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Old 24th Jan 2006, 1:07 am   #21
Sideband
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Default Re: Motorboating

Make sure the HT is not too high. It could just be that causing the valve to 'take off'.

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Old 24th Jan 2006, 7:32 am   #22
Neil Purling
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Default 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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Old 24th Jan 2006, 8:08 pm   #23
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Default Re: Motorboating

An educated guess would be around 40-50mA. That's the normal area for class A output stages in later valve radios.
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Old 24th Jan 2006, 8:18 pm   #24
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Default 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.
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Old 25th Jan 2006, 7:46 am   #25
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Default 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?
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Old 25th Jan 2006, 10:36 am   #26
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Default 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.
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Old 25th Jan 2006, 11:36 am   #27
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Default Re: Motorboating

Quote:
Originally Posted by Neil Purling
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?
If you've got plenty of ECL80s around then that would work - as I said earlier I did this once for a simple stereo amp employing two ECL80s with the triode section of one ECL80 driving the pentode of the other, and I had no problems with stability. Many ECL80 amp circuits employ negative feedback by means of a resistor connected between the anodes of the triode and pentode stages (680k - 3.3meg being common) which helps improve stability, although you will loose gain from what is, essentially, a low gain amp anyway.

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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Old 25th Jan 2006, 12:37 pm   #28
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Default Re: Motorboating

Quote:
Originally Posted by quantum
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...
I've used just the *pentode* section of an ECL80 on it's own in the past. It worked very well as a low power output valve. You could use pretty much any valve in the junk box as a preamp stage - EF80, EF91 etc - but an ECC81/82/83 will give you 2 suitable triodes to play with. You don't need to use both. The ECC83 would probably be best.

Good luck, Paul

Last edited by paulsherwin; 25th Jan 2006 at 12:43 pm.
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Old 25th Jan 2006, 2:59 pm   #29
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Default 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?
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Old 25th Jan 2006, 3:07 pm   #30
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Default Re: Motorboating

Quote:
Originally Posted by Neil Purling
What do I do with the other grids when a pentode is wired so it behaves as a triode?
Connect them to the anode, apart from the control grid of course.... . If you can't get the ECL80 stable, why not use the ECL82 or ECL86 ?, they have separate cathodes. .

Regards, Mick.
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Old 25th Jan 2006, 3:10 pm   #31
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Default 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
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Old 25th Jan 2006, 4:38 pm   #32
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Default 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.
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Old 25th Jan 2006, 5:20 pm   #33
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Default Re: Motorboating

Quote:
Originally Posted by Neil Purling
Paul:
On the basis of that description the pentode section of the ECL80 is wired as a triode in the circuit I used.
My understanding is that it's wired as a pentode with the screen grid connected directly to the HT. If it were triode wired the screen grid would go the the anode side of the output transformer.

I'm no expert - others may know better

Best regards, Paul
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Old 25th Jan 2006, 11:05 pm   #34
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Default Re: Motorboating

Quote:
Originally Posted by paulsherwin
My understanding is that it's wired as a pentode with the screen grid connected directly to the HT. If it were triode wired the screen grid would go the the anode side of the output transformer.
The screen is only connected to HT directly if the screen voltage for the valve is the same as the HT voltage for the circuit. Otherwise there is a dropper resistor between the HT line and the screen which will drop sufficient volts as the screen current flows through it. If the valve is triode-strapped the screen is connected to the anode.

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Old 26th Jan 2006, 11:40 am   #35
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Default 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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Old 27th Jan 2006, 3:02 pm   #36
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Default 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.
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Old 24th Mar 2006, 12:51 am   #37
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Default 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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Old 24th Mar 2006, 12:24 pm   #38
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Default 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.
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Old 27th Mar 2006, 8:27 pm   #39
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Default Re: Motorboating

Quote:
Originally Posted by Neil Purling
I have followed the reccomendations by member 'radioman'.
Look at the attached image. It is a modified version of the circuit I first posted.
I have managed to stabalise the amplifier by putting a resistor 'X in. The existing pentode grid resistor was left where it was. The thing doesn't motorboat now. The value is presently 2.2 Meg. I think further experimentation is warranted.

However I have been using the output from my turntable as a test signal.
What is the rated output of a Sonotone 9TA.
I would like to know if any of the TRF builders on the forum have measured the peak-to-peak signal output of the detector with a scope. I wondered how it compares to my test signal?
When a resistor in the grid line stops the motorboating, it is most probably RF oscillation in the VHF band. It happens more. ECC83 and 82 valves are notorious for that problem. A lot of times (most times actually) you do not notice, but the amps distorsion is higher. Normally a 10 k resistor will be more than sufficient, 2.2 M is very high. I should not take tha value, it may increase the sensiivity for hum.
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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Old 27th Mar 2006, 8:41 pm   #40
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Default 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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