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Old 29th Mar 2023, 2:58 pm   #1
dougietamson
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Default ECC81 with broken glass envelope meets 2 x LND150 transistors

I have a broken Mullard ECC81 valve where the glass envelope had split neatly around where the bottom mica plate was supposed to be.
I un-clipped the the internals down to the copper wires, tinned them and soldered in a couple of LND150 high voltage N-Channel depletion mode transistors, 1.10 each.

It's now a solid state FrankenECC83

Currently only sellotaped together, any suggestions for glue?

Here it is in the phase inverter socket of my JCA22H amp.
https://www.youtube.com/watch?v=VohL5NQni2I
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Old 29th Mar 2023, 3:01 pm   #2
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

Heat shrink adhesive sleeve around the join.
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Old 29th Mar 2023, 3:04 pm   #3
G6Tanuki
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

Superglue dribbled into the crack, left to dry and any excess shaved-off using a scalpel?

In times-past there was a glass-glue type stuff available in kits for filling chips in windscreens; it hardened using UV light and then you shaved it down with a razor-blade before polishing it. Not sure if it's still available but it was specially designed so its refractive-index was closely matched to that of the glass so the repair would be invisible.
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Old 29th Mar 2023, 3:18 pm   #4
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

Quote:
Originally Posted by G6Tanuki View Post
Superglue dribbled into the crack, left to dry and any excess shaved-off using a scalpel?

In times-past there was a glass-glue type stuff available in kits for filling chips in windscreens; it hardened using UV light and then you shaved it down with a razor-blade before polishing it. Not sure if it's still available but it was specially designed so its refractive-index was closely matched to that of the glass so the repair would be invisible.
I have a tube of the windscreen stuff to repair a stone chip, no UV source so left it in the sun for ages, seems ok, the old Saab passed her MOT every time
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Old 29th Mar 2023, 3:19 pm   #5
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

Here's the pinout:

Anode pins 1&6 = Source
Grid pins 2&7 = Gate
Cathode pins 3&8 = Drain
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Old 29th Mar 2023, 5:29 pm   #6
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

Don't forget an LED to mimic the heater!
Rob
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Old 29th Mar 2023, 5:49 pm   #7
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

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Originally Posted by robinshack View Post
Don't forget an led to mimic the heater!
Rob
Good idea!

Maybe I should use one of my many xmas tree lights or a proper pilot light for a warm glow.

This particular amp has 12v dc for the filaments so, I could add 2 like a real ECC83, one for each triode
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Old 29th Mar 2023, 6:49 pm   #8
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

Nice!

There is a newer version of the old glass glue, Loctite super glue for glass, its rubbish!!! I stuck a GEC radio dial back together with it, the cracks were still clearly visible, and after a few years it fell apart! I had noticed when it dried it was sort of soft and rubbery, unlike the old glass glue which dried hard, and is still holding a wine glass together after 20 years!

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Old 29th Mar 2023, 8:09 pm   #9
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

All it needs is a tiny, tiny sign inside reading "In case of emergency, Break Glass."

David
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Old 29th Mar 2023, 8:43 pm   #10
Cruisin Marine
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

Where is a glass blower when you need one?
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Old 29th Mar 2023, 9:28 pm   #11
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

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Originally Posted by Cruisin Marine View Post
Where is a glass blower when you need one?
I was thinking of getting the blow torch out, would have to shield the transistors though and would end up melting the solder. Crimping might work.
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Old 29th Mar 2023, 11:22 pm   #12
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

Waiting for my LND150s to arrive from China, 13th April they say. I will use them as temporary replacements for impossible to get 1920s valves.
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Old 30th Mar 2023, 7:19 am   #13
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

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Originally Posted by boiss View Post
Waiting for my LND150s to arrive from China, 13th April they say. I will use them as temporary replacements for impossible to get 1920s valves.
I bought mine from a UK supplier, 1.09 each and 2.99 p&p

https://www.switchelectronics.co.uk/...00v-30ma-to-92
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Old 30th Mar 2023, 2:30 pm   #14
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

Here's the results from a simple test circuit with sine wave input

DC voltage measurements.

Anode resistor 100KΩ
Cathode resistor 2.2KΩ

HT supply - 200.3
Anode voltage - 161.2
Voltage drop across anode load - 39.1 = 0.39mA

Input 500Hz sine 0.62Vp-p 0.22Vrms (47KΩ grid stopper)
Output (at 0.022F 400v coupling cap) 19.33Vp-p 6.64Vrms
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Old 30th Mar 2023, 5:33 pm   #15
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

Here's the voltages for a 12AX7, Brimar 12AU7 (1950's seems ok) and the SolidStateAX7

HT = 200v - 100KΩ load resistor - 2.2KΩ Cathode resistor - 0.022F coupling cap - 47KΩ Grid stopper - 12Vdc heater pind 4/5

12AX7
Anode v drop = 56.9
cathode v drop = 1.22

Brimar 12AU7
Anode v drop = 108
cathode v drop = 2.38

12ax-fake-7
Anode v drop = 38
cathode v drop = 0.823

Doug
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Old 30th Mar 2023, 7:53 pm   #16
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

It would be interesting to see how far up the frequency scale you could get with the LND150 before the gain starts to drop.
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Old 13th Apr 2023, 3:56 pm   #17
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

How to connect this 2 x LND150?
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Old 14th Apr 2023, 7:56 am   #18
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Default Re: ECC81 with broken glass envelope meets 2 x LND150 transistors

As shown in post #5.
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