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Old 26th October 2006, 08:28 AM   #21
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Cascode, eh Giaime? We must be drinking the same wine

I'm trying to keep this fair minimalist. Cascode input too, 6AQ4 (EC91) bottom, ECC88 top. Sure it has a lower Gm than using a whole ECC88 on the input, but the extra mu (100) and far lower noise and microphonics is worth the tradeoff. Total gain of the input cascode is 180, so that sure won't be a problem. 3 tubes/channel, unless incorporated into an existing preamp with decent linestage, then there'll be only 2 tubes/channel
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Old 26th October 2006, 01:20 PM   #22
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Great minds and all that. The preamp I just retired to make way for the new was a FET-tube hybrid cascode input, the tube being an ECC88 variant, feeding passive RIAA, and then a Berning-type tube-FET gain stage, with a buffer very similar to the Heretical.

Hey, this is going to be a very interesting winter, guys!
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Old 26th October 2006, 01:28 PM   #23
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Greg, you made me look up that 6AQ4. Interesting tube, an equivalent noise resistance of 400R, input-referred. That should be fine for MM and higher output MC. Have you tried it "straight"in the first stage or was the ca. 250pF of Miller too much to deal with? The reason I ask this is that at 10mA, the rp is 12k, which seems rather high for the bottom bit of a cascode; that's reflected in the gain, which is only a few dB higher in the cascode than the "straight" tube, CCS-loaded.
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Old 26th October 2006, 07:29 PM   #24
Giaime is offline Giaime  Italy
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Quote:
Originally posted by Geek
Cascode, eh Giaime? We must be drinking the same wine
I think I'm drinking too much of that wine, because I'll go with an hybrid cascode (aaargh heretical! ) and I'm strongly thinking of learning good solid state design to make a good sounding discrete all FET phono amp
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Old 27th October 2006, 12:13 AM   #25
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Giaime, you want me to go on heart pills, don't you


Hi SY,

Quote:
Originally posted by SY
Greg, you made me look up that 6AQ4. Interesting tube, an equivalent noise resistance of 400R, input-referred. That should be fine for MM and higher output MC. Have you tried it "straight"in the first stage or was the ca. 250pF of Miller too much to deal with? The reason I ask this is that at 10mA, the rp is 12k, which seems rather high for the bottom bit of a cascode; that's reflected in the gain, which is only a few dB higher in the cascode than the "straight" tube, CCS-loaded.
I'm using a Shure cartridge which can handle 250pF OK, but after reading about the responses at various loads, it seems that one is better with as little C as possible. The resultant peak is over 32Khz, which is way out of the EQ's band and is moot. I might try a switchable loading C switch, for full control. Have a Grado F3+ too, but want to find a belt-drive TT to put that one in
(I'm too mechanically inept to build one)

If you have 10mA through a 6AQ4, you've probably killed it. Noise goes up exponentially above 4mA (played with this one for a mic pre.. dyn-o-mite at 1.3-2mA). I'm running it at 1.4mA with an LED on the cathode.

Should be able to post a circuit by tomorrow
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Old 27th October 2006, 12:42 AM   #26
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Cool! I've never used the tube, just going off the data sheet. That's a great piece of practical info.

Are you giving the LED a bit of extra current to get its impedance down?
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Old 27th October 2006, 01:24 AM   #27
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Quote:
Originally posted by SY
Cool! I've never used the tube, just going off the data sheet. That's a great piece of practical info.
Glad you found it useful. Email me your addy, I'll send you some


Quote:
Originally posted by SY
Are you giving the LED a bit of extra current to get its impedance down?
No. Good idea though. Would I do this by adding a resistor from the LED/cathode junction to B+?

Cheers!
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Old 27th October 2006, 01:32 AM   #28
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Yes, that's it. A CCS will be even better than a resistor. What you don't want to do is inject power supply noise, so the high Z of a cascode CCS (I suspect you might have a board or three laying around!) will be beneficial here.

Now, it may well be that decreasing the dynamic impedance of the LED might have the opposite effect- rdf did some interesting measurements and found that there was some distortion cancelling in some setups.

I'll be delighted to take a couple tubes off your hands!
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Old 27th October 2006, 02:39 AM   #29
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The 6AM4 has similar specs to the 6AQ4 (though no noise spec), and is intended for the same type of grounded grid RF service. Unike most other tubes, the elements are mounted horizontally instead of vertically, so it looks like a little barrel inside the glass envelope. The glass itself is the small size, like a 6AK5. Since I have some of the 6AM4s around, I'm tempted to build up a phono preamp with them based on the comments here.
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Old 27th October 2006, 07:10 AM   #30
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Hi SY,

Quote:
Originally posted by SY
Yes, that's it. A CCS will be even better than a resistor. What you don't want to do is inject power supply noise, so the high Z of a cascode CCS (I suspect you might have a board or three laying around!) will be beneficial here.

Now, it may well be that decreasing the dynamic impedance of the LED might have the opposite effect- rdf did some interesting measurements and found that there was some distortion cancelling in some setups.

I'll be delighted to take a couple tubes off your hands!
OK, we'll give it a go and see, thanks!

Got a couple wrapped up and ready to go off to you


Quote:
Originally posted by wrenchone
The 6AM4 has similar specs to the 6AQ4 (though no noise spec), and is intended for the same type of grounded grid RF service. Unike most other tubes, the elements are mounted horizontally instead of vertically, so it looks like a little barrel inside the glass envelope. The glass itself is the small size, like a 6AK5. Since I have some of the 6AM4s around, I'm tempted to build up a phono preamp with them based on the comments here.
That's the super thing about those V/UHF triodes. Almost all of them have some use in audio and I'd say a good 1/2 are suitable for phono service

I call those TV tubes "hidden treasures", since it's unlikely the audio guru's will pay any attention to them, ever, despite the electrical/physical superiority of many. After all, it's just a lowly, worthless TV tube....
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