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Zenith 6BQ5 finished!

Thespeakerdude8

Thespeakerdude8

Super Member
I'd been putting it off for a while and decided to finally finish re-wiring this zenith 7D31. All I replaced were the .1uf coupling caps and some out of tolerance resistors. The 12AX7's anode resistors, oddly enough, were all shot and tested into the Mohm region (original 68k.) All I had were 75k's, so thats what I used. The OT's are from a Conn organ amp (PP 7868's) and measured 6.6k A/A. I still have the original OT's and I want to try to figure out how to wire them up because the original configuration was very confusing.

The amplifier sounds very good actually, and has wonderful imaging and sound stage. It is also dead quiet and I can't tell its on, other than the heat it produces on my computer desk. The original circuit is a paraphase splitter, and I wonder if I should leave it as is to keep the Zenith heratige, or change it over to the Magnavox 6BQ5 version?

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not copper chassis, but metallic copper paint. Original chassis was steel and had some pretty bad stains on it.
What say you to 1/2" oak skirt, or 1" oak? Trying to decide between the two sizes..
 
byoung334 said:
I vote for 1" oak like this one I modded, A Magnavox 9300
Click to expand...

Thanks! I was leaning towards 1".. :thmbsp:

Not bad sounding at all for $5! It was a console pull from goodwill. I just stuck the Amperex 6BQ5's that came with it back in it :music: Wish I had a quad of mullards though :scratch2:
 
I wouldn't bother changing out the output tubes. Those Amperex EL84 tubes probably sound just like their Mullard brethren. Made to essentially the same design (all by Philips).

I WOULD be on the lookout for a pair of Mullard, Valvo, Philips or Amperex input tubes. That'd make it REALLY complete, IMHO. :thmbsp:

Regards,
Gordon.
 
I'm using JJ 12AX7's right now and they are the best sounding of my bunch. As far as output tubes go, I don't really like these Amperex's very much honestly, they were just the only quad I have. They test ~80% and I didn't like them much in my SE amps- the bass seemed grainy. Midrange was nice and so were the highs, but they don't sound as good as my Blackburn Mullards. Unfortunatly, I only have one pair of those.
 
Sweet.
And The OPT swap makes it better than the Zenith design I'll bet.
A surround of some sort and rubber feet will go along way to a proffesional look.
Time for a 12 step program soon. :D


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Nice work on that. :thmbsp:

Did that start out as one of those with the strange OPT interconnects & feedback, hence the change out to the Conn OPTs?
 
I was just thinking, how about a nice wooden frame around it. Amps with metal sides like that are fairly easy.:yes:
 
Curious- has there been any tweaking done on the feedback loop? That kind of thing can be used to "null out" a lot of the perceived differences between output tubes. It's amazing how much BETTER and more CONSISTENT my 6BQ5 amp (on a repurposed Pilot chassis) with the 6973 transformers sounded, after I upped the feedback a bit. Bass became a LOT better defined, and it became a lot more tube-independent...
 
Thanks for the replies guys. Tinkerbelle, yes it is one of the amps you mention. I have not figured out the silly OT's that it came with, so I replaced them for now. Not sure how the tertiary winding for the feedback affects the amp.

Gordon, I have not had a chance to mess with the feedback.. It has 22k/.047uf from the 8 ohm tap to the cathode of the first 12AX7 stages. I will post a schematic and maybe you guys can help me a bit with the NFB loop?
 
Nice amp, Does it sound better now with the new trannys? I need to find some for mine, I dont think it remains stereo with the factory setup. Kinda wierd if you unplug one of the inputs it still produces sound from both channels. I'm interested in a feedback setup for mine too. I made a quick wood base for mine and I think I'll end up painting it with some hammertone paint or something since mines kinda pitted too and I think I burned through whatever its plated with trying to polish up the chasis. Heres a couple kinda lousy pics with my phone... The wife likes to hide the camara from me for some reason.



 
Nice work on yours too! I am going to use the oak face from an amp I never finished and some new oak sides similar to what you did. I have the original COOL chrome knobs from the receiver so I am going to use one for a volume control.

The OT's sound pretty decent! I have about 6 db of boost on my computer's master EQ at 20hz (small room with tile floor tall ceilings and poor response) and it digs pretty low and seems to start saturating around 10 watts or so. If I keep the EQ flat, the amp dishes out close to 15-18 watts/channel it feels like. I still need to add cathode bypass caps to the outputs which will help output power and LF response tremendously. The original OT's from what I remember sounded pretty good, but they had less iron by about 6 laminations each so the LF would have been more limited. There is surprisingly no crosstalk with the original circuit after replacing the OT's.
 
I keep forgetting to ask- what size screws did you use to mount the tube sockets? I'm working on a very similar Zenith amp (7K31) that I stripped the chassis for painting, and I don't have the tools for doing rivets. My local hardware only has down to 4-40 screws, and they are a bit too big.
 
I don't remember what size I usually use but the local ACE hardware and Do-It-Best has a nice selection of small stainless screws in the bins. I didn't however use them, I actually used those black coarse thread screws used on plastic electronics. They hold just fine in the fiber sockets for the 12AX7's, and work for almost everything else without nuts. The only thing I used machine screws for were the terminal strips and the bias adjust pots.

What do you guys think about eliminating those bias adjust pots? Or simply going to -12v fixed bias?
 
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