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Solid State Amplifiers => Schematics and Layouts => Topic started by: 5thumbs on November 10, 2009, 02:52:23 PM

Title: Kustom Tube 12A Schematic
Post by: 5thumbs on November 10, 2009, 02:52:23 PM
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!
Title: Re: Kustom Tube 12A Schematic
Post by: joecool85 on November 10, 2009, 04:27:28 PM
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?
Title: Re: Kustom Tube 12A Schematic
Post by: phatt on November 11, 2009, 07:31:49 AM
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.
Title: Re: Kustom Tube 12A Schematic
Post by: J M Fahey on November 11, 2009, 08:54:26 AM
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.
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 13, 2009, 03:34:58 PM
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*
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 13, 2009, 03:49:16 PM
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. 
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 14, 2009, 01:03:11 PM
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...
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 14, 2009, 01:32:49 PM
OK, here are the gut shots (Part 1) ...
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 14, 2009, 01:33:56 PM
OK, here are the gut shots (Part 2) ...
Title: Re: Kustom Tube 12A Schematic
Post by: Brymus on November 14, 2009, 06:34:16 PM
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.
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 14, 2009, 11:26:17 PM
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.
Title: Re: Kustom Tube 12A Schematic
Post by: J M Fahey on November 15, 2009, 08:09:15 AM
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.
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 15, 2009, 09:37:06 PM
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 1F film cap in series with -> 2.2KΩ (use 2.2KΩ pulled from R6, 2.2KΩ is closer to ground than 1F 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 0.047F 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.
Title: Re: Kustom Tube 12A Schematic
Post by: Brymus on November 16, 2009, 12:58:56 AM
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.
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 16, 2009, 11:17:15 AM
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).
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 16, 2009, 11:54:46 AM
...
I think 1k5 cathode resistors would be an improvement like your OPA hifi opamps(which I want to try now after your glowing endorsment)
...
You can also stack (solder Pin 1 on Opamp A to Pin 1 of Opamp B, Pin 2 to Pin2 2, etc) two lo-fi op amps to get better response.  (Just make sure they can handle a supply voltage of +24V.)  NE5532 opamps are a popular one to stack, but I'd avoid them here at first due to their low input impedance.  I'd recommend trying any of the FET-input dual opamps (TL072, OPA2134PA, etc) first, then try non-FET input dual opamps and pick the one that makes your ears happiest.

Just for the record, the OPA2134PA isn't an end-all-and-be-all opamp for me.  I just have gotten into the habit of using it before others because I love the flat EQ and highly-detailed output I get from them.  Sure, you often have to tweak the pedal EQ to get a little warmth back in the final tone, but that's a small price to pay for the other positives the OPA2134PA brings.  YMMV, of course. :)
Title: Re: Kustom Tube 12A Schematic
Post by: J M Fahey on November 16, 2009, 05:55:50 PM
Quote
You can also stack (solder Pin 1 on Opamp A to Pin 1 of Opamp B, Pin 2 to Pin2 2, etc) two lo-fi op amps to get better response.
A *very* curious idea, who/where was that suggested?
I'd love to hear the technical explanation.
Thanks.
Title: Re: Kustom Tube 12A Schematic
Post by: J M Fahey on November 16, 2009, 06:28:44 PM
Quote
6) Remove and jumper the following: C3, C7, R5, R6, R14.
Jumpering : C3, C7, R5, R6 will effectively remove needed DC isolation,  considering that pins 1,2 and 3 of IC1A normally "live" at around +12V.
The B1Meg suggested as new VR1 will scratch heavily most of its travel, thumping loudly (and killing all sound) on either extreme, whether shorting Pin 1 to ground or placing around -11VDC (relative to Pin 3) on Pin 2, effectively hard switching the output to practically +24V.
Title: Re: Kustom Tube 12A Schematic
Post by: phatt on November 16, 2009, 11:40:42 PM
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.
Hi JMF, If you are looking for ideas on Voltage Double circuits using cheap parts then have a look here.
http://ozvalveamps.elands.com/ava100/ava100psu.htm#andrew_k (http://ozvalveamps.elands.com/ava100/ava100psu.htm#andrew_k)

I've built a valve HT kit which delivers 260VDC using an auto Tr Smode circuit to run some preamp experiments. The Vtwin being one such circuit I built.
Phil.
Title: Re: Kustom Tube 12A Schematic
Post by: phatt on November 16, 2009, 11:49:09 PM
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.
The 1k5 k resistor will just turn the gain of the stage up a bit (even with low HT)
Depending how big the incoming signal is,,you might even get some half wave clip at 80 VDC by turning tubes up.
but as should be obvious,,, very hard to get triodes to work as full on distortion units on there own,,the diodes are doing that here.
Phil.
Title: Re: Kustom Tube 12A Schematic
Post by: Brymus on November 17, 2009, 01:40:26 AM
Quote
You can also stack (solder Pin 1 on Opamp A to Pin 1 of Opamp B, Pin 2 to Pin2 2, etc) two lo-fi op amps to get better response.
A *very* curious idea, who/where was that suggested?
I'd love to hear the technical explanation.
Thanks.
A guy(DavidAlanCole)(I think) at DIYstompboxes had a thread about this,which I havent had time to try yet.
But apparently stacking is a "trick" that is used to up the output of the opamps too.
His example used several NE5532 chips stacked to give a 5 watt output for a practice amp.
A couple others commented they had done this too.
They all raved about the sound quality this produced and they all claimed to have gotten the idea from the same book,which I cant remember the name of.
It was also mentioned that sometimes a 100r (or K,again its been awhile)was needed at the output of each opamp when stacking.
I think it was said it took a stack of ten to get 3 or 5 watts.

5Thumbs that diagram was helpful.
So if I understand right this setup would replace hard clippers to ground ?
So I am guessing the signal comes in at the top corner of the bridge and the bottom corner would connect to ground?
I appreciate you sharing that as I really like the sound of the Tonebone pedals but they are way too pricey for my means.
Title: Re: Kustom Tube 12A Schematic
Post by: phatt on November 17, 2009, 08:47:39 AM


Hi Brymus,
              If it helps just look at how some reverbs are driven.
Phil.
Title: Re: Kustom Tube 12A Schematic
Post by: phatt on November 17, 2009, 08:53:40 AM
Forgot,,
 Re Bridge for clipping?? I breadboarded a few a lot of those clippers and the extra diode did little for me but heck try it out and make your own judgement. 8)

Marshall used this idea in JCM 900's I think. The only way they could get more crunch from the amps as triodes alone can't do that trick.

Edit: please note I drew this the wrong way :-[ Phil
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 17, 2009, 11:30:48 AM
Quote
6) Remove and jumper the following: C3, C7, R5, R6, R14.
Jumpering : C3, C7, R5, R6 will effectively remove needed DC isolation,  considering that pins 1,2 and 3 of IC1A normally "live" at around +12V.
Could you please explain what this "DC isolation" is in the feedback loop?  The NJM4558 chips (and OPA2134PA) can handle a supply voltage of up to 30V, so 24V single supply should fall within spec.  Also, what purpose does this DC isolation serve?  (Please bear in mind that my experience is largely from the stompbox realm.  I have run SD-1/TS808 circuits at higher voltages than 9V (as have many others) and not observed any problematic behaviors.  However, I could be completely ignorant to the phenomenon you are describing.)

The chip can handle the +24V by spec, so I'm a little unclear on the rest of this assertion.  If you could please clarify, that would be greatly appreciated.

The B1Meg suggested as new VR1 will scratch heavily most of its travel, thumping loudly (and killing all sound) on either extreme, whether shorting Pin 1 to ground or placing around -11VDC (relative to Pin 3) on Pin 2, effectively hard switching the output to practically +24V.
If that's the case, then why doesn't a BOSS SD-1 "scratch heavily most of its travel, thumping loudly (and killing all sound) on either extreme...".  The 1M pot (and C3,C7,R5,R6 removals) make the IC feedback loop resemble that of an SD-1 (and TS-808/TS9 a little more distantly.)  A fixing resistor should keep from killing the sound at minimum resistance; at max resistance, well, the pot resistance takes care of that end too.

There are millions of SD-1 examples out there that bear testament that this arrangement does not produce the aforementioned artifacts when turning the pot.  However, just because I haven't personally seen a million dollars does not make me believe a million dollars does not exist...so I'll try to stay open-minded on this as well. :)
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 17, 2009, 11:37:13 AM
Forgot,,
 Re Bridge for clipping?? I breadboarded a few a lot of those clippers and the extra diode did little for me but heck try it out and make your own judgement. 8)

Marshall used this idea in JCM 900's I think. The only way they could get more crunch from the amps as triodes alone can't do that trick.
The diode polarities on your diagram do not match mine.  On the two "left side" diodes, they need to "point" inwards towards each other (and the + side of the "middle" diode).  On the two "right side" diodes, they need to point outwards away from each other (and the - side of the "middle" diode.)

The diode arrangement in your diagram will block flow from the signal-side through the bridge rectifier to Vref/GND (except once reverse-breakdown voltage is met in the diodes, but I doubt you'd ever hit that voltage in this scenario. :))

EDIT:  If you "rotate" the 4 bridged diodes 90-degrees to the left (and reposition the middle diode accordingly, but flip its polarity), your bridge rectifier clipper would work as expected.
Title: Re: Kustom Tube 12A Schematic
Post by: J M Fahey on November 17, 2009, 06:14:36 PM
Hi 5Thumbs.
Now I'll speak about what I see in this particular circuit; whenever I have a little more time I'll download and see the DS1 schematic, which by the way is a Pedal I like a lot (the SD1 too).
IC1 is single supply powered (+24V) so it's biased to approximately half that; pins 3 and 5 respectively going to a +14.4V point through 220k and 100k resistors.
R4 joins pins 1(output) and 2(inverting input. They are both DC decoupled or separated from anything else by C3 and C7, so the DC gain is 1,or in other words: whatever appears at the input, appears at the output, within 2 or 3 millivolts (0,001V).
We have 14.4 V on pin 3 so we have 14.4V on Pin 1.
If I short/jumper R6 and C7, the wiper of the pot will ground those 14.4 V suddenly, thumping loudly. On its travel there, it will scratch a lot, like any pot that carries DC .
Almost the same happens on its other end, travelling to the left,because of jumpering C3 and R5.
The only difference, although the end result is the same, is that the 33k added will avoid it being a full short to ground. Anyway, IC1 becomes a DC amplifier, with gain = 1+(220k/33k)=7.66 .
Since this DC amplifier has a DC voltage of 14.4V applied on Pin 3, it will *try* to have 14.4Vx7.66=110V DC. It can't, saturating its output very close to the +24V available. The VR1 wiper will scratch all the way to the left position, thumping when the output saturates.
If you leave the wiper ungrounded, no scratch or thump but no gain/distortion either, the pot becomes a fixed 1M resistor.
I guess the DS1 schematic you saw has errors,something very common on the Internet.
Even this schematic (Tube12) has *another* one, which I've just noticed: IC2 (LM1875) should be biased to 1/2 of *its own* supply (40V), yet it is shown direct-coupled to Pin7 of IC1B, which carries 14.4V!!!!
Remember the ones already pointed such as unexisting/duplicated D45 and not connection of D41/42/45 .
As you correctly indicated, we are working below the permissible 30V supply.
Hi Phil and Brymus: the *outputs* of regular Op Amps *can't* be joined, they'll suicide fighting each other for control. In the reverb driver posted, see that the first op amp is the master, all others are unity gain slaves, with their +inputs following the master output, each with *its own* feedback loop and all outputs summed through 100 ohm resistors, never wired straight together.
The same happens with regular power amps, which are "Big Op Amps"
Please somebody post a link to an SD1 schematic, so we all talk about the same.
Thanks.
Juan Manuel Fahey.
 
Title: Re: Kustom Tube 12A Schematic
Post by: 5thumbs on November 18, 2009, 12:23:39 AM
I've tried posting this info several times, but keep getting something like "server error 500."  So if the info below is posted like 5 times before this post, that's what happened.  (I'd apologize, but it's not my fault. :))  OK, assuming the server didn't get the info below posted, here it is again:

Here's the BOSS-original SD-1 schematic (http://www.godiksennet.com/images/sch/SD1PG2.JPG).

Here's a link to R.G. Keen's 'The Technology of the Tube Screamer' (http://www.geofex.com/Article_Folders/TStech/tsxtech.htm) as well, since the TS-808/TS9 use the same feedback loop configuration as the SD-1 and the proposed mod to the Tube 12a.

The proposed configuration is not due to a mistake on a schematic.  It's been used successfully in thousands/millions of Tube Screamer clones.
Title: Re: Kustom Tube 12A Schematic
Post by: J M Fahey on November 18, 2009, 06:53:25 AM
Hi 5thumbs, good morning.
Let's go over it bit by bit.
To begin with, thanks for the factory schematic.
Going on: DS1 is not the same as the Tube12 mod you suggested, there are quite a few significative differences:
1)
Quote
1) Replace VR1 (A10KΩ) with B1MΩ pot + 33KΩ (fixed) in series.
Just replacing the pot will not turn the original Philips type gain+volume control into a TS/DS1 gain control, the architecture is different and you should change it too.
Note1: Numbers between quotes indicate knob positions, not electrical value, such as "0"(min), "5", "10"(max); "x" after a number indicates gain, as in "10x" (gain of ten)
Note2: DS1 and Tube12 indicate the respective schematics.
Besides the different value and taper, the pots are wired differently and are in different parts of the op amp feedback network:
DS1: it is wired as a variable resistor, one end is pin 1, the other pins 2 and 3 shorted, its value goes from 1MΩ (on "10") , to 0Ω (on "0").
The 33K resistor in series sets that as the minimum value.
The gain varies smoothly from "0" to "10", because the pot is logarithmic.
The minimum gain is 1+(33K/4K7)=8x
Maximum gain=1+(1M/4k7)=214x
Note3: that architecture is very good (with a Log pot) to control gain smoothly, but it can not be used as a volume control because minimum gain, even if you short the 33K resistor, is 1x.
Note4) DS1: the gain network, VR1/R6 is DC decoupled from ground through C3 ; you suggested shorting the equivalente capacitor C3 on Tube12:
Quote
Remove and jumper ...C3
which would DC couple it, causing the effects I mentioned.
DS1: the IC output, pin 7 is DC decoupled from ground through C4 and C5; you suggested
Quote
Remove and jumper ...C7
which should also cause scratch and thump . That will not happen on DS1 because they adequately place their capacitors wherever needed.
In DS1 we vary the gain varying the output-to-input resistor; the -input to ground (R6) is fixed.
Note5) Tube12: VR1, the gain control, is very different from DS1. The output to input resistor (R4) is fixed. The pot works as a dual pot.
One is the section from Pin1 to cursor/wiper (Pin2), it varies the resistor to ground part of the gain network.
Gain is 1+(R4/R5+(wiper-to-pin1 resistance))
Gain goes from "10"= 1+(220k/2k2)=101x to "0"= 1+(220k/12k2)=19x.
The beauty of this Philips design is that the second pot section works as another pot, working as an attenuator grounding signal after R6, and allowing the output go to 0. This arquitecture can be used as a volumen control, although the gain variation is not as smooth as the DS1 type.
Of course if we short C7, R5 , we are not only shorting audio to ground, but also the 14VDC present there.
I think this is enough to show that the mod needed , although basically a good idea, is even much more difficult and radical than what was posted, if one tries to turn the original Tube12 preamp into a built-in DS1.
Practical suggestion: to get 90% of the sound wanted with 5% the complication, I'd rather just pull D3/4/5/6 , wire 3 of them as a "DS1 diode network" and solder it in parallel with R4. The added beauty is that SW1A is still active, giving me a "quasi DS1" distortion when ON, and a unity gain, clean preamp, when OFF.
Please try it and tell us how it sounds :).
It has the added bonus of still avoiding those pesky diode clippers killing the first triode tone.
Juan Manuel Fahey.
 
Title: Re: Kustom Tube 12A Schematic
Post by: phatt on November 18, 2009, 09:31:05 PM
Forgot,,
 Re Bridge for clipping?? I breadboarded a few a lot of those clippers and the extra diode did little for me but heck try it out and make your own judgement. 8)

Marshall used this idea in JCM 900's I think. The only way they could get more crunch from the amps as triodes alone can't do that trick.
The diode polarities on your diagram do not match mine.  On the two "left side" diodes, they need to "point" inwards towards each other (and the + side of the "middle" diode).  On the two "right side" diodes, they need to point outwards away from each other (and the - side of the "middle" diode.)

The diode arrangement in your diagram will block flow from the signal-side through the bridge rectifier to Vref/GND (except once reverse-breakdown voltage is met in the diodes, but I doubt you'd ever hit that voltage in this scenario. :))

EDIT:  If you "rotate" the 4 bridged diodes 90-degrees to the left (and reposition the middle diode accordingly, but flip its polarity), your bridge rectifier clipper would work as expected.

Whoops!! yep right you are ,,, I've gotta stop drawing stuff late at night. :-[
Phil.
Title: Re: Kustom Tube 12A Schematic
Post by: phatt on November 18, 2009, 10:08:40 PM
Simply put,,, ""Audio is an *AC signal* riding on a *DC potential*.""
You have to isolate the DC.
Took me quite a while to understand it but once I read that simple line it made a lot more sense.
So when you look at circuits you need to think in both terms.
Phil.