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Old 9th June 2005, 01:38 PM   #21
Danko is offline Danko  Hungary
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Sy,

- why did you put a 4uF/400V in the "B+" ? What if I put there a 200V capacitor?
- what voltage rating should be the output capacitor?
- what voltage rating should be the input capacitor, if I would like to omitt the transformer ?
- what specific transistors do you recomment for the current source?
- If I have a separate winding on my transormfer for heating the tube, do I need that 2x120 resistors paralell near the current source?

Thanks! :-)


P.S.: I tried to edit my previous post, but it took me too much, and the 30minutes editing period expired, I couldnt edit that post
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Old 9th June 2005, 01:56 PM   #22
SY is offline SY  United States
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You're right, I forgot to spec the transistors. I used NTE199. There's nothing special about that choice- it was based on what rattled out of the coffee can first when I shook it. Anything with a BVceo of 50V or better, an ft better than 10MHz, and a beta of 100 or greater will work. The NTE is a 50V device, ft of 90MHz, beta of 400, and costs under a buck.

The plate bypass cap can absolutely be a 200V device. I used a 400V cap because that's what I had on hand. If I were to rationalize, I'd say that it was to give me more flexibility on tube choice. For this tube, the extra voltage rating isn't needed.

The output cap will never see more than 15V. It can have any voltage rating you like. I had some Rifa caps with a 160V rating that I used, but a 63V Wima will be perfect there.

Likewise, any input cap will never see high voltage.

The heaters here are running on DC. If you use your winding (presumably 6V?), rectify and smooth it, chances are that you'll still need the dropping resistors, but you'll have to recalculate the value.
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Old 9th June 2005, 01:58 PM   #23
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Quote:
Isolation made a big difference- the horrible grounding of my satellite TV system made the old one buzz a bit. That's gone now. I was probably not clear enough, and I'll edit the post later to fix this, that the input and output grounds are totally isolated from one another. That's got nothing but good sonic consequences: the disappearing buzz bears testament to that.

Very interesting. I also believed that galvanic isolation must be beneficial. I had my TVC hooked with an isolated ground for a while but in some source configurations this added a noticeable hum. Before switching to a common ground i listened very carefully and could not hear any difference at all when the input and output grounds were shorted. Ok, the hum disappeared when shorting. More recently i have been using the TX102 in an autoformer configuration in which there is obviously a single earth connection with great success.

These experiences make me question any inherent advantages in keeping earth loops galvanically isolated. It makes good sense in theory but success seems dependent upon the specific curcumstances.
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Old 9th June 2005, 02:05 PM   #24
Danko is offline Danko  Hungary
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[QUOTE]Originally posted by SY
You're right, I forgot to spec the transistors. I used NTE199. There's nothing special about that choice- it was based on what rattled out of the coffee can first when I shook it. Anything with a BVceo of 50V or better, an ft better than 10MHz, and a beta of 100 or greater will work. The NTE is a 50V device, ft of 90MHz, beta of 400, and costs under a buck.


What about BC546B ?
Quote:

The plate bypass cap can absolutely be a 200V device. I used a 400V cap because that's what I had on hand. If I were to rationalize, I'd say that it was to give me more flexibility on tube choice. For this tube, the extra voltage rating isn't needed.

Thanks!
Quote:
The output cap will never see more than 15V. It can have any voltage rating you like. I had some Rifa caps with a 160V rating that I used, but a 63V Wima will be perfect there.

Likewise, any input cap will never see high voltage.
What do you recommend? What "MKx" type?
Or it doesnt matter?

Quote:
The heaters here are running on DC. If you use your winding (presumably 6V?), rectify and smooth it, chances are that you'll still need the dropping resistors, but you'll have to recalculate the value.
Yes, the heater is running from stabilized DC. Do I need thoose series resistors, even if I'm using soft start for the heating?

In Post #20, you wrote:
Quote:

Yes, you can build one without the input transformer, but you may find it of great value even with your simple setup. I'd probably RC couple if I were to go the non-transformer route.
You mean under "RC couple" a single series capacitor with the input, and a resistor, between the GND and the capacitor, aren't you?
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Old 9th June 2005, 02:10 PM   #25
SY is offline SY  United States
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Choice of caps is a religious issue. I'll stay neutral on that one- my own first choice would be FKP, followed by MKP, but that's just my religion. YCMV.
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Old 9th June 2005, 02:34 PM   #26
Danko is offline Danko  Hungary
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Originally posted by SY
Choice of caps is a religious issue.
Like some people are religious about cables :-))

Thanks your advises, I will choose MKT MKS4.
It's 10uF/100V (theresn't smaller voltage rated in this capacity)

What about that resistor, at the heating-part?

What do you think about putting BC546B into the constant current generator?

What values do you recommend into the input R-C ?
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Old 9th June 2005, 05:04 PM   #27
SY is offline SY  United States
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I just used some off-the-shelf rectangular power resistors for dropping the heater voltage. In theory, since the ECC88 has parallel heaters, it should be fed by a voltage source, not a current source. I'm not convinced that it's a big deal either way, so a resistor splits the difference in a Solomonic fashion.

Input RC? My personal inclination would be to use 0.1uF and 1M, inserted between the wiper and the grid. That choice is based on careful consideration of the position of the low frequency rolloff traded off with grid leakage current and my religious belief in not using large coupling caps whenever possible. And dictated mostly by what I have in my coffee can, which at the moment is flush with 0.1uF caps.
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Old 9th June 2005, 05:32 PM   #28
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Originally posted by SY
No. For 10K/1nF worst-case load, the output transformer brings nothing to the table that I can see.
Brings nothing to the table? Well, unless you're assuming everyone has input trannies on their amps, how 'bout a big plate full of isolation?

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Old 9th June 2005, 05:41 PM   #29
SY is offline SY  United States
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Don't you think it would be better, if that's where a system problem is, to add the input transformers to the power amps? We're diyers. We can do that, we've got the technology.

IIRC, input transformers do a better job of isolation. But if that's not right, I'm confident you'll gleefully correct me.
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Old 9th June 2005, 08:12 PM   #30
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Quote:
Originally posted by SY
Don't you think it would be better, if that's where a system problem is, to add the input transformers to the power amps? We're diyers. We can do that, we've got the technology.

IIRC, input transformers do a better job of isolation. But if that's not right, I'm confident you'll gleefully correct me.
Well personally I prefer to use input transformers when it's practical. But trying to incorporate an input transformer into an existing piece of equipment often isn't terribly practical compared to including say an output transformer in a ground-up project such as this one.

Anyway, I was just addressing your comment that you didn't see that an output transformer would bring anything to the table.

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