New SE With Triode Mode KT120/150

[Ray P]

I've a single nos philips 9710a driver (800 ohm) been looking for a second for years, if I was building an otl i would be based on that driver, most of the philips full range drivers have a good sound.

I've looked at those sorts of drivers at various times in the past Steve, always gave up because finding a decent pair seems to be on a par with winning the lottery.

[Ray P]

and as for winding voice coils.....

[steve s]

and as for winding voice coils.....

I'm a member of a lowther modification group (by chance). it does not look like rocket science in fact it looks quite simple, as with all these things it's about knowledge and good planning. I've a few lowther drivers that need a recone for practice

[Ray P]

...in fact it looks quite simple, as with all these things it's about knowledge and good planning. I've a few lowther drivers that need a recone for practice

You’re a better man than I am, Gunga Din!

[Cressy Snr]

Heaters are operational:

MH4s have undergone final adjustments for 4V DC at their heater pins.

Always nice to see a set of glowing heaters; it means you’re on your way to getting things finished.

[Paul Barker]

[/quote]

I'm a member of a lowther modification group (by chance). it does not look like rocket science in fact it looks quite simple, as with all these things it's about knowledge and good planning. I've a few lowther drivers that need a recone for practice
[/quote]

What you’re group using for suspension spider and surround?

[Paul Barker]

Heaters are operational:
49119139-4BE6-4CCF-B6C7-5C5AED0567EA.jpeg
MH4s have undergone final adjustments for 4V DC at their heater pins.

Always nice to see a set of glowing heaters; it means you’re on your way to getting things finished.

Nice

[Paul Barker]

Many years ago a man claimed he was supplied. Cone and suspension for lowther he came and collected my pair pm6sa . Never came back, lost his details. Oops!

[Cressy Snr]

Input stage power supply is completed and tested.
VR tubes (series combination of 0A2 +75C1) light up to give 228V output, prior to the anode chokes.

Allowing for the drop across the DCR of said chokes, there should be 214V at the plate of each MH4, when the amp is operating.

[Cressy Snr]

Have made a start on the output stage components:

Piece of white paper is threaded onto the free lead on the right hand coupling cap.
This is to remind me where to start tomorrow. I seem to need these kinds of visual reminders these days.

The home straight is within sight.

[Cressy Snr]

Finished......Deafening hum!

Then I realised that I’d used an old TKD 250K pot that had previous form for those sorts of shenanigans - Dagnabbitt!

Anyway the offending pot was removed, punted into the bin, a brand new Maplin 100K carbon track jobbie was fitted and the hum reduced dramatically. I have almost zero hum on the right channel, but the hum in the left is audible from the listening position, which is frankly not good enough.
May have to rotate the left channel anode loading choke to get the hum level acceptable on that channel, but otherwise there are no serious issues. However, I’ll try a valve swap over before messing with choke orientation.

Here’s a pic of it playing:

[Cressy Snr]

Damn, blast and buggery!

It’s a MH4, Cleaned all sockets, cleaned all pins, buzz moves with valve. Offending valve makes intermittent loud bangs and low freq rustlings in speaker, and this too moves with the valve. Hum stops and starts, then stops again.

Back with the NVA A20

[Nick]

Don't know the valve, but have you ant grid stoppers?

[Cressy Snr]

Don't know the valve, but have you any grid stoppers?

Yep, I use them as a matter of course.
The other MH4 is fine, so I can’t see it being anything other than the valve, because the problem moves with the valve.
I just feel guilty and embarrassed that one of them doesn’t work. Feel I’ve let Steve down somewhat, as he was so generous in the first place.

It sounded great, under the hum too.

[Cressy Snr]

Traced fault to an intermittent centre pin (cathode) which is why the fault moves with the valve.
Suspect that wire inside pin could be broken and is making intermittent contact. I had this on an old GZ37 once. But it’s impossible to be sure.

There’s no way I’m touching anything on the valve, it’s too old and too valuable to be messing with.