[Phil Allison[_3_]]

"John L Stewart"
'Phil Allison


Sounds like by either intent or serendipity the 300B and 6SN7 2'nd
harmonics are canceling.-


** Bingo !!

Seems an eccentric Russian gent was responsible for the circuit design.


There was an article on 2H cancellation in SE amps published about 10
years ago, I think in Glass Audio. Used the curvature of the driver to
cancel curvature in the OP Tride. One would think this possible at only
one level on the output.

** More testing with the FFT feature on a Rigol scope:

3H is dominant from about 9 watts through to 21 watts.

From 1watt to 4 watts, 2H dominates, then 4H, then 3H.

If load R is changed to 16 ohms, 2H dominates all the way.

2H cancellation is occurring over a fairly wide range, but it is not
complete.

I can swap the 6SN7 with almost no change.

Neat eh ?


..... Phil

[patrick-turner]

On Monday, 5 November 2012 14:03:31 UTC+11, Phil Allison wrote:
"John L Stewart" 'Phil Allison Sounds like by either intent or serendipity the 300B and 6SN7 2'nd harmonics are canceling.- ** Bingo !! Seems an eccentric Russian gent was responsible for the circuit design. There was an article on 2H cancellation in SE amps published about 10 years ago, I think in Glass Audio. Used the curvature of the driver to cancel curvature in the OP Tride. One would think this possible at only one level on the output. ** More testing with the FFT feature on a Rigol scope: 3H is dominant from about 9 watts through to 21 watts. From 1watt to 4 watts, 2H dominates, then 4H, then 3H. If load R is changed to 16 ohms, 2H dominates all the way. 2H cancellation is occurring over a fairly wide range, but it is not complete. I can swap the 6SN7 with almost no change. Neat eh ? .... Phil

Its actually possible to exploit the set up of a driver tube to cancel 2H of the SE OP tube. But while maximum 2H cancellation plus probable 4H cancellation can be arranged for the nominal load to be used, the cancellation may not be much when RLa is above of below nominal RLa. Roughly the same Vg input to 300B is needed over a range of RLa and a typical SE triode driver makes roughly the same 2H for most RLa values. But 2H is most severe in 300B when RLa is low, and then least when RLa is high. Some 3H is generated by intermodulation process and you can end up with higher 3H than you'd have if the driver triode was more linear. I found the cancellation of 2H with SE tetrode & pentode amps with about 15% local CFB could reduce THD to less than the same OP tubes used in a PP circuit. But with the multigrids, 2H is highest with low RLa, then as RLa is raised it nulls out at some centre RLa value even if the driver is perfectly linear, and then for RLa above the centre value 2H increases and its relative phase is opposite to low RLa values. Therefore the 2H of the driving triode adds to that of the multigrid OP tubes. But after spending much time studying this phenomena of THD cancellatiuon, I decided it was not worth deliberately setting up a driver tube to be slightly non linear to give maximum cancelling, and I concluded its better to make the driver as linear as possible no matter what driver and output tubes may be. I found that the CFB OPT connection forced THD production to be very much lower than for triode or UL SET, and then you'd still get some 2H cancelling with low RLa so the end result was THD equal to many class A PP amps over a useful range and for a similar amount of low GNFB applied.

Nearly all SE amps with 300B are ideally used where average PO 0.1 x max clipping PO so then average THD 0.2% and mainly 2H and lower 3H. This assumes you may have 10dB GNFB. But in fact if you remove the GNFB the sound change is very subtle if any can be percieved at all. I have had ppl say, "Well, now we have disconnected the NFB it sounds the same" - even though N&D has risen maybe +3 times for a given volume level. If RLa for the 300B is say 3k6, and Ra at the bias point is 1k2, then DF = 3 without any FNFB, not to be sneezed at. Ppl will argue the GNFB either gilds the lily, or makes it into pumpkin. My conclusion is that more ppl like adequate GNFB well applied than dislike it, citing better bass tighness and a creamier top end, ie, the NFB allows the tubes to blossom, so good GNFB is natural organic electronic fertilizer....

Some amps such as the EAR509, a 100W Paravicini PP concoction with a total of 44dB NFB including the unity gain OPT, does not sound as well as something with less NFB and far lower PO but with much more pure class A1. I think its pointless playing silly tricks with nearly all class B and huge amounts of NFB. But hey, that's what is done with OTL amps, and ppl rave that they can't hear the veil of the OPT. But they don't understand the tubes work mostly in class B and with terrible load matching and with huge amounts of NFB.

There is truth in that GNFB can increase the THD spectra so creating higher H that were not present when no GNFB is used. But the effect seems to rear its ugly head when the THD without NFB is high at say 10%, and not much NFB is applied, say 10dB, and 10dB is quite typical for triode amp GNFB, so it can be said use of this FB in a triode amp invites sonic horrors. There is a point where even if open loop THD = 20% before clipping, and there was still sufficient open loop gain, then increasing NFB to say 26Db will usually reduce THD to 1%, and as THD is about proportional to Vo, then at 1/3 full Vo and at 1/10 full PO, THD should be say 0.3%. But its not ideal though to start with 20%. Its better to have open loop THD 2% if possible and then the figures and sound all get better without huge dollops of NFB, and the mild 10dB GNFB does not make things sound worse. Along with applying GNFB come the rigours of making sure the amp is stable and does nt have a peaked response just below and above the AF band. One does not want LF peaking to upset dynamic DC core magnetization. I suspect many pp hate NFB because it forces them to think of the amp as an active bandpass filter with NFB, and think about Nyquist stuff.

There have been attempts to cancel 3H production of most PP amps where typically wave peaks are flattened slightly. Therefore one could make a driver PP amp stage which produced oppositely phased 3H, ie, peaks on waves became "more pointed" I tried all this about 12+ years ago and concluded it was a totally hopeless design aim. Just make it linear, said my brain, without tricks.
I also tried positive current FB and its easy to get a negative output resistance. But THD rises even though Ro becomes negative. And if there is a shorted output, the +current FB goes sky high and amp violently oscillates. No sir, no tricks, right?
Patrick Turner.

[John L Stewart]

Originally Posted by Phil Allison[_3_]

"John L Stewart"
'Phil Allison


Sounds like by either intent or serendipity the 300B and 6SN7 2'nd
harmonics are canceling.-


** Bingo !!

Seems an eccentric Russian gent was responsible for the circuit design.


There was an article on 2H cancellation in SE amps published about 10
years ago, I think in Glass Audio. Used the curvature of the driver to
cancel curvature in the OP Tride. One would think this possible at only
one level on the output.

** More testing with the FFT feature on a Rigol scope:

3H is dominant from about 9 watts through to 21 watts.

From 1watt to 4 watts, 2H dominates, then 4H, then 3H.

If load R is changed to 16 ohms, 2H dominates all the way.

2H cancellation is occurring over a fairly wide range, but it is not
complete.

I can swap the 6SN7 with almost no change.

Neat eh ?


..... Phil

I’ve found four articles covering Second Harmonic Correction in SE Amplifiers. All are now in a pdf of about 2M. If anyone would like a copy please email me direct as follows-

johnnhelen4 at gmail dot com

In Glass Audio 3/96 we find a fix authored by Graeme J Cohen. It uses a pair of side by side SE amplifiers, driven in antiphase. The combined output is added to drive the speakers. At first look it appears the even order harmonics are simply cancelled at the secondaries of the OPTs rather than in the primary of a single PP OPT. Redrawing the cct it looks a lot like a PP amp but needs two expensive SE OPTs. And it doesn’t get the cancellation of the zeroth order harmonic which is also even order, the DC component.

In Glass Audio 4/96 the cct is a 5842 driving into a 300B, authored by Reid Welch. It is a fact of life that in an RC coupled amplifier the AC load line is critical & is a cause of distortion. Interstage transformer coupling can avoid that problem at the expense of cost & BW limiting. An improvement possible with IT coupling is by reversing the leads driving the 300B grid. Curvature in the 5842 may subtract from the curvature in the 300B. Reid comments that the 5842 driver does not have the best transfer characteristic. But his amplifier measured a lower THD after that fix.

In Sound Practices Fall 1994 John Atwood of One-Electron discusses the SE 3x 2A3 amplifier developed for testing the OPTs to be built by Electra-Print Audio. It is RC coupled thru 6J5 input followed by the paralleled sections of a 6BL7GTA for the driver.
The 6BL7 family is much like a 6SN7 on steroids, same mu with twice the G.

In the same issue of SP is an article by Jack Elliano of Electra-Print. The amp in this case is again a 300B OP. The driver is a rather complicated mu-follower using paralleled sections of a 6AQ8 as the gain stage & a pentode 12HG7 as the constant current source. In my opinion the reason for the improvement in the THD figures is due solely to the mu-follower stage & nothing to do with curvature correction of the OP stage at all. For a triode (And pentodes too) 2H declines as the load R is increased. In this cct the 12GH7 looks like a very good current source at AC, hence a high Z plate load for the 6AQ8. And the mu-follower OP to the following grid is low impedance so easily drives the 300B.

In all these I find a lot of work sorting for the best sounding tubes & parts. Many of the tests are subjective. I could not find any tests for individual harmonics or intermodulation distortion.

My opinion, curvature correction is smoke & mirrors. Might work for some for a while & might not.

Phil has shown us that the harmonic content of the amp he has for repair varies quite a bit as the power level is changed. I would not depend on curvature correction as a cure for amp distortion. The THD may go down but what about the higher order harmonics & IM? It has been shown by many tests that these are the objectionable components in the reproduced sound.

Cheers to all, John

[Phil Allison[_3_]]

"John L Stewart"


Phil has shown us that the harmonic content of the amp he has for repair
varies quite a bit as the power level is changed.

** Duh !! The percentage goes up with output level.

You cannot stop paraphrasing a false characterising - can you ?


I would not depend on
curvature correction as a cure for amp distortion.

** Pompous bull**** and a classic " straw man " fallacy.


The THD may go down
but what about the higher order harmonics & IM?


** Converting a lot of 2H into a small amount of 3H can only be good.


It has been shown by many tests that these are the objectionable
components in the reproduced
sound.

** Blah, blah, blah....

Another ****wit "straw man" fallacy.

10% 2H distortion ( as seen with many SET amps) represents a HUGE amount
of IM with a complex wave - maybe as much as 50%.

Reduces music to purest mud.


..... Phil

[patrick-turner]

On Tuesday, 6 November 2012 11:31:56 UTC+11, Phil Allison wrote:
"John L Stewart" Phil has shown us that the harmonic content of the amp he has for repair varies quite a bit as the power level is changed. ** Duh !! The percentage goes up with output level. You cannot stop paraphrasing a false characterising - can you ? I would not depend on curvature correction as a cure for amp distortion. ** Pompous bull**** and a classic " straw man " fallacy. The THD may go down but what about the higher order harmonics & IM? ** Converting a lot of 2H into a small amount of 3H can only be good. It has been shown by many tests that these are the objectionable components in the reproduced sound. ** Blah, blah, blah.... Another ****wit "straw man" fallacy. 10% 2H distortion ( as seen with many SET amps) represents a HUGE amount of IM with a complex wave - maybe as much as 50%. Reduces music to purest mud. .... Phil

The very worst designs of SE amps generate 10% 2H at 1dB below clipping.
The best SET amps maybe make 4% at -1dB. This is without GNFB. Using adequate GNFB of say 12dB and using average PO limited to 1/10 of the clipping PO means N&D are usually tolerable at least, and below limits of audible perception by most ppl. But when used with horn loaded speakers the SET performs with typical PO averaging 1/100 of clipping PO, ie, say 0.2Watts out of a possible 20W. Since Dn reduces with Vo, it sounds OK.

In many past long tortuous arguments here and especially at aus.hi-fi there were those who could never grant any SET amp hi-fi status, and they labelled SET amps as illitimate, ie, ******* audio products. The loudest and most stubborn accusers of SET amps tended to have incomes derived from selling solid state gear, and they were never ever to agree the purchase of an SET amp was sensible in any way - bad for business. But despite the flames the SET amp buyers have kept on buying SE amps. They refused to accept they ever needed more than a few watts of triode power.

I have heard fabulous music from SET amps when used wisely and sensibly. But There is no point comparing one lone 300B making 8 piddly watts to say a 100W SS amp when both are tried at very loud levels with speakers rated for 88dB/W/M. Horses for courses.
Patrick Turner.

[John L Stewart]

Originally Posted by Phil Allison[_3_]

"John L Stewart"


Phil has shown us that the harmonic content of the amp he has for repair
varies quite a bit as the power level is changed.

** Duh !! The percentage goes up with output level.

You cannot stop paraphrasing a false characterising - can you ?


I would not depend on
curvature correction as a cure for amp distortion.

** Pompous bull**** and a classic " straw man " fallacy.


The THD may go down
but what about the higher order harmonics & IM?


** Converting a lot of 2H into a small amount of 3H can only be good.


It has been shown by many tests that these are the objectionable
components in the reproduced
sound.

** Blah, blah, blah....

Another ****wit "straw man" fallacy.

10% 2H distortion ( as seen with many SET amps) represents a HUGE amount
of IM with a complex wave - maybe as much as 50%.

Reduces music to purest mud.


..... Phil

As always, Phil is the contrarian. Hey Phil, I tried to send you a copy of the curvature correction stuff I found but the address I had for you at tpg.com.au no longer worked. Pls send your new address direct to me at-

johnnhelen4 at gmail dot com

And I will pass it along for your inspection & comments.

Cheers, Jojm

[patrick-turner]

On Tuesday, 6 November 2012 02:46:34 UTC+11, John L Stewart wrote:

snip for brevity,

The THD may go down but what about the higher order harmonics & IM? It has been shown by many tests that these are the objectionable components in the reproduced sound. Cheers to all, John

Well, you are possibly about correct about IMD being worse sounding than THD. All music generates many harmonics of fundemental F produced by instruments. An amplifier or speaker adds to the THD but is not all that bad itself. If a saxaphone's 3H is increased by 1% then the tone won't seem to have changed much. But IMD products, ie, the sum and difference between various F are often quite an-harmonic, and their presence can have a far greater effect.

If you have an OP tube which makes 5% of 2H near clipping, then for 2H cancellation you need the driver to make 5% of 2H. That 5% cancels the OP 5%, but in fact the sum and difference between the driver 2H and fundemental are produced, ie, 1H and 3H, ie, more fundemenmtal plus 3H, and so you can get more 3H than was present to begin with even if you had a perfect driver. But 2H + 5H yields 3H and 7H, and suddenly you have lots more H than you started with although their levels tend to be lowish, and which are called second order. AFAIK, there is no need to worry about their production where the amp had low THD without any FB, as is the case with most well made class A amps used where they are meant to be used at average power 1/10 clipping power. One can always analyse to find just how many H a single 6SN7 might produce even when set up optimally. One may find H extending to 10th H. So what? We still must use devices to amplify sound, and you can't avoid the H production by one weay or another. RDH4 gives a guide to IMD significance and cites that IMD is around about 3 times the % of THD where you have the standard old IMD test where there the output V0 has say 4V of 80Hz and say 1V of 5kHz. So if THD was say 1% at 4V of 80Hz, then expect 3% IMD relative to 5kHz levels. One can display the 5kHz tone on a CRO, and without any 80Hz by means of using a high pass filter, and then you can see what % depth of amplitude modulation exists, ie, presence of sidebands at 4,920Hz and 5,080Hz. Neither of these F are harmonious with 5,000Hz, and one may think they'd sound bad, but IMD also is generated within instruments, and our ears seem used to such slight amplitude modulation of high F tones by the usually much larger amplitudes of bass tones.

Anyway, All I know is that if you have an SE amp which measures well without using much GNFB, it usually sounds just fine at levels for which it has been designed for. I have found that the best way use multigrid tubes to get basic linearity is to employ CFB windings on the OPT, and not to rely solely on the triode connection for FB via the screen, which is of course the same as the triode internal NFB in a real triode such as 300B. With say 20% CFB with a multigrid and SE amp, THD just under clipping can be routinely less 2% with correspondingly low IMD.The CFB converts the OP stage to have quite low voltage gain like as if it was a triode. So the driver stage which is usually a signal triode then has to make a substantial amount of drive voltage, say 60Vrms and so it also naturally generates around 2% THD even if fairly well set up. So some substantial 2H cancelling occurs inevitably. Usually an OP triode tube at near clipping may make far more 2H than the driver tube could ever make unless someone has set up the driver with a low value of anode load to provoke production of 2H.

I've long since given up being too concerned about the unavoidable issues of THD/IMD in tube amps. All one can do is minimise them "legitimately", and I find 99.999% of everyone likes the sound. And if they change to something solid state and THD becomes 100 times less because an extra 40dB NFB is used, they don't hear the slightest betterment, and in fact are likely to dislike the sound.

As I said previously, I working on drastically reforming a pair of Jadis 300B SE amps. The owner is currently using Atmosphere OTL with sensitive speakers and it will be interesting to see if he finds the Reformed Turnerized Jadis to be better. Perhaps he'll find it different, rather than better/worse, but my main function has been to make the Jadis work properly, quit smoking, and stop being a sulking and silent and expensive French loungeroom companion. I have thought of sending the original Jadis OPTs to Jadis in France but methinks they'd take no notice at all, and pretend i didn't exist, and probably never admit they ever could have forgotten to use an air gap. I'm trying not to be unkind. Anyway, Jadis are free to take my advice in future when I post up my reforms for all to read. BTW, the Jadis have CLC filtering for main 300B B+. The choke used is a 20mm stack of 20T E&I, and C are 2 x 470uF/500V. Nice caps, but shame about the choke. The choke air gap was 10 times too large, reducing L value to a negligibly effective amount. I was able to re-gap the little choke to give nearly 2H, and thus it is now useful. PSU resonance is now 5Hz, Q not too high, and just passable.

I could perhaps use the Jadis OPT for a future parafeed amp because available wasteless winding Z ratios are 888:1 and 222:1. So 888:1 gives 5k0 : 5.6r, and Lp is high enough with no air gap at 88H, and RwP = 76 ohms only, so losses are low enough. The OPT would them be useful for an 845. But with 222:1 I get 1.25k : 5.6, OK for 3 x KT88/6550 or 4 x EL34 in triode for about 30Watts. Using 2 x KT120 in triode also comes to mind, because one may idle the KT120 at Pda at 35W and possibly 40W because Pda rating = 60W. A single KT120 is potentially able to do what 2 x 6CA7/EL34 can do in SE.

But KT120 triode Ra is not really all that low to allow a low RLa value, and so RLa must be high with probable Ea at +500Vdc, and Ia up to say 70mA, with load for one at 5k0, giving PO = 12 Watts. Theroetically 2 x KT120 in parallel should give 25Watts into RLa = 2k5, just right for a Hammond SEA1627, but not that easy with the old Jadis OPT because there are just not enough sensible and available load matches.
Patrick Turner.

[Phil Allison[_3_]]

"Phil Allison"


** More testing with the FFT feature on a Rigol scope:

3H is dominant from about 9 watts through to 21 watts.

From 1watt to 4 watts, 2H dominates, then 4H, then 3H.

If load R is changed to 16 ohms, 2H dominates all the way.

2H cancellation is occurring over a fairly wide range, but it is not
complete.

I can swap the 6SN7 with almost no change.


** One final test result:

300Bs removed, 10:1 scope probe on grid pin, input 260mV rms to 6SN7.

2H = -24 dB = 6%

3H = -42dB = 0.8 %.


..... Phil

[patrick-turner]

On Tuesday, 6 November 2012 14:02:15 UTC+11, Phil Allison wrote:
"Phil Allison" ** More testing with the FFT feature on a Rigol scope: 3H is dominant from about 9 watts through to 21 watts. From 1watt to 4 watts, 2H dominates, then 4H, then 3H. If load R is changed to 16 ohms, 2H dominates all the way. 2H cancellation is occurring over a fairly wide range, but it is not complete. I can swap the 6SN7 with almost no change. ** One final test result: 300Bs removed, 10:1 scope probe on grid pin, input 260mV rms to 6SN7. 2H = -24 dB = 6% 3H = -42dB = 0.8 %. .... Phil

Well, with 10:1 probe, presumably you have 2.6Vrms at 6SN7 grid. If the 6SN7 gain is say 17, then its drive to output tubes is 44.2Vrms, ie, 62.5Vpk, or getting close to the dc bias voltage for 300B. The THD you measured in 6SN7 seems about right. It would be interesting to measure the THD of the 300B output stage when driven by a source with less than 0.1% THD which would seem easy to do if you had a spare SS amp laying around and capable of say 28Vrms output and if you had a 1:2 step up tranny giving you 56Vrms drive max to 300B, ie, up to 80Vpk. Without the driver amp disguising the performance of the 300B, you would get to work out if the 2H from 6SN7 is reacting in 300B to increase the 3H of the 300B. My guess is that there would be some intermodulation and increase in 300B 3H. Whether music sounds worse because of IMD products formed because of effective presence of 3H or just 2H us a moot point, and it all depends on the % of whatever THD is present. When I made an SET amp with a pair of parallel 845 in 2008, I needed 105Vrms drive to the 845 grids. So I used 3 x EL84 in parallel with a dc load resistance PLUS a 30H+ choke to prevent whatever other pure dc carrying resistance loading down the EL84, and then I was able to make the 845 grid bias R = 23k, so prevent serious dc generated across Rg if there ever was some "reverse grid current. I recall the THD produced by EL84 in triode was low, but whatever it was, it cancelled whatever 2H the 845 produced, thus helping THD figures. for full details and all schematics,
See http://www.turneraudio.com.au/monobloc845se55.html

If ever you were worried that 300B grids were fragile, you'd be right. Some would suggest one have cathode biasing always in class A tube amps. One may then connect a silicon diode from grid to cathode so that if ever the driver tube tries to drive the grid positive, the grid cannot go positive, relative to cathode. It measn also that if any arc occurs between anode and grid, and I have recently seen just that, anode then becomes connected to cathode, and fuse may blow or active protect circuit trun off the amp. The down side is that when 300B get tired and old they stop developing their full proper Ek, and then the diodes cause premature limiting of grid swing. NFB causes the diodes to charge up coupling caps even faster at clipping, but then we mustn't let hi-fi amps clip. Usually where you have parallel 300B all with separate cathode biasing networks there will be considerable variation in Ek, perhaps +/- 10V where Ek bias is meant to be say 90Vdc. I get very nervous about amps needing more grid bias than that except in case of transmitting tubes like 845 which are happy with grid bias = -190Vdc, and with Ea at 1,200V, and in a PP amp where 40mAdc idle is just fine with a load of say 12ka-a. In about 2010 I totally rewired a pair of 100W PP Ming Da amps with 845PP. These amps had input tube = SET 6SN7, then 6SN7 as common cathode LTP, then foloowed by pair of 300B as a balanced amp with resistance loading, each with only 7mAdc which was plenty to get the wanted grid swing for 845, about 141Vrms max to each grid.
The 6SN7 would have been better with a CT choke plus R loads to get their Ia higher, but in this case the available B+ was over 1kV, so the 300B could have 50k anode loads from B+ to anode. The Chinese became utterly muddled up as they often do when attempting to make grandiose amps like this, and a complete re-design was needed - long story, no time now to tell it. I discovered that the PP 845 with 3 stage input&driver driver was fabulous sounding, the owner was very happy, and so who needs SET amps? But then life is about appreciating all drinkable wines, seeing beauty in what may not be needed but nevertheless worth having. I have an SET EL34 giving me my kitchen music, seems to sound just fine with an old sensitive speaker. Never mind the old redhead - if she can cook, but of course I am assuming a 300B is a bit temperamental like the old redhead. Having two of them in the same kitchen invites complications.
Patrick Turner.

[Phil Allison[_3_]]

"patrick-turner"


Well, with 10:1 probe, presumably you have 2.6Vrms at 6SN7 grid.


** No, the 10:1 probe was used merely so I did not load the signal at the
300B socket grids.


If the 6SN7 gain is say 17, then its drive to output tubes is 44.2Vrms, ie,
62.5Vpk, or getting close to the dc bias voltage for 300B.


** Wrong again.

The two halves of the 6SN7 are direct coupled, so 260mV input becomes 74 V
rms ( 200V p-p) at the 300B grid pins.



..... Phil

[patrick-turner]

On Tuesday, 6 November 2012 20:35:05 UTC+11, Phil Allison wrote:
"patrick-turner" Well, with 10:1 probe, presumably you have 2.6Vrms at 6SN7 grid. ** No, the 10:1 probe was used merely so I did not load the signal at the 300B socket grids. If the 6SN7 gain is say 17, then its drive to output tubes is 44.2Vrms, ie, 62.5Vpk, or getting close to the dc bias voltage for 300B. ** Wrong again. The two halves of the 6SN7 are direct coupled, so 260mV input becomes 74 V rms ( 200V p-p) at the 300B grid pins. .... Phil

I've often mis-interpreted some of what youv'e typed. So there are two 6SN7 stages and gain of both is 284, or average of 16.8 each, about typical. If you'd described the circuit more fully, I may not have made my assumptions.. However, 74Vrms seems right because you have -105V bias, and peak grid drive max could be about 108pk volts (76Vrms) allowing for some few volts of positive grid drive before grid current clips the wave, or coupling cap begins to charge up to push the grid bias up and 300B into cut off, when Ia is then found to decrease. My prvious assumptions didn't make a nonsense of my recommendation you test the 300B with a linear drive voltage source to test just how much 2H and 3H is made without cacellings, and to see how 3H is increased due to IMD action because of the feeding in of 2H from the driver with 1H.

Have fun playing with 300B, but methinks Ea = 550V with Eg1 = -105V is using voltages that are too high, even though allows a high RLa, near maximum anode efficiency, high natural DF, even though Ra at Ia = 50mA is much higher than the data figure of 800 ohms, and low THD. I might be a lot happier if the OPT tubes were KT120 in triode, IMHO.
Patrick Turner.

[Phil Allison[_3_]]

"patrick-turner"

If you'd described the circuit more fully, I may not have made my
assumptions.


** FFS, Pat READ the description in my first post.

" The tube line up is two 300Bs in parallel and one 6SN7 per channel,

260mV sensitivity, "


..... Phil

[patrick-turner]

On Wednesday, 7 November 2012 13:44:12 UTC+11, Phil Allison wrote:
"patrick-turner" If you'd described the circuit more fully, I may not have made my assumptions. ** FFS, Pat READ the description in my first post. " The tube line up is two 300Bs in parallel and one 6SN7 per channel, 260mV sensitivity, " .... Phil

I'm either forgetful, because I did read your first post, or, like so many reluctant examples of the species Homo Wrectus, I am not perfect, and sometimes need someone to repeat something they typed.

Though this is a little OT, whenever I think about 300Bs and SETs, I can't help think about the pair of rare spare Jadis OPTs I have here. I think they'd each work well with a pair of KT120 in triode and in series with cap feed to the OPT primary. About 22Watts of pure class A triode is possible. Although the Futterman type of driver could be used, or the Technics version which makes each series tube work as cathode followers, I'd prefr an EL34 in triode with a Hammond IST, so the secondary can float on Vout. The driver triode then has an easy task, no need to aim for 2H cancelling, and some mild GNFB would fix their minor mistakes to let tubes perform more better.
Getting 800Vdc is easy with an existing CT winding meant for 5U4 and 400Vdc.. Just use a bridge with 8 x 1N5408, and two elcaps, and take CT to join of caps. But I doubt I'll ever find the time.
Patrick Turner.