Tube console help

barberbasses

barberbasses

New Member
First post here, long time reader and audio nerd.

I decided to start my journey into the tube audio format. Before I spend multiple thousands of dollars on a brand name boutique set of mono blocs, I've looked for an alternative that may include a little elboy grease. I dabble in woodworking and am an avid antique motorcyclist so I know my way around tools...for the most part.

A console stereo came to my attention that was headed to the trash pile, it's cosmetically in bad shape and has a significant amount of wear and tear. I know there are guys that don't want to part these out but......this one is in bad shape so it's what I would call a good donor. I wasn't looking for this as an option into the tube arena but sometimes these things land in your lap.

I took a few photos and tried to get some numbers, but without taking it out of the console this is what I got.

Here's where I need the collective knowledge of the Audiokarma empire.

Is this a mono bloc or a stereo amp?
Can anyone identify a manufacturer, or at least the type of tube circuit we're looking at?

The back of the console shows a tag with wattage at 115, seems high for an early sixties tube amp in a console.
The console itself is a Curtis Mathes....There's no sound coming out the speakers, though some of the smaller tubes have power to them. one of the tubes was taken out and was laying on the bottom of the unit. I didn't touch anything as i wasn't sure if there was voltage hangin around which is why the photos aren't that good.

Any help is appreciated.
 

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It is a stereo amp. what brand is the stereo? Whatever brand the console is, that is the brand of the amp.

It looks like the power transformer has been replaced. The original one might have burned up due to bad capacitors but who knows. It needs to be recapped anyways if all the capacitors are original.

That wattage is not the speaker output watts but rather the watts pulled from the wall outlet. This amp uses what looks to be four 6BQ5 output tubes (2 ea for each speaker). Its probably good for about 10-12 watts per channel.
 
Due to the age, I had planned on recapping it. Any idea why type of circuit it is?

Thanks for the quick response century!
 
barberbasses said:
Can anyone identify a manufacturer, or at least the type of tube circuit we're looking at?
The console itself is a Curtis Mathes....
Click to expand...

century tek said:
what brand is the stereo? Whatever brand the console is, that is the brand of the amp.
Click to expand...

The amp looks to be original so I'm going to say it's a Curtis Mathes, that being said I don't know if CM had other entities make their amps for them.
 
Thanks guys. I'll get it home and pull out the electronics, clean it, get some better pictures, and start to order parts.

I'll keep this thread going and provide photos and overall progress for the forum.
 
No sound is because it is missing its rectifier tube. That one's gonna be a 5U4GB.

That power transformer looks like a replacement. I have seen hundreds of CM tube stuff but never with a power transformer mounted like that using pin risers.
 
I got the unit out last night, documented tubes, their placement, and took a lot of photos for reference, adding just a couple here. inside the console was a tube diagram, I pulled it and have it in a plastic liner to keep it intact as it's brittle.

more to come.
 

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Is there a model number on this? I can't see the tube diagram well enough to tell. Or it might be either on the chassis itself or on the back of the console somewhere? There is probably a Sams schematic for it although the voltages would probably be inaccurate since the PT has been changed.

Definitely worth fixing up.
 
6bq5 push-pull stereo. I'm going to guess that the amplifier itself isn't quite complete on the chassis though. Its short a tube for a typical configuration. My bet is that you'll find a connection from the speaker terminals up through the tuner umbilical to the last tube on the tuner chassis.

If you can find a chassis number stamped on there someplace, a schematic might be found for it

Power trafo doesn't look original but its probably considerably more beefy than the original. The stockers always looked a bit feeble to me, and the fact that they had to add fins to cool them enough tells me it was probably just barely enough to start with.
 
I'm not sure it is actually push-pull stereo.

As gadget says, it is missing a tube for that configuration.

Also, there do not seem to be enough wires around the OPTs.

Parallel SE would explain both.

There would need to be at least 10 wires total (CT primary and at least one secondary winding each) associated with the OPTs if it is stereo P-P.
 
I don't think any top of the line console and amp manufacturers ever made anything parallel on the output tubes but I could be wrong but I have never seen it in my 50+ some years. This chassis looks like one of those Zenith or Voice of Music chassis's that I have seen with four output tubes and one preamp tube. The other driver(s) needed are usually on the separate tuner or preamp chassis.
 
. More power output with the same tube count, and it allows for less filtering in the supply if you go push-pull. As far as the mfg is concerned, more power output with less cost on their part is a win all around.
 
Definitely push pull. From the under side picture, you can see two black wire that go to the two output tubes. OPT has 5 wires, plates, CT, and speaker.
 
so, barberbasses, it looks like you have a great find there, and a great introduction to tube amps.

Apparently, the original power transformer was underrated, as evidenced by the fact that yours has been replaced with a larger one, and the large heatsink "ears" on power transformers in the other thread.

I would do exactly what was done on the last page (as of right now) of that 2014 thread -- specifically:

-- Drill or punch another hole for another 12AX7 (See below*). Install socket.
-- See if you can fit that larger power transformer down into the chassis. (Cutting required.)
-- Completely re-cap. Filters and coupling caps. Installing separate, modern caps will get rid of that big can cap, and thus help make room for the bigger power transformer to mount flush. If you end up with an oddly shaped hole, you may have to do a little metal fabricating.
-- Check all voltages. Did they keep the fixed bias arrangement? Everyone here will probably tell you to delete it, but if the parts are there, and the new power transformer has the windings, I'd recommend keeping it. I was talking about this for a 6BQ5 P-P amp in another thread, and from the schematic in the other thread, it looks like they did exactly what I was talking about.
-- If you do keep the fixed (-) bias, then replace any old (selenium) rectifiers, if present.
-- Install nice speaker terminals, RCA inputs, Line cord, Power switch, etc.
-- I would not add/keep a volume pot, but that depends on preference, and what you are going to drive it with.

Incase you have not worked on these before, the warnings about high voltage in tube amps are, of course, applicable.

*Not sure why the schematic in the other thread shows a 12AU7. Should work with 2 12AX7's with proper component values.
 
Allright guys, stick with me close here and I'll keep you posted on my progress. Here's what I'm thinking for an end result, I won't lie but I want to do as little changing as possible simply so I can get this up and running so it's safe, stable, and sounds relatively good. This is my first one of these so I'll have to take it step by step.

I'd like to keep the Tuner/Preamp all together, and hooked to the power amp. I've added a photo with the two units side by side on my bench. I want to get a solid piece of cherry or mahog and mount them with some good spike feet under the wood mount. The entire unit would able to sit on top of my current console and could be disconnected and stored for Wife Approval Factor level 2. I'd prefer not to cut anything just yet, I'd like to clean it and get it running safely. this entire unit has already been messed with a little so I'd like to keep it as stock as I can....."it's only original once." see bench photo for layout. I'll leave the tops of the units exposed.

Changes I'm planning:
- recap
- rewire the speaker leads
- rewire the RCA's that connect the signal between the two units, the wires are in bad shape
- clean as much as I can without changing around anything at a high level.
- new power cable with a ground

The only thing I may do that would alter the main config would be to rewire the speaker leads, which come out the "front" of the unit as it's set now, and move them to the back of the unit so they come off the rear. I may do this with the RCA's as well....

My current set up is an NAD CD player and Receiver. I run a Garrard Turn Table with a Grado stylus. My mains are Monitor Audio RS-6 speakers, for vinyl I'll hook up a set of full range speakers( photo) I made based off the website Frugal Horn. I currently have an NAD phone preamp.

http://www.frugal-horn.com/index.html

First order of biz -
Schematic

#2
order caps

#3
clean

#3A
clean more

#4
practice my soldering.....on something other than this unit.

#5
locate inexpensive tube tester

I've ordered electronic parts from https://www.surplussales.com/ for other projects, are there any other prefered sites you guys like? stupid question but admittedly I'm new to this so cut me a break.
 

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FlaCharlie said:
Is there a model number on this? I can't see the tube diagram well enough to tell. Or it might be either on the chassis itself or on the back of the console somewhere? There is probably a Sams schematic for it although the voltages would probably be inaccurate since the PT has been changed.

Definitely worth fixing up.
Click to expand...

35A is stamped on the chassis on the RHS of this photo.
 

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I usually source most of my bits and bobs from Mouser or Digikey.

Looks like there is a Sams for a 35AMX tuner chassis, can't quite tell if its the same as what you need or not.

Is there a number on the amplifier chassis as well?
 
gadget73 said:
I usually source most of my bits and bobs from Mouser or Digikey.

Looks like there is a Sams for a 35AMX tuner chassis, can't quite tell if its the same as what you need or not.

Is there a number on the amplifier chassis as well?
Click to expand...

No number on the amp chassis
 
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