Hello all,
I'm looking for a way to determine the appropriate speaker impedance for an unlabeled tube amp. I have an amp from a late '50s Webcor phonograph that I'd like to turn into a guitar amp. The model it came out of is the Webcor Musicale Coronet. I'm not trashing a turntable -- it came to me as a bare amp.
I've been unable to find any technical info on this little guy. No schematics, nothing. First, is there anyone out there who is familiar with this model? It's a tiny chassis with the power transformer mounted on the end, two 12AX7s, two 6V6s and a rectifier tube.
If no one has one to refer to, is there a way to figure out the necessary speaker impedance from the amp?
I'm looking for a way to determine the appropriate speaker impedance for an unlabeled tube amp. I have an amp from a late '50s Webcor phonograph that I'd like to turn into a guitar amp. The model it came out of is the Webcor Musicale Coronet. I'm not trashing a turntable -- it came to me as a bare amp.
I've been unable to find any technical info on this little guy. No schematics, nothing. First, is there anyone out there who is familiar with this model? It's a tiny chassis with the power transformer mounted on the end, two 12AX7s, two 6V6s and a rectifier tube.
If no one has one to refer to, is there a way to figure out the necessary speaker impedance from the amp?
Can't go wrong with eight Ohms.
It's not easy to guess the design impedance; you would have to decide what load to put on the tubes and then see what the turns ratio of the transformer is. I wouldn't bother.
I suggest you go through some of the common guitar amplifier schematic diagrams to see how this one is put together; they are almost identical. Of course, many guitar amplifiers have reverberation and tremolo added, which you could easily do if you chose.
I just built a guitar amplifier from an old speaker and a Lafayette stereo amplifier. Works great. The amplifier didn't have enough gain so I added an opamp, which works fine.
I can direct you to make this thing into anything you like. I assume it's mono?
It's not easy to guess the design impedance; you would have to decide what load to put on the tubes and then see what the turns ratio of the transformer is. I wouldn't bother.
I suggest you go through some of the common guitar amplifier schematic diagrams to see how this one is put together; they are almost identical. Of course, many guitar amplifiers have reverberation and tremolo added, which you could easily do if you chose.
I just built a guitar amplifier from an old speaker and a Lafayette stereo amplifier. Works great. The amplifier didn't have enough gain so I added an opamp, which works fine.
I can direct you to make this thing into anything you like. I assume it's mono?
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I wonder if 8 Ω is really the best choice. A lot of tubed consumer electronics used 3.2 Ω voice coil drivers. Please inspect the chassis for labels, markings, etc. A clue to the model, and therefore the schematic, might be present. Tom Bavis has an extensive collection of schematics.
This thread provides some useful clues. It would seem that the Webcor Musicale Coronet was fairly upscale, as it came with a GE moving iron magnetic cartridge. Piezoelectric carts. are the norm for cheapy products, of that time frame. The tube complement makes sense. A 12AX7 was used for the RIAA preamp. The 2nd 'X7 is the power section voltage amplifier and phase splitter. PP EL84 "finals" were employed. Pretty serious stuff for late 1950s consumer electronics.
This thread provides some useful clues. It would seem that the Webcor Musicale Coronet was fairly upscale, as it came with a GE moving iron magnetic cartridge. Piezoelectric carts. are the norm for cheapy products, of that time frame. The tube complement makes sense. A 12AX7 was used for the RIAA preamp. The 2nd 'X7 is the power section voltage amplifier and phase splitter. PP EL84 "finals" were employed. Pretty serious stuff for late 1950s consumer electronics.
Would need a model number or chassis number to find a schematic. With two speakers it is very likely 4 Ohms... it would be possible to estimate by connecting an AC source to the two primary terminals of the transformer (tubes removed) and measuring the secondary voltage, or vice versa... The impedance ratio is the square of the voltage ratio.
For example, 1V applied to secondary, 35V measured on primary. Z ratio will be 35^2 or 1225. Since the primary will be 8-10K for push-pull 6V6s, the secondary would be 8 Ohms.
For example, 1V applied to secondary, 35V measured on primary. Z ratio will be 35^2 or 1225. Since the primary will be 8-10K for push-pull 6V6s, the secondary would be 8 Ohms.
Hmmm...
Hi guys,
The only marking I've been able to find is on a paper tag attached to the chassis: 14X237 3. I only know it came from a Musicale Coronet from looking at photos.
The power tubes are 6V6s on this one -- maybe an earlier version? Here's the photo I used to ID the chassis:
It sure looks like there were two speakers wired in series, which says to me the overall load must need to be at least 8 ohms. Did they use any 4 ohm drivers back then?
Hi guys,
The only marking I've been able to find is on a paper tag attached to the chassis: 14X237 3. I only know it came from a Musicale Coronet from looking at photos.
The power tubes are 6V6s on this one -- maybe an earlier version? Here's the photo I used to ID the chassis:
An externally hosted image should be here but it was not working when we last tested it.
It sure looks like there were two speakers wired in series, which says to me the overall load must need to be at least 8 ohms. Did they use any 4 ohm drivers back then?
Yes, if they're in series they are likely 4 Ohms each, so amp would be 8 Ohm output.
14X237 does look like a Webcor number, but it isn't listed in Beitmans or Sams.
14X237 does look like a Webcor number, but it isn't listed in Beitmans or Sams.
Hmmm...Revisited
Yep, that's what I assumed too. I hooked it up last night to an 8 ohm speaker, plugged in my guitar, and it passed signal! On the down side, the volume was really low. I'll be looking at the resistors this weekend -- I left all the carbon comps in place for the time being.
Bob91343, I may need your assistance for my next step. I have to think that some key changes to the existing circuit should produce a lot more gain/volume. Two AX7s driving two 6V6s should make for a loud guitar amp! I see three possibilities: the RIAA equalization needs to go, and/or I need to boost the gain on the pre tubes, or the resistors have drifted. This is the first time I've tried to convert an amp designed specifically for a phonograph. Usually I start with a PA amp, so I can choose which input I want to use, ie whichever has the most gain.
The sound I'm getting is very clear, very "hifi", just not very loud.
What I don't get is why an amp built to amplify a magnetic cartridge should pass such a low signal with a guitar as the source. Wouldn't the signal from a guitar pickup be stronger than the signal from a magnetic phono cartridge? There's an unmarked RCA jack on the top of the chassis -- I'll try plugging into that tonight. Maybe it's an input for a radio or reel to reel?
Yep, that's what I assumed too. I hooked it up last night to an 8 ohm speaker, plugged in my guitar, and it passed signal! On the down side, the volume was really low. I'll be looking at the resistors this weekend -- I left all the carbon comps in place for the time being.
Bob91343, I may need your assistance for my next step. I have to think that some key changes to the existing circuit should produce a lot more gain/volume. Two AX7s driving two 6V6s should make for a loud guitar amp! I see three possibilities: the RIAA equalization needs to go, and/or I need to boost the gain on the pre tubes, or the resistors have drifted. This is the first time I've tried to convert an amp designed specifically for a phonograph. Usually I start with a PA amp, so I can choose which input I want to use, ie whichever has the most gain.
The sound I'm getting is very clear, very "hifi", just not very loud.
What I don't get is why an amp built to amplify a magnetic cartridge should pass such a low signal with a guitar as the source. Wouldn't the signal from a guitar pickup be stronger than the signal from a magnetic phono cartridge? There's an unmarked RCA jack on the top of the chassis -- I'll try plugging into that tonight. Maybe it's an input for a radio or reel to reel?
Thanks Tom
Thanks for looking up the chassis number. I haven't had much luck finding anything either! This thing is very nicely built, VERY compact, and seems to have a lot of potential as a really dirty little guitar head. I have a little 1x12 cabinet with a Celestion black back in it, and I hope the tube set and comparatively small OT will make for some super distortion.
Thanks for looking up the chassis number. I haven't had much luck finding anything either! This thing is very nicely built, VERY compact, and seems to have a lot of potential as a really dirty little guitar head. I have a little 1x12 cabinet with a Celestion black back in it, and I hope the tube set and comparatively small OT will make for some super distortion.
Darned tootin the RIAA EQ circuitry has to go. The RIAA spec. has a 20 dB. roll off at 1 KHz. and the roll off gets worse at higher frequencies.
Assuming the rectifier is a 6X5, you have 70 mA. of B+ available. That should be enough of 4 'X7 sections and pair of 6V6s set up near Class "B". Forget about Webcor's schematic, as this is a guitar conversion. How about something based on a Fender Deluxe. The early models used 6SC7s, but the 'X7 is close enough.
Here is what I know
My Webcor Phono is model 333. Uses 2 6V6s in output. Has 3 speakers in series that total 8.2 ohms DC resistance. Output transformer is center tapped. The Brown Center tap goes to a 200 Ohm 2 watt resistor.. Green and red wires go to the Cathodes of the 6v6s. Transformer code is 239-340, which is a Merit transformer from 1953. Other stamped code , I presume, is the Webcor part #: 67Po368, as unfortuntely that does not correspond to any of the codes I see in the Merit transformer catalogs that I can find on the web by Googling.
I too want to make this into (yet another) homebrew amp, like a Silvertone or early Harvard. Although I have successfully built a few homebres, and a 5E3 and Matchless clone, working with what is essentially a "mystery output transformer with a center tap is beyond my experience.
If I cap off the center tap, my concern is the voltage ratio is doubled, but the impedance is 4 times the impedance of either end.
Here is the source for above conjecture: Output Transformers and Impedance
So all this is meant to help add more info re the Webcor Musicale, but also chip in with the question of how to wire it. My default will be to use the center tap, with its resistor, maybe adding a bypass cap, into an 8 ohm load. Thoughts?
My Webcor Phono is model 333. Uses 2 6V6s in output. Has 3 speakers in series that total 8.2 ohms DC resistance. Output transformer is center tapped. The Brown Center tap goes to a 200 Ohm 2 watt resistor.. Green and red wires go to the Cathodes of the 6v6s. Transformer code is 239-340, which is a Merit transformer from 1953. Other stamped code , I presume, is the Webcor part #: 67Po368, as unfortuntely that does not correspond to any of the codes I see in the Merit transformer catalogs that I can find on the web by Googling.
I too want to make this into (yet another) homebrew amp, like a Silvertone or early Harvard. Although I have successfully built a few homebres, and a 5E3 and Matchless clone, working with what is essentially a "mystery output transformer with a center tap is beyond my experience.
If I cap off the center tap, my concern is the voltage ratio is doubled, but the impedance is 4 times the impedance of either end.
Here is the source for above conjecture: Output Transformers and Impedance
So all this is meant to help add more info re the Webcor Musicale, but also chip in with the question of how to wire it. My default will be to use the center tap, with its resistor, maybe adding a bypass cap, into an 8 ohm load. Thoughts?
OOps- Typos galore. as you will see from schematic I posted, it is a 220 ohm resistor tying the center tap and cathodes to ground. 2nd, the Output trannies secondaries seem to tie into both the plates AND the cathodes of the 6v6s, showing 205V to the plates and 12V to the cathodes. I will appreciate any guidance on this unique (to me) output trannie configuration. I assume this is a cathode bias design, but am stumped as to what range of schematics I can adapt to a guitar amp homebrew.
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