Turning a solid state unit into a hybrid guitar amp

[Jim Coash]

Greetings:  I have an old solid state guitar amp which recently smoked.  I have isolated the problem to the power amp which is conveniently on the rear panel of the amp with the preamp on the front.  The harness that connects them would seem to be ideal for driving a completely new tube amp I could build in place of the original.  I think this would yield a very useful hybrid with a solid state preamp with plenty of features and a much better sounding tube output section.  I would like a 15W (Princeton) or 40W (Super) design that would marry nicely in this unit.  Has anyone ever attempted something of this nature?  There is plenty of room inside and I could easily modify the chassis to fit a pair of 6V6, 6L6, KT88 or EL34 output tubes.  I expect I might need a tube driver to get the impedance to match.  I will also need a new transformer.
I would appreciate some advice on this.  Jim

[kagliostro]

Give a look to the schematic section here on the forum

there are schematics like those of the old italian Davoli Krundaal that use SS preamp in junction with vacuum tube PI and Power Section

K

[terminalgs]

two transformers.   PT & OT.  That was a big advantage of SS power stages, no OT. 


If you don't have the real estate for all that iron, you could find a PT that has a winding for SS amps (red knob fenders of the 80s/90s, like a Super 60, or a Peavey tweed) but just like you'll have to do for the HT winding for the tube side, you'd need to match the VAC of your existing SS power supply.


Check out Musicman schematics, like the RD50.

[eleventeen]

There are many many considerations involved. You're surely going to need 2 transformers, if not a choke. You're right about needing a general impedance "transformation". There's the strength of the chassis sheet metal which in many SS amps is a little on the light side. And whether there is room for all that you'd need.


We'll assume the power supply of your SS amp is OK as you said and it's the power output section that is cooked, you just want to patch around it, in effect.


Personally, I think I would build the tube power section in an entirely separate chassis that could, worst case scenario, be built up to a functioning amp (eg; with room for controls and room for a few preamp tubes) all by itself just in case the "bolt in the tube section project" could not be made to work.  Indeed, if the SS amp has a "preamp out" and you can figure out a spot in a tube amp you own where you can patch to a "power amp in" it would be highly desirable to see if you can Frankenstein the two parts together with interconnecting wires before building something. Obviously, you can not allow 250 volts typical preamp B+ to leak back into the SS section, you'll obliterate it!

It could be an interesting project, but it could also be a blind alley consuming a load of time. All depends upon how skilled and how well-equipped you are. You'd for sure want an oscilloscope.

[MadMax]

Jim,
I've pondered the same kind of madness. If you go for it, I hope you document it here!

-MadMax-

[Jim Coash]

Thanks for the input.  The amp in question is a Kustom K-200 series unit I got from Jack here on this site.  I cleaned it up, replaced the reverb tank and it worked fine for several hours on my bench.  My son took it to drive my Fender Dual Showman clone cab with a pair of E/V-15M speakers at 8 ohms each for a 4 ohm net load.  It sounded normal, for a while, reminding us of why we like tube amps so much better.  It had that early transistor edge and harshness.  Then it began to distort and by the time he switched it off, smoke was issuing from the rear.  As it happens, I have an old Simpson PA amp with good iron that would easily fit inside of the Kustom cabinet.  I would begin by removing all of the solid state amp; power transformer, power transistors (4), filter cap and one large output board.  The entire tube chassis would drop into the empty space easily.  I could just remove the existing controls from the Simpson along with the three preamp tubes and then find a way to match the impedance.   The Kuston has two channels, reverb, OD and several other features we would like to keep.  Jim

[terminalgs]

Like this one?


http://www.ebay.com/itm/John-Fogerty-s-1968-Kustom-K200-A-4-amplifier-from-Creedence-Clearwater-Revival-/281536359747

[Jim Coash]

Yes, very much like that one.  I do have a Carver M-4120 automobile amp (4 X 120WRMS).  I can be run mono.  That would fit nicely in the Kustom chassis.  What I would need is a 12VDC power supply to run it.  Any good ideas for building one?  Jim

[eleventeen]

Wait, now you're saying you want to "patch around" the dead power amp section and replace it with another SS power section? That's different from what you originally proposed. Starting with a SS amp, there are no tubes in that picture.


If you need a 12 v power supply, yes, of course you can build one. If you want a 50 watt amp, you'll need (assuming 100% efficiency which does not exist) 4-5 amps, more practically nearly 10 amps.


While it's a piece of cake to build a 1 amp 12 volt supply, it starts to get a little trickier to build a 10 amp one. You can buy lab-quality 12-15 volt high-amp supplies on ebay for next to nothing, surplus. They would be switching power supplies, but the technology on these has gotten very reliable. Arguably, you most likely have a +/- 35 or 45 or 60 or 75 volt supply within your amp head as is and could theoretically reduce said voltage down to +12 volts. But you'll still need to incorporate parts of highish current capacity, and that can and will get expensive if you blow them up repeatedly.

[shooter]

starts to get a little trickier to build a 10 amp one

here's a couple schematics I found on the web, haven't built either, fwiw a car battery would make a SS amp "feel" like a tube amp 

[eleventeen]

Oh, there are plenty of schematics out there; and the great majority use a 3-terminal regulator to regulate bias on a stack of pass elements, and the thing doesn't seem particularly complicated. Just as you've shown. Go ahead and build a few of those, you'll see that for good sized output current, you'll need a largish-sized heatsink, and, for example, if your power transformer/rectifier outputs say 40 volts (because he might try to use his existing PT and whack down the output volts) and you need to drop 28 volts at 10 amps = 280 watts, you'll need a reasonably serious heatsink.


All I am saying is that by the time you buy 5 qty 2N3055 transistors (used to cost 35 cents, now they are $3-5) and silpads for each at $1.79 * 5 or a $7 tube of silicon goop and mica insulators and a $20 heatsink and the various ancillary parts.....you can go buy a 12 volts Lambda or Power-One or any one of 2 dozen other brands power supply for under $20 delivered, bolt it in, and be done. 

[shooter]

2N3055 transistors

I took 10 1 gal zip-loc bags full of 'em to a scraper about 8 yrs back!, think I got around 35cents a pound!!
You are right though, why pay twice as much for something 1/2 the cost

[sluckey]

Have you considered just fixing the amp?

[eleventeen]

In the early 80's I used to work at a company that built school-type indestructible record players and various small CRT monitors (like as found in early ATM machines and those little 5" ones you used to see in cash registers. They had to do parts inventory every month, I think, certainly every quarter.


Their way of resolving any inventory discrepancies was to throw out any parts that were over what they thought they had.


In the dumpster on inventory days was tens of thousands of dollars in parts. 1000 piece bags of LM317s, 5000 piece bags of 2N4124 and 2N4126 very nice small signal transistors (same as 2N3904/3906 except with formed leads) reels and reels of 2-3 watt resistors, *any* parts that potential suppliers had sent as samples. Huge styrofoam flats of 2N3055's, hundreds of them. 5 lbs bags of threaded chassis standoffs (those have REALLY gotten expensive) It was absolutely sick. Had there been an ebay back then I'm sure I could have cleaned up, but there was no real way to get rid of the 99.7% of the stuff that was beyond what I could use.

[shooter]

all mine came from big water cooled split rail PS's, one NPN or PNP would short, insert steal fuse n blow it up, then spend 4hrs taking off the other 39 and replace with all new.  With 10 systems each having 3 supplies it was a monthly thing!!!

[Jim Coash]

That is a lot of useful info.  Yes, I have considered just fixing the amp but while it was working I wasn't very happy with it.  I am much happier working on tubes and I am a true believer in tube sound over solid state.  One thing I do know is that running my Hafler Hellraiser tube guitar per-amp through any one of my Carver Pro power amps sounds very good.  That Kustom design is almost 50 years old and I suspect I would continue to have problems with it.  The previous owner said it was noisy and he was done fooling with it.  I think a radical change is in order.
There is a large power transformer, filter cap, bridge rectifier and regulator already there.  If I can use that to build a power supply for the Carver car amp, great.  BTW, it is a Carver M-4120 not a M-2120 so it has 120WRMS X four with built in high and low pass filters, bridged mono options for both stereo amps and is stable into 4 ohms or even less.  I have quite a few options.  I am leaning toward running both stereo amps mono for 2 X 240WRMS and having two separate outputs for speaker cabs.  Perhaps even driving each of the two with the two different channels on the pre-amp.  If I could find a suitable power supply that would generate enough amperage for the Carver that would be a good solution but finding or building one that would mimic a 12volt car battery with perhaps several hundred amps is a tall order.  Jim

[terminalgs]

what you are talking about sounds like complete insanity.

I am much happier working on tubes and I am a true believer in tube sound over solid state.

then why concoct this 12V auto supply SS amp to a guitar amp??


The source amperage required is bananas. you might as well strap a Tecumseh 5HP tiller engine  and a '73 Chrysler alternator to the cabinet.  thats easier that figuring out the 120VAC:12VAC transformer and power supply you need.


Are you trying to build a PA for open mic night and karaoke ?

[Jim Coash]

Well, you are of course, correct.  My wife of 44 years would certainly confirm that I have been certifiable as long as she has known me.  Bear in mind that I have always had and will always have a very limited budget to work with.  If I had money, I would buy nice new amps and all the cool parts I need to do things the "right" way, but I do not.  In this case, I must find a way to build a 12 volt power supply that will be as good as the one in an automobile.  I can only use what I have or at the very most spend a few dollars.  I have a Carver M-4120, gratis and a Kustom K-200 that I traded for.  I feel that there is a way to make a power supply using what I have to provide a stable, well filtered and regulated supply equal to the needs of the amp.  I have seen it done at CES shows where car stereo companies brought "tricked out" cars and trucks on to the convention floor and ran huge sound systems at full out put all day long.  They used battery chargers when needed but they still had something that was like a "wallwort" on steroids that converted standard A/C to 12 volts at high amperage.  How can I do that?  I do have several transformers salvaged from solid state amps including a Carver Pro plus very large filter caps and bridge rectifier blocks.  Where do I start?  What will yield a workable supply and not cost me big bucks?  I plan to take the pre-out that feeds the blown power section in the Kustom, strip out all of the power amp parts and install the Carver.  I just need a supply to run it.  I cannot believe it can't be done.  Jim

[sluckey]

Bear in mind that I have always had and will always have a very limited budget to work with.  If I had money, I would buy nice new amps and all the cool parts I need to do things the "right" way, but I do not.  In this case, I must find a way to build a 12 volt power supply that will be as good as the one in an automobile.  I can only use what I have or at the very most spend a few dollars.

It will cost WAAAY more to build a proper 12VDC supply capable of producing the juice needed to power a 4 X 120W amp. The only sensible thing to do is fix that Kustom amp as it is. Better yet, put it on eBay.

[Glennjeff]

Hi Jim,

Of course it CAN be done, the first circuit in reply #9 is how you would do it but it would cost a bit and you would need a big mains to 12 V transformer.

The actually relevant question is SHOULD it be done.

You could always carry around a couple of big marine grade deep cycle 12V batteries to power the PA, they should give a couple of hours running time before recharge. Thy are fairly expensive as well.

No offense meant, but it sounds a bit like swatting mosquitoes by shooting them with a 12 gauge shotgun.

[eleventeen]

Have you ever tried going to an e-recycle place? People throw out old stereo amps all the time. You have to be polite and you have to not take up much of their time and wiggle waggle hesitate over price. many times, the folks who work at such spaces are not exactly the most polite and don't want you looking around. Most of the stuff you will find will be dead computers & printers and screens which do you little good. But now and again you'll find a SS power amp. Some of those have very chunky power transformers. If you pick one up and it is heavy, it's likely to have a real power transformer versus a switching power supply. Again, most likely it will be a 35-45-60 volts plus/minus supply, but often the transformer will have a 120-220-240 volt primary and if you jumper it for 240 volt but run it on 120 volt, the output will be cut in half, at least getting you closer to your 12-volt target.


If you don't have much money to throw at a problem, you just have to be that much more clever.

[Jim Coash]

I have two large transformers, one from a late model Denon receiver which is substantial in size.  The other is one from a Carver PM-600 power amp.  I hadn't thought of the 220V idea.  That just might give me something close to 12 Volts.  I also have the rectifier and filter caps from both amps.  I guess my next step is to identify what I have as far as output goes on those two.  BTW, I am the kind of guy who uses a 12 gauge for just about anything.  Varmints never stand a chance with that.  Jim

[terminalgs]

If I were you,  I'd mock it all up , speakers, guitar, power amp, and some pre-amp powered off your running car/truck in the driveway. see if that power amp will sound good with guitar input and guitar speaker output.   If it sounds good, then worry about building the power supply with your parts on hand.

[Jim Coash]

Actually, that is just what I did over the weekend.  The amp ran fine using my Hafler Hellraiser as a pre-amp.  I just converted the Hafler 1/4" outputs to RCAs for the amps input, used my car for powering the amp and I tried several different cabinets.  I also tried all channels for 120 WRMS outs, both mono at 240 WRMS, with and without the high and low pass filters.  The variable gains on the Carver made it easy to fine tune.  My son and I both thought it sounded very good.  Certainly better than most solid state guitar amps we are familiar with but still not as nice as tubes.  There was little heat even after some hard play for over an hour.  I also took the time to search the web for power supplies and I found one advertised as a perfect replacement for a 12 volt battery system, designed for car stereo.  13.2 volts, 50 amps, well filtered and regulated, $175.  That is an option but I still think I have parts on hand to build something adequate or better.  The Kustom transformer and filter cap is already there but I do have a Denon and a Carver Pro transformer along with the related electrolytic caps and diode block.  Next I need to figure out what I have in terms of output on all three transformers and then find a schematic for a suitable supply.  Jim

[shooter]

FWIW, car DC is the way to go, I built a redneck camper with 500w solar array, 4 deep cell marine batteries and a 3.6kw invertor.  plugged in 3 different guitar amps to the invertor and the modified sine wave is WAY to noisy to get a quiet amp!  So I need to try a dc-dc convertor and "tap" into the amp after the rectifier and see how that works.  But it is sorta cool playing music with solar 

[eleventeen]

Just the first one I found. Lab quality, bulletproof, you could pass this on to your kids. Look a little harder and you'll find one or more for $20. These supplies are for the most part indestructible.

Your 13.2 volt 50-amp supply is probably a linear supply. I bet it weighs 30+ pounds.