Fender Vibroverb clone Vibrato issue

[Minnievirginny]

Hello Amplifier people.

I am hoping that someone may be able to assist with a technical problem I am experiencing with a Vibroverb clone which I bought about 6 years ago.

The Vibrato (tremolo actually) has ceased to work.

This in itself is a minor irritation but I didn't realise that unless the intensity control is set to absolute minimum, the power valve bias goes heavily positive and would appear to have done for one set of 6L6s.

I am including a circuit portion of this amp as although it adheres closely to the Fender 6G16 circuit, the creator of this amp has incorporated a bias voltage control which allows accurate adjustment.

Looking at the schematic, I am at a loss to see, once the intensity control leaves the minimum setting, how bias can be maintained and indeed does go far too high pushing the power tube current up easily above 100 ma if I let it. (Current measured accross a 1 ohm resistor between cathode and earth on power tubes now = 47ma)

The oscillator section appears to be working OK and provides a good sine wave fluctuating between +2v and -24v at the intensity pot when set to max (see diagram - red mark). Advice as to whether this is the correct amplitude and voltage at this point would be appreciated.

Intensity pot has been removed and tested. Alternative power valves and oscillator valve have been tried. Reflowed most joints but his was a quality build so joints look new and shiny. B+ line = 446 volts.

I have biased the power valves and have a stable current of 47ma yielding 70% bias. The sound quality and power is as expected (good). Rock solid bias reference voltage is 42 volts at green mark.

I suspect I should be seeing something different at red mark but am stuck. Shouldn't the LF at red mark modulate the bias voltage but from what to what?

Thanks for having a look,

Regards to all,

Larry

Vibroverb schematic marked.jpg

[sluckey]

Turning the INT pot will not affect the bias in a properly wired/functioning 6G16 amp so that indicates a problem. Maybe that .1µF cap is leaky. Disconnect the cap from the INT pot. Does the pot still affect the bias? If no, replace the cap. If yes, we need to see the bias mod.

[Stevem]

Not that it’s part of your problem ( yet ) but if that amp is built with a 50 volt rated bias filter as in the original Fender, that’s not a high enough voltage rating.

[pdf64]

Just to note that 47mA idle current seems very high for fixed bias AB.
Design max limits are intended to be derated, in order to accommodate supply and component variance.
When the root cause of the issue is sorted out, cooler idle typically helps to achieve a good trem depth with bias vary type trems.

[Minnievirginny]

Thank you all for your advice.
I will carry out suggested tests today and report back.
Also I noticed that there appear to be grid stopper resistors of 1.5k in series with pin 5. I do not see them on my Fender diagram.
I will draw out an accurate representation of the actual power supply and bias components.

I don't know if the amp was trying to give me a clue but prior to adjusting the bias, the Trem function worked as soon as the standby switch interrupted the supply, obviously for a brief period.


PDF64:
I used the Biasing Tube Amplifier Calculator @ https://www.amplifiedparts.com/tech-art ... calculator
This suggests that for a plate voltage of 446 volts and a 6L6GC tube a current of 47ma will yield a 70% bias. Have I missed something or misunderstood?

Stevem:
I thought that as long as the bias voltage was adjusted to set the ecommended quiescent current through the 6L6GC as above then the amp would be happy. Your yet comment suggests that I may be looking forward to problems in the future?

Your advice is greatly appreciated
Larry

[Stevem]

My info states that for a 6L6 and it’s 30 watt rating this should be observed.
450V 47ma = 70%, 40ma=60%, 33ma = 50%.
With 475V 70% =44ma.

Don’t forget that it’s not just the tube your running harder, because if you play the amp loud enough to have the power supply voltage start to sag then your also running a lot more average current thru the OT and it will run hotter also and with a reduced life expectancy.

In regards to the bias supply , that PT winding supplies 40 volts , so long term a 50 volt rated filter is getting pushed pretty close to its limit possibly when you consider that there’s a minimum 10% tolerance in its rating.

[Minnievirginny]

Hello Folks,

Here is the bias circuit in my amp as an attachment (Vibroverb clone bias circuitry).

Yesterday I replaced the .1uf cap from the vibrato circuitry with no improvement. This morning, as recommended, I disconnected the cap from the 250k intensity pot and moving the wiper away from the 55v bias end resulted in the voltage on the wiper going progressively positive into dangerous territory so there is clearly something amiss.

In terms of my playing volume, it is usually quite quiet (jazz stuff). Incidentally apart from no trem the amp is sounding great. I am very unlikely to be playing at 'foot to the floor' volume but it woulld be great to get the thing working as it should.
Anybody any views on those 1.5k grid stoppers?

Again, many thanks for taking the time to help me on this.

Regards,
Larry

[sluckey]

Are you sure the bias pot is 250K? If so, that is insanely dangerous because you can adjust the bias voltage from -55V down to -2.1V. A more appropriate bias pot value would be 10K to 25K, definitely not 250K.

If your actual circuit is the same as your drawing, changing the INT pot cannot change the negative voltage sent through the 220K resistors to the 6L6 grids. Pull the 6L6s and set aside. Now monitor the voltage on pin 5 of a 6L6 socket and rotate the INT pot from one end to the other. The voltage should not change. If it does, what is the minimum and maximum voltage?

While still monitoring pin 5 voltage, turn the bias pot from end to end. What is the minimum and maximum voltage?

[Minnievirginny]

Hello again,

Firstly apologies. you are of course correct in that I carelessly specified the bias pot as 250k. It is in fact 10k as you guessed.

pin 5 wrt chassis :
INT from -41.2 Volts to -8.35 Volts
Bias from -54.6 volts to -29.5 volts

Trem cap was disconnected during these tests.

I suspect this is not good.

Thanks again, I am sure you could not think of a better way to celebrate Easter!

Cheers,
Larry

[sluckey]

pin 5 wrt chassis :
INT from -41.2 Volts to -8.35 Volts

I suspect this is not good.

Me too. It's a very simple circuit. If it's wired IAW your drawing then something must be touching something that it should not be touching. Some hi-rez pics might reveal some clues.

[Minnievirginny]

Thanks so much for your advice on this.

I am thinking back to when the problem occured.

The amp has always had a tendency to hum but about 2 months ago whilst using it I noticed what looked like the beginning of one of the 6L6s to head towards red plating. having removed the valve in question I notice severe scorching down one side of the tube and this is when the vibrato function ceased to function. Unfortunately, I cannot think of anything that happened that could have brought this on. It has not been gigged or moved about, It has not been subjected to temperature extremes so I was inclined to think of it as a failing component. With the Vibrao INT set anything beyond zero, it was obviously getting a serious kicking from massive under biassing.

As you say, a simple linear power supply circuit which, if considered as a negative grid bias seems to function iscollated from the Trem cap just fine but I was unsure (not having had any reason to 'scope the Vibrato voltages) what I should be seeing coming from the .1uf cap.
There is a decent HT swing on the other side of it.
If the Vibrato was working and intensity was fully advanced, if the dc bias was, say -50 volts, what would you expect to see in terms of voltage swing at the two 220k resistors? Something like a sine wave going from -30 volts to -60 volts perhaps?

I will carry out further investigations focussing on possible shorts/solder joint failure and hopefully find something.

hanks again, really appreciate the help

Regards,
Larry

[sluckey]

This morning, as recommended, I disconnected the cap from the 250k intensity pot and moving the wiper away from the 55v bias end resulted in the voltage on the wiper going progressively positive into dangerous territory so there is clearly something amiss.

This statement should tell you that the tremolo circuit is NOT causing the problem. Just leave that .1µF cap disconnected and concentrate on fixing the simple bias circuit. Maybe start by replacing the INT pot. Once you have fixed the problem that causes the INT pot to change the bias voltage you can reconnect the .1µF cap.

[Minnievirginny]

Yes, that makes sense. I suspected that the oscillator circuit was Ok.
I will try that recommendation and report back
Thanks again,
Larry

[Minnievirginny]

Well,
Much to my total astonishment and as a result of your suggestion, I replaced the INT pot and everything behaved as it should!
I had previously removed this pot and tested it to ensure that the end to end resistance was good and the wiper gave a smooth variation accross the sweep. It appeared to come up with a clean bill of health.

It was clearly faulty

So a massive thank you for engaging in this adventure with me with your helpful and knowledgeable advice.

Next: Fender Twin overhaul that I blackfaced from Silver a couple of years ago has developed a bit of a hum!

Regards,
Larry

[Stevem]

Still, it’s not making sense to me that the pot would just up and fail!
Had you been in the amp and doing some other stuff and then a given amount of time later after you buttoned it up realized the trem did not work?