switch basically just switches the batteries between the amp and the charging circuit.
I know Teres, the TT kit people, have an autosensing circuit for that which I think is supercool.

Quote:
Um, you WOULD go much lower or you WOULDN'T go much lower?
Keyboard sick too....WOULDN'T it is.

Quote:
That's good news. Means I can try a little tube rolling as well!
You'll be rolling allright...

Quote:
The Amperex 6DJ8s are expensive little *******s. $90 a pop at TubeDepot.
Hey....Don't blame me for wanting to spoil your aural senses.

Seriously, the Amperex is probably the easiest to buy for you being on the wrong end of the good tube world and all that....

Good ECC88s and family (ahem) don't come cheap but the good news, or so I've been told, is that the cryoed ECC88/E88CC JJ (formerly Tesla) are on a par with the best NOS and are considerably errr....cheaper.
IIRC, Steve Massett from SAS AudioLabs is using those and seems thrilled with them.

You could send him an e-mail if you like:

SAS AudioLabs

Cheers,
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Old 28th September 2004, 05:24 AM   #10
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Default Re: Re: HOLD YOUR HORSES....

Quote:
Originally posted by Steve Eddy
Um, you WOULD go much lower or you WOULDN'T go much lower?
It would be happier with more. For my VBIGC i've got a =/- 70 V supply lined up.


Quote:
Originally posted by Steve Eddy
Daayyyamn! The Amperex 6DJ8s are expensive little *******s. $90 a pop at TubeDepot.
Often cheaper to just buy a surplus tech oscilloscope... the last one i got had 4 in it. The Sovtek 6922/6H23pi* is the workhorse of this tube family... and with the right care & attention (off to PEARl for the full SPA work out)....

* (last batch i got were a buck each)

dave
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