JTM45 transformer and choke specs

[martinw]

My next build is going to be a JTM45-a-like on a budget.
The transformesr are a bit of an issue here in the UK, but I have found a maker who will make me a set to my specs.
I have, or can work out most of it, but could someone please confirm some bits for me:
PT: HT should be 350-0-350 ? @ 250mA ?
htr 1 6.3V @ 6A ?
htr 2 5V @ 2A ?

OT: primary 6.6K ?

Choke: Ha! anything from 3 to 20H.
100 or 200 mA?

I haven't managed to finf anywhere I can get these trannies cheaper, nor anywhere where all the specs are laid out clearly.
Any help appreciated. thanks.

[kd]

Here are the specs on the PT I used.

350 0 350 @ .250 Amps
6.3 Volts CT @ 5 Amps
5 volts @ 3 Amps

If you intend to use KT66 tubes take a look at using an OT that has an 8K primary impedance.

I don't know what you're paying but you might check the Marstran.com site. The transformers are wound by Heyboer to vintange specs.


Ken

[martinw]

Thanks Ken.

I can get a Hammond in 6.6k but not 8k.
Would 6.6k be ok?

And can anyone help on the choke spec?

[zaphod_phil]

Don't know about the Hammond, but probably a 36W OT would work, if you do the trick of running an 8 ohm speaker on the 4 ohm tap, etc. Not sure about the choke, as I'm no expert on JTM45s....

[Coco]

We have JTM45 OT with 8K & 6.6K for that very reason. They come with the PT/OT/choke set. email if interested.

[martinw]

Done.

[GREEN_CANOE]

We have JTM45 OT with 8K & 6.6K for that very reason. They come with the PT/OT/choke set. email if interested.

Furthering my education:

What is the difference I would hear between the 8K & 6.6K OT?

[Phil_S]

Choke: Ha! anything from 3 to 20H.
100 or 200 mA?

I think, perhaps you need at least a 5H 100ma choke. I used a 7H from an only Heathkit in mine and it works just fine. I think there is a cheap Hammond in this range that should work well.

For filament winding capacity, look up the heater current requirements for each tube and just add them up. This will take you 3 minutes and then you will know for sure what you need. You may be able to control cost a bit by not over spec-ing the tranny. As for the main HT secondary, again, lookup the max current on the power tube plates and add them up. For example, on a KT66 in PP, per tube, max plate current under load is 125mA and screen is 9.5mA. With a little rounding, you need 270mA plus something for the preamp tubes. I'd be looking at something close to 300mA to insure you don't over stress the power tranny. In all likelihood, a 250mA tranny will be adequate, even if a bit under rated. In a PP amp, each tube is only "on" part of the time.

[Coco]

What is the difference I would hear between the 8K & 6.6K OT?

It's more about matching the tubes impedance / power transfer. Not so much the tone - but it can affect the tone. Going lower than 8K on KT66 mght add an edge to the sound. Best Impedance for 2-KT66 is 8K.

Just nice to have options.

[Coco]

Don't know about the Hammond, but probably a 36W OT would work, if you do the trick of running an 8 ohm speaker on the 4 ohm tap, etc. Not sure about the choke, as I'm no expert on JTM45s....

3H or more @ 100ma choke is good.

You can hook the 4K of the 36W but don't you connect an 8 ohm speaker to a 16 ohm tap to increase the reflected impedance from 4K to 8K.

Not JTM45 spec, but it would work.

[keith]

...Going lower than 8K on KT66 might add an edge to the sound....

Can you describe edge?

[Coco]

A slight edge to the sound. More crisp/less smooth. Some bite.

[Coco]

Transformer specs at: http://www.cohrs.ca/var/www/html/staged ... ost&p=6878