Weber 6M18 TBM PI Voltages

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eljerman
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

Thanks once again. I will definitely experiment with the value of R39 and post the results.
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

Tried changing the value for R39 from 2.7k to different spots along the way up to 20k with no change in the B+.
However I tried a 100 ohm resistor in place of the 200 ohm B+ dropper and the B+ elevated from 333 to 339 and 10.6 rose to 10.8.
Seems like the 100 ohm difference is a pretty drastic change. Do you think it might be tube swapping time? Don't know where to go from here.
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

hello,
the R39 refers to tone not your B+ so did you hear any difference between 2.7K and 20K? i.e. was the tone still muddy or any brighter?
also have you tried altering the 125 ohm bias resistance yet to raise the 10.8v closer to 12v? swapping the 2 250 ohm resistors for a 150 ohm will bring your pin 3 voltage up and also your plate & grid voltage.
keep at it!
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eljerman
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

I'm going to leave the 100 ohm cathode resistor in. Going to try and reduce the 125ohm resistor pair as you suggested. I think once I get the voltages correct...then I can experiment with tone. Thanks again to all that have had the time and patience to help me and others.
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

My bad....I meant raise the value of the bias resistors.
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

okay,
replacing the 2 bias resistors with 1 will give you some room to bring the capacitor in as well instead of it hanging over the edge of the board.
if we get your pin 3 voltage closer to 12V then we can concentrate on the muddy tone,have you checked off the coupling caps values against the schematic/layout?
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eljerman
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

I had to use a 200 ohm for a bias resistor to raise the voltage to 12.3.
The B+ is now at 343 vdc. The cathode resistor remains at 100 ohms. No red plating and running pretty quiet. The tone however is muddy and not the greatest sounding so I will now experiment with R39 values next.
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

hello,
the 100 ohm you refer to is the dropper for the grid voltage not your cathode! so what plate & grid voltage do you have now on the el84's?
keep safe working around the high voltages and just ask if you are unsure with anything.
good luck
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

340v and 329v on the el-84's. I have been careful probing. One hand behind my back. Going to use a potentiometer in place of R39 to dial in the best R for tone then solder in a resistor. Thanks again for the help.
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

good! better B+ and plate voltage now and just over the "magic" 12V on the cathode! i would be tempted to try a 180 ohm as well on the cathode bias if you have'nt already.
keep us posted with your tone experimenting,i'm sure there will be other people with webers who will find it of interest.
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

Running steady with 12.4 v and 344 v at the B+.
Now turning my attention to the mud-tone. The normal channel seems fine. Low noise, good headroom nothing crazy. Volume is less than the TBM channel which is expected(right?). Experamented with R39 and did not hear any sigmnificant change in the sound so I left the value at 2.7K.
The TBM channel is another story. Squealing when the treble is turned up past "3" and/or the volume is turned up past"6". Also noted on the schematic was the suggestion to try different values at R38 which is probably next on my list.
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

yeah notice R40 adjusts the amount of signal into the phase inverter for the TMB channel,you might want to try a higher value resistor there.
good luck
say's optional on schematic but looks easy enough to add in,looking at your pics you have it soldered to the wiper of the treble pot.
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

I am pretty confidant that the Normal channel is up to par. I know it should have less gain than the TMB which it does and the tone control works fine. No hiss even on "10". Sounds correct. My problems lie in the TMB side where I get hiss and hum and mud! Where to look first? Get better caps? Try different values at R38 and R41? Tubes? All of the above? Once again looking for advice.
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

hello,
using good quality capacitors on the cathodes does contribute to overall tone so i would recommend sprague atom or F&T's,you will notice the difference! also i've been going thru the trinity amps 18W build guide and there is some good info & tips in there,one is replacing the 100uf bias cap "your C2" with a 50uf to clean up muddiness.
i would also replace the 200 ohm bias resistor with a 180 ohm just to keep within certain parameters,most of the clipping/breakup comes from the power tubes in an 18W and having 200 + resistance reduces this.
your TMB channel is higher gain so it will be noisier than the normal channel and you have higher B+ now so you will have to go through and maybe re-tweak certain area's.my amp's a lite2 so i'm not familiar with the TMB channel but maybe someone who has one can help you.
keep at it you're getting there!
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Re: Weber 6M18 TBM PI Voltages

Post by Daviedawg »

I agree that you will get more hiss on the TMB channel. But not hum and mud. It is always hard to judge the level of noise as expressed here. But if it is noticeable I would suspect something else. Have you chopsticked that preamp section? I might suspect a cold solder joint somewhere before changing values. Tap and poke a bit to look for noises. Other suspicious signs are "clunks" when you switch standby or pedals on that channel.
I suggest that route before changing component values.

Dd
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Re: Weber 6M18 TBM PI Voltages

Post by 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?
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by 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
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Re: Weber 6M18 TBM PI Voltages

Post by 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
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eljerman
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Re: Weber 6M18 TBM PI Voltages

Post by 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.
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by 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!
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