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[5thumbs]

I work on Kustom Tube 12A combos from time to time and have a couple of these amps kicking around my house at any given moment.  If you replace the 4558 IC with something hi-fi like an OPA2134PA, these $100USD amps are positively charming and useful as a practice, recording or workbench amp.

Since nobody else has previously posted this schem, I figured I'd share it with y'all.  Good luck!

[joecool85]

I work on Kustom Tube 12A combos from time to time and have a couple of these amps kicking around my house at any given moment.  If you replace the 4558 IC with something hi-fi like an OPA2134PA, these $100USD amps are positively charming and useful as a practice, recording or workbench amp.

Since nobody else has previously posted this schem, I figured I'd share it with y'all.  Good luck!

Whats the difference you get when you change that chip?  Better quality?  Better tonal reproduction?  Louder?

[phatt]

Nice ,,similar in concept to the Mesa VTwin pedal.
BTW I think the schematic has a bobo next to *R40 and R42* looks like a dead short to me   as drawn that shorts out the secondary winding of the power transformer. Just thought I'd mention it.
Cheers, Phil.

[J M Fahey]

Hi 5thumbs (I'd *love* to hear you tapping a 5 string bass he he), thanks for sharing.
If someday you could post a couple of pictures, including some "guts", they will be welcome.
We can always learn more, anyday.
Hi Phatt
Yes, you are right.
Another error is the undefined connection between D42 and D45.
I usually fiund many "official factory" schematics, with dumb unexplained errors.
I guess that they are there on purpose, so any technician will recognize them as such, and in a service situation, the dead amp is there, on the bench, and the actual wiring can be checked, but a non-tecnician, who can read a schematic and solder but not really understand design, will fail when trying to clone something, based only on an Internet downloaded schematic.
Even worse are "reverse engineered" schematics, when some component is not accurately read or some tracks get mixed.

[5thumbs]

Whats the difference you get when you change that chip?  Better quality?  Better tonal reproduction?  Louder?

If you've used the OPA2134PA to replace *4558 chips in stompbox designs, the difference in this amp is similar to the qualitative changes seen there.

The *4558 chips, in my opinion, are rather lo-fi on the high end, as well as having that "mid hump" that it is famous for.  Once replaced with the OPA2134PA, you get better high-end clarity and lose that midrange mud of the 4558.  Since the Tube 12a is kind of mid-heavy already (due to the small 8" speaker), the addition of the 4558's mid hump is just a wrong move, IMO.  The flatter EQ and cleaner high end (as well as higher slew rate and all the other esoterica that you may or may not care about) of the OPA2134PA add some much-needed clarity to this little amp.

Finally, in all honesty, some of the additional top/bottom end performance changes and clarity might well come from the JFET input stage on the OPA2134PA.  This gives it a higher input impedance than the *4558 chips, which would likely result in more top and bottom end response. *shrugs*

[5thumbs]

If someday you could post a couple of pictures, including some "guts", they will be welcome.

I think I'll have a chance to open up one of the amps tonight.  I'll try to get some shots posted here later on.

BTW I think the schematic has a bobo next to *R40 and R42* looks like a dead short to me   as drawn that shorts out the secondary winding of the power transformer. Just thought I'd mention it.
Cheers, Phil.

Hi Phatt
Yes, you are right.
Another error is the undefined connection between D42 and D45.
I usually fiund many "official factory" schematics, with dumb unexplained errors.
I guess that they are there on purpose, so any technician will recognize them as such, and in a service situation, the dead amp is there, on the bench, and the actual wiring can be checked, but a non-tecnician, who can read a schematic and solder but not really understand design, will fail when trying to clone something, based only on an Internet downloaded schematic.
Even worse are "reverse engineered" schematics, when some component is not accurately read or some tracks get mixed.

When I'm taking pics tonight, I'll try to get some close ups on the R40/R42 and D42/D45 neighborhoods.

Thanks!

Hi 5thumbs (I'd *love* to hear you tapping a 5 string bass he he), thanks for sharing.

My handle is more about how my chops are suitable for silent movies and the hearing-impaired and not much else. ("I play like I have 5 thumbs." )  But that doesn't stop me from building/playing/enjoying it all just the same. 

[5thumbs]

OK, I got to take things apart and get some pics.  First off, attached is the schematic corrections for the R40/R42 and D42/D45 neighorhoods.

The "dead short" between R40 (which is actually R41) and R42 is valid.  There is also a union between D42 and D45 (as was previously theorized.)  There is also an erroneous duplication of D45, but I've Red X'd the duplicate diode that is not present in the actual amp.

I'll post the gut shots next...

[5thumbs]

OK, here are the gut shots (Part 1) ...

[5thumbs]

OK, here are the gut shots (Part 2) ...

[Brymus]

What kind of sound do the 4X4148s to ground give after the first triode?
I just tried something similiar with red LEDs and found 2 sounded great but 4 only added a little dirt not very noticable.
How does the low plate voltage affect the clipping the make?
My triodes are closer to 250V- 300V plate voltage 80v seems low.

Also why are they biased with 2k2 instead of 1k5 or 820r?
Is it because of the low plate voltage?

And finally have you tried added a bypass cap to either triode?
Seems like the first one could have a 4.7uf on a switch for a nice warm boost or even a .2 - .68 for some top boost .
Thanxs for sharing the schematic and pics,Its nice learning about new stuff.

[5thumbs]

What kind of sound do the 4X4148s to ground give after the first triode?

Not that great.  I'd planned to mod them into a quasi-bridge rectifier arrangement to boost the proximity-to-clip a bit, but before I actually tried that out, I decided to design my own amp.  (I'll post that project here at SSGuitar once I've got it all done.)

I just tried something similiar with red LEDs and found 2 sounded great but 4 only added a little dirt not very noticable.

I usually like a proximity-to-clip around 1V on both sides of the waveform.  A bridge rectifier w/five 1N34A diodes is my current favorite.

How does the low plate voltage affect the clipping the make?
My triodes are closer to 250V- 300V plate voltage 80v seems low.

I don't think the valves are involved in the distortion much at all.  I think the majority of the distortion on this amp comes from the opposing 1N4148 diodes.

Also why are they biased with 2k2 instead of 1k5 or 820r?
Is it because of the low plate voltage?

While the tube in the Tube 12a appears to be more than a "marketing tube", but then again, at these moderately-low voltages, I doubt they factor much, if anything, to the distortion of this amp.  I've never tried to mod the tube parts mostly because of this belief.  Another thing is that they can't boost the signal too much before the LM1875, which they have to cut in 1/2 as it is to meet the max input voltage on the chip amp.

And finally have you tried added a bypass cap to either triode?
Seems like the first one could have a 4.7uf on a switch for a nice warm boost or even a .2 - .68 for some top boost .
Thanxs for sharing the schematic and pics,Its nice learning about new stuff.

Nope, never tried modding around the tubes.  To be honest, I've been a bit lazy with this amp, since it is pretty nice with just the upgraded op amp.  All the mods I was going to do I've put off in favor of building a backpack amp head instead.

I like the ideas you've mentioned and I'll look at them once I get my other amp project completed.  Luckily, this amp is so cheap, you can afford to have a couple to tinker with.

[J M Fahey]

Thanks for the schematic and comments.
I'm always learning, everyday, everywhere.
I liked the simple voltage doubler idea very much, and if used carefully, they can power some 12AX* to get something useful.
Here, the real distortion comes from the diodes.
It's a pity, they could have done more, but I guess the Marketing dept. pressures were greater than the Sound dept ones. Oh well.
Most of my "bread and butter" 100W SS amps use around +/- 40 volts, so I'll get those "free" 80V and experiment some tube preamps.
Personal opinion: those +9V "tube pres" are toys, but 80V *start* to look reasonable.
Up to now, the only tubes I could get "serious" results from with +42 to +35V are 12AU7.
This trick opens aup the field.

[5thumbs]

FWIW, here are the mods I considered for the Tube 12a.  These mods would allow the gain stage to be active all the time.  These mods will also make the clipping boost phase into a soft-clipper (similar to the tubescreamer), as well as provide a "dist warp" control to control the amount of hard-clipping.

I doubt I'll do these mods myself, but they would likely improve the distortion on this little amp.

Mods:

 1) Replace VR1 (A10KΩ) with B1MΩ pot + 33KΩ (fixed) in series.

 2) Replace R4 with SW1A-controlled circuit for feedback loop clipping diodes (2 ->|-, 1 -|<-), use 1N4148 diodes from replacement of D3 thru D6 (below.)

 3) Rewire SW1A to turn feedback loop clipping (R4) on/off.

 4) Replace R7 (220KΩ) with 1µF film cap in series with -> 2.2KΩ (use 2.2KΩ pulled from R6, 2.2KΩ is closer to ground than 1µF cap.)

 5) Replace C6 (150pF) with 51pF or 47pF.

 6) Remove and jumper the following: C3, C7, R5, R6, R14.

 7) Replace C8, C10, C16, C18 with 0.047µF film caps.

 8) Replace with 1N34A Ge diodes and invert polarity of D3, D5.

 9) Replace D4, D6 with 1N34A Ge diodes.

10) Install 1N34A Ge diode on solder-side of PCB between the union point of D4/D5 and D3/D6; Diode polarity points towards D3/D6.

11) Replace R15 (470KΩ) with 12mm B500KΩ pot; drill hole in front panel at 10:30 relative-position to GAIN KNOB; install DIST pot in hole.

12) Install 2200pF film cap on solder-side between R13 (union of R13/D5) and SW1B (side connected to union between R14/R15); disconnect and remove wire from union between R14/R15 and SW1B.

13) Replace and socket IC1 (4558) with an OPA2134PA or other stacked, dual ICs.

[Brymus]

Its kinda late to go over those mods you listed,I will later.
I appreciate the answers.
I asked elsewhere about the common 2k7 cathode resistors and was told the cooler bias was a trade off for the filter effect when using a bypass cap.
However since the triodes here dont have cathode bypass caps,I wonder why they would use that value over 1k5,
I think 1k5 cathode resistors would be an improvement like your OPA hifi opamps(which I want to try now after your glowing endorsment)

Can you enlighten me on the 5 x 1n34A set up you refered to?
I to like using them on occasion,but am unfamiliar with a 5 diode bridge rectifier,sounds very interesting.

[5thumbs]

It's a classic bridge rectifier arrangement, with the exception of the addition of the 5th diode in place of the battery/mains differential load.  (The diode going between the two middle points is this "exception" diode.)  I first saw this clipping arrangement in the Marhsall JMP-1 preamp, then in the Radial Engineering ToneBone Classic.  I tried it in my DS-1 "Huevos Grandes" mod and liked it...and started using it in selected distortion circuits.

Attached is the diagram of the bridge rectifier clipper in question.  The dotted diode is there to show how the bridge rectifier replaces a hard-clipping diode in a distortion stompbox.  (In this case, replacing diode D5 in the BOSS DS-1.)

In this application, you're getting the clipping shoulder contour and proximity-to-clip of three 1N34A diodes in each direction, while only using 5 diodes (instead of 6).

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