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Old 14th Aug 2006, 10:41 pm   #1
Steve_Bell
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Default Stereo Valve amp

Based on the Radford STA25, it gives about 35w RMS per channel. Complete with OTT regulated HT supply! Chassis and PCBs all my own work.
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Old 14th Aug 2006, 11:22 pm   #2
yestertech
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Default Re: Stereo Valve amp

Ooh ! fab ! I must dust off my chassis punches immediately ! Did you have the chassis made or was it a stock item - The PCB's look great ANDY

PS I have one of those OP TXF's somewhere - Aren't they the Maplin ones ?
How does the bass end hold up at 35 W ??
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Old 15th Aug 2006, 9:15 am   #3
JoshWard
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Default Re: Stereo Valve amp

That looks great! And very neatly layed out too! Josh.
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Old 15th Aug 2006, 10:07 am   #4
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Default Re: Stereo Valve amp

Steve,
Looks a lovely job. I bet it sounds good too!

Well done

Aub
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Old 15th Aug 2006, 10:35 am   #5
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Default Re: Stereo Valve amp

Looks like a professional bit of kit. That reminds me about 5 years ago I assembled a full kit of parts to build a similar project (stereo p-p valve amp using KT66's) but never got round to building it! Maybe this Winter I'll have a go and try and get it going. Possibly...

Biggles.
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Old 15th Aug 2006, 3:53 pm   #6
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Default Re: Stereo Valve amp

Hi Gents, for those who do not have the price of KT66's. There has been a re-vamp of The Mullard 5-10 and 5-20 amps. These now used fixed bias and a lower level of feedback (and also eliminate some coupling caps). This has improved a decent amp no end. I've built one of each for friends and they were delighted with them.

Ed
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Old 15th Aug 2006, 6:15 pm   #7
Steve_Bell
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Default Re: Stereo Valve amp

Thanks for the plaudits! One of my better efforts, and I still use it.

The output transformers aren't as good as real Radfords but are actually OK. And yes, they are the Maplin ones. They do lack full power output at the lower end of the original frequency response. Can't remember any figures though.

As for the chassis, it was made by me, the basis being half of a miniature PC case, as in this link http://www.datasalen.se/Utstallning/...ild/vib111.JPG. You can see the slots at the back of the amp are those on the top of the PC. The front and rear panels are 3mm aluminium and there are several bracing pieces in the amp. Hole punches then applied! Then sprayed with a metallic purple paint,a Ford colour ISTR, then lacquered.

PCBs made at home too but still some point-to-point wiring for heaters and connection to transformers, etc.

As Ed says, its EL34s in fixed-bias, you can see the multiturn pots to adjust. The voltage test points are across the cathode resistors, the black wire-wounds at the top of the PCBs, adjacent to the large valve holders.

Last edited by Steve_Bell; 15th Aug 2006 at 6:21 pm. Reason: stated transformers were from Maplin
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Old 15th Aug 2006, 6:25 pm   #8
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Wink Re: Stereo Valve amp

Well, compared to some commercial kit that I've seen over the years, this certainly holds it's own!
Well done!

Al / G8DLH
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Old 15th Aug 2006, 6:39 pm   #9
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Default Re: Stereo Valve amp

Wonderfull piece of work chap!
I too use the Maplin output transformers and am very pleased with the sound they produce.
I'll have to dig out some pics of mine to show off!!!!!
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Old 15th Aug 2006, 6:46 pm   #10
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Default Self-built Valve amp.

As promised in my response on my message to Steve_Bell about his lovely self-built valve amp, here's mine!
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Old 15th Aug 2006, 6:59 pm   #11
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Default Re: Stereo Valve amp

What a load of rubbish. The ECF82s are Brimars and the 2 channels have EL34s from different manufacturers

It must sound awful, even with an oxygen free mains lead and a gold plated plug. Some people have no understanding of high fidelity engineering at all

Paul
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Old 15th Aug 2006, 7:10 pm   #12
Steve_Bell
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Default Re: Stereo Valve amp

Actually, mine did have a full set of Mullards but guess what? I sold 'em to the "golden-eared ones"! That probably paid for most of the transformers and hardware....tee-hee
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Old 15th Aug 2006, 7:30 pm   #13
percival007
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Default Re: Stereo Valve amp

Sorry Steve,
i didn't realise i'd put my pictures in your thread!
Don't want to steal your thunder as it were.
Maybe the moderators could move it??
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Old 15th Aug 2006, 7:41 pm   #14
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Default Re: Stereo Valve amp

Quote:
Originally Posted by percival007 View Post
Sorry Steve,
i didn't realise i'd put my pictures in your thread!
Don't want to steal your thunder as it were.
Maybe the moderators could move it??
No you didn't

I moved it to keep all relevent items together, which makes sense to me, at least I think so, if you want it separating, drop me a PM and I'll have a go.

Cheers
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Old 15th Aug 2006, 7:42 pm   #15
percival007
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Default Re: Stereo Valve amp

No, if you think it's ok there and Steve doesn't mind then leave it where it is
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Old 15th Aug 2006, 7:46 pm   #16
Steve_Bell
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Default Re: Stereo Valve amp

Yes, by all means keep percival007's amp here.
What is the chassis made from?
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Old 15th Aug 2006, 7:52 pm   #17
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Default Re: Stereo Valve amp

There is no real chassis as such. The box is of MDF construction, hand veneered. I have attached metal boxes inside, into, and on to are mounted the electronics. The valves "poking" through!!
The metal boxes are used to prevent interference being induced in to the circuit and to help prevent "crosstalk".
Sorry, no pics of the internals, and i'm not "un-plumbing" it as it's my "everyday" amp!!!
Cheers, Percy.
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Old 15th Aug 2006, 8:38 pm   #18
Jim - G4MEZ
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Default Re: Stereo Valve amp

Quote:
Originally Posted by paulsherwin View Post
What a load of rubbish. The ECF82s are Brimars and the 2 channels have EL34s from different manufacturers


Paul
Speaking of which, I see one pair of EL34s have evacuation pips on top of them. Never seen that before! What make are they?

An excellent piece of construction. I'm very impressed...
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Old 15th Aug 2006, 9:02 pm   #19
Steve_Bell
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Default Re: Stereo Valve amp

I had never seen EL34s with pips either. They're Siemens.
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Old 15th Aug 2006, 9:05 pm   #20
percival007
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Default Re: Stereo Valve amp

how do they sound?
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