New Vibroverb AB763 build

[SleepLess]

Hi there!Just a few pics of my new build. I have a volume issue (the amp lacks volume and doesn't sound as punchy as it should) I need to fix. I have to play it on 6 in order to sound as loud as it should on 2...

[mresistor]

Nice build, very tidy. Looks like a chassis kit form Songwut in Thailand.

[SleepLess]

Looks like a chassis kit form Songwut in Thailand.

Spot on! ;)

[mresistor]

He really makes nice chassis and plates..  Very authentic.   
So does your low volume occur in both channels?   Do you have an idea what the problem is? 
Bias set correctly ?

[SleepLess]

Yes, it's on both channels. My B+ is too high at 495V but that's not the reason why there is a lack of volume...

[bmccowan]

Nice! I'm sure someone here can help you find the problem.
Built to the standard Fender schematic, or some mods?
Have you checked and recorded voltages at pins?
Have you checked all the resistor values? It's pretty easy and common to make a color band mistake.
Have you considered renumbering the vol knobs?  Sorry.

[SleepLess]

Have you considered renumbering the vol knobs?  Sorry.

I'm just getting into the fixing so yes, I need to do the whole voltage chart of the amp... The sound is good though, so I'm not really used to an amp that works but not perfectly. Usually they just don't work. 

[mresistor]

Well HVolts being that high would indicate that the tubes aren't conducting much. Try checking bias again and setting it around 65% max P diss at idle.

[SleepLess]

This is the layout that I used for this build, by Rob Robinette:
Here's my voltage chart. Can you spot anything?

[Willabe]

Great way to double check a new build with problems;

https://el34world.com/Forum/index.php?topic=17701.msg178630#msg178630

[bmccowan]

+1 with Willabe. Quick look - V3 - missing voltage on pin 8. V5 - volts are wacky. V6 - pin 2?
The Vibroverb schematic I have is hard to read - but it does have the Fender intended values on it.

[SEL49]

see pic

[stratomaster]

Have you calculated the clean output power of the amp?  I'm willing to bet what you're experiencing is the difference between pot tapers, namely the 30% vs 10% log pots. If you can dial in the full power and go up from there to see a distorted waveform at max volume, then I'd look at the pot taper.

Alternatively you can just map the resistance as you turn the pot up.

[SleepLess]

Hi everyone and thanks for your input!
I do have an 8V reading at pin 8 of V3, my mistake sorry! The readings on V5 are still the same, no mistake on my part there and V6 pin 2 does read 65V.
@SEL49, thanks a lot for spotting this! I added a small wire to the ground bus wire, it's now grounded.
I haven't sold the volume issue and @stratomaster: I'm sorry but I don't know how to measure the clean output power of the amp. The amp starts delivering a fine sound/volume at 4 on the knob (I should already be deaf by then!) and its sweet spot around 7. Above that it starts sounding messy and the distortion is not pleasing to the ear.
Thanks again!

[mresistor]

It looks like you haven't installed 1 ohm 1 watt 1% resistors on the cathodes of the power tubes. Or maybe I just can't see them. Having those makes it easy to set a safe bias level.

[SleepLess]

It looks like you haven't installed 1 ohm 1 watt 1% resistors on the cathodes of the power tubes. Or maybe I just can't see them. Having those makes it easy to set a safe bias level.

Indeed I haven't, but I have the Hoffman bias checker so it's not a problem.

[SleepLess]

OK. So I have checked all the component values and my wiring, including a continuity check. I didn't spot any mistake. I don't understand why I don't get any voltages on V5 pins 3 and 8. I put another tube there and got the same readings. I don't know if that's why the amp sounds like a small amp volume-wise though since V5 deals with the tremolo.

[bmccowan]

Have a trem pedal? Same result with trem on or off?

[stratomaster]

OK. So I have checked all the component values and my wiring, including a continuity check. I didn't spot any mistake. I don't understand why I don't get any voltages on V5 pins 3 and 8. I put another tube there and got the same readings. I don't know if that's why the amp sounds like a small amp volume-wise though since V5 deals with the tremolo.

If there's a problem with the tremolo that is causing the LDR to go low resistance then you'll be bleeding a ton of signal to ground. This would affect the Vib channel more than the Normal though.  However the mix resistors act as voltage dividers here, and the normal channel has one less gain stage--so the Normal channel isn't completely unaffected.

What happens when you disconnect the wire going to the wiper of the Intensity pot?

[SleepLess]

Hi!I'll be away from home for a couple of weeks, so I'll get back to it when I'm back. Thank you!

[SleepLess]

What happens when you disconnect the wire going to the wiper of the Intensity pot?

I'm back.
Nothing changes. The volume is the same whether the wiper is connected or not and I still have 0V readings on V5 pins 3 and 8.

[SleepLess]

Have a trem pedal? Same result with trem on or off?

Yes, same results

[mresistor]

Do you have voltage on the right lug of the intensity pot?

[SleepLess]

Do you have voltage on the right lug of the intensity pot?

Here is what I have, I don't get it...

[stratomaster]

What happens when you disconnect the wire going to the wiper of the Intensity pot?

I'm back.
Nothing changes. The volume is the same whether the wiper is connected or not and I still have 0V readings on V5 pins 3 and 8.

Ok so that eliminates the entire tremolo circuit as a potential cause for low volume.

Do you have a signal generator, or a way to inject a sine wave into the input of the amp?

[SleepLess]

Unfortunately no, I don't have any way to generate a signal generator.

[stratomaster]

Unfortunately no, I don't have any way to generate a signal generator.

Phone app to headphone jack to 1/4" plug?

Alligator clips to the tip and sleeve of a 1/8" stereo cable plugged into the headphone jack of a laptop?

An old Casio keyboard where you plug into the amp and tape a key down?

Think outside the box.  We need a steady sustained signal--preferably sine wave, but sawtooth will work in the case of the keyboard.

[SleepLess]

OK, I'll try. And so once I have a sine wave, what do I measure in the amp?

[stratomaster]

OK, I'll try. And so once I have a sine wave, what do I measure in the amp?

AC voltage (RMS if your meter has it) at the grid and plate of all of triodes except those in V5.

I'm looking for relative magnitudes here, so eq at 5 with reverb and Vibrato turned off and volume between 3 and 6 should be fine for this purpose.  Just don't change volume or EQ between measurements.  Pick a frequency between 400hz and 1kHz. The exact one doesn't matter for this purpose.

[SleepLess]

Hi!Here's a little update on this issue. A friend of mine and I looked up into the amp and we've narrowed the issue down to the tremolo section. Disconnecting the tremolo from the circuit doesn't increase the volume at all, it's even the opposite. We're still not getting any voltage readings on pins 3 and 8 of V5. We changed the capacitor that goes there but to no avail. I'm currently in the process of removing the whole components board to check if I haven't made any mistake with the hidden underboard wiring. It takes a lot of time for a very hard to fix problem.

[pdf64]

Does the amp have the expected level of gain when the volume is set to 10, full CW?

[SleepLess]

I suspect it does.

[stratomaster]

What's with the suspecting and not measuring?  I gave you an easy way to measure the gain of each stage without the need for a scope.  This is a simple test that would have taken a few minutes to perform and instead you're swapping caps and taking the amp apart. And reexamining areas of the circuit that we ruled out earlier. 

To be clear, you do have a problem with your Tremolo circuit.  But by disconnecting the wiper to the Intensity pot you removed the effect of that circuit on the signal while maintaining the 50k load from the pot. This decoupled the two circuits, so if you still had the volume problem it's not from the tremolo.

I think what pdf64 is getting at, and your answer seems to confirm, is pot taper.  If you're at full gain at max volume then there is no other explanation than the pot taper isn't matching what your reference amp has installed.

[SleepLess]

OK stratomaster. I have a 1/4" stereo cable and a 1/8"s stereo cable. Can you walk me through with what you would like me to do with them? Thank you!

[stratomaster]

Generate a sine wave. The quickest way I can think of is to download a signal generator app for you phone, tablet, or laptop/PC.

Connect the 1/8" to the headphone or speaker out of your device.  Use an adapter or alligator clips or anything you can think of to get the signal into the amp.  Tip is signal and sleeve is ground.  You can ignore the ring.

Connect plug tip to jack tip and plug sleeve to jack sleeve on input 1. You can even use a plug that is connected to nothing to defeat the shunting of the input jack.

I don't care how you do it, but get the sine wave into the amp.

Once there set the frequency to 400Hz.  And adjust the volume if your device until you get 150 to 200mV AC.  You can measure this with your meter.

Set all EQ controls to 5, volume to say 3, and reverb/trem off.

Now go to every triode grid and measure vAC, except V5.  Write them down.

V2a:  xx V
V2b:  yy V
Etc

Then without changing anything on the amp or the device feeding the signal measure the vAC on the plates of each triode. Write these down too.

If your gain stages are set up properly you'll see a 50-60x difference between the vAC at the plate vs the grid of any given triode.

This will establish whether or not your amp is properly amplifying. 

If this sounds like a lot, then just pull out the volume pot and measure the resistance between the wiper and the two outer lugs with the pot set to 50% rotation.  I'm willing to bet you have a 10% taper pot and are comparing it to an amp with a J taper. If this is the case, then the only problem with your amp is the tremolo.  We can tackle that next.

[labb]

Just a suggestion: build your self an audio probe. See R.G. Keen’s Geoflex. You can hook it to a”Walkman” and use it to inject a signal. Start at the PI work back to the input. Also see listening amp over on EL 34 world. Last time this happened to me I had a whisker on the shielded cable shield grounding at one of the pots.

[SleepLess]

stratomaster, where am I supposed to measure 150 to 200mV AC? Thanks!

I suppose I should measure the AC voltage at the tip of the input jack. If so, I measure 60mV AC. I can't go above that, my mobile phone is set to its max output already. Would that be enough for the measurements?

[stratomaster]

Can't hurt to try.

[SleepLess]

OK, so here's what I have in AC with the 400hz signal going through:
V1    pin2: 9.4mV      pin 6: 0mV      pin 7: 5mV  V2    pin 2: 60mV      pin 6: 0mV      pin 7: 151mV  V3    pin 2: 350mV    pin 6: 0mV      pin 7: 2.6V  V4    pin 2: 64mV      pin 6: 0mV      pin 7: 50mV  V5: not measured  V6    pin 2: 0mV        pin 6: 0V         pin 7: 0.1mV. Thank you!

[stratomaster]

Good first attempt but you'll have to redo the measurement.

You're missing the plate (pin 1) measurement for every "unit 2" triode, and your pin 6 measurements shouldn't be 0.  I think you're out of range on your meter if you manually set it to mV. Also V3 pins 2 and 7 are wired together so they should read identical.  This tells me you have either an unreliable input signal connection or problems placing the probes securely enough to get a good reading.

I'll take a step back and explain what's happening as I think that's what's missing.

Signal is entering each triode at the grid and exiting at the plate.  The signal is amplified in the process.  A healthy triode in this amp will produce a voltage at the plate that is about 50-60x higher than the voltage present at the grid.  The reverb driver will be the oddball, but measure it anyway.  So if you're hearing signal through the amp then there's no way to not have signal present at the plates.

[SleepLess]

OK, I have done it again but the only pin 1 I can read is on V1, all the rest go down to 0V...I should probably add that clipping the alligator clip on V6 pin 2 shuts down the signal...
Hope this helps...

[stratomaster]

I don't think you're doing what I asked.  The signal only goes into the input of the amp. You don't inject the signal into every grid independently.

In this exercise you're giving a steady input signal and measuring how the voltage changes through the signal path. 

You should only be touching probes to the tube pins.  I'm unsure why you would be clipping anything to V6, and your description of shunting the signal implies something is wrong with what you're doing.

[SleepLess]

I'm clipping an alligator clip instead of touching the pin with the probe so that my reading is more accurate. The signal is just going into the input 1 of the normal channel.

[stratomaster]

What is your meter set to?

The only mV range measurement should be V 2a grid. The rest should be volts to tens of volts (except in the reverb circuit).

[SleepLess]

I change the setting to get the most accurate reading possible. Most of the time it's in the 200mV range but sometimes I use the 2V or the 200V range. For instance for V1 pin 1 I used the 200V range.

[stratomaster]

If your grid voltage is 0.06v (mV range) and the expected amplification factor is 55, then your plate voltage will be 3.3v.  This is out of range for your mV setting and could be why you're not getting plate measurements.

Also I just noticed you are only injecting at Normal channel.  You need to inject into the Vibrato channel to measure the voltage gain in that portion of the amp. 

[SleepLess]

Will do it again tomorrow with the signal injected in the Vibrato channel. The lowest mV range I have on my MM is 200mV so yes, it may not  be accurate enough to measure 0.06mV... Crap!

[stratomaster]

Will do it again tomorrow with the signal injected in the Vibrato channel. The lowest mV range I have on my MM is 200mV so yes, it may not  be accurate enough to measure 0.06mV... Crap!

Reread. 0.06V this is the same as 60mV.  You don't need to set your meter lower. You need to set it higher. Look at your numbers for V1a.  It looks like you're getting a voltage gain of 40 through the stage.  This looks like a reasonable measurement and was likely done correctly.  You should see numbers like though through the rest of the stages.

[SleepLess]

Hi!Here's a new one. I injected the signal in the normal channel to get proper readings for V1 and then injected the signal in the Vibrato channel for all the other tube sockets measurements. I hope I'm doing things right this time and I hope it helps! Thank you!

[stratomaster]

These look better. The stand out is V4B.  This is the final gain stage after the reverb circuit and before the PI.  You're showing 300mV out and 18mV in.  This is a gain factor of 16.7.  Based on the other measurements you should be expecting about 40. Even 40 seems low to me, but it's been a while since I've done this myself on an AB763 style amp.

Try a different tube in V4.  If that doesn't work, then look at the components around V4b to make sure there are no build errors. Double check resistor values on the plate and cathode.  Ensure the cathode cap isn't backwards or has come disconnected.  A lot is happening this area of the circuit and just downstream (vibrato circuit ties in, normal channel mixes in, etc) so it can be easy to make an error.

Edit:  I found the Fender Vibroverb Custom schematic online and it has this exact information on it by stage, AC in and AC out.  It's the measurements given in an oval flag at each triode symbol.  Divide the out by the in for gain factor. It's not a 1 for 1 between this schematic and your amp (the Cesar Diaz mods affect gain), but maybe the Super Reverb Reissue, which has similar voltages on the schematic, is closer.