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added extra 5y3 and 6v6 to 5f2 ...problem?


 :
7/19/2003 2:05 AM
Gary Gustin
added extra 5y3 and 6v6 to 5f2 ...problem?
I just finished adding another 5Y3 and 6V6 parallel  
to the others. I followed the Angela super 6V6 instructions fairly closely, but am using a hammond 272DX (300-0-300 125ma, I believe)and a hammond 125E  
OT. Before I did the mod, the amp sounded pretty good and full, with a nice break-up. But now the amp seems barely, if at all, louder, less low end, and less breakup...its kinda wimpy sounding now. Also after 5 or so minutes of use, I am getting a "hot" smell coming from the amp.  
My measurements are with 1 12ax7, 2 jan Philips 5Y3WGTA,some nos 6V6 tube from France or Italy?-(marzzonisomethingoranother-can't remember the name), and another 6V6 -Electro-harmonix  
 
1.plate to ground= 350VDC  
2.plate to cathode= 328VDC ( I installed a 240 ohm 5W cathode resistor)  
3. screen to cathode= 342VDC  
4. cathode to ground= 22.3VDC  
 
Nothing too out of the ordinary, is it?  
Help!  
 
Gary
 
7/19/2003 6:09 AM
Gary Gustin
kind of fixed problem, but still has a "hot" smell
I went and put a 10uF/150V cap parallel with the 240 ohm 6V6 cathode resistor, and put 25uF/50V caps parallel with both cathode resistors of the 12AX7, and changed the primary impedance of the transformer to make 3150K ohms for an 8 ohm load, and also reversed the two wires going to the the speaker jack just to see what would happen. Now I'm getting a nice cranked amp tone that is considerably louder than before. But I'm still getting a " too hot" smell from the amp. I don't see any smoke or anything like that. My measurements are now different:  
 
1. plate to ground= 357 V  
2. plate to cathode= 335 V  
3. screen to cathode= 349 V  
4. cathode to ground= 22.8 V  
 
so... 22.8/240= .095  
 
.095x 335= 31.825 /2= 15.91 Watts  
 
This seems kind of high? Would this cause the hot smell? The trannys and the choke don't seem to be getting hot to the touch, although the tubes seem pretty dang hot, but the don't look like they are orange plating( at least I don't think so...) ?
 
7/19/2003 5:51 PM
Bruce /Mission Amps
Re: kind of fixed problem, but still has a "hot" smell
Not that it's a deadly problem, but why do you think you have 349vdc from screen to cathode and only 335vdc plate to cathode? Can you spell Choke?  
When you fixed that "problem", raise that cathode resistor from 240 ohms to about 270 - 300 ohms, (maybe even as high as 360 ohms) and bypass what ever you resistance with a 68uF or 100uF, 50v to 100v cap.  
Also you didn't say what speaker load you are running with that 125E but I can tell you that OT is borderline freaking out with two 6V6s in parallel at 30 watts in idle!!! It's pretty much clapped out at 80ma and at that it's low freq -3dB is around 100Hz-150Hz. Not so good for that big thumpin' bass response.  
If it's an 8 ohm speaker, blue to brown for the plate leads and B+ with 1 and 6 leads as the speaker loads.  
Or better yet, use a better OT.  
 
Bruce
 
7/21/2003 6:50 PM
Gary Gustin
thanks Bruce for response
Actually I do have a choke hooked up. Its a 2H 200ma stancor, but the Angela schematic says to use a 5H 125ma. I put a 319 ohm 10W wirewound in and it seemed to help. I got it to about 13.5W. I took it to band practice, and played on it for about an hour, and it still seems to work(no smoke or flames).  
But,Just as Bruce said, the bass response sucks, although it has an interesting dimed out lead sound from the burdoned tranny. Guess I will put it back to the normal 5F2, and then build something else. I was trying to get some more volume for a small portable practice rig, but I'm not sure I like the overall sound I'm getting with this circuit mod. I guess it was a learning experience anyhowz.
 
7/27/2003 10:49 AM
neatpete

part of the problem is that your screens are higher than the plate. this should not be so, as most of the signal will be going into the screen.  
 
 
peter
 
7/27/2003 1:01 PM
Newt

That's the way the 5F2 is designed. I don't understand it, but the plate and screen both come off the same power supply node, and the plate voltage drops across the OT primary.
 
7/28/2003 10:04 PM
Bruce /Mission Amps

quote:
"That's the way the 5F2 is designed. I don't understand it, but the plate and screen both come off the same power supply node, and the plate voltage drops across the OT primary."
Well, yeah but that's not the way his 5F2 is built.  
I was trying to let him figure it out...the choke, the OT, two 6V6s etc...  
Newt I think you're kind of missing the point too... he is using TWO 6V6s, with the current of both going through that OT, rated at only 80ma.  
Now think about your voltage drop statement and it should start making more sense to you as to why the plate voltage is lower then the screen voltage.  
5v-15v is not such a big deal but in his application it's causing the two parallel power tubes to pull even more plate current, again through the OT with the same DCR for even more voltage drop.  
Sort of like a dog chasing his tail only he really has it in his teeth.  
 
Bruce
 

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