TOS OT Problem !!!!!!!

[TIMBO]

Hi Guys, I've MADE a big mistake. I just realised i've ordered the wrong OT for the TOS . It only has a 2ohm tap. Can i do any thing to at least make it a 4ohm .Thanks

[stingray_65]

where did you order from?
I've had luck with quite a few places fixing a mistake before it's shipped.

[TIMBO]

I ordered it from tubesandmore. I live in australia and it has already arrived.

[stingray_65]

Weber sell an impedance matching transformer for just such issues.

at the price they are asking it would be just as cost effective for you to order the OT that you want and shelve the one you ordered.

other options : 2x12 cab with 4 ohm speakers, 4x12 cab loaded with 8 ohms

that too would be more expensive than a new OT unless you had the speakers on hand

[TIMBO]

Hi guys, I've done some investigating into my OT purchase. I did order the right OT as per the TOS BOM, which is the Super Reverb 002855.This OT only has a 2ohm tap. I am now thinking of puschasing another OT , a hammond 1750m but it has a 4200ohm primary with 4,8,16 ohm taps which is what i was wanting. So can anyone tell me if the 4200ohm primary is ok for this project. Thanks

[sluckey]

4200 ohm primary will be fine. In fact, the OT you just bought will be fine if you use a 2 ohm speaker load.

[WiderGates]

Hello TIMBO,
as far as I know is the 1750M with 2, 4 and 8 Ω secondary.


Ciao

[tubenit]

from Kevin O'Connor  FAQ

Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.

A: It is awfully expensive, and awful that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.

As stated throughout the TUT-series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.

[EL34]

I did order the right OT as per the TOS BOM, which is the Super Reverb 002855. This OT only has a 2ohm tap

That is exactly what a super reverb tranny is, a 2 ohm load with four speakers.

IMO, a much better choice is the 018343 output that I stock
It has 2, 4 and 8 ohm taps, so you can set up your amp cabinet with more choices
http://www.hoffmanamps.com/MyStore/catalog/parts1.htm

The 018343 tranny I sell is very popular choice and I sell lots of them.
Sizes, mounting holes, spec's are all listed on the library page

[tubenit]

I changed the BOM (in the SCH file) to reflect Hoffman's 018343 OT with the multiple taps.

With respect, Tubenit

[EL34]

Thanks Jeff

I think that 018343 tranny will work out better than a dedicated 2 ohm load
I stock both though if someone wants just a 2 ohm load.

The 018343 is what I would use if I was building one, not the 022855, which is the 2 ohm only tranny.

I have several different cabinets and it's always nice to be able to switch the impedance and hook up to different speakers cabs.

[TIMBO]

Thanks guys, The feedback from this forum is great .I'll keep the ot i have for another build for maybe a combo or 4 speaker cab. All is great in the amp building world :wink:The TOS is taking shape. Thanks