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Got my Clubman clone going


 
6/27/2003 3:47 PM
hasserl
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Got my Clubman clone going
I finally got it going this week, but I'm still trying to sort out the power supply. The original RCA pa amp I started with ran the entire B+ through a choke. The plate voltages were in the high 300's. I first changed this to a more traditional circuit with the B+ connecting first to the first stage of a multi-stage can style filter cap and out to the OT center tap, then through the choke and out to the second cap stage and the screens and so on. But when I did this the B+ voltage went up well above 500v! It caused some popping noises in the filter cap and I believe I may have damaged it. So I rewired it back to having the entire B+ run through the choke. So my plate voltage is back down, now with the 6L6GC's only at about 360v. It sounds good, but there's not a lot of volume. So with such low voltage on the plates the output power must be pretty low. I think I'm going to swap out the 5Y3 rectifier with a 5AR4 and see if it raises the B+ much. Maybe I should go back to running the first leg out to the cap before the choke, and lower the B+ with zener diodes on the PT center tap to ground.  
 
Besides this though I have to say the amp really sounds good. I was worried that with just two gain stages it wouldn't have much of a dirty overdrive, that it might just stay pretty clean so that I would have to push the front end with a pedal to get it to crunch. There was no need to worry about that! It gets all the distortion I need, and cleans up very nicely by backing of the volume. I've only had a short time with it, and I'm still tweaking it, obviously. But so far I'm really happy with the tone. I just need to straighten out the power supply woes, and get some better speakers too! :D
 
6/30/2003 12:35 AM
hasserl
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Well, with a 5AR4 in place I now have 376v at the plates of the output tubes. All other voltages are up slightly too. It does sound good like this, it just is not as loud as I expected it to be. That doesn't mean it won't be loud enough, just not as loud as I expected. I think I will leave it like this for awhile, it does allow me to crank it up without blowing out the neighbors. Later I might fiddle with the power supply some more, maybe add Bruce Collin's Hollow State rectifier by-pass to get a few more volts and less sag.  
 
After spending more time with the amp, I have to say that this amp gets just a little too much distortion. I've added back into the circuit a grid leak resistor after the pentode pre-amp stage I left out originally, that helped some. I'll probably tweak it some more to try and smooth it out a little. Low distortion settings sound great, but with the volume up past 6 0'clock it starts getting muddy fast. I'm also not thrilled with the Bass control, it doesn't offer much effect on the tone. But I am liking the Brillance control, that is a cool option.
 
 
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6/30/2003 6:48 AM
MJ Harnish
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Glad to hear she's working with only a few minor niggles.  
 
What are the tubes biased at? Using a pair of 6L6's you'll have to get the bias voltages up there to be running them in Class A (though that may not improve your bass problems). With those low plate voltages you shouldn't have any problems running the stock EL34's.  
 
That said, you may need to push your plate voltages higher in order to get some more headroom out of the amp; that may be what's contributing to your excessive distortion. A SS-rectifier would increase your voltages a bit more plus probably help tighten the bass a bit.  
 
Did you use a cathode decoupling cap on the input stage? If so, try taking it out since this is going to increase the gain significantly.  
 
What sort of OT are you using?  
 
MJ
 
6/30/2003 12:45 PM
Alexander
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Hey Hasserl-  
Congrats on gettin' it singing right away- feels great, don't it :)  
What model RCA PA did you start with ? I have one in the basement but haven't paid much attention to it other than keep it around for a possible trade someday. Thanks !  
 
Alexander
 
6/30/2003 3:57 PM
hasserl
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What are the tubes biased at? Using a pair of 6L6's you'll have to get the bias voltages up there to be running them in Class A (though that may not improve your bass problems). With those low plate voltages you shouldn't have any problems running the stock EL34's.  
 
I’ve tied the cathodes together with a 220 ohm resistor, and I’m seeing 127ma across it. At 376v on the plates, what do you think I should see across the cathode res to run in class A?  
 
re: the Bass, I just find that the control doesn’t offer much effect throughout it’s range. In fact, I hooked it up backwards like you did to a 1m audio pot, since I don’t have a ra pot, and only in the first ¼ turn of the pot is there any change in tone, the other ¾ turn doesn’t seem to change anything. With the pot turned all the way clockwise (which should be minimal bass because it is wired backwards, I wonder if the pot is mislabled on the schem) I get a very thick chunky tone. At about 9 o’clock it thins out a bit, but from then on there isn’t much effect. I need to spend more time playing it, and through better speakers, before I condemn it.  
 
I am not using a cathode by-pass cap on the first stage. I think I am going to play with the biasing of the 6AU6 second stage in an attempt to smooth it out a little. I do have the 25uf by-pass cap on it, but not the .1uf cap added to it. The OT is the stock OT from the RCA pa amp, there are number codes stamped into the end cover: 459654-1 & 138839. I'd love to find out what they mean, any links to manufacturer's codes?  
 
Alexander, thanks for the congrats. There is a small tag on the back of the amp that reads: 15 watt PA Amp, there is another number under the heading of MI: 12156A, then also a serial no.
 
7/1/2003 5:43 PM
MJ Harnish
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I’ve tied the cathodes together with a 220 ohm resistor, and I’m seeing 127ma across it. At 376v on the plates, what do you think I should see across the cathode res to run in class A?  
 
What kind of voltage drop are you seeing across the cathode resistor? I can tell you with a shared resistor, those low plate voltages, and 6L6's you're probably not running anywhere near Class A.  
 
I can't say much about your tone stack b/c you're using a 6AU6 which I'm not sure is going to match up with the stock values on the schematic since those are for a 6SH7 (I used an EF86 like the later Clubmans used). We could be talking about apples & oranges here comparing the tone of my amp to yours given the significant differences in the tube complement. The .1uf cap on the cathode decoupling cap is most likely a typo and is guaranteed not to improve your bass response. You could try decreasing the value of that 25uF cap though....  
 
Finally, it sounds to me that your PT is very under-rated for this application; in fact if the amp indeed was originally a 15w amp you're liable to fry it. You are also liable to have an impedance mismatch going on; what were the original power tubes used in the amp?  
 
MJ
 
7/1/2003 6:56 PM
D. Hiatt Collins
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In some of my clubman research, I've found out that the 6AU6 is an electrical equivalent of the 6SH7 and is actually, of the two, the one recommended for audio. I'm not sure why matchless chose to go with the 6SH7 when 6AU6's are plentiful and cheap, besides being the preferred version for this application. I'd guess that the octal 6SH7 may have had a mellower or better tone than the 7-pin mini 6AU6.  
 
Hiatt
 
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