#124 help understanding OT

[duriehill]

Hi

I'm building the #124 ODS, a 50w version mainly by the ceri@tone layout and layout on here.

Now, I'm choosing my output transformer and am trying to get my head around the primary impedance.

I guess I'm trying to understand if this is effected by going from the 100w version (where I read it's about 2k) to the 50w version? Should I be still aiming to source a 2k primary 50w to best represent the tonal characteristics or does the primary impedance change? If so - can anyone provide a brief explanation of why?

I've seen many 50w builds where the owners have used approx 4.2k primaries.

I'm looking at either the Hammond 1760w (2k, 100w) or going with the Hammond 1750M (4.2k, 50w).

Thanks in advance

[JazzGuitarGimp]

You want the 4.2K primary with two 6L6GC output tubes.

When going from a two-valve, 50W amp to a 4-valve, 100W amp, we double the number of tubes on either side of the OT from one tube to two tubes. This allows us to pull twice as much current through the OT. If you are familiar with Ohm's Law, you know that resistance is equal to voltage divided by current. Compared to the 50W circuit, the 100W circuit wants to have twice the current, but the B+ voltage stays the same, which means the only way to increase the current by a factor of two is by halving the resistance. Thus 4K for 50W, and 2K for 100W. The difference between 4k and 4.2K is negligible, and I think 4.2K is more common, so that is what people use.

[David Root]

My two cents worth sez get a BF Bassman OT. Magnetic Components makes one, Hammond does (not the 1750, look in their Guitar amp transformer section), Edcor makes a very high quality one IIRC. Mojotone's are made by Heyboer, also a good choice.

If you really want THE tone and can live with 4 ohm only secondary, there's an original '60s Bassman OT on the Bay right now. Mucho deneiro $$ compared with the current production but you can't beat it with two sticks for the application.

[M Fowler]

With Magnetic Components you can get about 6 or 7 different versions of the Bassman depending on what secondary loads you want which is nice depending on circuit and chassis a guy is using. If I want a 4 ohm secondary only MC provides that.

Mark

[duriehill]

Jazz - that's really helpful. I'm a relative newbie, but absolutely love it. It's an addiction now.

Thanks David for pointing out the bassman style ot better suited. I think I'm going to go with the Hammond 1760L. I have good access to Hammond transformers over here in the UK.

[Structo]

Please put your location in your profile so we can help you better with parts selection that may be local to you.

Thanks