Problems Lundahl LL1660 and 6c45 to 6c33 Triode!Need help! - diyAudio
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Old 27th October 2003, 01:07 PM   #1
barossi is offline barossi  Germany
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Default Problems Lundahl LL1660 and 6c45 to 6c33 Triode!Need help!

Hello ,
i have problems with my LL1660/20ma interstage transformers to drive directly the 6c33 triode.
I get a freq. cutoff 1-2 db continously from 10Khz. The -3dB point is 23Khz. I use a 6c45 with 40mA in ALT S 1:1,25 connection (gain ~50). I also tried the 5842 with 20 mA in that connection. On the scop you see a cutoff 1-2 dB continously from 10Khz. Probably it happen because the input capacitance of the 6c33 is ~11pF. But what could i do? I also need a gain in this circuit near 50 or more!

Thanx a lot

Barossi

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Old 27th October 2003, 06:14 PM   #2
barossi is offline barossi  Germany
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Hello,

the schematic looks like this:

http://www.republika.pl/diytriode/EC8010_GM70.gif

but i use 6c33 triode instead of GM70. The 6c33 runs with Ra=600Ohm experience electronics high quality transformer:

http://www.experience-electronics.de...ager.htm#a6c33 (A6c33)

Ub~200V, I~175mA
At the grid of the 6c33 i get a cutoff beginns from 10Khz with 1-2dB! I need help.
The 6c45 runs with 17ma, 175 at anode, interstage is LL1660/18mA in 1:1,25.

Thanks Barossi
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Old 27th October 2003, 11:54 PM   #3
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Hi

A 6C33 has an input, (grid-cathode) capacitance of 33pF not 11pF. The total input capactance that will load the driver stage is the Miller capacitance, that will be 33 +(1+Av)*30.5, where Av is the voltage gain of the output stage and 30.5 is the grid to anode capacitance. With Av=2 the Miller capacitance is ~124.5 pF.

A 6C45 has an output impedance of ~1.2kohm, this will be transformed by the transformer to ~1.5kohm, the -3dB point will therefore be ~850kHz and therefore the input capacitance is not the real problem in your case.

I would assume that the problem is the transformer itself, you can try checking the frequency response of the driver circuit including transformer by disconnecting the 6C33 and put for instance a 10kohm resistor across the transformer output and measure the frequency response that way.

Regards Hans
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Old 28th October 2003, 05:39 AM   #4
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The 6C45 has a quite high input capacitance that may be the cause of your problem.

Check the data sheet and do the same calculations as Hans but for the driver stage.

Carl
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Old 28th October 2003, 05:43 AM   #5
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By the way, I think 6C45 need more current than 17mA for good sound. I used it at 30mA.
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Old 29th October 2003, 03:17 PM   #6
barossi is offline barossi  Germany
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Hello,

i checked again the hole circuit. The LL1660/18mA 1:1,25 was connected right. At pin 16 i connected the bias for the 6c33. I now lowered the gridstopper of the 6c45 to 150 ohm. I removed the 6c33 and grounded the LL1660 on pin 16. I paralled a 10KOhm resistor to the output of the LL1660. I measured 24KHz -3dB upper frequency limit, without (minimal) cutoff . Also i tried different Ia for the 6c45 15-22mA. Whitout Load on the 6c45-LL1660 circuit i measured upper frequency limit up to 28KHz without cutoff.While i measured a cutoff on the grid of the 6c33 begins from ~10KHz. The total circuit shows a cut off under max. Load 21,25Vpp into 8 Ohm Load (~ round about 7 Watt output power):

14Hz 70% (-3dB)
1KHz odB
15KHz 77,5%
20,5KHz 70% (-3dB)

What could i do? Seems that the driver circuit including LL1660 doesn't like the heavy capacitivy load from the 6c33. I think this is possibly the 80% problem the other 20% causes the output transformer of the 6c33.
I think i have to build a new circuit with a input tube (gain ~ 20) and a stable output driver stage with low Output-R, while the current consumption is limited by the LL1660/18mA 1:1,25 to ~20mA for this tube.
Any ideas? Or could i drive the 6c33 with one tube and the LL1660?

Many Questions?!?!
Thanks a lot!

Barossi
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Old 29th October 2003, 10:08 PM   #7
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Hi Barossi,

It could be so that the problem is the LL1660, If the leakage inductance is high it can resonate with the input capacitance of the 6C33 and create a resonant circuit. Above the resonant frequency the response will fall rapidly.

In order to improve the situation I think you can do 3 things.

1 Connect a relatively low value resistor across the secondary of the LL1660, this will reduce the effect of the resonant circuit thereby giving a flatter response, but the gain will also be lower, try with 2.2kohm.

2 Buy a better transformer with less leakage inductance, maybe something from Tamura or Tango or maybe a better one from Lundahl.

3 Remove the transformer all together and build a different drive circuit, for instance the one that I showed earlier in the 6C33C SE tread, the circuit can be seen here: http://www2.gol.com/users/tube/se.html

Without interstage transformer coupling the high frequency response will be determined almost entiraly by the output transformer and with the Tango transformer I used the -3dB point is ~ 60 kHz

Regards Hans
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