High gain output valve - Page 4 - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 26th Nov 2015, 10:32 pm

The SP61 is rated to pass around 10mA of anode current.

Assuming your HT voltage is 250 and you're running the valve biased in pure Class-A then you can expect a good Watt or so of audio (Class-A can never be more than 50% efficient). Treat it like an EF91 - various editions of "Practical Wireless" in the 1960s had receiver designs where the EF91 was used as an audio-output valve, and I believe at least one commercially produced TV receiver from the era did likewise.

Neil Purling · 27th Nov 2015, 10:38 am

I have a 50:1 transformer, well it is marked 50:1 in pencil. It could have come out of something like a record player.

Herald1360 · 27th Nov 2015, 2:08 pm

You can try any transformer with a load on the secondary without damaging thevalve. Just have a play. A 240:6V mains TX will work surprisingly well.

Neil Purling · 27th Nov 2015, 11:08 pm

Was the 240V-6V transformer for a 8 ohm speaker or 3-4 ohms used with the SP61?
If 4 ohms then I only have one speaker that has this impedance & that is from a low power valve portable that used a DL9* as o/p.

Bazz4CQJ · 1st Dec 2015, 5:26 am

Can anyone say what are the pro's & cons of running something like the SP61 or a EF91 either as a pentode or connected as a triode (screen grid connected to anode) for the AF output role?

B

Herald1360 · 1st Dec 2015, 1:25 pm

Quote:

Originally Posted by Neil Purling

Was the 240V-6V transformer for a 8 ohm speaker or 3-4 ohms used with the SP61?

Sorry, no idea really. I've no data as to what the ideal load for SP61 used as audio output valve would be. I was just saying that such a transformer would work as in you'd get something out whatever speaker you used.

Neil Purling · 4th Dec 2015, 10:21 pm

An EF91 will give portable radio audio strength w/o need for a volume control.
When I was trying it I forgot the grid-leak!
The penny eventually dropped when I wondered why such tiddler of a valve was getting so damn hot!

It survived the abuse..........

The higher gain valves I have mentioned would probably need a volume control

Neil Purling · 14th Dec 2015, 9:34 am

I have the SP61 with the same cathode bias resistor and grid resistor as the EF91: These are 150 ohms and 470K ohms respectively. The cathode resistor is bypassed by a 22uf 63V electrolytic. The valve is wired as a triode: g3 to cathode and g2 to the anode.
Would it be more effective with different wiring as a pentode, or with with different cathode bias? The o/p transformer is a 240:6V transformer driving a 8 ohm speaker.
The SP61 runs fairly hot, as you might expect of a output stage. I just wondered if the beggar is running too hot.

Bazz4CQJ · 14th Dec 2015, 2:11 pm

In using valves like EF91 and SP61 as output triodes, there seems to to be some divide of opinion about g3. That could be connected to the anode or to the cathode, but I've seen at least one datasheet for the EF91 saying g3 must always be connected to the cathode. You might like to evaluate what difference you see/hear between those two configs.

B

Bazz4CQJ · 14th Dec 2015, 2:53 pm

Whoops; have mis-read Neil's last post; he already has g3 to cathode.

PJL · 14th Dec 2015, 3:42 pm

You will lose quite a bit of gain running it as a triode. As a pentode it looks like it will drive 5K ohms according to the datasheet so with the 40:1 transformer the load should be greater than 3.1 ohms. However, even an 8 ohm load will not result in clipping.

Neil Purling · 14th Dec 2015, 9:07 pm

Is that SP61 going to be OkK with sucha a low value of cathode resistor, whether it is wired as a triode or as a pentode?

Herald1360 · 14th Dec 2015, 9:47 pm

Measure the anode current (or cathode current ( volts across cathode R)) and multiply by the anode voltage. Compare this with rated anode dissipation (2.75W at least) and see how it stands.

150R cathode resistor is OK for pentode operation but if resulting anode current in triode mode is > 12mA at 250V Va, you're overdoing it.

Neil Purling · 23rd Dec 2015, 10:22 am

I will increase the value of the cathode resistor if I find I need to.

26th Nov 2015, 10:32 pm	#61
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: High gain output valve The SP61 is rated to pass around 10mA of anode current. Assuming your HT voltage is 250 and you're running the valve biased in pure Class-A then you can expect a good Watt or so of audio (Class-A can never be more than 50% efficient). Treat it like an EF91 - various editions of "Practical Wireless" in the 1960s had receiver designs where the EF91 was used as an audio-output valve, and I believe at least one commercially produced TV receiver from the era did likewise.

27th Nov 2015, 10:38 am	#62
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: High gain output valve I have a 50:1 transformer, well it is marked 50:1 in pencil. It could have come out of something like a record player.

27th Nov 2015, 2:08 pm	#63
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: High gain output valve You can try any transformer with a load on the secondary without damaging thevalve. Just have a play. A 240:6V mains TX will work surprisingly well. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

27th Nov 2015, 11:08 pm	#64
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: High gain output valve Was the 240V-6V transformer for a 8 ohm speaker or 3-4 ohms used with the SP61? If 4 ohms then I only have one speaker that has this impedance & that is from a low power valve portable that used a DL9* as o/p.

1st Dec 2015, 5:26 am	#65
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	Re: High gain output valve Can anyone say what are the pro's & cons of running something like the SP61 or a EF91 either as a pentode or connected as a triode (screen grid connected to anode) for the AF output role? B Last edited by Bazz4CQJ; 1st Dec 2015 at 5:36 am.

4th Dec 2015, 10:21 pm	#67
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: High gain output valve An EF91 will give portable radio audio strength w/o need for a volume control. When I was trying it I forgot the grid-leak! The penny eventually dropped when I wondered why such tiddler of a valve was getting so damn hot! It survived the abuse.......... The higher gain valves I have mentioned would probably need a volume control

14th Dec 2015, 9:34 am	#68
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: High gain output valve I have the SP61 with the same cathode bias resistor and grid resistor as the EF91: These are 150 ohms and 470K ohms respectively. The cathode resistor is bypassed by a 22uf 63V electrolytic. The valve is wired as a triode: g3 to cathode and g2 to the anode. Would it be more effective with different wiring as a pentode, or with with different cathode bias? The o/p transformer is a 240:6V transformer driving a 8 ohm speaker. The SP61 runs fairly hot, as you might expect of a output stage. I just wondered if the beggar is running too hot.

14th Dec 2015, 2:11 pm	#69
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	Re: High gain output valve In using valves like EF91 and SP61 as output triodes, there seems to to be some divide of opinion about g3. That could be connected to the anode or to the cathode, but I've seen at least one datasheet for the EF91 saying g3 must always be connected to the cathode. You might like to evaluate what difference you see/hear between those two configs. B

14th Dec 2015, 2:53 pm	#70
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	Re: High gain output valve Whoops; have mis-read Neil's last post; he already has g3 to cathode.

14th Dec 2015, 3:42 pm	#71
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: High gain output valve You will lose quite a bit of gain running it as a triode. As a pentode it looks like it will drive 5K ohms according to the datasheet so with the 40:1 transformer the load should be greater than 3.1 ohms. However, even an 8 ohm load will not result in clipping.

14th Dec 2015, 9:07 pm	#72
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: High gain output valve Is that SP61 going to be OkK with sucha a low value of cathode resistor, whether it is wired as a triode or as a pentode?

14th Dec 2015, 9:47 pm	#73
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: High gain output valve Measure the anode current (or cathode current ( volts across cathode R)) and multiply by the anode voltage. Compare this with rated anode dissipation (2.75W at least) and see how it stands. 150R cathode resistor is OK for pentode operation but if resulting anode current in triode mode is > 12mA at 250V Va, you're overdoing it. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

23rd Dec 2015, 10:22 am	#74
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: High gain output valve I will increase the value of the cathode resistor if I find I need to.