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Starving Student and tube questions

post #1 of 5
Thread Starter 

Hello Head-Fi, I hope y'all are ready 'cuz I have a lot of questions.

 

So this started when I picked up a pair of 12B4A tubes from an antique store because I was interested in trying out "tube sound" on the cheap. (They didn't have any of the well-known tubes)

After spending some time on the internet, I think I have a very basic understanding of vacuum tubes and schematics understanding from years ago. Seeing as how its so popular, I've decided to try my hand at Pete Millit's Starving Student amp and here begins my questions...

 

The Starving amp uses 19J6's which are dual triode tubes while my 12B4's are single triode. In looking at the schematic for the Starving though I saw this

To me it looks like both triodes are being used for the same signal, which feels like it would be redundant. Is it or is the a reason?

 

Next  was the heater circuit

To me it looks like the heater is more directly connected to the output audio jack than the tube anode is. Am I reading it wrong? I've never seen vacuum tubes in operation before so I have no idea what I'm doing.

Oh yeah, and I get the capacitors, resistors and whatnot, but this?

What is this, a MOSFET? I don't even know. I have no idea what those things do or how they work.

So that's it for the amp, but i have some questions about the tubes

 

So the 19J6 looks like this...

While my tube looks like this...

Back to my original question, if the use of two triodes per channel is redundant, would I be able to modify Pete's design to a lower voltage so I could use my tubes? Also, is there something special I should know, it looks like I have two heaters...

 

Hopefully you guys could help, it would be amazing I could pull this off. Thank you in advance

post #2 of 5

I am probably going to really screw up the terminology here, but I see what millet did. He is essentially combining both triodes together to increase the gain of the signal. (IDK if gain, power, or volume is the right word here). But essentially he is putting both triodes in parallel. If you use only a single triode stage, then you might no get enough volume out of your headphones. 

 

And yes, that is a mosfet. 

 

Not really a tube expert here (if you could not have already guessed), but may I ask why you are even bothering?

 

If you choose to go with the 12au7 mod for this amp, it takes care of all of the guess work for you and the 12au7 does not cost more than a few bucks.

 

I would not blame you for trying to use your new tubes, but it just seems like you would save yourself the headache if you simply went with the new design. 

 

http://diyforums.org/SSMH/variants/SSMH-12AU7.gif

post #3 of 5
Paralleling tubes doubles the current available, which doubles the power gain, but not necessarily the voltage gain. Where the tube is acting as a driver, it means that there is more current available to drive the subsequent stage's capacitance, which can be significant.

It can also affect the effective voltage gain, because, depending on the exact implementation, it can reduce the output impedance, which affects the effective gain due to the interaction with the driven stage's impedance.

w
Edited by wakibaki - 12/24/13 at 12:07am
post #4 of 5

There is a very practical reason for paralleling triodes.  Many (if not most) triodes are not matched within a tube.  Using a single tube for both channels often results in a channel mis-match.  You can adjust bias (or use an auto-bias circuit) and that will "center" the sound, but the stronger triode will always respond more strongly to peaks.

 

Paralleling the triodes effectively averages the two triodes and allows one to easily match it with another dual-triode tube.

post #5 of 5

EEEh, to the OP: I really like 12b4(a) tubes. They have nice low gain, and low plate impedance which should both help you well in a circuit like this. 

 

On the other hand, as you probably know, you don't sound like you know enough to write your own recipe. 

Follow the recipe for the 12au7 variant of Pete's circuit and enjoy building something. 

While you are building it read as much as you can about as much as you can to learn what you are doing. 

After a bit you should be able to answer your own questions, and using a 12b4(a) tube in a hybrid like this should be very doable. 

 

I would personally save the 12b4a for a transformer coupled "mostly tube" amplifier, but to each his own.


Edited by nikongod - 12/24/13 at 7:08am
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