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Old 30th Mar 2012, 2:40 pm   #1
greenstar
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Default 12v ECC88 multivibrator

Hi, I would appreciate expert comment on this! I've knocked up a basic multivibrator with a tv valve and 12volt supply. It's the first valve thing I've made since my teens - too long ago. I made one then with two SP61's which was very useful. The idea of a 12v version, and use of common valves at low HT appealed to me. I may need to increase the HT, as the output doesn't seem strong. It causes an aubible buzz when applied tothe audio input of my transistor radio, but it's not overwhelming. Nothing on the aerial. Photos below. Ths 'scope is set to 5ms and 2v. It may get a coat of paint if it behaves. Apologies - had trouble getting some of the images clear. And they were the right way around!

Tony
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Old 30th Mar 2012, 4:41 pm   #2
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Default Re: 12v ECC88 multivibrator

Although the ECC88 will work at low HT, you won't get much switching action at 12V as you have found. Have you tried plotting a load line?
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Old 30th Mar 2012, 5:37 pm   #3
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Default Re: 12v ECC88 multivibrator

A whatter? I may trya battery pack with an increasing number couple of PP3's.12 v is handy as I have some 12v cells.

I've tried several ECC's. ECC82 is similar. ECC83 very little, but corrugates the mains hum. ECC85 not much, and 88 as prev.

Tony
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Old 30th Mar 2012, 5:45 pm   #4
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Default Re: 12v ECC88 multivibrator

how about the ECC86 that is designed for a low anode voltage
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Old 30th Mar 2012, 8:18 pm   #5
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Default Re: 12v ECC88 multivibrator

At least with an ECC82 you can halve the heater current by wiring in series mode.
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Old 30th Mar 2012, 8:43 pm   #6
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Default Re: 12v ECC88 multivibrator

Laurdys, the ECC86 is the one I don't have! Bit pricey on Bay. I could get a whole sig gen for that. I have 82, 3, 5 and 8. I'll probably just try adding a couple of PP3's, but it's not so simple. Dave, I'm sorry again, but haven't a clue what series mode is .... there's only one heater, I'm using a 15 Ohm res in series at 12v ... what would that be? Thanks, this is educational for me if you can stand it!
Tony
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Old 30th Mar 2012, 10:00 pm   #7
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Default Re: 12v ECC88 multivibrator

27v HT it's a lot more lively. This may well be ok.
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Old 31st Mar 2012, 11:43 am   #8
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Default Re: 12v ECC88 multivibrator

The ECC81, 82 and 83 can have their heaters wired in parallel for 6.3V 300mA or series for 12.6V 150mA. Have a look at the datasheet: pins 4, 5 and 9.
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Old 31st Mar 2012, 12:45 pm   #9
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Default Re: 12v ECC88 multivibrator

Cheers Dave, I should have checked. I'll probably use 30v or higher HT, as it runs pretty well on 3 PP3's. I'll make up a battery pack, and it will do for other gadgets. Will have a think about the most convenient LT.
Thanks for your input, and to Laurdys for the ECC86 suggestion. I might stay with the ECC88, having fitted a dropper - I make this the same power drawn unless I am wrong.

Tony
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Old 31st Mar 2012, 7:30 pm   #10
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Default Re: 12v ECC88 multivibrator

ECC88 takes 365mA heater current at 6.3V. ECC82 takes 150mA, when wired for 12V. Up to you.
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Old 31st Mar 2012, 11:53 pm   #11
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Default Re: 12v ECC88 multivibrator

Quote:
Originally Posted by greenstar View Post
I might stay with the ECC88, having fitted a dropper - I make this the same power drawn unless I am wrong.

Tony
Only if you run the ECC88 from 6V! On 12V you dissipate as much again in the dropper resistor. Of course, now you have 27V HT, you could run the heater from 6V anyway.....
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Old 1st Apr 2012, 3:52 pm   #12
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Default Re: 12v ECC88 multivibrator

An ECC88 can only be run from 6.3V; it doesn't have the more flexible heater arrangements of the ECC82.
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Old 5th Apr 2012, 10:52 am   #13
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Default Re: 12v ECC88 multivibrator

Thanks, Herald and Dave, the penny has dropped - can see the advantages of the ECC82. The reason I have been so stuck on 12v is that I have several lead acid sealed cells, ex my former employers alarm system, and a UPS. I've also a car battery system topped up by a solar cell, that I run 'cordless' drills with. I had a few of these from the tip - Bosch's, Machitas', etc, chucked when the cells failed. Currently making up a battery box, with a regulator taken from a TV, which gives 0-24v. Am interested in running valve projects from this, but I think will settle for separate HT for far better results.

Tony
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Old 5th Apr 2012, 11:11 am   #14
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Default Re: 12v ECC88 multivibrator

What was the ECC83 like? I happen to have one.
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Old 5th Apr 2012, 5:03 pm   #15
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Default Re: 12v ECC88 multivibrator

Neil, very little output from the ECC83 at 12v HT, so not tried at higher. This was an untested valve though! My 'scope was picking up a mains hum, and when the ecc83 kicked in you could see there was some output, as the sine wave had a kind of corrugated edge! I imagine with a bit more HT it might work. I can try it if you like.

Tony
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Old 5th Apr 2012, 5:32 pm   #16
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Default Re: 12v ECC88 multivibrator

Yes, at 30v my ECC83 works as well as the ECC82 at 12. With a bit more HT I should think it will do well.
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Old 6th Apr 2012, 6:59 am   #17
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Default Re: 12v ECC88 multivibrator

What frequency does the device give & is it supposed to be a sine wave output?
Do you have a sharper image of the circuit diagram? I can't read the component values .

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Old 6th Apr 2012, 8:25 am   #18
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Default Re: 12v ECC88 multivibrator

Neil, my knowledge is fairly basic, and people can correct me, but I understand a multivibrator gives a very rough and hearty output, switching on and off between two valves, generating low frequency audio with lots of noise and harmonics up to HF. To the ear it's a buzz. Useful for testing, as it gives an input that doesn't need tuning and contains audio and RF, so can work through the stages of a radio, for example, and see where the signal appears. It's not a nice sine wave! I'll try and post a better image, but my scanner's being a pain.
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Old 6th Apr 2012, 9:00 am   #19
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Default Re: 12v ECC88 multivibrator

Just to add to that, I'd guess this might work fine with 90v HT or higher and an ECC83, but I'd want to use an insulating case and earth the metalwork. 30v won't do much harm, but maybe towards 100 one needs to start being more careful. Reducing the value of the 15k resistors might help use a lower HT? Mine was an experiment. There are transistor, chip and battery valve versions on the web. It really depends what you need it for.
And please bear in mind the above discussion re the filament supply - I've wired the filament to the 12v supply with a dropper, you will need to have a separate LT. Good luck!
Tony
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Old 6th Apr 2012, 12:47 pm   #20
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Default Re: 12v ECC88 multivibrator

The astable multivibrator gives a square wave output, in the ideal case. Lots of harmonics. With a perfectly equal mark-space ratio you only get odd harmonics (3rd, 5th etc.), but any asymmetry includes even-order too.
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