5C5 build hum and fried dropping resistor

[Jason C]

Hey guys I finally got my PA turned 5C5 wired up and when I turned her on and played through it yesterday there was a pretty substantial hum that would get louder or softer with the controls (vol or tone). I thought I would tackle that problem today but this time when I turned it on I heard a hum that wasn't through the speaker then I smelled something and shut her down. After the power was off I saw a little smoke, opened her up and found one of the 10K dropping resistors had been fried. Where do I start and are they related problems maybe? Prior to turning it on today I double checked my wiring and everything apears to be on the level. So now what? Thanks.
Jason

[stokes]

Sounds like you have a shorted filter cap,or a bad tube.To fry that resistor there has to be a short,unless you are using too low watt rating on that resistor.I am assuming it is the resistor between the OT tap and the screen tap.

[Jason C]

Thank you for the help. I was using a 1/2 watt carbon film there. I did have doubts about the tubes as they are the ones that came with the amp (old PA). So do I simply replace the resistor, use different tubes and hope for the best? Thanks again.
Jason

[tubeswell]

A 1/2 watt resistor isn't up to the job for that first supply resistor.

You need probably at least 2W (to 5W). If you don't have a 10k 2W resistor, try putting 2 x 22k 1W resistors in parallel here. Or you could go to a 4k7 5W resistor (or better still a choke instead of the first supply resistor), and a 470R-4k7 5W screen grid resistor at the screen grid pin(s) for a more chimey tone.

You might have created other problems frying your supply resistor. But replace it anyway with a higher-wattage resistor (setup) and watch carefully to see if anything else catches fire when you switch it back on. Also watch the tubes now for arcing, and watch the PT for smoke etc. I'd also measure the plate, screen and cathode voltages when you get it running again to see if you haven't had flow-on effects from the fry-up.

[Jason C]

A 1/2 watt resistor isn't up to the job for that first supply resistor.

You need probably at least 2W (to 5W). If you don't have a 10k 2W resistor, try putting 2 x 22k 1W resistors in parallel here. Or you could go to a 4k7 5W resistor (or better still a choke instead of the first supply resistor), and a 470R-4k7 5W screen grid resistor at the screen grid pin(s) for a more chimey tone.

You might have created other problems frying your supply resistor. But replace it anyway with a higher-wattage resistor (setup) and watch carefully to see if anything else catches fire when you switch it back on. Also watch the tubes now for arcing, and watch the PT for smoke etc. I'd also measure the plate, screen and cathode voltages when you get it running again to see if you haven't had flow-on effects from the fry-up.

Thanks. I'll replace it with a 10K 2W and measure those voltages. About where should those voltages be? Also, do you think the hum and this resistor going south are related or am I chasing two problems at once? I also looked at the tubes more carefully and realized that one is a 6L6GB the other is a 6L6GC. I assumed they were matched as they are what came in it but that's what happens when one assumes things.
Jason

[tubeswell]

do you think the hum and this resistor going south are related or am I chasing two problems at once?

Don't know for sure, but if the hum is present without guitar plugged in, then its likely to be a grounding related hum (which could be unconnected to the resistor blowing, as long as the resistor blowing was only related to its' inability to handle the power dissipation. If on the other hand the resistor blowing was related to a short in wiring the filter caps etc, then the hum could be related. But if the hum was caused by shorting the B+ to ground, you'd definitely hear a loud hum in the speaker,... shortly before the amp heads south )


I also looked at the tubes more carefully and realized that one is a 6L6GB the other is a 6L6GC. I assumed they were matched as they are what came in it but that's what happens when one assumes things.

They output tubes are both 6L6Gs, the "B" and "C" designations probably just relate to marketing hype. Unmatched tubes will produce a bit of hum, but can also add character to the amp in other ways - i.e. a cool sounding phasey 'swirl'.

On the voltage question, looking at the Fender 5C5 schematic and layout diagrams, on http://www.ampwares.com/ffg/schematics.html it doesn't show what the measured voltages are, but on the Fender Transformer Chart at http://www.unclespot.com/FenderXFMRchart.html it does say the 5C5 used a 6516 PT, along with 5D5 and 5E5A, and on the 5E5A schematic (which also uses a 5U4G rectifier, albeit a '5U4GA' as opposed to a '5U4G', and that this distinction probably meant something in the old days, but maybe not too important with a modern 5U4G) the B+ is shown as 390V. My guess therefore is that the plate voltage on the 5C5 is somewhere about the same (i.e. 380-400V) , and the screen voltage is maybe 25- 50V lower (because of the 10k dropping resistor). My further guess is that the cathode voltage may be around 18-20V (based on 250R biasing two parallel cathodes). What voltages do you measure?

Jason


Cheers

[Jason C]

Thank you for all the help. I won't get a chance to work on it until the weekend, if then, but as soon as I do I'll tell you what I have. When it blew up I didn't have a guitar plugged in and there was very little if any hum other than the hum not coming out of the speaker.

[stokes]

If we are talking the dropping resistor between the first node and the screen,I would recomend 5-10watt there.I would certainly suspect the hum is related to that resisitor doing a slow burn before it finaly fried.It could very well be related to unmatched power tubes.After you change the resistor and get it running,check the current draw on each tube,if they arent within about 5ma's,the mismatch could cause the hum.Also be aware that the B and C designation are more than just marketing hype.The 6L6GB and 6L6GC have different max output and max plate voltage ratings,with the GB being rated for about 360 plate volts and about 23 or so max watts,The GC is good for 400+ plate volts and 30 watts output.They can be used together so long as the amp is designed for the GB and not the higher rated GC.

[Enzo]

Is anyone actually making 6L6GB tubes today. I know there are some with that on the label, but that doesn't make them really that. We have plenty of tubes that are really something else inside these days. Change the pinout, and voila, your 6L6 is now a 7027. Need a 7591? We'll find something to stick in a bottle for you, it'll say 7591 on the label.

I tend to think that distinctions that were real in the 1950s are nothing more than memories now. When I see a Groove Tube 6L6GB, I tend to doubt it.

[stokes]

When talking current production tubes,yes the 6L6GB from GT is a code of their own,and is likely a GC,but since he is talking about a PA converted to a 5C5 and he does say these are the tubes that came in the PA,I am taking the liberty of assuming it is an older PA and he might have some old stock tubes there,where the GB and GC do actually mean something and are very different.

[Enzo]

Fair enough.

[Jason C]

I meant these are the tubes that were in it when I bought it but they are not the orginal tubes. The schematic I have for the PA just says 6L6G's. It looks like it's going to have to wait as I can't find any resistors around town and will have to order some. It's hard to believe that Radio Shack is the only place in this town that sells any kind of electronic component!

[Enzo]

Actually I find it surprising that ANYONE sells electronic components locally, unless a parts distributor happens to be based in your town. The retail market for components is extremely small. Even Radio Shack is edging out of it.

[Jason C]

My brother happened to have some 10K 5W resistors, so I replaced the 1/2W's that were in there and measured some stuff. I ended up with 457v on the plates (which seems awfully high to me anyway) 380v on the grid and 27v on the cathode. Do these numbers seem right to you guys. Also I fixed a stupid wiring mistake in the input jacks and that cleaned up some of the noise. I'll probably order some new output tubes here in the next day or two and try and dial it in when I get those. Also, how can I maximize my clean headroom with this amp? Thanks guys.
Jason

[tubeswell]

Well, if you compare it with the 5D5 and the 5E5A, 457V on the plates and 27V on the cathode seems high at first glance. What's your B+? and what's your Ht winding putting out (i.e.; when measured either end to ground with all the tubes plugged in and switched on)?

Using those values from what I understand you'd get

27V/500 Ohms = .054 (54mA tube current for each output tube) (I think that's right because your cathodes are in parallel and biased with a 250R resistor, so if you look at it on a per-cathode basis, its the equivalent of having a 500R resistor biasing one cathode?)

If that's the case, with each tube drawing 54mA current, then the tube dissipation with a plate voltage of 457V is; 0.054 x (457-27) = 23.22W per tube.

A Svetlana 6L6GC can handle 30W plate dissipation according to this;

www.drtube.com/datasheets/6l6gcsvet1996.pdf

So my guess is that your output voltages are okay.

No doubt someone else can correct me if I'm wrong

[stokes]

Did you have the power tubes in when you took those measurements?They arent too high for a 6L6GC of any brand,but if you are using a 6L6G or GB,which is closer to the original 6L6's the amp was designed for,then you are kind of high.If the volts are with no tubes in the amp,they will come down with tubes.If you measured the plate volts to ground,you can subtract the 27v's on the cathode to get what voltage the plate actually sees.

[Jason C]

Yes, the tubes were in it when I took those measurements. I ordered some JJ 6L6GC's today so hopefully those will be able to handle it. Is there a good way of dropping those voltages a little bit and get them a little closer to where they should be and will that give it more or less headroom? I fixed the hum today, it was a simple grounding issue. Now this thing really sounds good even with the mismatched tubes. Thanks again for all the help you guys.

[tubeswell]

Is there a good way of dropping those voltages a little bit and get them a little closer to where they should be and will that give it more or less headroom? .

See this page for tips on dropping the B+/plate supply voltages

http://www.aikenamps.com/BackBiasing.html

Putting a lower-gain PI tube in will give you more headroom in the output stage (because you'll reduce the clipping of the signal being fed into the output tube grid(s)).