ECC81 with broken glass envelope meets 2 x LND150 transistors

[dougietamson]

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

[Station X]

Heat shrink adhesive sleeve around the join.

[G6Tanuki]

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.

[dougietamson]

Originally Posted by G6Tanuki

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

[dougietamson]

Here's the pinout:

Anode pins 1&6 = Source
Grid pins 2&7 = Gate
Cathode pins 3&8 = Drain

[CambridgeWorks]

Don't forget an LED to mimic the heater!
Rob

[dougietamson]

Originally Posted by robinshack

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

[Lloyd 1985]

Nice!

There is a newer version of the old glass glue, Loctite super glue for glass, it’s rubbish!!!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!

Regards
Lloyd

[Radio Wrangler]

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

David

[Cruisin Marine]

Where is a glass blower when you need one?

[dougietamson]

Originally Posted by Cruisin Marine

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.

[boiss]

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.

[dougietamson]

Originally Posted by boiss

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

[dougietamson]

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 0.022µF 400v coupling cap) 19.33Vp-p 6.64Vrms

[dougietamson]

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 - 0.022µF 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

[Silicon]

It would be interesting to see how far up the frequency scale you could get with the LND150 before the gain starts to drop.

[Station X]

As shown in post #5.

[Radio Wrangler]

Re-opened at Julesomega's request.

Some JFETs and D-FETs are symmetrical, so source and drain are interchangeable. I have no idea if these are.

David

[dougietamson]

Originally Posted by Radio Wrangler

Re-opened at Julesomega's request.

Some JFETs and D-FETs are symmetrical, so source and drain are interchangeable. I have no idea if these are.

David

I'll try swapping the component around, ie pivot on the gate which is connected as the 'control grid'.

When I built them into the repurposed glass envelope I connected them up with the flat face/writing facing into the glass envelope.

I used the mini tester to verify the pinout after soldering, just to make sure there was no issue once assembled.

There is 'talk' of people having issues with these low cost testers and certain JFETs, so far I've been ok.

I had major issue with my 4 x EL34 guitar amp over xmas, I signal traced the preamp stages then found a failed J113 jfet switch cutting off the preamp signal just before the PI, the mini tester showed it as 2 x resistors (54? and 260?). The replacement tested ok and was correctly identified by the mini tester.

Doug

[Julesomega]

Originally Posted by dougietamson

Here's the pinout:

Anode pins 1&6 = Source
Grid pins 2&7 = Gate
Cathode pins 3&8 = Drain

Thanks for the discussion: there was a lead time of many months for the LND150 transistors to arrive at the local RS, and then I started looking for the 'circuit', which is defined only as above, and would mean N-chan MOSFETs being reverse pol'd. Further, the photo of the device being tested seems to show the devices with the round side facing out through the envelope, and the pin connections are indiscernible.