The Super Champ XD club

[Gnobuddy]

Originally Posted by davidchagrin

Thanks for the diagram. I'm going to give it a shot. I assume the amp should be on while testing. Is this correct?

David - as was just mentioned, there are deadly voltages in there. The amp does have to be on when setting bias, and those lethal voltages are exposed at several points quite near to where you will be measuring bias. For example, some of the solder dots in those white rings on the board (the undersides of the tube sockets) carry nearly 400 volts, enough to have you knocking at the pearly gates if you make contact with them.)

So please, please, be careful, okay? We really don't need any more musicians dead before their time. If you're not 100% sure how to be safe around and open tube amp, don't do it. Much better to take it to a tech, or to live with less than perfect bias, than to be electrocuted.

Personally, I won't go anywhere near an open, powered on tube amp chassis without a pair of electricians class 0 or class 1 gloves on my hands. You can find those gloves on Ebay under $30 if you're lucky (Salisbury is a godo brand). I consider that very cheap life insurance, and well worth buying.

Here is an example of these gloves: http://www.ebay.com/itm/Salisbury-E0...item3a741bbe7d

Of course having class 0 gloves on still doesn't guarantee your safety. For example you could accidentally knock the chassis into your lap while measuring bias, and the gloves on your hands would not help in the slightest.

I also recommend wearing safety googles. If you make a mistake you can short out a capacitor charged to 400 volts, and that can sling white hot sparks at your eyes. Capacitors can also explode under that sort of abuse, flinging toxic chemicals and metal shards around.

Treat an open tube amp chassis that's powered on like a loaded gun without a safety catch - it can kill or maim without warning if you make a tiny mistake.

-Gnobuddy

[Gnobuddy]

Originally Posted by JDRNoPro

Sorry not to have responded sooner Gnobuddy, but just got back from a little vacation......

Lucky man, hope the vacation was good!

Originally Posted by JDRNoPro

Certainly there is a lot of tonal similarity, but as you noted, the bass response on the PRRI is stronger and I think it's overall tone is somewhat thicker (richer) and more complex.

Thanks for the report!

What you say makes sense to me. On this thread many of us found the SCXD has to be pushed pretty hard before you hear a little "tube amp" sound to it. I found my SCXD sounded almost as clean as a solid-state amp at low volumes. In other words, it makes sense that it sounds a little thinner and less complex than the tone from a PRRI.

Recently I modified an SCXD, replacing the digital and solid state front end with a 12AX7 and regular old Fender tone stack. It now has a little more texture to its tone than it did before, so those preamp tubes did something. I don't expect this modified amp to sound exactly like a PRRI either, but I suspect it's a little closer than a stock SCXD.

Originally Posted by JDRNoPro

So, back to your original question, I still think there's more than 10% difference in clean tone between the two amps, but it's hard to quantify in terms that everyone would probably agree on. I think it's more accurate to say that they definately produce similar clean tones in the classic BF way....and in a very good way! I also would say that the SCXD sounds more like a PRRI than a DRRI or other BF circuit.

I agree, it's not something that can be quantified. Your observations are helpful, though.

You may already know this - the tube circuitry in the SCXD is pretty similar to the corresponding tube circuitry in a PRRI, i.e. the schematics are very similar.

That surprised me, given the huge price difference between the two amps. But it does match with your observation that the SCXD sounds more like a PRRI than other blackface Fenders. It makes sense, they share quite a few family genes.

There are definitely some differences, of course - the SCXD gets a cheaper transformer with less bass response, the SCXD power tubes get a little less voltage, the SCXD preamp tube gets a hundred volts more than the preamp tube in a PRRI. And of course the SCXD has a solid-state front end, unlike the tubes in the PRRI, as you mentioned.

Originally Posted by JDRNoPro

2) they really know how to use the tone controls (and vol controls) on both their amp and guitar. That second point may sound a bit snooty but that's not my intent - I'm just saying that some folks don't spend enough time really learning how the tone controls interact on their particular amp to achieve optimal tone the amp is capable of producing.

Good point. The Fender tone stack (treble, mid, bass) I added to the SCXD I recently modified is actually almost my first experience using this vintage circuit from half a century ago. I'm still at the beginning of that learning curve you mention, trying to figure out how to dial in a good tone. It actually helped to play with Duncan's Tone Stack calculator software on my computer - that gave me a little better idea what to expect from the interaction between the knobs.

The only other experience I have with the Fender TMB tone stack is on a little solid-state Fender Frontman 25 amp. I'm still learning what's possible with those tone controls too. I recently found my Dean Palomino archtop worked very well and produced a bright but attractive clean tone with the Frontman tone controls set with treble all the way up (on 10), bass all the way down (on 1), and mid also all the way down (1), and a splash of spring reverb.

I never expected such extreme settings to produce any kind of usable sound - but surprisingly enough, they do! It sounds a bit like the clean guitar tones you hear several times in the background music from John Hughe's film "16 Candles". Thin, but chimey and "spanky" and quite usable for some kinds of lead guitar.

-Gnobuddy

[Mr Perch]

Originally Posted by Gnobuddy

I never expected such extreme settings to produce any kind of usable sound - but surprisingly enough, they do!

With Wilde Bill Lawrence pickups in my Tele, I get the best results with both treble and bass set at "0" on the amp.

[davidchagrin]

Originally Posted by Gnobuddy

David - as was just mentioned, there are deadly voltages in there. The amp does have to be on when setting bias, and those lethal voltages are exposed at several points quite near to where you will be measuring bias. For example, some of the solder dots in those white rings on the board (the undersides of the tube sockets) carry nearly 400 volts, enough to have you knocking at the pearly gates if you make contact with them.)

So please, please, be careful, okay? We really don't need any more musicians dead before their time. If you're not 100% sure how to be safe around and open tube amp, don't do it. Much better to take it to a tech, or to live with less than perfect bias, than to be electrocuted.

Personally, I won't go anywhere near an open, powered on tube amp chassis without a pair of electricians class 0 or class 1 gloves on my hands. You can find those gloves on Ebay under $30 if you're lucky (Salisbury is a godo brand). I consider that very cheap life insurance, and well worth buying.

Here is an example of these gloves: http://www.ebay.com/itm/Salisbury-E0...item3a741bbe7d

Of course having class 0 gloves on still doesn't guarantee your safety. For example you could accidentally knock the chassis into your lap while measuring bias, and the gloves on your hands would not help in the slightest.

I also recommend wearing safety googles. If you make a mistake you can short out a capacitor charged to 400 volts, and that can sling white hot sparks at your eyes. Capacitors can also explode under that sort of abuse, flinging toxic chemicals and metal shards around.

Treat an open tube amp chassis that's powered on like a loaded gun without a safety catch - it can kill or maim without warning if you make a tiny mistake.

-Gnobuddy

I sincerely thank you for your concern. This is why I'm a member at TDPRI!!!
I am familiar with electrical work, but had never biased an amp. I was extremely cautious. It was barely off anyhow and I can't even notice an audible difference. But, I am happy to know how and will try it on my other tube amps now.

[JDRNoPro]

Glad my comments made some sense to you, Gnobuddy. I've been quite interested in your posts about the circuit similarities in the PRRI and SCXD - frankly, I never would have even thought about that possibility if not for your investigations, given the origins of the two amps. The tonal similarities are not accidental. Also, nice project and experimentation with Mr. Perch's reincarnated SCXD !

[Gnobuddy]

Originally Posted by JDRNoPro

I've been quite interested in your posts about the circuit similarities in the PRRI and SCXD - frankly, I never would have even thought about that possibility if not for your investigations, given the origins of the two amps.

Yeah, that surprised me as well! But if you forget about the advertising and sentimental value of the PRRI and just look at the engineering part, it makes sense. My guess is that Fender quickly knocked together the SCXD design by cutting and pasting together designs that they already had in-house. They needed a DSP modelling preamp, and a low wattage push-pull power amp - and I think they grabbed their own off-the shelf designs to start with, rather than have to spend money to engineer something new.

In other words, I think they started with the solid-state preamp from a Fender GDEC amp and the tube power amp from a Princeton Reverb. Put them together, tweak a few component values, use cheaper transformers, make a few changes to eliminate the LCD display, offshore the manufacturing to China, and voila - the SCXD is born!

Originally Posted by JDRNoPro

Also, nice project and experimentation with Mr. Perch's reincarnated SCXD !

Thanks!

Now if only I could find a way to cram spring reverb in there as well!

-Gnobuddy

[Gnobuddy]

Originally Posted by Mr Perch

With Wilde Bill Lawrence pickups in my Tele, I get the best results with both treble and bass set at "0" on the amp.

Very interesting! According to the GM Arts website, having bass and treble at 0 (and turning up the mid to maximum) actually sets the guitar amp to have a flat frequency response - and usually, that doesn't sound good with an electric guitar:

To hear the natural sound of a pickup on an amplifier with a tone stack, set the middle set to full, and bass and treble on zero. This actually sets a flat response in the amp (see below), and I expect you will hear a muffled and muddy sound.

This is supposedly because most electric guitar pickups have a midrange peak in them, which is normally somewhat cancelled by the midrange dip Leo Fender put into the tone controls on the amps he sold.

Are the Wilde Bill Lawrence pickups low-output types, by any chance? Those can have less of a peak and might be expected to sound better through amp set for flat frequency response.

Recently I've been doing some experiments with guitar pickups to find out what that built-in midrange peak looks like. I wound myself a test coil that lets me measure the pickup 's response. So far I've done preliminary measurements on two pickups - a stock humbucker off a cheap Epiphone LP II and a rails-type pickup (manufacturer and model unknown).

Here's the interesting thing: one pickup (the stock Epiphone one) has a huge peak near 4 kHz. The other has a huge peak near 3 kHz. And the dip in the Fender tone controls is apparently somewhere around 0.5 kHz (that's 500 Hz). Nowhere near the peak from either pickup!

In other words, the Fender tone stack doesn't even come close to cancelling the peaks in the two pickups I measured - the dip is in entirely the wrong place.

Did pickups evolve and change since the 1950's when the Fender tone stack was designed? Did the dip and peak line up in 1955, but not any more today? Or did Fender get it "wrong" from the start? Or did Fender design it to sound good, and not to actually cancel the peak from the pickup?

I have no idea, but the whole situation is pretty interesting!

-Gnobuddy

[TMoxness]

Regarding the re-bias of the SCXD;

I've owned my SCXD for about 18 months. I did the above mentioned bias on stock tubes and had them crap out in about 3 months.

I now have some old Philips 6V6 tubes in the amp that I had laying about, biased them to 40mV again, and now 6 months later I'm geting a loud sound feedbacking in the amp. Like a low rumble that starts occasionally. It doesn't seem to happen when I use an extension cab.

Keep in mind that I don't use this amp frequently and when I do it's usually at super low volumes.

I'm starting to think that this 40mV bias isn't worth it if it's going to go through $35 worth of tubes every few months.

I took the amp apart today, and the bias is at 39mV, and the bias pot is all the way up. Does this mean my tubes are on their last legs?

Thanks.

[Hiker]

TMoxness, the idea is to set the bias after installing a new tube, or tubes. After changing your bias setting, the question is whether or not to recheck bias again in the future-if tubes are replaced with the same make & model of tube(s).

My bias was much too high from the factory, and I reset the bias soon after it arrived at my door.

[Mr Perch]

Originally Posted by Gnobuddy

Are the Wilde Bill Lawrence pickups low-output types, by any chance? Those can have less of a peak and might be expected to sound better through amp set for flat frequency response.

I believe so. Although they are "stacked humbuckers," they are equivalent in volume to the stock single coils and they are said to be low inductance -- I'm not sure exactly what that means, but I told that it works to my advantage because I am using a volume pedal and there is no discernible loss in tone.

[Gnobuddy]

Originally Posted by TMoxness

I've owned my SCXD for about 18 months. I did the above mentioned bias on stock tubes and had them crap out in about 3 months.
<snip>
I'm starting to think that this 40mV bias isn't worth it if it's going to go through $35 worth of tubes every few months.

TMoxness, sorry about your bad experience. But it's not normal for that amount of bias (40 mV for a pair of 6V6 tubes) to kill output tubes.

I measured a B+ of about 390 volts in my SCXD. To find the power each tube is handling, multiply the B+ voltage by 40 mA (that's the bias current that corresponds to 40 millivolts across the one-ohm resistor R20. 40 mA is the same as 0.04 amps).

The answer is 15.6 watts, for the pair of 6V6 tubes. That means each tube is running at half that, i.e. about 7.8 watts. The 6V6 datasheets I looked at say that the tube is rated for a maximum of 12 watts of continuous power. 7.8 watts is 65% of that maximum, which is a safe bias point.

One rule of thumb I've heard is to bias the tubes to no more than 70% of the maximum power - in this case, that would be 8.4 watts. The 7.8 watt bias point recommended for the SCXD is a little lower than this, and should be safe for the tubes.

This is backed up by the experience of most users - we're not experiencing abrupt tube failure after setting 40 mA bias. For example I used the original tubes in my SCXD for well over a year, maybe closer to 2 years. It got played a few hours a week on average. At the end of that time I checked the bias, and it was something like 39 mV - basically perfect from the factory. So the tubes had been operating at that bias level for well over a year with no ill effects.

Perhaps the reason your tubes died is because they already had 21 months of use on them, and not because you re-biased them to 20 mA each (40 mV across R20)?

Originally Posted by TMoxness

I took the amp apart today, and the bias is at 39mV, and the bias pot is all the way up. Does this mean my tubes are on their last legs?

Thanks.

Unfortunately, I think you're right. I think that means the tube cathodes are low on emission - not spitting out as many electrons as they should.

You mentioned "old Phillips tubes" - were these tubes NOS, or had they already been used in the past?

While the math and the data sheets do confirm that 40 mA for a pair of 6V6 tubes running on 400 volts or less is safe, if you're concerned, you can always turn the bias current down.

By the way I think the "40 mA, NOT 39, NOT 41" is a little over the top. Both those numbers (39 mA or 41 mA) are within 2.5% of 40 mA - and I'm quite sure that a 2.5% change in bias current is insignificant!

-Gnobuddy

[Gnobuddy]

Originally Posted by Mr Perch

I believe so. Although they are "stacked humbuckers," they are equivalent in volume to the stock single coils and they are said to be low inductance -- I'm not sure exactly what that means, but I told that it works to my advantage because I am using a volume pedal and there is no discernible loss in tone.

Well, low inductance basically means fewer turns of copper wire in the coils. That usually translates to lower output and a cleaner and brighter sound, with a wider and more even treble response. It's a sound closer to the sound the strings themselves actually make - most magnetic pickups made for acoustic guitars are wound this way, to let the sound of the strings come through relatively unchanged.

I'm guessing the wider and flatter treble response of your pickups (as opposed to, say, the midrange peak and treble roll-off from a typical Gibson humbucker) has something to do with the fact that they sound good through an amp with the treble turned all the way down.

I should really check one of those stacked humbucker types out some time. With my Squier Standard Strat and SCXD, I usually can't use switch positions 1, 3, or 5 because of intrusive hum. Fortunately the humbucking positions 2 and 4 work fine, and I usually prefer their tone.

-Gnobuddy

[Gnobuddy]

Originally Posted by Hiker

My bias was much too high from the factory, and I reset the bias soon after it arrived at my door.

Wow. Not the greatest quality control on FMIC's part, eh?

There have been a lot of reports of low ("cold") bias on this thread, but I don't remember any other cases of too high a bias from the factory.

-Gnobuddy

[Mr Perch]

Originally Posted by Gnobuddy

I'm guessing the wider and flatter treble response of your pickups (as opposed to, say, the midrange peak and treble roll-off from a typical Gibson humbucker) has something to do with the fact that they sound good through an amp with the treble turned all the way down.

I should really check one of those stacked humbucker types out some time. With my Squier Standard Strat and SCXD, I usually can't use switch positions 1, 3, or 5 because of intrusive hum. Fortunately the humbucking positions 2 and 4 work fine, and I usually prefer their tone.

I like the sound of the individual pickups, and the hum from the stock pickups was too much for me to tolerate. I bought the Bill Lawrence pickups primarily due to the testimony of his devoted fans here on this forum. They are dead quiet. They sound funny at extremely low volumes, but you needn't worry about that because of your L-pad. With my amp cranked, they sound great, especially the bridge pickup. They are also far less pricey than other noiseless pickups, mainly because Bill and Becky don't advertise, they just rely on word of mouth (this forum, for example.)

[TMoxness]

Thanks for the info Gnobuddy.

I had the factory tubes crap out on me after only about 40 hours of use. Now this old set of Philips are crapping out on me after 20 or 30 hours of use. Granted I don't know how much use these Phillips tubes faced before installation.

I'll try some new tubes and I may bias a bit less than 40 mV and see how much life I get on these 6V6.

[JohnS]

You might consider JJ 6V6's. They are really rugged and can handle high voltages.

[t-luxe]

I've had Tung Sol RIs in mine for 2 years @40Mv with no issues.
JJs are are good as well.

[JohnS]

Originally Posted by JohnS

Well, I've been playing and tweaking for a couple of days now. Very nice, but sounded a bit constipated. Checked the bias this morning and it was WAY off. Put in some Hytron NOS 6V6's I had and a new Tung Sol 12AX7 and biased to 40 mv. MUCH better. I have a broken in Weber Sig 10S alnico on the way. That should remove the last bit of constipation. Havin' fun now!

Yep, the Weber Sig 10S alnico is "the bee's knees" as they say. Nice full sound on the clean channel and the distortion stuff sounds just fine as well. For $40 are you kidding me? Pics coming some day.

[Mr Perch]

Originally Posted by Gnobuddy

Well, low inductance basically means fewer turns of copper wire in the coils. That usually translates to lower output and a cleaner and brighter sound, with a wider and more even treble response. It's a sound closer to the sound the strings themselves actually make - most magnetic pickups made for acoustic guitars are wound this way, to let the sound of the strings come through relatively unchanged.

I think this is why the Bill&Becky pickups sound really weird at low volume -- the pure, unaltered acoustic tone of a solid body guitar is generally not a desirable one. However, they start to sound nice when you pump up the gain.

[Gnobuddy]

Originally Posted by Mr Perch

-- the pure, unaltered acoustic tone of a solid body guitar is generally not a desirable one.

I agree!

It makes me wonder why the inventors who built the first few solid-body electric guitars didn't immediately give them up as a bad idea when they heard that thin cold tone.

Come to think of it, Les Paul's signature guitar tone was pretty thin and cold and lifeless, at least to my ears. And we all know Les Paul preferred custom low-impedance pickups.

-Gnobuddy