18Watt.com

Hi everyone
i've been listening to the samples on the "zounds" section and I really liked the TMB
I have a couple of 12ax7s, El84 and a PT so i was thinking of building it but i don't know if the transformer is suitable for this

its a 150ma 315-0-315 with 6.3 taps (SS rectifier needed)

what do you think?
Any recomendations?

Thanks a lot!

315 might be a little high for an EL84 output section, especially if you use SS recitfication. You could get away with a 6V6 output section though.

Why do you think you need a ss recto? The 6CA4 is a 6.3v tube with cathode heated indirectly. You put it on the heater circuit without a problem. It requires 1.0A. That could be too much demand and not leave over enough for the rest? With a tube rectifier, you will drop the voltage some.

While I imagine 315-0-315 is a bit high, it would probably work. Use JJ EL84's as they have a rep for being able to take the voltage. The 6V6 suggestion is a good one, too. I've got an amp with 6AQ5's (like a 6V6) running in the 330-ish range and it works great. Go ahead, give it a try.

aren't 6v6s more clean sounding?

i have a 2x6l6s that works pretty well for cleans and was looking towards a more "british" sounding amp until i found this gorgeous page

the thing is my transformer is to low for EL34s and to high for el84s...
i thought it could be fixed by using some resistors though.

i can't use a tube rectifier because they're pretty hard to get here in Argentina and they are quite expensive (30 dollars and more), plus it's almost impossible to find sockets for double diodes.

Well, the 6CA4/E81 goes in a regular noval socket like a 12AX7 would. I understand about cost, though.

My opinion is that you get more dirty sound from 6V6 than you do with EL84. I would go ahead and try it. If you have decent EL84, they should work, too. Resistors won't drop the B+ supply enough to matter. You need to put a big Zener on the center tap and some people think there are reasons why you shouldn't do it. One reason is that it generates a great deal of heat. I don't know enough about Zeners to advise on this.

Phil_S wrote:Resistors won't drop the B+ supply enough to matter. You need to put a big Zener on the center tap and some people think there are reasons why you shouldn't do it. One reason is that it generates a great deal of heat. I don't know enough about Zeners to advise on this.

I think UR12's VVR circuit would work for this. It's meant for variable power to lower the output volume, but it does that by dropping the voltage. Other members have used the circuit to drop hundreds of volts (from 300v to less than 50v) so I would think this could work for you as long as you pick a MOSFET that is rated for over 500v(if you ss retify it you'll have 441v). You'd also have the side benefit of being able to lower the output volume.

Check out the VVR thread for more info on UR12's circuit ...
http://www.18watt.com/modules.php?ame=F ... 48&start=0

There aren't that many parts (3 resistors, a zener diode, a 1meg pot and the Mostfet) and although the NTE2377 mosfet is about $10 - $15, there are other cheaper mosfets listed in the thread that will work, for example:

ur12 wrote: Just about any Nchannel enhanced mosfet that has a rating of 500 - 900 volts and 4- 8 amps will work just fine. Mouser has a 2sk3675-01 for less than $3 that works great.