John Wood Audio, a tale of woe

[Audio Al]

Hi All,
Just an update, unfortunetly i havent had chance to inspect the amp in question yet, been very busy the last few months, and having to deal with a long standing illness, anyway, i am hoping to get to it soon, once i have inspected it, i will of course put up a full report of my findings here for all those interested!
Anthony,TD...

OK

Looking forward to the findings / report and auction

[anthonyTD]

Hi All,
Well, its been a while, firstly i Appologise for the wait, at last i can confirm that this morning i unpacked the John Wood amplifier for the first time, and it is now sat on my bench, i will follow up later today with an indepth explanation of my first impresions, and findings.
Anthony,TD...

[Audio Al]

Hi All,
Well, its been a while, firstly i Appologise for the wait, at last i can confirm that this morning i unpacked the John Wood amplifier for the first time, and it is now sat on my bench, i will follow up later today with an indepth explanation of my first impresions, and findings.
Anthony,TD...

Woo Hoo , look forward 2 this

[anthonyTD]

Hi All,
After initial unpack, and visual inspection, unfortunately, it became obvious that this amplifier was not treated kindly in transit, the transformers on this amplifier are very heavy, and therefore they’re weight and hefty size have buckled the chassis slightly, and it has distorted significantly in certain areas etc.

Now I have had chance to inspect the amplifier, I would first like to put to bed some of the aspersions that may have been banded about at the time AOS members first read about this amplifier.
First, it is not a modified Chinese amplifier, [even I was somewhat originally under this impression]
Secondly, it is a true hard wired amp.
Thirdly, it uses high quality components.
[refer to picks I will post later]

The first thing I did after getting the amp on the bench was to test the valves.
The amp uses 2 x ECC82/12AU7 in the first stages, 2x 6SN7 in the second, and driver stage, And 4 x KT88 in the output stage. The output stage is grid biased, with individual bias setting controls on each output valve, which in some respects negates the need for closely matched output valves.
The amp also has adjustment for balancing the driver circuits, which offers the advantage of being able to adjust the signal being fed to each output valve quite accurately , which in turn helps in reducing the over-all distortion.
The gain stage valves all have DC supplied heaters to ensure low noise.

On first inspection of the four output valves [Sovtek KT88] I found that three of them had signs of being severely over-run, ie; the logo’s which are usually prominently red on these valves had turned almost white on three! This is what usually happens with output valves when either’ the bias has not been set correctly, or is non existent, or a fault has developed in the valve itself. After testing all four, it was found that three did indeed have issues. Next I powered up the amp [no output valves fitted at this stage] via a variac on the mains input, and measured the voltages everywhere on the circuit as I increased the input voltage. Everything seemed fine, even all the bias voltages were there on the output valve bases, a good sign.

I decided to discard the four original output valves,[for obvious reasons]and instead’ I fitted a good tested set of 6550’s I had lying around, that would be good enough to put in for the first power up and test etc.
Next, I wired the amp up to some dummy loads, a scope, and signal generator and switched the amp on.
Well, I am happy to report that after initial warm up, the amp came on fine, with no drama’s, and tested very well on both outputs, with a flat frequency response in excess of 50hz to 50khz tested at quarter power output.
This amplifier runs quite a high Anode/plate voltage, in excess of 600v, however, it runs the screen grids at about half that of the Anodes, this is usually a good compromise for sound and power output.

My first impressions with this amplifier are; it is very heavy, always a good sign with valve amps.
It has been put together very well, with real thought gone into the layout, and use of high quality components in key areas etc.
It is mechanically quiet when powered up.

All in all, I am pleasantly surprised by this amplifier, in its quality of build, its component content, and test wise.
I will leave it on for a while longer to see if it develops any issues, but so far, so good!
Anthony,TD…

[Audio Al]

Great work Anthony

I do hope we find the real reason and justice is done

[anthonyTD]

Hi All,
I have now re-opend this thread, and all recent, and therefore relevant info will be transfered here!
New posts start from 312 above.

[struth]

PICTURES OF THE AMPLIFIER

[sq225917]

What a very well made looking thing that is. I'd give it space on the shelf, lovely tag strip hard wired construction.


So does the bloody thing hum like the OP stated it did or was he just looking to send it back at no cost to him.?

[Gordon Steadman]

What a very well made looking thing that is. I'd give it space on the shelf, lovely tag strip hard wired construction.


So does the bloody thing hum like the OP stated it did or was he just looking to send it back at no cost to him.?

Doesn't look bad for a second hand amp does it?

[Wakefield Turntables]

Looks pretty good to me, mind you we havent heard the full report from Anthony yet.