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thebes

Restore or Mod A Maggie 6V6 Console Pull

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Gift from a friend I visited last weekend. I have the preamp/tuner and the amp, but am primarily interested in the amp.

 

It came with quite a layer of dust, but all original tubes except for one 12AX7. To be honest, I haven't gotten around to testing the tubes yet.  Under the dust was no rust, and underneath it's very shiny and looks to be all original, so probably never serviced.  My buddy's had the console for over 20 years but has never fired it up.  Has a choke and a 5U4GB rectifier, push pull and somewhere around 10 to 12 watts.

 

This is amp 175-67.  I've done a bit of searching around the web and seen quite a few mods, but I've also heard that these can be sweet little things un-restored, just recapped, safetied etc. I'm thinking the power supply wouldn't even need beefing up because there would be no current draw from the preamp/tuner.

 

Any thoughts from you folks one way or another?  Thinking of it as a deep winter project.

 

Here's some pics before and after the initial topside cleaning and a shot of the underside.  I'll post a schematic in a minute and I have a Sams Photofact puled from the web.

post-12696-0-50880000-1448146470_thumb.j

post-12696-0-10120000-1448146481_thumb.j

post-12696-0-41440000-1448146490_thumb.j

Edited by thebes

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Schematic for the various model numbers.  I've just glanced at it but it appears just minor variations in the power supply.

post-12696-0-23440000-1448146583_thumb.j

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Yeah I'm inclined that way, but maybe swapping out the OPT's or other things would yield greater sonic benefits, but the existing circuit might be quite good too. 

 

Hence my post. :D

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Notice the quasi-cathode bias. Where all four output tubes are tied to one cathode resistor and cap. The driver/splitter stage is called floating paraphrase, but I'm probably wrong. The PS has a choke, so that's kind of a bonus.

Makes me want to drag out my 9300 series, the 6BQ5 version.

 

Get it going stock, and then after that make it into a 6V6 single-ended parallel amplifier, with different output transformers. 

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I honestly wouldn't even get it going stock. If you are looking for a winter project I would just gut it and keep the iron and sockets and tubes. Once everything is removed you can then clean and polish the chassis. If the transformers test okay then clean and paint them too. There are much better topologies for better AC balance. A LTP w/ a CCS in the tail has near perfect balance that never needs adjustment. Maybe we can have you measure the load impedance of the output iron and see where we are at for options, if they are 5k then you could go triode mode and ditch the feedback around the transformer. If the load impedance is higher, say 8k then maybe stick with full pentode and either keep the global feedback or opt for another form like "Schade".

 

The only reason to keep it stock is for collector purposes where you don't want to ruin the value of the amp.

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The only reason to keep it stock is for collector purposes where you don't want to ruin the value of the amp.

 

Evidently, some must like that stock Magnavox house sound.

 

I would guess that Marty will use this console amplifier with his Cornwalls. I have the 9300 series Magnavox which is the 6BQ5 PP version. The power supply on the 9300 is close to same, 5U4, with a smallish choke. The amplifier has different type OPTs, which are still marginal in size. The driver tubes are 6EU7, which is a 6.3 volt low noise version of the 12AX7.

 

Other than that, the amplifier topology is the same. Quasi-bias, and the weird paraphrase splitter. I had went through and re-capped the amplifier with Russian Military caps, and replaced other bad passive parts.

The feedback had to be readjusted, because the stock feedback circuit is tied into the tone pots in the pre-amplifier/tuner of the console stereo.

I put a strong set of EL84s in the amplifier, and I listened to it off and on for around a year or two.

 

The amplifier sounded pretty good on my Cornwalls, with respect to midband and top-end. That was the thing, the lower bass was really light...at 10-12 watts a side with Cornwalls, I should be able to pound out the bass. Not the case...I have more deep strong bass with a DHT 2A3 amplifier. I should drag the amplifier back out again and give it another go sometime.

 

I dunno....keep in mind that Magnavox voiced these amplifiers for the home console stereo. Where you have a woofer with a stiff cone on a open baffle more or less. The lower bandwidth (Fs) of these types of woofers is around 50-60Hz at best. And the amplifiers are voiced around that.

I don't think the maggotbox console amplifiers were voiced/designed for flat bandwidth of 20Hz-20kHz, I'm thinking they were voiced for the console stereos that they were designed for. A guess...

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Can you post the schematic to your amp? I don't see the feedback going through any tone controls. The input paraphase looks pretty straight forward to me, 1/2 the 12AX7 is the voltage gain and sends it's signal to one half of the push pull output stage and also through a -26db potential divider (470k 6v6 grid leak and 22k) through the other half of the 12AX7 which inverts phase 180 degrees and brings the gain back up to feed the other half of the output stage.

 

We should just focus on thebes amp and not draw any conclusions from other completely different amps. Maybe the output iron saturates before 15 watts @ 20Hz but it still will pass 20Hz up to a certain point, with that said we could focus on building him an awesome 6 watt amp if he can live with that.

Edited by xxJPMxx

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Here I outlined the signal path, where it turns red is the -26db signal being sent to the other half 12AX7 which is another inverting gains stage bringing the signal back up and also inverting phase to drive the other half of the output stage.

 

OUTt4qc.jpg

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You can see how poor the AC balance can be, it relies on the second gain stage to be exactly the inverse of the 470k/22k potential divider. Gain changes with tube age, and all tubes will not have the same exact gain. For an average 12AX7 tube I calculate voltage gain of 23.69 for the second 12AX7 and with exact resistors of 22k and 470k I calculate -26.9db attenuation (-22.6 voltage gain). Not very accurate to say the least. So like I said ditch this design.

Edited by xxJPMxx

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Can you post the schematic to your amp?

 

I'd have to scrounge a schematic up from online, but I don't think it will show that part of the circuit unless one has the schematic for the pre-amplifier/tuner.

 

If you look at the underside of the amplifier in Marty's pics, you can see the tag strip at the bottom, with the feedback resistors and ceramic caps. Notice there is a yellow colored wire coming off each feedback R and C into the molex connector. I think that is what ties into the preamplifier/tuner.

 

What is strange, is the feedback values for R and C are different for each side.

 

 

 

We should just focus on thebes amp and not draw any conclusions from other completely different amps

 

Okay.

Edited by mike stehr

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What is strange, is the feedback values for R and C are different for each side.

 

What's even stranger is that I can't even see the 2k2 feedback resistor in parallel with the cap on one channel, all I see is a cap in series with a 470 ohm resistor. I see the other channel has the 5k6 resistor in parallel with the ceramic cap and then the series 470R after it.

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Interesting comments guys, now I have a nice splitting ( :D ) headache.

 

I'm getting a sense that the tube guys here are more inclined towards SET/SEP topologies. Thanks to copious help from Maynard, I've built two SEP's so would be inclined towards a push/pull build, and yes they would be run through either Cornwalls or Heresies.

 

Let me also post the Sam'so n this:

post-12696-0-15200000-1448225135_thumb.j

post-12696-0-86880000-1448225155_thumb.j

post-12696-0-56400000-1448225170_thumb.j

post-12696-0-62280000-1448225182_thumb.j

post-12696-0-45680000-1448225198_thumb.j

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OK I checked the parts numbers and they are the same as posted in the parts list attached above. 

 

Here's what I get for the OPS:  8000k CT 4-6ohm

 

Main Transformer Primary:    117v @1.05a

                               Section 1:  500v CT @.160a

                               Section 2:  5v @ 3a

                               Section 3:  6.3v @ 2.6a

                               Section 4:   6.3a

 

The choke appears to have .250a current 50ohm (?) DC Resistance and is 1 Henry

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Hello thebes, did you want to keep it as is and just service it or did you want to build something that performs better? I say keep the output pentode and reconfigure the front end, I have some schematics for you if you desire to go this route :D  

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Hey Marty!  I agree with JP that the stock circuit isn't overly wonderful.  I've restored many of these for guys.  They're ok, but as Mike pointed out above, the bottom end isn't prodigious.  I'd venture that both of your single ended amps will easily out perform this one in every respect, within their power capabilities of course.  If you decide to keep it stock, install a couple of volume pots in the two unused holes near the 12AX7s, and drive it directly with your CD player, or other source.  For a few dollars in parts you can at least play with it.  In replacing C19A/B, be sure to go with class Y caps, especially if you choose not to add a fuse!  Worst case, you can always blow it out on Ebay for a decent amount.  But, if you want a winter project, why not take JP up on his offer to give you a total re-design?  Even that will be easy to implement, and you may wind up with something that you really enjoy using (I still think my designs will be better sounding! :D  :D  :D  :D  :D- had to get that one in JP........).

 

Maynard 

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(I still think my designs will be better sounding!

 

 

Them are fightin' words Maynard!!!!!!!!!   :P  

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I gotta tell JP, Maynard's no slouch when it comes to ginning up some serious sond!

 

Tube wise I am the luckiest man on the Forum. Over the years I have had MIke Beaser, Craig Otsby, and Maynard all help me with various builds and restoration projects to name a few, not to mention countless other bits of advice from guys like Mike Stehr.  These poor souls have endured endless emails as "The Dunce Of Tubes"  created yet another way to screw up a build and short out my city's power grid.

 

I t appears that yet another masochist is about to offer themselve up for sacrifice, and who would I be to object to their self-destructive ways.  After all, I'm going to be the one running around the room with my hair on fire.

 

So let's get to started with some basics. What is this Pentode you keep referring to. I thought that was a type of tube.  Does this relate to the fact that I want to do a pushy/pully build, or does it refer to the four horseman of the Apocalypse?

 

Oh, and can I do one of your builds keeping the current OPT's  to cut down on costs?  After all I'm dating hot Twins with ever expanding monetary needs.

 

Oh and even though Maynard's not a particular fan of tubed rectifiers, I like them. Also I have this monster Hammond 193k, 2.6 Henry 300ma choke that's never been used just lying around. 

 

Oh and speaking of Maynard, he has taught me well and there will be a y gizmo, 3 pronged cord, and a fuse even though it really cuts down on the fun factor.

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Hey thebes, sounds good! Lets catch your hair on fire :D

 

 

 

 

So let's get to started with some basics. What is this Pentode you keep referring to. I thought that was a type of tube. Does this relate to the fact that I want to do a pushy/pully build, or does it refer to the four horseman of the Apocalypse?

 

You are correct in that a pentode is a type of tube. But a pentode is not stuck being operated as such, you can wire it's grid #2 to the plate and it behaves like a triode. The way the tubes are being used in your amp are indeed pentode. There are special output transformers with taps to feed the second grids, this mode of operation is called ultra linear, we won't get into that now.

 

 

 

h, and can I do one of your builds keeping the current OPT's to cut down on costs? After all I'm dating hot Twins with ever expanding monetary needs.

 

Yes you can, that was my initial intention :)

 

I will most likely present a few options for everyone to look over and we as a team can settle on the final build idea. I know Maynard hates push pull but he has no choice, he is participating in this one ;) In the end it will more or less boil down to trade offs. I like to do things cheap so don't worry about your "twins", I am resourceful and promise to not break the bank.

 

 

Sound like fun?

Edited by xxJPMxx

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Maybe like you mentioned...a different driver/splitter of some such. Like a long tailed pair with a constant current source.

 

I've used this driver circuit with decent results. But it's another type of paraphrase, and uses resistors for balancing. But seems to be a better set-up as far the divider is concerned.

 

post-4542-0-20680000-1448342645_thumb.pn

 

I'd mention the cathodyne type, but it's another resistor balancing act situation I'm thinking.

 

The quasi-bias is something Marty may consider changing, and have each output tube share a cathode resistor and cap. Instead of all four tubes sharing a cathode R and C.

It's another part of the Magnavox "house" sound I suppose...or a bean counter type of engineering.

 

 

 

 

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The quasi-bias is something Marty may consider changing, and have each output tube share a cathode resistor and cap. Instead of all four tubes sharing a cathode R and C.

 

I don't know how thebes feels about sand in his amps but if he wants to use the stock output tranformers it would be wise to get rid of any DC current through them for better low end response. Four IXCP10M45S, one for each cathode, all they need is two resistors and a bypass cap to set current. That way there he can just use any tube and they should still have nearly perfect DC offset. No more matched tubes to worry about. Thing with this is that you are stuck in Class A operation, which isn't a bad thing.

 

Another big question is the quality of the output transformers and having them in the feedback loop, empirically finding the feedback cap w/ square waves isn't too difficult if thebes is up for the task. Or we can try to come up with another means of feedback, or if thebes doesn't need much power and is happy with 6 watts out per channel then strapping the 6V6's as triodes is an option too. I will have time later to sit down and weigh the options.

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