Power Pentode Output Choke - UK Vintage Radio Repair and Restoration Discussion Forum

SurreyNick · 13th Sep 2014, 11:45 pm

I have here a couple of pictures of a Telsen Power Pentode Output Choke. 30/15H - 40mA with a 1.3/1 ratio and a 1.7/1 ratio.

Can someone tell me it's purpose within a circuit and where it would be located? Is it an amplifier for the audio output (pentode) valve? It's not an output transformer for matching with a speaker is it?

Thanks
Nick

Herald1360 · 14th Sep 2014, 4:21 pm

I'd hazard a guess at it being a load choke for an output valve feeding a hi-z loudspeaker via a capacitor. The strange ratio tap might have something to do with hum cancellation.

SurreyNick · 14th Sep 2014, 5:23 pm

Thanks Chris.
It's definitely an LF Choke but it confuses me because it's almost certainly iron-cored, which would indicate it would be used for valve coupling, or for connecting a loud-speaker to the output valve, but my understanding was that for either of these two functions it should have an inductance of NOT LESS than 50 henries. This choke is much lower. So was it intended for smoothing to cancel hum as you suggest? In which case I was interested to know how and when it would have been employed in a circuit as it looks like it was a late 1920s / very early 1930s component.

I don't have sufficient knowledge or experience to tell if it was designed for D.C. current only, or A.C. only, or both. And if it is A.C. then I believe the field of uses could be greater

I consider it part of my education and learning to find out.

Nick

Ed_Dinning · 14th Sep 2014, 7:58 pm

Hi Nick, a consultation of the Telsen "Radiomag" shows it at 10/6 and used with a capacitor to couple to a speaker.
They were also advertised for classB stages as well.
They could also be used without a capacitor with certain high Z speakers.

Ed

kalee20 · 14th Sep 2014, 8:15 pm

Quote:

Originally Posted by SurreyNick

It's definitely an LF Choke but it confuses me because it's almost certainly iron-cored, which would indicate it would be used for valve coupling, or for connecting a loud-speaker to the output valve, but my understanding was that for either of these two functions it should have an inductance of NOT LESS than 50 henries.

It's not humongously lower than 50 henries, only by a factor of 3 or so, for your 15H figure. So it would work as an output choke, as others have suggested. Bass response would be curtailed with the lesser inductance, but the late 1920's and early 1930's were not the era of hi-fi. Efficiency and maximum gain was the task then!

AC/HL · 15th Sep 2014, 12:38 am

For circuitry of this age, any of FJ Camm's many books are a good guide. There's always at least one set that matches near enough to be of use.
This one just specifies a Pentode output choke, no ratio mentioned. I expect yours has four terminals.

SurreyNick · 15th Sep 2014, 2:49 pm

Thanks Bill. I will have a rifle through his books.

Quote:

Originally Posted by Ed_Dinning

...used with a capacitor to couple to a speaker.
They could also be used without a capacitor with certain high Z speakers.

Mmm. That's good news. Do you reckon it would couple with that small 5" speaker you sent me? If so, I'll use it instead of the small transformer for the TRF - much more handsome.
Nick

Ed_Dinning · 15th Sep 2014, 7:33 pm

Hi Nick, no it was intended to be used with a high Z speaker; used with the OP trans as well it should be OK for a demo.

Ed

SurreyNick · 15th Sep 2014, 8:43 pm

Think I'm going to have to go away and do some additional reading on speaker matching!
N.

Ed_Dinning · 16th Sep 2014, 9:38 pm

Hi Nick, the speaker matching transformer ratio is defined as the square root of the impedance ratio valve required load/ speaker impedance.
Confusion will arise over the various symbols used (ra, Ra, RL).
ra is usually the valve anode impedance. The desired load is often taken as 50% of this value for minimum distortion, often written as Ra or RL.
Confused yet?

In practice, for most low power stages using a 3R speaker any ratio between about 20:1 and 50:1 will produce sound and not damage anything.
Your reading will give you all the theory about maximum power transfer and minimising distortion.
In general pentode will need a higher ratio transformer than triodes.

Cheers, Ed

SurreyNick · 16th Sep 2014, 10:33 pm

Thanks to each of you for the information and advice. I now understand what a power pentode output choke is, and when and how it is incorporated into a circuit

Typically, this new-found knowledge has now raised additional questions on speaker matching!

As it naturally flows from this thread, but is on a different topic, I have started a new thread here (https://www.vintage-radio.net/forum/...d.php?t=109468)

Nick

13th Sep 2014, 11:45 pm	#1
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Power Pentode Output Choke I have here a couple of pictures of a Telsen Power Pentode Output Choke. 30/15H - 40mA with a 1.3/1 ratio and a 1.7/1 ratio. Can someone tell me it's purpose within a circuit and where it would be located? Is it an amplifier for the audio output (pentode) valve? It's not an output transformer for matching with a speaker is it? Thanks Nick Attached Thumbnails

14th Sep 2014, 4:21 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Power Pentode Output Choke I'd hazard a guess at it being a load choke for an output valve feeding a hi-z loudspeaker via a capacitor. The strange ratio tap might have something to do with hum cancellation. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

14th Sep 2014, 5:23 pm	#3
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: Power Pentode Output Choke Thanks Chris. It's definitely an LF Choke but it confuses me because it's almost certainly iron-cored, which would indicate it would be used for valve coupling, or for connecting a loud-speaker to the output valve, but my understanding was that for either of these two functions it should have an inductance of NOT LESS than 50 henries. This choke is much lower. So was it intended for smoothing to cancel hum as you suggest? In which case I was interested to know how and when it would have been employed in a circuit as it looks like it was a late 1920s / very early 1930s component. I don't have sufficient knowledge or experience to tell if it was designed for D.C. current only, or A.C. only, or both. And if it is A.C. then I believe the field of uses could be greater I consider it part of my education and learning to find out. Nick

14th Sep 2014, 7:58 pm	#4
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Power Pentode Output Choke Hi Nick, a consultation of the Telsen "Radiomag" shows it at 10/6 and used with a capacitor to couple to a speaker. They were also advertised for classB stages as well. They could also be used without a capacitor with certain high Z speakers. Ed

15th Sep 2014, 12:38 am	#6
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Power Pentode Output Choke For circuitry of this age, any of FJ Camm's many books are a good guide. There's always at least one set that matches near enough to be of use. This one just specifies a Pentode output choke, no ratio mentioned. I expect yours has four terminals. Attached Thumbnails

15th Sep 2014, 7:33 pm	#8
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Power Pentode Output Choke Hi Nick, no it was intended to be used with a high Z speaker; used with the OP trans as well it should be OK for a demo. Ed

15th Sep 2014, 8:43 pm	#9
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: Power Pentode Output Choke Think I'm going to have to go away and do some additional reading on speaker matching! N.

16th Sep 2014, 9:38 pm	#10
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Power Pentode Output Choke Hi Nick, the speaker matching transformer ratio is defined as the square root of the impedance ratio valve required load/ speaker impedance. Confusion will arise over the various symbols used (ra, Ra, RL). ra is usually the valve anode impedance. The desired load is often taken as 50% of this value for minimum distortion, often written as Ra or RL. Confused yet? In practice, for most low power stages using a 3R speaker any ratio between about 20:1 and 50:1 will produce sound and not damage anything. Your reading will give you all the theory about maximum power transfer and minimising distortion. In general pentode will need a higher ratio transformer than triodes. Cheers, Ed

16th Sep 2014, 10:33 pm	#11
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: Power Pentode Output Choke Thanks to each of you for the information and advice. I now understand what a power pentode output choke is, and when and how it is incorporated into a circuit Typically, this new-found knowledge has now raised additional questions on speaker matching! As it naturally flows from this thread, but is on a different topic, I have started a new thread here (https://www.vintage-radio.net/forum/...d.php?t=109468) Nick