Weber 6M18 TBM PI Voltages

[eljerman]

I'm beginning to think the amp is possessed!
Today "IT" decided to produce a 353 B+ and a 12.9v without changing a part. Amazingly, no red-plating. Should I make this my priority to reduce those readings?

[geoff 1965]

hello,
i think 200 ohms bias is too much as i mentioned earlier,i would of thought going from 125 to 150 or 180 max would have been sufficient to bring your pin 3 voltage closer to 12V. did you try those values or just use the 200 ohm dropper you replaced?
this voltage surge is strange! what is your mains voltage,120?
notice your PT has 120 and 125 volt primary windings and looking at your pics it looks like your wired to the 120 "brown" so where is that blue wire going to through that hole in the chassis?
also have you checked the spare PT wires going into that plastic cap are seperately insulated with heatshrink?
good luck

[JMPGuitars]

After measuring the power outlet, don't measure your amp voltages immediately. Make sure the amp has a few minutes to warm up / settle in. Then check your voltages. Include pin 2 of the power tubes and report the voltages here.

If they're still inconsistent, I would suggest looking over the quality of your solder joints. A bad solder connection (or 5), or fray wires could cause this sort of problem. If you have a bad solder joint, it might behave well until it's influenced by vibration, or heat...

Thanks,
Josh

[eljerman]

Found some problems. First in response to Geoff,
I did try other values between 100 up to 200 ohms for the bias resistor and the 200 ohms was the only one to get me to 12.4v and 344v for B+. The amp is back to those values apparently after I re-soldered that resistor. Don't ask me why but that's the only thing I did and it corrected itself. I was able to reduce hum by moving the heater wire away from another lead. A result of chop-sticking. Also the high feedback was helped a ton by re-soldering the lead from the center lug of the volume pot to pin 2 of V3. We are only talking TMB channel here as the normal channel is exactly that...… normal. I'm going to try a 12AT7 for V3 and see if that calms it down some as there is a little fizz in the tone. The channel breaks up pretty early and I'm feeling that it is supposed to be that way. Reminds me of a 50w Marshall that I had where it didn't get louder after 2
1/2 on the volume knob but kept getting more compressed.
Once again thank's to all for your time and patience. Can't wait till if feel good enough to put this into a cab.

[geoff 1965]

hello,
regarding the 200 ohm bias,i just think your concentrating more on boosting your B+ rather than actual biasing of the el84's.i've lost count of the amount of 18W schematics i've studied and have'nt seen one with 200 ohms plus biasing,remember the more resistance will increase your voltages but will also "cool" your dissipation of the el84's.
i've been looking at this weber on other forums and they mention the lead dress especially for the TMB channel so it's good that you are finding things in that area and dealing with them. the average voltages look to be 335 on the plates and 11.5 on the cathode and another area you might find of interest is one guy was getting hiss/fizz on the TMB channel and he used a 1meg resistor in the R40 position "treble pot wiper" which cured it.
if you can update the voltages like Josh said it will be useful,
glad to see your getting nearer to sorting things!

[eljerman]

I have had the 1meg resistor for R40 and used shielded cable.
My voltages at the el-84's are pin2=0, pin3=12.8, pin7=349 and pin9=338.
By the way my bias voltage mysteriously went up to 12.8 and the B+ up to 353. Something else is intermittent and I have to find it.

[JMPGuitars]

Don't get hung up on bias resistor value. The only thing that matters is the actual bias. You need to actually calculate the tube dissipation before you decide to change anything. Arbitrarily changing bias resistors based on what random schematics use doesn't mean anything to your amp. You can't tell your power tubes "be happy with this bias resistor, I saw it on a schematic somewhere."

Assuming these values are still the same:
Pin 7 (Anode): 340VDC
Pin 3 (Cathode): 12.4VDC
Cathode Resistor Value: 200 ohm
Then:
Plate Dissipation (per tube): 9.6W
Plate Dissipation (per tube): 80%
Plate Current (per tube): 29mA

Yes, the higher resistor value does cool the dissipation, but when you do the math it's at a decent place. The max wattage for an EL84 is 12W, but running the tubes biased too hot will reduce tube life, and of course raises the risk of tubes failing. At the end of the day, plug your guitar in and play it, and see if it has the right sound. Biased too cool will not, and biased too hot might sound cool until it dies....or sound bad and then die. Either way. BTW- 29mA might be considered biased hot already depending who you ask.

Right now, everything else including any other solder problems, or lead dress issues need to be worked out...

[JMPGuitars]

I have had the 1meg resistor for R40 and used shielded cable.
My voltages at the el-84's are pin2=0, pin3=12.8, pin7=349 and pin9=338.
By the way my bias voltage mysteriously went up to 12.8 and the B+ up to 353. Something else is intermittent and I have to find it.

With those voltages you're up to:
10.2 watts / 85% dissipation per tube. Plate current goes up to 30.2mA.

Change your voltage meter to mV to read pin 2. I usually get around 10 to 20mV there.

It sounds to me like you either have bad tubes, or bad solder joints, or both. Good luck with the hunt, and be safe.

Thanks,
Josh

[geoff 1965]

yeah Josh,
i agree the biasing looks pretty decent based on that calculation if the tubes are fairly matched.i was thinking biased too cool can ruin the tone/breakup, so with 125 ohms bias that weber must burn out el84's pretty quick!
as always it's hard to try and fault find when you don't have the amp in front of you to listen to.

[JMPGuitars]

yeah Josh,
i agree the biasing looks pretty decent based on that calculation if the tubes are fairly matched.i was thinking biased too cool can ruin the tone/breakup, so with 125 ohms bias that weber must burn out el84's pretty quick!
as always it's hard to try and fault find when you don't have the amp in front of you to listen to.

Well, 125 or 200 doesn't matter. HIS amp with 125 would be terrible. Most schematics show what they show based on averages and testing at the time the schematic was drawn. For example, the old original schematics have a different value than newer schematics. One reason being that wall voltages have changed, and that affects the whole amp.

The ONLY thing that matters is what you can consistently measure. If my wall voltage is 120, and yours is 110, we're not as likely to bias the same (all else being equal). The bias resistor values you see are essentially suggestions of where to start. Maybe they're safe to keep in, maybe they're not. That's why we should measure and calculate. The tone we seek is based on a little math, not just one variable of it.

Thanks,
Josh

[eljerman]

I can't thank all of you enough.
I went back and read pin 2 on the el-84's with the meter on MV and after warm up get a reading of 6.5. Looks like a problem if it is supposed to be between 10 and 20. Other voltages remain the same as my previous quote.

[JMPGuitars]

I can't thank all of you enough.
I went back and read pin 2 on the el-84's with the meter on MV and after warm up get a reading of 6.5. Looks like a problem if it is supposed to be between 10 and 20. Other voltages remain the same as my previous quote.

No, lower is BETTER. If you're reading 6.5mV, that's great! This should be as close to 0 as possible, and 6.5 is obviously closer than 20. Don't mess with that. Move to the next thing to test other joints.

[eljerman]

I felt like it was time to mess with the tubes and since I have a stash of old Mullards, RCA's, Telefunkens and Bugle Boys it was time. I first replaced V3 with a 12AT7 as recommended on the schematic and it helped a bunch. Next I replaced V1 and heard more improvement. When I say improvement it is the tone, the hissing and the headroom et al. My ears are telling me it's definitely close to where it should be. Going to swap out the PI and then the power tubes next and post results.

[eljerman]

I am very satisfied with the results of the amp build. I couldn't have done it without all the help on this site. I learned a lot...….. and that's a good thing! I would also like to pass on to readers the importance of "chopsticking". Found so many problems this way. Thanks again to all and I will surely continue to support this site...….. John

[JMPGuitars]

Congrats on the build! I'm glad it all worked out. You should show it off and put a demo in the "Hear it!" section.

Thanks,
Josh

[eljerman]

Just thought of a question I have to ask. On the Weber schematic it shows the 5vac lines from the tranny as not being used due to the SS rectifier. I am using an EZ-81. Does that mean anything as far as using the 5vac taps?

[geoff 1965]

hello,
this takes us back to your PT which weber will use for different amps beside the 18w,the 5 volt supply is for a GZ34 etc rectifier so does'nt apply to your amp. good news is if you ever wanted to try a GZ34 you already have the supply,using a calculator you would have a B+ boost of around 20 volts.
enjoy playing the amp & glad you sorted it.

[eljerman]

Never stop learning on this site. Can't say thank you enough.....John