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Old 20th January 2003, 09:20 PM   #11
dhaen is offline dhaen  Europe
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Hi 7n7,

We eagerly await..
Who's the guru? It's not the wife is it

Cheers,
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Old 20th January 2003, 09:57 PM   #12
7N7 is offline 7N7  United Kingdom
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Quote:
Originally posted by dhaen
Hi 7n7,

We eagerly await..
Who's the guru? It's not the wife is it

Cheers,
Wife? WIFE??

No woman would put up with this.

The guru's identity has to remain a secret - sorry!

7N7
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Old 20th January 2003, 11:02 PM   #13
7N7 is offline 7N7  United Kingdom
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Default Still not there

Update.

I have at least located the problem; curing it is proving elusive.

The problem relates to the g2 supply to the pentode cathode followers. At -12dB input all is well. At -10dB input the whole thing goes beserk; instead of +96V at g2, suddenly I get +200V, which rather changes things.....

More investigation required

7N7
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Old 21st January 2003, 07:28 AM   #14
dhaen is offline dhaen  Europe
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Hi,

Quote:
At -12dB input all is well. At -10dB input the whole thing goes beserk;
Then listen at low volume..

Come on, wheres the schematic then?

Cheers,
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Old 21st January 2003, 09:06 AM   #15
7N7 is offline 7N7  United Kingdom
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Quote:
Originally posted by dhaen
Hi,



Then listen at low volume..

Come on, wheres the schematic then?

Cheers,
As I said, there will be a circuit diagram when I have the thing working.

Only then can I draw it up, since component values and voltages are bound to change.

And you wouldn't want a circuit diagram without voltages would you?

7N7
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Old 22nd January 2003, 10:00 PM   #16
7N7 is offline 7N7  United Kingdom
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Default Update 2

This amplifier has been something of a struggle.

I have not been able to solve the problem with the pentodes, so for now they are triode-strapped and running at 13mA each.

Initially results were rather poor: the cathode feedback prevented the amplifier from producing more than about 1W, so I removed it. This got me up to about 7W, unfortunately -5.5dB at 10kHz: OK for a very early cassette player or an AM radio I suppose.

The fault lay with the output transformer. I have one other I could try: now this one is a rather crummy old thing, but I can now get 12W (target) with only 295V HT.

Initial tests suggest that distortion is quite low at this level (0.5%) but this wants checking again - if only I could depend on the mains!

Next move is to remove/change all the grid stoppers etc., I included when I detected oscillation, mount the transformer and get those pentodes working.

7N7
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Old 24th January 2003, 09:57 PM   #17
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Default 13D3/D3A/6CK4 ANother Update

I finally found out what was wrong with the pentode cathode followers; that person deserves to be sacked; so I sacked myself for a stupid mistake.

I eventually got 12W out of it, but THD was disappointing - that sacked person also made a mistake in calibration. THD at 12W is 3% but mostly 2nd & 3rd harmonic. The higher harmoincs were very low.

So I put it all back together and now the mains transformer has thrown in the towel.

A friend is bring another transformer which will necessitate re-jigging..

And there will have to be an outboard heater transformer.

I may try an alternative output stage: a single 6528 double triode in push-pull.

However, several things have been established: you can make a diff pair with active loads and a CCS underneath - and it works beautifully as designed. The D3A makes a superb cathode follower; in fact the driver stage is a success.

I have been let down by the output transformer and now by the mains transformer

Oh well no-one ever said it was easy - onwards and upwards

7N7
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Old 26th January 2003, 11:58 PM   #18
7N7 is offline 7N7  United Kingdom
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Some decent progress to report.

The new transformer arrived yesterday, and very fine it is. It has 2 x 126v outputs @250mA each plus a 208v output at 160mA and a 204v output at 50mA.

So I built two bridges and by combining the 208v and one of the 126s, together with a 4uF cap folowed by the 20H choke and a 220uF capacitor, I can now have 320V at 200mA.

The pentode cathode followers are now working as intended.

I have replaced the 4 6CK4s with a 6528A. This is a big regulator double triode. It is rated at 30W per section, has graphite anodes. Gm is 35 and mu 9. It requires 6.3v @ 5A to light it up.

I am running it p-p into 4k a-a. Bias is circa -37V and with +5dB input I am getting 16.8W into 4.7 ohms just before clipping.

The cheapo antique output transformer offers a choice of primaries: 4k, 6k, 8k.

Tomorrow, I shall do some distortion testing and try alternative loads.

Once all is finalised, I'll measure the voltages, draw it and post the circuit diagram.

7N7
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Old 27th January 2003, 12:23 AM   #19
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Default 6528A

Hi,

Well done.

That is another valve that is hardly used these days.
Along the lines of the 6336A but with higher mu if I remember correctly.

Happy building,
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Old 27th January 2003, 09:46 AM   #20
7N7 is offline 7N7  United Kingdom
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Default Re: 6528A

[QUOTE]Originally posted by fdegrove
Hi,

Well done.

That is another valve that is hardly used these days.
Along the lines of the 6336A but with higher mu if I remember correctly.

Happy building,
[/QUOTE


Thanks.

6528A is similar in appearance to 6336A and indeed has the sme Pa rating.

As you say, mu is higher (9 instead of 2) and gm is much higher at 35mA/V max. This means an ra of about 245 ohms.

The price as usual for all this is Ca-g, which is claimed to be 23.8pF (but is probably more). This means that a cathode follower is de rigeur to drive it.

7N7
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