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Old 25th September 2007, 04:35 PM   #11
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Quote:
Originally posted by anatech
Hi rich,
How many people do you think read tube manuals these days?
-Chris
Hang on .. Measuring NOS GE 6550's I got 1.3A heater current at 6.3V..(def on the low side and yes I measured it correctly). Another wild upwards variation is with the KT90 where I got 2A. (That's more like it).
The RCA tube manual for the 6550 quotes 1.6A at 6.3V, so what's up ?
Sounds like I've been given out of spec tubes, but my supplier says Nup. >Checking with another = same reply.
Simply have to accept.

richj
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Old 25th September 2007, 05:11 PM   #12
anatech is offline anatech  Canada
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Hi rich,
Well, that's the thing. Nominal specs should only apply to one brand. The type registration should list the maximum heater current allowed (in a perfect world). Types like 12AX7 heater currents should always be bang on.

Quote:
Another wild upwards variation is with the KT90 where I got 2A.
Well, a KT90 isn't exactly a 6550A. A KT88 should be closer - no?

If I look up 6550's in GE and RCA, I get (GE) 6.3V 1.8A and (RCA) no listing as of RC-27. A Genalex lists the KT88 as 6.3V and 1.6 amperes also.

I expect there may be some differences in tubes intended for parallel heater operation simply because the cathode coating recipe used by some manufacturers may require a little more current to emit the required amount of current. Your GE's may be late production which allowed them to drop the heater temperature a little. This would be a good thing as far as reliability is concerned.

I am looking at a matched quad of Westinghouse 6550A's in their original boxes. All broken when I moved.

I used to have a Westinghouse tube manual (still at old shop). Anyone know where I can get a copy? I would have looked up their spec too. I'll bet it's 1.6 amperes.

-Chris
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Old 25th September 2007, 05:55 PM   #13
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Quote:
Originally posted by anatech
Your GE's may be late production which allowed them to drop the heater temperature a little.
-Chris

SNAP. I'll leave it at this ! Incidentally the B version with button getter/clear glass made by Svetlana also behaves the same. A copy `?
richj
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Old 25th September 2007, 06:02 PM   #14
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Quote:
Originally posted by richwalters


Not from this typical output stage;
richj
If you read the original post by hihopes you may notice that his amp uses Mosfets, not tubes, as the output devices. The EL34's are voltage amplification stages wired as triodes.

As you noted, an AB output stage will clip sooner and have more distortion when heater voltage is low; however, his description of the problem he is having is not that .

You are talking about a tube output stage; his is solid state.
Your scenario involes clipping, his problem is crossover distortion.
As I noted twice in earlier posts, the EL34 heater voltage is low,
and should be corrected. It is not the cause of his problem, however.
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Old 25th September 2007, 06:11 PM   #15
anatech is offline anatech  Canada
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Hi diy4,
Quote:
If you read the original post by hihopes you may notice that his amp uses Mosfets, not tubes, as the output devices. The EL34's are voltage amplification stages wired as triodes.
I know that. It's wise to fix known problems as you find them. Amazingly, other faults tend to disappear sometimes.

With low emission, he may be having trouble with the mosfet gate charge issue. It is entirely possible that heater temperature may make this less of a problem.

One thing I can not understand is this. Why use an EL34 when a 6L6 type has much less heater current demands? There are other tubes that would probably suit this use even better. An EL34 would be my last choice here.

Quote:
As I noted twice in earlier posts, the EL34 heater voltage is low,
You are certainly not the only one who has noticed this.

-Chris

Edit:
Hi Rich,
Quote:
Incidentally the B version with button getter/clear glass made by Svetlana also behaves the same. A copy `?
With any luck! The Electroharmonix brand has been very good this way. May they continue to bring us good tubes at good prices!
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Old 25th September 2007, 06:15 PM   #16
anatech is offline anatech  Canada
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Hi hihopes,
Is there any way you can post the schematic or a link to it? This will help us help you out better.

-Chris
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Old 25th September 2007, 09:09 PM   #17
hihopes is offline hihopes  South Africa
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Thanks for all the input guys. Sorry, I would love to get hold of a full schematic myself. (The guy who designed and built the amp gave me one about 10years ago when i bought the amp, but my overzealous wife must have "tidied" it away into file no.13. I have asked him for a copy on a few occasions, but he is pissd that I lost the one he gave me. He is overseas for another month or so. What is it that you want to look at? Can I describe something for you?
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Old 25th September 2007, 09:13 PM   #18
hihopes is offline hihopes  South Africa
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I think I need to clarify that the problem I am trying to address is not very obvious, nor is it very often apparent. The information that has been provided by so many here seems to confirm that it is most probably the mosfets that are ringing/oscillating under very particular circumstances only.
BTW anatech, the LM317's are fine. 4 x Lm317's for a total of 3amps @ 6v drop should not have been a problem according to my calculations. (Of course, i could always be proved wrong) It was the heaters of the EL34's that suffered. Maybe they drew too much current when they were cold? (I guess 4 x LM317's should be able to supply 6A minimum)
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Old 25th September 2007, 09:16 PM   #19
hihopes is offline hihopes  South Africa
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Correction. I don't know that it was the heaters that suffered. When I power those tubes up, the heaters seem to be operating normally, but the outpust from the tube is only a fraction of what it previously was. So I guess its the cathodes that have suffered?
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Old 25th September 2007, 09:22 PM   #20
anatech is offline anatech  Canada
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Hi hihopes,
Quote:
4 x Lm317's for a total of 3amps @ 6v drop should not have been a problem according to my calculations.
If each heater runs at 1.6 amperes then you have a problem. You need to confirm the voltage while it's running. Also, they may go into limiting as they heat up.

Quote:
So I guess its the cathodes that have suffered?
Without more information it's impossible to say. I can't see why they would suffer at all.

-Chris
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