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Re: 300B SE Project, Part 7 - Common cathode 6C45Pi triode driver [message #31775 is a reply to message #31774]

Mon, 27 February 2006 13:44

Damir
Messages: 1005
Registered: May 2009

Illuminati (2nd Degree)

I`d like to Doug explain it, but I`ll try. It probably means that it isn`t "normal" signal (only of very HF) present on the input grid and amplified. It can be "extreme" and so "high" that the stage "overloads" (grid current). Sound still can "pass through", but "weak, undynamic, distorted, glassy..."
Everything can oscillate, no matter class of work, or device used - but high gm parts more likely. Even anode-stopper (between the CCS and anode, say 100 Ohms) isn`t over the top measure...

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Re: 300B SE Project, Part 7 - Common cathode 6C45Pi triode driver [message #31776 is a reply to message #31775]

Mon, 27 February 2006 19:01

Manualblock
Messages: 4973
Registered: May 2009

Illuminati (13th Degree)

So how does these measures affectr the sound? Do grid stoppers have any effect on sound?
Can the amp ocsillate more easily with a CCS load or can it oscillate easier with a resistor loaded plate?

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Re: 300B SE Project, Part 7 - Common cathode 6C45Pi triode driver [message #31777 is a reply to message #31776]

Tue, 28 February 2006 03:26

Damir
Messages: 1005
Registered: May 2009

Illuminati (2nd Degree)

Huh, MB... of course that probability of oscillations is larger in the circuit consists of one tube and two mosfets then in the circuit with just one tube. But, it`s a simple circuit(s), and there`re no reason why we couldn`t make it to work properly...

Grid stoppers - they have impact on sound, larger values sounded more "soft" and "rolled off". I used 1kOhms on both driver and power tube position, but now I lowered this. Currently I`m using 560 Ohms on 300B grid and 150 Ohms on 12SN7- why? I have those on hands... The "goal" is the minimum value that can still performs its job - to "damp" the resonant circuit consists of (Miller) input capacitance and inductance in the grid wiring.

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Re: 300B SE Project, Part 7 - Common cathode 6C45Pi triode driver [message #31778 is a reply to message #31777]

Tue, 28 February 2006 06:21

Manualblock
Messages: 4973
Registered: May 2009

Illuminati (13th Degree)

Ahh; thats the first time I heard exactly why we use the grid stoppers.
Now it is making more sense to me. So what happens if the depletion mode mos-fet overheats?

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Re: 300B SE Project, Part 7 - Common cathode 6C45Pi triode driver [message #31779 is a reply to message #31778]

Tue, 28 February 2006 11:22

Damir
Messages: 1005
Registered: May 2009

Illuminati (2nd Degree)

Similar as with overvoltage - I burned one, maybe both. B+ is over 400V, little mistake and puff... Luckily, I have a few more, but it`s a pain to resolder it from the PCB/heatsink. Now I must waiting for another group buy of DN2540N5...

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Re: 300B SE Project, Part 7 - Common cathode 6C45Pi triode driver [message #31781 is a reply to message #31779]

Tue, 28 February 2006 12:25

Manualblock
Messages: 4973
Registered: May 2009

Illuminati (13th Degree)

O'Kay; thats what I needed; as long as I know that a catastrophic failure of the mos-fet won't take out my transformers or my speakers then I am o'kay with it. I was told that could be a problem so that is why all these questions. You know those OPT's ain't cheap! Not to mention the loudspeakers.

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Update :-) [message #31796 is a reply to message #31765]

Fri, 03 March 2006 13:11

Damir
Messages: 1005
Registered: May 2009

Illuminati (2nd Degree)

Today I received my third pair of "Sovtek" 6C45Pi-E. It`s a new production tubes, former pairs were NOS from 70`s/80`s. This pair is well matched and closer to the specs, Ua is about 160V, and µ=43/45. I tried changing Ia through the tube(s) in 1mA steps. IME - lower current "sounds" darker and less sharp/rough. About 9-9,5 mA sounds best for my ears, although I can "swear" that I heard a little sound differences between the 6 tubes I have, too.
We can (safety reasons) "drop" the B+ a bit, on about 400V. I didn`t hear the difference.

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