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Another spitfire question: OT


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8/11/1999 10:56 PM
dave
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.  
 
 
 
Dave
 
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8/12/1999 1:09 PM
Mike B

Dave,  
 
 
 
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
dave

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.  
 
 
 
dave  
 
 
 
 
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
dave

10-4, good buddy, I'll heed your advice.
 
8/12/1999 11:02 PM
SpeedRacer
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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