Fender 57 Twin Amp reissue

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xtian
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Fender 57 Twin Amp reissue

Post by xtian »

Schematic attached. The output tube circuit attracted my attention: the screens AND the OT primary CT are connected to B+1, right after the choke, and the plates are powered through the OT primary. Meaning the plates will be at lower potential than the screens!

Any comments on how this affects tube life, sonics, setting bias?

fender-57-twin-amp-guitar-amplifier-schematic.pdf
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Re: Fender 57 Twin Amp reissue

Post by JazzGuitarGimp »

The plates will be significantly lower than the screens on the negative going half-cycles of the audio waveform. What's even weirder, is there are no screen grid stopper resistors to limit screen current. Scratching head....
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Re: Fender 57 Twin Amp reissue

Post by pompeiisneaks »

That IS super weird if you look way back to the first twin:

https://tubeamplifierparts.com/schemati ... ematic.gif

the 5C8, it had no resistors, but it had a dropper between B+1 and B+2 so that there was at least a voltage difference. That's got to be a typo!
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xtian
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Re: Fender 57 Twin Amp reissue

Post by xtian »

This Twin Amp reissue is the 5E8-A schematic. Looks like the only modern updates are a fuse in the 5v recto heater circuit, filter caps upped to 22uF, 12AX7s instead of 12AY7s in the preamp, and bias adjust pot.
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martin manning
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Re: Fender 57 Twin Amp reissue

Post by martin manning »

JazzGuitarGimp wrote: Sat May 13, 2017 7:06 pm The plates will be significantly lower than the screens on the negative going half-cycles of the audio waveform. What's even weirder, is there are no screen grid stopper resistors to limit screen current. Scratching head....
The plate voltage drops below the screen voltage on the positive signal swing. On the negative swing the the tube goes into cut-off and then voltages will be equal. Several of the early 1950's Fender amps were set up this way, with the plates and screens fed from the same node, downstream of the choke.
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JazzGuitarGimp
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Re: Fender 57 Twin Amp reissue

Post by JazzGuitarGimp »

martin manning wrote: Sat May 13, 2017 8:08 pm
JazzGuitarGimp wrote: Sat May 13, 2017 7:06 pm The plates will be significantly lower than the screens on the negative going half-cycles of the audio waveform. What's even weirder, is there are no screen grid stopper resistors to limit screen current. Scratching head....
The plate voltage drops below the screen voltage on the positive signal swing.
I think you're talking about the grid swing, whereas I was talking about the plate swing.
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martin manning
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Re: Fender 57 Twin Amp reissue

Post by martin manning »

JazzGuitarGimp wrote: Sat May 13, 2017 8:21 pmI think you're talking about the grid swing, whereas I was talking about the plate swing.
Yes we are in agreement then. :^)
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Re: Fender 57 Twin Amp reissue

Post by tictac »

Maybe a bit harder on the tubes but it does sound good with this setup....

TT
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xtian
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Re: Fender 57 Twin Amp reissue

Post by xtian »

Would the engineers of the 50s say they were doing it wrong?
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Re: Fender 57 Twin Amp reissue

Post by JazzGuitarGimp »

xtian wrote: Sun May 14, 2017 4:22 pm Would the engineers of the 50s say they were doing it wrong?
Probably not, but they had 1950's 6L6's at their disposal. Many people believe the current production tubes are not as stout as the tubes made in the 50's, 60's and 70's. If current production tubes are to be used for the build, tube life could be an issue.
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Re: Fender 57 Twin Amp reissue

Post by tubeswell »

The president of the local blues club has one which he brings in every so often for a re-tube. Here are some gutshots I took a couple of years back. These RIs are nice sounding amps, and they did a decent job on the RI spec (waiting for the comments on the ICs LoL)
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rooster
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Re: Fender 57 Twin Amp reissue

Post by rooster »

Note the RI, unlike the original, is wired such that you can run EL34s if you want. That's pretty cool - except - unlike 6L6s, the plate voltage should not drop below the screen voltage if you use EL34s so scratch that idea unless you want to add some large screen resistors. I think these amps put out about 35 watts if that.

But by design, the 6L6 can exist with screen voltages higher then the plates. 6L6s sound interesting wired this way (more distortion), but exhibit more power and more headroom wired the way we see in the later HP Twin of '59, 65 watts! (Also note, the 58 Twin added 100 ohm screen resistors after the choke, not quite the Fender 'norm' of 470 ohm that came in '59.)
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xtian
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Re: Fender 57 Twin Amp reissue

Post by xtian »

Owner brought his Custom Twin Amp, no sound. Recto tubes were not lighting up. At first I measured 40vAC on the 5v circuit. What?!? Turns out the fuse holder on the 5v circuit had come loose from the board, giving a false reading. Imagine that, poor soldering on a $3K amp. Other solder points looked a bit sloppy, too, but not terrible. Amp sounds great. Speakers are very nice. Pine cab is also good, and tweed is immaculate.
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Re: Fender 57 Twin Amp reissue

Post by Leo_Gnardo »

xtian wrote: Sat May 27, 2017 5:19 pm Imagine that, poor soldering on a $3K amp.
Well y'know they gotta crank 'em out fast as they can. Plus they're probably using that politically correct no-lead solder. :evil: No worries, more guarantee that we fixit folks will have plenty of work in the future, thanks Fender! Don't forget to charge accordingly. ;)
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