bias vary tremelo distortion

[Toxophilite]

In one of my AB763 deluxes I have the 6G11 bias vary tremelo circuit
I have 6L6s in that amp and I seem to be getting distortion on the tremelo when the intensity is turned up
I've tried swapping in a red LED instead of the cathode cap/resistor but that seems to exacerbate the problem (likely through deepening the tremelo)
The 6L6's are biased around 32ma
I've also tried biasing them as low as 20 and up to 42 just to see if it makes a difference and it doesn't really seem to


I can't seem to find info on this
Any ideas?

[Toxophilite]

Hmm
I do have a jack for a tremelo footswitch and it's working perfectly
The sound is clean with the tremelo turned off via the footswitch or by turning down the intensity


Tremelo works, but it's distorted with the intensity turned up


I'm pretty sure the jack is brand new


Does that eliminate that possibility??

[sluckey]

Toxo, meet gonzo.

[Toxophilite]

I think there might be a misunderstanding


This is a 6G11 tube tremelo circuit in an AB763 deluxe(with no reverb)
THE bias pot is easily adjustable and I've done it many times


I have NO Distortion with the tremelo turned off


NO tremelo = NO distortion


The Speed control has not been mentioned and does not affect the distortion


The distortion ONLY happens when the tremelo is turned on and the intensity is turned up past 5 or 6(tremelo can be turned on and off using the switch or by using the intensity control.). The distortion increases as you turn up the intensity


Tremelo ON + Intensity up = distortion


More intensity = more distortion


The tremelo controls do not affect the normal sound once the tremelo is turned off




Anywho
The problem is distortion on the tremelo when the intensity is turned up and my first post relates a few of the adjustments/fixes I have tried


if anyone has anything more to add after reading the symptoms and fixes tried I would really appreciate it


If not I will continue to research and experiment


Thanks

[shooter]

This a clone type or original, in the original, I'm thinking your light activated R might be old?
since speed is good guessing the osc is doing it's thing fine, everybody downstream is common to both channels.  did you swap the tube for the heck of it?

[sluckey]

The distortion ONLY happens when the tremelo is turned on and the intensity is turned up past 5 or 6(tremelo can be turned on and off using the switch or by using the intensity control.). The distortion increases as you turn up the intensity

Do you have sufficient good tremolo with the intensity set to 5?

With bias vary tremolo there is a trade off between bias adjustment and good tremolo. Can you fiddle the bias and get good trem?

[Fresh_Start]

The distortion ONLY happens when the tremelo is turned on and the intensity is turned up past 5 or 6(tremelo can be turned on and off using the switch or by using the intensity control.). The distortion increases as you turn up the intensity

Do you have sufficient good tremolo with the intensity set to 5?

With bias vary tremolo there is a trade off between bias adjustment and good tremolo. Can you fiddle the bias and get good trem?

I had to dial bias for slightly less current (cooler bias) to get effective bias vary tremolo in both Princeton Reverb and Suoer Reverb derivatives. It's the only downside for bias vary tremolo IMHO. If you are going for roaring power tube distortion, you may need to consider a different tremolo circuit. Works great in the AB763 circuit for my purposes.

Cheers,

Chip

[Toxophilite]

Thanks for the ideas
My tremelo starts at around 4 on the intensity pot


I have not yet swapped it out for a linear 250K like 6G11a circuit so it could be that, as it's a goodly amount when tuned all the way up, though it wants to whup whup through the JBL.

I'm not looking for raging power tube distortion  from this amp, just looking for as much nice clean headroom as it will supply ...(some of those ^%$^&% smileys are kind of funny)

I tried biasing the amp like a 6G11 would be biased (around -45 from the bias circuit) giving me approx 27ma on my 6L6s
I can't hear any distortion at the more moderate level I can play at this time of the eve in my apartment so I'll give it a better test tomorrow.


I just went over the tremelo circuit yet again and make sure it's good

[Toxophilite]

Is the circuit used by on Hoffman's AB763's the 6G16?
i read this somewhere and I was curious if it was a better circuit, or at least better with the higher voltages
The 6G11 circuit typically runs on 365 while the 6G16 is 440 i think
They have a lot of similarities with a few marked differences


Any opinions on these two circuits?

[sluckey]

The two circuits are very close. I don't think the difference in B+ is contributing to your problem. The 6G16 has one difference that may smooth the trem signal a bit more than the 6G11. It has a 470K (rather than a 220K) in line with the trem signal headed to the Int pot and it has a .1µF connected to ground (6G11 has none). See if  changing the resistor from 220K to 470K and adding the .1 cap helps. It will definitely decrease the level of the trem signal which should allow you to turn the INT pot up a little more.

I'd like you to try this amp with a less efficient speaker than the JBL. Does that help?

[tubeswell]

I tried biasing the amp like a 6G11 would be biased (around -45 from the bias circuit) giving me approx 27ma on my 6L6s
I can't hear any distortion at the more moderate level I can play at this time of the eve in my apartment so I'll give it a better test tomorrow.

Can you describe the 'distortion' you speak of a bit further?

Is it like overdriven distortion?

Is it like a 'warble' that develops over the sound as you drive the signal harder with the tremolo on?

Or is it like something else?

(Can you upload a soundbyte of the distortion to somewhere?)

[Toxophilite]

Well I swapped out the intensity pot for a linear one..Excellent suggestion, now the tremelo starts around 2 on the dial and progresses smoothly up to the top


On a non-JBL (but still quite efficient Utah (more efficient than my JBL D123s) no whoop whoop whoop
Mostly seems to do that on the D130
and some on the D123 when the speed is close to fastest and the intensity is maxed
It's currently biased at about 33.5ms 38 ish (the sovteks I have in there are about 4 ma apart)
That's about -42 on the bias circuit


It seems to be not making much distortion now.
Perhaps it's mostly when it's at the top of it's clean limit and the oscillating bias puts it over the edge..it does go up to about 65 on the top end when the tremelo is on


The distortion sounded like the amp was overdriving.

[tubeswell]

On a non-JBL (but still quite efficient Utah (more efficient than my JBL D123s) no whoop whoop whoop
Mostly seems to do that on the D130 and some on the D123 when the speed is close to fastest and the intensity is maxed

Sounds like its to do with particular speaker(s) not being able to handle the trem depth

[Toxophilite]

I might also just be that's very very efficient
it's the loudest speaker I own!!
JBL D130s are pretty sturdy

[mresistor]

[quote author=sluckey link=topic=18541.msg189443#msg189443 date=1428360422]
Toxo, meet gonzo.
[/quote]
I am sure with your experience, you know the answer, so tell the forum the answer, and quit being a troll.

https://www.youtube.com/watch?v=Y9A4W7RLXak

[Toxophilite]

"How do you do, boys and girls....."


I had an excellent Alfred Hitchcock record when I was wee
if I remember correctly he had a leaky faucet and was slowly being submerged throughout the entire record

[Toxophilite]

Is it normal for bias vary tremelo to be less intense as it gets slower and visa versa??
When I have my tremelo speed control at it's slowest and the depth at it's most intense the tremelo seems pretty mild,
If I JUST crank up the speed it starts to get way more intense and eventually whop whops some

[Fresh_Start]

Is it normal for bias vary tremelo to be less intense as it gets slower and visa versa??
When I have my tremelo speed control at it's slowest and the depth at it's most intense the tremelo seems pretty mild,
If I JUST crank up the speed it starts to get way more intense and eventually whop whops some

Are you seeing this on an oscilloscope or hearing it?  My guess is that the effect may just seem more intense with a faster oscillation.

Chip

[Toxophilite]

I was wondering the same thing

But I'm pretty sure that isn't the case
The slowest speed isn't as deep as a similiar speed on my super(1980 different circuit)
quite light
but the faster speeds can be  too deep


Just curious if anyone else had noticed this with 6G11 circuit
  I should test it with the bias because that would might be a good objective indicator too to see how deep it swings at both speeds

[sluckey]

I've noticed that with all the phase shift oscillator circuits. You can see why with a scope or a voltmeter that can read frequencies as low as 3 to 10HZ. My Fluke can. Try measuring the AC voltage level at the hot end of the intensity pot (not the bias input end) with your meter. Vary the speed and you will see the voltage reading change.

The trem signal level is not constant across the speed range. It drops considerably on the low frequency side and will even drop low enough to cause the oscillator to quit. Does the same thing on the upper frequency range. The signal is strongest at about mid speed.

Replacing the oscillator cathode resistor/cap with a red LED will much improve this. Not only will the LED provide a big boost in trem signal, it gives a flatter frequency response and allows a slower speed and faster speed.

[Toxophilite]

Cool
Learned something and it's good to know it's mostly normal
I have a red LED in lieu of the cathode resistor/cap , looks cool too!

[Fresh_Start]

I've noticed that with all the phase shift oscillator circuits. You can see why with a scope or a voltmeter that can read frequencies as low as 3 to 10HZ. My Fluke can. Try measuring the AC voltage level at the hot end of the intensity pot (not the bias input end) with your meter. Vary the speed and you will see the voltage reading change.

The trem signal level is not constant across the speed range. It drops considerably on the low frequency side and will even drop low enough to cause the oscillator to quit. Does the same thing on the upper frequency range. The signal is strongest at about mid speed.

Replacing the oscillator cathode resistor/cap with a red LED will much improve this. Not only will the LED provide a big boost in trem signal, it gives a flatter frequency response and allows a slower speed and faster speed.

Thanks!

Chip