Magnavox Flea Power: Getting More Out Of The 8600 Series - A Lot More!

that only works on things with 2 inputs. My Sherwoods and the Fisher with the bank of input jacks that are all jammed together isn't quite so easily solved. I need an excuse to make up some cables that aren't crappy anyway.
 
Hi all. I modified an 8600 and loved it and so I tried to do a scratch build. At the moment my scratch build does not work. I put it in the closet and left it for a few months out of discouragement but I just pulled it out again and figured I would run it by the experts. I am a newbie and I don't understand a lot of what you guys are talking about when it comes to details.
My amp initially makes music but it is quickly overtaken by a ringing so hideous that I have to shut it down. I can't really even keep it on long enough to check voltages although I know that I am definitely over on ALL my voltages because of the power transformer I used to build it with. I have ran the amp with AC of 94v as well as 66v though and still get the hideous ringing, so it's not a high voltage issue. What are causes of ringing? It's actually like a squeal, all the same tone and constant until I shut'r down. Any help?
 
Look at my amp though. I think it looks really cool and I don't want it to go to waste! I built it into an old cash box with some brand new Edcor iron.
 

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That does look neat. Not sure about the squeal tho. But once I had one just making a horrible racket. Looking it over I realized I had the left feed back going from the left channel output transformer to the right channel side of the 6EU7 and vice versa.
 
My suggestion is to switch the two transformer connections.....plate and B+. This sounds somewhat like a 'polarity' issue with respect to the feedback. You can also swap the two transformer 'output' wires......so the one formerly to ground becomes 'hot' and the one formerly to 'hot' now becomes 'ground'. BUT, in either case, leave your feedback connection alone. If you try swapping it around too you'll just muddle things up even more. I'd start with the two 'primary' side wires, swap those connections, and see what happens. BTW, a post with a detailed pic of your actual build wiring might be helpful as well. FWIW, WC
 
I've built a few using Edcor iron, including the one I listen to in my shop. If need be, I can double check the way I wired that one in order to provide some polarity info to you. And, like Gadget says, you also DO need to make sure that the 'negative' side speaker connection from the transformer's secondary is effectively in the circuit to 'chassis' ground as well. If you have the transformer wires going to speakers and 'not' connected into the circuit ground, then there's no proper feedback circuit......so.....a potential problem there as well. Again, a shot of your underside build wiring might help clear some of this up. Good luck! Tom D.
 
So I had my output transformer secondaries reversed. Once I fixed that all is perfect. No more hiss, just lots of that good SET sound!!! Thanks guys. Now my only issue is the 30 extra volts coming out of my ez81? I am at 300v where the schematic calls for 270v. What are my options? I know I could change the rectifier, but that would require quite a bit of work on my part. I do have a couple extra 5u4gb's sitting around. I know a couple other folks have done scratch builds, what did you guys do?
 
I think the 5U4 is the way to start. Mr. Gillespie has very kindly pointed out that this amp is NOT sensitive the DC resistance factor in the PS......as long as you can draw the 100 ma current 'up front' (so to speak)....you're good to go. The voltage drop from the 5U4 will certainly help you with getting closer to the target voltages. Also, I might add that in my humble experience, as you continue to adjust bias on the output tubes, you'll find the B+ voltage fluctuating as well. So, it becomes kind of a balancing act. Of course, I decided to use the 5AR4 so as to keep the 'slow start aspect of things within the PS....then I 'added' power resistors to massage the DC voltage to the proper level. Mr. G suggested that I add a 4th PS smoothing cap at the rectifier socket.ahead of my first 'dropping' resistor. When I use the Hammond PTs I get about 300 VDC at the rectifier.......so would put a 20uf 450V cap at the socket, then use something like a 10 to 20 watt dropping resistor of anywhere from 400 to 600 ohms, then another filter cap and the connections to the output transformer. After that it all follows the schematic. Hope this helps. WC
 
Thanks Wharfcreek, you are always a big help. I may have to revisit your advice if and when I try the 5u4gb if it doesn't drop my voltages down to an acceptable level. Can anyone say what an acceptable level of voltage is for this circuit? What is my extra 30 volts across the board doing to/for the amp? The reason I ask is because I am having no problem biasing the output tubes to 5.85volts and I just can't believe how good the amp sounds in its current state. To what detriment if I just run it as is?
 
One more in the Club. Thanks Dave for the project as a whole. Thanks Tom for pushing me over the edgeIMG_5906.JPG

I did get the bottom on it after these pics. I'm surprised how loud 3 and change watts can be on only medium efficiency speakers can be. If i didn't know i'd assume this thing pushed 3 x 5 times the power it does.:idea:
 
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Can anyone say what an acceptable level of voltage is for this circuit? What is my extra 30 volts across the board doing to/for the amp? The reason I ask is because I am having no problem biasing the output tubes to 5.85volts and I just can't believe how good the amp sounds in its current state. To what detriment if I just run it as is?
I've got an 8601 and 8608 and another SE amp converted to dcgillespie's design and they all have a b+ rail of +/-300 coming off the rectifier. They all sound great. I wouldn't worry about it. What is the voltage at your output and preamp tube plates?
 
While the effect is not huge, having a B+ voltage that is higher or lower than specified without making other corresponding changes effectively moves the operating point of the output stage off of dead center on its load line, which is always where it should be located with any true Class A amplifier. The net effect is that the amplifier will then clip unevenly as full power is approached. If you look back at the beginning of this thread wherein I document the performance of the stock design with some scope pics, you will see the uneven clipping produced due to the lack of coordination between the B+ voltage, standing current, and plate load used. Look ahead in the thread and then see the greatly reduced distortion, increased power, and even clipping produced when these elements are in fact properly coordinated. Those shots represent exactly how a real Class A amplifier should operate. Again, the amount of B+ deviation discussed won't ruin the soup, but coordinating these elements properly does take on more significance when flea power is the discussion, as a much greater percent of the available power is typically always being used.

Dave
 
Just came in to report that I built this little gem of an amplifier from the modified schematic, and WOW.

Thanks for this thread. It sounds great. I'm pretty shocked that this little thing can produce so much sound.
 
Akim -- Thanks for the feedback! I've had comments like yours from folks literally all over the world now who've built the modified design and were similarly impressed. I listen to the original developmental unit regularly and continue to enjoy it now as much as I did when I first heard it.

Thanks for the interest, and the willingness to give it a try!

Dave
 
Another one in the club. Thanks Dave. Such a cheep and cheerful way to learn about tubes. Moved the rca jacks to the back. Picked up a little hum. So shielded cable will be going in to see how that works out. I really like all the cables in the back. Legs off my old 1956 electric frying pan. I knew they would come in handy some day. The legs provide clearance for the fan. Plus I like the look. I picked the wood, it looks like a rattle snake skin. So I named the amp rattle snake . Small but a big bite with La Scallas. First wood work I ever tried. That amp is more then worth the effort.SAM_0617.JPG SAM_0618.JPG SAM_0621.JPG SAM_0622.JPG SAM_0615.JPG SAM_0616.JPG
 
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