magnavox console stereo

[Zephedone]

Anyone have any experience converting these to a guitar amp ?
Recently aquired 3 of them . 2 are maggies with 4-6v6gta output tubes and 6eu7 PIs . The other is an "Andrea" with 2 single ended el84 / 12ax7 PIs . All fired up but one of the maggies PT burned up in front of me . I found it behind a thrift store and it had been rained on several times . After inspecting the Xfrmer it was obvious it had water in it .
I'm unsure of how to wire it up [the maggie that still works] . I've already added a grounded AC cord , fuse ,PWR and Stndby switch along with 2 output jacks for each OT . Drilled holes for and installed input , volume and tone controls . I've built an 18 watt Stout with a turret board and scrabbled another one together from my ever growing pile of parts for "free" , or sort of free .
Not interested in the stereo layouts Ive seen so I'm wondering how to wire up the PI . I'll be using the Stout pre [cause I'm hooked on it ] to feed the PI . Changing the 6eu7s to 12ax7s . How to wire the input to the 4 6v6gta tubes to accept signal from the PI is where I'm having the most trouble . Also will this have any effect on the dual OTs .?

[Defdaddy]

Your one maggie sounds like what I use for a stereo, a model 8802 amp with the p/p 6V6s and 6EU7s. I actually like the 6EU7s, if for no other reason than the heater wires are conveniently off to one side - they don't straddle the socket like a 12AX7. And the 6Eu7s that Magnavox often used were made by Mullard. As far as I know that tube has about the same amplification factor as an AX7.

I never converted it to a guitar amp because.....
The separate preamp + tuner in one location and then power amp in another location is unwieldy.
While the PT had lots of filament amperage, mine does not put out as high voltages such as we use in the 18 watters.
The OTs are puny.
And it seems to do just fine as a console amp.

But feel free to blaze the trail and show us as you go.

I have the series 88 schem if you need it.

[Zephedone]

Thanks for the reply Defdaddy .
Would love to have the schematic as I have 2 of these chassis . One I've already started modding and the other with burnt tranny I will use as an amp for my cd/phono/tape deck . I have another tranny from my pile for this one .
I also had a 4 el84 version of this made by magnavox that I built 2 Stout 18 watters with the OTs . They both sound fine to me but not having anything to compare them to , what do I know ....
I lack some basic theory skills as the only electrical exp I have is changing out light switches and AC sockets . I can copy a schematic all day long , layout , build , fabricate , etc .....
have had pretty good luck with not letting the smoke out of anything yet on any of my "builds" and everything fires up first time ...so far ....
Since last post I went ahead and wired up PI as per a few old silvertone schematics I found . The PI is not really what I'm having a problem with as I've wired up several along with the pre in my eperiments with various old tubes that I found on the curbside during bulk trash day . A lot of radio tubes , 50l6 ,25l6, 35c5 etc .....
How to connect the 4 power tube configuration is where I havent had any exp . Tried leaving the pwr tubes as they were already wired and got a real nice sound , a little dark sounding , not as "chimey" as my 18 watt stout . Reminded me a little of an Ampeg SVT sort of . Next I tried hooking up pwr section as per silvertone schematic , I think 1485 is the model and the output was greatly diminished .....

[Zephedone]

By the way if you could send that schematic to zephedone@aol.com I would be much obliged .
Thanks, Mike

[Defdaddy]

Mike - email and schem sent.

DD

[Zephedone]

Thanks Defdaddy ,

The schematic is a big help . If I'd had it earlier things may have been easier . You seem to know a little about these amps so I'll run some ideas by you if you don't mind . What i think I should have done was to leave one PI as is , lets say V1 , and jumper pins 5 together on V2 and V5 , and the same with V3 and V6 . Now with V4 I could convert it to a preamp to feed the PI . What I believe is a negative feedback circuit from the cathodes of both PIs to the secondaries of the OT I would remove . A few of the things I dont understand are the circuit running between pins 5 of the PIs to the resistor network at R113 and R114 . Also on the plates of the PIs it shows 95 volts at the top plate and 120 volts at the bottom plate . Why the difference in voltage drop ? HT is connected to all PI plates thru 220k resistors so where does the extra 25 volts go ? Does it have something to do with C105 and C205 ? Also the coupling caps at C104 and C106 , .0047 and .047 , Should'nt I change these to like values ?

By the way , maybe you could post the maggie schematic up on this thread . I can't seem to make it happen .....

[olddawg]

Here's what I would do. I would use the tubes, power transformer, and one OT for the basis of a scratch build. Make or get yourself a chassis. You could re-use the tube sockets but I recommend new ceramic ones, they're cheap. With a few pots, jacks, and making a turrent board you have a nice amp for around $100. Less if you recycle more parts. Converting it would be a big monster.

[Defdaddy]

Mike. the reason the PI looks asymetrical is because that circuit uses a cathodyne phase inverter, with only one triode. The other triode in that bottle is a driver stage.

With the 18 watt amps we are used to seeing a long-tailed-pair PI, which is more symetrical and uses both triodes. This is a big part of what gives the 18 watt it's sound.

This brings us to the point of....... what amp are you hoping to build? If you want to build an 18 watt circuit using the schematics that are found on this site, by all means have at it and I agree with Olddawg's advice of using what parts you can into a new chassis, and a turret board that you wire up to resemble something along the lineage of the Marshall 1974.

If on the other hand, you want to use as much as possible of what is already there and get it to make sound with minimal re-machining, then PPWatt.com or AudioKarma or music-electronics-forum would be more appropriate places to post your design and ask questions. I don't mean to shunt your efforts - you've got a fun project on your hands!

[Zephedone]

Thanks guys ,
I was trying not to gut this thing and use as much as possible of what is already there . I posted here simply because of the four tube output section . Thanks anyways for the info and advice . Defdaddy by the way the two chassis I have are numbered 8802xx . Let me know if you would like any other numbers from them for the magnavox site you mentioned . Also thanks for the schematic . I'll go ahead and move on over to PPwatt .

[Shrapnel]

Got a Magnavox console about a decade back that I gutted. Didn't know much about it then, but now I've traced it down to be a AMP175-55 power amp chassis. The OT's are kinda small, the 6V6GT's had 340 on the plates, 20 on the cathodes with the screens connected directly to B+ (same power suppy point as the OT CTs.)

Nice thing is the PT is rated @ 290-0-290, with a 5v 3A (Mine used a 5U4) winding, a 6.3v winding that supplied 4 6V6GTs, 2-12AX7s, and a #44 pilot lamp. The HT windings also fed B+ to the preamp/tuner section as well as I'm assuming another 6.3 winding. For better or for worse though, 6.3 winding(s) need an artificial CT. (the known 6.3 used a 750R pot as a hum balance, the assumed 6.3v winding has no artificial CT on the power amp chassis (on the preamp chassis, I'm not sure... don't have the CR154 schem handy.))

IF one was to use a 5Y3 (or had the right AMP175 pwr amp chassis, you'd get two of these paralleled) Could make for a beefy 18w and probably a good 36w PT.

as far as the OTs go, I'm not sure what they're rated, and I'm going to soon find out the impedance ratios on them. They are small but I'm assuming that they handled about 15w... only a single impedance tap on them though.

[Defdaddy]

as far as the OTs go, I'm not sure what they're rated, and I'm going to soon find out the impedance ratios on them. They are small but I'm assuming that they handled about 15w... only a single impedance tap on them though.

Hey Shrap on the Model 175 parts list the OTs are described as 6000 ohm primary with 4-6 ohm secondary. Standard green wire designating 4 ohms - note that it also has a negative feedback line on it.

Post your email if you want the Sams pages for that amp.

Def

[Shrapnel]

Defdaddy,

email link is on the bottom of my posts (button provided by forum software.) Yeah, I'd like them schems. I'd appreciate it. Thanks.