Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ALBATROS1234 on January 29, 2022, 03:37:27 pm
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I was given a 1957 sylvania 4305M consolette record player. The changer is trashed and it crossed my mind that this may be a candidate for guitar amp conversion. It has a 12" speaker although the voice coil and magnet are quite small, the amp is 1x 12ax7, 2x 6v6 and a 5y3. It is presently set up with Volume, bass and treble controls and as for the amp, 1st half of the 12ax7 is the preamp,second half P.I. and the 2 6v6 in P.P. I fear that this setup for guitar may be a bit lacking in gain. So I was wondering if I set it up to have both halves of the 12ax7 as gain stages and did a self split P.I. setup like some early radios did or some magnatones and even a couple early gibsons did. another option is add a transformer as the driver/P.I. so which tradeoff would you guys suggest to produce the best guitar tone. I suppose I will try to see what it sounds like with some cap changes and a few tweaks but I wanted opinions on other options as well. Thanks.
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I have an old record changer rebuild with the same tube line up.....it severely lacks umph......even with pot after first half of ax7
And 1meg at the input it's just barely above bedroom volumes.....its going to need a complete overhaul...also a 10" Rajun cajun
Didn't really help....
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yep, get it working well 1st, otherwise you'll be chasing your tail.
what's the goal? noodlin in your room or gigging the night clubs?
test it with a gain/boost pedal up front. the original design is probably good in the 10-12W range already.
fwiw i used an IT as a PI in a couple builds, this build was well liked. the IT states you can operate as a step-up, 1:2 or unity, 1:1
measuring showed at 1:1 I lost about 10%, at 1:2 I got unity though the PI
EDIT: fixed schematic. TS is Marshall '87. Kinda sucked in this application.
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fwiw i used an IT as a PI in a couple builds, this build was well liked. the IT states you can operate as a step-up, 1:2 or unity, 1:1
measuring showed at 1:1 I lost about 10%, at 1:2 I got unity though the PI
That schematic looks to have an error on the tone stack, ie the 33k slope resistor is connected to 0V common.
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Fix whatever is broken. These high-end home players were often very musical. Put an LPB in front so you don't sprain your fingers trying to bring it to full output with a mere e-guitar.
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:laugh:
That schematic looks to have an error on the tone stack
*****WARNING check schematic for errors, posted only as a reference for phase invertor, not build accuracy******
I just grabbed the 1st image in line, there's about 12 variants, but the basic design is solid.
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I restore old tube radios and I have seen a handful of them with different methods of self splitting for push pull without a tube. Also there is the magnatone 109 and other early guitar amps with self split PP. My first thought was I could use the 12ax7 as 2 gain stages but not sure how much the self split methods are a gain killer. The magnatone takes off of the screen to feed the grid of the 2nd power tube. But there's also other methods one where the kathodes are tied together so the kathode acts as the grid. My question for the serious pro tubers was wondering out of the different self split PP circuits which method performed the best. Maybe it falls to me and I have to do the ground work and report findings for posterity.
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Maybe it falls to me and I have to do the ground work and report findings for posterity.
Bingo! :icon_biggrin:
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Fair enough and I don't mind, just thought I would ask the guys with much more experience than me if they had any experience in this matter which would guide me and save time. Much like reading a technical manual if there are pitfalls and dead ends that others before me have found I wanted to know. I will do my due diligence and report my findings.
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... just thought I would ask the guys with much more experience than me if they had any experience in this matter which would guide me and save time. ...
PRR says a lot with few words.
He posted a schematic for your Sylvania 4305 that shows both halves of the 12AX7 are used after the tone controls as a paraphase inverter. It also shows a Phono input runs to those tone controls, which implies there was a phono amp between the changer & the Phono Input that had a large enough output to drive this arrangement.
He then suggested using a simple boost pedal (https://www.ehx.com/products/lpb-1/) between your guitar & the Phono Input to avoid having to add a tube socket and construct additional gain stage(s).
And he said "fix whatever is broken" just within this section of the 4305 between the Phono Input and the speaker. An unstated bit of wisdom there is "get it working as-is before making major changes," which Shooter also said in the 2nd reply (https://el34world.com/Forum/index.php?topic=28482.msg313676#msg313676).
Seems to me like you've got a complete plan laid out for you.
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> implies there was a phono amp between the changer & the Phono Input that had a large enough output to drive this
No. Consumer recordplayers used high output needles, either magnetic or crystal. Often in the 0.2V to 0.6V range.
Electric guitarists like sensitivity (for full output) as low as 0.020V, for full tone on light strum, or gross distortion on not much more strum.
So it wants another gain of 10 or 30.
Yes we can self-split but that is usually a half-good plan. Transformers, but they are very rare in guitar, only the oldest and the biggest (and the weird little Fender). A MOSFET cathodyne is great on paper, and may work smoothly, but hey why not make everything Silicon!?
For TODAY, to get a sense of what it wants to be, just get it working right the way it was built, throw a booster in front, and play it for a hundred hours. It may inspire.