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24th Jan 2006, 1:07 am | #21 |
Dekatron
Join Date: Oct 2003
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Re: Motorboating
Make sure the HT is not too high. It could just be that causing the valve to 'take off'.
Rich.
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24th Jan 2006, 7:32 am | #22 |
Nonode
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Re: Motorboating
Yes. I realise the HT is too high now. The diagram reccomends 200V and I have actually 250V so I need to lose 50V. What sort of current will the ECL80 drawfrom the HT line? I need the figure to select a resistor.
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24th Jan 2006, 8:18 pm | #24 |
Rest in Peace
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Location: Dartford, Kent, UK.
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Re: Motorboating
I would suggest around 20 mA total current, given the low ratings of the ECL80...... :-
http://tdsl.duncanamps.com/show.php?des=ECL80 Regards, Mick. |
25th Jan 2006, 7:46 am | #25 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
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Re: Motorboating
Would feeding the triode section of one ECL80 into the pentode of another be a more stable option?
Or, alternatively the triode section feeding a 6V6? |
25th Jan 2006, 10:36 am | #26 |
Dekatron
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Re: Motorboating
I hadn't realised what a hopeless valve is the ECL80.
The fundamental problem is that having a common cathode means that any resistance in the cathode circuit makes negative feedback from the triode and positive feedback from the pentode. Presumably this circuit would go unstable if the valveholder made suboptimal contact with the cathode pin! Perhaps the triode is normally used for something else rather than to drive the pentode? (my communications receiver uses the cathode voltage of the audio output as a handy bias voltage for another circuit). As a general guide to layout - try to imagine what would happen if all the wire and joints had a significant resistance. Consider current flow and then see where the resultant voltages would be "seen" and wire it so that this does not cause trouble. The crudest way to do this is for all earth connections to meet at a single point (star point earthing mentioned above) so that earth current in one part cannot generate a voltage seen by another. |
25th Jan 2006, 11:36 am | #27 | |
Retired Dormant Member
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Re: Motorboating
Quote:
I can't see much point in using the triode stage of the ECL80 to drive a 6V6, as there are so many better triodes available as drivers... |
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25th Jan 2006, 12:37 pm | #28 | |
Moderator
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Re: Motorboating
Quote:
Good luck, Paul Last edited by paulsherwin; 25th Jan 2006 at 12:43 pm. |
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25th Jan 2006, 2:59 pm | #29 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
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Re: Motorboating
I happened to buy several EF91's so I had better try one of them as a pre-amp.
'Scuse the stupid question: What do I do with the other grids when a pentode is wired so it behaves as a triode? |
25th Jan 2006, 3:07 pm | #30 | |
Rest in Peace
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Re: Motorboating
Quote:
Regards, Mick. |
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25th Jan 2006, 3:10 pm | #31 |
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Re: Motorboating
The simplest thing to do is to 'triode wire' the pentode. This involves connecting the suppressor grid (g3) to the cathode and the screen grid (g2) to the anode. You could wire it as a proper pentode though, connecting the screen grid to HT via a resistor decoupled with a capacitor. A quick read through Paul's section on 'How Valves Work' would probably help: http://www.vintage-radio.com/repair-...lves-work.html
HTH, Paul |
25th Jan 2006, 4:38 pm | #32 |
Nonode
Join Date: Jul 2005
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Re: Motorboating
Paul:
On the basis of that description the pentode section of the ECL80 is wired as a triode in the circuit I used. |
25th Jan 2006, 5:20 pm | #33 | |
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Re: Motorboating
Quote:
I'm no expert - others may know better Best regards, Paul |
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25th Jan 2006, 11:05 pm | #34 | |
Hexode
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Re: Motorboating
Quote:
Sam
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26th Jan 2006, 11:40 am | #35 |
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Re: Motorboating
Going back to Neil's experiments, I suppose you could use an ECL80 as a double triode driving a 6V6, and that should give sufficient gain for most applications. Connecting the pentode stage as a triode, with the screen grid connected directly to the anode, and I would advise using grid bias to avoid any instability problems caused by the common cathode. Thus the cathode would be connected directly to chassis, and the control grids of both sections would connect to chassis via large value resistors, 4.7-10 meg or so. Smaller value interstage caps would be used here to avoid grid blocking (0.005uf might be suitable for the basis of experimentation) , and if there is any instability decoupling in the anodes should solve the problem.
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27th Jan 2006, 3:02 pm | #36 |
Octode
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Re: Motorboating
Hi
refering back to an earlier comment Are you sure thet the output transformer is the correct ratio as I think the ECL80 requires 11k which is higher than most other valves Also is it stable with no input signal if you disconnect the coupling capacitor from the pentode grid? This may help to isolate the problem. Regards Peter. |
24th Mar 2006, 12:51 am | #37 |
Dekatron
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Re: Motorboating
Interesting thread. I am only a novice so shoot me down if I am way off track, but the instability should not be associated with the 10n path as the time constant is too short. I can see the fundemental instability problem of shared cathode but could it be related to the phase lag in the triode anode path due to the 22uf?
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24th Mar 2006, 12:24 pm | #38 |
Dekatron
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Re: Motorboating
and...to see if this is the issue perhaps you could either:
- Temporarily remove the 22uF but I guess this might result in HF instability. - Either add another 22uF or halve the 3.3K and see if you get a corresponding halving of the motorboat frequency. |
27th Mar 2006, 8:27 pm | #39 | |
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Re: Motorboating
Quote:
In order to check for VHF oscillation, measure the current (cathode voltage) and ground the control grid with e.g. a 1 nF ceramic capacitor. When the current changes slightly, you have oscillation. An RF oscilloscope does not always show because the tip capacitance may influenece the oscillation. Sorry for the long response, I work in RF valve circuits and bin there a lot of times. Regards, Nico |
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27th Mar 2006, 8:41 pm | #40 |
Dekatron
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Re: Motorboating
Nico, makes more sense than my answer...I assumed motorboating would describe a low frequency instablility. I did say novice didn't I.
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