Valve Amp Identification? - UK Vintage Radio Repair and Restoration Discussion Forum

Stamford · 17th Oct 2020, 10:18 am

Could anyone shed some light or give any info on this Amp? Hopefully The photos provide a clear picture.

All help welcome.

Thanks

lesmw0sec · 17th Oct 2020, 10:49 am

It looks like a home-brewed one-off to me!

barrymagrec · 17th Oct 2020, 10:54 am

Chassis is nicely made but the wiring is definitely functional home made. Components say mid late fifties very early sixties.

Seems to be missing a PSU.

Aub · 17th Oct 2020, 11:04 am

Seperate PSU required?

Station X · 17th Oct 2020, 11:19 am

It appears to have smoothing capacitors. Is that a a mains transformer or an output transformer?

Silicon · 17th Oct 2020, 11:23 am

Is it safe to assume that it is an audio amplifier for loudspeakers and not a modulator for a transmitter?

ms660 · 17th Oct 2020, 11:24 am

Looks like ultra linear, the b7g valve looks interesting.

Lawrence.

barrymagrec · 17th Oct 2020, 11:30 am

EL90 or N78? could be a screen regulator for the EL34s.

However there is a corresponding empty base on the other side.

Edward Huggins · 17th Oct 2020, 2:59 pm

Are those 3 vertically mounted pots used for varying the valve/s bias?

Stamford · 17th Oct 2020, 5:40 pm

Thanks guys... your help is gratefully appreciated.

Radio Wrangler · 17th Oct 2020, 6:53 pm

Looks ultra-linear pair of EL34s. Power must be from something external.

Surprising number of trimmers. I wonder if this is not self-biased for max power and efficiency.

The switches are mysterious.

Measurement of transformer turns ratio should show whether it's for driving a speaker or modulating a transmitter.

Modulating a full legal power transmitter usually took a pair of KT88 to do comfortably, producing 100W You hd atransmitter with 150W DC input (legal limit) which meant about 100W output and 400W PEP, so the modulator had to boost the TX power rather dramatically. With a pair of EL34, it was a harder task.

I'd guess it was a modulator. Interesting what the transformer might be THe Woden UM series are easily recognised.

Full welly transmitters would use something like an 813, and the mod transformer secondary and all cabling from it would be up in the 2kV region with deadly current availability.

This looks to be from someone getting their power up, but a bit below the licence limit.

David

Diabolical Artificer · 18th Oct 2020, 8:10 am

Interesting beast. That looks like just the amplifier that needs a separate PSU. looking at it closely there seems to be a balance pot on the OP to either balance AC or set bias, there's a wire to it from the chassis plug. But then and again there's two big resistors (150r?) on the OP valve bases which with the the 16u caps could mean it's cathode biased.

The balance/trimmer pots, possible screen regulation and what looks like a zobel R/C on the OP point to an audio amp, g2 wouldn't be regulated in a TX would it?

If you could trace the circuit out that'd be great, it looks a really interesting amp, if not the OPT, if that's what it is looks nice and chunky.

Andy.

17th Oct 2020, 10:18 am	#1
Stamford Diode Join Date: Aug 2018 Location: Chichester, West Sussex, UK. Posts: 6	Valve Amp Identification? Could anyone shed some light or give any info on this Amp? Hopefully The photos provide a clear picture. All help welcome. Thanks Attached Thumbnails

17th Oct 2020, 10:49 am	#2
lesmw0sec Octode Join Date: Jul 2009 Location: Carmel, Llannerchymedd, Anglesey, UK. Posts: 1,509	Re: Valve Amp Identification? It looks like a home-brewed one-off to me!

17th Oct 2020, 10:54 am	#3
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,562	Re: Valve Amp Identification? Chassis is nicely made but the wiring is definitely functional home made. Components say mid late fifties very early sixties. Seems to be missing a PSU.

17th Oct 2020, 11:04 am	#4
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,040	Re: Valve Amp Identification? Seperate PSU required? __________________ Life's a long song, but the tune ends too soon for us all.

17th Oct 2020, 11:19 am	#5
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Valve Amp Identification? It appears to have smoothing capacitors. Is that a a mains transformer or an output transformer? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

17th Oct 2020, 11:23 am	#6
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,171	Re: Valve Amp Identification? Is it safe to assume that it is an audio amplifier for loudspeakers and not a modulator for a transmitter?

17th Oct 2020, 11:24 am	#7
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Valve Amp Identification? Looks like ultra linear, the b7g valve looks interesting. Lawrence.

17th Oct 2020, 11:30 am	#8
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,562	Re: Valve Amp Identification? EL90 or N78? could be a screen regulator for the EL34s. However there is a corresponding empty base on the other side.

17th Oct 2020, 2:59 pm	#9
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,340	Re: Valve Amp Identification? Are those 3 vertically mounted pots used for varying the valve/s bias? __________________ Edward.

17th Oct 2020, 5:40 pm	#10
Stamford Diode Join Date: Aug 2018 Location: Chichester, West Sussex, UK. Posts: 6	Re: Valve Amp Identification? Thanks guys... your help is gratefully appreciated.

17th Oct 2020, 6:53 pm	#11
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,905	Re: Valve Amp Identification? Looks ultra-linear pair of EL34s. Power must be from something external. Surprising number of trimmers. I wonder if this is not self-biased for max power and efficiency. The switches are mysterious. Measurement of transformer turns ratio should show whether it's for driving a speaker or modulating a transmitter. Modulating a full legal power transmitter usually took a pair of KT88 to do comfortably, producing 100W You hd atransmitter with 150W DC input (legal limit) which meant about 100W output and 400W PEP, so the modulator had to boost the TX power rather dramatically. With a pair of EL34, it was a harder task. I'd guess it was a modulator. Interesting what the transformer might be THe Woden UM series are easily recognised. Full welly transmitters would use something like an 813, and the mod transformer secondary and all cabling from it would be up in the 2kV region with deadly current availability. This looks to be from someone getting their power up, but a bit below the licence limit. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

18th Oct 2020, 8:10 am	#12
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,670	Re: Valve Amp Identification? Interesting beast. That looks like just the amplifier that needs a separate PSU. looking at it closely there seems to be a balance pot on the OP to either balance AC or set bias, there's a wire to it from the chassis plug. But then and again there's two big resistors (150r?) on the OP valve bases which with the the 16u caps could mean it's cathode biased. The balance/trimmer pots, possible screen regulation and what looks like a zobel R/C on the OP point to an audio amp, g2 wouldn't be regulated in a TX would it? If you could trace the circuit out that'd be great, it looks a really interesting amp, if not the OPT, if that's what it is looks nice and chunky. Andy. __________________ Curiosity hasn't killed this cat...so far.