heater wire for a Kalamazoo reverb 12 I just got

[conkface]

So i just picked up a kalamazoo reverb 12 which hums like crazy and which apears to have had some tampering done to the circuit.

The heater wires come from 2 green wires from the power transformer. One green goes to the white wire of a twisted pair of black and white wires to pin 4 from tube to tube starting with the first power tube and ending with the preamp tube farthest away. The black wire of the twisted pair is conected to the other green transformer wire and goes to pin 5 of the tubes.

Is this right?

I found this schematic (bottom right) which apears to show the heater circuit but I don't understand how to read that part of the schematic.

http://www.rru.com/~meo/Guitar/Amps/...everb12-b.jpeg (bottom right of schem)

Any ideas?

[52 Bill]

Welcome to the place.

The drawing is wrong in the heater wiring section.

The heaters of the 12AX7's are wired with one side of the 6vac going to pin 9 and the other side of the 6vac supply going to pins 4&5.

The two 6BQ5 power tubes should have one side going to pin 4 and the other side wired to pin 5.

[conkface]

Thanks for replying so fast.

Could that be why I'm getting a very loud hum regardless even when the volume is down?

My power tubes are red-plating a bit as well so I think I've got a few issues to deal with yet.

I've replaced the big caps and re-wired to a 3 prong plug and after the kids are in bed I'll look into things a little more.

[52 Bill]

Could that be why I'm getting a very loud hum regardless even when the volume is down?

It could be, but it could also be any number of other things.

Did you check the actual wiring to the heaters in your amp?
Which caps did you replace, the main filters?

[conkface]

The two green wires coming from the transformer are now wired to the heaters. one green going to pins 4&5 of the 12ax7's and pin 4 of the el84's and the other going to pin 9 of the 12ax7 and pin 5 of the el84's

in the schematic it shows that on v1 pin 8 is grounded. on my amp pin 8 was bridged to pin 9.

I thought that was weird so I snipped the connection and wired my heater wire to pin 9 with no ground.



I turned the amp on and it still buzzed loudly but this time made a fizzy sound and it seems like I fried the 12ax7 in v1 position.

The caps I've changed so far are just the big caps: 40, 20, 20 450v's

Don't know why I fried that 12ax7. starting to get a little frustrated with this project.

[52 Bill]

Pin 8 of V1 should be grounded, and one side of the heater secondary should be grounded as well. The convenient spot was at the tube socket. I would reconnect the jumper, that may get rid of the fizzy sound.

If you have replaced the main filter caps, double check your wiring. Are all of the grounds good and tight?

Are all of the tubes ok? Have you checked them or swapped them out with a set of tubes that you know are working?

Try pulling out the driver tube and then powering it up. Is the hum better or worse? If it is no better, the problem is in the power supply or output stage. If it gets better or goes away, the problem is in front of the power amp.

[RWood]

I recently did some work on a Kal 12 and reworked the weird heater wiring/grounding scheme. I undid all the heater-to-ground connections, then created an artificial center tap on the final power tube, connecting it to the cathode.

On one of the 6BQ5s I used a 100 ohm resistor from pin 4 (heater) to pin 3 (cathode) and a second 100 ohm resistor from pin 5 (heater) to pin 3. I just wired it right to the socket, and it removed the heater buzz.

Another opportunity to lower noise on this amp is to move the power switch off the treble control and get it away from the preamp. I put a toggle switch where the fuse was and moved the fuse to the rear panel. The new toggle is a center-off DPDT, so it does standby in the down position and full on when all the way up. Very nice with a ss rectified amp such as this. And the unused switch on the treble pot? That's the secret weapon. Use it to lift the tone controls from ground and see what happens!


RWood

[conkface]

Thanks for all the info.

Rwood, I'm not sure I understand fully how you re-wired your heaters but would the heater circuit still work properly if wire it like 52 Bill and others suggest?
That is one green wire conecting to pins 4&5 of the 12ax7s' and then to 4 of the el84's and then the other green going to pins 9 of the 12 ax7's (and also connecting to pin 8 to ground of only the first preamp tube) and then pin 5 of the el84's.

Out of curiosity, how were the heaters wired on the reverb 12 originally?

Since I can't find much info or any pics on the internet about this amp I would gratefully apreciate any insite into it!

[RWood]

would the heater circuit still work properly if wire it like 52 Bill and others suggest?
That is one green wire conecting to pins 4&5 of the 12ax7s' and then to 4 of the el84's and then the other green going to pins 9 of the 12 ax7's (and also connecting to pin 8 to ground of only the first preamp tube) and then pin 5 of the el84's.

I'm not saying those ways won't work but the way I did it worked well for me and lowered the noise. The way you describe it above is the way I did it, with one exception and one addition.

I did not do the part that I bolded above
but the heater still needed a ground reference.
So the way I accomplished that was to create one by taking a 100Ω resistor from each heater leg and connecting it to ground. That is referred to as an "artificial center tap" and is common to see when the PT has no filament center tap.

Finally, the addition that I gave it was instead of connecting the artificial center tap to ground, I connected it to an elevated voltage source, namely the cathode of the power tube, which sits at around 12 volts above ground. This, too, can help remove some buzz.

On the original Model 12s the heater was grounded on one leg, as in that schematic. It works but is noisy.

Miles O'Neal has some great info on his site and has yet a different take on the heater wiring. So you can see there are several ways to skin this cat. Or kal.


RWood

[conkface]

Thanks again to RWood and 52 Bill for the responses! Things are becoming clearer now.

Okay, so I rewired the heaters and everything works but I still get a loud 60hz hum.

What I've done so far is to replace the 3 big filter caps in the power supply,
replaced the 100ohm 2watt resistor and 25/50cap in the power tube bias circuit as well as replaced the other 10/25 electrolytics.

I've pulled the tubes one at a time. the hum goes away to almost nothing when I pull out v2 the phase inverter tube.

With v2 in I tried pulling out one and then the other el84 and still the hum persists.

The other wierd thing is that I can change the tone of the hum and increase/decrease it a little by turning the reverb control. This happens whether or not there is a tube in the v3 reverb circuit position.

One thing I haven't done yet is to confirm whether the 2 el84's are any good as I have no way of testing them short of buying a new set and sticking them in. Is that the next step?

Also, I haven't checked the measurments to see if the bias is good on the power tubes. Maybe I'm going about this whole thing ass- backwards..i' don't know but it's fun learning!

[52 Bill]

Don't worry about the output tubes for now, by pulling out V2 you have proven that the hum is probably not related to the output tubes.

I'm guessing that the hum you are getting is signal ground related.

Have you tried pulling out V1?
Have you checked the input jacks and related wiring?
How about the reverb tank and wiring?
When you replaced the caps where did you ground the negative ends?

[conkface]

I did pull v1 with v2 in and it still hummed. with v2 out the hum stops when v1 is in.

The amp didn't come with a reverb tank so the transformer leads are dangling through the hole in the chassis.

When I replaced the caps I grounded them to their original location which is were on several terminal strips located on the chassis.

[52 Bill]

The amp didn't come with a reverb tank so the transformer leads are dangling through the hole in the chassis.

Perhaps this is part of the cause of your problem.