More Fun With Magnavox: The 9300 Series

ok, so ~15w output is plenty reasonable. Its actually better than I get from mine, so you've got something right. And thats with 0.46v input ?

Input sensitivity seems about right for the amp with feedback in place. I'd expect less than .05v input for full output with no feedback in place. It should be ~12db NFB which is more than a 10x reduction.

How much output do you get with the feedback connected before it goes funny?
 
That’s what I was confused about, with the NFB hooked up I can only drive the input to about .1 v before distortion. I’m headed to work I’ll see if I can get out to the shop tonight and hook up feedback and get some data to look at.
 
i wonder if the phasing isn't reversed. That will make it oscillate and be far more sensitive than it should be. If the speaker side negative isn't grounded that makes it stupid too.
 
Voltages
Power supply
367.3 VDC 363 VDC 329 VDC

OT
Right Left
Bl-Br
364-365 365-366
Screen pin 9
363-363 362-362

Phase Inverter
Pin 1-6
193-192 183-191. (14% high)
Pin 3-8
1.70-1.65 1.66-1.60. (25% high)

Buffer
294
Pin 8 116. Pin 3 112
 
This shot is both RT (1) and Left (2) channel with just the NFB resistor connected. And input set at .12 v rms.
The right has a 2.7K installed and left is 3.9K.
I added a x10 zoom on the time line.
 

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Next I removed the signal and the amp didn’t return to a quite state, but continued oscillating. Power supply voltage was sinking (280 VDC) it was pulling line voltage down (98 vac) and power line current was .95 amps when I shut the amp off. These are images of the scope while this was happening.
 

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The last thing I could think to check was for VDC on the EL84 side of the coupling caps. This is what I found.
RT -6.7. -4.5
LT -.08 -.10. Climbing
 
i wonder if the phasing isn't reversed. That will make it oscillate and be far more sensitive than it should be. If the speaker side negative isn't grounded that makes it stupid too.
Gadget here’s a pic that shows the speaker output wiring.
 

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try reversing the brown and blue leads to the output tubes. If it stops oscillating, its a phasing issue. If it gets worse, its something else. It really sounds like the amp is in full blown oscillation though by the symptoms you describe.
 
I’m feeling a little bit silly. Nerdorama sujested checking the phasing back on my first post. And I did I don’t know the number of times I’ve been though this build looking for a wiring error and I found nothing.

Then gadget73 and Dave jumped in and helped with troubleshooting. Still no real improvement.

This morning gadget73 again brought up phasing and swapping the blue and brown leads. I was just going to give it a try and see what happened. I grabbed the Edcore drawing just to double check if the swap would effect anything else. As I was reviewing I noticed the white output lead was in phase with the brown lead. So I swapped the output leads and built the NFB just as Dave’s drawing.

Audiokarma we have a amp. So far about a hour on it and wow I really enjoy listening to this amp.

Thanks to everyone who offered help and a giant thank to our engineer, R&D, drafting and technical support Dave.
 
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awesome. Possible my transformers are backwards then. I know the leads on mine are opposite Dave's, but I was thinking it was an Edcor color coding thing.
 
Hello Guys,

Will 12ax7 work instead of the 6eu7? I know I need to rewire the heaters etc... I have quite a bunch of 12ax7 thus Id like to use them on the buffer stage and driver stage.

Thank you
 
Same tube, different pinout. I rewired one that I rebuilt for someone for use with 12ax7's for exactly that reason. I left mine 6EU7 since the chassis is stamped and I have nothing else that uses them, which means they won't get stolen.
 
Socket converters are convenient in this case. I thought I had problems with the 6EU7 tubes in my 9304. I got some socket converters to use 12AX7's. Turned out the original 6EU7's were fine and the problem was with the amp. That got fixed and I put the 6EU7's back. Want to buy two converters?
 
I have a set of spares that actually came from the gentleman who's amp I converted to 12ax7's. I've got 4 spares for an amp that wants 3 of them. Other gear i own uses as many as 6 12ax7's per, so I find myself chronically short. I've got "enough", but I have a number of random singles where I'd prefer more pairs. A few things I own are fine with singles, and I appreciate that gear very much.
 
Socket converters are convenient in this case. I thought I had problems with the 6EU7 tubes in my 9304. I got some socket converters to use 12AX7's. Turned out the original 6EU7's were fine and the problem was with the amp. That got fixed and I put the 6EU7's back. Want to buy two converters?


Thank you for the offer for the sockets, i will have to pass for now... ill wire the amp with 12ax7s, the chassis is rusted so i will just take everything out and clean the chassis. I very much app your response...

Thanks
 
Same tube, different pinout. I rewired one that I rebuilt for someone for use with 12ax7's for exactly that reason. I left mine 6EU7 since the chassis is stamped and I have nothing else that uses them, which means they won't get stolen.


Thanks, im excited about this build as I have a few nice 12ax7 that i hope will work great.

Thanks
 
Good luck! They are great little amps. I had considered rewiring for 12AX7 tubes but it looked like too much work in too small a space. If you are doing a major rewire that shouldn't be any more work that redoing the sockets for the 6EU7's.
 
If you are rewiring the sockets, I might suggest replacing them. Those wafer sockets just aren't that robust, and shuffling wires around is somewhat likely to make them fail.
 
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