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Aikido hum

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pchw said:
...Also, did you bias your heat to 1/4 of the B+?
I am just starting my Aikido and didn't completely understand this (relative noob to toobs). Does this only apply to a DC supply? If I am just using a 6.3vac 4A trafo for AC heaters do I need to do anything special here other than connect the secondary to H+ and H- or does it somehow need to be referenced to a 1/4 B+ point? My tubes are 6cg7's (6.3v heaters).

Tom
 
tms0425 said:

I am just starting my Aikido and didn't completely understand this (relative noob to toobs). Does this only apply to a DC supply? If I am just using a 6.3vac 4A trafo for AC heaters do I need to do anything special here other than connect the secondary to H+ and H- or does it somehow need to be referenced to a 1/4 B+ point? My tubes are 6cg7's (6.3v heaters).

Tom
Hi Tom,

This applies to both AC and DC heater supply. Tubes have a limitation of the voltage between the heater and the cathode. In the Aikido topology (or any totem topology for that matters), the top cathode is typically at around a little over 1/2 of the B+. Use the ECC88 as an example. According to http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/ECC88.pdf
when the cathode is more possible than the filament, the max diff is 150V, the other way is 50V. Let say we have a 300V B+. If the filament is referenced to the ground (zero potential), then the diff between the lower cathode and the heater will be well within the specs; however when you look at the upper, it is not because the upper cathode will be a little more 1/2 of the B+ which implies 150+ over the ground. This exceeds the 150V max rating. By biasing the heater at 1/7 (not 1/4 as suggested in John's user guide!!) of the B+ (approx 43V in this example), the lower cathode will be approx 40V more negative than the filament which is within the 50V limit. The top cathode will be approx 120V more positive to the heater, so it is good, too. Be careful, even the same type of tube, you can find different limitation in this regard. So, don't plan at the border line.
For 6cg7, the heater negative with respect to cathode is 200V. There, the 1/4 magic number works well here.

If your heater trafo has CT, then you connect this CT tap to the DC bias point. If not, use 2 100 ohm resistors (1W will last forever), one on each leg of the heater and connect the other ends to form the pseudo CT and connect this point the DC bias. That should do the trick.
 
Thanks Fred!

My apologies for not using the search button first. I just started looking through the gazillion Aikido posts and threads and found some other discussion on this topic too.

I have two options available:

- Hammond 185C12 which has two non-CT 6.3v 4a windings. I could use both in series for 12.6 2a and use the CT from that, though I'm not sure that's enough current. I'd also have to series the two 6.3v heaters on each PCB to 12.6.

- I also have an extra Modwright external power supply Dan uses for his Signature tube CD player output stage mods. It has both a regulated +285vdc supply and a 12.6vdc regulated supply that share a common toroid transformer and have a common ground on the load side. I think I could separate those grounds and bias the negative side of the regulated 12.6vdc up using the 300k/100k divider trick in John's manual. I'm not sure what the coupling of the two secondary windings on the trafo would mean to that though. I'd love to just use the Modwright supply. Would this work for my two mono PCB's assuming I series the two 6CG7 heaters?

Tom
 
Hi Tom,

Wired in series, the Hammond is a 12.6VCT 2A. That will be enough to drive all four 6cg7's, heaters wired in series in each bpard and then parallel the pairs.
The Modwright PS looks quite suitable for this application. Since the specs says the heater negative wrt cathode is 200V, give that a try before doing any mod. It has a good chance to work as is. The heaters will be wired as if using the Hammod - series in each board and // the heaters of 2 boards. Please let us know how it goes!!
 
Biasing Aikido

Hi pchw

From your answer in post 23 I do feel that you have a god understanding about what is correct biasing for a Aikido stage.

My Aikido play very well, but in spite of that, I am very keen on having the correct biasing.

B+=200 V.
Between GND and CT on 12,6 V I measure 65V (Bas powersupply).
Bas recommend a voltage divider made of 100/200 kohm
I have changed this to 100/300 kohm (this is what John B recommend to obtain one fourth the B+ voltage)

One fourth of a B+=200 would be 50V.

Shall I increase the 300k resistor to get a more correct bias voltage (50 V)??

Regards

Eivind Stillingen; Norway
 
Hi Eivind,

You didn't specify what tubes are you using, but let look at some numbers. You have 200VDC B+, the mid-point will be 100V. At 65V bias point, roughly the cathode of the bottom triode is approximately a little less than 65V on -ve side of the heater. For the top triode, the cathode is roughly a little more than (100-65)=35VDC more +ve than the heater. These numbers should be good to many many twin triode tubes such as 6SN7, 6922 etc. To be sure, look at the tube's data sheet, the subject to look at will be something like:
Maximum Heater-Cathode Voltage
Heater positive with respect to cathode (for bottom triode in this case)
Heater negative with respect to cathode (for top triode in this case)

My gut feeling is that, your set up is fine.

Edit: using the same logic, bias at 50V should be no issue as well. Check the spec sheet for your tube to confirm.

Good luck!!
 
B+=200 V. Between GND and CT on 12,6 V I measure 65V (Bas powersupply). Bas recommend a voltage divider made of 100/200 kohm I have changed this to 100/300 kohm (this is what John B recommend to obtain one fourth the B+ voltage)


I've got 100/200k ohm on Bas's board running 6N1P/5687 tubes.
virtual CT connected to heater bias at around 83 V.
B+ is around 258 V.

Peak Cathode to heater voltage for 6N1p is 100V.

Do I need to reduce the heater bias voltage? Maybe with a 100/300K on the psu to get closer to the 1/4?
 
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