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Old 4th March 2006, 12:25 PM   #1
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Default Upper and lower triode in E88CC

Hi all,
I've built Thorsten's tube stage for TDA1545 (see pic), but at the time of bulding it I was sure that the triodes in the envelope were interchangeable.

Now, after reading some datasheets I'm not that sure...

Amperex datasheet for the 6DJ8 says that triode 1(pins 6,7,8) should be the grounded cathode amplifier and that triode 2 (pins 1,2,3) should be the grounded grid amplifier in a theroretical cascode amp.

All the datasheet I read about E88CC/6922 don't mention this limitation.

My implementation, by Murphy's law, uses the triode "2" as the first stage (at lower voltage) and the other for the output cathode follower.

Now the question is: am I worrying too much or it is mandatory (or at least suggestable) to re-do all the work?


Cheers

Andrea
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Old 4th March 2006, 01:03 PM   #2
EC8010 is offline EC8010  United Kingdom
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The crucial thing is leakage between the heater and the cathode. As you increase the voltage between them, the leakage current rises. People tend to treat Vhk as a hard limit, but it's really very soft. Given that your DAC is stereo (making an assumption here, but I expect it is), the best solution is to use one envelope for the cathode followers and another for the gain stage. Then, you feed the heater for the cathode followers from a supply sat on 120V or so (thus dramatically reducing Vhk), and the other envelope from a heater sat on 0V to 40V. (+40V turns off the diode between heater and cathode.)
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Old 4th March 2006, 01:52 PM   #3
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Quote:
....Given that your DAC is stereo (making an assumption here, but I expect it is), the best solution is to use one envelope for the cathode followers and another for the gain stage. Then, you feed the heater for the cathode followers from a supply sat on 120V or so (thus dramatically reducing Vhk), and the other envelope from a heater sat on 0V to 40V. (+40V turns off the diode between heater and cathode.) [/B]

I'm using this solution in my implementation of this I/V stage (the Dac is stereo of course).
Due to the fact that I've drawn a pcb for this stage, I've found this solution more comfortable, not only to provide the right heaters reference but also to draw a compact layout.
The ground grid stage has the haters ref. set to 0V, the cathode follower to 110V.

Now a question that is a bit OT. Is there a miniature double triode that is similar to the ecc88?
I was thinking to use the 6111Wa in place of the ecc88.
I've found the gain of this stage a little too high in my chain (I've try to chance the anode load of the first stage) so the lower mu of the 6111 could be an extra bonus for me...

Mark
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Old 4th March 2006, 03:43 PM   #4
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Mark,

Perhaps the 6GM8/ECC86, which is a LOW (max. 30) voltage relative of the 6DJ8/ECC88 would work for you. Like the ECC88, the ECC86 is high gm/low Rp, but it has a lower mu.

A different tack is to leverage the "excess" gain that the 6DJ8 yields. Step down trafos could be placed at your unit's O/Ps. The spare Volts get converted into improved drive capability.
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Old 4th March 2006, 04:28 PM   #5
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Originally posted by Eli Duttman
Mark,

Perhaps the 6GM8/ECC86, which is a LOW (max. 30) voltage relative of the 6DJ8/ECC88 would work for you. Like the ECC88, the ECC86 is high gm/low Rp, but it has a lower mu.

A different tack is to leverage the "excess" gain that the 6DJ8 yields. Step down trafos could be placed at your unit's O/Ps. The spare Volts get converted into improved drive capability.

Hi Eli, thank you for the ideas.
Well, when I've said that I'd like to find a miniature substitute I was thinking to something like these one:

http://frank.pocnet.net/sheets/079/6/6111.pdf

http://frank.pocnet.net/sheets/137/6/6814.pdf

These miniature tubes should be a good solution to modify the output stage of a commercial player due to the low power dissipation and really small dimensions.

Regarding the gain of the first stage I'm thinking to a different solution:

1)add a ccs as anode load. In this way I'll provide better PSRR, higer gain, better linearity and the chance to fix in a better way the input offset from the dac ( I need 3.3V at the cathode of the input stage if I want to connect the dac in dc)
The gain of a grounded grid is (mu+1)Ra/Ra+rp so, with a good css, now is near (mu+1).

2) to lower the gain, distortion, noise and output impedance, I'm thinking to add a parallel feedback path, from plate to grid.

Look at the pic Ive attached

Mark
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Old 7th March 2006, 06:58 AM   #6
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Default Yes but is there any difference....

Quote:
Originally posted by EC8010
The crucial thing is leakage between the heater and the cathode. As you increase the voltage between them, the leakage current rises. People tend to treat Vhk as a hard limit, but it's really very soft. Given that your DAC is stereo (making an assumption here, but I expect it is), the best solution is to use one envelope for the cathode followers and another for the gain stage. Then, you feed the heater for the cathode followers from a supply sat on 120V or so (thus dramatically reducing Vhk), and the other envelope from a heater sat on 0V to 40V. (+40V turns off the diode between heater and cathode.)

... between the triodes in the same envelope (in the cathode-heater ratings)?
This is the point I wanted to know...

Cheers

Andrea
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Old 7th March 2006, 08:39 AM   #7
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Some data sheets suggest that there is a difference between the Vhk ratings of the two triodes within the same envelope, and others don't. I say it's better not to stress that particular rating at all. Use a separate heater supply for the valve with an elevated cathode.
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Old 8th March 2006, 06:51 AM   #8
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Default Got the point...

Hi,
so unless you have good reasons not to do so (e.g. Aikido style amps) it is always better to reference the heater to a voltage close to the cathode(s)...

Thanks

Andrea
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