New SE With Triode Mode KT120/150

[Paul Barker]

I’m sure you’ll make a good job of it!

[Cressy Snr]

I’m sure you’ll make a good job of it!

Aye, should be a nice little build.

I’ve got the NVA A20 rather than the A60 clone back in the system at the moment. Actually, I’d forgotten what a sweet sounding little power amp it is. Interestingly, when we compared my A20 and Steve’s at his place a couple of years ago, mine had the nicest treble; Steve’s being somewhat arid and bleached in comparison. We didn’t get to the bottom of why.

The clone I built with twin power supplies, is also coarser up top, though a bit more more detailed in the mids. I prefer the sweeter presentation of the A20 into my Fanes at the moment. Given the efficiency of the Fanes, the 60W of the clone is way too much; the less powerful A20 sounding more comfortable with them.

I’ve opened my A20 up and the power transistors in the positive rail are of different manufacture to those in the negative rail. In the clone I have devices from the same manufacturer in both rails. Maybe that’s the answer to the different sound mine had to Steve’s and the clone. I know next to nothing about solid state, so I could be talking nonsense; who knows?

I’ll put some parts orders in for the new amp tomorrow.

[steve s]

I've noticed differences differences between makes steve, my view the tda2030 has some big differences , whilst the mofo I'm struggling to tell,
I think matching could well be important too depending on the circuit.

[steve s]

My a20 is now at my Lads driving a pair of ns1000's, he much preferred it to a relatively far more expensive cyrus seperates pre/poweramp/powersupply set up

[Cressy Snr]

It’s no good!
I’ve had to put the KT120 amp back in the system. The solid state amp was just not doing it, and had begun seriously to get on my wick! Didn’t take long for that to happen.

I’ve borrowed a set of digital callipers from our Ant and will use these to measure the dimensions of the major iron components, then cut out card shapes to get the layout for the new amp; that way the valves will be out of the system for the absolute minimum amount of time.

[Cressy Snr]

Using the digital callipers I was able to produce actual size base areas for the iron and was also able to measure the diameters of the valve envelopes I intend to use. It was then pretty straightforward to produce a layout for the new amp.

The KT120 diameters are actually done to KT150 data sheet measurements, in case I ever decide to splash the cash on a pair.
Shaded areas represent the actual thickness of the sides, plus the internal corner fillets, as the top is rebated into the surround to give a half-inch reveal all round.

[Cressy Snr]

Front end power supply will be shunt regulated with a simple VR tube setup.This will provide the 200V at the anodes of the MH4s after allowing for the drop across the hammond loading chokes.

[steve s]

Front end power supply will be shunt regulated with a simple VR tube setup.This will provide the 200V at the anodes of the MH4s after allowing for the drop across the hammond loading chokes.

Those old globe mh4 still take 250 volt,(most amp triodes of that era are 200v)
Running them around 200 will give them any easy time Steve

[Ant]

Remember the is a pot to go in there and it will be somewhere under where one of the mh4s is

[Cressy Snr]

Remember the is a pot to go in there and it will be somewhere under where one of the mh4s is

Yes, it’s gonna be tight.

[Paul Barker]

Usual excellent finished product well on the way.

[Cressy Snr]

Those old globe mh4 still take 250 volt,(most amp triodes of that era are 200v)
Running them around 200 will give them any easy time Steve.

Yes, I certainly don’t want to be thrashing them unnecessarily; not irreplaceable things like these.

[Cressy Snr]

Ordered a few bits this morning: a couple of Mills 750R cathode resistors for the MH4s, four new ceramic octal sockets; two for the output valves and two for rectifiers, plus a few tag-strips.

That’s all the pieces I really need to get it built.

[Cressy Snr]

Will start marking out the top plate today. There will have to be some serious thinking done about what goes where, below decks. This is all because of the twin power supply setup and the fact that there is no room up top for the caps to poke through, so their bulk will have to be accommodated underneath, plus the chokes for loading the MH4s. The twin power supply is an area with which I am not prepared to cut corners.

One Hammond 150H choke will provide 11Hz -3dB point, for the 11k internal resistance of the MH4, which is reasonable, but I can get down to 5.8Hz with 2 in series per anode, one bucking the other to minimise hum pickup.

Question: is it worth going for two series chokes per MH4 ?

[pre65]

One Hammond 150H choke will provide 11Hz -3dB point, for the 11k internal resistance of the MH4, which is reasonable, but I can get down to 5.8Hz with 2 in series per anode, one bucking the other to minimise hum pickup.

Question: is it worth going for two series chokes per MH4 ?

Had you considered a CCS on the anode ? Single or cascaded.

I use Ixys 10M45s but other chips are available.