tweed blues meister OT recomedation

[Garycbet]

I am building off of the 3-2012 schematics which specifies 6v6 tubes but the BOM for this in the store specs out 6L6 tubes.  I would think that there would have to be a different OT spec for the 6v6.  anyone have a recommendation from the transformers that Doug stocks?

thanks for the help.

[tubenit]

IF you are going with 6V6 tubes (which sound great with the Tweed BluezMeister), then Doug's Deluxe OT  041318 or Mojo 768sp or his 18w OT would all be good choices.

With respect, Tubenit

[2deaf]

The speaker load you are planning to use could be a factor in deciding which transformer to use.  The 041318 only has an 8 ohm tap, so you would have to use an 8 ohm load.  Both the 768sp and the Heyboer 18W have 4, 8, and 16 ohm taps, so you would have choices for the load.

[Garycbet]

thanks all

[tubenit]

From Kevin O'Connor's London Power

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.

[drgonzonm]

Tubenit,
Could you expound on Conner's comment regarding reflected load.  2deaf mentioned two transformers, one an 8 ohm transformer, and the other a 4,8 16 ohm transformer.  I believe the discussion is relevant only to an 8 ohm tap.   

It is my understanding, that match the tap to load, you are maximizing the "efficiency" of the amp? So, I wonder if my assumption is correct, if you have cabinets of various loading, then to use an O/T with various taps maximizes the efficiency of your equipment. 

[tubenit]

My take away from KOC's comments is that some of the amp building "cultural" belief that OT & speaker ohms have to be a perfect match is not accurate in that it's not as critical as is sometimes indicated. 

Yes, I think there may be an optimal ohm rating for an OT for a particular individual for a particular amp looking for a particular tone. 

I'm not sure what the definition of "optimal" is for others?   For me, if it sounds great and doesn't blow up and is affordable, I'm thinking it's optimal.
 

I've not spent anytime worrying about mismatching on my amp builds and have been happy with my results.

As I understand it, Howard Dumble purposely mismatched OT and speaker to achieve a specific desired tone.

I've heard there is a range of mismatch that is "safe" & I've heard some specific amps seem to have a history of OT's frying.

With respect, Tubenit

[2deaf]

The gentleman asked what transformer would be appropriate for his situation and Tubenit gave him three correct choices that all have eight thousand some-odd ohms across the primary when the secondary has the intended load.  I doubt that anybody in the know would have recommended a 4K primary or a 17K primary for this gentleman's amplifier.

Mr. O'Connor started off in the above quoted passage as though he was debunking impedance matching, but then proceeded to offer a good argument in favor of impedance matching.  He stated that you will suffer a loss of power and a narrowed bandwidth if you mis-match intended impedances.  I don't want that, do you?

I believe the discussion is relevant only to an 8 ohm tap.

I know you were not addressing me, but why would the discussion be relevant to an 8 ohm tap only?

[drgonzonm]

The gentleman asked what transformer would be appropriate for his situation and Tubenit gave him three correct choices that all have eight thousand some-odd ohms across the primary when the secondary has the intended load.  I doubt that anybody in the know would have recommended a 4K primary or a 17K primary for this gentleman's amplifier.

Mr. O'Connor started off in the above quoted passage as though he was debunking impedance matching, but then proceeded to offer a good argument in favor of impedance matching.  He stated that you will suffer a loss of power and a narrowed bandwidth if you mis-match intended impedances.  I don't want that, do you?

I believe the discussion is relevant only to an 8 ohm tap.

I know you were not addressing me, but why would the discussion be relevant to an 8 ohm tap only?

Conner states "given transformer." Then he uses an 8 ohm tap as an example.  His statements regarding reflected impedance and the changes are only correct for that tap.  If you have multiple cabinets with different nominal impedances, then a multiple tap o/t is strongly suggested.  This gives you the ability "match" intended impedances.   You and I have stated the same thing, but in somewhat different words.   

Bottom Line,  Use the 4-8-16 tap o/t for maximum flexibility.  Avoid plugging a 2 or 4 ohm load cabinet in the 16 ohm tap.