Metropoulos Forum

I don't know if this belongs here, if it doesn't please direct me to the right board.

Anyway, over the summer I built a 50 watt kit from Triode Electronics, and after a lot of troubleshooting I got it to work and it sounds great.

What I'm wondering if it's possible to add on the the amp. I was thinking it would be really sweet to have two circuits in the same amp and be able to switch between them. Like, have the 50 watt circuit and then a JTM 45, or even an 18 watt circuit.

Is this possible? Basically the two circuits would share the transformers and tubes, and I'd have a switch to go back and forth between them.

Thanks for the help!

It is a pretty tall order to do what you ask.
The 18w option is not a runner although you can use THD Yellow Jackets to use EL84s in your 50w. These self bias and your amp will sound more like an 18w.

It is possible to mod to have a JTM45 and JMP50 in the same amp, Denis Cornell makes one although I have seen inside one I have not seen a schematic.

depending on what 50w you built (plexi vs 2204, lead vs bass) there are switching options:

shared vs split cathode on V1
cascaded vs non cascaded V1 (2 in 1 on SDM's comcast website)
switchable bypass (bright cap) on Vol1 pot
switchable tonestack (SDMs site)
switchable bypass cap on V2a
variable NFB (additional 100k pot in series with NFB resistor)
switchable valve vs ss recification and filtering (see Shatki's thread)

All possible but you might have to think hard about how many switch options you have.

Most simple and versatile mod, IMO, is the SDM 2 in 1, but omit the plexi bright cap and perhaps use the NFB pot as well.

You could map out the differences and then try using some relays to jump between certain preamp circuits. I started down the path of making a 50w and switch between tone stacks, diode vs tube recto and cathode cap at V2. I basically gave up on all that after I heard what amounts to a tube rectified JMP50, (EL34).

Anyhow, to do that type of channel switching requires relays, otherwise it can get a little clunky.

Mike

I built a 50 watt plexi.

Here's the schematic for a JTM 45 http://metroamp.com/wiki/images/6/69/JT ... ematic.gif" onclick="window.open(this.href);return false;

This is the layout of the 50 watt plexi I built. http://site.triodestore.com/50WPlexiLayout.pdf" onclick="window.open(this.href);return false;

Can someone point out what I'd have to do to make this happen?

Going from memory, the transformers are different in a SuperLead vs JTM45, so right there you have to make some choices. Split cathode in the SLead and shared in the JTM45. V2 cathode bypass cap in SLead and none in JTM45. Tone stacks are different. Mix resistors are different. Phase inverter caps are different. NFB is different. EL34 in SLead and KT66 in JTM45. Filter caps are different. There are enough differences that you can't just "switch" tones. Generally, you can make some compromises between the two designs and figure out what you want to "switch".

To do two full circuits is like two full amps in one chassis, (input to transformers and power tubes). With some creativity, you might get a flair for each design, but in the end, this is a significant project. Tonestack switches, cathode bypass cap switches and even diode/solid state recto switches can do quite a bit to alter tones and much easier to accomplish. Good luck!!

Mike

OK, I think I might just make a couple switches then rather than two circuits. You think a switchable rectifier and a tonestack switch would give me a bit more versatility?

I'm just curious why you are considering this. Do you really need that much flexibility in one amp or are you just wondering if it can be done from a technical standpoint? I assume you would use the amp in a live band situation because there would be little need for this when recording.

I suppose almost anything is possible but having all those circuits without compromising any of them individually would require a LOT of planning, sophisticated switching circuits and a very well-designed physical layout. Plus, it would be a rather heavy amp with all those transformers.

The only "switching" options I've tried are split/shared cathode and tone stack. I removed the split/shared as I didn't use it much because it makes a LOUD popping noise unless you turn the volume down first. There is probably a way to eliminate the switch noise but I didn't look into it seriously.

The tone stack switch is pretty useful and makes very little noise when switching. I use 56k/250pF most of the time, but if I want more mids and gain for certain songs I use 33k/500pF.

The V2a cathode bypass cap switch is probably useful also but I haven't done that because I much prefer no cap there.