New Vibroverb AB763 build

[mresistor]

The K bypass caps are not in backwards.


It appears in your pictures that your feedback from the speaker jack isn't connected to the 820ohm resistor.  What is the 820 ohm resistor connected to?

[SleepLess]

I have checked (again) everything that is connected to V4. All the resistors are correct and within values. The caps are placed how they should be. The wiring too.
@mresistor the 820ohm resistor goes to the speaker jack.
I have also tried another tube in V4 and it didn't change anything.

[mresistor]

Oh ok sorry  I zoomed in on the full view of your chassis and now I see the wire from sp jack to under the board and then to 820R.


Curious   have you disconnected the feedback to make sure the OT is in phase?

[SleepLess]

Curious   have you disconnected the feedback to make sure the OT is in phase?

No I haven't tried that. So I simply disconnect the yellow wire that connects the 820ohm resistor and the speaker jack?



Yes.

[stratomaster]

Will you please measure the volume pot taper?

You can either measure the resistance to ground of the wiper with the amp powered down or reinject a sine wave and measure the AC volts into the pot vs out of the wiper.  Set the pot to about 50% rotation for either measurement.

If you measure around 100k using the resistance method then you have a 10% taper pot, and the response will be different from the J taper that Fender used in these originally. With a 10% pot "7" sounds more like what "4" should sound like.

[SleepLess]

Will you please measure the volume pot taper?

You can either measure the resistance to ground of the wiper with the amp powered down or reinject a sine wave and measure the AC volts into the pot vs out of the wiper.  Set the pot to about 50% rotation for either measurement.

If you measure around 100k using the resistance method then you have a 10% taper pot, and the response will be different from the J taper that Fender used in these originally. With a 10% pot "7" sounds more like what "4" should sound like.

I've just measured the resistance. With the volume on 5 I get 78k (red probe on the middle lug of the pot, black ground probe on the left lug). I rotated it from 0 to 7, I get 0k on 0 and 198K on 7.

[stratomaster]

I've just measured the resistance. With the volume on 5 I get 78k (red probe on the middle lug of the pot, black ground probe on the left lug). I rotated it from 0 to 7, I get 0k on 0 and 198K on 7.

This is your problem. Wrong taper pot (as mentioned very early in this thread).

Put in a J taper and you'll get the behavior you're used to.

Now we get to work on the tremolo.

[SleepLess]

I've just measured the resistance. With the volume on 5 I get 78k (red probe on the middle lug of the pot, black ground probe on the left lug). I rotated it from 0 to 7, I get 0k on 0 and 198K on 7.

This is your problem. Wrong taper pot (as mentioned very early in this thread).

Put in a J taper and you'll get the behavior you're used to.

Now we get to work on the tremolo.

OK. So, to understand what I'm doing, with the right pot I was supposed to get 500K at 5 and not 78k? Am I right? I've always ordred CTS 1Meg pots and never had that issue before. How strange! Maybe they sent me the wrong ones this time!I don't have CTS pots to replace them but I have Alpha ones.

[stratomaster]

There are a few versions of audio taper pots. The most common are 10%.  This is what you have and what most amps use.

The next is J taper or 30% audio. This is what Fender used on the volume and treble pots. You'll see "2-35" on the old schematics near the pots. These pots reach about 30% of their max resistance at 50% rotation.

If you ordered J taper but got 10% then they sent you the wrong thing.  If you ordered 10% then you ordered the wrong thing.

The pots you have will work just fine, just know that they will behave differently from what you expect of a vintage Fender. All of the sounds are still there, just at different pot settings.  If you want the vintage Fender behavior, put in the right pots.

[SleepLess]

This is what I ordered:https://www.tubesandmore.com/products/potentiometer-cts-audio-solid-shaft
They never mention J taper or 10/30% anywhere. So was the volume pot supposed to read 500K at 5 so I know if the Alpha ones are the right ones?

[stratomaster]

30%.  So ~300k.  The Alpha you have sounds like a linear pot of it's 500k @50% rotation.

This is what you're after: https://www.tubesandmore.com/products/potentiometer-fender-1m-j-taper-solid-shaft-panel-mount

They have a CTS branded option as well, but that one seems to be a low torque pot and liable to be unsuitable in loud combo amps.

[Willabe]

They have a CTS branded option as well, but that one seems to be a low torque pot and liable to be unsuitable in loud combo amps.

This; 

https://www.tubesandmore.com/products/potentiometer-cts-j-taper-solid-shaft-panel-mount

If so, how are you getting "low torque pot"?

[mresistor]

Curious   have you disconnected the feedback to make sure the OT is in phase?

No I haven't tried that. So I simply disconnect the yellow wire that connects the 820ohm resistor and the speaker jack?



Yes.

[pdf64]

I've just measured the resistance. With the volume on 5 I get 78k (red probe on the middle lug of the pot, black ground probe on the left lug). I rotated it from 0 to 7, I get 0k on 0 and 198K on 7.

50% rotation on a 1-10 knob is 4.5.
To measure this properly, the right lug needs to be disconnected from the circuit ie treble pot wiper.
Whatever, it points to the pot having a 10% taper, where 50% rotation would be about 100k on a 1M pot.
Whereas these Fenders normally have a 25-35% taper, eg 250k to 350k at 50% rotation on a 1M pot.

Perhaps just turn it up a bit more to achieve the desired level?

[stratomaster]

They have a CTS branded option as well, but that one seems to be a low torque pot and liable to be unsuitable in loud combo amps.

This; 

https://www.tubesandmore.com/products/potentiometer-cts-j-taper-solid-shaft-panel-mount

If so, how are you getting "low torque pot"?

User review. I said "appears to be" for this reason.  I know I have CTS 1M J tapers in my shop somewhere. No clue if they are the ones sold on that page or purchased elsewhere or if they were the Fender branded CTS.  The ones I have seem ok for amp use, but I've used low torque pots accidentally before and had the amp self adjust on stage. 

[SleepLess]

Thanks everyone! I'll change the pots tomorrow and I'll also disconnect the feedback to see what happens. Then hopefully we can tackle the tremolo issue!

[SleepLess]

Hi.
This amp makes me want to cry.  I changed the volume pot for the Alpha 1MAudio one that I have but unfortunately on 5 it only gives 100k, not much better than the 78K I had with the CTS. It's not a J taper either. It reaches 300K only on 8. There really is a problem with the volume anyway. On 6 it starts giving out a playable volume but only with the strat on 10. If I lower my strat's volume to 6 then it's like playing an acoustic guitar...
I disconnected the feedback wire and to my surprise the amp worked as badly as it does when it's connected... Nothing changed!
I need to order good J taper pots. In the meantime if you have other ideas, please shoot! I've built dozens of amps, it's the first time that I'm having such a hard time debugging one...

[pdf64]

... I changed the volume pot for the Alpha 1MAudio one that I have but unfortunately on 5 it only gives 100k, not much better than the 78K I had with the CTS. It's not a J taper either. It reaches 300K only on 8.  ...

...
To measure this properly, the right lug needs to be disconnected from the circuit ie treble pot wiper.
 ...

Alpha Taiwan standard audio taper is 15% IIRC.

[SleepLess]

Alpha Taiwan standard audio taper is 15% IIRC.

I'll try and find the proper CTS pots here in Europe so they arrive more quickly.

[mresistor]

Hi sleepless   I read through this thread again and notice we still don't know what the power dissipation is at idle. 


The Hoffman bias checker does the same as 1 ohm 1% 1w resistors do connected between the cathode and ground.  Having these installed makes it a ton easier than breaking the connection of the tube to socket and then inserting a bias checking device. I have a Hoffman bias check tool that I use for clients amps but mine have the resistors installed on the Ks.


Please tell is the mv reading with your tool for each power tube and the Plate voltage at that time. Both Vol knobs to 0.

[SleepLess]

Hi sleepless   I read through this thread again and notice we still don't know what the power dissipation is at idle. 


The Hoffman bias checker does the same as 1 ohm 1% 1w resistors do connected between the cathode and ground.  Having these installed makes it a ton easier than breaking the connection of the tube to socket and then inserting a bias checking device. I have a Hoffman bias check tool that I use for clients amps but mine have the resistors installed on the Ks.


Please tell is the mv reading with your tool for each power tube and the Plate voltage at that time.

Hi mresistor. I installed the 1ohm 1 watt resistors on the 6L6GC sockets two days ago just in case. What measures would you like me to take precisely? Thank you!

[SleepLess]

Please tell is the mv reading with your tool for each power tube and the Plate voltage at that time. Both Vol knobs to 0.

V5: 26mA      V6: 22.7mA    Plate voltage (pin 3 of the power tube socket) is 438V for both V5 and V6.

[mresistor]

You are running the amp ice cold.   438 x .027 = 11.86W  at idle.   6L6 is a 30W tube and 70% of 30 = 21Watts.   


I would bias the amp much hotter.



crank the bias up.

[mresistor]

Also remember that you are measuring K current and that it includes screen current. So going 70% at idle is really less than 70% subtracting out the screen current.

[SleepLess]

Hi!I've just set the bias at 45mA. It's much better in terms of volume. Much better.
There still is a problem with the amp though: I still don't have any voltage reading on pins 3 and 8 of V5. They read 0V where the schematic shows I should get 2.3V.Thanks for your help!

[pdf64]

I still don't have any voltage reading on pins 3 and 8 of V5. They read 0V where the schematic shows I should get 2.3V.Thanks for your help!

Perhaps the trem needs turning on?
ie plug in footswitch and try it both ways

[stratomaster]

Hi!I've just set the bias at 45mA. It's much better in terms of volume. Much better.
There still is a problem with the amp though: I still don't have any voltage reading on pins 3 and 8 of V5. They read 0V where the schematic shows I should get 2.3V.Thanks for your help!

Take a look again at your wiring and voltages. 2.3v is just for pin 3 and 12v on pin 8.  Your measured plate voltages are indicative of essentially no current flow as well.  They should be 280v and 400v respectively.

With both cathodes reading zero, and with the tubes not sharing plate or cathode wiring, whatever is wrong has to be affecting both triodes.  My money is on a bad tube. Check if it's gone atmospheric (white cloudy residue).  And try swapping in another tube even if it isn't visibly bad.

[Willabe]

I've just set the bias at 45mA. It's much better in terms of volume.

How did you set the bias at 45mA?

You can't just set it there without checking the power tubes plate dcv and doing the math for what ever dissipation % you want. 

[stratomaster]

I've just set the bias at 45mA. It's much better in terms of volume.

How did you set the bias at 45mA?

You can't just set it there without checking the power tubes plate dcv and doing the math for what ever dissipation % you want.

His plates were at 450v per post 8 with bias cold. He's ok at 45mA even if the plates didn't drop with hotter bias.

Granted I don't know how he settled on 45mA.  I just know he won't hurt anything there.

[Willabe]

Your teaching him the wrong thing.

You ALLWAYS do the work of doing the math. It's soooo easy these days, you can use an on line bias calculator. Doesn't get any easier than that.

https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

Tube Bias Calc

[SleepLess]

Hi everyone!I calculated my way to 45mA. 435V plate voltage x 0.045mA = 19.5W. I'm a bit below 70% for a 30W (21w) tube so I'm good I think.

I now have 1.9V on pin 3 of V5 with the tremolo on using the footswitch (pdf64 you were right!)
Now I have a loud hum on the Vibrato channel as soon as I turn the volume up. Normal channel sounds quiet. A tube maybe?
I've swapped V5 for another tube and I get the same hiss/hum. It's not the tube!

[Willabe]

I calculated my way to 45mA. 435V plate voltage x 0.045mA = 19.5W. I'm a bit below 70% for a 30W (21w) tube so I'm good I think.

That's good that you didn't just guess.

You should have then calculated them for 60% dissipation next just to see where it came out.

[AlNewman]

So, just to recap.
Setting the bias to 70% solved your volume issues?
Now the normal channel works as expected, but you have a hum on the vibrato channel only, which is only affected by the volume control?
I'd look at ground connections in V2 and the tone stack, as well as the inputs, check to make sure your input jacks are switching properly. 

[DummyLoad]

Maybe still missing another ground? See attached - It's part of V5, the CF for trem driver and the ground for V6, the LTPI - if it's missing, the ground path for the LTPI is taking a path through the NFB 820R via the OT secondary. Also, can't see pots, is one side of the Intensity pot grounded? 


--Pete

[pdf64]

So, just to recap.
Setting the bias to 70% solved your volume issues?
 ...

Working back from the voltages and referencing them to anode characteristics, it doesn't seem that Fender ever intended to idle 6L6GC at such a high dissipation.
Bear in mind that 60% of a design maximum anode dissipation rating (eg as per 6L6GC's 30W) is equivalent to 70% of about what would be its design centre rating (26W).

Certainly more recent Fenders with that info available seem to idle somewhat cooler, eg below 15W here https://el34world.com/charts/Schematics/files/Fender/Fender_65_super_reverb.pdf

[SleepLess]

Hi!
Yes, it seems that setting the bias at 45mA solved the volume issue. I think I'm nearly done with this amp. The only remaining problem is the hum/frying noise that I have. I said before that the normal channel worked properly but actually it does have a little hum as well, though a lot quieter than the vibrato channel.

@pdf64  I can lower the bias down to 40mA, I would be slightly under 60% max dissipation.
Here's a pot-side picture. @AlNewman: the ground connections are there. The little bridge between the ground buss and the 47ohm resistor is underneath the board.
I also shot a short video so you can hear the noise I'm getting.Thanks again for your help!
https://youtu.be/w9g6NB-I6Qs?si=J-4O2IcSyJ1vHDWC

[pdf64]

Idle the 6L6GC as hot as you like, I'm just noting that it shouldn't be necessary to do so in order to get the amp working right.
eg the Super Reverb RI is specified at 433V on the screen grids with 33mA cathode current.

[SleepLess]

Idle the 6L6GC as hot as you like, I'm just noting that it shouldn't be necessary to do so in order to get the amp working right.
eg the Super Reverb RI is specified at 433V on the screen grids with 33mA cathode current.

I get you. Need to keep looking then!

[Willabe]

That sounds like a bad carbon comp plate resistor.

[SleepLess]

That sounds like a bad carbon comp plate resistor.

Should I test or replace each one of these?

[Willabe]

You can try pulling the preamp tubes, 1 at a time, starting with the 1st tube. Pull the tube and listen if the hiss went away. If hissy/crackling went away, it's thse plate resistors. If it doesn't go away, then put that tube back in, pull next tube, and so on.

If you think you've found the tube stage that's hissing, change those plate R's. It may be a few.

Could even be a dirty pot, but those usually only act up as you turn the pot. But sometimes their so dirty they still hiss when not turned. It can also be a coupling cap leaking dcv to a pot. When you turn a pot that has dcv on it it does that, as long as there's current flowing it make a hissing/crackling noise.       And that could be why the amps not acting right volume wise, dcv on pots at the grid changes that tubes bias.

It could also be a hissy/noisy tube. You have to poke around until you find it.

[SleepLess]

Ok, I'm not getting any noise when I pull V6 out.

[stratomaster]

Pulling V6 cuts all signal from reaching the power amp.  It tells you very little diagnostically except the noise is in the preamp--which we already knew from it responding to the volume knob. 

Since it's common to both channels I'd be inclined to cover the chassis with something conductive (a baking sheet is what I use) to rule out ambient signals as the source. 

If you have no luck with that then you'll need to strategically ground grids to pinpoint where the noise is coming from. 

[mresistor]

I would use 1W metal film resistors on plates. The only reason I see to use CC resistors there is for aesthetics.  MF are much quieter.  There are some cool Dales that would look great in there.


CMF60100K00FKEK

[SleepLess]

Maybe still missing another ground? See attached - It's part of V5, the CF for trem driver and the ground for V6, the LTPI - if it's missing, the ground path for the LTPI is taking a path through the NFB 820R via the OT secondary. Also, can't see pots, is one side of the Intensity pot grounded? 


--Pete

Yes the grounding is good in those places.
I will order metal film resistors for the plates but honestly I've always used carbon comp there and never had such a noise issue with them. I tested all the plate resistors and they're all giving me the right value. Some are a bit above 100K, others a bit below 100k but they're within a 10% margin, so unless a resistor can give its proper value and still be faulty, they're not the culprits here.

[Willabe]

The old cracked carbon R's were the most problematic. Something about the cracked carbon chunks expanding and then the R absorbs moisture? Then they sputter. Carbon comp. R's can do the same.

So the R can measure to spec but still be noisy. Sometimes you can get a brand new carbon comp, CC, R and it will be bad. Or it's been sitting around in inventory for years and has gone bad.
 

[stratomaster]

I've had noisy plate resistors of all types except metal film. The vishays referenced above have served me well.  However, that doesn't mean your noise is from the plates. I think you need to do some more troubleshooting and investigation before you swap them out.  I've been able to leave original plate resistors in well used 65 year old amps because the source of noise was something else.

Cell phones, wifi, digitally controlled soldering stations, fluorescent and LED overhead lighting, etc are all electrically noisy.  You can't judge how noisy the amp is until it's shield is complete. 

You may find, as Fender did later in the decade your build reproduces, you need shielded wires to the grids for the first 2 stages to get good quiet performance. 

Additionally, I find the noise floor on most (easily 3 of 5) new production 12AX7s to be atrocious.  You'll likely have to roll your tubes to find the quietest combination.  Put your noisiest one in V5.

[Willabe]

The only reason I see to use CC resistors there is for aesthetics. 

There is a reason to use carbon R's. They will distort when they have a high dcv on them. But only with a high dcv on them. So for the most past you would only use them for plate and CF K R's.

Ken Fisher used to tell people in Vintage Guitar mag. if you get a hissy/crackling carbon plate R, make sure to put in a carbon R, or the amp will sound different.

There's a short tech paper, only a few paragraphs, that talks about it. Might be on R.G. Keens web site?

[Willabe]

I also shot a short video so you can hear the noise I'm getting.Thanks again for your help!
https://youtu.be/w9g6NB-I6Qs?si=J-4O2IcSyJ1vHDWC

Stratomaster, have you listened to this?

It's very bad.

[Willabe]

And yes I love those Dale R's too.