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**Chuck H** · 09-26-2008, 07:25 AM

Those voltages are fishy. But I think you need to narrow things further. If you take some more voltage readings, plates and cathodes of each triode for the preamp and PI tubes and the power tubes plate, screen and cathode it will help alot.

Chuck

**MWJB** · 09-26-2008, 09:30 AM

Fender layout voltages cannot really be trusted...but as Chuck says voltage readings from plates & cathodes etc will help identify if anything is amiss.

Try the other 6SC7, you can still buy them new in box rather than "Tested" by dubious methods (the real test is how they sound like compared to other tubes of the same type), yes they cost a bit but then you knew that you were using an obsolete tube before you started this.

The amp will be distorted - you have built a cathode biased amp, with low preamp voltages, from a time in history when there realy weren't ANY guitar amps capable of a clean (BF type) tone, or the kind of volume you would associate from even a post 1955 2x6L6 guitar amp.

250K was the correct value for R4, but 125K would work too.

**zhyla** · 09-26-2008, 03:42 PM

Ok, here's some voltages:

Code:

preamp (6SC7) voltages:

Plate, V1a	113
Plate, V1b	113
Cathode  	1.64

phase inverter (6SL7GT) voltages:

Plate, V2a	60.6
Cathode, V2a	1.07
Plate, V2b	61.4
Cathode, V2b	1.04

power amp (6L6) voltages:

Plate, V3	416
Plate, V4	413
Cathode, V3/V4	0.502
Screen, V3/V4	-0.994

The tubes were tested with a tube tester by a guy who sells a lot of tubes on ebay, but of course you can't always trust what you hear. I will try the other tube when it cools down.

**MWJB** · 09-26-2008, 03:50 PM

Sure you haven't got a 100K in the power supply instead of 10K?

You should have more like 30vdc at the 6L6 cathodes, look for shorts to ground.

You've got no 6L6 screen voltage, check connections from the power supply.

**zhyla** · 09-26-2008, 04:47 PM

Wow, that was a dumb mistake. With that fixed my B+ is now at 320 V instead of 400 V. I've got a 120 V zener in there because the B+ used to be 500 V, but looks like this was my problem all along. Should have checked more thoroughly.

I've got 23 V at the 6L6 cathodes now and the amp is much happier. Actually, it sounds great. I've still got to correct the power supply voltage but I'm very happy with the way it sounds right now. I'm sure I'll have some less grievous issues later - thanks a lot.

**zhyla** · 09-27-2008, 10:50 PM

I'm hunting for the source of a background hum. It doesn't scale with the volume, leading me to believe the preamp stage is clean.

I read for push-pull you want the power tube heaters in phase. This means heater pin X on one tube is connected to heater pin X on the other tube, right? The original heater wiring was the opposite way. It was a little loose anyways so I rewired all the heaters with tightly twisted wires. I also raised the heater center tap up to the power tube cathodes (25 V). None of this made any difference, though I guess it may have reduced some hum that isn't yet audible because of the large background hum.

The filter caps are ancient (50's?) but measured well above the spec'd capacitances (60 uF, only 32 uF needed). I used a fancy-schmancy multimeter to measure them, not sure how much to trust that or whether a 60 uF reading with the multimeter means the caps will perform that well at 400 V. I've got new caps I can use instead but if this was the problem wouldn't the preamp be humming like crazy (e.g. hum would scale with volume)?

Lastly, it could be the PT inducing hum into the OT. This is an old organ amp originally so I wouldn't have expected a lot of hum from this given it's a production amp. The transformers are at right angles to each and about 3 inches apart. I would have placed them farther apart but it doesn't seem crazy. A picture of the amp is attached. I can slide the PT over about 4 inches with some work.

Anything else I should think about before I go modify the chassis? Thanks.

Attached Files

**tubeswell** · 09-28-2008, 04:14 AM

I think putting some distance between the PT and OT is a good place to start. If the hum is there pretty much constantly it could also be the aged filter caps. Try some new filter caps. You can tack-solder them in parallel to the old caps for a short while, and if the hum goes away, then you'll know that was it.

**zhyla** · 09-29-2008, 05:46 AM

New caps didn't help. Swapping the power tubes out did help some, enough that I'll leave the PT placement for a rainy day. The background hum is low enough I can tell I've got a bit of noise in the preamp somewhere that scales with the volume knob. I poked at every moveable wire in there with a chopstick but didn't find anything. The PT is on the opposite side of the chassis from the preamp and phase inverter tubes. I'm beginning to worry that a gigantic PT is a bad idea - does EM interference scale with the size of the transformer? Or just the current drawn out of it?

Another annoyance is that when I adjust the volume pot I hear a white noise scratchy sound that goes away when I stop moving the pot. I've tried two pots. I remember having the exact same problem on an old V-M amp I cleaned up for my uncle a long time ago. Brand new pot didn't help in that case either. What could this be? I

Here's an easy question. My heater line is at 6.9/7.0 VAC, a little on the high end. I figured I'd get a couple diodes to knock it down to 6.2. My tubes all together are spec'd at 1.5 A max. Will any old 2 A (or more) diode work?

Thanks a lot for help with my laundry list of problems.

**MWJB** · 09-29-2008, 11:42 AM

"Another annoyance is that when I adjust the volume pot I hear a white noise scratchy sound that goes away when I stop moving the pot." Try replacing the coupling cap that feeds the pot, if it's leaky that might be the problem?

"Here's an easy question. My heater line is at 6.9/7.0 VAC, a little on the high end." No, not really. It's what I would expect from a vintage Fender. Tubes are usually rated at 6.3-6.9VAC, I'd only be considering remedial action if you were well over 7VAC.

**zhyla** · 09-30-2008, 03:48 AM

Replacing the coupling cap didn't help with the scratchy thing. Really odd.

I noticed another funny thing. I was poking around with a wooden stick and noticed if I get it near near the grid pins on the preamp tube (or even the wires connecting the grids to the input jacks) that the preamp hum gets a bit louder. Metal pliers have the same effect but it's like 20 times louder. I don't know if this is just that I'm making an antennae to focus the hum from the PT or heater lines or if this indicates something wrong with my input stage. Obviously this isn't a problem as long as I don't poke around in there while playing guitar but maybe it's a clue?

**Enzo** · 09-30-2008, 05:33 AM

Your link is dead. For reference I posted the 5B6 schematic on www.ampix.org in the Enzo gallery.

Isolate the problem. Once you know where it is, then it is easier to solve.

Is it power supply? Set your meter to AC volts and measure the B+. Measure the first node and the screen node. There should be exssentially no AC on the screen node, and maybe just a few on the plate node.

Caps when old often still measure the same capacitance or even higher than when fresh. But that doesn't tell you that they won't leak like a screen door when voltage is applied. Your hand meter is not a test for old caps other than to see if shorted.

And would this be worst in the preamp stage? Not necessarily. You have at least three stages of filtration here. They may be old and crappy, but they also may add up to more or less clean on the preamp node. Leave the old can caps there for appearance but wire some nice new caps under the chassis for the actual filters. Disconnect the old caps

Is it the power transformer coupling into the output transformer? Pull the power tubes, not the only thing that can make hum come out the OT is coupling from the PT or a OT shorted to frame. I doubt there is a short to frame if the amp otherwise seems to work, so if it still hums now, it might well be coupling. (Looks to me they are mounted at right angles and should be OK)

Is it the power amp stage? Pull the phase inverter tube. How much hum is there now? That is the hum not related to the other stages. Push Pull stages rely on the side to side balance to cancel B+ ripple. Mis-matched output tubes won't cancel because they won't be balanced. Try a pair of fresh relatively matched tubes.

By the way, those old metal 6L6s look spiffy, I think they look cool, but they are notoriously crappy for audio. Also, look in like the RCA for tube specs. Where it says 6L6, that means the metal tubes, while 6L6G ( and GA, GB, GC) refer to the glass tubes. Note the specs for voltage and cetera are lower for the mteal tubes.

Is it the preamp? WIth all other tubes back in, pull the input tube. Did the hum diminish? That was the contribution of the first stage. Actually zeroing the volume control should also kill any input stage noise. APparently you have deviated from the schematic with the heater wires - you did not ground one side but have used a center tap instead (good) - and you also elevated it DC. (also good) That should lower some input stage hum.

I would not expect nearing the grid wires of the input to hum much if the grounding contacts on the input jacks were wired right and working. With something plugged into one, then yes, coming close will induce hum.

Your 6SC7 could be hummy, have any others to try in there?And for that matter the 6SL7 could be hummy.

And your scratchy pot? Turn it all the way up and measure for DC voltage across it. You could have a half a volt or even a whole volt of grid leak voltage there. If so, then live with the scratch. The cure is worse than the disease. DC across a pot will make it sound scratchy in audio circuits.

**Chuck H** · 09-30-2008, 05:37 AM

It could just be a grounding issue. The way your trannies are oriented is really quite typical and I wouldn't expect a problem. But where things are grounded can be. If your using a regular metal input jack thats grounded to the chassis via it's installation, and you have the input jack - lead grounded at a buss or star point, you could have a ground loop hum. I wouldn't rely on the chassis connection for a ground, so you may want to try isolating that jack from the chassis with a non conductive collared washer or change to a plastic Marshall type jack. No guarantees, but this could fix it. Of course, you still need to have the input jack grounded in the right place in the circuit too. And a shielded input lead is also helpful in rejecting external noise if you still suspect proximity.

Chuck

**zhyla** · 10-01-2008, 02:24 AM

Enzo, thanks for the very logical approach to hum hunting. Here's a live schematic URL.