|
|
|
Dec 22, 2008 13:29:40 GMT
|
Having a bit of a sort out I found the valve amp I built as the centrepiece of my hifi when I was a student. It's a simple class A design I stole from a 1960s valve telly, with a single PCL86 triode pentode on each channel. Power is about 5 watts RMS per channel. I built it in a chassis from a broken piece of valve-era modular test equipment. What you see is the modular bit, the case it plugs into has the power supply. Unfortunately the power supply died, I must fix it one day. Either way given 12v for the heaters and 150v HT what you see would still perform as a hifi amp. Quality? Not too bad, good frequency response. Phase response is curse word, mainly because I had to give it some feedback to cut out the midrange "hump" from the cheap output transformers. But it was pretty good though I sez it myself, and among my peer group of geeky early-90s EE students it gave me SERIOUS bragging rights! Look! A DIN socket, how quaint!   |
| |
Last Edit: Dec 22, 2008 13:30:16 GMT by herald948
"Jeremy Clarkson, a man we motor enthusiasts need on our side like Lewis Hamilton's F1 car needs a towing ball and a Sprite Musketeer" My motor |
|
|
|
|
|
circa 1991 stereo valve ampBenzBoy
@benzboy
Club Retro Rides Member 7
|
Dec 22, 2008 23:59:21 GMT
|
Nice! I'll admit the first time I ever saw a valve set-up was when I opened up an old Dansette turntable the other week. Really cool to see it glowing like that!  Unfortunately the Dansette needs work and is probably (definitely) beyond my capabilities. |
| |
|
|
|
|
|
|
|
Nice! I'll admit the first time I ever saw a valve set-up was when I opened up an old Dansette turntable the other week. Really cool to see it glowing like that! Unfortunately the Dansette needs work and is probably (definitely) beyond my capabilities. What's up with it? They're usually pretty simple. Those turntables are not always the best for your vinyl though. |
| |
"Jeremy Clarkson, a man we motor enthusiasts need on our side like Lewis Hamilton's F1 car needs a towing ball and a Sprite Musketeer" My motor |
|
|
|
|
Dec 25, 2008 12:03:32 GMT
|
|
What did you use for the power supply? I only see two transformers?
|
| |
|
|
|
|
|
Dec 26, 2008 20:38:47 GMT
|
|
I used a modular enclosure, that's the module unplugged. The enclosure came with a pretty standard valve-era PSU, nothing special, a filtered 150v. The heaters came from a 12v line intended in the original enclosure for transistor circuits, the "P" in PCL86 means it has a 300mA heater chain, in the case of that valve about 13v.
|
| |
"Jeremy Clarkson, a man we motor enthusiasts need on our side like Lewis Hamilton's F1 car needs a towing ball and a Sprite Musketeer" My motor |
|
|
|
|
Dec 26, 2008 20:41:32 GMT
|
|
Those are new valves BTW, Pinnacle branded, made in Yugoslavia in 1975. I originally used two different age Mullard valves, but when I saw a valve dealer at a radio rally I bought a couple of new ones for ISTR the amazing sum of 50p each. "P" heater valves never go for much because outside of 1960s TV sets not many were used.
|
| |
"Jeremy Clarkson, a man we motor enthusiasts need on our side like Lewis Hamilton's F1 car needs a towing ball and a Sprite Musketeer" My motor |
|
|
|
|
Dec 26, 2008 20:46:48 GMT
|
|
I also remember high voltage rated components being surprisingly difficult to find in an early 1990s university electronics department stores. That's why the keen-eyed amongst you will notice big yellow mains suppression capacitors, probably the only time they've been used in a hi-fi amplifier.
|
| |
"Jeremy Clarkson, a man we motor enthusiasts need on our side like Lewis Hamilton's F1 car needs a towing ball and a Sprite Musketeer" My motor |
|
|
|
|
|
|
|
Yeah I used to think they would be trouble to get hold of, at least audio grade anyway, but Maplin used to do a whole range of HV components and new valves. I'm keeping an eye on eBay at the moment for some power transformers to get my amp back up and running because Maplin discontinued the transformer they used in their kit amp. It's quite a big transformer so I might just end up making a smaller amp with EL84's for simplicity and I can sell the bigger amp on as a unit for spares/repair.
|
| |
|
|
|
|
|
|
|
With the resurgence of interest in valve amps through the last decade it's become a lot easier to pick up valve-spec components. Audio transformers for instance, those transformers are slightly nasty ones that RS used to stock, I have no idea why they still carried them in the early 1990s as they're of little use for transistor circuits but nowadays a simple Google search reveals suppliers of far better alternatives that just weren't available to me then. www.vvttransformers.co.uk/output_se_tx.htm for example. Not cheap though. What HT transformer do you need? Guesses... 300-0-300@250mA? Or is it a bigger amplifier than that? I'm lucky with my little single ended PCL86, less volts required so the 150v instrument supply was fine. |
| |
"Jeremy Clarkson, a man we motor enthusiasts need on our side like Lewis Hamilton's F1 car needs a towing ball and a Sprite Musketeer" My motor |
|
|
|
|
Dec 28, 2008 11:26:11 GMT
|
|
Blimey, I wrote the spec on the transformer ;D
6.3V @ 5A and 0-350V @ 200mA which is full wave rectified.
I reckon I could use one off an old Gibson amp, they turn up every now and then on eBay, the other option is I underdrive it on mains 230V on the HT and plumb in a 6V or 12V transformer driving the heaters. I think that should just lead to a reduced output, but since it used to pump out a hell of a lot more than its 25W per channel I don't think I'll actually lose much?
|
| |
|
|
|
|
|
|
|
Dec 28, 2008 11:33:02 GMT
|
|
|
| |
|
|
|
|
|
Dec 28, 2008 13:37:27 GMT
|
Blimey, I wrote the spec on the transformer ;D 6.3V @ 5A and 0-350V @ 200mA which is full wave rectified. I reckon I could use one off an old Gibson amp, they turn up every now and then on eBay, the other option is I underdrive it on mains 230V on the HT and plumb in a 6V or 12V transformer driving the heaters. I think that should just lead to a reduced output, but since it used to pump out a hell of a lot more than its 25W per channel I don't think I'll actually lose much? If not then the "standard" 250-0-250 and 6.3 and 5v for the rectifier as found in most off-the-shelf valve-era mains transformers should provide you with enough volts. I would not expect the slightly reduced voltage to cause you any damage, after all once it's rectified and filtered it'll be more than the RMS voltage. |
| |
"Jeremy Clarkson, a man we motor enthusiasts need on our side like Lewis Hamilton's F1 car needs a towing ball and a Sprite Musketeer" My motor |
|
|
|
|
Dec 28, 2008 19:00:45 GMT
|
|
I think the power from the output is determined by the HT voltage though, just the valve can only supply a maximum current based on the size of the valve. That's what my brain works it out as, otherwise you'd be able to drive a miniature valve like an EL84 pentode at 1400V and get a massive current output from it just based on the voltage rather than step up to the larger EL34 or a KT88. Not sure on the hows and whys there ;D
|
| |
|
|
|
|
|
Dec 30, 2008 10:06:12 GMT
|
I think the power from the output is determined by the HT voltage though, just the valve can only supply a maximum current based on the size of the valve. That's what my brain works it out as, otherwise you'd be able to drive a miniature valve like an EL84 pentode at 1400V and get a massive current output from it just based on the voltage rather than step up to the larger EL34 or a KT88. Not sure on the hows and whys there ;D I'm afraid I don't know the full story on valve design either  Is power alone such a consideration for you? Remember a valve amp can be driven to a greater percentage of its capacity while still sounding good than can a transistor amp so if your speakers are well matched to it you shouldn't notice your reduced power. The amp above for example is impossibly weedy by modern transistor amp standards but the crucial difference is that I can turn it up to 11 without sacrificing my tweeters. I do know the maximum voltage you can use on a given valve comes down to the distance between the anode and cathode governing the voltage at which discharge occurs. When I built the first prototype of the amp above I used a 250v transformer and silicon rectifier, followed by a very period dropper resistor to give me the ~160v the pentode in a single ended PCL86 needs. What I got was an extremely pretty purple glow from the PCL86 for the 10s or so until the valve heated up. Back in the day they would have used a valve rectifier so the HT would only have arrived as both valves heated up, my silicon rectifier meant the HT was there straight away and since there was negligible current passing through the dropper resistor until the PCL86 heated up it got the full 250v for the first few seconds. It didn't seem to do any damage and it certainly looked rather impressive, but I figured a better PSU was in order |
| |
Last Edit: Dec 30, 2008 10:08:25 GMT by herald948
"Jeremy Clarkson, a man we motor enthusiasts need on our side like Lewis Hamilton's F1 car needs a towing ball and a Sprite Musketeer" My motor |
|
|
|
|
|
|
|
Mine is bridge rectified with diodes so the valves get the full 350V from switch on. It doesn't seem to bother them at all either.
|
| |
|
|
|
|
|
|
|
Mine is bridge rectified with diodes so the valves get the full 350V from switch on. It doesn't seem to bother them at all either. If it's a double ended design then each valve can only ever get half the HT and then only when the heater on the other valve has started up. The way I had it as described above the valve got the whole lot at first, dropping to the working voltage as the current started to flow when the heater came up. |
| |
"Jeremy Clarkson, a man we motor enthusiasts need on our side like Lewis Hamilton's F1 car needs a towing ball and a Sprite Musketeer" My motor |
|
