Weber 6M18 TBM PI Voltages

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eljerman
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Re: Weber 6M18 TBM PI Voltages

Postby eljerman » Thu 01/31/19 11:39 am

Thank's again to all for your time and patience. I will take the info and go over everything. I did try a whole new set of tubes but I am not sure of all the sockets and wire dressing. One thing at a time.
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eljerman
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Re: Weber 6M18 TBM PI Voltages

Postby eljerman » Sun 02/03/19 4:23 pm

Did some troubleshooting yesterday and wanted to add this info to the discussion. Re-measured all component values and checked all grounds.
Pulled both V1 and V3 and the hum was still there. Very loud too.
Can I assume that the Input jacks are good since the hum was unchanged when the tubes were removed? The 2 EL-84 sockets seem really loose. Should I be automatically changing them? All the voltages remain good.
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Re: Weber 6M18 TBM PI Voltages

Postby Daviedawg » Mon 02/04/19 3:28 am

Unfortunately it does not rule out any cause of hum. But it does suggest looking at the PI stage first. Something is not right so a slog through the schematic is required. Take a copy of the schematic and a marker pen and look, tap and check values for every component and its connections. My inclination is towards a bad joint or a whisker shorting. So examine the PI stage in minute detail, then the output stage.

Can you post a layout as built so we can see the ground scheme you used?

Dd
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eljerman
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Re: Weber 6M18 TBM PI Voltages

Postby eljerman » Wed 02/06/19 4:46 pm

Once again my gratitude to all. Before posting pics I wanted to go through every component and chopsticked the daylights out of it. Did find one of the 68K's on the jacks was open. Also ran the input shielded wires from the inputs to V1 over the top of the turret board instead of under. Hum is still there but definitely cut about 60% lower. I will still have at it until I post photos.
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eljerman
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Re: Weber 6M18 TBM PI Voltages

Postby eljerman » Fri 03/01/19 4:06 pm

Here are the photos after spending a month away from this beast.
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Daviedawg
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Re: Weber 6M18 TBM PI Voltages

Postby Daviedawg » Sat 03/02/19 3:10 am

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

Postby eljerman » Sat 03/02/19 3:31 pm

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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Daviedawg
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Re: Weber 6M18 TBM PI Voltages

Postby Daviedawg » Mon 03/04/19 3:22 am

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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eljerman
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Re: Weber 6M18 TBM PI Voltages

Postby eljerman » Sat 03/16/19 4:57 pm

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

Postby JMPGuitars » Sat 03/16/19 5:10 pm

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

Postby eljerman » 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 ?
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Daviedawg
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Re: Weber 6M18 TBM PI Voltages

Postby Daviedawg » Sun 03/17/19 3:11 am

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

Postby JMPGuitars » Sun 03/17/19 8:30 am

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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eljerman
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Re: Weber 6M18 TBM PI Voltages

Postby eljerman » Sun 03/17/19 10:05 am

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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