Dear Teemuk, thanks a lot for the link. It´s often difficult to get "modern, commercial" amp schematics on the net. The MG was even cheaper than I thought, it´s very similar to the VS8040 minus the 12AX7, period. I was for ages the official service for Import Music, the argentine Marshall importers, but lately we´re not in talking terms, because of some unpaid bills. I still can´t believe in the skimpy 1000uF power supply filter capacitors. They must have about 10V PP ripple on full power, and the 50Hz hum, mixed with the guitar sound, must be easily heard . And the TDA7293 in no way can output more than 80W RMS , by their own admission (6.5A Pk limits output power into a theoretical 84W into 4 ohm resistive load, much less into a real world speaker). Thanks again.

Hola TChrist, sí, soy Argentino, ¡aguante el Perú!, nuestro buen amigo desde hace más de 200 años.

K-15´s are great little amps. They sound specially good when coupled with a somewhat larger 10" or even 12" guitar speaker. What saves the distortion, which "should" be buzzy, is that it has no Master control, so when you distort it you´re also clipping the output stage, which clips the buzzy peaks; besides you can´t distort without putting out at least 8/10 watts.
Some answers :
1)R1 suppresses RF interference by a)working with the OpAmp´s input capacitance and b)"Killing" or at least degrading the input junction diode  which would have demodulated parasite RF.
2)473=47+"three zeros"=47000 pF=.047 uF
22/16= 22uFx16V
3)Clean gain= pot value/33k or approx. 250/33=8x  max.
Dirty gain: 250/1.5=166 ; besides, because of the diodes, the clipping peak voltage lowers from almost 15v to around 700 mV .
4)Log pots preferred (same as Fender uses)
5)I think C19 is a drawing error, maybe carried over from an earlier single supply version. It iss not necessary and can actually harm the sound.
6)R9/10 and C10/11 lower slightly and filter the preamp supply.
Good luck with your project.
PS: the distortion footswith is very crude but actually works; remember to isolate it from ground and use a screened cable, with the screen connected to C5.
PPS:I almost forgot: the gain of the output chip is =(5100/180)+1= around 30. Part of the magic is that the distorted signal (which is attenuated by the tone control) barely drives the power amp; the mix of preamp (buzzy) and power amp (flat and boring) distortions gets interesting: smoother than one and sharper than the other.

They´re similar but the BIG difference lies in the bias: FETs require typically around -2 V on their gates; so they can "self bias" (like tubes) by just tying their gate resistors to ground (0 volts) and using resistors from source to ground. Study your circuits. MosFets require Positive bias (around 2 or 3 volts), so basically you should tie the gate resistors to around + 4 volts supply (Use a preset to adjust it)

Nice work, congratulations. Running your preamp from "only" the +B supply is no problem, you still have a lot more than enough signal to overdrive your 1875. I think you´ll have a smoother EQ range using Log pots (what Fender uses).

For these and many others check: http://www.mif.pg.gda.pl/homepages/frank/

Noval dual triodes generally go: 1,2,3,4,5,6,7,8,9= plate, grid, cathode, filament, filament, plate, grid, cathode, filament center tap (if applicable)

Joecool, you´re in the right path. 50C5´s and many other "radio output tubes" are cheap, plentiful, and "output iron" is easy and cheap to get. Usualy 1 to 2 watts (perfect bedroom power) but you can always add an SS 50 or 100 W power stage after them, keeping the tube sound.

I often repair Valvestates (I have a VS100 on my bench right now) and Fenders, but to be precise, the problems are more mechanical than electronic. PCBs flex and develop invisible cracks just where the thin track (around 30 mils) joins the pot/jack/switch solder pad. Laneys, with a similar construction, are more reliable because they have fatter tracks (40/50 mils). Old Peaveys and most 70´s / 80´s amps, with their hand drawn PCBs, are most reliable in that respect.
Today I also have an Acoustic 118 with beautiful hand-drawn boards (and blown output transistors).  Those cracked track problems are often mistaken as "cold solder joints", specially because they are repaired by resoldering, but truth is that resoldering just bridges both sides of the crack. I use a jeweler´s loupe to detect them. Sometimes I also find blown output transistors and very infrequently some blown TDA1514, very hard to find. I have made a replacement mini-board with an LM3886, which is cheap, available, and far stronger, and wire it to the corresponding pins (+-B, ground, etc.) on the original board.

I think that rather than "emulate tubes", the real challenge is "get good sound", which is often (but not always) found in tube amps, without using tubes themselves. I think Mr Pritchard is a genius, has *very* good sounding amps, but he misses the point, he often tries to emulate *everything* a tube does, which is not the real problem. Someday he´ll emulate a spirally wound 6.3V heated filament, a glass envelope, a 9 pin socket, the sound of high vacuum, and that´s not really necessary. I think Runoffgroove´s approach is basically flawed: Fets do not even approach triodes, (if any, they´re closer to pentodes); they copy (should I say mimic) classic circuits, horribly biased, horribly powered, that have ten times less gain than the originals, clip radically different, (just use a scope; I´me sure they don´t), harsh, dry, ugh!!  An example of good engineering (and audibly good taste) is found in the SansAmp guys, who do not use Fets , CMOS or even diodes but do get a very acceptable sound. I agree that Op Amps (universal gain blocks) with suitable components around them, skilfully used, are one of the safe ways to go.

It´s confusing, but googling MC1303L brings data of a "single channel power amplifier" whichi it very obviously is not. In fact I vaguely remember (from the Stone Ages, at least 25 years ago) something about it, and it as an early op amp type, similar to a dual 709. Perhaps it is so old and so discontinued that they even recycled the name!!. And the SC17063L brings similar confusing resulta. Other possibility is that it was a custom special part number, unrelated to any commercial product. Anyway, first of all, test it; if it works, don´t touch it. The worst problem is that on the power stage. Good amp, classic design, absolutely no short protection and very dead. With the amp off , measure (on the "diode" or "continuity" or "buzzer" resistance scale) the C-E resistance in the Tip33 s and Tip 29, with the red pin on the collector and the black one on the emitter; and on the Tip 30 with the colors reversed. Any short there means a dead transistor, replace them. Be careful with the micas, insulation, etc. If in doubt, pull them carefully from the board and re-measure them: infinite resistance (open) C-E both ways, around 600 mV B-E and B-C one way and infinite resistance the other way. After you´ve replaced them, hook the amp with a 25/40W lamp in series and check voltages  (no speaker connected). You´ll have somewhat less than +-35V on the supplies, and close to 0 Volt on the "hot" speaker out.
Check some pictures that may help:   http://www.geocities.com/nbl968/GibsonG80/


Post your results here. Good luck.

Dear kernalflagg. I think I can help you with your amp, but to make sure we´re talking the same, post here the schematic *you* have, so I´ll suggest you some things to check, to see how close it is to your actual amp, and then suggest some voltage and resistance measurements. To begin with you´ll need a "series lamp" fixture, and your digital multimeter. Bye.

Dear Teemuk: it would be great if you could post some MG schematic here, especially in the 60 to 100W range. I understand they´re basically "tubeless valvestates", I wonder what they replaced the 12AX7 with. Probably nothing too different from a 9004 preamp, but I haven´t repaired one yet, so I don´t know for sure.  Thanks.

Hola TChrist, ¿de dónde sos?

Polytone speakers *are* OEM Eminence; find a small white label that´s usually glued to the magnet ring itself (not the Polytone label), with around 10 digits and often a barcode: that´s the Eminence "part number"; e-mail them and ask for a close modern equivalent. Most of their classic OEM speakers are now sold to the public under the "Legend Series" label, browse through their online catalog and you´ll find your answer. That´s provided that you want the original sound. Bye.

Please post the "hmmmm"  schematic.

Post the schematic (or the relevant part of it) to be sure, but probably it´s a 100 ohm preset. Usually they are part of the output transistor biasing circuit.

Hi Darwin. Nice transformers. Many are *very* strange, one wonders where were they pulled from. Maybe most (or all) are factory excedents. The 60V 5A CT one is excellent for a serious 2x100W amp. (+-42V supplies); the 80V 4A is good for a 200W/4 ohm PA or Bass amplifier.