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Old 10th Apr 2021, 9:13 am   #21
qualityten
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Default Re: Mullard 3-3 voltages query

Thanks Andy. Iím going to look for a variable capacitance box.
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Old 10th Apr 2021, 9:21 am   #22
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Default Re: Mullard 3-3 voltages query

You'll find you only really need a range of about 1p to 1n so you could build your own in 100p steps but a decade box is really useful, mine was £20. When do the 1khz/10khz square wave test you might find a 560p is good at 1khz but when you jump to 10khz it needs lowering or whatever. It's just a case of mucking about till it's good compromise for both.

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Old 11th Apr 2021, 9:18 pm   #23
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Default Re: Mullard 3-3 voltages query

Thanks Andy, I've dug out an old tuning capacitor that gives me a range from about 0 to 380pF. At the moment, square wave testing shows a HF boost on an otherwise nice square wave. My build does not have tone controls. I guess the treble cut control would see to that.
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Old 11th Apr 2021, 9:41 pm   #24
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Default Re: Mullard 3-3 voltages query

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gives me a range from about 0 to 380pF
Does it really go down to '0'?

I think that's rather unlikely myself.
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Old 12th Apr 2021, 7:51 am   #25
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Default Re: Mullard 3-3 voltages query

That sounds good, a bit of HF boost is handy,the opposite is often the case. You might need a tad more capacitance so could try a 220p or 330p with your varicap in series, I've attached a quick guide to give you some idea as regards square wave testing.

What I've found is at lower frequency square waves you can dial in the FB cap to get a nice sq wv but at 10khz might have to trim it back a bit. It is an interesting exercise though to do this, tweeking the capacitance & watching the response change.

Lastly a bit of a faff but you could fit a switch to shunt out the FB cap to give you a crude HF boost,very handy to have for old ears.

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File Type: pdf square_wave_testing.pdf (102.4 KB, 16 views)
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Old 12th Apr 2021, 8:35 am   #26
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Default Re: Mullard 3-3 voltages query

Contrary to the belief that the layout is ultra critical I can say without a shadow of doubt it's not.

Be sensible with the layout. My stereo 3-3 uses two busbars, one HT the other ground and returned to chassis at one single point. Its hum free and bang on spec, it works beautifully with any combination of EF86/EL84 even Russian valves. I also have a 3-3 test amp that only has 250v HT although it's only just over 2 Watts the distortion level is only marginally raised and sounds excellent, they are actually less effected by low HT than some think. I've made many 3-3s now and for a single ended amp are brilliant, knocking spots off those two grand triode amps with 300B valves. I did a comparison last year with the stereo 3-3 and a 300B amp with a HiFi dealer and he came away shaking his head after the comparison. He even asked me to go into production and make some, which I declined.
Why not try an EF37A & EL34 3-3?
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Old 12th Apr 2021, 8:40 am   #27
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Default Re: Mullard 3-3 voltages query

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That sounds good, a bit of HF boost is handy,the opposite is often the case. You might need a tad more capacitance so could try a 220p or 330p with your varicap in series, I've attached a quick guide to give you some idea as regards square wave testing.

What I've found is at lower frequency square waves you can dial in the FB cap to get a nice sq wv but at 10khz might have to trim it back a bit. It is an interesting exercise though to do this, tweeking the capacitance & watching the response change.

Lastly a bit of a faff but you could fit a switch to shunt out the FB cap to give you a crude HF boost,very handy to have for old ears.

Andy.
Hi Andy.
My 3-3s all have treble boost rather than cut. Easy to do with a fixed value cap and 500 oum pot. Great for us pensioners with cloth ears.
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Old 12th Apr 2021, 9:41 am   #28
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Default Re: Mullard 3-3 voltages query

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Originally Posted by murphyv310 View Post
Contrary to the belief that the layout is ultra critical I can say without a shadow of doubt it's not.

Be sensible with the layout. My stereo 3-3 uses two busbars, one HT the other ground and returned to chassis at one single point. Its hum free and bang on spec, it works beautifully with any combination of EF86/EL84 even Russian valves. I also have a 3-3 test amp that only has 250v HT although it's only just over 2 Watts the distortion level is only marginally raised and sounds excellent, they are actually less effected by low HT than some think. I've made many 3-3s now and for a single ended amp are brilliant, knocking spots off those two grand triode amps with 300B valves. I did a comparison last year with the stereo 3-3 and a 300B amp with a HiFi dealer and he came away shaking his head after the comparison. He even asked me to go into production and make some, which I declined.
Why not try an EF37A & EL34 3-3?
I made mine with an EF36 and a KT61 - worked a treat.
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Old 13th Apr 2021, 6:13 am   #29
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Default Re: Mullard 3-3 voltages query

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Contrary to the belief that the layout is ultra critical I can say without a shadow of doubt it's not.
Depends on the OP transformers Trev and OP devices. A lot of SE OPT's are cheap and cheerful and need a bit of voodoo to get them to work.That's been my experience anyway.

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Old 13th Apr 2021, 9:32 pm   #30
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Default Re: Mullard 3-3 voltages query

Thanks for the further input.

Techman, you're right of course, though I did say 'about' 0. I just couldn't be bothered to check how low it went.

Andy, thanks for the info about square wave testing. Good to hear that a bit of HF boost is handy and fixable. My dummy loads are just 8ohm 10W wirewound resistors and Morgan Jones frowns on them for being inductive. I can't find a source of the 50W non-inductive thick film resistors that he recommends. What do others use for dummy loads?

Murphyv310, interesting to hear that lower HT is fine too. I have a lower power transformer standing idle, so I just might build another one of these days! Once my error was spotted and fixed, mine has no hum or hiss.
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Old 14th Apr 2021, 5:37 am   #31
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Default Re: Mullard 3-3 voltages query

My dummy loads are bog standard 2r 50w ali panel mount resistors in series, any inductance they have is tiny and will only effect very HF readings. There's probably more stray inductance in the amp, wires going to dummy load etc, and loads of stray capacitance. I think non inductive dummy loads are a red herring, if it effects test results it's insignificant. Unless your testing amps at 10mhz can't see it'd make a difference.

There's more good info about square wave testing on Rod Elliots site.

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Old 15th Apr 2021, 9:17 pm   #32
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Default Re: Mullard 3-3 voltages query

Thanks Andy. Good to know what to look for for the dummy loads, apart from wirewounds.
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Old 15th Apr 2021, 10:14 pm   #33
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Default Re: Mullard 3-3 voltages query

Why would an inductive dummy load be bad?
Surely a loudspeaker is more inductive by definition?
What have I missed?

A.
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Old 16th Apr 2021, 11:50 am   #34
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Default Re: Mullard 3-3 voltages query

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Why not try an EF37A & EL34 3-3?
Wouldn't that be a 3-6?

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Old 16th Apr 2021, 4:24 pm   #35
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Default Re: Mullard 3-3 voltages query

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Why would an inductive dummy load be bad?
Surely a loudspeaker is more inductive by definition?
What have I missed? A.
I don't know; I'm just going on what Morgan Jones says in Building Valve Amplifiers (see the section on Performance Testing). He is pretty emphatic about not using wirewound resistors.
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Old 16th Apr 2021, 4:51 pm   #36
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Default Re: Mullard 3-3 voltages query

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Quote:
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Why not try an EF37A & EL34 3-3?
Wouldn't that be a 3-6?

A variant of this was used on the (very rare) Pye Super Black Box with a SE EF86/EL34. Some cribbing of the Mullard 3-3 circuit was used by the Pye Labs.
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Old 17th Apr 2021, 6:53 am   #37
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Default Re: Mullard 3-3 voltages query

There are quite a few references to using non inductive dummy loads in various tomes like in Morgan Jones Valve Amps and on Utoob. It makes sense on one level, IE you want your test load to be purely resistive and have no reactive element lest it skew your test results. I too was spent ages looking for non inductive resistors a few years ago, until learned more and had a think.

As mentioned any inductance is small and will only effect RF frequencies possibly, that and there are reactive capacitances and inductances in your test leads and as Andy says a speaker is reactive anyway so does it really matter. For someone just wanting to do some home testing I'd say the answer is no.

BTW David, you could just use a few 1/2w resistors in a cup of water as your load, something like two 10r's in parallel with a 3r3 in series with the them.

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Old 17th Apr 2021, 2:21 pm   #38
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Default Re: Mullard 3-3 voltages query

The Pye Mozart is my favourite single ended amp as it's just so fiendishly clever and works so well. The OPT has windings driven by the cathode as well as the anode which not only reduces distortion but the two windings are arranged so that the magnetising currents cancel. The output is taken from the cathode winding, auto transformer style (no doubt with a little DC at output but prob only say 50mV) and several feedback loops give a very high for a valve amp 34dB NFB and a damping factor of 30 along with 0.2% THD at 8W.
The power amp sounds surprisingly "modern"!
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Old 17th Apr 2021, 6:30 pm   #39
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Default Re: Mullard 3-3 voltages query

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Contrary to the belief that the layout is ultra critical I can say without a shadow of doubt it's not.
I'm in 100% agreement with that! A few years ago, just for fun one afternoon, I 'slung together' a 3-3 on a scrap chassis using just what I had available. I didn't pay too much attention to the layout although with the few components needed and the large chassis I had, it wasn't difficult to keep space where it was needed. The output transformer was a small one from a scrap radio. The only thing I knew about it was that it was a Philips transformer and suitable for an EL84. Apart from having the negative feedback connections the wrong way on initial try out, it worked amazingly well except that the output was limited because of the small transformer. Couldn't fault the quality though and it enabled me to quickly check a couple of EF86's that I had lying about.
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Old 17th Apr 2021, 9:43 pm   #40
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Default Re: Mullard 3-3 voltages query

I have to agree re the Pye Mozart what a superb design Using a single EL34 its simply a brilliant design In some respects half a quad 22 in that it has a negative feedback loop in the output valve cathode by the split winding
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