Guitar Amp to Guitar Amp Conversion

[BackwardsBoB]

Hi. New here. I'm a circuit designer, and I've been studying tube amps. I grok how they work, and have tweaked a few. I'm finishing Kevin O'Connor's TUT series, and Richard Kuehnel's books. I'm getting strong on the electronic theory, but I anticipate much experimentation to master how the circuits/tubes sound.

I find I'm more interested in working on the tone in my rig than the tone in my fingers. I like guitars with air in them, and jazzy or bluesy tones, but I like torturing a 100W EL34 based half-stack as much as the next real man. I find that EL84s are annoying, EL34s are only useable somewhere above 100dB, 6L6s are very nice, and 6V6s sound like angels (but I still respect you. It's just me). Power stage distortion is much sexier than anything you could do with 1.5 12AX7s.

Anyway, I note that home-built amps almost always look like cheese and feature an EL84 or something running SE class A with 2 knobs max, putting out enough wattage to really annoy your wife, but not enough to play over drums. I also note that buying power and output transformers, a chassis, speaker, reverb tank, knobs, case, etc. makes a mediocre kit as expensive as some of the nicer amps already available.

So instead of trying to make a kilowatt amp out of an arc welder, I figure why not make a guitar amp out of a guitar amp? To this purpose, I have assembled a disturbingly large collection of Carvin X-Series amps, all vintages and configurations. These amps don't suck, and you can get them for $200-$450. I got a faux-Tolex XV-212, no speakers or tubes, minty, with cover, for $199 with free shipping a few days ago. The little X30s and X60s are charming, the heads are loud, and they all have a solid-state rectified clean channel that's just lovely. The XT and XV 112s might as well be Boogies for my purposes. Other features include windings for a +/- 15V supply.

So for $200-$300, all I have to do is add custom tube circuitry to open my own boutique, and a single board layout (do it for a livin') can be a teeny 112, a larger 112, a 212, or a head, with 60W or 100W transformers.

Using GMX, I can convert even the 100W heads to a pair of 6L6s with no loss of power, no need for an output transformer change, and an instant abundance of spare heater current. I can add power scaling to the X60s and make the ultimate grab-and-go amp. I can add a sag resistor, 6L6s and a decent speaker to the X30s and make blues babies. And they all need a better drive channel pre-amp. Concentric-shaft pots solve my additional control needs for a price, and a pair of custom front-panel overlays (for the heads and the combos) are worth the price as a finishing touch.

Of course, nobody's going to pay me what it's worth for a butchered old X100B, but they'll certainly make lovely gifts, and I want 6 for myself (X30, X60, XT112, XV112, and 6L6 and EL34 X100B heads).

I figure if I get only a little lucky, with careful planning I can do a single board layout that will let me experiment for years, with various overlaid circuit provisions.

Party. Anybody out there actually implement GMX?

[Bruce / Mission Amps]

Hi. New here. I'm a circuit designer, and I've been studying tube amps. I grok how they work, and have tweaked a few. I'm finishing Kevin O'Connor's TUT series, and Richard Kuehnel's books. I'm getting strong on the electronic theory, but I anticipate much experimentation to master how the circuits/tubes sound.

I find I'm more interested in working on the tone in my rig than the tone in my fingers. I like guitars with air in them, and jazzy or bluesy tones, but I like torturing a 100W EL34 based half-stack as much as the next real man. I find that EL84s are annoying, EL34s are only useable somewhere above 100dB, 6L6s are very nice, and 6V6s sound like angels (but I still respect you. It's just me). Power stage distortion is much sexier than anything you could do with 1.5 12AX7s.

Anyway, I note that home-built amps almost always look like cheese and feature an EL84 or something running SE class A with 2 knobs max, putting out enough wattage to really annoy your wife, but not enough to play over drums. I also note that buying power and output transformers, a chassis, speaker, reverb tank, knobs, case, etc. makes a mediocre kit as expensive as some of the nicer amps already available.

So instead of trying to make a kilowatt amp out of an arc welder, I figure why not make a guitar amp out of a guitar amp? To this purpose, I have assembled a disturbingly large collection of Carvin X-Series amps, all vintages and configurations. These amps don't suck, and you can get them for $200-$450. I got a faux-Tolex XV-212, no speakers or tubes, minty, with cover, for $199 with free shipping a few days ago. The little X30s and X60s are charming, the heads are loud, and they all have a solid-state rectified clean channel that's just lovely. The XT and XV 112s might as well be Boogies for my purposes. Other features include windings for a +/- 15V supply.

So for $200-$300, all I have to do is add custom tube circuitry to open my own boutique, and a single board layout (do it for a livin') can be a teeny 112, a larger 112, a 212, or a head, with 60W or 100W transformers.

Using GMX, I can convert even the 100W heads to a pair of 6L6s with no loss of power, no need for an output transformer change, and an instant abundance of spare heater current. I can add power scaling to the X60s and make the ultimate grab-and-go amp. I can add a sag resistor, 6L6s and a decent speaker to the X30s and make blues babies. And they all need a better drive channel pre-amp. Concentric-shaft pots solve my additional control needs for a price, and a pair of custom front-panel overlays (for the heads and the combos) are worth the price as a finishing touch.

Of course, nobody's going to pay me what it's worth for a butchered old X100B, but they'll certainly make lovely gifts, and I want 6 for myself (X30, X60, XT112, XV112, and 6L6 and EL34 X100B heads).

I figure if I get only a little lucky, with careful planning I can do a single board layout that will let me experiment for years, with various overlaid circuit provisions.

Party. Anybody out there actually implement GMX?

Hmmm... You remind me of Dr. Evil (coming forward to 2008 from 1965)... and holding the entire world hostage for.... one million dollars.. snicker .. whoa ha ha ha whoa ha ha ....


I think you need to find your #1.

[PRNDL]

May I suggest that you bring these wonderful amps back to life by putting in an eyelet board based on the original? This way you'd be doing a vintage restoration.

It's also possible to put in a Marshall or Fender circuit board that is similar to the original circuit. For instance: "This is a Carvin amp with a Pro Reverb circuit" Also, going from four to two 6L6's is smart these days.

I believe you are about to make a major leap in your amp building skills - from novice hack to semi-pro.

I'm in the same situation, but am taking my own path ... I'm designing and building 1 and 2 watt tweed amps. My latest project involved rebuilding a P2P amp into a tweed style chassis with an eyelet board on red G03. I picked up the tweed today and plan on gluing it on tomorrow.

[BackwardsBoB]

PRNDL wrote:

May I suggest that you bring these wonderful amps back to life by putting in an eyelet board based on the original? This way you'd be doing a vintage restoration.

It's also possible to put in a Marshall or Fender circuit board that is similar to the original circuit. For instance: "This is a Carvin amp with a Pro Reverb circuit" Also, going from four to two 6L6's is smart these days.

The amps aren't dead. They generally just need a bit of pot/connector clean-up, stiffer screen resistors, maybe some new sockets, a bias pot...

I don't really think they're vintage either, except for the X30s. They date from around '84 to '93, but they are plentiful and little-valued, except by owners. I like the amps alot. Otherwise, I never would have bought 15. They're quiet and well built.

They're PCB based, and feature about 10 op-amps, mostly in the tone controls, but sometimes used as buffers or mixers in the signal path and reverb, some CMOS inverters, and some JFETs in the channel switching. The result is a low tube count and great tone controls (4 controlling both channels, and a switchable 5-band EQ on all but the X30s and X60s) with low interaction and +/-12dB range you can use as a boost.

The clean channel is special. The pre-amp is a single 12AX7, without cathode bypass. Headroom on this channel is great, and with the silicon rectification, they are a chicken-picker's dream. There's enough warmth to get you by, especially in the earlier 6L6 based amps. I think this is a result of their descendance from a long line of steel guitar amps (another great application). My Takamine jumbo 12-string sounds really good through it.

The drive channel is a bit of a mess, and went through a few revisions, apparently in search of Boogie tone, which they found elusive. Many players stick to the clean channel, and feed it with pedals, or just turn it up, run the drive channel pre-amp pretty clean, and overdrive the output (earplugs and a farm recommended).

The exception is the little X30. It's a 30W amp, about as small as you can make a 1x12 combo with a short-tank reverb, identical to the X60, except that the B+ is reduced with a different power transformer. It gets crunchy on the clean channel when turned up, and the drive channel is over-the-top. This was actually marketed in the early '80s as a recording amp. I've seen 4 on eBay in a year, and snapped them all up. Both the X30 and X60 love to shake themselves to death, but tight sockets, tubes with low-microphonics, a light touch on the reverb, and frequent maintenance keep them running.

I count myself in with Carvin and London Power. Tubes are great, it's OK to support them with silicon, and PC boards don't have to suck. I can do things with PC boards for grounding and shielding I just can't do point-to-point or on an eyelet board, and Hiwatt, London Power, and even Carvin have shown it can work, though the board you need ain't cheap.

Replacing the whole circuit board on these babies is definitely a drastic act, but most of the mods I'm considering can be bypassed or switched out:

• Sag - need it to get a good smooth jazz tone for my jazz box.
• Power Scaling - I love the cry of overworked power tubes, and I don't live in Montana. I'm married too.
• Drive channel preamp improvements - I need lots of options here, and a PC board can provide them. I'm not against cloning the Legacy or a JCM800 in this area.
• Support for mutiple tube types - Individual tube biasing and pair balancing, and cathode switching for multiple pairs.
• Support for Class A operation and cathode biasing.
• GMX support to get higher output from 6V6s and match the output transformer impedance.
• Optional Microprocessor control and storage of setups. I want knobs for everything, but I play guitar two-handed, and I'm just not that well endowed.

The result will still do Carvin clean, with the operating point of the preamp tube shifted a bit toward cutoff with a smaller cathode resistor (I like it like that), sag and power scaling can be turned off, and the amps can still be set up to do Class AB push-pull with the tubes the designer intended.

Different guitars need different amps, and different amps need different circuits. With the X-series, I've got everything from little 30W 1x12 combos, up through 100W heads and 2x12 combos.

I want one of the larger 1x12s with sag and 6V6s for my jazz box and my Holdsworth, my tele and acoustics need stock Carvin clean through 6L6s on the larger 1x12s, with a sag option and a clean channel that goes to crunch using power scaling of the output, and my humbucking mahogany solid bodies need 100W of my choice of 6L6s, EL34s, or both, a bit of sag, and a decent drive channel and power scaling on the heads and 2x12 combos. And I want an X30 and an X60 with power scaling tuned for blues.

Imagine showing up somewhere with what looks like a joke practice amp and a joy masquerading as a stock Lite-Ash Tele, waiting for people to laugh, and then ripping face.

Or showing up with a gold metal-flake jazz box you bought new for $200, and what looks like a $300 combo amp, putting everyone to sleep, and stealing their wallets.

Or stacking a grey fuzzy head on a grey fuzzy 4x12 cab and rendering somebody's Marshall inaudible.

Good clean fun.

Carvin recently re-introduced the X100Beast as a 100W head and 2x12 combo ($999 for a half-stack!), stuffed with EL34s and switchable to other tubes, but the combo weighs 37#, the head is smaller, they've replaced the Accutronics long tank reverb with something digital, and they converted the pull-pots to teensy mini-switches. They're available in lots of colors, but there's no grey fuzzy option. They're just beginning to ship, so who knows what else they did? The white ones look cool though, in a metrosexual kind of way. I hope to be more respectful.

PRNDL (if that's your real name) - If you're rebuilding classic vintage amps respectfully, returning them to their original reliability, using modern components when they have an advantage, and making small tweaks to the design that offer clear advantages without altering the amps' souls, I think that's great. May your supply of vintage parts be plentiful.

I'm the kind of guy who can't own an old JCM800 because the urge to add reverb would drive me crazy, and can't own a '57 Les Paul because I'd want to change the pickups and re-wire it. There's tons of other great stuff out there that nobody loves for me to play with. You want to preserve a minty old X100B 100W head like Vai and Zappa used to use? There's one going off on eBay Sunday. Bet you can snag it for $373. And another one that's more my style. Great deals on Quad X-Amps coming up Sunday too. One minty and one needing love. Very good stuff.

[Steve Conner]

What the heck is GMX?

[BackwardsBoB]

You know what Gm is, and X can be the unknown, or it can be the multiplication sign for non-programmers. It's a tube transconductance multiplier circuit. You put a little cathode resistor on a power tube, providing a way to measure the tube current, and have an op-amp drive a cascode of MOSFETs with feedback so that the MOSFETs drive some multiple of the tube current in parallel with the tube. Wango! A high-power, solid-state circuit with tube characteristics. It's buried in the back of Kevin O'Connor's TUT4.

So I can take a 100W Carvin X100B, host two 6V6s in it, and still drive the 100W output, with proper impedance matching to the stock transformer, using two of these circuits. I'm left with amps of unused heater current that I can use for more tubes and other things.

There are heatsinking issues, etc., but as long as I'm doing a new board, I think I'd be daft not to support it as a stuff option.

Pretty sacreligious, huh?

[Steve Conner]

There is an old adage that says: If you want high voltage with transistors, or high current with tubes, be prepared to pay.

This means that transistors naturally prefer low-voltage, high-current operation, and tubes vice-versa. You'd need to spend a lot on high-voltage MOSFETs and heatsinks to implement the GMX scheme reliably. It strikes me as an equally deranged mirror image of the reasoning that led to OTL tube hi-fi amps: "Transistor amp with an output transformer whose sole purpose is to make the output devices more expensive and harder to source"

I suggest a more radical solution: take the money you'd have spent on this stuff, go to the store, buy four 6L6s, stick them in the four sockets provided and let 'er rip. You still have 2.8A of heater current left over, if the amp was designed for EL34s.

IMO, the place for transistors in a hybrid is between the output transformer and the speaker. The voltage levels here are suitable for 3 dollar types like the MJ15024 and MJ15025. Now if you took the voltage across those cathode resistors, and somehow sneaked it round the O.T. to a high-power current source that drove the speaker directly, then you'd be cooking with something.

[hasserl]

I think this is the first time I've seen a (fellow) Carvin geek on Ampage. Don't get me worng Bob, I'm pretty heavily into Carvin's too; but my attention has been focused on the amps that followed the X series amps, the Valve Master and Vintage series amps. Good platforms to play with. But it's pretty rare to see anyone pay much attention to Carvin around here.

Hey, didn't Musicman do something similar with tubes & resistors in a hybrid output section? Getting a claimed 100 watts out of two 6L6's and some transistors in a cascode arrangement?

[BackwardsBoB]

Hasserl wrote:

I think this is the first time I've seen a (fellow) Carvin geek on Ampage. Don't get me worng Bob, I'm pretty heavily into Carvin's too; but my attention has been focused on the amps that followed the X series amps, the Valve Master and Vintage series amps. Good platforms to play with. But it's pretty rare to see anyone pay much attention to Carvin around here.

You're definitely the god of EL84-based Carvin amps. I just can't wrap my ears around EL84s. I'm more 6l6s and 6V6s. I need to hear whether the guitar is mahogany or koa. I suppose if I played in a band, I'd be more amenable to EL-84 bite. The ValveMaster definitely has X-series roots, and the drive channel's even worse. It's a great platform for mixed quads though, and it comes in fuzzy.

The X-series are great amps, slightly crippled by their lead tone, but easily patched up with pedals. I almost always run the clean channel. And what do you want for $350? I've got pretty much 4 of every configuration, X30, X60, XV and XT 112 and 212, and 60W and 100W heads.

You can tweak them a little and get anything from smooth clean to chicken pickin' to classic Marshall-inspired rock tones.

Carvin stuff on eBay is really good. Great amps and guitars, really cheap. My Holdsworth Fatboy, CT6 and DC400 are exceptional instruments at any price. You've got to admire a company that would develop and sell the Quad-X preamp. It's pretty far out there.


Hey, didn't Musicman do something similar with tubes & resistors in a hybrid output section? Getting a claimed 100 watts out of two 6L6's and some transistors in a cascode arrangement?

There's nothing new under the sun. Just configurations of pieces, hopefully aimed at a goal.

[BackwardsBoB]

Steve Connor wrote:

There is an old adage that says: If you want high voltage with transistors, or high current with tubes, be prepared to pay.

Might be an old adage. My college "Integrated Electronics" text says that "FETs with on resistance as low as a few Ohms are now available."

A custom transformer would let me run the FETs at a lower voltage, but I want to use the standard X-series transformers. A second pair of 6V6s would only get me to 44W if I lie, which is disrespecting the X100Beast. I'd need 10 6V6s to get 100W. I'd only need 6 $3 MOSFETs going solid state. But... I could substitute a pair of 6L6s for the MOSFETs, and a pair of 6L6s would get me into the 60's, with 6V6 sound. That's still an extremely buff 6V6 amp. I wanted to drop the screen voltage a bit for the 6V6s though, and it's right where it should be for the 6L6s. Shouldn't be too hard.

You're full of good ideas!

[Steve Conner]

On-resistance has nothing to do with it. The on-resistance of any MOSFET is vanishingly small compared to any power tube.

The problem is that you'll need FETs capable of standing twice your B+ voltage plus a safety margin. So 1200V for a 100W amp.

They also need to be rated for linear operation at high voltage. The garden variety MOSFETs for switching power supplies aren't specified for this. They may work, but they're not guaranteed to.

In practice you seem to get away with it: I've used switching MOSFETs in linear regulators myself, and Yorkville seem to use an IRF-something as a capacitance multiplier type thing in some of their tube amps.

I'd be real interested to know what parts Ko'C recommends and/or what parts you plan to use.

[Gtr_tech]

PRNDL wrote:


The amps aren't dead. They generally just need a bit of pot/connector clean-up, stiffer screen resistors, maybe some new sockets, a bias pot...

I don't really think they're vintage either, except for the X30s. They date from around '84 to '93, but they are plentiful and little-valued, except by owners. I like the amps alot. Otherwise, I never would have bought 15. They're quiet and well built.

The value of these X amps is going *way* up these days. I used to get them for $100 or less back in the early/mid 90s....you couldn't *give* them away back then 'cause they sounded so bad everyone thought there was something wrong with them. I don't understand why they're collectable now, I think they sound horrid stock. One of my main gigging amps is a mid 80s X100b, but its gone through a good bit or re-engineering and really screams now.

They're PCB based, and feature about 10 op-amps, mostly in the tone controls, but sometimes used as buffers or mixers in the signal path and reverb, some CMOS inverters, and some JFETs in the channel switching. The result is a low tube count and great tone controls (4 controlling both channels, and a switchable 5-band EQ on all but the X30s and X60s) with low interaction and +/-12dB range you can use as a boost.

Vintage snobs scoff at op amps in the signal path, but I think thats what makes the X amps great. The mid control is actually a mid control, not just another treble control that starts further down the audio band.

The clean channel is special. The pre-amp is a single 12AX7, without cathode bypass. Headroom on this channel is great, and with the silicon rectification, they are a chicken-picker's dream. There's enough warmth to get you by, especially in the earlier 6L6 based amps. I think this is a result of their descendance from a long line of steel guitar amps (another great application). My Takamine jumbo 12-string sounds really good through it.

The drive channel is a bit of a mess, and went through a few revisions, apparently in search of Boogie tone, which they found elusive. Many players stick to the clean channel, and feed it with pedals, or just turn it up, run the drive channel pre-amp pretty clean, and overdrive the output (earplugs and a farm recommended).

Yep, seems they were going for Booger territory, but had the good sense to leave the tone stack out of the preamp gain stages. Its fairly easy to redo the front end to get them to smooth out and focus the gain better. There's too much low end forced through the lead channel and that makes them sound tubby and loose.

The exception is the little X30. It's a 30W amp, about as small as you can make a 1x12 combo with a short-tank reverb, identical to the X60, except that the B+ is reduced with a different power transformer. It gets crunchy on the clean channel when turned up, and the drive channel is over-the-top. This was actually marketed in the early '80s as a recording amp. I've seen 4 on eBay in a year, and snapped them all up. Both the X30 and X60 love to shake themselves to death, but tight sockets, tubes with low-microphonics, a light touch on the reverb, and frequent maintenance keep them running.

I count myself in with Carvin and London Power. Tubes are great, it's OK to support them with silicon, and PC boards don't have to suck. I can do things with PC boards for grounding and shielding I just can't do point-to-point or on an eyelet board, and Hiwatt, London Power, and even Carvin have shown it can work, though the board you need ain't cheap.

Replacing the whole circuit board on these babies is definitely a drastic act, but most of the mods I'm considering can be bypassed or switched out:

Imagine showing up somewhere with what looks like a joke practice amp and a joy masquerading as a stock Lite-Ash Tele, waiting for people to laugh, and then ripping face.

Heh...I've done that several times at jam nights with a rebuilt/redesigned Alamo Montclair. The looks I get when I walk in with it are expected...the looks I get after I fire it up are priceless.

Or showing up with a gold metal-flake jazz box you bought new for $200, and what looks like a $300 combo amp, putting everyone to sleep, and stealing their wallets.

Or stacking a grey fuzzy head on a grey fuzzy 4x12 cab and rendering somebody's Marshall inaudible.

Good clean fun.

Carvin recently re-introduced the X100Beast as a 100W head and 2x12 combo ($999 for a half-stack!), stuffed with EL34s and switchable to other tubes, but the combo weighs 37#, the head is smaller, they've replaced the Accutronics long tank reverb with something digital, and they converted the pull-pots to teensy mini-switches. They're available in lots of colors, but there's no grey fuzzy option. They're just beginning to ship, so who knows what else they did? The white ones look cool though, in a metrosexual kind of way. I hope to be more respectful.

PRNDL (if that's your real name) - If you're rebuilding classic vintage amps respectfully, returning them to their original reliability, using modern components when they have an advantage, and making small tweaks to the design that offer clear advantages without altering the amps' souls, I think that's great. May your supply of vintage parts be plentiful.

You want to preserve a minty old X100B 100W head like Vai and Zappa used to use? There's one going off on eBay Sunday. Bet you can snag it for $373. And another one that's more my style. Great deals on Quad X-Amps coming up Sunday too. One minty and one needing love. Very good stuff.

The Quad-X thing is getting pricey on Ebay as well. HAte the way those sound as well. I have one but its been rebuilt/redesigned and sounds the way I want it to. Only issue with it is theres a little hum I can't track down. It may be a grounding issue...haven't looked that far into it yet.

[Gtr_tech]

Hmmm....that whole damn post came up as a reply. Read between the lines kids...I flubbed it.

[BackwardsBoB]

Steve Connor wrote:
On-resistance has nothing to do with it...

You don't think that Marketing told the guys who dropped the on-resistance by a factor of 1000 "Damn the cost and breakdown voltage! We need lower on-resistance!" do you? The cost of tube solutions has only come down a little, mostly due to capacitor pricing, but semiconductor solution cost is always coming down. The MOSFETs around when an old adage was young cost more. Check the selection table at the IR site for 400V NPNs. It has pricing. I was surprised. Soon all life will be BGAs, MOSFETs, little tiny inductors, and the R's and C's to make them happy. Actually, it's been that way for a few years now. I need new glasses.

I'd be real interested to know what parts Ko'C recommends and/or what parts you plan to use

You pretty much sold me on the 6L6s for my application. KO'C suggests cascode triples of IRFP460 for a 300W solution, and he goes on to sketch additional refinements with custom transformers. TUT4 is wild stuff.

[BackwardsBoB]

Grtr_tech wrote:
The value of these X amps is going *way* up these days. I used to get them for $100 or less back in the early/mid 90s....you couldn't *give* them away back then 'cause they sounded so bad everyone thought there was something wrong with them. I don't understand why they're collectable now, I think they sound horrid stock. One of my main gigging amps is a mid 80s X100b, but its gone through a good bit or re-engineering and really screams now...

The Quad-X thing is getting pricey on Ebay as well. Hate the way those sound as well. I have one but its been rebuilt/redesigned and sounds the way I want it to. Only issue with it is theres a little hum I can't track down. It may be a grounding issue...haven't looked that far into it yet.

It sounds like most of you dissatisfaction is with the lead channel pre-amps. The X-series clean channel is good. But you're better off with the clean channel and a tube screamer than you are with the drive channel. The drive channel pre-amp is where you've been focusing your efforts, right?

$100 would have been great. The mint X100B heads are going over $400 now, and a Quad-X and matching 100W tube power amp will set you back over $700 now. I kind of wonder how much of that is my fault. I've bought at least 25% of the X-series amps on eBay in the last year. All the X-30s. Maybe 60% of the X-60s. I think $350 is still a great deal.