Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: munkeyboy on November 26, 2019, 10:08:47 am
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I have an old (1948) Magnavox Illustravox Junior. It has what looks to be a PP 25L6 section, though I don't quit understand it (no PI, so it's self inverting push pull right?). Also has a cool 6" field coil.
I have an isolation transformer already hooked up and caps replaced, but it still has some bigger issues I was going to work thru. The rectifier was bad, but I believe i have a working spare. I think the 6SC7 is bad, but I don't have a back up yet. However, I do have a few 12sc7s and one 6sj7 if I want to rewire. Grounding issues or a short seems to be another issue (only getting 20vdc B+).
I love the case/cab this is in, but before I go any further, any opinion of wether it worth the effort to make this into a tiny watt practice amp? Is it just gonna sound horrible no matter? I tried finding PP 25L6 guitar amp examples out there, but so far nothing too close. I don't mind adding another socket for a PI to get around the whole self inverted PP. However, I think the Giboneette amp was a self inverted PP, but of course that is using 6V6s (and a PT).
Here is the Illustravox schematic: http://www.imperialclub.com/Repair/Lit/Master/48Illustrovox/Page11-big.jpg (http://www.imperialclub.com/Repair/Lit/Master/48Illustrovox/Page11-big.jpg):
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self inverted PP
the 6SC7 appears to be your PI, the preamp section is somewhere off circuit
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Hmm that was my first thought too, since the PP schematic didn't look like the gibsonette or the magnatone 109. However, there is no preamp that I can find (physically or in the manual/schematic). Must there be a preamp or did the projector/phono feed directly to the PI? (don't have the projector part, but I had the phono and no preamp there.) However, that makes thing easier then... just add a preamp tube. I have a nice hole where the old cap can is.
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never mind, that's a dumb question :icon_biggrin:
There must have been a separate amp. I think that is the difference between the Magnavox Illustravox Junior and the Magnavox Illustravox.
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The PI is on another chassis. I wouldn't be surprised if the PI is a transformer. It sends the two out of phase signals to the two grids of the 6SC7 via socket 220 (upper left). The 6SC7 is just a post PI amplifier.
This is worth what you are willing to invest in it. That price will be different for everyone. Unless it has a cool cabinet I would throw it away. There's nothing cool about that schematic. But if the cab is cool I would gut the whole chassis and build a small cool amp to fit into it. Then you be two cool! :icon_biggrin:
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Yeah, I have a back up plan. Kinda silly, but i have a cheap danelectro battery powered honeytone that I'll transpose the guts off into this cab. Already test fit and temp wired it up to another 6" speaker inside this cab. Sounds pretty good for a cheap $20 ss amp. That 6" speaker is quite and improvement on the 3" one. Might also wire up the guts of cheap danelectro corned beef reverb pedal.
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I am 99% certain that Illustravox is a HOT CHASSIS amplifier. Rarely a good idea for radio/phono/film. Can be DEADLY for a guitarist where all the metal on the guitar is connected to amplifier chassis.
So: do you really know what you are doing? You wanna bet your life?
Yeah, the HoneyTone transplant sounds like a better plan.
BTW, I think there is an error where the field coil connects to the second main cap (not).
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I am 99% certain that Illustravox is a HOT CHASSIS amplifier. Rarely a good idea for radio/phono/film. Can be DEADLY for a guitarist where all the metal on the guitar is connected to amplifier chassis.
Yes sir, it is. First thing I hooked up was an isolation tranny.
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BTW, I think there is an error where the field coil connects to the second main cap (not).
Not following, are you saying it should not be connected to the first cap (yellow path)?
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> yellow path
No... the other side:
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Hi, I've started the project up again because I found a few inspiring schematics. I found the gibsonette early with the self split PP and the danelectro leader with isolation tranny and voltage doubler.
I've modelled most the circuit on the gibsonette, but the power on the danelectro leader. However, I'm not clear on if I got the power right, especially with the heaters (since I want to keep them after the isolation transformers) and the field coil. Can someone review my schematic and give some feedback?
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Ugh, forgot about the ripple. Those 47uf will need to be bigger. I was hoping to use what I have laying around. Maybe I'll start with just a half wave rectification first.
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Well, the self split Gibsonette circuit with stock half-wave rectification works, but at ~115 B+ and that field coil speaker, there isn't much volume. It is enough volume to hear over the pluck of my strings. However, bypassing the field coil by using an 8ohm speaker helps. Still using FC as choke. Not sure why the field coil speaker is quieter (imp is 4ohm), just less efficient I guess.
It sounds good for as much as I can hear it. There is some buzz as the tiny chassis and off board transformer/volume pot/input doesn't help my already poor lead dress skills.
Newer schematic attached. Photos of amp here: https://photos.app.goo.gl/PkgdF6tZPq9PhT7i8
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The 25L6 is an interesting tube. Except for the heater rating, it's the same as the 50L6. As a single ended tube with 110 volts on the plate and screen grid, you can get 2 watts from it. With 200 volts on the plate and 110 on the screen grid, you can get 3.8 watts from it, both rated at 10% distortion. With 2 of them in parallel single ended, you should be able to get 6 to 7 watts from them. In a push pull configuration, you should be able to get more than 7 watts from a pair. That's more power than the old Fender Champ...
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Here's an interesting example of a push pull amp with 25L6 power tubes.
It obviously could use an isolation transformer.
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https://www.edcorusa.com/xpwr270
If you want to go 50L6s, this tranny has 6v and 50v windings, as well as 110v windings.