Which PT for Overtone Special?

[Kassie]

Hey Guys,

I am building a normal (non hrm) overtone special and can't wrap my head around the PT I should use...

The Ceriatone transformer secondaries are 345V while a Hammond Twin has 320. There will be a 50volt difference on B+ Which is the better?

Furthermore.. Hammond sells the 373HX model. which has 350, and even an extra 5 volt tap which could be used for the relais. However it is a standing transformer, and will not fit in the pre drilled holes in the current chassis, it might work by drilling extra holes.

Should I go with the hammond fender replacement, extra relay transformer, and 2x 100ohm center tap. Or just take the 373hx model with 50volts more and drill some extra holes?

Thanks in advance.

[steve_p]

You could use this model from classictone that gives you the choice of 320 or 345 secondaries:
http://www.classictone.net/40-18042.html

It also has a center tap on the heaters. You would still need a secondary transformer for the relays though. I use a small triad transformer that I get from mouser usually.

In my first 100W ODS, I used this classictone:
http://www.classictone.net/40-18004.html

320 secondary, no center tap on the heaters and 120V primary only.

My voltages worked out fine, so I don't think you will really need the 345 tap. That higher voltage might be helpful if you were using a tube rectifier and had a bit more voltage drop.

Good luck,
Steve

[Kassie]

Thanks for the information Steve

I live in the Netherlands... so I reckon shipping from the US will be quite expensive for a couple of transformers. Shame we don't have transformer dealers of Magnetic Components, Edcor etc. here in Europe. It's either Hammond or Mercury.

What would you say is the better? Original used 320 right? Why would Ceriatone do 345? More headroom? If 320 is allright, I might go with a normal Fender replacement.

[steve_p]

Ohh, i missed looking at your location.

320 is the original spec. Not sure why ceriatone has the slightly higher voltage.

veel succes

Steve

[Guitarman18]

Hi Kassie,

I'm not sure how much the Ceriatone trannies go for but the Heyboers from Mojotone are also worth considering.

The UK supplier is here:

http://www.allparts.uk.com/online-shop/ ... -5489.html

spec:
http://www.mojotone.com/amp-parts/amp-t ... fGCPY3VApk

I've used them on a number of builds and they work great.

Cheers,

Paul.

[Kassie]

Ohh, i missed looking at your location.

320 is the original spec. Not sure why ceriatone has the slightly higher voltage.

veel succes

Steve

Haha Wicked! Dankjewel! (I'll take it as a compliment you didn't look at my location, means my English is up to par).

@ Paul.

Thanks for the tip. I am going to look into it right now.

Cheers,

Erik.

[briane]

centertap on heaters quieter than 100 ohms off legs.

make sure it has a bias tap - artificial bias tap also humms a lot.

[steve_p]

centertap on heaters quieter than 100 ohms off legs.

make sure it has a bias tap - artificial bias tap also humms a lot.

Funny, I've heard that 1% resistors as an artificial center tap is quieter.
Reason being is that the two precision resistors provide a better balance than the two windings.

[eazilyled]

I used a Hammond PT 291DEX for my 50w non HRM build, so I had to drill the extra holes. It worked fine but it was a bit of a pain so I would probably go for a drop in PT next time.

The secondaries on that PT are rated at 325V, and I ended up with 477V at the power tube plates which is a little bit high (compared to #124 which had 440V at plates). I was considering using a Zener to drop the B+ down, but Nik thought it was fine and in the end I adjusted the dropping resistor string to get the preamp / PI tube voltages down. I got V1 down to 190V and the amps sounds fine, so I left it at that.

[eazilyled]

BTW take a look at http://www.ceriatone.com/voltage/VoltageChart.xls

It's a voltage chart for all of the Ceriatone amps.
It looks like that with the stock Ceriatone PT, you get around 440V on the power tube plates (for both the 50W and 100W models).

I think the way in which secondary voltage ratings are measured varies for the different manufacturers, so it's hard to predict what voltages you will actually end up with.

BTW I got my trannies from Bluebell audio in the UK.

[Kassie]

BTW take a look at http://www.ceriatone.com/voltage/VoltageChart.xls

It's a voltage chart for all of the Ceriatone amps.
It looks like that with the stock Ceriatone PT, you get around 440V on the power tube plates (for both the 50W and 100W models).

I think the way in which secondary voltage ratings are measured varies for the different manufacturers, so it's hard to predict what voltages you will actually end up with.

BTW I got my trannies from Bluebell audio in the UK.

Thanks, really usefull information.. that means that the Transformer doesn't drop so much voltage under load. Can you tell me something about the temperature of the PT when you play it for a while? I think I'll order from jukebox-revival. They have good prices. I have a 372jx in my express build and it stays cool for days. Comforting thought if you compare it to my Marshall Silver Jub which PT tends to get very warm.

I was going for the 350 model, but now I tend to go to with the 325 model.

The dropping resistor is the 3k3 right across the 2 22uf caps? http://www.ceriatone.com/images/layoutP ... iatone.jpg

Edit: I checked the model.. and it's a 50watt OT. 200ma for B+ and 4A on the heaters is not enough for a 100watt.

[sergio]

HI , this also in Europe:

http://www.tubeampdoctor.com/en/shop_Ma ... an_100_111

S

[eazilyled]

It always stays pretty cool when I play the amp.

Yes that's the dropping resistor I changed - went up to 4K7.

[Kassie]

It always stays pretty cool when I play the amp.

Yes that's the dropping resistor I changed - went up to 4K7.

I see your trafo also had an extra 5v tap? Did it work out fine for to power the relays?

[eazilyled]

It always stays pretty cool when I play the amp.

Yes that's the dropping resistor I changed - went up to 4K7.

I see your trafo also had an extra 5v tap? Did it work out fine for to power the relays?

Yes, the 5V tap works fine for the relays.