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Old 25th July 2007, 07:41 PM   #1
yan24 is offline yan24  France
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Default ampli 20w cathode follwer

Hi
I'd like to do a vacum valve ampli about 15 to 20w.
I like single end for good sound and cathode follower for performance on DHT and Zin out.
I think it 'll be easyer to do using valves like 6c33 or el 519
I knowsthat it 's possible to use boostraping to help driver to do his hard job : 200 to 250 vott cc
What do you think about this projet.
Thanks
Yan
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Old 25th July 2007, 11:19 PM   #2
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^^^^^
Need more details. Were you thinking of going OTL, or is an OPT OK? Cathode follower outputs are doable, but it is nowhere as easy as using a common cathode final. 15 -- 20W of output is 845 territory, and that will really complicate matters to say the least.
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Old 26th July 2007, 01:32 AM   #3
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Bruce Rozenblitt has a SE cathode follower design in his "Audio Reality" book. This design uses 3 X KT88 in parallel to generate about 20 watts. I have not built one, so I have no experience with the design.

I have been experimenting with cathode follower output stages. I fine that you need a tube that has a relatively high GM and can handle high peak currents. I do not have any EL519 or 6C33 to test, so I can't comment on those, but I have tested a bunch of different tubes, and come to similar conclusions. I have not made the triode - pentode decision yet, but I have decided that the triode would be a 6336A, or a pair of 5998's. Both are similar to the 6C33. My pentode choice would be a 6LW6, not to far from the EL519.

For the driver circuit, I am using a two stage 6V6 SE amp. I replaced the output transformer with a choke, and used capacitor coupling into the output tube grid (a parafeed connection). My choke is too small (kills the bass), so I plan to switch to a CCS load when I have more time to experiment.

The optimum load impedance looks like it is in the 500 ohm to 1K ohm range, but it depends a lot on factors that I haven't nailed down yet, like the choice of tube, B+ voltage, and tube current.
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Old 27th July 2007, 01:33 PM   #4
yan24 is offline yan24  France
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hi
Without schema : it's difficulte to discribe .
1 er stage : a usual commun cathode or SRPP with 12at7 or 6dj8 and + - 300v
2 eme stage a over commun cathode in direct link with 12b4 and + - 800 volt
3 eme in C F 2 EL519 tiode connected and OPT about - + 400 Ohm

There is a over way to drive the output stage with boostrap : a self induction is in beetwin +U and plate's charge resistor of driver.

In this way you can choice all power valves and using high charge impedance and use KT 88 KT90

I'd like to know if this way gives good sound

Thanks

Yan24
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Old 27th July 2007, 04:45 PM   #5
coresta is offline coresta  France
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Hi! There was an interesting description of such an amp in the defunct Audiophile review .. from Guy Marec if i'm right . He said it was astonishing . The main difficult remains in the driver ...
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Old 27th July 2007, 04:47 PM   #6
Jeb-D. is offline Jeb-D.  United States
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My first tube build was a 6c33c cathode follower. I tried bootstrapping, but had a problem with turn on thump from when the drivers stage cap charged. Though I'm sure it would work if done under different conditions. Bootstrapping is a form of positive feedback, which will raise the output impedance and add distortion, which is what you are trying to get away from when doing the cathode follower approch. However, if the driver stage is using an active load(CCS) it seems, you may not encounter the negative effects of bootstrapping. Since the CCS would in a sense isolate the outputs audio from the driver tube. Eventhough I now perfer passive loads to active and would suggest just going with a larger voltage transformer for the driver stage.

The hardest part about building a 6c33c SET(at least in the US) is sourcing transformers. Both output and power trannies. The outcome was rewarding though, the amp sounds very clean and detailed. Eventhough there are a couple things I'd do different if I were to do it again. It benches at .1xx% THD up till it starts soft clipping. Oh, forgot to mention the tight bass and that it can handle 3way speakers.

If you decide to go that route I can help you out.
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Old 27th July 2007, 07:16 PM   #7
7N7 is offline 7N7  United Kingdom
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Quote:
Originally posted by yan24
hi
Without schema : it's difficulte to discribe .
1 er stage : a usual commun cathode or SRPP with 12at7 or 6dj8 and + - 300v
2 eme stage a over commun cathode in direct link with 12b4 and + - 800 volt
3 eme in C F 2 EL519 tiode connected and OPT about - + 400 Ohm

There is a over way to drive the output stage with boostrap : a self induction is in beetwin +U and plate's charge resistor of driver.

In this way you can choice all power valves and using high charge impedance and use KT 88 KT90

I'd like to know if this way gives good sound

Thanks

Yan24
I have never built an amplifier of this type, but judging what Tubelab has to say on this thread, I would think that 13E1 would be a good valve to use. A regulator beam tetrode of 90W Pa, its gm max. is 35.

Bonne chance

7N7
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Old 27th July 2007, 09:12 PM   #8
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Heres just a stab in the dark, but here goes.
When looking at tubes like the 6080,6C33 and such like I would guess that a common cathode with a normal output transformer will produce almost all of the benefits of the cathode follower, but with extra power and easier design. I say this from some personal experience with low output impedance valves and the resultant low ratio transformers.
The basis for my assertion been that the low output impedance gives maximum bite on the transformer inductance, and the low ratio minimises stray nasties which tend to bleed off the high frequency response.
My conclusion, for what its worth, is why bother with a cathode follower (with its degenerative feedback) when a common cathode will probably sound as good.

You may disagree.

Shoog
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Old 27th July 2007, 09:31 PM   #9
coresta is offline coresta  France
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And just add some total local feedback as CF does and ... You french guys (and others ..) have a look at my OTLs on Radiofil forum / realisations personnelles / un demi mètre carré d'OTLs
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Old 27th July 2007, 09:33 PM   #10
rdf is offline rdf  Canada
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Quote:
Originally posted by tubelab.com
I have not made the triode - pentode decision yet, ...
A third option? http://www.mif.pg.gda.pl/homepages/f...084/6/6HS5.pdf
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