6G16 Vibroverb - Thunderstorm noise

[Elad]

Hello all,

I've built a 6G16 '63 Vibroverb clone about a year ago using Mojotone's Heyboer iron except for the PT which is a ClassicTone.
all resistors are modern CC, F&T 16uf filter caps, Sprague Atoms for cathode caps and a mix of Sozo Royal Blues and Mallory caps for the various coupling caps.
Tubes are all Tung Sol Reissues.
I used these schematics for building the amp:
http://ampwares.com/schematics/vibroverb_6g16.pdf
changes to the circuit were a bias pot between the bias circuits' diode and the 250K intensity pot leftmost lug, a lar/mar Master volume and 1 ohm resistors to ground on the power tubes to check bias.

Recently the amp began making loud thunderstorm noises while the guitar signal becomes faint. It sounds like strong wind blowing on a microphone while someone turns a scratchy pot violently.
I'm pretty sure the problem is in the power section as the noise remains even when all preamp tubes are pulled out except the PI.
this same problem happened a few months ago and I eventually got rid of it (albeit, temporarily) by reflowing the NFB tail resistor (470 ohm) ground connection.
Voltages across the amp seem ok except for the V6 (PI) pin 1 which has about 160Vdc instead of 250V as in the layout (the other plate's pin 6 has 240v) - this problem also appeared the last time I had the problem and it was remedied when the grounding problem was cured.

I'd appreciate any help I can get as I tried a bunch of stuff to no avail, here is some of the stuff I tried so far:
1. tried known working 6L6s and different preamp tubes
2. switched around the filter caps on the power board
3. bypassed the 10k bias pot
4. changed intensity pot
5. subbed new CC resistors in the NFB
6. reflowed all solder joints in the power section and power board
7. changed the coupling caps in the PI
8. hooked up a different speaker cab
9. changed the 25uf 50v bias cap for a 100uf 100v cap
10. checked 470 ohm screen resistor - they are fine
11. changed rectifier tube
12. redid the filter caps' ground point
13. disconnected the lar/mar PPIMV and went with the original wiring.

[shooter]

what was the 160vdc when it worked?


what is your bias set at?  can you measure it at the tube sockets without tubes?
what happens when the problem occurs, with oven mitts on, wiggle the PA tubes?

[Raybob]

reflowing the NFB tail resistor (470 ohm) ground connection.

IIRC that should be 4.7K.  The mistake is on the layout, not the schematic.

[Elad]

what was the 160vdc when it worked?


what is your bias set at?  can you measure it at the tube sockets without tubes?
what happens when the problem occurs, with oven mitts on, wiggle the PA tubes?

When it worked the voltage at pin 1 was approximately 220-230Vdc.

I tried wiggling the power tubes with oven mitts while the problem occurs - no effect on the problem.
bias voltage at the power tubes' pin 5 with no tubes is -35v.

[Elad]

reflowing the NFB tail resistor (470 ohm) ground connection.

IIRC that should be 4.7K

I'm pretty sure it shouldn't, that's a typo in the schematic, the layout has the right value - 470ohm.
the schematic also omits the 100pf cap between pin 1 and 6 of V6.
I noticed Mojotone's recently released 6G16 kit's layout diagram features both of these mistakes.

[shooter]

verify your current through the 1 ohm'rs
when the problem occurs, disconnect the NFB  (so prep by disconnecting, then gator clip back in circuit)


sounds like a PI issue, or a parasitic oscillation?

[Elad]

verify your current through the 1 ohm'rs
when the problem occurs, disconnect the NFB  (so prep by disconnecting, then gator clip back in circuit)


sounds like a PI issue, or a parasitic oscillation?

Disconnected the NFB but nothing changed (with power tubes installed) - the bias voltage stays the same, V6 pin 1 still around 160v, plate voltage of power tubes still 414Vdc and bias on the 1 ohm remains approx. 47mV.
maybe the NFB wasn't working to begin with...

edit: another change made to the original design is 1.5k metal film resistors between pins 1 and 5. that being said I did it the last time I had the problem so I can attest it didn't help at all.

[tubeswell]

Chopsticks

[sluckey]

Does the noise stop when you pull the PI tube V6? Have you tried another tube for V6? I would change the 82K plate resistor for V6 pin 1.

[Elad]

Chopsticks

I chopsticked every component in the power section (incl. filter caps leads) as if it was a bowl of Ramen - didn't hear any change whatsoever (I don't have an oscilloscope).

[Elad]

Does the noise stop when you pull the PI tube V6? Have you tried another tube for V6? I would change the 82K plate resistor for V6 pin 1.

yes, there is no noise when V6 is pulled out.
I changed both the plate resistors (82k and 100k)- nothing changed...

[sluckey]

...Have you tried another tube for V6?

[Elad]

...Have you tried another tube for V6?

yep, just to be on the safe side I swapped another one just now and got the same results both with and without the NFB.

[shooter]

anyone have an example of the "balance cap" that goes plate to plate for testing?


I found this example, look at C52 on the "top" PI tube, 47pF grid to ground.  might want to solder it instead of gator-clipping

[Raybob]

reflowing the NFB tail resistor (470 ohm) ground connection.

IIRC that should be 4.7K

I'm pretty sure it shouldn't, that's a typo in the schematic, the layout has the right value - 470ohm.
the schematic also omits the 100pf cap between pin 1 and 6 of V6.
I noticed Mojotone's recently released 6G16 kit's layout diagram features both of these mistakes.

I've found that when there is a discrepancy with Fender layout to schematic, always go with the schematic.  In other 6G_ 2x6L6 Fenders like Concert or Super, they have 4.7K tail resistor. 

[Elad]

anyone have an example of the "balance cap" that goes plate to plate for testing?


I found this example, look at C52 on the "top" PI tube, 47pF grid to ground.  might want to solder it instead of gator-clipping

What should I look for after soldering the 47pf cap?

[Elad]

I've found that when there is a discrepancy with Fender layout to schematic, always go with the schematic.  In other 6G_ 2x6L6 Fenders like Concert or Super, they have 4.7K tail resistor.

You are right but it seems the other 4ohm Brown amps with the 4.7k tail have different NFB resistors (Concert, Super, Pro, Tremolux).
The 6G9a Tremolux, basically a Vibroverb in head format without reverb powering 2x10 inch speakers as well has 100k and 4.7k which form the same ratio as the Vibroverb's 10k+470ohm.

that being said, I replaced the 470ohm with a 5.6K (the closest resistor I had) and the static noise is still there...
I'm basically out of ideas - might just order a new OT and new tube sockets and hope for the best.

[shooter]

hopefully you "find' a working amp 
a scope would be real handy, my rabbit hole detector is pointing to a parasitic osc.
a 47pF "might" shunt it to ground before it gets amplified, fed back, amplified, fed.................
there's a "version" that spans plate to plate, but i haven't looked for that


EDIT:  the 160vdc still bothers me

[sluckey]

EDIT:  the 160vdc still bothers me

Me too.

[Elad]

what could be the problem causing the low plate voltage on pin 1 then?
the power node for the plates (where the brown wire connects) has 330volts, and pin 6 of V6 has the correct voltage.
both 100k and 82k resistors measure within spec (99k and 78k respectively).
what makes the voltage at the 82k (pin 1) drop by 50 volts?
is there anything going on with the grid (pin 2) that I should lookout for?

[sluckey]

is there anything going on with the grid (pin 2) that I should lookout for?

That's where I'd look next. You can't reliably measure voltage on pin 2 though. Just replace the grid resistor. Maybe disconnect the input coupling cap.

[PRR]

Be sure that is the right type tube.

[Elad]

is there anything going on with the grid (pin 2) that I should lookout for?

That's where I'd look next. You can't reliably measure voltage on pin 2 though. Just replace the grid resistor. Maybe disconnect the input coupling cap.

Thanks y'all, problem solved!

I replaced the 1M grid resistor, the 1M resistor that is jumpered to V4's pin 6 back from V4 (Y) and the 500pf cap (which I believe was the real culprit) and the amp now plays great, the voltage on pin 1 of V6 is as expected and it seems the amp is considerably less hissy when I run the volume high.

If anyone has an idea what was the problem and can explain it in layman's terms I would be grateful.

Edit: Now that I got everything sorted out I tried a 4.7k NFB tail resistor and got a high pitched squeal - I'm back to the 470 ohm tail.

[tubeswell]

Edit: Now that I got everything sorted out I tried a 4.7k NFB tail resistor and got a high pitched squeal - I'm back to the 470 ohm tail.

Swap the OT primarily ends around and reinstall the 4k7

[Elad]

Swap the OT primarily ends around and reinstall the 4k7

I'm puzzled - how come I didn't experience squealing for the 1.5 year I've been playing the amp with the 470 ohm NFB tail R?

also, I dug the pics I accumulated of original '63 VV innards - could only find ones where the tail resistor is 470 ohm (where you can make out the pics)

[Elad]

and another one.

[sluckey]

I'm puzzled - how come I didn't experience squealing for the 1.5 year I've been playing the amp with the 470 ohm NFB tail R?

The 470Ω gives much less feedback than the 4700Ω, probably even low enough to not go into oscillation if the feedback phase was wrong.

You did swap the OT plate leads with the 4700Ω in the circuit? And the squeal went away?

[Raybob]

I'm puzzled - how come I didn't experience squealing for the 1.5 year I've been playing the amp with the 470 ohm NFB tail R?

also, I dug the pics I accumulated of original '63 VV innards - could only find ones where the tail resistor is 470 ohm (where you can make out the pics)

This was 1st year of Vibroverb and circuit was changed in July 63 for blackface. If layout was never corrected, all production models must have gone out with anemic 160v PIs.

[Elad]

This was 1st year of Vibroverb and circuit was changed in July 63 for blackface. If layout was never corrected, all production models must have gone out with anemic 160v PIs.

Not sure about the math but mine has the 470 ohm and I have around 230v on the PI - pretty much in line with the numbers on the layout.
Amp sounds glorious now - if anything I'd rather try lower feedback.

btw, the BF Vibroverb has a similar ratio between its NFB resistors 820/47 = 17.44 (6G16's NFB has 21.2)