Can I build this?

sbcellis · 8th March 2007, 12:47 PM

Hi.
I am modding a music angel kt88 china amp.
New OPT = tamura F-683 .
New 12au7 and 12at7.
Now i want to mod the driver. Point to point wire, new resistors and caps.
I want to use the driver scematic below.
Is there a 'old hand' there will be willing to say ok or ?. before i go ahead.
NB: the first tube is a 12at7, but that tube is not in my circuitmaker program.
Thanks for reading and best regards
Soren

ulibub · 8th March 2007, 01:19 PM

Hi Soren,

don't see anything why it shouldn't work. ECC81 (12AT7) as pre-stage and cathodyne and ECC82 (12AU7) as a driver are the right choices, and resistor and capacitor values are ok, too. Grid stoppers needn't be thus high (10k), I found 1 k usually sufficient. I have used almost the similar layout in my first tube amp with two EL36 in the power stage.

Good success!

Uli

sbcellis · 8th March 2007, 02:05 PM

Thanks.
I will go ahead.

Do you think i will need to elevate the heather to the driver stage ?.

Soren

ulibub · 8th March 2007, 10:14 PM

Hi Soren,

no, that shouldn't be necessary. 150 V is quite high already, but heaters like it more if the cathodes are on a high positive than on a high negative potential respective to them. So if you would lift it, the cathode of system 1 would be negative to the heater. Datasheets often show that heaters allow only for half as high a negative potential of the cathodes than for a positive one.

Uli

sbcellis · 9th March 2007, 08:25 AM

Thanks Uli.

The datasheet say 100v and there is the ac sving so i was a bit confused.
Thanks
Soren

ulibub · 9th March 2007, 09:46 AM

Hi Soren,

oh, yes, indeed, I was assuming about the same values as with ECC 82 and 83 (180V). Maybe you better lift the heater potential on 50 V. Or - the best solution - provide a negative voltage in your supply to connect the cathode resistor of the cathodyne to, to tear down cathode potential. Certainly the best but the most complex solution.

But it is well possible that it works also "as it is" now. In my experience, cathopde-to-heater voltages are conservatively rated. I am operating a PL 84 in the mains supply of my preamp (regulator tube) with +300 V difference to the heater for nearly 20 years now.... - its max Vcf is 200 V (AC?) in the datasheet..... I didn't know it better that time, certainly no example to imitate, but I never have had any problems with.

Excess cathode-to-heater voltage causes sometimes strange noise in the speakers, a high, hissing sound, crackling noise, etc.

Uli

sbcellis · 9th March 2007, 12:23 PM

Hi Uli.

I have seen it done, in the Michaelson & Austin TVA-1, but i did not understand why, until now. I will try the negative option.
Thanks
Soren

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8th March 2007, 01:19 PM	#2
ulibub diyAudio Member Join Date: Nov 2006 Location: Lorraine	Hi Soren, don't see anything why it shouldn't work. ECC81 (12AT7) as pre-stage and cathodyne and ECC82 (12AU7) as a driver are the right choices, and resistor and capacitor values are ok, too. Grid stoppers needn't be thus high (10k), I found 1 k usually sufficient. I have used almost the similar layout in my first tube amp with two EL36 in the power stage. Good success! Uli

8th March 2007, 02:05 PM	#3
sbcellis diyAudio Member Join Date: Apr 2005 Location: denmark	Thanks. I will go ahead. Do you think i will need to elevate the heather to the driver stage ?. Soren

8th March 2007, 10:14 PM	#4
ulibub diyAudio Member Join Date: Nov 2006 Location: Lorraine	Hi Soren, no, that shouldn't be necessary. 150 V is quite high already, but heaters like it more if the cathodes are on a high positive than on a high negative potential respective to them. So if you would lift it, the cathode of system 1 would be negative to the heater. Datasheets often show that heaters allow only for half as high a negative potential of the cathodes than for a positive one. Uli

9th March 2007, 08:25 AM	#5
sbcellis diyAudio Member Join Date: Apr 2005 Location: denmark	Thanks Uli. The datasheet say 100v and there is the ac sving so i was a bit confused. Thanks Soren

9th March 2007, 09:46 AM	#6
ulibub diyAudio Member Join Date: Nov 2006 Location: Lorraine	Hi Soren, oh, yes, indeed, I was assuming about the same values as with ECC 82 and 83 (180V). Maybe you better lift the heater potential on 50 V. Or - the best solution - provide a negative voltage in your supply to connect the cathode resistor of the cathodyne to, to tear down cathode potential. Certainly the best but the most complex solution. But it is well possible that it works also "as it is" now. In my experience, cathopde-to-heater voltages are conservatively rated. I am operating a PL 84 in the mains supply of my preamp (regulator tube) with +300 V difference to the heater for nearly 20 years now.... - its max Vcf is 200 V (AC?) in the datasheet..... I didn't know it better that time, certainly no example to imitate, but I never have had any problems with. Excess cathode-to-heater voltage causes sometimes strange noise in the speakers, a high, hissing sound, crackling noise, etc. Uli

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