50 Watt amplifier

[John Caswell]

Hi all,
Does anyone have a need for a 50Watt amplifier based on KT120 or the like?
Sensitivity about 500mV, u/linear with feedback.
Answers in a brown paper envelope here please

John

[pre65]

I know a friend who was thinking of building something along those lines.

[Ali Tait]

How much are you looking for John?

[John Caswell]

Interesting!
I am at present cobbling together one/two to try out. I have a WD 300B pp, the early type, but really needed a bit more power for my ProAc D28s so am trying the GEC 88/50, but using KT120 instead. Should have them both finished in a week or so, so wondered if there was any interest should WD decide to make them as a kit.
Before anyone else leaps on me, I have no idea if Matthew would want to go this route and no idea of cost. Considering you can buy something similar from Icon audio for about £2K this route may not be commercially viable
That's all really.

John,

[justin brown]

Hi John,
This is very interesting! I would love to hear a more powerful valve amp driving my Quad ESLs, but in this instance it has been suggested that, ideally, the ESLs are driven by an amp of about 80-100W. So that means something comparable to the Quad 11 eighties, or the Icon Audio MB845s. I have heard the MB845s, but it was a long time ago, before I really appreciated valves.
I have often read articles in HiFi World by Noel Keywood and have been told by Ron & Steve at One Thing Audio that the Quads are much happier with more power (although the 30w output by the 300bpp stereo amp is adequate), and that there is less damage to the speakers over time.
I too would be very interested to hear what Matthew has to say on this matter.


Justin

[pre65]

How's the new amp progressing John ?

[John Caswell]

Hi Phil et al,
Well both are built, and after a bit of minor twincing frequency response is flat ±1/2dB from 3Hz out to about 80KHz. There is a bit of a rise from 80kHz up to 100kHz but as it is so far outside the "normal" bandwidth, I haven't bothered to look at it yet.
Residual hum and noise -70dB unweighted and -85 "A" weighted wrt to 1watt (2.83V//8R) ie can't hear it, which in absolute terms is well below 1mV. Sensitivity 575mv for 50 Watts max at clip. Distortion at 1Khz = 0.013% and about 0.15% at 20kHz
Both use ECC83, ECC82, 2X KT120, all ac heating of valves but heater windings are centre tapped, however there are slight variations on a theme.
Amp1 has 2X (cheap) 6d22S as rectifiers amp2 has a (b****y expensive) GZ37, but it looks like options for rectifiers are going to be 5U4,5R4GY, 5T4, GZ37, 2x 6D22S, 2x EY84 plus of course SS (not sure about that).
Obviously using SS and Directly heated rectifiers the capacitors are a bit of a problem and would need at least 800V ratings, so 2 X 450V in series is the answer here.
I am using standard Hammond 1650RA OPTxs with a variation of available power tx and chokes to give 500V HT.
The 6D22s heaters are run off a small 30VA toroid but would possibly run off the 5V winding according to Eric Barbour. Haven't tried that yet!
The basic amp cct is identical to the GEC cct with minor changes to C7/8 to flatten lower LF hump.
I cannot see any reason why you could not use a 6SL7 for the ECC83 and a 6SN7 for the ECC82.
I am pretty sure I am going to build these in anger for myself as monoblocks in which case I would probably punch the chassis for Octal/B9G valves (6SN7/6SL7/6D22S/EY500) and make some small sub plates to fit the B9A types (ECC82/ECC83/EY84).
Each amp power requirements are 500V/180mA, 6.3V/4A and 5V/3A.
I am passing the amps over to Matthew Snell to listen to, but frankly for a design first shown in Wireless World April 1957 I am seriously impressed.

John

[Richard]

Hi John, all,

Yes following this with great interest! I too am thinking a bit more power might be good having changed from the Rogers LS6A to Castle Conways recently although they're both nominally 89dB/W.

I've been collecting a few bits together as and when for 5-20 or these same GEC 50W monoblocs though it won't happen very soon.

Be interesting to hear what you and Matthew think of them finished in a system and some pics would be great

[John Caswell]

Hi all,
Well at the moment they are lashed up on a pair of gash chassis I happened to have handy, but I need to get a semblance of ideas for the real thing.
I thought either long and thin chassis a la KEL80 with PSU at rear and amplifier at front, or a more squarer type with PSU at R/H side and amplifier on left with a mirror image for the other channel.
Obviously if WD were to supply them as a kit then both amps would be laid out identically to keep costs down.
Will try to get photos asap but do bear in mind these are lash ups, they will, however, give some of you ideas as to how to layout and maybe start things going. They are all hand wired/tag-boarded and none of the wires/components leads are at present shortened for neatness.
Reservoir (straight after rectifier) capacitor is a 25µF motor run cap and main smoothing is a 51µF 1000Vish polypropylene.
The cct calls for a cathode bias resistor of 525R 6watt and this is easily made up of a 560R 10Watt in parallel with and 8K2 5 watt = 524R close enough I feel. Everything else is readily available from all the usual suppliers, and at present no fancy audiophile components/wire, but I will probably use silver plated PTFE wire and some PIO or WD caps, there are really only 4 to worry about 2 off 0.22µF and 2 off 0.47µF all 630Vdc rated
Feed back at present is about 20dB to give a sensitivity of 575mV for 50watts into 8R and you could juggle with that a little to perhaps increase sensitivity to 400mV, but for those of you driving it with a Pre 3 where you have oodles of gain to play around with I suggest leaving the amp alone for starters.
More to come as and when.

John

[Richard]

Ah now I've gone about it backwards. I saw these heavy steel chassis on Mike Holmes site and took it from there. I particularly liked the dimensions 12" x 8" x 2 1/2" which will allow 2 side-by-side, making 16" x 12", on a standard rack shelf which in my case is 18" x 15" if that helps in your thinking.

You'll recognise the sockets John, thank you kindly, they fit the holes perfectly but it has taken about 3 years so far as I keep getting distracted. The rear mains TX holes are cut but the rest is plain and I've yet to settle on a circuit so it could easily take me another 5 years... you know the old saying, I used to be indecisive but now I'm not so sure...