CONN organ tube amp guitar conversion

[Ran Randle]

Greetings,

Looking for some direction in modifying this amp for guitar. It's a CONN organ tube amp model 56665. I'm just starting out and this will be my first tube amp conversion. I would like to do as much as I can myself and will attempt to draw out a schematic for learning purposes and to help the process. I have read up on amp safety(draining filter caps of voltage) but still a bit nervous about getting started so bare with me. Some general wiring guidance for changing to a 3 prong plug, 1/4 input(s), volume, output jacks 4ohm, 8ohm, 16ohm speaker connections, (is a preamp needed or not?) would be a good place to start. I realize some(if not all, lol) options and variations are beyond the scope of my knowledge so please be kind.

included link to description and photos.

VINTAGE CLEAN CONN ORGAN TUBE POWER AMP 56665 6L6 - eBay (item 380235256333 end time Jun-01-10 15:25:47 PDT)

Thanks for any help!

[loudthud]

I'm worried about the black goop that looks like it oozed out of the power transformer. The first thing to do is check the fuses. Are they the correct rating and are they blown?

[Gtr_tech]

Looks like sealing varnish/tar on the pwr xfmr. Sometimes they do a sloppy job and goop it onto the lead wires. Rectifier tube is bad accroding to the seller.....they're easy to find tho...no shortage of 5U's around.

[Ran Randle]

Thanks for the replies...do you know if this type of amp has self draining caps or would require draining? I read a similar conversion post on a conn amp where this was the case. I still have yet to fire this thing up (waiting on the OA3 tube) to see if it all works, PT, OPT, etc... Also, plan to change the plug to a 3 prong before any testing. Here are some pics and any advice, tips would be great, thanks.

I'd like to use an external preamp unit to plug into this or possibly wire in a simple preamp if noise is not a problem.

[Ran Randle]

Done!

Hey there, I just wanted to show everyone the finished product for this conversion.

The only mods I did were the AC ground plug, 1/4 input, power switch, pilot light, 500k master volume pot, 8 and 16ohm output jacks. I made the box with mid 1800's yellow pine that were old church pews. My skilled and lovely Cara Lisa did awesome upholstery work with blue marine vinyl. I didn't think it would turn out like this and the best part is that it sounds great as a guitar amp too . Looking forward to doing another conversion soon.

Cheers,

[Ran Randle]

The amp repairman who checked it out said the black stuff coming out of the PT was probably sealing wax and must have gotten really hot at some point due to a power tube failure then melted and oozed out. The fuses were in good shape when I got it. Lastly, I just added a stand by switch so I think I'll leave it alone for now. I was concerned about damaging the tubes using it without one since its purpose is to let the tubes warm up before the full current is put to them. One of the power tubes is a RCA McIntosh 6L6G and I was told that tube is pretty valuable or sought after by collectors. The other power tube is an original Conn 6L6GC.

[Gtr_tech]

I wouldn't mix a 6L6g with a GC. Get a new pair of 6L6s that match up. The "g" version is not up to the specs of the "gc" and may well redplate and burn up in a circuit intended for GCs. IE, the G has a 19w plate, trhe GC has a 30w plate.

[Ran Randle]

Thanks for that tip for sure. Since the lettering on the glass has long worn off, I really don't know if its a G or GC to be honest. I'm pretty sure its a GC (I think I just wrote G by mistake) but is there a way to tell otherwise by just looking at it? Looks like it was replaced a long time ago when the other Conn tube went bad so hopefully whoever replaced it knew what they were doing. I need to double check that they match but haven't any problems thus far. I just ordered a matched pair of NOS 6L6GC 1981 USSR military tubes.

[Cygnus X1]

Very nice work Ran!

I just acquired the very same amp.
Just looking at it right now to get a feel for the terrain.

Question-are you using this as a fully integrated amp, or just as a power amp?
In other words were you able to just tie in the input to the RCA jack and add a volume pot?

And do you happen to know what ohm the OT is rated for?
Thanks in advance.

[Ran Randle]

Very nice work Ran!

I just acquired the very same amp.
Just looking at it right now to get a feel for the terrain.

Question-are you using this as a fully integrated amp, or just as a power amp?
In other words were you able to just tie in the input to the RCA jack and add a volume pot?

And do you happen to know what ohm the OT is rated for?
Thanks in advance.

Thanks Cygnus!

Yes, the 1/4 input was wired from the RCA so it still could be used if needed. I disconnected the wires going to the rotary "big wheel" potentiometer and wired them to a 500k volume pot.

The OT is green=16 ohms yellow=8 ohms

Adding bass and treble pots too eventually. Good luck with it!

[Cygnus X1]

Thank you Ran, I'll give it a shot.
4th amp I've worked on in two days...I'm on a roll.

[Cygnus X1]

OK, lit up just fine, and sounds good.
BUT...that OA3 rectifier tube glows very bright...but not on the circular plate.
Like an orange/red Christmas tree bulb, circa 1959...

Never seen one of these tubes, is it ok?

Edit, I googled images of the OA3.
Should be fine, but it is scary looking!

[Ran Randle]

Oh yeah, the OA3 is a voltage regulator tube that glows bright orange but it is normal, looks pretty cool too. I was a little wary of it too at first but its a sign of a higher quality amp design or so I've read.

[Cygnus X1]

I went through it, cleaned it up a bit and put a tone and volume control in along with the 1/4" jacks.
I need to do more work but it has a great sound as it is.





That PT goop is on mine as well-don't think it is a problem or that it ran hot in the past.
The tone layout is Princeton 5D3.

[Ran Randle]

That looks great, nice work! That chassis is in great shape. Inspired me to clean up my conn and add a tone stack, thanks.