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Old 10th Sep 2011, 8:55 pm   #1
maninashed
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Default Linear Conchord/RSC A11 30 watt amp

Hi everyone, the Linear Conchord 30 watt amplifier was very common in the 60s/early 70s and was sold ready made or in kit form by RSC Hifi centres all round the country and was very popular. The ready built versions used the
EL34 output pentode but the kit form used the 807 tetrode valve which at the time was plentiful in the surplus market. Its original use was as a transmitter PA valve I believe. My question is, does anyone have a copy of the original circuit diagram of the RSC/ Linear amp using the 807? It used a different biasing arrangement to the EL34 versioin. Im not interested in the PW version as I think its inferior. My intention is to build a stereo valve amp of approx 40w per channel using the 807, with some circuit modification to improve performance. Any advice or assistance would be welcome.
Bill
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Old 11th Sep 2011, 5:21 pm   #2
Ed_Dinning
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Default Re: Linear Conchord/RSC A11 30 watt amp

Hi Bill, I have a Geloso version of this type of amp using 807's and it sounds very good. It runs at about 450v on the anodes and the fixed bias is approx -36V.

Ed
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Old 11th Sep 2011, 7:04 pm   #3
Valvepower
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Default Re: Linear Conchord/RSC A11 30 watt amp

Hi Bill,

The early Vortexion 50 Watt amplifier used 807 valves. Iíve attached the circuit if it helps. Sadly there are no voltages marked on the diagram.

Iíve seen a fixed bias circuit for a 75 Watt 807 amplifier in one of the Brimar Valve and Teletube manuals. Maybe if you think it's of interest Iíll see if can find it and scan it for you?

Terry.
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Old 11th Sep 2011, 8:32 pm   #4
maninashed
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Default Re: Linear Conchord/RSC A11 30 watt amp

Thanks lads, Ed, I've got a mains tx that will put 450 or so on the anodes, the 807 will take a high voltage. What I need most of all, is the biasing arrangement using a standard type mains tx, ie 450-0-450 6.3 5.0, I'm aware that the concord used a standard tx but the centre tap was lifted from ground to give the bias for the 807. Terry, if you could send me the schematic for that amp I would appreciate it, I can pm you with contact details. Many thanks.
Bill
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Old 11th Sep 2011, 9:13 pm   #5
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Default Re: Linear Conchord/RSC A11 30 watt amp

Quote:
Originally Posted by maninashed View Post
Thanks lads, Ed, I've got a mains tx that will put 450 or so on the anodes, the 807 will take a high voltage. What I need most of all, is the biasing arrangement using a standard type mains tx, ie 450-0-450 6.3 5.0, I'm aware that the concord used a standard tx but the centre tap was lifted from ground to give the bias for the 807. Terry, if you could send me the schematic for that amp I would appreciate it, I can pm you with contact details. Many thanks.
Bill
You might might need to ensure a decent bias regulation if deriving -Ve the bias supply from the centre tap of the HT transformer as the HT current draw will vary a lot with push pull O/P as the valves swing between low and high current draw.

Hope this is of some use.

Lawrence.
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Old 12th Sep 2011, 10:41 am   #6
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Default Re: Linear Conchord/RSC A11 30 watt amp

I've got a schematic I traced from an actual amplifier that I own awaiting renovation.
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Old 12th Sep 2011, 6:45 pm   #7
Ed_Dinning
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Default Re: Linear Conchord/RSC A11 30 watt amp

Hi Bill, the bias supply can be derived from a resistor in the centre tap, but it is not a good design.
By far the best way is to use a small (say 5va) transformer to derive about 50 volts, even voltage doubling to get a higher output is OK, as the current requirement is so low.
If you want the Geloso circuit PM me with your e-mail and I'll send you a scan.

Ed
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Old 12th Sep 2011, 8:07 pm   #8
ukcol
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Default Re: Linear Conchord/RSC A11 30 watt amp

Hi Tony

That is a very nice drawing you have produced. What drawing program did you use?

Please use this link to a new thread for your reply, to avoid taking this one off topic.

http://www.vintage-radio.net/forum/s...703#post464703


Regards
Colin
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Old 13th Sep 2011, 9:38 am   #9
maninashed
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Default Re: Linear Conchord/RSC A11 30 watt amp

Tonyharker, thats the one! Nice drawing too, thanks for posting that. I recall though, that the smoothing capacitor on mine was only a 32uf 450v component isolated from chassis) Also your version employs far more smoothing components on the ht rail than I ever remember! Its true that this was not a 'quality' amp, it used cheaper components and transformers, however I do still own a couple of them which use EL34s and I have to say they do actually sound quite good. (I remember some years ago, I used 2 of these amps for the school dances etc and there was plenty of volume in quite a large hall through large banks of 12" speakers) Although far from HiFi! there is far too much hiss mainly from the high gain circuitry of the EF86s. My intention now is to construct from scratch, a stereo version using 807s, high quality transformers and improved circuitry. I think the suggestion re the seperate biasing transformer is a good one, although Im not sure what effect this may also have with crosstalk. Anyone any ideas?, also suggestions for circuitry would be appreciated.
Bill
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Old 13th Sep 2011, 9:53 am   #10
MarcoBerti
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Default Re: Linear Conchord/RSC A11 30 watt amp

In these days I'm working on a Linear L50, on which I've experimented stabilized B+, with astonishing effects on dynamic range - I'll talk about it in a separate thread. I too had too much hiss but cured it removing the cathode bypass capacitor on the first EF86.
Cheers

Marco
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Old 13th Sep 2011, 10:13 am   #11
maninashed
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Default Re: Linear Conchord/RSC A11 30 watt amp

Hi Marco, that sounds really interesting and it would be nice if you could share that design here with the lads on the forum, also I would love to see that schematic if you could send it to me.
Bill
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Old 13th Sep 2011, 11:03 am   #12
MarcoBerti
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Default Re: Linear Conchord/RSC A11 30 watt amp

Thank you Bill,
as I've said I'm going to start a new thread on it this evening.
However I'd like to anticipate a shot of my "Blue Linear": the added radiator hides a BU931 darlington.

Marco
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Old 14th Sep 2011, 5:44 pm   #13
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Default Re: Linear Conchord/RSC A11 30 watt amp

Quote:
Originally Posted by maninashed View Post
Tonyharker, thats the one! Nice drawing too, thanks for posting that. I recall though, that the smoothing capacitor on mine was only a 32uf 450v component isolated from chassis) Also your version employs far more smoothing components on the ht rail than I ever remember! Its true that this was not a 'quality' amp, it used cheaper components and transformers, ---snipped
Bill
There one an EL34 type explode on stage as the output had been a number of extra capacitors added to the HT before I got it presumably to reduce hum?
I used one of of these and two EL34 variants, in my youth as guitar amplifiers and PA. I had the PA cathode decoupling caps failed. Foil and electrolyte everywhere. I cleaned it up and replaced the caps, and was ready for the next gig the following week. We were very short of money in those days (mid 60s) and didn't get paid much for playing!
Incidentally I used MS Visio to do the drawing.
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Old 14th Sep 2011, 10:22 pm   #14
maninashed
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Default Re: Linear Conchord/RSC A11 30 watt amp

Hi Tonyharker, I too have had them go wrong but they were so easy to fix that they would be ready for another gig the next night. It was amazing how many bands started out using those Linear/RSC/ Northwood cage amps there were hundreds of them and many still survive to this day. Other disco lads and bands would bring their amps to me during the week with instructions that they needed them for gigs on a Friday night, much to the consternation of my old man, when he saw the guys coming up his path with long hair and greatcoats! Good days though, but as you say money was tight and you had to make do with whatever parts and spares you could lay your hands on. I recall that my uncle, who was an area manager for DER (TV rentals) used to bring me chassis from old sets that were scrap so I could canibalise them for spares! How times have changed. Its nice to 'keep your hand in' though.
Bill
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