JTM45 transformer selection for 5881s

[BenTobith]

For a JTM45 build using 5881 power tubes (e.g. NOS Tung Sol), what power transformer and/or B+ do you recommend? (I won’t have enough room in my 1x12" combo shell for KT66s.)

I’m considering Mercury P4550JT-G4 (380VDC) and G2 (420VDC), as well as Hammond 291KX (345VDC I think?).

Similarly, which output transformer would you recommend for 5881s? I am currently looking at the Mercury O45JT-16 and Hammond 1750Q.

I am going to be using the Metro chassis from ValveStorm.

[Roe]

I recommend 400-450v at the plates. 5-7k primary should work well

[stephenl]

Be careful with the plate voltages on 5881's, I think they're only rated for 400v.

[pdf64]

Be careful with the plate voltages on 5881's, I think they're only rated for 400v.

My understanding is the voltage limits to anode and g2 apply under conditions of simultaneously being at their dissipation limit. Such that as electrode dissipation reduces, more voltage can be tolerated, eg see Ao and A voltage limits p6 https://tubedata.altanatubes.com.br/she ... e/EL84.pdf
Empirically, we may see that output valves of class AB amps don’t tend to fail at idle.

The point being that at high signal power output, the amp’s HT characteristics should be such that it sags to 400V or less. So it all should then work out ok.

As their experience with them increased, Fender seemed happy to use 5881 at ever increasing voltages, eg https://el34world.com/charts/Schematics ... _schem.pdf

[stephenl]

I had a discussion with tech support at Tube Depot, specifically about the new production Tung-Sol 5881's. I wanted to put them in an amp with a 500vdc B+, he cautioned against it - stating that they would likely have a greatly reduced lifespan.

[bepone]

5881 is 6L6 family so 4k or 4k2

[Roe]

5881 is 6L6 family so 4k or 4k2

yes, but the early jtms that came with 5881s use the RS deluxe OT, with a 6k6 primary, which actually measures closer to 5k7 on the 8ohms output. Later jtm45s had a 8k primary. 4k2 will give you a bassman, not a jtm

[bepone]

so you want to say the same mentioned OPT 4k with the 8 ohms speaker inserted to 4 ohm tap to have 8 k?
looks difficult to make
the topic is started 5881 for JTM and you cannot escape from "bassman" style. 5k6 recoomended from datasheets from 6L6s and those 25-30W doesnt sounded good to my ears , never, but 4k is the impedance. also from harmonic content chart is also seen the difference

[BenTobith]

Hi all,

So I've decided to go with the PT and OT from Modulus. I believe both are Dagnall, UK-made?

Dumb question, but their PT (https://modulusamplification.com/JTM45- ... 95817.aspx) has two 120v leads on the primary, as well as two 0v leads. Which pair would I use for 120v wiring (USA), or does it matter?

Hopefully it comes with a more detailed schematic, too.

[sluckey]

PT (https://modulusamplification.com/JTM45- ... 95817.aspx) has two 120v leads on the primary, as well as two 0v leads. Which pair would I use for 120v wiring (USA), or does it matter?

Usually on a dual winding primary you will use both windings. Connect in series for 240V or connect in parallel for 120V.

[ViperDoc]

Dumb question, but their PT (https://modulusamplification.com/JTM45- ... 95817.aspx) has two 120v leads on the primary, as well as two 0v leads. Which pair would I use for 120v wiring (USA), or does it matter?

Both 0-120V primary leads should be wired in parallel for 120V operation. Wire both 0 V leads and 120V to their respective sides of a properly rated DPDT switch and then you can obviously run single leads from there to the mains fuse and mains connections.

[BenTobith]

Dumb question, but their PT (https://modulusamplification.com/JTM45- ... 95817.aspx) has two 120v leads on the primary, as well as two 0v leads. Which pair would I use for 120v wiring (USA), or does it matter?

Both 0-120V primary leads should be wired in parallel for 120V operation. Wire both 0 V leads and 120V to their respective sides of a properly rated DPDT switch and then you can obviously run single leads from there to the mains fuse and mains connections.

Thanks, both. Very helpful. So basically, instead of just twisting one 120v wire from the PT primary to one side of the mains switch w/ one wire from the neon lamp, I should twist both 120v wires w/ one of the neon lamp wires (so 3 wires twisted together total) and connect those to the right lug of the mains switch (closest to standby switch).

And then instead of just twisting a single 0v wire from the PT primary together with the other wire from the neon lamp, and running those to the “side” lug of the fuse holder, I should bundle both 0v wires w/ the other neon lamp wire (3 wires twisted together total) and connect those to the “side” lug of the fuse holder.

(This is based on the Valvestorm/Metro build guide I’m following, for context.)

[BenTobith]

Here’s a PDF of the DIYLC file I’m working on for this build for reference: https://www.dropbox.com/s/exckq65717m0 ... t.pdf?dl=0

[martin manning]

If your pilot lamp has solder lugs, I would be inclined to use those to tie the transformer primaries together (0 and 0, 120 and 120) like this. Note the fuse and power switch are on the hot lead, in that order, with the hot lead from the IEC going to the tail of the fuse holder.

[BenTobith]

If your pilot lamp has solder lugs, I would be inclined to use those to tie the transformer primaries together (0 and 0, 120 and 120) like this. Note the fuse and power switch are on the hot lead, in that order, with the hot lead from the IEC going to the tail of the fuse holder.

It is one of those amber neon lamps with the two attached wires, no solder lugs.

http://valvestorm.com/sites/default/fi ... icator.jpg