Weber 6M18 TBM PI Voltages

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eljerman
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Weber 6M18 TBM PI Voltages

Post by eljerman »

I have been in the process of my Weber 18w build for a couple of months and have been reading post after post and progressing with getting closer to a completely working amp. Thanks for great info guys.
Anyhow I have been getting sound out of each channel but with a loud hum too. Now I used the chart on this site where 6 members posted their voltages at the pins of the tubes. I was pretty much in line but a bit lower in readings for example my V4 and V5 ...Pin 3= 10.0, Pin 7=311, Pin 9=299.
However my V3 readings were wacked with Pin 6=241, Pin 7=155, Pin 8=156.
Note that Pin 7 and 8 are not shorted. I measured every component involved with the PI and they all were in spec. Any suggestions ? Anyone experienced the same issue ?
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Re: Weber 6M18 TBM PI Voltages

Post by Daviedawg »

Hi. Your output valve voltages are low but that only affects the tone of the output. It will make a reasonable and is not the cause of your noise but probably another symptom. Target is 11.5 to 12v on pin 3 and 325 or 330v on pin 7. The jump from 11v to 12 on the cathode makes a big difference to the 18 watt's tone.

V3 is I assume your trem channel preamp. If it is not then we need to know. One half of it should look like the voltages on V1 as it is doing much the same job. The other half can vary depending on the trem circuit in the schematic. However the voltages should not look like yours. So first we need to see the schematic you used to see what that valve should be doing.

Dd

Edit: I looked just now for a schematic online. This may be a TMB kit. If so you cannot compare with all of the voltages recorded in the table. Also I notice in your title it is the PI valve.
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

I have been trying to attach the schematic with no success. It is the schematic that I believe you looked at online. Weber 6M18TBM(non tremolo). The voltages of concern are on V3 which I have readings as follows ….Pin 1=154
Pin 2= 0
Pin 3= 1.2
Pin 4= 3.2vac
Pin 5= 3.2vac
Pin 6= 241
Pin 7= 155
Pin 8= 156
Pin 9= 3.2vac
Don't know if the hum will be related to this. Thank's for your help.
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eljerman
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

Added note: Hum increases in both channels as volumes are turned up.
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Re: Weber 6M18 TBM PI Voltages

Post by JMPGuitars »

eljerman wrote:
Mon 01/28/19 4:26 pm
Added note: Hum increases in both channels as volumes are turned up.
Let's take a step back and learn to read the signal path of the schematic for a moment.

If you look from left to right you see V1A and V1B separated by two sets of jacks. 12AX7 tubes are called "dual triodes" because they're two triodes in one tube. So the first half of V1 is for channel 1's input, and the second half is channel 2's input (each half being 1 triode).

So what happens next?

V2 connects V1 to the power tubes. So we know V2 is the phase inverter, this affects both channels.

Now if we look back at channel 2, we see that the second half of valve 1 (V1B) is connected to V3, which (V3) then goes to the second half input of the phase inverter (V2). That tells us that V3 is only used on the second channel. Looking at the connections of this tube you should see your TMB tonestack.

Now that we know this, how does it help?

Well, if the hum is there on BOTH channels, and V3 is only channel 2, how can V3 be the source of the problem? Logically we can rule it out. If you want to test that theory, you can turn off the amp, pull out V3, and turn the amp back on and test channel 1 and you'll almost certainly still have your hum.

In most cases hum is caused by lead dress and/or ground schemes. Sometimes a bad solder joint (or 20). Sometimes a bad tube. Sometimes other stuff, but rule out those things first.

Number 1: finish the build. Number 2: verify the build is correct (correct value components and connections - even when the schematic or layout has errors).

Once that's complete, try chopsticking.

And if that doesn't satisfy you, you may need to apply a better ground scheme.

Thanks,
Josh
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

hello,
V2 is your phase inverter if that is the correct schematic,have you used the weber copper cap WCA4 rectifier? also is your B+ filtering the same as the schematic with the multi can cap? that's the first area i would check for hum. if you could tell us the voltages at pin3 of the WCA4 and positions A,B,C,D on your B+ it might be helpful,your power tube voltages seem quite low.
also it helps if you can add some pics of the amps internals.
is yours the head version or the combo?
good luck
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

Thank's guys. You give me hope.
Done quite a bit of the chopstick thing but it won't hurt to do more. I do walk away from it for a few days before attacking again so not to burn myself out.
Geoff, My voltages on WCA4 pin 3= 320. A=323, B=309, C=263 and D=245.
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

i've been having a look on the weber site and found details for the PT,so what HT voltage do you have before rectifier?
w025130sch.JPG
this is what weber tells you about the WCA4,
copper-cap-nine_3.jpg
Weber Copper Cap Rectifiers are solid state rectifier replacement devices for use in place of vintage tube rectifiers. Each model emulates the forward conduction resistance curve of the tube type it replaces and also has linear in-rush current limiting to simulate the warm-up of a tube rectifier, however, it is in the 1 to 2 second range, rather than the 7 to 9 seconds of a typical rectifier tube. The envelopes are solid hard-drawn copper with a high thermal capacitance that allows a 100% continuous overload condition.
i looked into the voltage drop of the ez81 when i was working out the sag resistor simulation on my amp with help from curtiss and if i remember right i'm sure it was 18 volts. the WCA4 drops 28V @ 150ma max current so it's an area to consider if you want to get nearer the desired 330-340 plate voltage on the el84's.
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eljerman
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

I'm currently away from my home and read your response. First of all, thank you. I will measure the HT but wanted to ask you this. With several vintage Mullard EZ-81's at my disposal, should I just switch to using a tube rectifier ? If so, what additional connections are required.
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

yes you could try an ez81 to see if you get a bit more B+ voltage,the pin wiring is the same for HT in and B+ "pin3" out,check the 6.3VAC heater supply is connected "pins 4&5".
if your HT is wired as the schematic with red/white wires to pins 1 & 7 of the rectifier and red/yellow wire to ground "CT" you should have an HT of 270-0-270, you won't have the resistance of the WCA4 so your B+ should be a bit higher,if you post your results we can see.
it looks like weber are making use of the dual HT transformer rather than a specific 18watt,normally you have 290-0-290 supply with a ez81 so your B+ won't be as high but you can also adjust the el84's biasing to bring the plate voltage up if needed.
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

Went to the EZ-81. Ht reads 280-0-280. B+ which is pin 3 rose to 333. Hum got louder, probably with the voltage increase.
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geoff 1965
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

okay so that's closer to the norm,what's your el84 plate voltage now?
so we need to focus on finding the hum,have you followed the layout for grounding? other than doing what Josh & Dd have already suggested if you can add some pics someone might spot something.
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

My El-84's now read......Pin 3 = 11.0, Pin 7 =332, Pin 9 =318.
Should I try to bump up to 12.0 or start concentrating on the hum ?
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Re: Weber 6M18 TBM PI Voltages

Post by geoff 1965 »

personally i would concentrate on getting rid of the hum first,your el84 voltages are better now and we can work on fine tuning that area later.
has the raise in B+ affected the preamp/phase inverter at all? you can easily change the 6.8K dropper resistor to bring the preamp voltages back to where they were if you have problems.
notice the schematic shows R39 as adjustable 2.7K to 22K for best tone & stability.
good luck finding the hum.
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Re: Weber 6M18 TBM PI Voltages

Post by JMPGuitars »

If you want to focus on the hum (which you should), read my post above and make sure you understand it - ask questions if you're not clear on anything. Then post some good quality photos of your build so we can take a look at what you've got going on.
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eljerman
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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

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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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

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

Post by Daviedawg »

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

Post by eljerman »

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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Re: Weber 6M18 TBM PI Voltages

Post by eljerman »

Here are the photos after spending a month away from this beast.
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