My Small Special Build

[Matt J]

Update:

Finished wiring everything up today and went through all the checklist procedures before giving it a full power up. Thankfully, everything came on ok!! No unpleasant pops or hisses. I biased up the 6L6GCs and everything worked well from there.

Tone wise, this amp is very interesting. First, it is very quiet in terms of background hum. My guitars all have humbuckers, but I'm really happy at how quiet the amp is. You really have to crank everything up before you hear any background hum. The amp is just begging to have some cool pedals put in front of it. Missing the overdrive channel, it is not really designed to distort like an ODS, and with the high plate voltages you have to nearly dime the preamp and master volume before it starts to clip. It doesn't enter into SSS type clean territory, but I find it really pleasant. I plan on using this as my clean/very slightly overdriven amp and build something else to handle the dirty rhythm and leads. Perhaps something close to a Plexi or JTM45.

I'm open to any suggestions for high-quality 12AX7s and 6L6GCs I might want to try in it!

The tone controls and switches only seem to have a limited impact, which is actually fine by me. I could have misswired something, but the sweep of the treble and bass controls isn't very wide, possibly because I used 500kA pots for them. The bright, deep, and accent switches only cause a very slight noticeable change in the tone. The rock/jazz switch actually sounds like it has more gain in the Jazz position, so I might have wired it backwards and not noticed it. I don't know if this was the original intent of the 2nd/3rd gen switching layout I used, but I could only see myself wanting to adjust them when I use my Gibson Firebird since it has vintage spec mini-humbuckers in them that can be too bright at times.

It is going to take a lot of burn in time to really hear the full voice of this amp, but that just gives me an excuse to play it more! Right now, the glue is drying on the wooden head cab I'm building for it. Once that's done I'll put a few coats of finish on that then fit the chassis it.

- Matt J.

[xtian]

Nice work, Matt!

If you want to play with the tone stack, decrease the value of the 10K bass-to-ground resistor. Like, 6K8 is closer to Fender value and will give more Q sweep in the tone controls.

[Matt J]

Nice work, Matt!

If you want to play with the tone stack, decrease the value of the 10K bass-to-ground resistor. Like, 6K8 is closer to Fender value and will give more Q sweep in the tone controls.

Thanks for the tip, xtian!

I'll have to pick up some extra resistors closer to that value and give them a try. Right now, the only one I have in stock is a 3.3k one.

I started playing with the controls again some more this evening. One thing I noticed is how interactive the switches are with how you've set the treble and bass controls, but what really shocked me is that I appear to have wired the switches incorrectly, because they work upside down!! Thus, the the bright deep switches are on when they are in the down position. I followed the layout that is posted on here for a 2nd/3rd gen as close as possible, and posted a drawing of the switch layout above. Any suggestions as to what I may have missed? I'm open to rebuilding the switch layout to a different design if anyone has any constructive input!

Thanks!
- Matt J.

[Charlie Wilson]

Hello, you missed the fact that slider switches are wired the reverse of toggle switches. Nice looking build.
CW

[Matt J]

Hello, you missed the fact that slider switches are wired the reverse of toggle switches. Nice looking build.
CW

Hah! I never realized that Charlie! (smacks forehead) Well, time to warm up the solder iron and get to work!!

[Matt J]

Alright another update!

I rewired the tone switches correctly and they work perfectly! I've spent most of the morning giving things a test run.
I do not notice much of a tonal difference when using the Accent switch. I'm currently using an 820r NFB resistor, per the 2nd/3rd gen layout. Should I consider trying other values? I've seen 100r, 3.3k, and 4.7k in other duel 6L6GC versions of Dumble amps, but they use a pull Presence control instead of a simple Accent switch.
- Matt J.

[Krinkle]

Very nice build, great job! Do you notice much change with the Deep switch? I have the same wiring as you and I really can't hear any change when switching it. I'm baffled, just reaching out to anybody who has this wiring to see if there's a particular setting or something/anything.

[Matt J]

Very nice build, great job! Do you notice much change with the Deep switch? I have the same wiring as you and I really can't hear any change when switching it. I'm baffled, just reaching out to anybody who has this wiring to see if there's a particular setting or something/anything.

Thanks for the question, Krinkle!

Well, there's always the chance you miswired it like I did by using toggle switches instead of the Fender slider switches. It not only worked backwards, but it seemed to only have a very limited affect. Also, what value pots are you using?

What I have noticed is that the deep switch (along with the bright switch) doesn't act like your traditional bright switch, especially if you are used to using them on old Blackface era Fender amps. From what I have noticed so far, the amount of boost depends on how the volume and bass controls are turned up. You really notice a volume/tone change when the bass and volume controls are fairly low. When the controls are closer to full, the effect less pronounced since more of the original signal is passing through. I guess the mentality behind the design is to let the switches compensate for limited frequencies and loss of low/high end when you are using the amp at low volumes, or with the bass pot pulled back.

Hope this helps! I'm still playing around with the NFB resistor to get a little more out of the Accent switch. Tomorrow I am going to try a 3.3, then a 4.7k resistor.

- Matt J.

[Matt J]

Update:
Played around with the NFB resistor a little more to see if it had any affect on the Accent switch. Changed the 820r to 3.3k. Still no change in the tone when flipping the switch. I have noticed that, whether the resistor was 820r or 3.3k, when I measured it on the board with my DMM after soldering it in, it always reads about ~82r. Very odd to see that. Any ideas?

[Matt J]

Here is a schematic I used based on all of the info I could find about Small Special amps on here. I drew it up in Paint using similar schematics people have posted on here (all credit goes to them). I've noted in the voltage values I am currently getting. I figure others on here may find it useful! Note that I changed the final dropping resistor from 10k to 27k to lower the B+ on V1 a little.
- Matt J.