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dude

Sixty Cycle Blues

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Well, I'm stumped. I've gotten ahold of yet another vintage PA amp to convert into a guitar amp, but this one has me at a loss.

The first thing I did when I got it was set up a temporary chassis ground, attach a proper speaker load, and power it up. Once the tubes warmed up, the speaker began to emit a (loud) drone, which by ear I'm gauging at about 60/120 Hz - the wall socket in other words. This is with all the volumes down to zero (bear in mind there is no MV on this thing). Adjusting the volume knobs has no effect.

Undeterred (and unsurprised), I replaced the old 2-prong power cord with a 3-prong grounded cable, attaching the ground to the chassis. Power it up and ... same problem. Just for grins, I plugged a guitar in. I was able to get sound out of it, but the alien frequency created some nasty oscillation and it sounded like some weird vibrato/terrible. Hooray for interference.

At this point I was still not worried. The amp is from 1955 and so far as I can tell, it is all original. In other words, the electrolytic can caps in the power supply are definitely shot. So I replaced them with some lovely modern electrolytic caps, got everything wired up all nice-like, and ... still the same problem.

So now I'm at a loss. I have no idea what might be the problem. I hesitate to blame the grounding scheme, as this thing was (at one point) professionally made.

the model number is a Dukane 1C460a

An additional problem: This is the rack mount version, and there was no bottom plate cover for the chassis - and hence no schematic attached. What's worse is that I have not been able to find anything more than a blurry photo online.

Does anybody have any idea what might be causing this oppressive hum?

Edited by dude

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The first two things I would check would be the tubes themselves and then the heater supplies. If your heaters are run on DC, you could have insufficient filter caps. If it is an AC heater scheme, you should check the lead dress to make sure there is no funny business there. Also, I would get a scope and look at the signal path starting at the input jack (with no guitar connected) and work my way through to see where the 60 Hz is coming from. Also, make sure you aren't in a room with big fluorescent lights while the chassis is open, as their ballasts will emit very strong 60 Hz EMI.

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So you're the owner of a Dukane 1c460. (PA, rackmount)

I found something of the Dukane 1u460. (PA, head)

So you need to check if it's based on the same schematic.

I wouldn't be surprised if your Dukane is the result of a u46025F6A(Bassman) conversion that went wrong.

If I have understood it well the flickr link provides enough info to get the Dukane working as a Bassman.

flickr not flikker

cheers

FF

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Thanks all. I've got a spare set of matched 6L6's I could swap in without too much trouble. A bad tube would certainly be an easy enough fix. Then again, if I'm somehow getting bad power supplied I may not want to throw a perfectly good pair away like that. I should measure some voltages first.

I unfortunately don't own a scope. I may be able to rent one from the university. We shall see.

That link there is very helpful. It only has part of the original schematic, but its the power supply, which is what has had me most concerned so far (since I'll be redesigning the preamp).

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Instead of renting a scope, you can buy a little digital one that will do exactly what you want for about $50. I think the brand is JYE. It is a rather popular scope in the DIY effects building world. Also, if you have hum on your heater supply, it most likely won't damage your tubes. It would have to be a bad bias or too high of a plate or screen voltage to really damage your power tubes.

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That's good to know. I'll look for one of those straightaway.

I just compared the schematic to the actual machine, and it appears that the rectifier RC1 is missing. This is probably not a good thing (I think its part of the bias supply).

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That's good to know. I'll look for one of those straightaway.

I just compared the schematic to the actual machine, and it appears that the rectifier RC1 is missing. This is probably not a good thing (I think its part of the bias supply).

It's a miracle that nothing burnt through. Is it because of the valve rectifier (GZ34)? Are the kathode resistors bigger than 1 ohm (some kinda class A setup)?

The 5F6A bias(& power) conversion should solve your problem when using 5881s.

Since you're experimenting with 6L6s the bias setup requirements could be different. I'm looking at a Fender vibrolux 6G11-A schematic. It has 365 volts at the anode and -36 volts as bias input.

FF

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That's good to know. I'll look for one of those straightaway.

I just compared the schematic to the actual machine, and it appears that the rectifier RC1 is missing. This is probably not a good thing (I think its part of the bias supply).

It's a miracle that nothing burnt through. Is it because of the valve rectifier (GZ34)? Are the kathode resistors bigger than 1 ohm (some kinda class A setup)?

The 5F6A bias(& power) conversion should solve your problem when using 5881s.

Since you're experimenting with 6L6s the bias setup requirements could be different. I'm looking at a Fender vibrolux 6G11-A schematic. It has 365 volts at the anode and -36 volts as bias input.

FF

No, sorry, My mistake. No, the valve rectifier was there. I'm referring to a tiny, solid state rectifier (similar to the item on the right of this picture).

selenium.jpg

As it turns out, it actually IS in the circuit, I just missed it (it looks kind of like a mounting lug).

Interestingly enough, I notice that the chassis is labeled for 5881's, while the tubes inside are 6L6's. They require different biasing, no?

EDIT: never mind. They don't. Foolish me. It still might be a bad set of tubes though.

Edited by dude

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Could also be due to bad lead dress. Maybe there is a wire picking up AC coupling and injecting it into the signal path. Also, the input jacks mightbe picking up hum if they are connected to the chassis. Try isolating those from the chassis, sometimes that helps a lot. Also, consider star grounding it. I did it to one old amp of mine and it made a huge difference.

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I'll have to check on that. I doubt it is the input jacks, as volume adjustments have no bearing on the hum coming out of the speaker. It is constant. Star grounding would be a good idea, though. I think I'll work on that. Actually, I'm contemplating knocking together the Bassman mod posted earlier in order to "start from scratch" so to speak, and then gradually transition it to more of a SLO-type (opposites, I know). In this case, I would do my grounding all via star ground and save myself a headache or two.

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I'll have to check on that. I doubt it is the input jacks, as volume adjustments have no bearing on the hum coming out of the speaker. It is constant. Star grounding would be a good idea, though. I think I'll work on that. Actually, I'm contemplating knocking together the Bassman mod posted earlier in order to "start from scratch" so to speak, and then gradually transition it to more of a SLO-type (opposites, I know). In this case, I would do my grounding all via star ground and save myself a headache or two.

The most likely is that your power valves are not a matched pair. They draw different currents at rest. As a result of this the remaining hum after rectification isn't cancelled out at the output transformer. You can check the currents through the power valves by measuring the voltage across the cathode resistors (& no input signal) and applying Ohm's law. Should be somewhere around 35 - 40 mA.

2 solutions: use a matched pair of output valves or use some bias circuitry with a trimpot per valve.

cheers,

FF

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Matched pair is probably the easiest. Well actually its not, but it involves less poking around inside the amp with the power on, which is always a good thing :D. I'll get on that later today. Hopefully it will all work out.

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have you checked for leaky caps emitting dc on the audio path

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have you checked for leaky caps emitting dc on the audio path

Not really. I replaced the electrolytics with no result, but I should probably check the rest. Power tubes first though, I think. I swear I WILL find the time this week to try some of these things. Or if nothing else next week. School's been busy recently.

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well dc on the lines can oscilate and cause the thing you speak of. also its quite easy to check the caps for leakage

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well dc on the lines can oscilate and cause the thing you speak of. also its quite easy to check the caps for leakage

How would you recommend? I assume NOT by hooking up a current meter in series and looking for DC current.

Edited by dude

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not looking for current but voltage on your audio path. at every tube plate in preamp right up to the grid of the power tubes you have a coupling cap that allows ac voltage to pass ie the guitar signal and dc voltage to be stopped ie high voltagte.

to test this. simply find where your high voltage ties into each tube. and with one end of meter tied to ground and the meter set on dc voltage at around 600-1000v use only one hand with the red probe to test both sides of the coupling caps. in a 12ax7 pin one and pin six. you should have your plate voltage on one side that touches the plate through the plate resistor. and the other side goes to a cap the output of the cap should have no voltage on it. if it does you have issues.

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Well its been two years since I started this thread. One electrical engineering degree and a fair bit of study and work with tubes, and I'm thinking its time to take another crack at this amp. For the record, you all were right last time - the problem was unbalanced power tubes. In this case, of course, "unbalanced" means "one is completely dead". This has been known to cause a less-than ideal sound.

Now, when I say "another crack at this amp", what I really mean is I'm going to yank the PT and the OT and build from scratch. I'll also be designing from scratch, because I can do that now. Why make a SLO-clone when I can be original, right? It will be a bit before I start the physical work here because I'm going to simulate everything first, but I'll keep y'all posted in case there's some interest.

Edit:

Oh, also there will probably be custom paper caps involved, dependent on whether I can get the formula right. They're not hard to make, but I'm not completely certain I trust them to block 300vdc.

Edited by dude

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i wouldnt worry too much about the caps. theres a ton of mojo credited to caps thers better places to focus your attention

i have always though it would be fun to design a amp with 6v6 in the preamp section run a 12ax7 in to a 6v6 (much like a champ) then regulate that down then run in to a good clean power amp. use the 6v6 in the preamp section to create power tube distortion. if you could build something like that it should have a nice bluesy sound at managable volume levels.

just a idea.

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That's a pretty cool idea. I was toying with a similar idea a few months back, though I was thinking I'd use an el84 (or a slow-cutoff pentode from a tv, since they're REALLY nonlinear) instead for the smaller form factor and (I think, though I don't know off the top of my head) the lower ideal quiescent current. Might be fun to try. Hell, I could even just put in a solid state power amp in that case, since the power tube distortion would already be there. That would save a bundle on transformers.

The caps I am considering simply because I built a paper cap out of newspaper, mineral oil, aluminum foil, and wax a while back and was surprised to see that it actually worked. My buddy and I built the thing in a half of a paper towel roll, and it came out to about 0.8 uF. Considering most coupling caps in a guitar amp are usually going to be on the order of .02 uF, it got me thinking it might be fun, and not too hard, to build an amp using only diy caps for coupling. This would probably be better done after building it with factory-made caps, however, so you could do a proper comparison. Maybe I'll build a jtm45 clone sometime and make the caps for it.

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i would suggest a smaller pentode can achieve the same type of output grunge. you could use a 6dx8 for instance but have a look at the late great fred nachbaur i think thats how you spell his name http://www.dogstar.dantimax.dk/tubestuf/dzindex.htm this is a mind funk of an amp. i have been using the compressor trick for years since learning it from him

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