JTM45 PT Filament Current question

[FuzzFaceBuilder]

Hey all,
This is a kinda lame question. I've been looking around on the site/internet and can't seem to find the total filament current of a JTM45 PT. Would it be able to handle the extra .9 amps from a 3 tube reverb circuit (with the standard JTM45 topology)? Or should I move up to a 50w Marshall PT?

Thanks again,
Hale

[paulster]

You need to ask the individual manufacturer of the transformer you're interested in, as they'll all specify them slightly differently.

They should all be prepared to give you that figure though, since it's hardly proprietary.

[tubeswell]

I think most of 'em are about 4A - but you could get by with 3A (assuming your lamp was rated 0.3A or less) (Edit - this of course assumes you'd be running 6L6s like in a 5F6A).

[Phil_S]

This is easy to figure. If you are going to build amps, learn how. (Please, this post carries the best of intentions, not trying to give you a hard time.)

Here's how. Find the tube data sheets for whatever you are using in the amp. To do that, download Duncan Munro's TDSL http://www.duncanamps.com/ or go to Frank's tube data sheets http://tubedata.itchurch.org/index.html.

I'll help you get started. You check what I tell you with one of the resources listed. Today you get the fish for free, but you also get the fishing pole.

The JTM45 uses 3 * 12AX7 and 2 * EL34. That's it for the 6.3V tubes. Ignore the GZ34 for a moment, as that's on the 5V filament winding.

A 12AX7 running on a 6.3V filament supply needs 0.3A. An EL34 needs 1.5A. Total them up, do the math, 0.9A for the preamps and 3.0A for the power tubes. You need a little something for the pilot light if you run it on the 6.3A circuit. The JTM45 needs 6.3V @ 4A.

If you use a 5V tube rectifier like a GZ34, your 5V winding needs to supply enough amps for that tube. If you use a 6.3V tube rectifier like an EZ81 (not in the JTM45, tho'), you'll need to determine if it needs a separate winding or if it can be run from the same winding as the other tubes. Directly heated rectifiers require a separate winding and indirectly heated do not. You can determine this by looking at the pin-out diagram. If it shows that the cathode and filament are the same, it is directly heated. If it shows a separate filament and a separate cathode, it is indirectly heated.

Now that you know how to figure that out, I'll leave you to answer the rectifier question on your own.

It's really that straight forward for the filaments. Just add them up.

Figuring the high voltage current requirement is a little less straight forward, but the answer can also be found on the tube data sheets.

Figure out what's needed. Make sure you buy a PT that's rated appropriately. The manufacturer will tell you what the ratings are.

[paulster]

Phil - I think the OP was asking about the typical rating of the transformer filament winding since he'd already calculated that he was going to add 0.9A.

Anyway, JTM45s used traditionally 6L6 tubes, then 5881s then KT66s.

The highest current draw of these are the KT66s, at 1.27A each, so the transformer will be rated for these.

Many people drop EL34s in, so it's also likely that the transformer will be rated for EL34s at 1.5A each, so that's an extra 0.5A of capacity you've got available.

If your transformer can handle EL34s and you use KT66s you'll be 0.4A over stock, which isn't likely to cause a problem. Go to 5881s (wonderful with these amps) or 6L6s and you'll be well within budget.

Regardless, the transformer manufacturer should be able to tell you the specific rating of the 6.3V filament winding and, if they can't, then find another manufacturer.

[FuzzFaceBuilder]

Hey Guys,
Thanks for all the responses!

Paulster: To the rescue! Thanks for providing the current draw for all the common power pentodes used in a JTM45! It's a great help! And for pointing that last post out !

Tubeswell: Thanks! Usually 4A, good to know! I'll be in touch with Brian @ Marstran to check out his JTM45 PT!

Phil: No harm, no foul ! In fact, if I didn't meet a few guys who are willing to share that kind of info (like you are!) when I started, I'd be MUCH farther behind the curve! Appreciate it!

As for the project, I'm going with 2x5881's, 5x12AX7's and 1x12AT7. So around 3.6A!

Thanks so much again!

Take care,
Hale

[Structo]

Also check the PT's at Metro since they offer JTM 45 amps and kits.

A lot of guys do like the Marstrans though.

http://metroamp.com/store/index.php?mai ... cts_id=270