Weber 6M18 TBM PI Voltages

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Daviedawg
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Re: Weber 6M18 TBM PI Voltages

Post by Daviedawg »

Thanks for the pics. I will look properly later but I would say right away to shorten the two input connections so they run straight to the valve base. And ground one end of the shielding.
I cannot see a ground point close to the input so perhaps we need to review your grounding scheme. I see two at the power end of the chassis, one close to the rectifier and one in the middle. Can you describe all of the ground points and where they are and how the pots are grounded or not grounded?

Dd

Edit. Looked again and can see one end of the shield may be grounded as it is.
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eljerman
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

Thanks for all your help. Very appreciated.
I will shorten the shielded cables from the inputs. The cables are grounded back at the brass plate that the pots(that require grounding 1 lug) are also using for ground. The a/c input is grounded to the chassis by way of a separate lug. Same with the tranny, a separate lug to the chassis. Both can be seen in the photo. My volume and treble pots in the TMB channel screech when turned up a bit. Should I use shielded cable from the treble pot?
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Re: Weber 6M18 TBM PI Voltages

Post by Daviedawg »

I am not familiar with the scheme using a brass plate for grounding but it should be no different from grounding the pots to the chassis.
The screeching may be feedback caused by the lead dress. It is easy to test by chopsticking the cables in that section of the amp looking for changes in the tone and tapping the connections looking for noise. If you do not find anything more detailed examination we will think again.

Ask if you are not sure what to do and as always be careful of the high voltages.

Dd
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

The hum is gone! I think it was a combination of things from lead dress to tubes and a few small pieces of solder here and there. Anyhow......on to my next issue that being tone. Its too muddy and I feel my low voltages are causing that. 10.6, 328, and 315 at the EL-84's. What would be the best way to up those numbers to 12, 345, and 330 assuming that the higher voltage will clean up the sound.
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Re: Weber 6M18 TBM PI Voltages

Post by JMPGuitars »

340 to 345 is the sweet spot for the B+. What's your B+ reading at?
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

Assuming I would measure it at pin 3 of the rectifier it is 333v.
Is that where the reading should be taken ?
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Re: Weber 6M18 TBM PI Voltages

Post by Daviedawg »

If you have around 325v on your EL84 plates you are doing good. But I would go for as near 12v as you can on the cathode. I have noticed big increase in clarity and crunch between 10.5 v and 11.5v. I installed a 150 ohm cathode resistor. Some TMB schematics have a 125 ohm and some up to 170 ohm that I recall depending on other mods. The original 18 watt design has 125 which I increased and felt the benefit. So depending what you already have it may be a simple route.

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Re: Weber 6M18 TBM PI Voltages

Post by JMPGuitars »

eljerman wrote:
Sat 03/16/19 7:05 pm
Assuming I would measure it at pin 3 of the rectifier it is 333v.
Is that where the reading should be taken ?
That isn't terrible, but the closer you get to the the 340 to 345 range the less muddy you'll be. A lower B+ than 340 is usually muddier tone, and higher than 345 is more headroom with less overdrive.

As Dd mentioned, chances are your power tubes aren't biased well and should be closer to 12V, but correct your B+ first and measure again.
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

Thank you once again. This site is a blessing.
I"m currently running a 200 ohm cathode resistor. Do you think I should try swapping rectifier tubes and output tubes or replace the 200 ohm with a 180 or so first.
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

hello,
first check your wiring against the schematic,you have 2 x 250 ohm bias resistors in parallel which gives you 125 ohms resistance for the el84's.
also the large 10W 200 ohm looks like it's the B+ dropper for the el84 grids "pin 9" the schematic shows that should be 100 ohm.
every component has a knock on effect so adjusting the bias voltage will also adjust your plate voltage and using a higher value cathode resistor i.e. 200 ohms can loose power tube distortion due to the bigger voltage drop.
remember your 280-0-280 supply regarding your 333 B+ at the rectifier so to get a higher B+ you will have to look at changing the rectified stage to hit the sweet spot Josh mentioned.
good luck
P.S. what value is your R39 resistor? schematic says you can adjust this between 2.7 to 22K for tone & stability.i would start with this and bringing your bias voltage closer to 12V like Dd said first before going into altering your B+ or swapping rectifier/power tubes.
6m18tmb_layout.jpg
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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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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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