vintage organ amp to guitar amp HELP

[doctorfixit]

CONN 48425 conversion

Would this work for a preamp? Also, should I add 1k grid resistors to the 6L6's?

[doctorfixit]

CONN 48245 conversion

Would this work for a preamp? Also, should I add 1k grid resistors to the 6L6's?

[chris mckenna]

You don't need to replace the grid stoppers with that value. The 470 OHM will work for that and 6V6s as well. Only if your thinking about running EL34s in there do you need to change that.

I am looking to incorporate an adjustable biasing scheme on the amp that I am rewiring and run it up on top of the chassis as well. Each tube will have it's own biasing vs. using a fixed bias that is shared.

I am now looking to rewire this amp to shunt regulated push pull vs the layout that they are running now. This will also give the amp more output than it's currently wired / biased for now. This is why the output is very low coupled with a possible low-output tranny.

As far as that preamp- any preamp will work so long as it's wired and laid out correctly and, if you've already used an AX7 based setup on the other amps you've modded, then, it will most certainly work in this application as well. While it may not be the best, it will work to amplify the high impedance signal. I will take a look at those drawings. What software are you using to create those? I don't like how it appears or how that is laid out to be honest. It just looks messy to me but, I can see what you trying to do there so, for that, it works.

I don't know about using a SL7 for a preamp tube personally. I use the 6SN7 for my phase inverters in my custom tube amps that I make for guitar but, I use 12AU7s and AX7s and BH7s. I've never run a 6SL7 for a preamp stage although, a friend of mine used that in his hi-fi audiophile phono amps along with AX7s. I don't think he liked Sl7s and went back to using the standard preamp tube types. FWIW

What's the gain factor in terms of MU rating on that tube?

[doctorfixit]

I have changed it to a 6SN7 which is the duo of the existing 6J5, mU 20.
Probably a safer bet. This preamp circuit is copied from an Ampeg Model M12, which used 6SL7s for both the preamp and the PI with similar RC values.

I changed to the original schematic for this version of the dwg. Hope that helps.

[chris mckenna]

If this is going to be a bass amp, I would simply use a 12AX7 CKT with a 68K R and 1M to ground at the input or, vary R according to your ear to tune it to suit. Of course, you'd need B+ to run the AX7 and, a standard self biasing CKT but, other than that, it's pretty straightforward.

Then, run the heater off a DC supply vs AC- less hum. This is what I do to minimize hum and noise. You may also want to incorporate negative feedback to reduce distortion when playing bass through it. If it were guitar based, I use no negative feedback thus allowing the amp to totally clip and saturate the tranny- this is where the good distortion comes from. There's nothing like it. No emulator is going to dupe that nor is any preamp CKT with cascading gain stages going to give you that type of smooth overdrive, harmonic overtones and dynamics.

Of course, the components have to be beefed up to run in this mode along with the use of high grade trannies to prevent a catastrophic failure. I would also change the biasing CKT to suit bass amplification and change out the OT as well to a Hammond 50W tranny for some clean power. You'll get around 36W RMS before clipping. This is why these little amps are so neat as they are not high powered so, you can really crank the crap out of them and get them to distort nicely. Personally, if I were looking for a bass amp- I would not use this amp at all for it is way underpowered for a solid, clean sounding bass amplifier. I'd be looking at at least 4-6l6s for the headroom and dynamics that bass possess.

Good luck with your preamp.

CM

[doctorfixit]

Updated schematic for Conn 48245

This is working quite well. I used a 6Sl7 for the preamp.

[chris mckenna]

This is working quite well. I used a 6Sl7 for the preamp.

Glad to hear it's working good for you.

I just got done testing this amp out and, the sound is quite inferior as is. In my tests, I think the entire driver section should be removed and replaced with a higher gain for guitar never mind bass guitar. A change to a nice AX7 such as a Amperex Bugle Boy would be nice and clean with superb top end response.

The other thing is that this amp distorts quite easily which can be problematic for loud, clean playing- there's not nearly enough headroom. I ran a line out signal into it off my load box and it distorted quite early for a dual 6L6 amp. It's quite loud once it goes into clipping but, other than that, for loud clean playing, the output is just not loud enough to reproduce the input signal.

I had compared it against a couple of other power amps that I have here including a 15W Japanese Hi-Fi power amp and, the Conn amp does not posses the deep rich tone that this little amp had. I also noted that I could run the input a lot hotter on the Japanese amp than the Conn amp. It was much smoother, way more open and wider sounding with a much smoother top end that was silky and smooth even when opened all the way up. To my ears, the Conn amp is dead and lifeless on the clean. It doesn't have that top end sparkle nor the depth that I was looking for. I was planning on a complete rewire with a few changes but, after testing, I don't think it's worth it. It would require gutting it and nearly replacing everything other than the tube sockets and some resistors.

In my opinion, the OT, the driver section, the filter supply caps, the PS should be changed out to improve the tone and be able to accommodate bass or electric guitar. I would up the B+ voltage and run the tubes a bit hotter along with putting in F&T caps to beef up the PS.

While the distortion is pleasing on this amp, it's a bit flubby on the low end. Changing out the recto to a beefier design alone was not enough to clear it.

In my opinion, the whole amp needs to be overhauled to not only improve the tone but, also improve the headroom before clipping on the stock design.

[jcart]

conn now for sale

thanks for all the help but for what I am looking for and what it will cost to get there I am selling this amp and getting something else..first $100 owns it

[Ran Randle]

I converted a 1958 conn organ amp

I have a 1952 conn organ amp.(model connsonata ) amp has two 6l6's.. i wanted to use this as a power amp for a guitar setup. anyone here have any knowledge as to where i can get a wiring diagram. or do you think it would be relatively easy for someone to mod this amp such as gounded ac line in, on off switch, maybe newer components?

This was my first amp conversion so I brought it to a local amp guy and had him test it so I knew it all worked first. I had to replace the OA3 regulator and 5U4 rectifier tubes but all the others tested good. First, I wired in the 3 prong plug grounded to the PT bolt. He pointed out where to wire the speaker outputs from the heater plug from the output transformer (yellow=8ohms green=16ohms in my case) and wired the jacks to the ground bus. I drilled holes for all the input/output jacks, power switch, pilot light, master volume pot, and it was 6 holes altogether. I am really impressed with how it sounds as is so I don't plan to do any tone mods at this point and I'm still leaning about options. I use a preamp in front of it as well. The pic with the coke can is the before and the other one is after.

[doctorfixit]

Final version of my converted CONN 48425

After a ton of research I finally have a really nice amp that I use for bass. It's plenty powerful , esp if pre-amped with an effects box. . I used a 35H choke coil instead of a long tailed pair on the PI. the key changes were an unbypassed 2nd half of the 6L7 gain stage, a partially bypassed 1st gain stage, with separate bypass cap for the highs & a 4.7k bass cut pot