Glass-ware Aikido 12vac build

[qguy2000]

I am getting conflicting advises regarding B+

From the WoS site

"If the voltage is not between 75 and 85 volts resistor R12 will need to be changed.
Use a lower resistance if the voltage is too low, use a higher resistance if the voltage
is too high."


From @Goatguy

"B+ of 104 volts unloaded, and 94 volts loaded does not sound out-of-spec."


if I do adjust the R12 to get between 75 nd 85v, from the 20ohm resistor, what would be an ideal resistance to use?

[qguy2000]

I replaced R12 from 470 to 1K and magically everything went to spec. So far so good

[Steve Morley]

Because of the common power supply for filament and B+ the values for R12 and R15 can be SLIGHTLY co-dependant. More important than the B+ voltage is making sure the filament voltage doesn't go too high. Personally I wouldn't let it drift above about 24.5 volts. B+ can go a bit higher than recommended and you shouldn't have any grief.

Once you are happy with the filament and B+ (only measured with the tubes in, tubes out measurements don't mean much) measure the voltages across R2 and R11 as detailed on pages 12 & 13 in the WOS link below.

Be aware that the Aikido circuit can be somewhat sensitive to tubes that have mismatched triodes. As well, because the input and output tubes are direct coupled (no capacitor) one tube can sort of "pull" the other around.

Steve

https://wallofsound.ca/wp-content/up...iring-test.pdf

[bhpistorqintorpm]

Hi All,
I'm in the middle of building the Aikido 12VAC preamp. In the build manual, it is suggested that R13 be calculated based on the following formula:
R13=R10 X ((33-2)/(33+2)) .
That results in a value of 300X(31/35) = 265 Ohms. But the recommended value of R13 is 88.7K ! But I see pics of several builds[pics] with R13 of 88.7K ohms and R10 of 300 ohms. Can anybody please confirm?

[qguy2000]

I used
300 ohms for R10
100 ohms for R13


I used this as reference for testing. This is part 4, please check out part, 1,2 and 3

[Steve Morley]

Some of the confusion is down to me and some to the relabelling of board components between revisions of the board. The initial release of the board DID NOT have the two Zener diodes. On this board the filament dropping resistor is labelled R11 and the B+ dropping resistor is R12. The board drawing on the Glass-Ware site is of the earlier version of the board.

The newer version WITH the Zener diodes labels the filament dropping resistor as R15. This relabeling avoids confusion with two other resistors in the amplifying section of the board labelled R11. The pictures on the WOS site are the later version of the board WITH Zener diodes.

The schematic, page 6, in my first version of the 12Vac has three R10s. The one in the "middle" that connects to ground should be R16, which is 100K ohms. R13 which connects to C1 and R10 (2x) and R16 should be 88.7K The middle resistor labeling has, I think, been corrected in later versions of the manual and definitely on the board.

I'm assuming you are building the 6DJ8 version and NOT the 12AU7 version of this amp. The 12AU7 version will have many different Component values compared to the 6DJ8 version.

I hope I haven't made this as clear as mud. If you follow all four parts (and the multiple attachments with each part) on the WOS site your should finished board should be correct and the amp work properly.

If the B+ runs a bit high don't worry. The circuit runs the tubes quite conservatively so even a bit more B+ won't put the tubes anywhere near their dissipation limits. Watch the filament voltage though. Running this too high will shorten tube life!

Cheers, Steve

[zacster]

I built a 6GM8 version of the Aikido on the original board that was sold. It worked the first time off the bench and has been running ever since. It's probably been over 10 years at this point. This one uses a 24vdc open frame linear regulated power supply that supplies both B+ and heater voltage. 24/4 = 6vdc, a little low but still works just fine. The only mod I've done over the years is to put in a remote selector/volume control. It doesn't actually change anything in the underlying circuit.

The biggest problem is that the price of 6GM8 tubes has gone through the roof. I bought my original quad for $4 apiece, and these were Amperex Bugle Boys. I bought another quad as backup soon after and all of these are still the ones I use. Some day I'll need to buy new again.

In a similar fashion, I've been using Sovtek 6B4G in my amp and after a few early failures I've had the same quad for a very long time now. But just today I got an email from TubeDepot and they have a black Friday sale on these so maybe it is time to buy a backup quad.

[bhpistorqintorpm]

Originally Posted by Steve Morley

The schematic, page 6, in my first version of the 12Vac has three R10s. The one in the "middle" that connects to ground should be R16, which is 100K ohms. R13 which connects to C1 and R10 (2x) and R16 should be 88.7K The middle resistor labeling has, I think, been corrected in later versions of the manual and definitely on the board.

I'm assuming you are building the 6DJ8 version and NOT the 12AU7 version of this amp. The 12AU7 version will have many different Component values compared to the 6DJ8 version.
...
heers, Steve

Yep, 6DJ8 version. And thanks Steve for clearing the confusion. I guessed it would be R16 in the formula as it was the only 100K resistor . But the build guide emphasised on the formula for R13 calculation, hence wanted to be sure.
PS: among the resistors supplied in the kit, only 88.7K seems to be of 1/4 watt rating. Will it be able to withstand the current or should i replace it with 1/2 watt one? Or bit of variation is fine as i see a 84k Dale resistor there in the pack meant for R13.
PPS: is it possible to post the soft copy of the latest version of manual here?

[Steve Morley]

1/4 watt is fine for R13 (the 88.7K). There is very little current flowing through this resistor and it is all AC, essentially the noise (AC) riding on the DC from the power supply. Likely in the range of a few millivolts.

Sorry, can't post the manual, it is copyright material.

Cheers, Steve

[Steve Morley]

I had a LV board and did a little creative hacking to boost the B+ to about 48 volts. I copied the power supply part of the 12Vac on a separate proto board. See pic. Using a 9 volt trannie lowered the B+ but still gave me a bit more than 24 volts for the filaments. The normal power in on the board got the B+ but the filament supply was connected to the board between V1 and V2. See pic. I also installed the zener mod here too. I had to cut a few board traces to separate the filament from the B+. Each triode of a 6GM8 will handle 30 volts so two "stacked" in an Aikido will handle 60 volts. With 48 volts R2 & 4 were set to 220 ohms, R8 & R11 180 ohms.
I also put in a remote volume kit, a cheapo found on eBay.
It sounds decent but doesn't quite have the drive and pace of a 6DJ8 version of the 12Vac.
If you need 6GM8s PM me. We might be able to do a deal.

Cheers, Steve