Which PT for Overtone Special?

[eazilyled]

Actually I'm using this one:

http://www.hammondmfg.com/pdf/EDB291DEX.pdf which has 230mA on the secondaries but only 4A on the heaters.

[eazilyled]

It's a shame that Twin PT doesn't have a tap suitable for the relays Kassie.
Sounds like some extra holes may be required after all.

[Kassie]

It's a shame that Twin PT doesn't have a tap suitable for the relays Kassie.
Sounds like some extra holes may be required after all.

There are some holes for the filament wiring.. so maybe only 1 extra drill according to the size.. so that is no big deal really.. it's better then the other possibility.. the 291FEX is cheaper then the last trafo I wanted. And an extra relay trafo would be around 20E.

I am looking at this site.
http://jukebox-revival.eu/catalogsearch ... 1223&y=-64

There is the 266m6. Which has 3A. Wouldn't the watt be too much for the voltage regulator to handle? The original used was 0.3A by Dumble? So the 266F6 would be the most suited, but it is not stated on the shop website. http://www.hammondmfg.com/266.htm

Edit: I measured the chassis holes. 6cm = 2.36''.. the hammond 266J6 would fit it with 2.38''. I'll send jukebox a mail.

[eazilyled]

For the 266m6, I think the 3A refers to the maximum current it can safely deliver. So I believe you would be fine with that one.

[rogb]

FWIW I used Hammond from Bluebell in my 1st OTS 50 build, sounded great.
The guy who bought it gigs most nights a week with it and him and his audiences love it.

I built an AB763 with Mercury iron. This also sounds great and everybody loves it.

Last 2 builds have been a 50w 102 and JCM800 2204 with Classictone. They sound great and everybody loves em.

Many people have good success with Heyboer/Mojo too, following Paul's suggestion, definitely worth a try.

So far I have liked all the iron I have used but if I had to pick one it would be the Mercury, seems to have some extra definition and "body" closely followed by the Classictones equal.

Go figure

[Structo]

You can also just use a cheap 6.3v filament transformer for the relays.

[Kassie]

FWIW I used Hammond from Bluebell in my 1st OTS 50 build, sounded great.
The guy who bought it gigs most nights a week with it and him and his audiences love it.

I built an AB763 with Mercury iron. This also sounds great and everybody loves it.

Last 2 builds have been a 50w 102 and JCM800 2204 with Classictone. They sound great and everybody loves em.

Many people have good success with Heyboer/Mojo too, following Paul's suggestion, definitely worth a try.

So far I have liked all the iron I have used but if I had to pick one it would be the Mercury, seems to have some extra definition and "body" closely followed by the Classictones equal.

Go figure

Nice info... I reckon the PT is not so much a tone factor as the OT.. but more a feel/stability/safety feature. I personally like a PT which supplies the correct voltages without sagging too much.

For the OT I have been thinking for 3 things... 1 Mercury Mag (sells for 230E), 2. Hammond Twin OT (sells for 130). 3. Hammon 1650T (Hifi Iron, very beefy, sells for 80E).

Well I read a few years back, that 1650T would not be so suitable for a dumble clone... but why not? I remember someone stating on a dutch forum that the Mercury swap vs stock ceriatone transformers improved clarity and bass... wouldn't a Hifi iron like a 1650T do that as well? Does a dumble actually need OT saturation?

Anyone experimented with that? The hifi Hammond line?

[rogb]

If you're going to use Hammond, go 1760W.
If you're planning on keeping the amp for a long time, suck up the bucks and go Mercury or Classictone IMO.

[Kassie]

Ok, I made the choice to go with the Hammond line.

PT: Fender Twin 291FEX
OT: Fender Twin 1760W
Choke: 194B
Relai Filament: 266J6

Might take a while before it gets here though, since the shop needs to order some parts. 2 of them weren't even on their site, but now they are.

I also considered ordering a mojotone OT, it was cheaper even, but the sending from the UK proved to be a little too much.

[martin manning]

This is all you need for the relay transformer, available from Mouser. They have a European site, but here's the listing from the US:
http://www.mouser.com/ProductDetail/Ham ... OWNNnyo%3d
Hammond 186B12, 12.6VAC @0.4A, 6.3VAC @0.8A $9.10

Use 6.3VAC secondary for 5V relays, or 12.6 for 12V.

I'd stay with the Fender Twin OT to keep it Dumble.

[Kassie]

Wow thanks Martin!,

Mouser also sends in NL. They have a centre in Eindhoven.
The prices are a lot cheaper. It saves me 50E, compared to Jukebox-revival, and more then 100 compared to Tubetown. And best of all, no sending costs.

You sir, are a hero!.

[martin manning]

Proost!

[David Root]

Just a postscript, glad it all worked out.

Most mfrs rate secondary voltages X-0-X at full stated load, Y mA. So at idle, sec voltage will be a little higher. If you check the description you might find some that rate the sec voltage at idle. Big difference! So it pays to check that. If the specs aren't clear and explicit, then ask before you buy.

Any headroom in your build between the stated mA at nominal secondary voltage will show up as higher voltage at idle. So while it's a good idea to have some headroom with an AB1 class design, which most MI amps are, don't overdo it.

Secondly, since I started using PSUD2 to model power supplies I get pretty accurate predictions of PT performance before I buy, and chokes too. PSUD2 is free and crunches the numbers in seconds that it used to take ages to do back in the Radiotron Design Handbook days. Graphical results display as well as numbers. Can't recommend it too highly.

For those who don't know about it, go to www.duncanamps.com and download it. There is a learning curve, but it allows you to plug in to the calculations the regulation of the PT and its source resistance too. Regulation is essentially the difference between the nominal sec voltage at full load and at idle, and varies from mfr to mfr and PT type. I use 7% for main PT and 15% for cheap heater and relay PTs.
Source resistance is not just the DCR of the primary so it pays to plug that in too.

I'm still finding my way with PSUD2 and I dont really get all the differences between using a resistive load on the output vs. a fixed current draw.

[Kassie]

Interesting Info David.

It'll be some more studying for me to fully comprehend all you say.
But I looked up the specs of the particular Hammond PT.

http://www.hammondmfg.com/pdf/EDB291FEX.pdf

It says. 665.26vct, 7.02v at no load.

So at load the voltages may drop to roundabout 640 as stated in the diagram? Maybe a bit less?

Interestingly the Ceriatone transformer is rated for 690. But shows a remarkable drop in voltage as stated by someone before. While the Hammond actually stays a bit higher in voltages than one may need. Though I do not have evidencial data to back that up.

Then there is quite a difference between the current rating. In the diagram it says 517.5ma, but in the chart next to it only 255ma. Or is that the primary current that the Transformer uses from the main power line?

[martin manning]

255 mA is the primary current. 640VAC is the secondary voltage at full-load, so yes you will see higher than that at idling conditions. It will be fine.