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Another spitfire question: OT

8/11/1999 10:56 PM
Another spitfire question: OT
I read on a post a little while back that Matchless put a fairly beefy output transformer in the Spitfire. Specifically, it was stated that the stock spitfire OT had a lower pri impedance than usual for a pair of el84s, in the neighborhood of 4K-5K; and that it is similar to something normally used for a 35-watt 2x6L6 type power amp.  
That being said, I figured I could get away with using a Fender OT from a blues deluxe in my spitfire clone. It has a 4 and 8 ohm taps. When I play the amp, it sounds better using a 4 ohm speaker load into the 8 ohm tap. Comments anyone? Is this a case of mismatchiing the speaker to balanced the mismatched OT? Any info will be appreciated.  
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8/12/1999 1:09 PM
Mike B

The measured primary impedance on a Spitfire OT was around 4K (I think Doc measured 3.8K). The blues deluxe OT that you are using probably has a primary Z of 8K or so. By connecting a 4 ohm speaker to the 8 ohm tap, you are effectively lowering the primary Z from 8K to 4K. The same result could be obtained by connecting an 8 ohm speaker to the 4 ohm tap.  
Why does it sound better? For a given tube type, there is an optimum impedance that provides a reasonably high level of output power with a reasonably low level of harmonic distortion. The 8K value is what the tube manufacturers specified as the compromise to achieve a good power to distortion ratio for the El84/6BQ5 (for hi-fi, not guitar!). By lowering the primary Z, distortion and power are both increased. It appears that Mark Sampson, the designer of Matchless amps, did a lot of experimenting with this to find the optimum value for guitar. I have played around with this quite a bit myself, and I agree with your findings - lower primary Z results in a 'beefier' tone, for lack of a better word. Switching back to 8K thins the tone out. There is probably a better technical explanation for exactly what's going on here, so hopefully someone can shed some more light on this. From a purely practical point of view I design all my EL84-based amps with a 4K primary Z, since the tonal difference is significant, at least to my ears. Hope this helps...  
Mike B
8/12/1999 2:01 PM

Yeah, I noticed more of an edge and a fuller tone with the mismatched load. For some reason, I thought that the blues deluxe OT had a lower primary Z, in the neighborhood of 4K. I think I saw data for the generic fender 35 watt OT and figured this is what they used in the Blues deluxe.  
8/12/1999 9:04 PM
Mike B

I thought you you were talking about the Blues Junior - that amp uses a 2xEL84 output stage and a fairly small OT with 8K primary Z. The Blues Deluxe is a 2x6l6 amp, right? Then, the primary Z is probably in the 4-5K range. Did you use this OT in a 2xEL84 circuit? Connecting a 4 ohm speaker to the 8 ohm tap would result in a primary Z around 2K or so. This is probably lower than you want to run from the point of view of reliability.  
Mike B
8/13/1999 12:27 AM

10-4, good buddy, I'll heed your advice.
8/12/1999 11:02 PM
To elaborate more on Mike's excellent post, the distortion and power do vary with load imp, and more to the point (of tone) the MIX of harmonics changes.. typically you have a greater % of 2nd at lower loads, which drops as you increase the load and while that occurs, the odd order HD tends to increase. Your max power point is (generally) a point where the 2nd HD is trailing and the 3rd (and higher) hasn't really climbed yet. There's a great chart in one of the RCA books with the 6L6's reponse at given loads. Your harmonic mix can have a profound effect on the tone of the transformer and how it reacts to the rest of your amp circuit. I say "can" rather than "does" BC I've found in my own research that load changes of as little as 10% can be very audible when incorporated into the wind of a tranny, but I've also mis-loaded the 2ndary (say on a Fender) by a factor of 100% and not noticed as big a change.. It's very peculiar.  
Reliability-wise, low loads will tend to increase plate current and can lead to reliability issues (as noted).

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