Aikido Questions

[Pyre]

I built an Aikido preamp for my first real from parts preamp project and had a few questions that I hope someone can help me with.

When I first fired it up I was using a Jung regulated power supply hooked to a Dynaco P211 transformer for the B+ giving about 300 volts (for 12AU7’s and 12BH7’s). For the heaters I just wired them up to the heaters on a nearby PAS3. Well there was serious hum and when I touched the tops of 100uf shunting caps the tone of the hum changed. So with some easy research on diyaudio I learned about the methods of grounding with the 20 ohm resistor and the adding of reference voltage to the heater circuit. I wired up a simple two diode two 2000mfd cap heater circuit to the P211. Went and got a 300K 2W, a 0.1uf cap and a 100k ½ watt and wired it up according to the diagram and on warming up the unit there was no hum. My main question is why does this work? Why does adding ¼ of the B+ voltage to the heater circuit cancel out the hum?

Also a question about the coupling capacitors. When buying all the parts for this project I had no idea which coupling caps to buy. The sheet that came with the board said 0.1 to 4uf and I did not want to drop 25 bucks a cap to find out that they did not sound good so I purchased two 0.1uf Hovland Musicap and two 0.33 uf Auricap’s. I figured this hit both ends and they were not that expensive. On listening one seems to have better lows and one seems to have better highs. Does anyone have any recommendations of Coupling caps to try? Or ones they use that sound great.

Last question. I had a set of Mono 1M 24 step attenuators left over and used them for the volume controls. They seem to work find and have enough range but I read on DIY that most people use a 100k pot. Are the 1M ones too high of a value? Should I swap them out?

I have attached a photo of my monstrosity. I have a nice case for it but wanted to get it sounding fine before starting the metal work.

Thanks in advance!

[dsavitsk]

I bet the larger caps have better bass. Coupling caps form part of a high pass filter, meaning that all frequencies above a certain point are passed, and those below are not. As the caps get larger, the roll off point goes lower., that is, more bass frequencies are passed Most people shoot for a roll off of 2Hz or so.

You can see some details of how to calculate this point, as well as my impressions of various (mostly inexpensive) coupling caps at http://www.ecp.cc/cap-notes.html

Oh, and the higher the pot impedance, the more noise. 50K to 100K seems to be the sweet spot of being high enough to present a decent impedance to the previous load and low enough to not be too noisy. Also, the 1M will be too high of a grid leak for some tubes.

[Pyre]

Wow thanks, thats a great link. Exactly what I was looking for.

[beau2317]

Looks good. How does it sound to what you had before (and what was that?)

I found that they always hum if you don't reference the heater to something. Does not have to be quarter b+, it would also not hum if you grounded it. The reason you reference it to quarter B+ is so you don't exceed heater to cathode limits.

Also, you could try both caps in parallel.

Hope you enjoy your aikido. Welcome to the club.

[zacster]

I see an ST-70 on the side there too. Is that your main setup?

I can tell you this. I gutted my ST-70, replacing the driver board with a Curcio CCDA board, the can cap with a Curcio board, but best of all I wired it to run 6B4G instead of EL34. It is an entirely different amp this way. No negative feedback, directly heated triodes, cathode bias. It only puts out about 10-12 watts this way but there is no contest on the sound. And it isn't hard to do once the boards are built.

I also run mine with an Aikido, the 24v version that Broskie spec'ed out on his website. It uses 4 6GM8 tubes running 24v on the plate, and 6v * 4 = 24vdc in series on the heaters. A single pre-built 24v power supply runs both. It could also run on battery but I haven't tried that. I use 100k dual pots for now, and also have a Welborne remote attenuator that needs to go back in.

And mine doesn't look any better than yours.

[Pyre]

Hi,

Yes that is indeed that is a St-70 its been modified by Analogique to be solid state on the power supply and all the caps updated. It sounds pretty good all and all. I got it on ebay for 250 so I could not pass it up. It definitly needs a new driver board. In shipping one of the 7199 tubes broke so instead of buying one I got out my dremal and converted it to 6GH8A's that I had kicking around. My main amp is a St-70-C3 but its an Audio Research rebuild. It sounds pretty good and does not really need any mods.

How did you convert your St-70 to 6B4G's? I would love to try that on a amp. 6B4G's are the same as 2A3's right? Was it that dificult? Can you reccomend any documatation?

Im going to buy a set of 100k pots just to try and see if they sound better.

My old preamp was a modified PAS3 I have 3 of them accualy. a stock one, a modified one and a super modified one. I like the sound of the aikido better already. Maybe its just because I built it all by myself. Im going to try putting in the jumper to run both the coupling caps.

[zacster]

The only documentation I can tell you about are my questions on AudioAsylum. Do a search there on 6B4G and zacster. You should get lots.

In a nutshell, start by replacing the 5ar4 with a 5U4 rectifier. Since you are using SS in the PS you have a problem right there. The goal is to reduce the B+ to about 390v on the power tubes. Then, run the center tap of the filament supply through a 560ohm/25w resistor. This provides the cathode bias. (If you have higher B+, increase this resistor to compensate, maybe 750ohm) The last part is the trickier part only because I don't know your driver board. You need to have a grid leak resistor from the grid of the 6B4G to ground. On the curcio board the wire from the bias pot is directly connected to such a resistor and I just swap that to ground. I think it is 120k. In any case disconnect the negative bias as this is cathode biased. Lastly, disconnect the negative feedback. You don't want or need it.

And one other thing, the 6B4G must be Sovtek, do NOT use NOS tubes as they are not rated to run this hot. Only the Sovtek can run this high as the internals are more like a 300b.

I'm not at home so I'm doing this from memory, so take it all with caution, not that I'm expecting you to run home and do it anyway.

[Pyre]

I will not run right home and try it mostly because I am home and I dont have the tubes just kicking around

The St-70 in the pic I just bought becase it was a good deal and I had a quad of Xf2 Mullards I got in a trade and I wanted to hear what all the fuss was about. My St-70-C3 takes 6L6-GC's

What im planning on doing is building an amp from the ground up based on the St-70 that matches the black cases that I got for my phono stage and Aikido. I have a set of ST-70 output transformers and dont mind dropping the 100 bucks for a new power trans. Everything else I was just going to buy new so the 6B4G's sound really interesting.

Wow a 25 watt resistor is that one of the fancy ones with the aluminium case?

I will search AudioAsylum and if anything is that confusing I will put a foot in the mop bucket and touch the B+, naaa I will email you

[zacster]

Yea, the 25w resistor is a gold finned one. I can't remember the name of the supplier, but they are available from a few places. Just remember though that my amp is based on the Curcio premium driver board, some others may not work as well. It was well suited as it provides enough voltage swing.

On another note, I just put a pair of Auricaps into my Aikido, replacing the Solens that I had. WOW. I never heard my system sound so good. The bass is sooo deep and fast. It's rattling my computer table. It just goes to show that you're never done, just waiting for that next thing to try. It's been at least 6 months since I've tried anything new with my main system. And to think, these Auricaps were sitting unused and I just never got around to trying them in the Aikido.

[Pyre]

What Size of Auricaps caps did you try?

The biggest I had to try were .33Uf I was thinking of ordering a set of 3-4 uf's and some similar sized Jensens or Audio Notes. I wonder if they have had any takers on the Audio Note 3.3 uf silver foils for $1598.40 each.

Do you have any pictures of your 6BG4 amp and Aikido preamp?

Shane