Yet another Small Special started

[Synchu]

Looks very nice!
I will be curios to hear how the added reverb sounds.
Niki

[lord preset]

Looks very nice!
I will be curios to hear how the added reverb sounds.
Niki

The reverb is working. It's inserted just before the master volume. At the moment there isn't much reverb available on tap but I have a 2M pot coming to replace the 1M Reverb control which should provide a more useful range. So far I'm not all that pleased with the quality of the reverb I do get (kind of tinny and jangly) - possibly because of the short tank, but I know others have gotten good results from them. The Dwell control isn't really usable since it has to be turned all the way up to get much reverb but I may be able to find a more pleasing setting once the 2M is in.

[Guy77]

If this 2M pot yields better results for you I will order one as well. Thanks for updating on your progress.
I wanted to share this website's info on a 2 tube reverb circuit that looks very interesting and may produce more reverb than a single tube system.

http://www.tdpri.com/forum/shock-brothe ... ayout.html

I am just in the process of adding choke and transformers to my build. I did a test with one of my other ODS #124 builds and found that the required Hammond 194B choke sounded good but a larger choke from a Marshall JCM800 (194C) sounded even better!

Guy
Keep the iron hot!

[lord preset]

I installed the 2M pot and it results in a bit more reverb but only a bit. But I have a bigger issue. On this build I have noticed that it distorts some even at very low volumes, not what you would expect from a Small Special. It turns out that the distortion goes away with the reverb tube removed but is there with the tube in even if the reverb tank is unconnected. I'm not sure what is going on. The reverb circuit is inserted just before the master volume and matches the tubenit/10thfx schematic.

[10thTx]

I always have my reverb after the master volume and just prior to the LTPI.
(with amps that have an LTPI)

http://el34world.com/Forum/index.php?topic=7957.0

With Respect, 10thtx

[Guy77]

I am building my 2x6L6 ODS 124 with 2 tubes for reverb and short tank like yours "lord preset" . I am placing it right before the LTPI ( after Master Vol)

12at7 to drive and half 12ax7 to recover and a bypass switch to eliminate reverb circuit. I will post the results in about 1 week.

Cheers

[lord preset]

For whatever reason this amp does not like the reverb inserted after MV - I get severe buzzy distortion on louder notes. I went back to pre-MV insertion and have been working on ways to get rid of the always present distortion I reported previously.

I had found that removing the cathode bypass cap on the driver triode (pin 3) eliminated the distortion. I tried lowering it from 10uf to 5uf but that didn't do the trick. Eliminating the cap reduced gain and thus available reverb even further. Reducing the cathode resistor to 1K gained some of that back without introducing distortion, and I found that reducing the recovery triode cathode resistor to 1K also helped a bit.

This may be as good as it's going to get for me, but I am also curious about whether my plate voltage on pin 1 may be an issue. It's 339v which seems high for a 12ax7. Is that a safe voltage in this application?

[Guy77]

I attached a schematic for a fender Super Reverb amp ( 2x 6L6 amp).
It shows the reverb driver tube running at 450 volts and the recovery tube running at 410 volts! You will see it in the bottom left hand corner of the diagram where it says "reverb unit".



Cheers
Guy

[lord preset]

450 volts

It guess that's why they call it a Super Reverb

[David Root]

And guess how long that poor little innocent 12AT7 will last! I know 3.8mA is not a lot for a 12AT7 but at that voltage that is pushing 2W plate dissipation, and that's not a good combination. You can push voltage at low current or current at low voltage but not both at once!.
Pd max=2.5W. Even a good NOS will wince, and I would imagine most current production would faint dead away real quick.

A better choice is a 6K6GT, which is what was used as the reverb tfx driver in the original Fender 6G15 reverb, I wonder why?
Two downsides to that in an exisring build as the 6K6 is an octal and the driver tfx needs to have a 20K primary whereas the stock for the 12AT7 is 10K IIRC.

There is a 12AT7 that can take it, someone on TAG told me years ago to try a JAN Phillips ECG 12AT7WC as it won't die in this spot. See pic. These were made in the '80s as the military was still buying tubes at that late date, and they are readily obtainable today for a reasonable price. This one is from '87.

[Guy77]

Finally got around to finishing my ODS #124 with 2 tube reverb ! The results are fantastic! Below is a layout of the reverb I used. This was done with a 3 spring short tank . The reverb was inserted after the master volume and after the effects loop just before PI. The reverb I get is a beautiful hall type reverb. I made some changes to the layout seen below.

1. I used a 2.2M mixing resistor with no cap in place of the 3.3M in the layout.
(I may change this later since I only had a 2.2M resistor handy)
2 The 12ax7 tube has 820 resistor with 10uf bypass cap for the cathode instead of the 1.5K in the layout.
3. I am using a 1MA pot for the reverb instead of the 100K in the layout. I also put in a dwell pot that i keep turned up to about 8.

I am running 420 volts on the B+2 and 219v on the B+3.

As you suggested David I will switch to the JAN 12at7 , I have a Grooves tube in there now but have heard a lot of good things about the tube you recommended.

I have some static noise when I strum chords hard. I will have to chop stick it to see where the noise is, probably a loose solder .

This 2 tube setup gives me all the reverb I would want! Its not a super strong surf type reverb i guess you would need a a fender 6g15 for that but I am pleased with the results !

I will post some sound clips once the hiss and pop are fixed.

Cheers

Guy77
Keep the iron hot!