18Watt.com

Hi i've just got a set of transformers to convert my trinity plexi
into a 36 watt.
I'm thinking of using el34's but i'm not sure what it's going
to sound like.
Has anyone else done this?
Cheers.

Yes, it's been done a few times before. Just do a Search for threads. The sound of your amp will be fatter and less ice-picky bright than with EL84s. So EL34s would be my prefernce. The downside is that you would need to cut bigger holes in the chassis for the two octal sockets, and you would need some real big resistor(s) on the cathodes. It may be more practical to split the cathodes of the power tubes and run with them separate resistors and caps.

Edit: FYI - Here's are a couple of clips of a might fine EL34 36W - http://www.18watt.com/modules.php?name= ... c&p=224508 complete with schematic

Zaphod i listened to those clips before,that's when i thought about doing
a new amp using the trinity plexi layout and try and work it out from there.
I've been using the 18plexi for 2 years nothing broken sounds great so i'm leaving it alone.
I'm just looking at the different pin layouts as i'm not familiar with el34's,
the values needed and where they should go.
I'm looking at this as a learning amp so no pressure on getting it built.
I'm using an ig 36watt mains transformer 290 0 290 he said using el34's is no problem and a valvepower output transformer,i want to use a gz34 rectifier.
can you give me a rough parts list so i can get the bits needed and
make a start.
Cheers.

Just follow the schematic for the "Phat Ass" in thread I just posted. He's only getting 26W (according to his estimate) because his PT is a little under-powered for the job. If you use SS rectifiers (and not a GZ34) with the PT you've got there, you will get higher voltages to get you closer to the target 36W.

Three questions for Z. Phil, thanks you please, that are related to this topic. I believe that I have read, if you use 2 each 6V6 tubes, as in the "Phat Ass" schematic, that you can use a standard 18W transformer 290-0-290 set. Is that correct?
Question #2, I saw somewhere on the net that Z. Phil advised that it is not necessary to change out the 8.2K power tube grid block resistors for the "usually used 1.5K's" on a 6V6 build. Am I correct on that assumption also?
And the last question. Usually I see a 270 ohm cathode resisor for the 6V6 power tubes, but the "Phat Ass" indicates that a 450 should be used with 6V6 tubes. Which is the preferred ohm's value to use? Thanks. Terry

1 - Correct
2 - Correct - we still want it to sound something like a Marshall 18W
3 - Depends on anode and screen voltages. Take measurements and calculate dissipation whichever value you start with, and then adjust from there. It's called the trial and error method.

Thanks for the info Z. Phil !

I did a 2x 6V6 amp with 18watt iron, and rectifier tube simulation (resistor inline). I ended up using a 200ohm resistor to bias the valves hot.

270 Ohm should be fine, or at least a good starting point.

That is my current build, with an EZ81 rectifier (290-0-290 PT). Should I stay with the EZ81, or would the extra gain in voltage with diode rectification make much of a difference? Just curious. Thanks. Terry

If you're planning on running EL34s in the amp, then solid state diodes (preferably fast recovery ones like UF4007s) would be a good idea, so you get your 36ish watts power out. A shared 150 ohm cathode resistor should be in the ball park for EL34s.

A rectifier question about the phat ass schematic,is there a sag resistor.
If i do the ss rectifier as in the phat ass ,where and how do you put a sag resistor in if i want to try it and what value.
Thanks.

lespaulblues wrote:A rectifier question about the phat ass schematic,is there a sag resistor.
If i do the ss rectifier as in the phat ass ,where and how do you put a sag resistor in if i want to try it and what value.
Thanks.

After the diodes, before the first filter cap. 75-100 Ohms is probably about right to get the feel of a regular 18W but with EL34s.

To figure out the power you need, P = I^2 * R. If your amp's drawing 200mA ( = 0.2 A) at max, 0.2^2 * 100 = 4 Watts. But you should probably double that for safety, so an 8 or 10 W resistor.

The PhatAss doesn't have a sag resistor. If you put one in you will get less power output, and have less of the big Marshall feel. The cathode bias itself will provide a degree of squish. Leave the tube rectifiers and sag resistor thing for the 18W end of the scale IMO.

Can someone tell me if a 250 ohm 7watt cathode resistor is ok to use instead of 10watt.
Thanks.

Don't know for sure. But it sounds a little risky.