Pye Black Box, can you spot the error. - UK Vintage Radio Repair and Restoration Discussion Forum

Michael Maurice · 7th Mar 2021, 11:25 pm

This beautiful Black Box with the Chinese lacquer in very good condition, came in for reconditioning and service.

It had never been opened since it left the factory.

I discovered a mistake which had been there from new.

There are two pictures of the underside of the amplifier pre restoration.

so can you guess what the error is?

wd40addict · 7th Mar 2021, 11:32 pm

Cathode resistor only connected to one output valve?

turretslug · 8th Mar 2021, 1:24 am

Do the output valve pair have a common cathode resistor that has been connected to a pin designated "internal connection" that here is fortuitously connected to the cathode? I'm not familiar with the circuit but it looks like push-pull ultra-linear B8a pair with a purple wire linking the two cathodes at the pin 7s but cathode resistor at pin 3 of one socket.

Also, 470 ohms seems quite high for a common cathode resistor, but maybe this is a low-power amp, so could be true!

Michael Maurice · 8th Mar 2021, 9:06 am

You are both correct, the manufacturer had left off the wire connecting the two cathodes.

Here is a picture of it together with new capacitors. all the resistors are well within tolerance.

snowman_al · 8th Mar 2021, 9:43 am

Might I disagree?
As turretslug noted the EL42 cathodes are on pin 7 and they are connected with a purple wire. Note the cathode resistor is on pin 3 not from pin 7.
The Philips data does not show an internal connection between pins 3 and 7, but the Mullard data does show them connected internally.

It did leave the factory with the pin 3 to pin 3 wire 'missing', as it was not necessary. A quick search for pictures shows some with (a few) and some without the wire (more)...

Alan

ms660 · 8th Mar 2021, 10:54 am

I can't see any error in the 1st photo's either, the two cathode pins 7 and 7 are connected together and the common cathode bias resistor is connected to pin 3 of one of the valves, pin 3 also being a cathode connection as shown in the Trader sheet valve base diagram.

Lawrence.

ms660 · 8th Mar 2021, 2:40 pm

Had to go out at short notice this morning, I forgot to post this Mullard data for the EL42, it also shows Pin 3 and pin 7 as cathode/g3:

https://frank.pocnet.net/sheets/129/e/EL42.pdf

Lawrence.

Michael Maurice · 8th Mar 2021, 7:37 pm

I bow to your superior knowledge folks

Nickthedentist · 8th Mar 2021, 7:55 pm

What a beautiful machine!

Wellington · 8th Mar 2021, 8:05 pm

Quote:

Originally Posted by Nickthedentist

What a beautiful machine!

I was thinking the same thing

Ed_Dinning · 8th Mar 2021, 9:22 pm

Hi Michael, I was confused by that on the first BB I did, but fortunately I used a Mullard valve data book.
I did have a later BB (PCL83) where the C/R networks were connected from HT to valve screens, not anodes and had been like that from production

Ed

Herald1360 · 8th Mar 2021, 9:28 pm

I went round the same loop myself a week ago, not the only "interesting" feature of these amps.....

https://www.vintage-radio.net/forum/...8&postcount=11

Station X · 9th Mar 2021, 10:47 am

The non-fatal error has been revealed, so I'll close this thread.

7th Mar 2021, 11:25 pm	#1
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,224	Pye Black Box, can you spot the error. This beautiful Black Box with the Chinese lacquer in very good condition, came in for reconditioning and service. It had never been opened since it left the factory. I discovered a mistake which had been there from new. There are two pictures of the underside of the amplifier pre restoration. so can you guess what the error is? Attached Thumbnails __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

7th Mar 2021, 11:32 pm	#2
wd40addict Octode Join Date: May 2009 Location: Stevenage, Herts. UK. Posts: 1,518	Re: Pye Black Box, can you spot the error. Cathode resistor only connected to one output valve?

8th Mar 2021, 1:24 am	#3
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,396	Re: Pye Black Box, can you spot the error. Do the output valve pair have a common cathode resistor that has been connected to a pin designated "internal connection" that here is fortuitously connected to the cathode? I'm not familiar with the circuit but it looks like push-pull ultra-linear B8a pair with a purple wire linking the two cathodes at the pin 7s but cathode resistor at pin 3 of one socket. Also, 470 ohms seems quite high for a common cathode resistor, but maybe this is a low-power amp, so could be true! Last edited by turretslug; 8th Mar 2021 at 1:30 am. Reason: Supplement

8th Mar 2021, 9:06 am	#4
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,224	Re: Pye Black Box, can you spot the error. You are both correct, the manufacturer had left off the wire connecting the two cathodes. Here is a picture of it together with new capacitors. all the resistors are well within tolerance. Attached Thumbnails __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

8th Mar 2021, 9:43 am	#5
snowman_al Octode Join Date: Dec 2018 Location: Llandeilo, West Wales, UK. Posts: 1,092	Re: Pye Black Box, can you spot the error. Might I disagree? As turretslug noted the EL42 cathodes are on pin 7 and they are connected with a purple wire. Note the cathode resistor is on pin 3 not from pin 7. The Philips data does not show an internal connection between pins 3 and 7, but the Mullard data does show them connected internally. It did leave the factory with the pin 3 to pin 3 wire 'missing', as it was not necessary. A quick search for pictures shows some with (a few) and some without the wire (more)... Alan __________________ Never Leave Well Enough Alone...

8th Mar 2021, 10:54 am	#6
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Pye Black Box, can you spot the error. I can't see any error in the 1st photo's either, the two cathode pins 7 and 7 are connected together and the common cathode bias resistor is connected to pin 3 of one of the valves, pin 3 also being a cathode connection as shown in the Trader sheet valve base diagram. Lawrence.

8th Mar 2021, 2:40 pm	#7
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Pye Black Box, can you spot the error. Had to go out at short notice this morning, I forgot to post this Mullard data for the EL42, it also shows Pin 3 and pin 7 as cathode/g3: https://frank.pocnet.net/sheets/129/e/EL42.pdf Lawrence.

8th Mar 2021, 7:37 pm	#8
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,224	Re: Pye Black Box, can you spot the error. I bow to your superior knowledge folks __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

8th Mar 2021, 7:55 pm	#9
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,846	Re: Pye Black Box, can you spot the error. What a beautiful machine!

8th Mar 2021, 9:22 pm	#11
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Pye Black Box, can you spot the error. Hi Michael, I was confused by that on the first BB I did, but fortunately I used a Mullard valve data book. I did have a later BB (PCL83) where the C/R networks were connected from HT to valve screens, not anodes and had been like that from production Ed

8th Mar 2021, 9:28 pm	#12
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Pye Black Box, can you spot the error. I went round the same loop myself a week ago, not the only "interesting" feature of these amps..... https://www.vintage-radio.net/forum/...8&postcount=11 __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

9th Mar 2021, 10:47 am	#13
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Pye Black Box, can you spot the error. The non-fatal error has been revealed, so I'll close this thread. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.