Low wattage Plexi

[Littlewyan]

What Amp is this John? Sounds pretty good.

[Smokebreak]

Have you considered the Marshall JTM1? I found this schematic, but cannot vouch for its accuracy.

There is actually a revision to this schematic by Matec and I built it up. The last of the discussion and the build is here with layout, pics, clips.https://tubeamparchive.com/viewtopic.php?t=24671 if you dig in the links it goes back to a Brazilian DIY forum....An international amp building affair!

[Littlewyan]

I'm thinking of installing a low power switch in my amp. Now I could just copy the JTM1 but how do I work out the impedance? Half power is easy but looking at that JTM1 Low Power Switch I have no idea how thats working. Doesn't look like it matches the impedance anyway.

[Smokebreak]

I'm thinking of installing a low power switch in my amp. Now I could just copy the JTM1 but how do I work out the impedance? Half power is easy but looking at that JTM1 Low Power Switch I have no idea how thats working. Doesn't look like it matches the impedance anyway.

I was having the same issue understanding how that all worked. Matec and Roberto gave a good analysis of the switching and how the loads stay roughly the same.

[Littlewyan]

If you change the Secondary Impedance then you change the primary impedance. Now if the primary impedance is already matched to the valves then you can normally safely go 100% either way. If you increase the Primary Impedance then I think you lower the power at the same time. Someone will probably correct me on this.

[Smokebreak]

I don't think the secondary impedance changes, nor does the primary. Good explanation on bottom of page 1,2 of that thread

[Littlewyan]

Ah sorry didn't read the posts, just the schematics. Now the 18R Resistor in parallel with the 16ohm Tap works but I can't work out how the 39R Resistor works?

[jazbo8]

Ah sorry didn't read the posts, just the schematics. Now the 18R Resistor in parallel with the 16ohm Tap works but I can't work out how the 39R Resistor works?

It's just a simple voltage divider, so Vo = Vin x 8//18 / (8//18 + 39) = Vin x 5.54 / (5.54+39) = Vin x 0.124. It's meant to drive a pedal, soundboard or another amplifier.

[Littlewyan]

Why do you have 18R and 39R? Also do you mean 8 divided by 18 when you put //? And I meant in terms of impedance but the voltage divider was going to be a later question

[jazbo8]

Why do you have 18R and 39R? Also do you mean 8 divided by 18 when you put //? And I meant in terms of impedance but the voltage divider was going to be a later question

The actual values are not so important, just remember the function of the voltage divider - to get a lower signal level. "//" means parallel, so 18//8 means 18R is in parallel with 8R when the 0.1W switch is on.

You can also read Santiago the designer of the 1W amps ('90s and '00s) talk about the low power switch here.

[Littlewyan]

But can you not use this to drive a speaker? In which case impedance matching would be important?

[Smokebreak]

You most definitely can. Those clips I did were with the ".1W" setting through a WGS ET65

[Littlewyan]

Yer but what about the impedance matching? With the 18R Resistor the impedance is fine but with the 39R the 16Ohm tap doesnt match up in low impedance.

[Smokebreak]

The revised scheme does not have the 39r. Just the 18r in parallel with the 16ohm tap. So let's see:
For the 8 ohm tap, you have 47r+8r(speaker) and 9r paralleled( half of 18r on 16r tap), so you have 9r paralleled with 55r = ~8ohm

For the 16 tap you have 82r + 16r(speaker), and 18 ohms paralleled, so you have 96//18 = ~15ohm . Close enough. My "16ohm" tap on the Hammond I used is actually 15K @ 21K5, so it's even closer to matching, in my case.

[Littlewyan]

Ah sorry guys, I thought they were both valid schematics for different amps! My bad. Well that all makes sense, I'll see if I have room to install this into my new amp. I will have a 4 Ohm tap as well but shouldnt be too difficult to work out what resistance is required.

In Jazbo's voltage divider equation why was 18R and 39R used? Was 39R supposed to be 47R or something?