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Trem only on standby?!?


 
6/21/2005 6:17 AM
Benboom Trem only on standby?!?
I'm pulling out my hair because of the tremolo in one of my amps. I recently built an 18 watt Lite (Marshall clone) and put a Matchless Hurricane tremolo circuit in it, It works great! THAT's not the problem. :-)  
 
The problem comes with an earlier amp I have that I put the same circuit into. It's the same power amp (very similar to the Hurricane, which is why I used that trem circuit in the first place) so I expected the same result. Wrong! The tremolo works, but it's too faint to be useful. I've checked the wiring and values and even replaced many of the components but with no improvement, also trying different tubes. There are two differences between the amps though: the "good" amp has somewhat lower B+ (about 30 volts lower, although they are both biased to give the same dissipation), and the "good" amp has a dedicated tube for the trem circuit (the other triode is unused), while the "bad" amp shares its preamp with the oscillator triode, although they have separate supplies (of course) and cathodes.  
 
Now here's what prompted this question: yesterday while I was fiddling with the amp I happened to put it on standby and continued to strum chords. As soon as I flipped the switch the trem functioned correctly - nice and deep. Of course, the sound faded out right away, but while it lasted I had a working tremolo.  
 
What does this imply? Sorry if this is a beginner question - I'm a much better player than builder.  
 
Thanks!
 
6/21/2005 9:03 AM
Dai Hirokawa
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quote:
"There are two differences between the amps though: the "good" amp has somewhat lower B+ (about 30 volts lower, although they are both biased to give the same dissipation), and the "good" amp has a dedicated tube for the trem circuit (the other triode is unused), while the "bad" amp shares its preamp with the oscillator triode"
 
 
might be the problem. They can interact inside the tube (high frequency electrostatic coupling from being close to ea. other, I believe), plus low frequencies coupling through a shared filter cap (if they share one). Tim G. had some noise problem traced to a dual triode sharing a gain stage and some other function.
 
6/21/2005 10:16 AM
Benboom
The two triodes are separate electrically, at least as separate as I can make them. Separate cathodes, plate supplies, not much left to separate. I tried changing the plate resistor to the oscillator tube and I also tried powering it off a different B+ node (I had two more to try and neither helped although one was quite a bit lower).  
 
But I'm fine as long as I don't need tremolo for more than about a second and a half. :-)
 
6/21/2005 1:54 PM
MBSetzer
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Can you point to the schematics of the two different amps?  
 
This may not be too similar to what you are doing, but here are some of my experiences:  
 
http://members.aol.com/tubestim  
 
Mike
 
6/21/2005 4:32 PM
Benboom
Mike, I had already found your page but I'm not sure it's applicable in this case, or maybe it is but *I* can't apply it. But I do have schems for the amp with the buggy trem and I have the Matchless schematic the tremolo was taken from; it's connected just like it is in the Matchless and the tremolo circuit is just like the Matchless schematic. The amp with the non-buggy tremolo has the same power amp as the one here.  
 
http://www.xprt.net/~benboom/InterociterCurrent.gif  
http://www.xprt.net/~benboom/hurr_bw.gif
 
7/1/2005 9:30 PM
MBSetzer
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Well, I'm back.  
 
It does look like these amps are very similar, are the cathode voltages on the EL84's fairly similar on both amps?  
 
IOW, the hotter the tubes run, the higher the cathode voltage at idle, therefore the greater the voltage differential between the grid and the cathode.  
 
Basically the tube just amplifies the difference between the cathode (K) and the control grid (G1).  
 
But if one amp has a greater G1-K difference than the other, then the same amplitude trem wave will give less audible variation in the amp with the greater difference at idle, since the grid variation would be a smaller percentage of the total standing voltage at idle.  
 
In addition, you are injecting the trem signal to the grids through a 250K pot to 220K grid load resistors in the Hurricane, but if you have 470K loads on the Interocitor that may be dampening the variation some also. I would try reducing the 470K to 220K or even 100K's for testing.  
 
Also you might benefit from reducing the 10K grid stoppers toward the 1.5K seen in the Hurricane.  
 
If all else fails, then you may need similar treatment to what I did on the Thomas organ, by adding more gain to the trem signal itself. I added a second triode for further amplification of the trem oscillation. This amp has 6V6's which put out about the same power as El84's, but have about twice the K-G1 operating difference so they just plain need more trem variation for it to be audible.  
 
If your trem tube is getting its B+ from too far down the preamp power supply chain, maybe it just needs to have its plate be supplied from closer to the screens of the power tubes so the trem tube has more voltage available to begin with. then it may swing wider without any other soldering changes. On my Silvertone 1472, the 6AU6 trem oscillator had its plate connected directly to the screens of the 6V6's. Which I didn't really like so I isolated it by about 2K and another dedicated filter cap just for the plate of the 6AU6, but that was still about as high a B+ as the raw screens had.  
Notice how on the Hurricane the trem B+ (point Y) comes from a direct node not downstream from the PI or preamp B+ nodes.  
Also, for getting trem from a single gain stage, the 6AU6 pentode has more gain than a single 12AX7 triode, so that would be an alternate consideration.  
 
Mike
 
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