Question about Output transformers

[duriehill]

Hi all

I'm a bit of a newbie to the amp building seen. My project at the moment is a TW rocket.

While looking at OT's, I've just a quick question..

I'm looking at these two options from Hammond (easy to source in the UK)

The first is the model 1750v:
http://www.hammondmfg.com/pdf/EDB1750V.pdf
It's basically an AC30 replacement, so it's 30W with a primary impedance of 4K.

Now I happened to stumble across this model, the 1650N.
http://www.hammondmfg.com/pdf/5c0051-52.pdf
This seems like more of a hi-fi transformer - which I believe is more of a rocket trait. And has a primary impedence of 4.3K - which I also understand is that of Ken's rocket. It also has secondary taps at 4, 8 and 16ohms. This transformer though is rated at 60W.

What impact does this have if any?

Thanks

[dartanion]

The 1650N sounds like the better option. 4.3k is correct but the A470 OT that Ken used was rated at 70W iirc.

[duriehill]

Thanks. And obviously the fact that the wattage is double what is required (60 V 30), won't have an impact on sound? It's just what the transformer can handle in terms?

I'm looking into a PT too. I am a little confused as to the correct high voltage spec. for the Rocket. Some mention 250V or 260V. From what I read it seems higher voltages give more headroom and I imagine sound more 'brown' at lower voltage? What have people been using and how do you find this effects the sound? What did Ken use?

[statorvane]

I'm not sure what Ken used, but the Rocket uses 4 x EL84s. Plate voltage is lower for a Rocket/Liverpool than say an Express.

[Matt D]

I used the 1650N on a Rocket build and it sounds pretty good.

[hans-jörg]

Hi,
i´m a little confused: didn´t has Kens Rocket ~6k6 primary?

I use a Hammond 1650P in the Rocket. Is this way to much? Sound is very streight, very nice and easy bloom and huge bottom end. Only the cleans I cant get "fendrish"- with LP though. V1 and V2 are 7025 to cool him down a bit

Hans-Jörg

[duriehill]

I'm not sure hans-jörg, from what I've read he used a 4.3K primary.

Matt D, did you use Hammond for the PT as well? Can I ask if so what model you went for and why?

[dartanion]

6.6K was the Express. 5.2K Liverpool. 4.3K Rocket.

[Matt D]

I used RJ's Edcor part for for the PT. I didn't go out looking for the 1650N specifically but a friend sold me one he wasn't using for cheap. I wouldn't say the amp I made with it is particularly magical or mind blowing but it holds it's own nicely next to my bandmate's English-made 90's AC30. Everyone who plays it says something like "Hey, this amp sounds pretty good," rather than "I must have this amazing amp."

[hans-jörg]

6.6K was the Express. 5.2K Liverpool. 4.3K Rocket.

damned! Looks like I had the Express sheets in hand when I ordered for Rocket. I built both amps one after another and mixed it up - somehow. Anybody else with such a stupid mistake

But I will swap them. Cince I had the Express I allways used the half of secondary (16 Ohm speaker - 8 Ohm at the amp) to meet the right impedance. But now I have the reason to change on by another.
My Express (K60) OT shows 4,2k prim. (measured), and is a Marshall 50W style (probably a replacement for 50W Plexi). I think it will work with 4xEL84 same as with 2xEL34 - or am I misleaded?

BTW: both are great amps even with slightly mismatched impedance

Last Question: because I don´t know when I find time to swap them: Am I right to use the 16 ohm out for a 8 Ohm speaker to come to a propper matching? Or is this to much too? That would be 6k6 down to 3k3, right? To low I find.

Best

Hans-Jörg