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Which console amp do I do first? RCA or Maggie?

avguytx

avguytx

Lunatic Member
I have a few different console amps that have been sitting idle and I really need to get one rolling soon. If you guys had the choice and had these 2 amps, which one would you do first? Which one sounds better or is there a difference? Pros and cons? I have a Magnavox 9304-20 and an RCA RS-177 (R I think?) and one of them needs to get started on. I have some solid oak for making a wooden frame around it, I just ordered Banana style jacks for the speaker terminals, a fuse holder, a rocker switch for power and some nicer board mount RCA's. I know there are caps (and resistors?) that need to be replaced which is no problem. What about the diodes? I'm a tube newbie so I wanted to ask the pros so be gentle!

I would imagine that both had some sort of equalization circuits in them that will need to be removed so, I know I'll be asking for help on that. I will be using a Carver preamp for the most part with either of these amplifiers. Attached are a couple of pictures of each amp and if I need to snap a couple of others, just let me know. I am really excited to be able to listen to some of the older speakers I have on some tube power...especially my Electrovoice Interface C's! Both amps were fully working when removed, also. The RCA seems to be heavier in the transformer department. Does that make a big difference in the two?

Yeah, I know. I still need to clean up the RCA amp. I hope it turns out as well as the Maggie did.

Thanks for any and all help!



Here's a couple of pics of the Magnavox amp

002b.jpg


005c.jpg



And a couple of the RCA amp

001.jpg


004.jpg
 
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I'm no expert and a newbie to these old console amps. That said, I'd work on the Magnavox. You've already have the chassis cleaned up, there's more space to work under the chassis, and those EL-84 amps are known for their great sound. That RCA has some pitting that may be hard to get looking as good as that Maggie chassis, unless you want to paint it.

I'll be watching your thread, for I am preparing to start on an old set of 6V6 maggies.

Good luck, and I look forward to learning with you. That Maganavox looks very nice, btw.

Ken
 
I think that Ken called it. El84 (6bq5) amps are really very nice. Do it right and you should end up being happy.

I also like 6V6 based amps too. Similar warm characteristics to the El84...just a bit more linear.
 
Man, I was hoping to get more input from you guys that may use these amps! I was going to order the replacement caps and resistors today so I could have them here by this weekend! Keep those opinions coming and thank you both for replying.

So the Maggie is the preferred of the two? The RCA is quite a bit bulkier and the room inside to work on it isn't too bad. But, it would take a lot more cleaning on it to get it looking right. Most likely it would take stripping it down and painting it.
 
The RCA is really hard to drive with just a regular pre (it really needs another tube or two as voltage amps). The 6BQ5s in the Maggie look a lot sweeter too (smoky :yes:), allthough the flash looks a bit worn on the rectifier (no biggie you just might want to get one while you're at it). The RCA output iron is probably a hair better, but the 6BQ5's look a bit tired. Either will be nice when fixed up and tweeked. :thmbsp:
 
Thanks jaymanaa. So it looking at the underbelly of the Maggie, I know that I replace the capacitors but what about the resistors? What are the tan discs? They look like poly switches like the ones used on tweeters for protection. Are they caps?

Also, did the 9304-20 have the EQ circuits built into it that will need to be bypassed? I've got to go back and do some more reading on this amp...I think I have some stuff downloaded on it.
 
The tan discs are ceramic capacitors which should be fine. I don't have any first hand experience with that particular Maggie, but I don't imagine it would have any EQ circuit that you need to worry about. From what I can see, it looks like it has RCA inputs that you can just plug straight into. You will probably have to figure out the power switch circuit (the amp was most likely turned on remotely from the tuner). A schematic will help a bunch (if you have one and can post it). I usually measure all the resistors and replace any that are off more that 10%, being careful to do both channels the same way. Ask lots of questions, and I'll help as much as I can, as I'm sure others will. :thmbsp:Jay
 
The ceramics are probably good as far as their values go...but they are also not the best sounding caps out there. I don't always replace all ceramics but if you do, then silver mylars come in the usual small values and sound better than ceramics. It's really a matter of what you want to do for sound and not a safety/working correctly issue.
 
If you look at the pic of the underside of the Magnavox amp, you will notice a green and white wire coming off the feedback loops to the molex plug on the right.

They more than likely went to a tone compensation circuit in the pre-amp/tuner.

When I got 9300 series Magnavox, I took it to a friend and put it on the bench.
One of the driver tubes,(or both) was overdriving the output stage. I can't recall if it was into oscillation or just overdriven. It didn't look good on a scope.

We pondered around and figured out that tone comp control is part of the feedback, daisy chained through the pre/tuner. He took a couple 1K R's and fixed them to ground after the driver tube cathode resistors.

The extra resistance worked. Then the compensation cap value was determined for the feedback, using square waves and a scope. The stock feedback R was fine.
Around 1000 picofarad was the value, I used silver micas.

The stock coupling cap values are .0015 or .0012uF? The ceramics.....the black ones are either .047 or .47uF.....
I think I remember using the same value coupling caps for each section, or used way too low a value for one section and came with a big motorboat.
That's when I was shown 1 divided by 2x3.14xRxC......

One of these days I'll rebuild mine again, I think the cap can is pretty much used up.

My main project of concern is to getting a Eico ST-70 configured as a power amp. And to quit oscillating with feedback connected.
It won't oscillate without feedback, though it don't sound too pretty...now I'm off topic....

I still got plans for the PP Maggotbox however.....Yahoo groups Magnavox friends has most the schematics one needs, including the 9300 series.
 
I just picked up a four 6V6 based Maggie last night. Looks like we will be traveling similiar paths soon.

I'll be watcing this post and others like it and asking many questions as well.
 
jaymanaa said:
A step by step thread (or 3) down below in DIY would be cool to watch and participate in. :yes:
Click to expand...

I'll get one of those going when I start mine. It's covered in about a half inch of dust right now so a clean up will be the first item of business.
 
OK......here we go....

I took a "survey" of all the parts on the underside of the Magnavox 9304-20 and have listed it below. A few make sense but some just look like part numbers but you guys may know what it means. This is the exact writing that are on each one. If I can get the list of caps/resistors together, I can add them to my (already free freight) order from Parts Express if they carry the ones you guys recommend. Here it is...

Ceramic caps (disc shape)
  • 1) 1500 10% 244
  • 1) -U- RMC .01 +80-20 1400v (biggest ceramic beside power transformer)
  • 2) RMC 100 N1500 20%
  • 1) Solar 0015M Z5F

Electrolytic cap(?)
  • 2) .047 400v 20%

Resistors
  • 1) 240080-49 100
  • 1) 240080-80 2000

The large cap on the top side has numerous different ratings on each line of it so here is everything that's written on it top to bottom per line.
  • 270021-56
  • 40 MFD 450VDC
  • 30 MFD 450 VDC
  • 10 MFD 450 VDC
  • 20 MFD 25 VDC
  • MAX. Oper. Temp. 85 C
  • GI (it looks like) 2406233

I also see from looking down on the top of it that this amp does NOT have the balance control...at least where the word "BAL" is. But, it DOES have a control for "HUM" on the far right side above the molex plug.

I can take close up pictures if need be. Any help on these parts will get me that much closer to starting so help a brotha' out! :D I'm excited to hear it on some older speakers and I do greatly appreciate any and all help.
 
OK, I'm looking at the underside picture you posted above. You need to measure the 240080-49 resistor with an ohm meter (it should read real close to 100 ohms). The other big white one should read 2000 ohms (or real close). You will want to put nice caps in for the two .047 ones. Orange drops would be fine, or you could spend a little more for something fancier if you like. Just make sure the voltage rating is the same or higher (400 or 600 would be fine), and the uf is the same at .047. The two ceramic discs that are connected to pin 2 of the EL84s would be a good candidate for upgrading as well. It looks as though one of them may have been replaced at some time (what are the numbers on them?).
 
jaymanaa said:
How about some close up pics. :yes: Did anyone ever find a schematic that they could point me to?
Click to expand...

Closer than the one in the first post? I can do that. Give me a little bit....

jaymanaa said:
OK, I'm looking at the underside picture you posted above. You need to measure the 240080-49 resistor with an ohm meter (it should read real close to 100 ohms). The other big white one should read 2000 ohms (or real close). You will want to put nice caps in for the two .047 ones. Orange drops would be fine, or you could spend a little more for something fancier if you like. Just make sure the voltage rating is the same or higher (400 or 600 would be fine), and the uf is the same at .047. The two ceramic discs that are connected to pin 2 of the EL84s would be a good candidate for upgrading as well. It looks as though one of them may have been replaced at some time (what are the numbers on them?).
Click to expand...

So the resistors can be measured with a DMM with them in place? Where do I get the "orange drop" caps and if I spend a little more, are we talking something like a typical 400v non polarized poly cap per chance? IF I replace the ceramic disc caps, do I use that same type or go to something like the orange drop caps?

Thanks for the reply!
 
For the big silver cap, there are some different options. Here's a link to AES http://www.tubesandmore.com/ You just have to figure out how you want to approach it. Usually you can't find an exact replacement, so you have to do some creative stuff. I try to find a can cap that will mount up right and maybe have two of the values I need. Then I will add a couple axial (or radial) seperate capacitors under the chassis. This retains the look up top while getting everything covered electricaly. It never hurts to use a slightly higher voltage rating here too (say 500 or more) due to the fact that house line voltages have typically risen since the time the amp was built.
 
The Maggie looks like it would be easier to work on. It also doesn't have those old wax and paper caps and a single filter cap to replace vs two on the RCA. Underside is a lot more spacious too, plus it looks good.
 
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