Pye BB output circuit - UK Vintage Radio Repair and Restoration Discussion Forum

Ed_Dinning · 29th Apr 2016, 12:58 pm

Hi Gents, I have a BB on the bench at present and the official schematic.
This is the unit with PCL83's, BB/A & BBT/A

The amp has output transformer C/R circuits from each valve anode to HT (0.004u + 3K9).
The schematic shows these networks, of the same values, from each anode to its associated screen.

Your thoughts please.

Ed

peter_sol · 29th Apr 2016, 1:03 pm

They are to keep the amplifier stable by basically rolling off the out of band high frequencies, (above the audio band)

Ed_Dinning · 29th Apr 2016, 8:34 pm

Hi Peter, yes, that is understood, But, where do they connect, anode to screen, or anode to HT. I have a schematic showing one method and a factory wired and working chassis showing the other?

Ed

peter_sol · 29th Apr 2016, 8:51 pm

I think the one with the anode to screen may be the ultra linear circuit and anode to HT, the non ultra linear circuit.
Wherever they are connected Physically in your amp will be correct for your circuit.

'LIVEWIRE?' · 29th Apr 2016, 10:42 pm

To my knowledge there were three different Black Box Valve circuits. As Peter says, one is of the ultra linear type in which the screen grids of the o/p valves are taken to taps on the o/p TX primary windings, whilst on the others they are connected to HT.

julie_m · 30th Apr 2016, 10:24 am

As long as there is a suitably low AC impedance to 0V, wherever those R-C networks are connected ought to be fine.

Ed_Dinning · 30th Apr 2016, 8:42 pm

Hi Gents, this is the UL circuit using PCL83's.
In the working amp the CR networks connect from HT to the respective valve anodes.
In the official company schematic they are shown connected from anode to screen. I believe this to be a drawing error. Anyone thoughts on this?

If they are anode to screen there is then the impedance of the screen to HT section of the op trans to consider.

Ed

Herald1360 · 1st May 2016, 8:46 am

Possibly they were physically connected Anode-Screen or at least drawn that way on the non UL version. The draughtsman creating the UL version drawing probably had a marked up print from the designer and didn't notice the relevant change (if indeed it had been marked!).

My memories of circuit draughtsmen are that they were much better at producing works of art than circuit diagrams that made sense, even to the bloke that designed them!

Ed_Dinning · 1st May 2016, 11:40 am

Hi Gents, no , no similar network in the EL42 versions of this amp (P-P but not UL)

This PCL83 version has a more complex feedback loop with a length of Nichrome wire (about 0.5R) in the circuit.

This set has been got at before and looks as if instability has been a previous problem.

Ed

29th Apr 2016, 12:58 pm	#1
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Pye BB output circuit Hi Gents, I have a BB on the bench at present and the official schematic. This is the unit with PCL83's, BB/A & BBT/A The amp has output transformer C/R circuits from each valve anode to HT (0.004u + 3K9). The schematic shows these networks, of the same values, from each anode to its associated screen. Your thoughts please. Ed

29th Apr 2016, 1:03 pm	#2
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Pye BB output circuit They are to keep the amplifier stable by basically rolling off the out of band high frequencies, (above the audio band)

29th Apr 2016, 8:34 pm	#3
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Pye BB output circuit Hi Peter, yes, that is understood, But, where do they connect, anode to screen, or anode to HT. I have a schematic showing one method and a factory wired and working chassis showing the other? Ed

29th Apr 2016, 8:51 pm	#4
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Pye BB output circuit I think the one with the anode to screen may be the ultra linear circuit and anode to HT, the non ultra linear circuit. Wherever they are connected Physically in your amp will be correct for your circuit.

29th Apr 2016, 10:42 pm	#5
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Pye BB output circuit To my knowledge there were three different Black Box Valve circuits. As Peter says, one is of the ultra linear type in which the screen grids of the o/p valves are taken to taps on the o/p TX primary windings, whilst on the others they are connected to HT.

30th Apr 2016, 10:24 am	#6
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Pye BB output circuit As long as there is a suitably low AC impedance to 0V, wherever those R-C networks are connected ought to be fine. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

30th Apr 2016, 8:42 pm	#7
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Pye BB output circuit Hi Gents, this is the UL circuit using PCL83's. In the working amp the CR networks connect from HT to the respective valve anodes. In the official company schematic they are shown connected from anode to screen. I believe this to be a drawing error. Anyone thoughts on this? If they are anode to screen there is then the impedance of the screen to HT section of the op trans to consider. Ed

1st May 2016, 8:46 am	#8
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Pye BB output circuit Possibly they were physically connected Anode-Screen or at least drawn that way on the non UL version. The draughtsman creating the UL version drawing probably had a marked up print from the designer and didn't notice the relevant change (if indeed it had been marked!). My memories of circuit draughtsmen are that they were much better at producing works of art than circuit diagrams that made sense, even to the bloke that designed them! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

1st May 2016, 11:40 am	#9
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Pye BB output circuit Hi Gents, no , no similar network in the EL42 versions of this amp (P-P but not UL) This PCL83 version has a more complex feedback loop with a length of Nichrome wire (about 0.5R) in the circuit. This set has been got at before and looks as if instability has been a previous problem. Ed