KT120 Tryout as SE

[Cressy Snr]

I'm going to have to investigate voltages and currents around the circuit, to be sure it is not something untoward going on.

Hmmm... There was something dodgy going on. Scoping the input stage after half an hour's power up time revealed intermittent bursts of 50Hz hum, mixed with higher frequency spiky stuff appearing at the cathode of the left hand valve, with the same kind of dirty waveform bursts at the right hand valve, but at a lower amplitude.

Further physical investigation revealed a dodgy looking situation between one of the 4V heater legs and the cathode tag of the left valve, where there was a double heater connection to carry the 4V over to the right valve.

Also the left hand valve grid connection wire from the volume control was trapped under one of the legs of the heater wiring.

Solder sucking the heater and grid tags of the left hand valve socket and rerouting the offending grid wire, seems (touch wood) to have cured the noise. The amp has been idling with 41MTL mesh plates in situ for 45 minutes with no further bursts of dirty noise from the speakers.

It is probably now possible to say that there is nothing wrong with either of the 41MTLs and by inference probably nothing amiss with the MH4s either.

The fault looks like it was either an intermittent cathode to heater or grid to heater short at the socket.

"Job's a guddun", as they say.

[Cressy Snr]

Amp with MH4s:

No probs.

Thank goodness for test gear.
It is useful sometimes

[Paul Barker]

Great. Not as bad as the time you ripped apart a perfectly good OTL design because of resistors taking on too much current and valves getting the blame!

It also demonstrates how tough valves are.

[JamesD]

Nice one, Steve.

Rescued two great valves and resurrected your amp!

Scopes are great!

[Cressy Snr]

Great. Not as bad as the time you ripped apart a perfectly good OTL design because of resistors taking on too much current and valves getting the blame!

It also demonstrates how tough valves are.

Aye, taking the Rozenblit Son of Beast apart because of a set of cheap ass badly manufactured 5W wirewound ceramic coffin resistors smarted big time.

As you say, it shows that nine times out of ten, the valve is the last thing you should be blaming when fault finding.
At BT we fault found by changing out whole modules to keep the service going, then went down to component level with the offending module out of the rack.
Applying the same mentality to home builds, is not the best policy.
You change out the valve, the fault comes back, then what?

Scopes are worth their weight in gold for fault tracing.

[steve s]

Dang it....one of my MH4s has gone noisy now...

Hi Steve
Half of this deleted by mistake makes Paul's comment below make no sense Sorry I was trying to post below Pauls post and ended up editing this one.

But yes Steve It would be nice to visit your place this summer. Would be good to see the new listening room and maybe you could invite Paul to bring his uTracer.

That would be good..i did not catch all the message but it looks like you don't need the valves now but more than welcome to have open day... Pauls latest gadget looks good
I' m hoping the new transformers turn up at the end of the month so it may be worthwhile waiting until then though...

Glad you have the amp sorted...

[Cressy Snr]

That would be good Steve.

[Cressy Snr]

I think I am getting to the age where, if I want to continue building things, I am going to have to address the issue of eyesight when dealing with close-up work.

One of those magnifiers on an anglepoise stem, with a ring light for illumination ought to be the next purchase. It'll reduce significantly the number of wiring faults I've been making lately.

[simon]

My eyes are starting to get a little tired too Steve and I find the extra lighting from one of those magnifiers really helpful, and also to hold small parts up to the magnifier to see what I'm looking at. But I've never got on with it for soldering with, just couldn't get it, the work and iron in the right places.

[Cressy Snr]

I got one of those ring light magnifiers today and I must say it is a godsend
for old gits like me with age-related near-sight problems.

It enabled me to do easily, something I have wanted to do for a while with this KT120 SE, Ultra-Linear amplifier.
I wrapped a small amount of cathode feedback around the output stage, which has created a useful
clean-up of the bass response and has cleared out the space around instruments and voices. Tone has remained unaffected, which is nice.



Feedback...a right old can of worms.

[Cressy Snr]

Just incorporating the bypass cap in the feedback loop, rather than both cap and cathode resistor, neatly avoids any DC offset at the speaker.

[JamesD]

I was wondering how many feedback loops there are around the KT120 now?

At first sight we might glibly say there are two - the ultra-linear connection to the screen grid and the cathode connection to the cathode but that is just the start...

I think we have:

1) Ultra-linear to G2, this is valve anode output signal % to screengrid.
2) Cathode feedback, this is speaker output signal % to cathode.
3) Cathode signal % to screen grid
4) Cathode output signal % to anode
5) Screen-grid signal % to cathode
6) Screen-grid signal % to anode

I think that is all

But am I right or wrong? I'm not sure...

Glad I don't have to analyse that!

J

[Cressy Snr]

Terrifying! A right bloody mess of spurious extras.

But as there is a feedback loop to the valve from both sides of the OPT, some signals are going to take that route across the OPT to get to the various destinations you listed
I have to agree.

Sounds great though.

[JamesD]

And that is the acid test!

The nice thing is that they are all short local loops and that minimises stability problems.

I think its great!

[Cressy Snr]

Here's a three stage variation with cathode feedback, using a 5687 direct-coupled to its partner, which then drives the output stage.
Dual power supply.

It won't get built (too heavy and expensive power supply) but I had nothing better to do today. Keeps the old brain cells active.