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    Building, troubleshooting and testing of these amplifiers should only be
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aikido problem

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jayme

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Joined 2005
#1
Ok. Just finished my Aikido-based preamp, using 4 6SN7s.

I plugged it in...and a bit of hum, and it is volume-knob level dependent.

But, the big problem is that the GAIN is outrageous...I barely move the volume-knob from the lowest level and the music is waaaay too loud.

The 6SN7 Aikido should give me around 20DB of total gain, which is not too much off of my Grounded Grid at 12DB of gain. And, on that preamp...I can use the entire range of the volume knob...

I am lost...Any thoughts as to where to start investigating?
 
#3
I had one loose connection on the board...fixed that which took care of the variable hum. There is still a bit of hum...but it's not bad now.

But the overall gain is still an issue.

My pot is wired correctly, and grounded. The rest of the ground connections check out.
 
#4
Hi Jayme

Are you able to measure in and output at the same time (using a 2 channel osciloscope)? Play a 1V RMS tone and see what exactly is coming out at the output. Or maybe just a DVM with fine AC reading...play a 60Hz tone (simple DVM's measure the best at this range, I've been told), measure input...don't change anything and than measure output. Crude, but can shed some light!


Erik
 
#5
I have a DVM, but no oscilloscope. I'll try to measure the output voltage...

I have found that my B+ is higher than I expected, at 337V. I was shooting for around 300V. I have 330R grid resistors, with 1000R cathode resistors on the input tubes and 300R cathode resistors on the output tubes.

I am new to tube math...does the higher B+ give a larger swing to the signal output voltage?
 
#7
jayme,

My point to point 6sn7 is very quiet. My speakers are over 100db
1W 1M and I can barely hear a very slight hiss if I get my head touching them.

As stated above you need to check the VC to see what the taper is. I run CD's at about 9-10 oclock setting without problems.

With respect to tubes.... the 5692's are sweet but the Russian 1578 is even better. The 1578 carries two numbers as its a military grade tube. 6N8S/1578 Nice tube with a nickle base. I picked up two pair on ebay.
 
#11
It's the standard pot from the Transcendent Grounded Grid kit...I used a stepped attenuator in that project instead of it.
Click to expand...

When your volume is completely down, what resistance do you measure between the grid of the input triode and ground? It should be just the Grid stopper (if you are using one). And 'increasing' the volume, how does it change?

It may sound stupid, but maybe you connected the pot the other way around....the correct way is that when the volume is completely turned down the wiper touches the ground.

Erik
 
#12
I wired a 100k resistor before the pot to knock down the gain. That should do for now while I continue testing...i'll replace it all with a good stepped attenuator later.

Next question: When I tap the case, the sound is amplified thru the signal. It sounds like a drum. Is this usually a problem with grounding? Or do I have a bigger problem?
 
#13
jayme said:
I wired a 100k resistor before the pot to knock down the gain. That should do for now while I continue testing...i'll replace it all with a good stepped attenuator later.

Next question: When I tap the case, the sound is amplified thru the signal. It sounds like a drum. Is this usually a problem with grounding? Or do I have a bigger problem?
Click to expand...

Sounds like you have mocrophonic tubes.
 
#14
Microphonics usually isn't too much of a problem, unless you place it ontop of a speaker cabinet :xeye:

Those "Tube Dampeners" can help, but go to your auto parts store and get a silicone oil seal ring instead - same thing, only $20+ cheaper 😀
 
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