5c3 Proluxe help

[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.