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Old 22nd August 2014, 04:45 PM   #1
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Default ideas for a simple ECL82/86 p-p amp?

Hi Gents (and ladies).

Please be gentle I'm new here and it's my first post.

I have collected a set of output transformers and a mains transformer suitable to build a push pull power amp using either ECL82 or ECL86 types. As I need to use a separate heater transformer, I can have the option of using PCL/UCL82 and PCL86 which could be a potential saving as these old tv valves seem plentiful in NOS form.

The mains transformer is rated at 240-0-240V x 150mA secondary and 6.3V x 1A which I could use to fire up an EZ81(6CA4) rectifier, or use as heater source for a preamp valve if used and silicon rectifiers.
The output transformers are Radiospares (UK) "Hygrade" devices and are 8k centre tapped (4-0-4k) and the transformer is rated at 60mA, I assume that's 60mA through each 4k winding which will give me plenty of headroom, hell I could possibly use EL84 in push pull!
I'd Ideally like a design that used just the pair of ECL8* per channel, maybe with a concertina phase splitter. I have seen designs using a long tail pair and no preamp valve but it's a bit insensitive at 800mV and I'd prefer around 400 to 500 mV but would be ok as low as 250mV. The Mullard designs are too sensitive at 100mA and I'd be attenuating sources too much I feel.
I hope thats enough information to go on and I'll be grateful for any info and tips.
I have a bit of experience and recently built a SE EL84 amp based on Alex Kitic's RH84 design which was very successful and amazingly completely hum free, no mean feat on an SE amp.

thanks in advance.

Andy.
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Old 22nd August 2014, 06:22 PM   #2
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This is what I'd do with it.

Sensitivity seems to be ok, full power at about 150mV in.

A balanced amplifier will have absolutely no B+ (or B-) hum. PSU design is not paramount, although I would still use a simple two MOSFET regulator.

Use IRF820 MOSFETs or what you have available.
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Old 22nd August 2014, 06:54 PM   #3
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Andy,

Go with the 6BM8 or its alternate heater voltage siblings. They are much easier to source than the 6GW8 and its close relations.

If you decide on SS B+ rectification, use 1200 PIV/2 A. Schottky diodes made by Cree. There are no reverse recovery spikes, when Schottkys are employed.

Round the schematic for the ASL AV-8 up. It's exactly what you have in mind.
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Old 22nd August 2014, 07:33 PM   #4
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Mr Curwen, thanks for the circuit but its not what I had in mind, with all those CCS etc.
Eli, I take your point on 6GW8, being European I will say ECL86! If I used that "toob" Then I'd use the PCL86 which is fairly plentiful being an old TV valve and its no problem for the odd heater voltage as I need to use a separate transformer for the filaments. I reckon I might even get away with a 12V AC transformer as the PCL86 seems to need 13.3V and although its a bit on the low side, I'd bet I was getting close enough especially as the UK mains tends to be nearer 250V than 230V. Of course I could go DC and feed them from a 300mA Current source. I would prefer an EZ81 rectifier but as you say there are solid state alternatives. A couple of UF5408 or some schottkies if feeling flush, I have had good results with my KT88 amp by snubbing the rectifiers with .022uF polywotsit capacitors.
Whats this ASL AV-8?

Andy
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Old 22nd August 2014, 08:33 PM   #5
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For AV-8 info., try Wave AV-8 Info. The link may be dead. Should that be the case, try the Way Back Machine.
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Old 22nd August 2014, 08:40 PM   #6
Bigun is online now Bigun  Canada
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This is the schematic from the link posted above:
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Old 22nd August 2014, 10:30 PM   #7
Jaap is offline Jaap  Netherlands
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The schematic looks awful but it sounds terrific (good)
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Old 22nd August 2014, 11:52 PM   #8
Matt BH is offline Matt BH  United Kingdom
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The RS "hygrade" output transformers are pretty good. I have a couple of them but have not built anything using them as they seem to be worth a fair few quid to the Marshall 18W guitar amp crowd.

You could certainly use the E/PCL86 with them but how about thinking out of the box a bit and using something like a single TT15? Or an 832/GU32?

Cheers
Matt.
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Old 22nd August 2014, 11:55 PM   #9
Matt BH is offline Matt BH  United Kingdom
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Forgot to ask, why does it need to be so sensitive?
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Old 23rd August 2014, 07:17 AM   #10
chrish is offline chrish  Australia
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Here is a copy of the wave 8 design mentioned above by Eli in .pdf format. I built this amp a few years ago and it works well. I am listening to it now in my study...
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