Console Amp

[Thatch_Ear]

Jim, your brothers amp and mine. Same amp, I put OPTs from a Curtiss Mathis and moved the tubes around, put new guts in but same schematic, same amp. Since I moved stuff around I used auto body stuff on it, put in military ceramic sockets, a gain control pot etc. The amp I pulled from the console had a fairly crispy OPT from the closeness of the 6BQ5. The OPTs I put in were larger so I had to move things.
I pulled some pics from the auction to show before and after. The amp your brother bought is in much better shape than the one I got.
I think I pulled it from an RCA but it has been a while and I don't know where the Sam's is, not that it really matters.


Well here goes: His

 

[Thatch_Ear]

Mine

 

[Thatch_Ear]

His

 

[Thatch_Ear]

The back of mine

 

[Thatch_Ear]

Underneath, mostly stock.

 

[Thatch_Ear]

And all the junk I got into mine. The amp works and sounds OK. These parts are all from a electronic closeout store and are not audio grade. So while it works fine and sounds good it isn't in the same league as amps made from all quality parts. The OPTs were free, the switch and fuse holder pulls, the sockets used and around 25 cents ea, so not counting Bondo, paint etc I put around $25 or so into this. Main thing was to build it from gutted chassis back into an amp, and like I stated before this is based on the original schematic.

 

[jt1stcav]

I'm very impressed...

Tom really likes his lil' amp. It sounds wonderful, it's smooth with lots of clarity and detail, and still maintains punch in the bass region. The seller seemed to know his stuff, and did what we think is a rather good job of restoring this Magnavox sleeper!

Since you did your work on your amp, have you noticed an overall improvement in its sound quality? I don't think Tom will have any other work done to this amp (like me, he's not electronically inclined to do anything mod-wise). He may do some tube rolling later on, but for now the original Magnavox-stamped tubes are doing their thing in style.

Our dad restores antique tube radios and has been working on tube gear for almost 50 years. Tom may have our dad give it the once over, and maybe do whatever he thinks it might need to bring it to spec (if it's not already). Tom was thinking of having the white plastic connector disabled or removed permanently, and maybe a wooden base built around its chassis. In any event, Tom is very pleased with this 40 year old SET amp, and it really brings out the best in his Klipsch KG 3.2s!

Thanks for sharing your modified Magnavox clone...it might inspire Tom to do something with it in the future...we'll see!

 

[Thatch_Ear]

Like I said one of the OPTs was toast. This amp was mounted sideways in the console with the OPT close and just above one of the 6BQ5s. Lots of use and possible drift on bias with resultant high heat....... well just speculation. It worked but I didn't trust it not to short and fry my ass, so I got a junked reciever from another console that was SE 6BQ5, pulled the OPTs and rebuilt the one like your brothers.
I at that point had to move the tubes because the OPTs were larger and the 6BQ5 would have been touching the waxed paper side. If I remember corectly I shifted one 6BQ5 to the socket of the 12AX7, moved the 12AX7 to a hole left by a plug and shifted one OPT back from the 6BQ5 that stayed in place.
It is a linear pot for volume (not good) and what ever I could buy cheap for caps, BPs etc. It sounds OK and a 5751 in place of the 12AX7 helps a lot.
Seriously though, I used parts from 2 amps, at least 2 stores and used good tubes when I was running it. It probably doesn't sound any better than your brothers but it is sure a lot better than what I pulled from the console.
These are really a good place to start with tubes. The parts used in them were not the best but the schematics are usually great. Upgrading isn't that hard. It just takes money. When I put that amp back together the only original parts used were the chassis (modified) the choke and the power transformer. Everything else came from another source and most if not of high quality were new.
It was fun. I got the amp out of a console at a garage sale for $10. Got the Sam's at the library and photo copied it, got the other console trash from a friend and had most of the stuff to rebuild it at home. Paint from Pep Boys.

 

[jt1stcav]

I was just thinking...this is really a single-ended pentode amp, correct? I said SET earlier, but the EL84s are pentodes, are they not? I think I read somewhere that this amp could be wired for triode operation with a little modification, but then it'd be even less output power than it already is, correct?

 

[Thatch_Ear]

It can be triode strapped and it would mean less power. I like the SE 6BQ5 fine, haven't heard one that was triode strapped and never felt the need to do it to one of my own.
I would triode strap a 6550, but if I want something with the power of a 45 I will listen to 45s.

 

[tubino]

Triode-mode pentode

I've had a lot of fun with little SE console amps and -- even more -- with underrated Japanese-made non-console units using 6BQ5, 6BM8, 6AQ5, etc. Occasionally I've used them as triodes, but it has never made a big difference, certainly not as noticeable as when you do EL-34s. No harm in trying it, obviously, but if I like how it sounds as a pentode, I wouldn't bother monkeying with it.

To me these little units are a fun cute way to get most of the good sound on the cheap. But if you really want to start making progress into that last 10 or 20% of high-end potential, with full bandwidth, power AND grace, I'd say start over with something else.

But fer gosh sakes, have fun with them!