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Old 27th Apr 2011, 10:47 pm   #41
glowinganode
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Default Re: 2C34 Project taking off...

Quote:
Originally Posted by tri-comp View Post
...or is 5687WA just not the tube for the job?
I think you're right, looking at the curves, it wants to be running 15 - 20mA's.
Try an ECC81 / 12AU7, and drop the standing current back down to 3mA.
I'll have a play this end if I get the chance.
Rob.
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Old 28th Apr 2011, 8:05 am   #42
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Default Re: 2C34 Project taking off...

Also consider a cascade bootstrap to keep the voltage across the c/f constant (~100V).
Rob.
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Old 28th Apr 2011, 11:29 am   #43
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Default Re: 2C34 Project taking off...

I was thinking about using a transistor to regulate the B++ for the CF.
Now there is a TO-3 LM317 mounted on the alu heat-sink.
It was for regulating DC for the 6AN8A filament, but I'm not sure it's necessary (Hum-issue?)
If not, I'll put a BU208A or something similar instead to stabilise the B++.
I agree about bootstrapping is best but also more complicated.

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Old 28th Apr 2011, 11:38 am   #44
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Default Re: 2C34 Project taking off...

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Originally Posted by tri-comp View Post
I agree about bootstrapping is best but also more complicated.
No more complicated than a dedicated stabilised supply.
I'll try and scheme something out.
Rob.
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Old 28th Apr 2011, 9:17 pm   #45
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Default Re: 2C34 Project taking off...

Sorry, no ECC81 on stock.
I do have a few NOS RFT ECC82 and the curves don't look all that bad with dropped B++ to +200V and CCS at 3mA.
Still room for an extra 10mA of Ia before hitting the Wa-ceiling

It'll have to be during the week-end.
A lot of work keeps getting in the way....

rgds,

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Old 1st May 2011, 7:23 pm   #46
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Default Re: 2C34 Project taking off...

Hi All,

It really isn't all that difficult once the bearings are right.
I wasted some mA in the Grid Bias circuit. Now I'm running 1mA and it's fine.
The Bias can be adjusted easily to the required cathode-voltage of -15V and it stays there.
The Bias-transformer could be beefier. It's rated at 24+24V/66mA but it has a high internal resistance of around 60 Ohm.
That doesn't exactly help stabilizing the Bias-voltage once you put some drain on the transformer.
With 4x10mA for the CCS the voltage-doubler drops far too much for the TL783 to keep regulating.
Oh, well at 4x3+3mA it's OK so that will have to do for now.
Otherwise I'll just drop the negative regulated supply to keep the TL783 regulating. I can probably get away with a lot less negative supply.
G1 requires around -45V plus the CCS voltage-drop of 6,2V + 10V + a little more, probably another 10V which adds up to around -75V.
With that low an output the TL783 will regulate.

With the shown resistor-values it puts -15V at the CF/cathode very near the center of the 100K bias-pots.

Attached is the modified power-supply and the ditto CF/CCS circuit.

rgds,

/tri-comp
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File Type: pdf 2C34 Power-Supply_II.pdf (18.8 KB, 572 views)
File Type: pdf 2C34_Modified_II.pdf (13.5 KB, 179 views)
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Old 2nd May 2011, 10:37 am   #47
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Default Re: 2C34 Project taking off...

Why are there 10R resistors shown in each cathode of the 2C34?
Glad it's working out, how does it sound?
Rob.
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Old 2nd May 2011, 3:00 pm   #48
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Default Re: 2C34 Project taking off...

1) Resistors are for measuring Cathode-current.

2) Sound.. Don't you think I should add the anode-leads for the 2C34' first?

I was very focused on getting the CF right. The rest is just a no-brainer (hopefully).
I'm just down with a LOT of work presently. It's already 4PM and I still have 3 more customers to visit before calling it a day.


rgds,

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Old 2nd May 2011, 5:29 pm   #49
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Default Re: 2C34 Project taking off...

OK, Anodes connected!

Bias adjusted for each CF/Cathode = -14V which makes for 20mA each 2C34-section. Since the 2C34-Cathodes are stripped together inside the valve, the combined voltage over ONE (NOT 2, as in the schematic!) 10-Ohm Cathoderesistor = 400mV
AND, it stays that way after heating-up.
That's pretty close to the spec's with the B+ settling at +335V, being drained around 46mA (3 + 3 + 20 + 20); roughly = 15Watt; one 2C34-section being installed, only.
Still without any audio. First I've got to wire the phase-inv. and the pre-amp sections.
I have NO idea what will happen once the 2C34 starts conductiong grid-current

That's all for today.

rgds and thanks to all faithfull followers of this thread.

/tri-comp

Last edited by tri-comp; 2nd May 2011 at 5:45 pm. Reason: Corrected measured data.
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Old 2nd May 2011, 11:43 pm   #50
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Default Re: 2C34 Project taking off...

Hi Torben, glad it's all coming together.
Here's a c/f circuit I put together, and works very well, but requires 2 extra valves.
I can get the grid of the 2C34 up to +15V on the scope, very loud at this point!
Valves are ECC81 / 12AT7, and are fed from the classic Mullard direct coupled EF86 - ECC83 preamp / phase splitter.
I put a jack socket with a shorting contact in the cathode lead to measure the current directly.
Rob.
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Old 3rd May 2011, 1:24 pm   #51
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Default Re: 2C34 Project taking off...

Unfortunately I don't have room for extra valves.
Something to consider for the next project.

This is how I intend to do the Concertino phase-inverter.
Any comments?

rgds,

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File Type: pdf 2C34_Phase-inv + Pre.pdf (9.9 KB, 159 views)
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Old 3rd May 2011, 7:55 pm   #52
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Default Re: 2C34 Project taking off...

What a strange circuit, where did you find it?
By the way you seem to have g1 and g2 (of the pentode) swapped over.
Rob.
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Old 3rd May 2011, 8:18 pm   #53
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Default Re: 2C34 Project taking off...

It'a alive....It's alive!!

Oh well, it doesn't sound all that good compaired with my EL34 Ultra-Linear.
I've only finished one channel and haven't had a chance to put a 'scope on it.
2C34 Cathode-current will only double at full output which sounds very, very terrible!
Actually it should triple or there abouts so probably the phase-inverter isn't fully doing what I'm expecting it to.
Soundquality can be classified as poor-to-bad when pressed and it feels like the amp is way too sensitive. Right now it's running w/o NFB so there will be plenty of dB's for that purpose.
Also it sounds like lack of control over the woofer like the output-impedance is too high (NFB will cure that)
One more thing: The 6AN8A is a bit microphonic. No feed-back from the speakers but don't tap the chassis!!
My EL34 amp must be hit with a forehammer to register microphonics.
Room for experimenting with the output transformers as well.
They are 8K-primaries with 43% and 20% taps. Maybe another configuration will suit the 2C34 better; now they are on 8K.
Also my speakers, come to think of it, are 8-Ohm and the transformer is tapped at 4-Ohm as that was the only high-power dummy-load resistor I had when building and testing w/o speakers. I'll move the tap and try again

Attached is a revised schematic for the Phase-inverter/Pre-amp.
You just don't use g2 for input !!!

I'll be away until Monday and have no chance to do further experiments until then.
Feel free to put forward ideas for improving things. Don't be shy; anything will be an improvement and sound better than what I have

rgds,

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File Type: pdf 2C34_Phase-inv + Pre.pdf (10.0 KB, 149 views)
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Old 3rd May 2011, 8:25 pm   #54
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Default Re: 2C34 Project taking off...

Hi Rob,

Yes, I've noticed the g1/g2-confusion.

The circuit isn't all that strange. It's a very nice way to avoid shortcommings of the cathodyne/consertina phase-splitter.

It does away with un-even output impedance (Anode and Cathode differences) and high-frequence drop-off on the Anode-output.
Also the Pentode is loaded a lot less because of boot-strapping the triode resulting in much higher pentode-gain.
Read the article.; clever stuff!

Have a nice week-end. I'm off the Sweden for a few days.

/Torben
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File Type: pdf Improved Split-Load Phase Inverter.pdf (255.1 KB, 1899 views)
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Old 3rd May 2011, 8:32 pm   #55
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Default Re: 2C34 Project taking off...

That's great news, but get it sounding ok before applying NFB (if at all).
Thanks for the info on the phase splitter circuit, I'll try it out.
Rob.
edit, just had a look at the site, I'm not sure the 10nF and the 100nF caps (C7&8) are the right way round. The cascode circuit shows the larger capacitor as the anode coupling capacitor which is what I would have expected.
Rob.
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Last edited by glowinganode; 3rd May 2011 at 8:44 pm.
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Old 3rd May 2011, 9:28 pm   #56
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Default Re: 2C34 Project taking off...

Ah, I knew I had another and much older acticle about the improved phase-inv. Just couldn't find it in a hurry.
This appears to be the original 1947 in-dept analyzis.

Cap's 7&8 are correct as I see it.
Read the older article here.

I'm listening to Dr.Hook/Baby makes her blue jeans talk & Robert Palmer/Addicted to love.
Sounds pretty good in Mono and 2-3Watts
The muddy woofer and screaming tweeter problems starts after turning up volume beyond 'normal' listening level.
How to lower the output-impedance without NFB?

/tri-comp
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File Type: pdf Hi-Gain Phase-Splitter (Cathode-Follower).pdf (71.2 KB, 384 views)
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Old 3rd May 2011, 10:29 pm   #57
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Default Re: 2C34 Project taking off...

Yes, thanks for that. The caps do indeed seem the correct way round, perhaps the cascade circuit is wrong?
I thought the idea was to avoid NFB, the use of triodes lowers the output impedance.
Glad you're enjoying it,
Rob.
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Old 3rd May 2011, 11:08 pm   #58
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Default Re: 2C34 Project taking off...

Nice looking piece of equipment you have there!
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Old 9th May 2011, 8:39 pm   #59
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Default Re: 2C34 Project taking off...

I managed to connect the NFB and I'm starting to believe in 15+ usefull and tamed Watts.

Haven't had a chance to connect a scope or a generator so it's purely based on experience and comparing it with my EL34-amp.
Of course I only have one channel to compair so far; it will be a lot better after I wire the other channel as well.
It really sounds good and the IMD (rolling of the bass and screaming of treble) almost gone after connecting NFB through a 10K trim-pot.
Of course I first wired the 2C34 anodes the wrong way around and had a very, very powerfull tone-generator, fully adjustable with the NFB-pot

To celebrate I've put Jay and The Americans on the T-T (Dawning, Cara Mia, Some Enchanted Evening etc.)
110% pure nostalgia

rgds,

/tri-comp

Last edited by tri-comp; 9th May 2011 at 8:45 pm.
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Old 10th May 2011, 8:21 pm   #60
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Default Re: 2C34 Project taking off...

Brilliant, well done.
Rob.
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