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Criticism please!!

82.217.1.124

Posted on May 13, 2004 at 13:35:33
Bas Horneman
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Joined: March 28, 2001

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general loud applause...., posted on May 13, 2004 at 15:06:18
keto
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Posts: 1006
Joined: September 29, 2001
Looks great! Just a couple suggestions.

The 100K may be a little large. You might try a lower value, say above 10K. Allen Wright suggested to me 1K, and he builds a lot of amps. You want that R to be at least 10X the output impedance of the preceding stage. I've tried a pseudo grid choke (primary of a cobalt B7--about 1/10 the inductance of the real thing) in that position and its fabulous.

The coupling cap should be about 3-5uF and >200V. With 0.47uF it will work, but you'll have some strange things going on around 30hz. You want those strange things at around 3hz, instead. Maybe try that 3.3uF PSU cap, which may not be of such vital importance to the sound (if the stuff is on hand already). I've got a chart someone made for me that shows the interaction, which I'll try to find and post.

One way of balancing the current 60mA-60mA between the OPT primary is to measure the DCR of the two halves, and make them equal by adding a precision resistor. Michael Percy has a little multi-turn Vishay trimmer, which I use in its 100R form in a constant current source to do a similar thing. A value of 50R would give you the right amount of flexibility and fine adjustment. If you can up end your amp (I C-clamp mine to the bench) so that the filament pins are in a vertical plane and heat-generating elements wont otherwise damage some other part, THEN, set up the little trimmer in some sort of stable module, and you can do the fine tuning on the run. I've found this fine tuning to be really stable, once its done and though perhaps independent of true zero DC offset, but at least close to it, humm should null to about 0.5mV. I've also found that even between TJs, the two tubes want to be in a particular order.

I'm not familiar with the 6AX4 function.

A single 220uF or 470uF instead of your 47uF+40uF might help. The bleeder draws less than 1mA and may take a while to drain voltage; especially if you go to a bigger cap, maybe change it to a 250K.

The 100K dropping R is ingenious, but I bet it doesn't look as good as an OB3!

As described, it should work fine; you might want to try my suggestions if convenient. I look forward to hearing your impressions.

--keto

 

Re: Criticism please!!, posted on May 13, 2004 at 15:19:07
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
You have the rectifier diodes reversed.

Ignoring losses, you need "356"-0-"356" to get 320 VDC with a choke I/P filter. A 375-0-375 winding seems indicated. Follow ARRL Handbook recommendations; use 2 LC sections in cascade for the 2A3 B+; the inductor in the 2nd LC section does not have to be rated for choke I/P service. A 10 nF. HIGH WVDC ceramic cap is plenty in front of the 1st inductor to deal with inductive kickback.

Get rid of the RC section after the 0D3. All that's needed is a 47 nF. cap. across the regulator to kill its noise. I'd put the 100 Ohm resistor between the 30 mA. CCS and the 0D3's anode.

Separate FLOATING filament supplies and separate hum balance pots. are in order for the 2A3s.

Eli D.

 

I was planning hum balance pots..., posted on May 13, 2004 at 16:46:04
Bas Horneman
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Posts: 4081
Joined: March 28, 2001
but what do you mean floating...? Seperate filament transformers?

I copied the 100R resistor from Lynn Olson. I think he got it from some handbook...

Thanks for the transformer tip.

Is the second choke really necessary for PP?

I liked what the RC section did for my preamp sound. However that was for feeding 2 channels from 1 psu... Maybe I'll change that afterwards and see what happens with the sound.

Thanks Eli.

Cheers,
Bas

 

The 100k is going !! ;-), posted on May 13, 2004 at 16:57:17
Bas Horneman
Audiophile

Posts: 4081
Joined: March 28, 2001
10k sounds good...will do.
47uF+40uF is enough I find for SE...I find it hard to believe my psu needs so much C and I ;-) especially for PP! And lytics are forbidden in my amps..except for the heater supply that is. :-)

You're right on the bleeder! Will do!

You mean the 100R? (Whatever it is ...I can't be ingenious...cause I copy most everything I see.;-)

--The coupling cap should be about 3-5uF and >200V.
That big huh...Ok!...will change it.

--I look forward to hearing your impressions.
Should be around the end of this year!

Thanks Keto!

Cheers,
Bas


 

Updated version..give it your best shot!!!, posted on May 13, 2004 at 17:14:18
Bas Horneman
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Posts: 4081
Joined: March 28, 2001

.

 

Re: I was planning hum balance pots..., posted on May 13, 2004 at 20:03:58
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
>>but what do you mean floating...? Seperate filament transformers?<<

Floating means that the winding is not grounded directly at either end or by a CT.

You don't need separate filament trafos. For monoblocks, a trafo with 2X 2.5 VAC windings is fine. Each 2A3 filament gets a dedicated floating filament winding and a dedicated hum balance pot. The only connection to ground for the whole shebang is via the wiper of the hum balance pot. In your case, the balance pot. wiper connects to the CCS.

Eli D.

 

Re: Updated version..give it your best shot!!!, posted on May 13, 2004 at 20:15:35
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
The cap. in front of the 1st filter inductor should be 10 nF HIGH WVDC ceramic, not 100 nF. That cap. serves only as protection against inductive kickback.

A 100 nF. cap. across the 0D3 is too big; it can cause oscillation. A 47 nF. cap. is enough to kill regulator noise and will not oscillate.

As I indicated previously, a 2nd LC section is in order. The 1st filter cap. can be 22 muF. The 2nd inductor can be 5 H. The 2nd filter cap. is where the bulk of the energy storage goes.

Grounding a 2A3 filament winding is a "no no", when balance pots. are used and doubly so when a CCS is used in the cathode circuit.

Eli D.

 

Re: Updated version..give it your best shot!!!, posted on May 14, 2004 at 03:20:44
Kuei Yang Wang


 
Konnichiwa,

Unless you want a high output impedance from the Driver valve, here a little trick. If you return the output pair cathode current to the driver cathode you only 12 Ohm to get the bias for the ECC99. This means the output impedance of the valve is raised only by around 250 Ohm vs. 1k6 for the 82R resistor.

Also, adding a 20 - 40uF capaitor (switcable if you like) between the output transformer center tap and the output pair cathodes often sounds better than not having it, subjectively speaking (we tested this several times).

Sayonara

 

Cool!, posted on May 14, 2004 at 04:38:31
pistoljoe
Audiophile

Posts: 269
Location: cambridge, ma
Joined: February 19, 2003
Bas-

Pretty much identical to my test mule I've been working over the past year. I thought the blocking cap sounded the same to my ears anywhere from 3.3u to 1.1u. The jury is still out on my .47u + .1u combo. I did like the Ecc99 alot, and was my favorite at 20mA and less, but I tried the 6H30. It sounded not as good at the same current, but really opened up when I took it north of 30mA, right now I'm happy at 34mA. And I'm running HexFreds instead of tubes, but will change those out later, chassis space getting to a premium.

If you use a 120mA current source, I'm not so sure you need another section on the supply. The CCS will really smooth out the ripple for anything under a volt. Havent played with sinks so much yet.

Try it out- Joe

 

Thanks KYW San.., posted on May 14, 2004 at 04:43:14
Bas Horneman
Audiophile

Posts: 4081
Joined: March 28, 2001
Will incorporate your suggestions into the design and post the schematic to see if I understand it properly.

Regards,
Bas

 

Hey Joe!, posted on May 14, 2004 at 04:55:52
Bas Horneman
Audiophile

Posts: 4081
Joined: March 28, 2001
Where are you going with that pistol in your hand! ;-)

--If you use a 120mA current source, I'm not so sure you need another section on the supply. The CCS will really smooth out the ripple for anything under a volt. --

I did not know that!..also when it is a current sink?

Regards,
Bas

 

Re: Updated version..give it your best shot!!!, posted on May 14, 2004 at 05:28:48
AnandR
Audiophile

Posts: 739
Location: North Carolina
Joined: September 8, 2003
Bas,

I'm obviously not as knowledgeable as some of the other folks here, but depending on what you are going for, you might wanna try replacing (2) of the 6AX4 damper diodes in this full wave bridge to (2) Hexfreds or Schottkey's 1200V types. This is the Hybrid Gaetz configuration that Andre Ciuoffoli first popularized (although designed by somebody else) and I know Kevin Carter uses to great effect on his designs. The sound and characteristics of the rectifier is now of the damper type and not the solid state Hexfred or Schottkeys. Might save you on some filament supply.


Just a thought. Nice job BTW.
Anand.

 

Re: Updated version..give it your best shot!!!, posted on May 14, 2004 at 06:47:24
Bas Horneman
Audiophile

Posts: 4081
Joined: March 28, 2001
Thanks for the tip Anand,

---Might save you on some filament supply.
Good idea..Will that increase the psu'
s abilitiy to deliver current quickly as well?
This rectifier config I have now is a little over the top isn't it. But with the Graetz bridge (Have used it in both my current pre- and power amp) the damper tubes still are the ones that up the impedance of the psu...this was an attempt to increase the capability of the psu to deliver current faster...if the Graetz bridge does that as welll...I'll go for it.

--Just a thought. Nice job BTW.
All copied from Keto and Jeff Lessard (And Mike Lafevre). They obviousely deserve the credit ...this is just an implementation of their spadework. I am merely the "paster" ...

Regards,
Bas

 

Re: The 100k is going !! ;-), posted on May 14, 2004 at 08:32:45
keto
Audiophile

Posts: 1006
Joined: September 29, 2001
I'm using the 6SN7 (mu=20) at the moment, with just the "grid-choke-lite" and find that almost all CD recordings will drive the amps into clipping, so my next experiment will be with the 27 (mu=9). If you're so inclined, you could probably put a volume pot between input and grid and a selector switch before that and call it an "integrated".

While quality of resolution is progressing in my amp, there are still lots of mysteries.

If you look at the shared 2A3 cathodes...

1. In terms of function, you can put a 375R/50W or nice 375R/20W Mills resistor, with whatever bypass cap is typical (2200uF/100V?), and achieve a performance similar to the CCS. An unbypassed resistor may add a "swimming" sound to the amp.

2. There are three elements of particular interest for refining the sound: balancing filament humm; balancing DC current between primary "half"-windings; and AC symmetry. I tried a 100R pot between separate filaments and a CCS and though it got pretty quiet, the pot seemed to add something unpleasant to the sound. Also, the idea was to balance DC between halves, but the value was too high I think and I lost some of the neat noise rejection of the PP output, turning the pot through what must have been multiple nodes. A fixed pair of 25R resistors and shared filaments has been the best solution, sonically, to balance filament humm. This is part of Lynn Olson's solution, it would seem. Gary Pimm uses 11 ohm resistors in his PP47. Of course you've got to have a well-behaved filament transformer. My next experiment (weeks away) will be to use the 25 ohm resistor pair, and apply the 50R Vishay trimmer between one 2A3 plate and its OPT "half"-winding; first, switching the pair of Valve Arts with a zero setting, to find out which way they want to go, and then CAREFULLY trimming-in a little series resistance. I'm expecting that just a couple ohms will do the job, but am prepared to find that, yet again, I'm mistaken. The Valve Arts, BTW will sound spectacular if the circuit is functioning well. AC symmetry, so far, seems like something you "enjoy" in the components themselves.

Your circuit seems to have a lot of high-figure-of-merit elements, but it might be worth it to start with only the essentials, about half of what's on your schematic, get the amp quiet and sounding good, and then start complicating things. All paths do lead to good sound, but that way you can first eliminate basic inefficiencies, and then refine each truly necessary element, one by one. My B+ filter consists of two 1N4007s, a BFFC-150, and a 470uF/450V cap. Yikes, you will say...

I found a 6N1P sounded lean and clean as an input and the 6C45 really dynamic in a really great way; both running at 10mA and 150Vp. That said, I'm fascinated with the fixed volume idea, from my limited experience with the 6SN7 and grid-choke-lite, and so have slated the 27, 26 and 12B4A for the next round.

So the 100R is from J Lessard? That's a good one. You know the OB3, OC3 and OD3 have the same pinout? With two octal sockets you can range from OB3+OB3=180V to OD3+OD3=300V. Also, you might want to draw 20mA through the VR, equal to the ECC99's draw -- this idea, like 9/10ths of what is in my amp, from the Bottlehead brain trust.

Again, looking forward to reading about your adventures... --keto

 

Re: The 100k is going !! ;-), posted on May 14, 2004 at 08:58:59
Bas Horneman
Audiophile

Posts: 4081
Joined: March 28, 2001
--Your circuit seems to have a lot of high-figure-of-merit elements, ---but it might be worth it to start with only the essentials

Touché, that is generally how I build...since I don't have the time and space to breadboard. I generally try to tweak before I even build...thanks to the internet that is possible (up to a point ofcourse)

I would very much like to start with the essentials and work my way up. (But the tube rectification is non-negotiable ;-)) That way you learn so much more. Maybe I will try that with this amp...

--So the 100R is from J Lessard?
Lynn Olson.

--Again, looking forward to reading about your adventures... --keto
Same here Keto...I enjoy seeing your schematics, thoughts and suggestions.!!

-- In terms of function, you can put a 375R/50W or nice 375R/20W Mills resistor

I am very much leaning that way at the moment. Allen Wright's latest amp being such a hit has swayed me a little to the ccs since that was one of the biggest changes from his previous amp to the current one...With the big improvement being in the DDR. :-) Giving you all the advantages of PP and none of the vices...sort of SE type sound with balls.

Cheers,
Bas

 

14-05-2004 version, posted on May 14, 2004 at 11:25:42
Bas Horneman
Audiophile

Posts: 4081
Joined: March 28, 2001

.

 

Re: Hey Joe!, posted on May 14, 2004 at 11:38:35
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
A CCS in the cathode circuit is a sink, while a CCS in the anode circuit is a source.

The conventional current flows from sources and it flows to sinks.

Eli D.

 

Re: Criticism please!!, posted on May 14, 2004 at 13:38:29
Dave Davenport
Manufacturer

Posts: 916
Location: North Carolina
Joined: March 20, 2002
Hi Bas,

Since you asked .... I have a couple of things for you to try.

First, try a 6H30 in place of the ECC99. Experiment with the current to get the best sound.

Second, try returning the 40uF ultrapath cap to the cathodes instead of ground.

Third, I see that the main cap is 47uF and the Ultrapath cap is 40uF. The purpose of the Ultrapath cap is to provide a very high quality path for the signal. So you must use a very high quality Ultrapath cap, like film, certainly not electrolytic. Remember, separation of the power loop and signal loop is a good thing. Getting back to the 47uF and 40uF thing, if you can provide a superb 40uF ultrapath cap, you can likewise provide a superb 47uF power supply cap and may not need the Ultrapath cap.

Now, there is a fly in the ointment as we say..... With the Ultrapath cap tied to the cathode and with the power supply cap tied to ground you have a capacitor across the CCS in the cathode circuit. This will lower the dynamic impedance of the CCS. So maybe it is or is not a good idea to connect the Ultrapath cap to the cathode. You will have to try both and listen.

My gut says the best will be a very high quality power supply capacitor with no Ultrapath capacitor.

Dave

 

Re: 14-05-2004 version, posted on May 14, 2004 at 15:02:46
keto
Audiophile

Posts: 1006
Joined: September 29, 2001
I think you'll want to connect the bottom of the CCS (rather than its top) to the ECC99 cathode, and I agree with the comment about maybe doing away with the 40uF-position cap. In some of Lynn Olson's schematics, the cap is there, but the joined cathodes are biased with an unbypassed resistor. --keto

 

Re: Updated version..give it your best shot!!!, posted on May 14, 2004 at 20:48:39
AnandR
Audiophile

Posts: 739
Location: North Carolina
Joined: September 8, 2003
Bas,

I can't vouch for the Graetz bridge delivering current any slower, but I do know from breadboarding as well as chatting with other builders that the Graetz bridge really doesn't sound too different from a full wave bridge of dampers. I imagine the actual impedance of the supply should be lower since you have replaced two tube rectifiers with two solid state ones.

Best,
Anand.

 

Version I will start collecting parts for......, posted on May 15, 2004 at 07:11:36
Bas Horneman
Audiophile

Posts: 4081
Joined: March 28, 2001

.

 

parafeed the output stage?, posted on May 15, 2004 at 19:41:07
MQracing


 
Hi Bas:

Just as a study excercise... to begin with... I'd like to see your schematic redrawn with a plate choke on top of each anode and a dc blocking cap in each leg of the OT...

the potential advantages as I see it are manifold;

1) gets the power supply out of the music business.... i.e., you have parallel paths for the ac and dc... and you get much, much greater PSRR.

2) gets rid of the problem of trying to balance the dc currents across the primary of the OT.

3) the OT will like the idea... once you remove the dc heating current from the primary winding... it is equivalent to increasing the wire size by a guage or two if we calculate out the copper current density without the dc current component.

4) you can dispense with that 40 mfd cap... which won't provide nearly the isolation that a parafeed arrangement would anyway... and use (typically) a 5 to 6 mfd cap in each leg of the OT.

5) conceptually... to me... I've never seen the elegance of dragging the ground path of the ac through the output trans and through the necessary blocking cap... and then tying it into the cathode ground path of the first tube... always struck me as a circuitous route to ground... and I've always thought that you should want a low impedance (sort of direct) path to ground.... i.e., that you want to get to ground as quickly and directly as you can... hence... my sense is always to take your ac ground from the primary of the OT (whether se or pp) and get to ground the immediately....

but i'm no circuit genius so perhaps I'm missing the beauty and elegance of the alternate method of grounding the ac.


Now.. back to parafeeding a PP output stage... biggest drawback.... is the obvious expense of getting two plate loading chokes... less expensive and perhaps as effective may be a beefy enough CCS on top of each plate.

but... my hunch is that there are (despite the increased cost) as many benefits to parafeeding a PP output stage as there is to parafeeding a SE output stage...

be interesting to see how this would look on paper and hear comments re: the idea.


Mike

 

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