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previous: Brian Its the vox AC41 SE... -- 6/1/2000 7:08 AM View Thread

Re: Can't escape crap distortion

6/1/2000 4:27 PM
Re: Can't escape crap distortion
Hi Brian:  
Your AC4 output stage seems a lot like the one on my Skylark, which I did some extensive experiments with.  
My RCA 6BQ5 is at 250V B+, screens also at 250, and my cathode resistor is 150ohms, with 35mf bypass.  
The GE book shows these same example conditions except for a 135ohm Rk, their reference load is 5200ohms, they never say what Ck, but I assume they were in the circuit also.  
On mine by using various precision power resistors and an auxilliary 125E, I was able to determine 6800ohms as the most efficient matched load, in listening tests loads over 7000 sounded better to me than those under 7000 though, and I test at every volume, especially maximum.  
With your slightly higher B+, I would expect an even higher load to be a good match, so I would pick an 8000ohm ratio from the 125E that uses the whole primary:  
Connect the blue lead to your plate, the brown to B+, no connection to the red one, tape it off and insulate it.  
Use a 16ohm speaker load on secondaries 2 & 6, this will use as much of the secondary winding as possible to reflect about 8000ohms of load to the tube. Using the whole secondary from 1 & 6 would be slightly more than the 5600ohms on the chart, but you still may like it better since it uses the whole winding, and it is closer to the *book* value.  
If you only had an 8ohm speaker, you would connect it to secondaries 2 & 5 for about the same 8000ohm load, but you would be employing even less of the secondary.  
With this type of universal transformer, as you use less of the available windings, the specifications need to be derated, not a problem with only 1 6BQ5, but this means that even though the unit has a fairly good size, if you use less of it, it acts more like a smaller transformer.  
I really wouldn't want to use less than 8ohms, especially without NFB, which the Vox circuits are supposed to be without.  
Focusing on crap distortion, one bad solder joint, even just the difference between soldering on a socket with the tube in versus having removed it first, can make a bigger difference than just the small incremental load steps available from the 125E. This seems to be also critical that the filter caps have far above average soldering as well. Although mine sounds highly superior with RCA, Ei, or Sovtek EL84M, when I put in the regular Sovtek EL84, it reverts to a fairly crappy shadow of its full capability.  
Another thing that might help if you are just breadboarding is to make sure you earth the 125E frame, making good contact through the resin coating, and also ground one of the speaker leads, if there is no NFB it should not make any differerence which one, but I always listen to it each way, keeping in mind speaker polarity as it relates to my playing position next to the cab, and how I expect the guitar to *feel* when it is exposed to the sound field beyond overdrive. Also the flying blue & brown leads from the 125E primary can be a major lead dress issue when breadboarding, until it is mounted in a chassis, oriented properly, and trimmed & routed optimally, slight movements of a few millimeters can really be heard.  
Plus with such a simple amp, each component has a relatively more prominent effect on the final tone. Your entire preamp (like mine) is only one tube, same with the power section, might as well be good ones. Plus the coupling cap(s), and other signal path components and their joints should be far above average. Not to mention the speaker, unless it is at least a 15 or 20Watt premium cone, preferably two, I would not expect to be able to fully test the amp without speaker limitations coloring the component effects.  
Hope this helps,  

Brian Thanks for the advice mike;... -- 6/2/2000 12:21 AM