1949 Magna Melodier amp build help request

Well damn! It works.....sorta. It sounds wonderful all the way up to about 90% at which point it starts to feedback a little and buzz. As soon as I touch the tone control, it squeals like a stuck pig. I can only play it with the tone as low as it goes. The original problem was that I had pins 5 & 6 wired backward. This doesn’t seem to be the situation here.
 
oscillation. That sometimes gets into stuff with lead dress, but make sure all the stuff around the tone and volume controls are wired properly. You may also get some improvement by adding a 1K resistor at the grid of each output tube. Put the resistor as close to the output tube as possible in between whatever is currently connected there and the tube. Ground the cases of the tone and volume pots if they aren't already.
 
Looked at all the connections to the pots and all is as it should be. I then fiddled around with the wires from the opt and who'd a thunk it? The green wire is not the neutral one. Switched the ground wire to the black wire and the problem went away. Still have a modest 60 cycle hum, but that’s a problem for another day.
 
Could be lead dress. Is your heater supply center tapped and grounded like the schematic shows? If not, you can use a pair of 100 ohm resistors to get it done. One from each heater leg to ground, or to the cathode of the output tubes to put some DC voltage on the heater supply which usually will reduce hum. The heater leads also should be twisted if you didn't do that.
 
Both the high voltage and the 6.3 volt taps have a center tap and are grounded. The 5v does not. All wires are wound as tightly as possible and I have separated the power supply from the output and the preamp tube as best as I could.
Would creating a virtual center tap on the 5v power supply help. I understand that ac and dc can coexist on the same wire so could there be ac leakage through the 1K and 100 k since they do seem to bypass the filter caps? ( are they acting as a form of voltage regulator?) If I understand correctly, the opt acts as an ac filter and therefore a coupling cap between the two 6v6s would serve no purpose. I have yet to try the 1K resistors from the plates on the 6v6s and as I’m flying up North for the next two weeks for work, it will have to wait until I get back.

Thanks Gadget.
 
the 5v cannot be grounded, virtual or otherwise. The high voltage DC rides on top of that, so any tie to ground would short out the plate supply.

HV must be otherwise you don't have a complete circuit.

The heater one is the only possible variable.

The 1K and 100K are not going to ground, they go between the + terminals of the different filter caps to create voltage drop. Its drawn kind of stupidly so its not as easy to follow as it ought to be. Basically the voltage travels around counter-clockwise from cap to cap to cap via the resistors. Normally it would just be drawn left to right so it was a little more obvious. What you've got is a 16uf from pin 2 (or 8) of the 5y3 to ground. A 1K resistor goes from there to the + of another 16uf to ground. From that point you have a feed to the output transformer and a 100K to another 16uf cap to ground. From that point its the 200k resistors to the 6N7.
 
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Almost done. Just a few connections to the pots and the jack left to make. I discovered that my cat’s eye is a perfect fit for the one hoke left over. If only I knew how to wire it up. Wink, wink. I am having a relative of my wife’s hand painting a logo on a piece of perspex with 1930’s font. Hopefully the tube glow will light it up.
 
Somehow I have managed to create a short somewhere in the circuit when I wired up the pots in the cabinet. The dim bulb lights up weakly after about 10 seconds or so and I hear a slight popping sound when I power it up. I pulled the tubes one at a time to try and narrow down the possible location of the short but it doesn’t go away until I pull the rectifier. I have tested all the caps and resistors and they, as well as the tubes all test good. I don’t have an ESR meter to fully test the caps though so I guess I will have to start from scratch and redo all the wiring. Good thing it’s a simple circuit.
 
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I have rebuilt the amp from scratch. The dim bulb still lights up and I have a loud hum as well, but the amp does work. I have checked each and every component before putting them in and have tested the tubes as well. I used a tag strip as a gound bus and the mains wire is grounded to the chassis about one inch away from where it enters the chassis. I get continuity between the two ground location indicating that they are both grounded through the chassis. What concerns me the most is the glowing dim bulb. Where is the short? The wiring is a bit messy because I was constantly soldering and unsoldering connections in an effort to track down the problem.
Confused in Calgary
 
The hum is a bad or improperly grounded filter capacitor right off of the rectifier. Just make sure that one or more are not wired backwards.

You don't have a short in your power transformer because if you did, nothing would work and it would get hot or smoke. You do have it fuse protected right?

What wattage bulb are you using in your DBT?
 
Thanks for the answer Century,

The dbt is a 250 watt bulb and the amp is fuse protected. I have checked the polarity of all the capacitors and as far as I can tell, they are wired correctly and test good. I will check again though. Since the amp is working after a fashion, I can only assume that the dim glow of the current limiter indicates too much current rather than a dead short.
 
Hope you get it figured out.. Loved the "Concert" post report. LOL. guitar player looked like he was under severe duress sitting down... My brain would wander, too. but not far enough to see a concert. :D

Is it an old transformer, that is if you think it matters on leaking current. I'm not a tube guy but ya need a scope I think or look over that power supply section again.. may be a mod needed as in beefed up?
 
I'd advise ditching the crimp connections for terminal strips with the wires soldered to them where you need to make a joint. I do not trust those things for high voltage.

The wiring is also fairly messy. Heater leads should be twisted and not routed near signal wiring, there are leads longer than are needed, and components spliced and hanging in space. Its not good construction practice to do that just because things can move and short out.
 
I wiggled all the wires until I found one that affected the hum. This wire connected the .02 cap from pin 5 of the 6N7 to the middle leg of the volume pot and after resoldering the connection, the hum is almost gone. As I turn down the trebble the hum gets louder and softer when I turn it up. Unfortunately, the current limiter still glows as soon as the tubes warm up so that issue still remains. Somewhere, something is still drawing too much current.
I guess that will be a problem for next month as I go back to the frozen North tomorrow.

Thanks for the help.
 
I'd advise ditching the crimp connections for terminal strips with the wires soldered to them where you need to make a joint. I do not trust those things for high voltage.

The wiring is also fairly messy. Heater leads should be twisted and not routed near signal wiring, there are leads longer than are needed, and components spliced and hanging in space. Its not good construction practice to do that just because things can move and short out.

I agree about the crimp connections. I swapped out the first output transformer I was using for an older one I had used in a previous build as I knew it was still good. The crimp connections were already on there and for the sake of expediency, I just left them there for the moment. I will be replacing the connections with properly J hooked, soldered and heat shrinked ones next time I’m back home. In my defense, the wiring did start out nice and neat, but got progressively worse as I went about rewiring sections over and over trying to find out where I went wrong. Now that most of the problems have been ironed out, I will be tidying and re-routing the connections. (The term “rat’s nest” comes to mind)

Thanks Gadget.
 
happens, and I've certainly built some rat nest things myself. I have an amp I knocked together a bunch of years ago that has now become a parts donor. Its an absolute disaster inside.
 
Bit of a necro-thread revival. After a long wait, my cousin in law finally finished the glass panel. Hand painted on 5mm glass. Nosback for the farm where I found the cabinet in pieces. (My wife’s family homestead). Barn find ‘cause I found it in the barn and 10-12 ‘cause I’ve made two baffles. One for a 10 inch speaker and one for a 12 inch.
I still have to rewire the amp and replace the rectifier which went to air. It must have taken a hit sometime over the past several years as it has a tiny crack at the base of the glass.

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