6g11 Vibrolux - fixing overdrive

[bullkelp]

I have a fairly original 62 vibrolux 6g11

I have reolaced the original oxford with a celestion type A. It has a replacement OT (mercury tone clone).
The electrolytics and filter caps have been replaced recently.

Up to about 4 on the volume it is a superb sweet fender brownface tone, but very clean.  Beyond 5 where I expected to find smooth drive out of the amp im getting a sort of unpleasant raspy, clipping, poppy breakup. All most like a blown speaker but its not. 

Whats the solution to this? Is it remediable?

[dogburn]

What's the timing for when this started happening: has it always sounded that way, or is it a new behavior that came after making changes?

I built a 6G2 Princeton, and it likewise has an unpleasant, raspy overdrive sound that I've been trying to address, so I'm interested in whether you find a solution with your amp.

[bullkelp]

Ive only had the amp a few months and its always been this way.

[Esquirefreak]

I owned a '63 6G9-b, which I think is pretty much the same amp. I think the overdriven/breakup tones from that amp was smoother than from most blackface amps I've owned.

Maybe a bad solder joint somewhere that reveals itself with more volume/vibration?

/Max

[Latole]

Are all tubes and the 6L6s are good ?  Matched 10 ma or less ?

What is the bias?

6L6 plate volt :
  "   cathode current on each tube:

Let say ; 365 volts X 0.042 A ( or 42 ma ) = 15,33 watts for each tube

Volts X current = watts
For a 25 watts tube I will not go over 15 watts

A correctly-biased amps will sound clean and tight at moderate volume, then at higher volumes break up and distort musically."
A correctly biased amp will run efficiently and maximizes the life of your tubes.
Under-biased ( hot )amps will lack punch and the tubes will run noticeably hotter shortening their lifespan
Over-biased (cold )  amps will sound thin and brittle, with the tube running too cool for proper performance.

[Latole]

Check the resistors values at the output of phase inverter circuit ; the 82k and the 100k

[tubeswell]

A number of scattergun thoughts


Are there any modifications to the 6L6 circuit? (like g1-k caps? and if so, are these leaky?)

What plate voltages have you got on the LTP?

Check for DC-leakage on coupling caps between LTP and 6L6s

Check the 0.1uF decoupling cap on the non-inverting grid of the LTP for DC leakage



Maybe scope V1b and V2b when you get above 4. Check that the cathode bypass cap for this stage is good.

[HotBluePlates]

I have a fairly original 62 vibrolux 6g11

Check for DC-leakage on coupling caps between LTP and 6L6s

Check the 0.1uF decoupling cap on the non-inverting grid of the LTP for DC leakage

Yellow Astron caps are notorious for leaking DC volts, and this amp is the right age to be to not have reached the change-over to the blue molded coupling caps.  I echo the suggestion to measure carefully for leaking DC volts.

[bullkelp]

All the caps are blue moulded
 

[tubeswell]

All the caps are blue moulded

Blue ajax caps can be leaky as well.

[bullkelp]

I need to revisit your instructions about how to check for leakage with a multimeter. Its still not part of my regular amp servicing lexicon.
Ill get the amp out of the cab after work today and give it a go

[bullkelp]

Righto

I replaced the filter caps as they were 20 yr old spragues. The original ground wire linking the first two for the resavoir section crumbled away upon touching...

Any how thats all good now.

I also checked all the coupling capos for DC leak on the grid side and im pretty happy they are good.

I have reflowed all the solder joints. One thing I did notice was some significant under board blobs starting to occur in a few spots. Could board conductance cause splatty unplasant decay on drive?

This board is largely original carbon comps. Im guessing drifted values are ok but hoe about in the phase inverter area where the schematic cals for 5% tolerance?
These are the readings of the four resistors in that area.
1. Spec: 220k measured 267k
2. Spec: 220K measured: 255k
3. Spec: 100k meaasured: 117k
4. Spec: 82K. measured: 99k

Ive rebiased to about 55-60% plate diss. Thats around 38mV on OT tube 1 and 39.5mV on OT tube 2. Plate volate 430v approx

Amp sounds sfantastic clean to slightly pushed but still goes into unpleasant drive sound above voleume = 4

Any where I should look next?

[passaloutre]

Any where I should look next?

What are the DC volts on all tube pins?

[dogburn]

You might try tweaking the bias up to 65% or so. Cold bias can be a factor in unpleasant overdrive/distortion.

[8wattjack]

"I have reflowed all the solder joints. One thing I did notice was some significant under board blobs starting to occur in a few spots. Could board conductance cause splatty unplasant decay on drive?"

When I built my Vibroverb years ago it did exactly what you are describing. I never could figure it out until one day I chop sticked it and found a wire that had a cold solder. Sometimes they hide even if you are good at soldering. Leaking dc in my experience usually causes static on the pots, and if the phase inverter caps are leaky it can cause bias drift. I just did two 65's with blue molded caps, and all passed with flying colors. Not likely your issue. Once I check some voltages and make sure an amp is stable I go staight to the sticks. You can uncover a lot of stuff with them. Louder volume= more vibration. A scope is nice if the sticks dont work.

[bullkelp]

Thanks
Yes I have chop sticked vigorously ! Thats what revealed need to seperate board with spacers so im still suspicious about conductance maybe even from solder stalagtites through the under board.
Nothing else shown up on chop sticking tho.

[bullkelp]

Any where I should look next?

What are the DC volts on all tube pins?

Seemed ok but I will do a proper record of them and report .
Im off over seas on confernce to korea for a week now.... with a cheeky little side quest to osaka for a couple days just to trawl guitar shops :)

[bullkelp]

You might try tweaking the bias up to 65% or so. Cold bias can be a factor in unpleasant overdrive/distortion.

I actually had it running hotter about 65-70 but thought id try backing it off t see if it helped at all.
These are old RCA 6L6s and are matched great. But they are old...

[stratomaster]

Do you own a scope? I'm asking because I've had two occasions recently where ceramic caps in the signal path distorted the signal enough to be both visible on the scope and audible as a harsh tonality. 

Is this happening on both channels?  If so, what happens if you lift the cap across the PI plates or temporarily install a different cap at the PI input? You can use anything from 500pF up to 0.1μF of sufficient voltage rating for testing purposes.

[tubeswell]

I also checked all the coupling capos for DC leak on the grid side

Did you check the DEcoupling cap on the non-inverting grid of the LTP? If dc is leaking through that it will affect the LTP duty cycle.