Custom SET Speaker and HP amp

[phergus_25]

Well I am kinda jumping the gun...
But Im so excited I just have to start singing Doug Savitski's praise.
I was posting trying to learn enough to build a tube amp over at DIYaudio.com. Dsavitski helped alot, but the idea was over my electronics know how.
I asked Doug if he ever built amps for other people. He offered to do a partial trade for a set of full range speakers for a tube amp of custom design.
I jumped on the deal and he started breadboarding the amp.
I am going to leave it to him to describe it right now, but I am very excited.
A pic so far:

ITs been tested with Grado phones and Iv been told it sound GREAT and no hum. And thats with the wires all over the place. So it can only get better!
-greg

 

[phergus_25]

Heres a schematic that dsavitsk sent me:

 

[dsavitsk]

I told phergus I'd comment on the design. The idea was to create a tube amp that would not cost too much but that could power efficient speakers and headphones. The design is pretty standard, but with a couple of tricks to improve performance.

It is just a triode driving a pentode strapped as a triode. The PS has less than a mV of ripple, which is quite low. Moreover, the CCS on the driver has a high PSRR, so very little (essentially 0) of that ripple gets to the driver. This is important as any hum on the plate of the driver will be amplified by the mu of the output tube. So, this keeps the amp quiet. The CCS also isolates the driver from the last cap in the PS which keeps the big electrolytic out of the signal path to some extent.

The driver is also LED biased which also keeps a cap out of the signal path which improves resolution considerably. The combination of the CCS and LED also allows the driver tube to set its own operating point, so lots of tubes can be swapped in with no changes being necessary.

For the output we are using a 6w6gt. 6l6, 6v6, el34, kt88, etc. could all be used too, but I happen to have a box of 6w6's, and replacements cost about $3, so it seemed like a good choice. It is slightly less power, but still should do 2 to 3 watts without trouble. The signature of the 6W6 seems to be very smooth with a lot of bass emphasis. I think it has better bass (though not more midrange clarity) than my 300b's to be honest.

OPT's are Edcors. To power phones, I just put a resistor (20R) in parallel with the phones. PT is from Allied. In the bread board pic above, the driver is actually a 5687 which was just being used temporarily. I think we are going to start with a 6BQ7a as the driver (again, I have a box), but as I say, there are tons of other option.

Total parts, not including the chassis, could be done for ~$200-250 (I went a bit higher due to experimentation and things exploding and some custom PCB's I made.)

-d

 

[Carl]

It's a nice design. Why'd you chose a resistor input to the filter rather than a choke input?

 

[dsavitsk]

Quote:

Originally Posted by Carl /img/forum/go_quote.gif Why'd you chose a resistor input to the filter rather than a choke input?

$

I really just put the resistor there because the cap is too big for the rectifier.

 

[Carl]

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif $ I really just put the resistor there because the cap is too big for the rectifier.

Yeah, makes sense.

 

[dsavitsk]

If you simply swap the choke and the resistor, ripple increases by a factor of 10 and voltage drops by 1/3 (~100V), so the resistor does not really act like a choke input filter.

 

[phergus_25]

Well the amp is coming along well
and heres the thing
Doug added a switch that would allow me to change the heater pin arrangment, so I can switch in both 6dj8
pin out as well as 12at7 tubes

 

[Carl]

Quote:

Originally Posted by phergus_25 /img/forum/go_quote.gif Well the amp is coming along well and heres the thing Doug added a switch that would allow me to change the heater pin arrangment, so I can switch in both 6dj8 pin out as well as 12at7 tubes

You might want to try other tubes with similar Mu, as I think you can do better than both those tubes. 5687, 6N30P, 6SN7, 6N6P all come to mind.

 

[cotdt]

no more small tubes! big ones! use big

 

[phergus_25]

Well Im always down for big tubes,
but I think alot of the idea behind this amp was for Doug
to take what he had, tube and everything else wise, and
experiment till he finds something that sounds good.
At some point I am going to get some custom transformers wound
with an 8 ohm tap and a 40 ohm tap, for the W5000s
If at that point it looks like some upgrades could be made
then Ill let Doug go to town
-greg

 

[dsavitsk]

Quote:

Originally Posted by phergus_25 /img/forum/go_quote.gif Doug added a switch that would allow me to change the heater pin arrangment, so I can switch in both 6dj8 pin out as well as 12at7 tubes

This does not mean just these tubes, but anything that shares this pin arrangement and that can live happily with 11mA while biased at about 3.8V. I did a quick search and found about 40 different tubes that will work. The 5963 and the 12AV7 have been good so far.

Quote:

Originally Posted by Carl /img/forum/go_quote.gif You might want to try other tubes with similar Mu, as I think you can do better than both those tubes. 5687, 6N30P, 6SN7, 6N6P all come to mind.

2 of those (6H30p and 6N6p) will work. I tried the 6n6 and found it too be too syrupy -- it needs a much higher plate current to sound good. I also tried the 5687 (which required wiring it up a little differently) and it sounded quite good. But, the pin arrangement is such that it locks you in, and the mu is actually a little low. The 12AT7 is nothing to scoff at. There are a lot of sleeper tubes out there.

And, just to stir things up, the 6SN7 is actually quite a poor driver. Distortion is high compared to something like a 417A or 5687, and Gm is too low to overcome Miller capacitance in many output tubes. May not be a huge issue here with the 6W6, but to drive something like a 300B you can do much much better.

Quote:

Originally Posted by cotdt /img/forum/go_quote.gif no more small tubes! big ones! use big

Some of the best tubes for audio, especially as drivers, are physically quite small -- 5842, 6688, 437A, 7788, etc. These were made during the space age, and have super high gm and super low distortion, and low noise and microphonics largely due to their small size.