Magnavox A9304 amplifier project

[guitargrump]

I finally got around to looking at my pile of planned projects and this is the one I have picked.



I bought the amplifier module and speaker set on Ebay last year... Fairly reasonable price...

The three 6bq5 tubes test as good...

But I'm unable to test the 6eu7 on my tube tester without making a 12ax7 to 6eu7 adapter... I ordered a 9 pin socket saver to modify for the task it will be here next week...

I previously Ordered a 5u4gt and matched quad of Soviet era reflector plant 6p14p-k tubes

I'm missing one 6eu7 tube but I'll probably just borrow it from another projects module...

I'm pretty sure that I have all the parts I need in Stock for a complete rebuild not sure how far I want to go yet.

I'd like to stick with mostly the original circuit design and components to start with get it running and modify it for stand alone useage...

Buck the power, add a fuse holder, power switch, power indicator, speaker binders, polarized two prong chord

And then listen to it a while as originally designed... Then I would like to do Dave Gillespie mods to the design and see the improvements and maybe learn a little bit more about why I don't know a enough...

I read Daves thread on the 9300 series up to page 70, lots great stuff in that thread.

I have been cleaning up and polishing the chassis there was some sort of acidic beverage spilled on the module at some point as well as the normal topside patina...

I believe that it's going to look really good when the polishing is done...



Plan is to gift the best sounding looking Magnavox amp module to a luthier a great friend and mentor of mine who has been helping me out with my guitar building and circuit design. He is always helping me out with my big little guitar problems that I am so grateful for... I hope it impresses him...

This amp will have a simple walnut case/shell surrounding it, rubber feet and alot of TLC...

 

[markmcmuz]

I can say that the DG mods with Dynakit OPTs sounds great! I've built two! Have fun!

 

[Lavane]

Post a picture of the original 6BQ5's. Mine came with Magnavox branded Mullards.

 

[guitargrump]

Post a picture of the original 6BQ5's. Mine came with Magnavox branded Mullards.

The tube that came with tested strong 80% but their unmatched in output and type...


That's why I bought a used quad of the 6p14p-k tubes their Gain matched and have a gold grid... 92-94%

 

[Lavane]

Thats funny, the one Amperex has scorch marks like the Mullards get.

 

[guitargrump]

With buying junk amps on Ebay I consider myself lucky to get any working tubes with the amplifier...

It's a losing gamble normally...

I believe this was a imperial model console there were 8 speaker drivers... Their 2 4ohm speaker cabinets now...

 

[Dswankey]

I...modify it for stand alone useage...Buck the power, add a fuse holder, power switch, power indicator, speaker binders, polarized two prong chord.....This amp will have a simple walnut case/shell surrounding it, rubber feet and alot of TLC...

Looking forward to the finished product, do share some pics when done

 

[gadget73]

The middle of that batch of old ones is an RCA. I have a worn but serviceable set for mine.
The Amperex could very well be Mullard. I've got 2 pairs of Amperex tubes in a set of mono amps, two are Phillips made in Holland, 2 are Mullards by the codes, but the print says Amperex Holland on all 4.
The Magnavox branded one I think is a Raytheon ?

 

[guitargrump]

I always get lost in my tube box... I assume most of these tubes were for TV's no 6eu7's in here...



Somehow I end up lost in a rabbit hole sorting tubes every time I start working on a tube based project...

I've already found the common ones I use but there's so many different types...

I'm getting better at finding the markings sometimes their pretty hard to see through the patina and I don't want to wipe them off because it removes the lettering... As usual a fun waste of time...

Hmm I need a gz34/5ar4 eventually too.

I need to look up the 6eu7, Russian 6n2p ,12ax7 pin outs and print them because I need to decide on how I want to proceed I have ten of the low noise Russian 6n2p-ev tubes here...

I'm thinking cheapest route to matched tubes would be to just remake a couple of socket savers into 6eu7 to 6n2p adapters... It's crazy how high tube prices are right now...

 

[FlaCharlie]

Hmm I need a gz34/5ar4 eventually too.

I need to look up the 6eu7, Russian 6n2p ,12ax7 pin outs and print them because I need to decide on how I want to proceed I have ten of the low noise Russian 6n2p-ev tubes here...

I'm thinking cheapest route to matched tubes would be to just remake a couple of socket savers into 6eu7 to 6n2p adapters... It's crazy how high tube prices are right now...

Actually, the 9300 uses a 5U4GB rectifier, not a 5AR4. So that will save you some money since 5U4s are common and even used ones often still test as new. As common as they are, you may even have a few.

The 6EU7, 6n2p and 12AX7 all have different pinouts. Rather than spending money on adapters, just rewire the sockets for which ever ones you have a good supply of.

In case you don't have it already, here's the Sams in pdf, which can be enlarged to show detail.

 

[gadget73]

Dave Gillespie's re-work uses a 5AR4.

 

[guitargrump]

Actually, the 9300 uses a 5U4GB rectifier, not a 5AR4. So that will save you some money since 5U4s are common and even used ones often still test as new. As common as they are, you may even have a few.

The 6EU7, 6n2p and 12AX7 all have different pinouts. Rather than spending money on adapters, just rewire the sockets for which ever ones you have a good supply of.

In case you don't have it already, here's the Sams in pdf, which can be enlarged to show detail.

Thank You for the PDF it's just what I needed...

I will be needing the gz34/5ar4 for when I do Dave's mods to it after I get it running...

I have a 5u4gt for the get it working first part of the build...

I have to make at least one adapter so I can test the 6eu7 my superior instrument tube tester was built before the 6eu7 was released so I ordered some socket savers already... It's okay I'll put them to good use...

 

[gadget73]

having a gizmo that can be re-wired for tube adapter purposes is handy. I picked up a couple of things that I used to make some 7199 to 6U8 adapters when I was working on a Dynaco. Not a clue what they were intended to be, but they're pretty convenient. If I needed these full time I'd just buy a set but these are more intended for temporary jobs.

 

[guitargrump]

having a gizmo that can be re-wired for tube adapter purposes is handy. I picked up a couple of things that I used to make some 7199 to 6U8 adapters when I was working on a Dynaco. Not a clue what they were intended to be, but they're pretty convenient. If I needed these full time I'd just buy a set but these are more intended for temporary jobs.

I'll bet that is really handy for quick testing voltages too

 

[gadget73]

I have different ones for that. Similar overall, but they are straight through with tabs sticking out at each pin for voltage checks.

Those aren't mine, but they are very similar. And yes, very convenient.

 

[63pontiacgp]

I suspect your tube tester will test your 6EU7's. Settings for 6EU7 are not in my roll chart but are in the supplement book for my tester.

Mike

 

[Dswankey]

..I'm unable to test the 6eu7 on my tube tester without making a 12ax7 to 6eu7 adapter...

I suspect your tube tester will test your 6EU7's. Settings for 6EU7 are not in my roll chart but are in the supplement book for my tester.

@guitargrump

If your TT is the one you posted pics of and referenced in post #3 of this thread, I'm willing to bet you can test your 6EU7 as 12AX7's without an adapter.

https://www.audiokarma.org/forums/i...retro-70s-battery-powered-tube-tester.980783/

https://bama.edebris.com/manuals/century/fc2/

 

[guitargrump]

@guitargrump

If your TT is the one you posted pics of and referenced in post #3 of this thread, I'm willing to bet you can test your 6EU7 as 12AX7's without an adapter.

https://www.audiokarma.org/forums/i...retro-70s-battery-powered-tube-tester.980783/

https://bama.edebris.com/manuals/century/fc2/

I still have the filiment checker it's untouched on the whatnot shelf... I just figure the 12ax7 to 6eu7 adapter will reliabily tell me more in my tube tester...

Thank you I downloaded the schematics manual tube chart and bookmarked the archive...

I actually just found the original century instruments tester circuit diagram stuck between two pages of one of the tube charts when I found out no 6eu7 test setting was available in any of my three tube charts...

I have been replacing the tester's capacitors and resistors already but there were some that I couldn't make out the colors of the codes too much patina....

Same results in this PDF tube chart... No 6eu7 setting...

It was a pleasure to read the original century instruments tube tester manual Thank You...

there's even a TV cathode tube tester cable...

I test my 6n2p-ev tubes with a 12ax7 adapter too...

 

[guitargrump]

Boy I can't believe I actually used to love working on computers...

My HP printer had been giving me trouble wouldn't print anything paper feed sensor errors...

I finally went and bought another cheapo Cannon printer after wasting way too much time on trying to get the HP printer to print...

I don't understand exactly why but I can't follow a circuit diagram very well on a computer screen...

So getting the circuit printed seemed like a easy task to get done so I could get started on the basic transformer and circuit checks...

Lol simple tasks sometimes become such stupidly Big Jobs...

sometimes it's the little things...

 

[trainbuftony]

Honestly computers frustrate me to the point I won't even play with them.
If I need a schematic of a something as simple as a 9304 I just draw it myself on graph paper to save time. Its generally more accurate too because its exactly what's in front of me. If I spend more than an hour trying to get the computer to spit it out, the pencil method ends up being faster.
It can be very relaxing to doodle your own schematics...