VHT 12/20 mods

OK, will have to give it a try... problem is, amp is away from home all the time at the rehearsal space, and there's really no time to sit around and tweak and test things during rehearsal.

 

I just ordered one of these amps - the non-RT version - I'm not really a reverb or tremolo fan.

I ordered the head version as I have several speaker cabs available - and I will also be using this amp live - so there will be a considerable amount of playing time before it sees a club just to ensure reliability.

But I haven't really heard of anyone having issues with transformers - so I probably won't worry about that much.

I purchased this amp as I had just a couple of months ago ordered the Special 6 amp and that little amp roars!
So when the opportunity for a new band arose I ordered the 12/20 head - along with so new 6V6's and a 5751 for V1 - I already have a couple of other 12AX7's I will use in V2 and V3.

 

I also just got the 12/20 nonRT head, and I am familiar enough with electronics to mod it. I make a LOT of pedals but have not modded a lot of amps, but this handwired platform is ideal for getting started.

There is a website called ronrobinette.com that shows how to turn the fixed tone stack in a Special 6 into variable resistors. The tones Resistor numbers are different, but the method is the same if you compare the schematics. Here are his instructions:

"For a bass tone pot you replace R15 with a 250k ohm audio (same as logarithmic) pot. Connect C16 to the bass pot's pins 2 & 3. Connect the wires from R14 and R16 to the outer two pot terminals (see diagram below).

For a treble tone pot you replace R13 and R14 with a 250k ohm audio pot. Connect where the two resistors were to the two outside terminals of the treble tone pot. The center terminal of the tone pot is connected to the white wire that runs to C18 (C18 is not visible in pic below). The white wire is connected to the volume pot so you can tap in there for a shorter wire run.

For a mid tone pot you replace R16 with a 25KA or 50KA ohm audio (log) pot. I couldn't find either in an audio mini-pot so I went with a 100k audio mini-pot but 25KA would be my recommendation. Connect where R16 was to the two outside terminals of the mid tone pot. The output side of R16 is connected to a red wire which runs to the volume pot--you can connect to the red wire at the volume pot to shorten the wire run. Connect C17 to the mid pot's pins 2 & 3."

His picture of his modified tone stack:


(BTW: Very many thanks to Robinette for the picture and I hope he does not mind me borrowing it. This is all from the https://robrobinette.com/Special6UltraMod.htm#Tone_Stack).

In the Special 12/20 the same tone stack exists but with different resistor numbers; R6, R7, R8 and R9. But you can see it is the same configuration (see the schematic in post #12 of this thread.) I do suggest you consult his entire page before you try this, because he also has a nice graphic on which lug connections to use on each pot. Keep in mind the Cap numbers will also change, but the locations remain the same (between the Special 6 and the 12/20), for example, when he says C16 in the 12/20 it is C5.

I would like to talk to people who have done such changes on the 12/20 - seems like much more info is out there for the Special 6.

 

I see Rob has posted here - thanks for your hard work, Rob, Great information on your web site!

 

FYI _ I just met the designer for this amp at NAMM. He is familiar with RobRob of course.

Some ideas he mentioned to me are; replacing R20 (a 3M3 resistor) with a similar size pot to possibly make a "gain" volume. He also mention replacing the 15R 10w resistor on the output (on the middle "texture switch" setting) with a variable pot. He said to keep in mind the speaker impedance and the amount of current you have there.

FTR: I completed the aforementioned tone control mod for the 12/20 - by taking RobRobs schematic and replacing the R numbers with the ones in the same places on the 12/20. It works quite well, but I noticed I do not get much bass response (I get it but only at about the lower 1/3 of the CCW radius - otherwise it is full on).

The designer (so sorry I did not get his name but he did not have a card and we were just chatting) suggested using a 50k pot (audio taper) for the bass, that it might yield a high frequency and so more control.

 

Hello cruisemates.
I just picked up a used 12/20rt head, so I'll be watching this thread.
I've got a couple other amps to repair and mod right now.
When they are done I'll start in on the 12/20rt.

So, keep posting and i'll do the same

 

Well... I have a non-RT head. I added a digital reverb using an accutronics brick. I also got the Tone Stack mod done. The tone stack mod is essential - I think it is one of the best amp nods I have ever heard.

If you don't know, the amp has "pre-set" tone stack where they put in fixed resistors instead of pots. They say it is built to replicate a Fender tone stack with all settings on "1:00". Doing the mod lets you turn them all up to "5:00" (full on). Just to compare, this is the same as pulling your boost knob out. BUT... having control over the amount of the different tones if much better. You especially get a lot of control in the midrange. The treble is the next one and you can definitely hear it working. The bass is more subtle - if the mids are on full you do not hear the bass working so much, but turn back the mids and you can really adjust the bass to the perfect spot.

I put new 6L6 tubes in mine, and now the amps just sings - especially with pedals. It is incredibly responsive, far more than even my JCM800 2204. I liken it to a German high-gainer. truly jaw-dropping tone.

I have it connected to just a single Greenback. Here is the tone mod --


The "labeled" cap numbers are the correct ones for the 12/20. I borrowed this from Rob Robinette who is referring to the Special 6. (C16 is for the special 6). But the yellow tabs are for the 12/20. So -

C16 is C5 in the 12/20
C17 is C6, R12 is R10, C15 is C4, C18 is C7.

All of the mods can be done on the circuit board - just look for the red and white wires going to the volume pot to find R10 and C7.

 

Yes, I mod'ed my special 6 ultra with both those.
I picked up the 12/20rt head cause I wanted the real reverb and tremolo.

I plan to do the tone stack mod on the 12/20rt also. Mainly because I frequently play into a iso cab and need more control on tone so that I can match the iso cab to the open air sound.

I'd really like to drive a preamp tube like the special 6 ultra can do. But the 12/20rt doesn't really have a good place to do that.

I picked up some old russian 6N6C tubes, they sound pretty good.
Also a couple low power el84 (jj el844), they sound good, but not great. I need to test some regular nos el84 tubes so I can compare.

I just got the amp (used) a few weeks ago so I haven't had a lot of time to play on it.

Have you messed around using the effects loop for pedals?
I'm really looking to move pedals (wah and volume) to the effects loop.

 

Hi Tromtek - how did you drive the preamp tube in the Special 6? Some kind of mod?

I do have my reverb pedal in the effects loops and I think it works great in there, however I want to figure out the method for putting a brick inside without need to use the loop. Volume pedals work really well in loops, but wahs usually want to see the impedance right straight off a guitar so they work better up front. The 12/20 is also weird in that the send & receive knobs are always in the circuit, and "normal" is to have them full on.

I have to say that I tried this amp with two different bands yesterday, and it was not quite loud enough. It was close, but I had to really push it and I could not maintain that sweet control over the tone that I could get a slightly lower levels. It does get very loud, good for a small club, but not for an average rock band, I am afraid. I had to go back to my JCM800 both times.

 

Overdrive
The Special 6 Ultra has an additional tube gain stage before the tone stack.
It was a little much for my taste so I put a 12dw7/ecc832 (or maybe it was the reverse version, a jj ecc823, i forget which one I have both).
The 12dw7 is; one half 12ax7 (gain 100) and the other half 12au7 (gain 20).

This was perfect because I could plug into the overdrive channel (12au7 side) and get clean with the 'ultra' pot turned down (below 9 o'clock).
But get a nice range of overdrive using from about 10 o'clock thru 5 o'clock.
Really really nice! It was the absolute best tube preamp distortion I've ever had.
I miss it on the 12/20rt.

Reverb
There is a mod for the Special 6 to put a Belton BTDR-1HL reverb brick inside.
I recall that it's pretty straight forward and should work on the 12/20.
I never did it because I like all analog and a real spring (it's what I grew up with).

Output
Yes, this amp with 6L6 is a little under powered for louder rock band.
But I wasn't looking for that.
I figured if I needed more volume I'd mic it into the soundboard and let that take care of the house.
That way I can play the amp's sound the way I want it without concern for volume.

It should be loud enough for you to monitor your sound and playing if you stay near your speaker cabinet. (but maybe not?)

Effect Loop
Yes, the wah is not perfect in the effects loop. But that's where I really want it.
I have a sonuus wahoo. I believe I can tweak it to make it work properly. Just haven't looked into that yet.

 

Yeah - can't find the exact schematic for the reverb brick mod. A guy was selling a kit on eBay but that is gone now. There is a YouTube that shows it. I would guess that whether you use the brick or a real spring the mod is the similar in terms of how you interface with the amp (except you might need to build a preamp for send & return for the spring unit). I would have gotten an RT because I do like reverb, but I actually did not realize there was difference when I bought my amp. Anyway, I built the perfect reverb brick pedal and with the 9v supply it plugs right in & works perfectly.

I am seeing this as more of a great studio amp than a band amp now. of course you can always mic it - but you have to set up some distance from the drummer if you really want to hear yourself without cranking it. This was MY one-day experience, though. It is also possible people farther away from me could hear me better than I could hear myself (that does happen)

 

I'll find and post the link to the reverb brick mod. I think it was powered by rectifying the heater supply.
Spring reverb is a whole other story. You need a driver and recovery circuit.

I haven't played with a band since I was younger (back in the 70's).
I was cranking thru a fender twin reverb and had lot's of power to spare.

 

Yup, it uses a simple L7805CV for 5v dc supply voltage, and I also would like to know about some of the RC-filter ideas people have used to roll off some of the lows hitting the brick. With my pedal I found that the reverb was very deep & long - which is not what I want. I prefer it snappy and shorter, almost indiscernible, it has to be there, but not a ocean of it.

I managed to control that on my brick pedal by dropping the resistance on the reverb control below the specs (I replaced 10k dual gang pot with fixed 7.5k resistors; it was madbean PING design with a 3H belton brick. It actually sounds really good, but can be a little challenging to control in the loop because the loop is always active and has its own affect on the tone.

I appreciate your effort in finding any information for me, though!

 

I’m hoping this zombie-ish thread doesn’t die....just got a 12/20 non-RT and want to dig into it.

Has anybody replaced the internal speaker w/ an 8 ohm speaker? I’ve got quite a few 8 ohm speakers to try, but none in 16 ohms. The internal speaker tap is 16 ohms, so is there a way to disconnect it and use the external 8 ohm jack for the internal speaker (like a Fender)?

 

Heh-heh-heh, I just picked up a 12/20 (non-RT) head, brand new, online from Sam Ash for $299. Makes me wonder if the Special series is being phased out. Not much VHT available, but I was able to also get a new VHT 112 empty open-backed cab on Amazon for $150. I was shopping for another Special 6, but upgraded instead.

I envision putting in an adjustable tonestack, eliminating the effects loop, adding a master volume...
I like the DEPTH control, and don't care for the variable power or texture controls.

 

I have the 12/20 HEAD, stock except I use a 5751 in V1 and 5881 power tubes on low power mode for now. VHT customer support said to try both low and high power settings to find the tone I like best. I wanted a lower power vintage type speaker(s) so I have 2 VHT 1x12 closed back cabs, 1 with a Jensen P12Q and the other a Weber 12A125A. I haven't tried both together yet but will at some point, both sound really good and should sound really good together.