Conn amp heater wiring issue, maybe

[Don Ho]

I have a conn amp that I decided to acquire for conversion to a guitar amp. I had been communicating with a builder/converter in Ohio through emails but cannot seem to get return communication any more. He had led me through initial stages, as well I have been reading extensively on the theory.

My issue is that I had wiring for the heaters from the PT at 6.3v, both green outputs rated at 6.3v with a center tap wire (grn/ylw). I wired the heaters just as instructed and everything seemed fine. I had three preamp tubes in their sockets along with two power tubes, a 6l6gb and 6l6gc (came with the amp that way). The gb will no longer show signs of the heater glowing. In the middle of this build I had acquired another amp, this time a Hammond, that had 3.2v heater taps with ct. I wasn't familiar initially with this but found info that checking pin to pin on the socket once wired yielded a result of 6.3v.

Once this 6l6gb stopped working I did the same test as was done on the Hammond, yielding 12.6v across the heater. Understanding that the preamp tubes will work with 6.3v or 12.6v on the heaters there would be no ill effect on them. But did I have the power tube sockets wired at 12.6v since I ran a lead of 6.3v pin to pin on one side and then the other 6.3v lead to the other pin to pin?

The conn amp had originally ran 4x 12v6's and then 2x 6l6's. Were the 6l6's wired in a fashion to drop the heater voltage? My problem is that the person helping instructed the teardown but I didn't take note of any difference on the 6l6 sockets to see if a change was made from the factory to regulate the voltage for these sockets vs. the preamp and 12v6's.

I would appreciate not only some direction on my issue stated above but also some direction on reading. I see many publications referenced for understanding tube amp building, what is really the best for the novice to understand the basics? I acquired yet another small amp and will be going forward with all three projects but have realized that there are so many differences that the net/web cannot pin down accurately enough. Thanks in advance, finding that this is a great site for info and the building is an exciting task. Can't wait to fire up my first amp!

[RWood]

Don,

I have one of those Conns and it is indeed a 12.6V heater supply. Which means you have your preamp tubes wired to pins 4 & 5 separately (nothing on pin 9), and then for the power tubes you wire them in series, not parallel. The 6L6s should have already been wired that way. Did you change the wiring?

On mine I've got one leg of the heater supply going to pin 2 of the first 6L6, the the other leg going to pin 7 of the other 6L6. Then a jumper connecting the other pin 7 and 2, respectively. It provides 6.3 volts on the 6L6s.

Whether or not this is the most effective way to accomplish it, I am not sure. I am still in the debugging stage on mine and although it works and has a nice tone (I built a Fender 6G6 circuit), it is rather noisy. I might swap the heater connections on one of the 6L6s, for instance, and listen to it to see if it makes a difference.

But the short answer is to go to series wiring on your power tubes.

Those 12V6s are cool. They came in real handy when I needed to replace a tweed Princeton PT and the one I scavenged, which fit perfectly in the chassis and was the right HT voltage, only had a 12.6V heater tap. No problem on the 12AX7 wiring, and thanks to the (set of four) spare 12V6, no problem there either! And NOS black plate, magic mojo, 12V6s are fairly cheap.

What circuit are you building in your Conn?

RWood

[Don Ho]

Thanks for the tip, I did jumper the preamp pins so I can rewire that section. I had been following the lead from the help I had, I had mentioned a person in Ohio was communicating with me on this project but for no apparent reason bailed out on the build. Initially I was given enough detail that the build was to be a dual rectifier that would use a 5ar4 tube and 1n5408 diodes. The build was to also be able to utilize EL34's or the 6l6gc's I had. That's as much as I was given initially. The last communication though that I had from him was that we were going to do a PA100 Fender, the schematic was attached to that email. So portions were modified and/or omitted. I really wasn't too excited with that direction. I had gone out on my own while waiting and found some project layouts, some were very close to the Bandmaster and others the Bassman. Weber amps had a couple I liked, layouts anyway. The other close builds I had considered were from ax84's website. Some were circuitry shem's with layouts that indicated 6l6's could be used.

At this point I've throttled back, realizing that if I don't get it right from step 1 that the end product will suffer. Getting these heaters right is the simplest circuitry and as crucial I've found as anything else.

One question, I had noticed on the conn that 7-8 were jumpered on the 6l6's then fed into a cap can that was grounded. Any chance that this had some bearing on the voltage being the issue since I only ran to pins 2-7 on the sockets this time?

[loudthud]

So you are trying to replace four 12V6's with a pair of 6L6's ? The 6L6 sockets need to wired so that you would measure 6.3VAC between pins 2 and 7. Since the center tap of the 12.6 VAC winding is probably grounded, you would need to connect (for example) pin 2 to one side of the winding and ground pin 7. To keep things balanced you should run each socket from different sides of the winding.

[RWood]

This amp has four 12V6 sockets AND two 6L6 sockets (three discrete p/p amps on this chassis) but I like your method or wiring the 6L6s.

Don, if you take the method I described in my post, and connect the jumpered pins 2 - to - 7 to the filament center tap, you'll accomplish what Loudthud is describing here. Right, Loud?


RWood

[Don Ho]

I plan to run the heater as mentioned on the 6l6's, one to the 7, the other to the 2 and then jumper the sockets for series. It makes perfect sense, just couldn't think straight and see the simplicity to that.

You are right about this amp, it did have some unusual capabilities. 6l6's for the main, 2 12v6's for the tremelo (or so it designated on the chassis), and apparently 2 12v6's for the line output socket.

From your perspective, being in the process with your conn, what would be the easiest route to take from this point?

I was thinking that I'd like a 5ar4 going to 3 12ax7's then the 2 6l6's. The amp came with the GZ34 rect, seems that it's more taxing on the pt from what I read. I'm looking for the bluesy glow of the tube rect with the 6l6's, might even trade the 12ax7's out if need be. Would still like to have some gain control, I do love the tube distortion but don't want it all the time. The idea of the ss rect seems feasable, just a bit sterile at times. I have a B-52 AT100 with the rect switch and am familiar with the sound this tube sequence produces. It does have 4 5881's, not exactly the same but close.

Anyway, thanks for your help. Very interesting to see the perspectives and knowledge on this forum.

[Don Ho]

Update, sorry I misquoted the factory rectifier as a GZ34. The rectifier was a 5u4gb, this is the one I heard can be used but can strain the pt. I don't know from experience with it though, have only used the 5ar4/gz34. The b-52 came with a new russian model 5ar4 which I replaced with a sylvania nos gz34, better but not significantly.

[Don Ho]

Advice worked on heater wiring

Thank you RWood and Loudthud. I wired the preamp tubes as suggested, pins 4-5 only and not jumpered. I ran off the last preamp from one pin to pin 2 on the first 6l6, the from the other pin to pin 2 of the second. I did a ground off pin 7 to the ground buss from both sockets. Everything checks perfect.

[RWood]

About the rectifier, if your amp is like mine it had two separate rectifier circuits, one set of windings to a 5U4GB and a second, lower voltage tap off the main winding to a 5Y3GT.

This is kind of handy because you can use one or the other, high or low.
On mine I took the higher of the two, disconnected the lower set, but paralleled over from the 5U4 to the 5Y3 socket. I used a pair of 5U4GB. I started with a 5AR4 but the voltage was a bit high for what I wanted. Plus, the guy I am building the amp for liked the two big bottles, lol.

Once I get the noise floor settled a bit and can crank it up, I am hoping that I'll find that for bass, the sag will be less with these two rectifiers (although the amp will work fine with just one).

Glad to hear you got the heaters sorted out!