5c3 Proluxe help

[Jason C]

Hey guys, I just finished putting together a 5c3 Proluxe (with a GA-75 preamp and tone circuit) together. The problem is a really loud hum, the signal gets through but with lots of hum and the tone controls don't seem to effect anything. Any ideas on where to look first? Also my voltages seem way high.....490v on pins 3 of the 6L6's. My first idea is a grounding issue, maybe? Thanks.
Jason

[tubeswell]

Got any pictures you can post?

[Jason C]

Here are some pics.
Jason

[Jason C]

And a couple more.
Jason

[tubeswell]

Where do you begin with any troubleshoot? Here's a couple of comments. (I might think of something else after a bit of a closer look). Got a copy of the proluxe layout and schematic there? - this is different from a Fender 5C1.

[Jason C]

Thanks Tubeswell. No those resistors aren't touching. This is the amp and layout I'm doing. https://taweber.powweb.com/store/5c3p_layout.jpg
And this is the preamp and tone circuit I'm using. http://www.schematicheaven.com/gibsonamps/ga75_old.pdf
I was going to run the input under the board but I thought it was bad to have it that close to the plate resistor.
Jason

[Jason C]

I pulled the phase inverter tube and the hum goes away. So that means my problem is in the preamp, right?
Jason

[tubeswell]

Hi Jason - Did you try another tube there?

[Jason C]

No I haven't but that tube came out of a working amp with no issues but I'll try another one. I did pull the preamp tube with no change in the hum. I also tried bypassing the tone controls with a jumper and still it hums. I dunno....just scratching my head here.
Jason

[tubeswell]

So everything is grounded to the brass grounding plate?

You could try a different grounding scheme and see if that reduces hum. Check out this one (courtesy of R.G. Keen - Thanks R.G.)

[loudthud]

In the picture prolux-010.jpg the top right jack appears to have a yellow wire that is not soldered. Also in that picture you have two wires from the volume control running right between the two pairs of input jacks. It would be better to route those wires from the volume control straight down towards the eyelet board, then along the chassis to the eyelets where they connect.

A couple of questions about the hum. When you flip the standby switch from Play to Standby, does the hum go away immediately or fade out over a period of a couple of seconds? Does the volume control have an effect on the hum? When you turn the volume all the way down does the hum go away, change slightly or get louder? Does the power transformer have a center tap for the heater winding (usually green with a yellow stripe) or is there a virtual center tap made up of two 100 ohm resistors?

[Jason C]

The input jacks are soldered. I took the picture just before I soldered them. I did reroute those wires going to the tone controls and also took the components they went to and mounted them on the pots. That shortened the grid wire to the PI pretty drastically (ugly now but what do you do?). The hum goes away imediately when the standby is flipped. The volume control has no effect on the hum. The PT has a filament CT that I grounded to one of the PT bolts along with the high voltage CT. Also, the hum did not stop when I pulled V1. I also swapped tubes and it still always stops humming when I pull the PI.
Jason

[Jason C]

I meant it does NOT go away immediately. It does take it little bit to fade away.
Jason

[loudthud]

Originally Posted by Jason C

I meant it does NOT go away immediately. It does take it little bit to fade away.
Jason

That usually points to a problem with the heater supply CT, possibly a bad tube, or a lead dress issue where a signal wire is too close to one of the heater wires. Your heater wiring looks pretty good. Make sure both of the green wires measure ~3.2VAC to ground. Another possibility is just a wiring error around the tone controls. Ohm all the points that are supposed to be grounded to the chassis.

A shielded wire from the tone controls to the phase inverter might help.

FYI: It looks like you used pin 1 on the 6L6 next to the rectifier as a B+ tie point. If you ever plug a metal 6L6 or 6V6 in there, the outside of the tube will have B+ on it!!! Another safety issue, you have the black wire from the line cord connected to the side terminal of the fuse holder. This allows you to touch the hot side of the line when inserting the fuse. Better to reverse those connections to the fuse holder.

[Jason C]

I tried clipping the filament CT to the positive terminal of a 9V battery with the negative clipped to the chassis....with no change. Thanks for the heads up on the fuse wiring. I'll never use metal power tubes in this but I may put that on some unused pin of the rectifier? I also fiddled with the bias and just made the hum a whole lot louder. Also, I'm not sure what you mean by ohm all the grounds. Do you mean measure the grounds resistance (sorry if that is a dumb question)? Here are my voltages V4 (6L6)pin 3=486 pin 4=463 pins 5 and 6=-61 pin 8=.011
V3 (6L6)pin 3=487 pin 4=465 pins 5 and 6=-61 pin 8=.008
V2 (6SL7GT)pin 2=277 pin 3 and 6=3.45 pin 5=279
V1 (6SL7GT)pin 2 and 5=441 pin3 and 6=7.91
all heaters are 3.3

[stingray_65]

It's hard for me to tell,
but each time i look i see that the heaters on the 12AX7's are reversed
unles i'm wrong the wire from V1 pins 4,5 connects to V2 pin 9

Ray

PS
I always use 2 different color heater wires, If I dont i will take a sharpie and run it along one of the wires to give it a black stripe

[Jason C]

It's an all octal amp. The PI and preamp are both 6SL7GT's and the junkie chassis I have has the preamp and PI sockets turned the wrong way. So the heaters are going to pins 7 and 8 on the 6SL7's.

[loudthud]

Jason, Yes, measure the resistance to ground of the points that are supposed to be grounded as a reality check. No matter how many times you have looked things over, a resistance check will often times expose a problem. Also, if there are wires on the back side of the eyelet board, check for continunity on the top side of the board between the two eyelets. I noticed that the Weber layout shows the minus side of all the filter caps connected on the back side but you have a separate ground wire on the right most cap. This is a good thing.

The Gibson schematic is a little hard to read but the voltages on V1 don't look right. It looks like you have the wrong cathode resistor (1.5K) or it's not connected to ground. The voltage on pins 3 and 6 should be less than 2 volts.

[loudthud]

It look like that cathode resistor on V1 is a 15K 1% resistor. 1...5...0...2........1

[Jason C]

The cathode resistor for V1 is 750K. I was told that since there is one tube instead of the two in the original gibson that I need a 680K instead of the 1.5K and when I was ordering parts the 750K was the closest. Also, like an idiot, I realized that I'm supposed to have 40uf, 16uf and 16uf for my filter caps and I have three 40uf's in there. I'm really concerned about my having 441V on my plates to V1. I double checked all my wires under the board for continuity and they're all fine.

[Jason C]

After typing my last response I just realized that he told me to use a 680 Ohm not 680K. So I guess I'll see if I have anything close to that and swap it out.

[loudthud]

Actually you need to go the other way. Something 3K to 4K is about right with both sides of the tube feeding a 220K.

[Jason C]

So there should be a 220K on each plate? I thought there was just one 220K for all four plates in the Gibson schematic. I also have pins 2 and 5 jumpered on my preamp tube going to the 220K because I thought (from the way the Gibson schematic is drawn) that all the plates are essentially tied together.

[loudthud]

I was thinking you would be using both sides of the dual triode into one 220K plate load resistor. I couldn't find the plate curves for the 6SL7 right away so I just looked at the RCA Resistance Coupled Amplifier tables. Since you have two triodes in parallel, I used the 300V B+ and 470K plate load and it suggests about 6K for a cathode resistor. Then I looked at the tubes that use the same tables as the 6SL7 and found some plate curves. You have a pretty wide latitude as far as biasing goes and I don't know what B+ you will end up with. So start with 3.3K or something close and adjust as needed to get the plate voltage about half the B+ or slightly higher.

You could get creative and wire the input jacks such that one jack feeds one triode and the other feeds both triodes in parallel. Or what many people do is split the cathode resistors and use a different cap on each side.

[Jason C]

I tried a 4.7K cathode resistor because I had it close and got the plates down to 245V. So that's progress, right? I also ran a buss across the backs of the pots just to see if that might help witht the hum but it didn't. I'll swap out my second two filter caps and see if that does something. The other thing I thought I might try is converting it to cathode bias and see what happens.

[Jason C]

OK I changed out the filter caps and still no luck . When I get another minute I'll try the cathode biasing angle. I'm on the verge of just taking this thing to somebody.

[loudthud]

Did you try the shielded wire. I'm not a huge fan, but sometimes it's the only way. I drew a schematic as an exercise with Windows' Paint. Shoulda converted to a bit map at first and then back to jpeg when finished.

[Jason C]

I did try shielded wire with no results. One question though. The speaker-out jack is a switching jack and when you plug the cable in the tip should have no contact with ground right? Because I get continuity to ground whether or not anything is plugged in and that can't be right can it? It is definitely wired correctly according to the weber layout.

[thumbs]

Jason, I'm not sure that it matters on an amp with no NFB, but I discovered a discrepancy between Weber's Proluxe schematic and layout. Check out the blue and brown wires connecting the OT to the power tubes in the drawings. If you have them reversed, your black and yellow wires will be reversed at the speaker jack. You might also check the 50uf cap that bypasses the bias pot. Note that it's polarity is opposite that of the filter caps, functioning as a "smoothing" circuit. I think it's important that it's wired as pictured in the layout and not simply run to ground at some other point.

[loudthud]

You'll always measure pretty close to a short at the output jacks because the OT has a very low resistance secondary. Usually a couple of tenths of an ohm. It's good practice to always ground one side of the speaker connections even if there is no negative feedback.

Back to the original hum problem. I recall that the volume control had no effect and the hum faded out when the standby switch was flipped off. Pulling the PI tube kills the hum but pulling the first tube has no effect. (please verify these symptoms)

You had 750K for a cathode resistor and now it's 4.7K. You should have way more guitar signal getting through now. If not, there must be something miswired or a wrong component between the PI and the input jacks. Try ohming across all the resistors in the preamp. You should always get a reading at or lower than the resistor you are testing. Some meters won't give a stable reading when there is a capacitor in the circuit. Some meters will show a low reading that creeps higher and higher as the capacitor charges up. Some of the resistors in the tone circuit will be affected by the setting of the pots. Set the pots midway.

The input jacks should be wired to short the signal wire to ground when nothing is plugged in. On the schematic I drew, I simplified the input wiring and added the 33K 'stoppers'. How are your inputs wired? Like the Weber layout? If there is a problem with input wiring, it should be effected by the volume control.

[Jason C]

This is where it stands now. It does indeed have A LOT more signal now with the new and improved cathode resistor. The tone and volume controls work on the signal just not the hum. Maybe it was that the signal was too low to hear it before but the signal now sounds like it's talking into a fan. I guess that's oscillation of some sort? Now I'm just tempted to rebuild the whole front end to the stock 5C3P circuit and see if I can get that to work and worry about the GA-75 preamp later....dunno. Yes, those symptoms were correct, PI kills the hum but vol. control has no effect on the hum and the hum fades away when the standby gets flipped.

[loudthud]

I looked carefully at your photos and have one question. The colors don't appear crystal clear on my PC but there is one 25K resistor (R16 on the Weber schematic) that looks like it might be 2.5K. In fact the 25K and the 2.5K (R9) look the same to me.

Edit: See next post before you try this stuff.

Just as an experiment, solder a wire from the input to the PI to the ground side of R9. You should have no hum.

Another thing to try is to use wires for all the grounds in the volume/tone controls and ground them to the ground side of R9.

Weber Schematic: 5c3 Proluxe help

[loudthud]

One thing I should mention. The problem might be ripple on the output stage bias supply. When the PI tube is pulled, the ripple looks like a common mode signal to the 6L6s but when the PI tube is installed, the bottom side PI tube sees it as an input. If you don't have a low voltage cap to try, you can use a 500V cap just to see if it kills the hum. Remember, + side of that cap goes to ground.

[Jason C]

OK status report. I took the bias supply out at "D" and unsoldered my 1 Ohm resistors off of my 6L6 cathodes and put in a 200 Ohm resistor (because I had one handy). No more hum. I do, however get a wierd tremolo kind of effect on some notes some of the time and that resistor was getting HOT but that's probably because it should be more like 250-300 Ohms right? So how do I fix the bias supply because I would really like this amp to be fixed bias.

[loudthud]

The cathode resistor for a pair of 6L6 needs to be 270 or 300 ohms 10 watts. You are probably getting "ghost noting" because the 6L6s are running hot and there is excess ripple on the B+. That should clear up somewhat with proper bias.

Looking back over all the Fender Tweed schematics, I'm not finding any examples where the 5C5 or 5D5 type PIs were fixed bias was used. And for good reason, it's hard to make it work! Ampeg did it slightly differently in amps like the SB-12 which is more like the 5D5 aka See-Saw inverter.

What Weber did is almost a combination as far as obtaining a signal for the second half of the PI. The 25K is on the big side, Fender used something like 6.8K in the 5C5. But the 25K is in the correct position to unbalance the resistors like the 5D5 if you ignore the impedance of the bias supply. Some of the siganl is going to get past the bias supply from the bottom 6L6s grid drive and it's going to depend on where the bias is set. I noticed that Weber's layout and schematic don't match in the way the bias supply is wired. You always want the pot on the shunt to ground side of the bias voltage divider so if the pot fails you have max negative bias, not zero bias.

My suggested solution is to increase the filtering in the bias supply per the attached schematic. I'm not sure how much AC is coming off that transformer tap so to get the bias in range, the 10K or 27K resistors might have to be changed. Along with this you should change the 25K resistor where the bottom side of the PI gets it's signal. It probably needs to be something between 5K and 10K and it will change how the amp sounds. I would solder a trim pot in the circuit and adjust by ear if I didn't have an oscilloscope.

Edit: Don't ground the bias circuit to the filter caps. Run a separate wire to the brass plate or to the ground side of the PI cathode resistor.