Super Champ X2: Transforming it to an all-tube 'vintage' style amp

Wharfcreek

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I recently ran across a thread here where another member was dealing with a Super Champ X2. Evidently it had quit working altogether, and a number of posts in response to his inquiry indicated that in all likelihood the problem was the digital front end. At one point I think someone suggested the OP just 'scrap' the amp and go buy another one as they are relatively 'cheap' and the complexity of the digital front end makes costs of repair almost prohibitive. I kind of jumped into the middle of the thread after looking at a schematic for an X2 and had a thought about just 'scrapping' the digital front end and just adding a single 12AX7 preamp tube and making the amp something along the lines of being between an old Tweed Deluxe (only with a Solid State Rectifier) or a new HotRod Deluxe (only with 6V6 output tubes). After a few more comments came in on the thread, a few saying that it couldn't be done due to 'space' constraints or potential problems with the current draw on the power transformer from the added tube..... I decided I'd give it a try! So, I started looking over the local ads on Facebook Market Place, Craigslist, and just getting the word out to my friends, and within a few days I had a potential 'donor' amp. So, now the project is about to start. My intention is to go out today and 'gut' this poor amp! I'll get some pictures of the starting point, and any progress made today. I intend to take my time with this part of the project as I'd like to try to salvage the front end PC board and maybe try adding another little power transformer down the road and making it into just a stand-alone pre-amp section. But, that's undeterminable at this point, as I'm not even sure what I'm going to see when I start hacking away. But, I've decided to give the project a name! I'm calling it the 'Tween Deluxe' in reference to the '..between the Tweed and HotRod Deluxe....' I stated above. Seems fitting! Anyway, this initial post comes more or less just to announce my intentions here, and if anyone has tried this with any results in the past (good or bad) I'd love to hear about it. If someone knows for sure that the PT either 'will' or 'will NOT' live based on doing this, I'd like to know that before I get too far into it. My belief is that adding 15ma of filament current to the PT while eliminating any draw from the Digital/IC side, that will be about a 'wash' as far as power consumption goes. So, I'm hoping the PT is both 'beefy' enough to support this, and if not, that the transformation will result in about an even state of current draw such that it doesn't overheat and fail. Either way...... onward.......... Tom
 

Wharfcreek

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Printer, thanks for dropping in here. I've read some of your other comments and clearly you've got some good info to offer.

So, here's my question to you: I'm looking at the SCX2 schematic, and I have to say, there are some 'oddball' things going on with it. To begin with, that diode and resistor set-up on the cathodes of the output tubes. If I'm understanding what I see, it looks to me like the diode off the one OPT is allowing voltage from the other OPT to pass through it to ground, but not 'backwards' up the cathode of the second OPT. 'Different'! I also see what appears to be a 1 ohm resistor in series with that diode, but both the diode and the resistor connect to ground. There also appears to be a voltage associated with that 1 ohm resistor (R20) of 40mv as measured on pin 8 of OPT (V2-A). I'm thinking that the current draw from both OPTs is going through that resistor, so a voltage of 40mV over a 1 ohm resistor would mean that the OPT 'pair' is drawing 40ma of current. For a Class AB 'fixed bias' amp, that's a bit low, but a good 'safe' level, but would you confirm that I'm correct about all this? Also, regarding your response above, what are you referring to when you mention the 150ma? I'm asking this because my take on this whole project is that the amp as designed will support what ever level of current draw the two output tubes are set at from the factory.....which appears to be about 20ma each. HOWEVER, I may be reading that improperly, and it may be 40ma each....which is HOT....but not uncommon for Fender to push tubes in it's amps. But, either way, the combined HV current draw is under 100ma, and under 50 if I'm correct at 20ma each. There's less than 2ma in a 12AX7, so I don't see adding another one as being a problem on the HV side. My concern was the filament supply side! A 6V6 draws 0.45ma on it's filament. So, 2 6V6 tubes is 0.9ma. A 12AX7 wired for 6V draws about 0.15ma. So, adding another one would take the total heater current draw from 1.05A to 1.2A, or an increase of 0.15ma. I just don't see the power transformer not being able to handle an additional 1.5ma of current draw on the 6V winding. BUT, there are two other windings available on that PT that I don't plan to use. My 'plan' was to wire up all 4 tubes (the two 6V6 tubes and the two 12AX7 tubes....heaters only) and fire up the PT and see if there's any sign of it getting too hot from the added heater draw. If not, then I was going to run with it. But, if so, then maybe I can repurpose one of the other two windings? I'm just not sure yet what voltage they are putting out. The Schemo of the PT doesn't show output voltages, just 'connections.

Anyway..... my amp is now 'gutted'.....and tomorrow I plan to drill a couple of holes for the 12AX7s, and mount the 4 tube sockets and wire up the filaments and see if I blow it up! Good plan, eh?
 

printer2

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So, here's my question to you: I'm looking at the SCX2 schematic, and I have to say, there are some 'oddball' things going on with it. To begin with, that diode and resistor set-up on the cathodes of the output tubes. If I'm understanding what I see, it looks to me like the diode off the one OPT is allowing voltage from the other OPT to pass through it to ground, but not 'backwards' up the cathode of the second OPT. 'Different'! I also see what appears to be a 1 ohm resistor in series with that diode, but both the diode and the resistor connect to ground. There also appears to be a voltage associated with that 1 ohm resistor (R20) of 40mv as measured on pin 8 of OPT (V2-A). I'm thinking that the current draw from both OPTs is going through that resistor, so a voltage of 40mV over a 1 ohm resistor would mean that the OPT 'pair' is drawing 40ma of current.

For a Class AB 'fixed bias' amp, that's a bit low, but a good 'safe' level, but would you confirm that I'm correct about all this? Also, regarding your response above, what are you referring to when you mention the 150ma? I'm asking this because my take on this whole project is that the amp as designed will support what ever level of current draw the two output tubes are set at from the factory.....which appears to be about 20ma each. HOWEVER, I may be reading that improperly, and it may be 40ma each....which is HOT....but not uncommon for Fender to push tubes in it's amps. But, either way, the combined HV current draw is under 100ma, and under 50 if I'm correct at 20ma each. There's less than 2ma in a 12AX7, so I don't see adding another one as being a problem on the HV side.

My concern was the filament supply side! A 6V6 draws 0.45ma on it's filament. So, 2 6V6 tubes is 0.9ma. A 12AX7 wired for 6V draws about 0.15ma. So, adding another one would take the total heater current draw from 1.05A to 1.2A, or an increase of 0.15ma. I just don't see the power transformer not being able to handle an additional 1.5ma of current draw on the 6V winding. BUT, there are two other windings available on that PT that I don't plan to use. My 'plan' was to wire up all 4 tubes (the two 6V6 tubes and the two 12AX7 tubes....heaters only) and fire up the PT and see if there's any sign of it getting too hot from the added heater draw. If not, then I was going to run with it. But, if so, then maybe I can repurpose one of the other two windings? I'm just not sure yet what voltage they are putting out. The Schemo of the PT doesn't show output voltages, just 'connections.


I had to break it up into three paragraphs, too hard for the old mind to keep my place. My mistake, I am used to thinking of a 12A*7 tube being 150 mA as I think 12V, 6V it is 300 mA. Even then if the 6.3V winding gets dragged down a little you still should be good with a -10% below 6.3V. The high voltage end the transformer won't even blink.
 

Wharfcreek

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Yup, I had it wrong too. I was recalling the .15A as being 15ma, and that on the 6v wiring. So, yea, 300ma on 6v is correct. Im going to just wire it up and see if the PT plays nice! I'll let ya know
 

printer2

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Yup, I had it wrong too. I was recalling the .15A as being 15ma, and that on the 6v wiring. So, yea, 300ma on 6v is correct. Im going to just wire it up and see if the PT plays nice! I'll let ya know
Forgot to mention last night, the 1 ohm resistor has both tube currents going though it. The diode? I guess they found a fault condition that will take out the resistor, I don't see much use for it.
 

JeffBlue

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My SCX2 head is working perfectly so I don't see any reason to repurpose to all tube for now. But if the amp somehow acquires an issue, I wouldn't think twice about converting to an all tube amp conversion. I'll be watching this thread.
IMG_0085.jpg

The only mods on this amp were removing and replacing the silly plastic logo for a metal one and replacing the rubber feet with taller ones to clear the handles on some of my speaker cabs. I have tried 5881s in this amp with good results without rebiasing.
 

archetype

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Forgot to mention last night, the 1 ohm resistor has both tube currents going though it. The diode? I guess they found a fault condition that will take out the resistor, I don't see much use for it.

I've never figured out why the diode is in the circuit. It's basically a Princeton output stage and the Princeton doesn't seem to require a diode, there. Fender did the same thing with the Deluxe Reverb RI, putting IIRC two diodes in the same place in the circuit.
 

printer2

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"OK, here's how it works: for each power tube cathode (which would ordinarily be connected to ground), there is a 1 ohm resistor connecting the cathode to ground instead. The purpose of the 1 ohm resistor is to provide a goof-proof safe way for someone to assess the quiescent cathode current in the power tube by taking a (low) voltage reading, while adjusting the bias voltage to obtain the desired amount of current flow.

If it was only a 1 ohm resistor connecting the cathode to ground, in the event of an interelement short in the tube, the 1 ohm resistor may end up with large amounts of current flowing through it due to the fault. The resistor will heat and possibly burn, potentially damaging adjacent wiring and parts.

To prevent the resistor from self destructing, and give some time for the mains fuse to open (these amps typically do not have a HV fuse), they install a common rectifier diode, like a 1N4007, in parallel with the 1 ohm resistor, connected so that the diode cathode goes to ground.

To forward bias a silicon diode, the anode needs to be at least 0.6V more positive than its cathode. So in normal operation, when the 6V6GT cathode current is typically somewhere between 20 and 30mA, the diode is never forward biased, and all current flows only through the resistor.

In a fault current condition where the current may be in excess of hundreds of mA, the diode will begin conduction when the current exceeds 600mA, and will hold the terminal voltage across the 1 ohm resistor to 0.6VDC, as any additional fault current diverts through the diode until the mains fuse opens and shuts the amp down."

 

Wharfcreek

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So here's progress so far! As you can see, full board w/ control has been removed and preserved. I'm even retaining the original spade connectors in the event this whole project is a bust-out. The only permanent modification so far is the hole in the chassis for the driver tube socket.
 

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Fretting out

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Bonus points for feet in the photo’s! ;)

I don’t know much about the inner workings of amps but find this very interesting

Woe and I just noticed this is from a fellow statesman
 

Wharfcreek

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Here are some pics of the new sockets wired with filament supply, tubes installed, and power applied. This is the 'burn-in' stage to see if the PT will overheat from the added 300ma filament draw from the added tube, or not! I'm bankin' on 'not'!! Anyway, been cooking for about an hour now and all seems well thus far. You may note that I also 'VERY CAREFULLY' removed the original faceplate sticker! I'll end up with just volume, treble, and bass controls, one input jack, a pilot light, and a two-position on/off switch that has a middle position for on w/o HV, and a full on position for 'on/run'. Anyway, Im now trying to figure out the 'how & where?' to build the Power Supply section, and then hopefully find a way to get some bias voltage out of the pair of brown wires. The blue set with the 3rd yellow center tap is the 12v+\12v- supply for all the SS stuff. Not my wheelhouse! So, progress! Hopefully all will continue as planned!!
 

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Wharfcreek

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Printer, EXCELLENT explanation on the diode thing! I'm sure some clever and careful tech-engineer type at Fender told the powers-that-be that those were absolutely necessary, and they've been incorporating them into the designs of remaining tube amps ever since!

So, it now appears that Fender was running the X2 at a modest 20ma per output tube, or even 'cold' by some people's standards. Perhaps this is a consideration going forward that I should NOT get too aggressive with bias level in this version, as again, there may be PT limitations here that need to be considered. More time on the bench with a dummy load when it gets that far, waiting to see if the PT cooks or not!
 




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