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Old 12th January 2010, 03:41 AM   #31
jrenkin is offline jrenkin  United States
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Jeff-
the closest I've seen is the Tubelab SE which uses a different CCS for the input tube (a 5842) and a MOSFET follower for the grid of the 300b.

I don't think the 300b would lend itself to a CCS as a load since the load has to be the output transformer. Maybe a CCS at the cathode?
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Old 12th January 2010, 03:50 AM   #32
jrenkin is offline jrenkin  United States
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My thought was to go with something like Paul's design with the mosfet follower. The tubelab site mentions that there really aren't other suitable tubes than the 5842, but I am not sure why. Seems that the D3a might be fine.
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Old 12th January 2010, 05:38 AM   #33
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jmillerdoc,

yes, there is a junction between g2, 27K, 47uF and top of zeners, to obtain about 150V on g2 D3a and 170V on g2 EL802.

Philippe
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Old 12th January 2010, 06:44 AM   #34
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Default Somedays it just all sounds kinda like this to me

There are days when all this starts sounding like the following to me:
http://www.youtube.com/watch?v=rLDgQg6bq7o%26NR=1
But just sometimes....
Jeff
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Old 12th January 2010, 06:46 AM   #35
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Sorry, that link seems to be broken, try this one...
YouTube - Turbo Encabulator
Again, lately I feel like I have been following along and wondering if I really understood that....???
Jeff
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Old 12th January 2010, 08:47 AM   #36
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Quote:
Originally Posted by ThorstenL View Post
I suggest you keep the fixed bias on the output. With a sufficiently quiet PSU and high quality caps from +B to Common this sounds unusually good (I have done it both ways often enough by now).

I'd even go fixed bias on driver stage, if it was me...

Ciao T
Thanks Thorsten - I will retain active bias. I am not sure what you mean by "high quality caps from +B to Common" though. I also have a few other questions, so will start a new thread in the next few days. I am tired of procrastinating about a build - the time has come for doing.

Cheers
Raymond
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Old 12th January 2010, 01:37 PM   #37
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Hi,

Quote:
Originally Posted by rcdaniel View Post
Thanks Thorsten - I will retain active bias.
You can use traditional fixed bias as well, it is just annoying to regulary have to re-adjust things.

Quote:
Originally Posted by rcdaniel View Post
I am not sure what you mean by "high quality caps from +B to Common" though.
Common or Ground is in the fixed bias amplifier the "reference" to which the 300B Cathode is directly connected. The signal current in the output loop circulates from cathode via the anode and output transformer back to the cathode.

As a result the capacitor in the power supply that offers the lowest impedance will carry this current. If the supply has a high impedance element (choke, electronic choke etc.) this capacitor will be the final PSU capacitor from the supply to the Transformer and 300B anode to ground. The differences in quality of this capacitor are quite audible.

Ciao T
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Old 12th January 2010, 07:22 PM   #38
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Quote:
Originally Posted by ThorstenL View Post
As a result the capacitor in the power supply that offers the lowest impedance will carry this current. If the supply has a high impedance element (choke, electronic choke etc.) this capacitor will be the final PSU capacitor from the supply to the Transformer and 300B anode to ground. The differences in quality of this capacitor are quite audible.

Ciao T
Hi, I am wondering what properties are the most important when you say it is audible. Losses, ESR, etc....what in you opinion makes the capacitor "high" quality in this regard. I guess I should also ask what your preference is in this final cap....Poly, poly in oil, Black Gate or other special composition electolytic types (SimilicII, etc..)? I have used either a Film in oil ASC type or Solen PP as my final cap before the output stage with excellent results. I have yet to try a BG mostly due to cost of the power tank types. Are there other electrolytic parameters that I should look for if I choose to use an electrolytic....as stated before, there are different specs I am aware of like the ESR but I am unsure which of these actually make the cap "good sounding"

I have recently considered the Obbligato Film power filter caps but I have a premonition they will behave like a Solen or any other descent PP cap.

Jeff
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Old 12th January 2010, 07:40 PM   #39
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Quote:
Originally Posted by ThorstenL View Post
Common or Ground is in the fixed bias amplifier the "reference" to which the 300B Cathode is directly connected. The signal current in the output loop circulates from cathode via the anode and output transformer back to the cathode.

As a result the capacitor in the power supply that offers the lowest impedance will carry this current. If the supply has a high impedance element (choke, electronic choke etc.) this capacitor will be the final PSU capacitor from the supply to the Transformer and 300B anode to ground. The differences in quality of this capacitor are quite audible.

Ciao T
Thanks again Thorsten.

I had not seen (or more likely forgotten!) the term "common" used in this context; signal "ground" I kind of understand. Your description of the current loop in fixed bias also helps. I have not really looked at fixed bias designs seriously; seems it is time for me to do some reading.

Jeff,

IIRC Thorsten does not care for Solen metalised, but has some regard for the Obbligatos... at least the film in oils. That said, Thorsten appears to have changed some of his design preferences during his absence for the forums, so this may have changed as well. As ever, impermanence (change) is a constant.

Cheers
Raymond
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Old 14th January 2010, 10:40 PM   #40
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Quote:
Originally Posted by jmillerdoc View Post
Pauldune,
Thanks alot it looks great. I think the only difference is I have 0.33 Russian PIO's I will be using.

I was wondering if I should maybe place some resistance to bring the voltage down some on the CCS....maybe a B+ of around 300-325 rather than the full 435v for the output stage. This should decrease the heat and sinking I will have to use for the CCS. Or am I mistaken? If it is a good idea to bring the B+ down a bit for the CCS is there a particular voltage you would target?

Is there any way to use the CCS for the 300B stage as well? Just curious, I haven't seen this before and I am sure there is a reason. Maybe it is just I haven't seen it too. Not sure. Either way, any use for this for the output stage at all?

Jeff
The 0.33 pio's wil be more than fine.

The lowering of the B+ seems like a good thing to do, but when I tried it, the CCS I used (the ixys 10m45s) started oscillating. Directly connected to B+ it didnt do that, so for me the choice was simple. Anyway, its not that much power, so with a nice heatsink it shouldnt be any problem.
With 300v B+ you still have about 200v pp swing, and thats more than you need to drive the 300b. just put a 6k8 10W resister between B+ and the CCS.
But I wouldnt do it, you shift 2.7 watt from the CCS to the resistor. Plus you need an extra resistor and an expensive cap.

You can't use the same CCS for the driver and the end tube at the same time. You can however use another one. You have to use the output stage parafeed with a decoupling cap. But it wont do the same thing as with a driver. With a driver, because of the very high impedance of the next stage, the current through the drivertube stays practically constant. The loadline becomes a horizontal line.
With an end-tube, its a different story. the current through the ccs remains constant, and for dc it flows all through the tube, but for ac it splits. part goes through the tube and part goes through the output transformer. so the load line isnt horizontal, because the anode current isnt constant. What the loadline would look like, I wouldnt know, but i suspect something in between.

One of the advantages of using a ccs for the output stage is the decoupling from the powersupply, its not in the signal path anymore. also, the powersupply sees a constant load; it doesnt matter if you play hard or soft, transients etc.

I tried to use those qualities in my design:here
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