Re: 845 use in SET amps, output tranny info.


"Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> a écrit dans le message de news:
43C92ACC.72350AE@xxxxxxxxxxxxxxxxxxxxx
>> The absolutly minimalist schemo is here:
>> www.dissident-audio.com/SE_845/Amp.gif
>
> That design is very minimalist for 845.
> The gain of the EL802 driver/input pentode must be very high since the
> peak voltage swing would be maybe +/-180V pk.
>
> I prefer to use an EL34 in triode with 6SN7 paralleled input,
> and If I wanted more gain a 6SL7 in µ follower will do.
>
>>
>> I rather concentrated my efforts on the OPT design.
>>
>> The first one had 2975 turns on primary in 7 equal sections interleaved
>> with
>> 6 secondaries, a total of 13 !
>> A 0.5mm teflon foil was used between each section
>> The measured leakage L was a decent 9mH, however, it exhibit a 12µS
>> irregular rise time:
>> (Topmost shot in www.dissident-audio.com/SE_845/3xSQ10K.gif )
>> I bet this is caused by too hi parasitic caps.
>> Primary L is 40H, impedance ratio is 1000.
>
> But what size was the core cross section and window?

Seen below you reached the target closely !

>
>
> Getting a decent square wave with any tube amp isn't always easy
> because of the combinations of the many amounts of leakage inductances
> between
> the many sectiuons and ditto for the capacitances.
> Its impossible to model the behaviour of the LCLCLCLC filter at HF, and
> usually
> some
> ring F will be present.
> However, if your square wave was at 10kHz at full power its not too bad
> since it looks like it will be -3dB at well over 30kHz.

Exactly ! There is a notch just above.

>
> I am surprised that your measured leakage L was 9mH.
> I'd have guessed all that interleaving and close coupling
> of insulation = 0.5mm only would give less LL.

My guess is that increasing interleaving above says, 10 sections has very
limited effect.

>
> NFB makes it harder to get a good square wave, especially if the load
> is a capacitor.
>
>
>
>>
>>
>> The second one has less primary turns (1999 x 0,28mm dia.) and use a much
>> simpler interleaving scheme, but with an half first and an half last
>> section, that is 333 + 666 + 666 + 333 turns with 0.1mm insulation at
>> each
>> layer.
>> 3 secondary sections are interleaved and the thickess of insulation was
>> rised to 1mm.
>> The leakage L was a bit less than 8mH, but rise time went down to 3µS.
>> (Center shot in previus pix)
>> Primary L is 25H, impedance ratio is 987
>
> Yes, but isn't the Lp a bit low relative to the load value of 8k?
>
> The HF looks OK.
>
>>
>>
>> The second OPT is better to my ears, more detailled, less agressive and
>> well
>> balanced.
>>
>> The bottom shot is from a "professional winder" unit. I'm not ashamed !
>
> Its not unusual to be able to do better than pro winders.
>
> Many ppl try to have thin insulation between P&S sections to help reduce
> LL, but C rises too much, so using 1mm of insulation is quite OK if you
> have
> enough sections, so C is kept low.

Curiously, this OPT shows a relatively hi Leak L (around 30mH) and a small
shunt cap (less than 300Pf) !
Low interleaving, large insulation ?
The core has 36mm tongue and probably at least 50mm stack. I can't measure,
it's potted !
Looks a bit too small.

>
>>
>> I'm not sure yet, but the next trial will probably have a bit more
>> primary
>> turns, says 2100 to rise the impedance ratio up to 1125 - (while keeping
>> the
>> same interleaving scheme) in order to reflect a 9000 Ohms load with a
>> typical 8 Ohms load.
>> This should reduce THD (already in the 3% range at 15 watts).
>> It should fit in the copper window by reducing wire dia to 0.25mm, ohmic
>> losses remaining lower than 5%.
>> Leakage L should not be affected so much.
>
> Any slight increase in Np won't change the thd very much.
>
> The only way to lower thd to a minimum is to settle for
> less max po and use say 16k for the load; maybe you make thd 1.5%
>
> But the driver tube is important with thd.
>
> Your pentode may or may not produce much 2H.
>
> Some pentodes have a lot of 2H when RL is small, then as RL is raised in
> value
> 2H declines to zero at some RL value then rises again when RL is increased
> beyond this RL value for minimum 2H.


Yeap, I admit it's even a bit provocative !
Look at this:
www.dissident-audio.com/Yves/EL802LdLine.gif

At first, this penthode is obviously a tetrode with a very sharp knee and
distorstion is almost H2.
I've used a 12K load (blue line) and, with a 500V supply, the voltage swing
is impressive !
Since the cathode is not bypassed, the linearity is not so bad and the
sensitivity (about 1,7V rms) remains sufficient for use with "modern"
sources.
I've obtained some global H2 cancelling by tweaking screen voltage and bias.
Indeed, I was paid for many years to count beans. Old habits are hard to
kill ;>)

>
> But the 2H when RL goes high is the opposite phase of that when RL is low.
>
> The 2H of the pentode driver tube when RL is high adds to ther 2H of the
> output
> tube,
> and when RL is low it tends to cancel.
>
> The odd numbered thd products don't get cancelled and just get amplified,
> so I would never use a pentode driver to make such a large drive voltage
> because
>
> you would have maybe several % of driver thd at max po.
>
> A driver triode tube always has very similar and mainly 2H in its thd,
> and it will always cancel the 2H in the output stage, with a slight
> penalty
> that some small amount of 4H is produced.
>
> Since the 845 requires such a large drive voltage, it souldn't be too hard
> to
> select
> a triode driver such as an EL43 in triode whose 2H increases from zero
> watts at
> the output
> in much the same % level as po is increased for most loads.
> So it should be possible to set up the EL34 so that it is nicely loaded,
> ie,
> has a load that is a high ohm load to ensure its thd isn't unpleasant,
> but which does cancel 2H in the output stage so that maximum cancelation
> occurs
> with a 6 ohm load.
>
> Its possible to get thd at 15 watts down to less than 1%, and at 1 watt
> it will be negligible, and you hear tubes behaving themsleves with
> Motzart.
>
>
>> Wait and see !
>>
>> Some amazing photos of this (yet unfinished) amp.
>> I'm not responsible for mechanical design ;>)
>>
>> www.dissident-audio.com/SE_845/Lateral.jpg
>> www.dissident-audio.com/SE_845/Rear.jpg
>> www.dissident-audio.com/SE_845/Protection.jpg
>
> What you are doing looks quite impressive.
>
> My approach is much more traditionally styled
> with a normal steel chassis with potted trannies and tubes on top and cap
> under
> with all hard wiring.
>
> Your OPT look like it has 44tongue, 50 stack, wasteless E&I GOSS material,
> and that explains the inductance figures you have got.

Exactly, a 42mm tongue !

>
> I used about 2,000 turns on 44T x 62S to get about 15H for a load of 1.2K
> to
> suit 4 x 6CA7 to make 35 watts; Ia is 240mA, so gap has to be about 0.28mm
> across all the E&I.
>
> But now I am making an amp for 50 watts with 2 x 845 in parallel,
> the Ia = 125mA approx, and since the Va = 700Vrms, the core must be huge.
>
> A colleague in Sydney is making an SE amp with 6 x GM70,
> and I think his OPT will be quite large for the 120 watts.

Mamma mia ! How much DC current ?

Yves.

>
> Patrick Turner.


.

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