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Questions about using a multi-tap output transformer (long)...

1/15/2000 5:05 AM
Mark Knapp
Questions about using a multi-tap output transformer (long)...
...or is it stupid speaker wiring tricks?  
I have had a 4 X P10Q cab for about 6 months now, and I could never get the thing to sound good with my Bandmaster Reverb when I turned up the volume. The distortion had a very ugly sound to it. I had originally intended to wire the speaker cab with a switching system so I could get different combinations, but I just wired it for 8 ohms (parallel-series) so I could use it with the Traynors OR the Bandmaster. It always sounds ok with the Traynors, even with the volume up, but not the Bandmaster. Well, tonight I finally put a switch in the speaker cab to switch between 8 and 2 ohms. The overdriven sound of the Bandmaster has become MUCH better. There is still a little bit of harshness, but there used to be an almost metallic grating sound to the distortion.  
BTY, The bandmaster has a multi-tap Super Reverb-style XFMR. I have a rotary switch in the ext speaker jack so that I can use any tap. As stated, I had been using the 8ohm tap, but here is the rub: I have the feedback resistor, the standard 820 ohm, connected only to the 2 ohm tap. Is it possible that the feedback was "not working" well enough with the connection being so isolated from the actual load connection? There are many amps that have multiple taps, but only one feedback connection. They usually take it from the highest Z tap. Or is it just that 4 speakers in parallel behave better than 4 in series parallel? I tend to think the bad distortion was due to the feedback being from the wrong tap. The next time I am plumbing around inside the Bandmaster Reverb, I may rewire the feedback so that there is a different resistor value for each tap, and the chosen tap will also be the feedback source. Does this make sense?  
Mark Knapp
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1/19/2000 8:58 PM

It makes sense to me. Feedback from the output transformer secondary to a point earlier in the amplifier circuit has the effect of reducing gain, reducing distortion, and linearizing the frequency response. The other reason feedback is derived from the secondary of the output transformer is to enhance speaker damping. In HiFi, the most direct effect is achieved when the feedback is taken from the speaker terminals. Next best thing is the same transformer tap, almost as good, especially for short speaker leads. Taking the feedback signal from a transformer tap other than the direct winding attached to the speaker is kind of like trying to read a pulse while wearing a glove. There's a desensitizing due to the magnetic field in the rest of the transformer secondary winding. Speakers that are fairly loose anyway will require more damping to keep them in check (return them to neutral) than speakers with thicker cones and stiffer suspensions.  
Is your amp running a high bias current in the output stage? Warm tone at low volumes turns into no headroomn and early distortion at high volumes. Beam tubes can generate some ugly distortion at max signal, unless they run with negative feedback, or an ultralinear transformer connection.
1/19/2000 10:35 PM
Jim S.

Assuming all component values in the neg. feedback circuit are stock, you should connect that 820 ohm resistor to the 4 ohm tap of the OT, in order to get closest to the amount of negative feedback you'd get if you were still using the original 4 ohm OT. By connecting to the 2 ohm tap, you are reducing the amount of negative feedback by a significant and audible amount. As Doc said, most class AB, non-UL amplifiers really need a certain amount of negative feedback or else they will sound harsh, mushy, or "out-of-control", in non-musical ways.
1/20/2000 3:12 AM
Mark Knapp

Finally! I was suprised to find replys. Thanks, Doc. and Jim S. My intent was to make this amp be as Super Reverb-like as possible, hence the connection to the 2 ohm tap. I still wonder why they used the same value feedback resistor for all of the models, reguardless of output Z. I may just connect the feedback to the output jack and just live with it. I thought there was another pole on my rotary switch, but there isn't. As for the bias current, it is around 35mA. Plate voltage is about 440V-445V. I want to get a 5AR4 to see if there is a big difference in tone there.  
As for the distortion, I may have jumped the gun on the post. It still gets that weird distortion a little, though the amp is a bit better with the tone set for t=6, m=6, b=3. It seems very picky. My other amps are not that picky about tone control settings and good overdrive tone. And it just seems to be the 4 X P10Q cab that has this raspy distortion. My 2 X 12 cab and this amp are a nice, crunchy pair...  

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