Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shaun on January 10, 2024, 06:39:21 pm
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Hi All,
Hope you're having a merry winter in your workshop wonderland.
A Victor Animatograph came into my possession and I couldn't resist doing a conversion. However, I could not find an original schematic, and I would have replaced most/all components in any case, so I did the unthinkable (which actually seems most practical to me) and started over, using whatever I could salvage after gutting it.
I have been wanting to build with octal preamp tubes for ages, so this was my opportunity. As the unit has two input jacks, I'm planning on using two separate input preamp tubes - a 6SJ7 pentode and a 6SQ7 triode (corrected - I thought it was a twin triode) - a dedicated input jack for each one. Then on to a 6SC7 driver (also bridged). Then to a 6V6 output stage, which was what it had originally.
Referring to my rudimentary attached schematic, I am unsure about one point of my preamp stages, particularly where they connect between the two 270k resistors. I am tempted to insert a switch there to keep the two signal paths completely separate, but maybe I don't need to. It's just that I've never seen anything like this - usually there is a vol pot and TS on each channel before the signal paths are joined (common in many Fenders), and I'm guessing there must be a reason why, but I can't see it. I want to get away with using a single Tweed style TS and Vol pot if I can, as I'm sure many manufacturers would want to also (less $$). The thinking behind my preamp design is mostly so that I don't have to drill the hell outta the front panel. It's so pretty and red :).
Beyond that, I'm certainly open to any criticism/feedback. The PT may be a bit overworked, especially as I plan to also reinstall the original electric eye (these are surprisingly easy to add and look really cool). But I don't think I'll be pushing the PT too much harder than it would have been, except that a modern 6V6 puts out more power. I do expect the PT to get pretty warm.
Instead of the 6X5 rec tube it had, I switched to an EZ81 because they draw less amperage while also offering a 1.3x B+ voltage, rather than the 1x offered by the 6X5 - gotta get some voltage onto that 6V6! (That small aluminum plate with the noval socket replaces the old rectifier octal socket.) I'm sure the EZ81's greater output could add some heat to the PT, but I guess I'll see how it holds up before modifying the chassis and further to install a bigger PT.
Thanks for any feedback, and have a good one. Pics and schem attached.
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If you want to play both inputs at once you want balancing control, perhaps more than you need a tweed "tone" knob.
Why does your 6SQ7 have two plates? Strapped together? The 6SQ7 has three plates. And two are for radio receiver working, of little use to you?
6V6 does not need huge voltage unless you play with a drummer or a dog.
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Why does your 6SQ7 have two plates? Strapped together? The 6SQ7 has three plates. And two are for radio receiver working, of little use to you?
Looks like he accidentally drew it the same fashion as he drew his 6SC7.
@OP Do you know that the 6SQ7 is a triode + double-diode? The triode section is equivalent to a 12AX7, and the two diodes are equivalent to the 6H6.
In AM radio circuits, it acts as the first audio stage, AM detector and AGC rectifier.
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Yes folks, thanks for the correction. I realized later that the 6SQ7 is a single triode, not a twin triode.
And I should have pointed out that I don't want to use both inputs at once. So maybe I'm overthinking things, as usual.