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using the 6BX7 at low'ish current in Aikido


2006-10-03 11:02 pm
The desire to dabble has caught me again.

I run my Aikido with four 6SN7's the inputs at 4.8mA using 1000R cathode resistors and the outputs at 8.1mA using 470R resistors. B+ is around 344V loaded.

Another user (Jayme, I think) recommended giving 6BX7's a try in the input position.

Now the 6BX7's draw more current than 6SN7's and my PSU trafo can only support 50mA current draw. Luckily, if I don't change the cathode resistors when I use the 6BX7's, I estimate around 15mA per 6BX7. To estimate this, I took half B+ for the plate voltage and looked for where grid voltage / current is around 1K.

So, 2 6BX7's at 15mA ea and 2 6SN7's at 8.1mA ea means around 46.2mA total current draw.

This current draw (46.2mA) is close to my trafos limit of 50mA, but I assume that the Hammond design can handle close to the limit without becomming too hot. Is this correct?

With increased current draw, my B+ should drop somewhat (maybe to around 320VDC), this would mean that plate voltage, grid voltage and current would also drop, resulting in lower overall current draw for the PSU. Does this sound accurate???

I suppose, I can replace the 6SN7 input with a 6BX7 in one side of the Aikido and measure voltage drop across the cathode resistor to calculate current to make sure that I can keep within the 50mA of the trafo.

Any answers, comments or advice would be much appreciated.



2006-10-03 11:02 pm
Well, I haven't tried a 6BX7 yet (I'll buy a pair tomorrow), but I did find a couple of NOS GE 6BL7GT's that were cheap.

Using the datasheet (in the method I asked about above) I estimated plate current using my existing cathode resistors of 1K and 470R with B+ of 350VDC as around 8mA and 15mA respectively. When I actually calculated the plate current using the measured voltage drop over the 1K and 470R resistors, I got 9mA and 13mA respectively!!! Phew

Anyway, what about the sound in my Aikido???

With the 6BL7 in the input position (1K Rk, grid voltage 9V at 9mA), the sound was not very good - I forgot the details, but it seemed thin and lacking in bass.

With the 6BL7 in the output position (470, grid at 6.2V and 13mA), the sound was full. However, I thought it too full at the lower mid / bass compared with my all 6SN7 version. My wife preferred the fuller sound, so I put on my test CD with chromatic scales at -20dB. It was quite revealing: with my all 6SN7, the highs were pronounced, but there was a depression in volume in the mids, a regain of volume in the upper bass, then distinct roll-off as frequency further dropped.

With the 6BL7 in the output position, everything was more level in terms of percieved volume across the chromatic scale - a little roll-off in the mids, somewhat pronounced highs and the bass is more extended.

When playing Alan Parsons (iRobot 2007 release), the bass does seem a bit agressive, however in Genesis - We Can't Dance (1991), everything seems to be in its place - if you know what I mean. Could the re-release of iRobot be mastered for fuller bass to satisfy the modern lust for low-end? I'll have to go back to the vinyl to find out.

If anyone has experience with 6BL7's and 6BX7's or any other more "exotic" octals that work in the Aikido, it'd be great to get some feedback.

Bruce Anderson......if you read this, I'd appreciate your comments on the differences, which means a visit if you're interested. I sometimes doubt my ears when it comes to subtleties and nuances.

I modified a Bada PH 12 headphone amp to use a pair of 6BX7 optimal WCF as output, driven a single 6SN7 ground cathode stage. It will also take 6BL7 in the output stage though it is not optimal. So far I have tried silvania 6BX7 and Ge 6BL7. The 6BL7 sounds more lively, while 6BX7 goes a bit louder in extreme bass. I might try some other brands I have later.


2006-09-07 12:15 pm
Though I have not built the Aikido, may I suggest that
you try the 6BX7 for input n output.
I have a parafeed linestage that sounds wonderful with
this tube. Tried the BL7's but it did'nt sound that good.
Maybe that BL7's need to be run at higher voltage but
my regulated PSU could only produce 300v dc.
Hi Charlie

Was about to call you. Not completely moved in but close. Would like you to stop by with your phono, etc. I did briefly play around with 6BX7 and 6BL7 when I first completed the Aikido line amp. I didn't optimize B+ or current flow. I believe it was even prior to his writings about optimization of cancellation resistors. If it was me, I would use either tube only in the second tube position, not the input. Neither measures as low in distortion as the 6SN7 but both can drive a BUS. (I came to the conclusion that the 6SN7 was perfectly adequate as a cathode follower. On the other hand, I use the 6DJ8 in my phono pre to drive a low resistance RIIA circuit.)
I am now set up in a 10 X 12 "bedroom" rather than my living room. I won't prejudice your impressions but it does some very interesting things and I have made some interesting changes since you last heard my system. Unfortunately with the system right in my lap the power amp is a little noisier than I would like. To put it in perspective, it is buried in record surface noise or even the gentle air flow noise of the heating duct but, like you, I know its there so I have some work to do. Do you still have my cell # or Yahoo address?