Goin all the way with these...can't believe how good they sound
RCA Theater Amps
- Thread starter Supercool!
- Start date
century tek
Addicted Member
Tease! 
More pics. 
More pics. 
Hey knockbill... any suggestions on the rebuild... I'm just starting...
knockbill
Lunatic Member
I think the rebuild thread is on here someplace... Hardest thing I had to deal with was finding a readable schematic, as my chassis was pretty trashed when I got it (inside bottom cover)... If you have one you are way ahead of me... I ended up finding a PS schem that matched mine from another RCA amp, and after I was done, someone posted the complete correct one!!
Couple things I "think" I remember! PT and FT on mine had 3 pair of primary input windings each, and it was set up for 117VAC, which was too low for modern line voltage... Output tubes would red plate, until I rewired it for 130V(IIRC) line in... Also, keep the voltages in mind for the caps,,, 600V is just marginal in some places, and some are in series to get the proper V,,, specially if its run without tubes in place... 6146s can be tricky to deal with, so I set and check the bias occasionally...
There are at least 3 or 4 versions of 6146,, (6146,, 6146A, 6146B, and 6146W (again I'm going from memory) I few of them are not subs in some chassis,,, mine uses 6146As only... I grab em at hamfests as it seems some ham radios use them...
Let me know if you can't find the build thread,, I'll see if its still around...
Some people convert those amplifiers to 6550, in fact, I think RCA even sanctioned this modification. I would be very tempted to do so if these were mine.
I have a pair of the older MI-12188A amplifiers with push pull parallel 807 and enjoy them very much. I wish you good luck with your project!
I have a pair of the older MI-12188A amplifiers with push pull parallel 807 and enjoy them very much. I wish you good luck with your project!
6550 is still made by lots of suppliers, and it doesn't have the reliability issues the 6146 is known for. Plus,a pair of KT88 would look way more cool than the ham radio looking 6146. I'm very conservative about modifying things, but this conversion to me seems like a no brainer. The only reason this amplifier would have been originally equipped with the 6550, is the 6550 was never made by RCA - it's a Tung Sol tube. Really a moot point at this stage in the game.
Be sure to use an NOS rectifier though, new production isn't really capable of handling the voltages in an amp like this one.
In my RCA amps I went with Russian PIO capacitors for coupling capacitors. I made sure to test them all for leakage first, and there was none. The old bumblebee caps it came with were junk. For electrolytics, I'll let someone else figure that out - I'm inclined to reform the old ones and if they measure OK leave well enough alone. If they're bad, maybe under chassis radials on terminal strips. In my amplifiers none of the resistors were bad, and all measured within tolerance, so I left them alone. Instead of modifying the chassis I just soldered a couple #18 wires to an RCA jack and attached it to the input terminal strip.
Be sure to use an NOS rectifier though, new production isn't really capable of handling the voltages in an amp like this one.
In my RCA amps I went with Russian PIO capacitors for coupling capacitors. I made sure to test them all for leakage first, and there was none. The old bumblebee caps it came with were junk. For electrolytics, I'll let someone else figure that out - I'm inclined to reform the old ones and if they measure OK leave well enough alone. If they're bad, maybe under chassis radials on terminal strips. In my amplifiers none of the resistors were bad, and all measured within tolerance, so I left them alone. Instead of modifying the chassis I just soldered a couple #18 wires to an RCA jack and attached it to the input terminal strip.
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Yeah, I do like the 807, I've had 8 of them heating up my room for 16 years already! If you want to see a real ugly duckling plate cap type tube have a look at the Russian 6R3S, it's like two 807s in a too small bottle. They don't sound bad, though!
Back to the original subject though, I think the issue in this amplifier is the 6550/KT88 is more durable.
Back to the original subject though, I think the issue in this amplifier is the 6550/KT88 is more durable.
PakProtector
AK member
Not sure if RCA corporate sanctioned 6550's but that amp's Chief Engineer sure did...
You'll probably find it useful to change the 0C3's for 0D3's just as he did.cheers,
Douglas
Can you elaborate??
PakProtector
AK member
The regulator tubes in the existing amp are type OC3, which set Vg2 to about 210V, which is what the 6146 needs.
If you change the output tubes to type 6550, you will need a higher Vg2, about 300V,, hence the different regulator tubes. Also may need to adjust the series resistance for the shunt regulator.
Xlnt! Thank you. That was what I was trying to understand...I wasn't very clear..sorry. I'm wondering if the whole mod is documented somewhere? I've decided to stick with the 6145s for now because they seem to be plentiful and cheap..and sound good...but I'm not ruling out a second tier resto that might involve the change over.
I attached this PDF to a thread a few years ago. Resolution isn’t best but may be serviceable.
http://www.audiokarma.org/forums/index.php?attachments/rca-mi12182_b-pdf.457978/
s-petersen
Scott
That's different. The only feedback is from the plates of the output tubes to the cathodes of the drivers.
