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Old 20th October 2005, 01:24 AM   #31
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Default Experimental Results from last night

The amp in question has a 12AU7 Common Cathode / Direct coupled Cathodyne feeding 2 pairs of triode strapped EL34s into a Plitron PAT4006 (VDV2100) Toroidal O/P Transformer (1900raa : 5 secondary). Cathodyne RA and RK are 22K and stage current is 4.4mA.

NOTE ALL Measurements except max power refered to 6V RMS across 3.5 Ohm load @ 1kHz (approx 10W out) - 0.8V RMS input. These levels set using my multimeter.

Baseline measurements with 12AU7:
Power into 3.5 Ohm dummy load @ 1KHz = 39W
Zout @10W, 1kHz = 4.4 Ohms
Gain = 5.5
High Frequency -3dB point is 225kHz

Assuming approx 100pF each side for the parallel EL34 inputs that tells me straight away that the effective source impedance from the cathodyne is NOT MORE than 7KOhms.

Change to ECC99 - Sound was a lot cleaner (The Common Cathode stage???)
Power Out - unchanged
Zout - some suggestion it was slightly higher at 1kHz (measured a value of 4.9 - shifted pole from cathodyne effective source Z ???)
Gain = 7.2
High Frequency -3dB point is 250kHz

While running the frequency response checks using oscilloscope I could see the effect of the feedback from the Miller capacitance of the EL34s. As frequency went up, so did feedback due to Miller Cap. This reduced EL34 rp which reflected to the secondary as reduced Zout and we saw a boost in voltage across the 3.5Ohm dummy load.

Zout measures (Vout open circuit =88V p-p):
Freq. Vout Zout
20 28 7.5
50 34 5.6
100 35 5.3
200 35 5.3
500 35.5 5.2
1000 37 4.8
2000 40 4.2
5000 46 3.2
10000 48 2.9
20000 49 2.9
50000 49 2.9
100000 42 3.8
200000 28 7.5

These were a consistent set of readings from the oscilloscope so Zout results are correct BUT for some reason it doesn't stack up with the 6V RMS into 3.5 Ohms @ 1KHz set using my multimeter. 6V RMS is NOT 37 V pk-pk????

I havent bothered BUT I think that you could calculate backward from this data to arrive at an effective Source impedance from the cathodyne.

Also: There was some indication (on the oscilloscope) of slewrate limiting starting at about 50kHz

The gain difference 7.2 vs 5.5 can be explained by difference in mu (22 vs 17).

I think the next step for me and this amp is ECC81 front end with 6dB of global feedback.

Hope there is stuff of interest to you in the data.

Cheers,
Ian
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Old 20th October 2005, 04:25 AM   #32
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Hi,

Johan, I agree with your post, as rp is dependant on the operating point you have to be a bit careful when selecting anode/cathode resistors.

Quote:
But Hans: You conclude that rp is as critical as RK (or RL), but according to the above it forms about 12% - 18% only of RK (RL). Do I misunderstand you here?
No it is not a misunderstanding but rather me being incomplete or unclear with my statement, it depends on what you mean by either rp or RK being most critical, what I meant is that a reduction of rp in ohms is as beneficial as the same reduction of RK, of course you could say that it is better to compare for the same reduction in % and in that case it would be more beneficial to reduce RK rather than rp, RK being larger.

Gingertube, I think you have some interesting results, a 12AU7 at 4.4mA should have a rp of about 13kohm and a mu of about 16 which should give a Zout of ~(13 + 22)/(16+1) or ~2.1kohm which woul give 2 poles close to 760kHz assuming 100pF in input capacitance of the EL34's. Changing to ECC99 but keeping the 22k's would then not change the poles so much but maybe a ECC99 allow for lower RK's as well?

BTW, 6VRMS would be 2*SQR2*6 or ~16.9V Pk-Pk

Regards Hans
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Old 20th October 2005, 05:07 AM   #33
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Hans,
The one major thin I learned from the experiments last night is that the effective source impedance of the cathodyne splitter is NOT a problem in this amp. It is quite adequately driving the input capacitance of the EL34s. This was worthwhile learning as I'd prviously assumed it would be a problem and I was all set to replace the entire front end.

The major problem with the amp is its Zout - thats why I'm now thinking about some global feedback.
I had thought to manipulate the tube type and RL,RK values to get the Zout of the splitter and the output tubes input capacitance to be my dominant pole and aim to set it at about 100kHz (about 1/2 the output tranny -3dB point). This now seems to be less practical than I'd hoped (since the cathodyne output Z is much lower than I expected) so may have to shift the dominany pole to the Common Cathode Stage. The one good thing about the Plitron output tranny is that its bandwidth is so high that ensuring stability is not a difficult thing.

I'm surprised at how good the amps sound into my nominally 6 Ohm speakers (DF=1.3). There is no evidence of bloated or wooly bass, it sounds a little bright but not objectionablly so.

Cheers,
Ian
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Old 20th October 2005, 08:16 AM   #34
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At the expense of a bit more drive, you might consider feedback from the OPT secondary to the output tubes' cathodes in lieu of global feedback.
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Old 20th October 2005, 08:23 AM   #35
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SY,
Good idea - unfortunately the Plitron PAT4006 (VDV2100) has ONLY a single 5 Ohm Output winding with no centre tap etc.

I do have a pair of VDV2100-CFB/H sitting on the shelf as well, which have both a centre tapped 5 Ohm secondary AND 2 separate 20 Ohm cathode feedback windings BUT I am saving those for the next project where I want to mess about with Ultralinear + Cathode feedback (Menno's "Super Triode" connection).

Learning some good stuff from this lot though.

Thanks,
Ian
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Old 21st October 2005, 01:58 AM   #36
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I worked through Morgan Jones treatment last night. Her is the summary of his work.

For equal loads on Anode and cathode

Zout = RL.ra/RL(u+2)+ra

The ra term on the bottom line is insignificant compared to RL(u+2) term so drop it. Then the RL terms top and bottom lines cancel leaving

Zout approx = ra/u+2

At typical values of u (>=20) U+2 approx = u, so simplify again

Zout approx = ra/u = 1/gm

If driving output stage directly The equal loads on Anode and Cathode will NOT be guaranteed if:
1) Output stage strays out of Class A (When a tube cuts off it has no gain so Miller capictance will change, particularly with Triode Mode Output, less so with Ultralinear and less so again in Pentode Mode)
2) Output Stage strays into grid current.


If the Anode load drops significantly then:

Zout cathode = RL+ra/(u+2) x ra/RL - ra/RL term insignigicant so

Zout cathode approx = RL+ra/u+2 - at uual values of u

Zout cathode approx = RL/u + ra/u = RL/u + 1/gm

That is it increases by RL/u

If the cathode load drops sifgnificantly then:

Zout anode = RLxRL(u+1)+RL.ra / RL(u+2)+ra

RL squared (u+1) is much larger than RL.ra and RL(U+2) is much larger than ra so

Zout anode approx = RLxRL(u+1)/RL(u+2)

and at reasonable values of u

Zout anode approx = RL

Summary:
As the loads on Anode and cathode become unbalanced then

Zout anode increases from 1/gm toward RL
Zout cathode increase from 1/gm by maximum factor of RL/u

Thats why its suggested that low u is better when driving other than Class A Output Stage.

Cheers,
Ian
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