1964 Fender Vibro champ volume problem

[Wally]

I think I helped cause the failure by resting the chassis on a table with the tubes and filter cap can bearing all of the weight.

why would I do this?


because I saw Gerald Weber do it.

I take care to heat the chassis metal thoroughly by apply the chisel tip to that metal directly With the edge of tip contacting the tab. I do not sit the chisel tip on top of the tab. Otherwise..from my pint of view, the solder could flow quickly but not make a good joint with the chassis.

 

[Antoon]

I also had to fix a friend's BF Champ. It's output was somewhat low and it did not sound as it should. Took me ages to find the problem. Checked all resistors, replaced the electrolytics, checked all voltages. In the end it turned out to be the output transformer (the only part that remained). Once it was replaced (ouch.. ) by a classic tone, it suddenly sounded great. I could not find what was wrong with the original OT. I suspect failure of the insulation of the secondary windings, causing shorts.

 

[maybeoneday]

I also had to fix a friend's BF Champ. It's output was somewhat low and it did not sound as it should. Took me ages to find the problem. Checked all resistors, replaced the electrolytics, checked all voltages. In the end it turned out to be the output transformer (the only part that remained). Once it was replaced (ouch.. ) by a classic tone, it suddenly sounded great. I could not find what was wrong with the original OT. I suspect failure of the insulation of the secondary windings, causing shorts.

I hope this is not the case, I would hate to replace the original one.

 

[maybeoneday]

@maybeoneday, if you want to hear the difference in the power tubes, simply switch them. There may or may not be a difference. It could be an interesting experiment. I like to know how a power tube is Biasing in these little single-ended amps. There can be a wide variation in plate dissipation, and that will determine what the amp sounds.

I have also measured the bias and it came up very high, around 90mA. I wasn't sure about the multimeter so I have taken all voltages plus the resistance between plate and pin 3 of the 6V6 and put them in the rob's bias calculator and it confirmed the multimeter.
I think it is time to bring it to my amp tech.

 

[Wally]

I have also measured the bias and it came up very high, around 90mA. I wasn't sure about the multimeter so I have taken all voltages plus the resistance between plate and pin 3 of the 6V6 and put them in the rob's bias calculator and it confirmed the multimeter.
I think it is time to bring it to my amp tech.
View attachment 1050504

90% of MPD is not high in a single-ended amp, imho and ime. It is not uncommon to see MPD numbers much over 100% with 16-18 watts of dissipation common. Below 16 watts yields something that I do not care for. At 16 watts and above, the amp gets much livelier and richer..without redplating.

Edit: I misread 90ma as 90% of MPD. My error and apologies.

 

[maybeoneday]

It could be that I dont understand the bias calculator properly but the plate dissipation is almost 200% with 29watt against the 14 of the JJ.
If I check the suggested current I should have around 40mA for a Class A amp and the JJ 6V6S tube.

 

[peteb]

90% of MPD is not high in a single-ended amp, imho and ime.

he didn’t say 90% he has 90 mA. Way too high.

It could be that I dont understand the bias calculator properly but the plate dissipation is almost 200% with 29watt against the 14 of the JJ.

your cathode resistor is too low. 268.8 needs to be at least 470 ohms per the schematic.

too low of bias will result in too high of current which will drop your B+ and drop your volume.

 

[Wally]

Thank you for the correction, @peteb. Good advice on the bias resistor.
I was looking at the numbers in the Biasing equations above and had concluded that the current draw was dropping the voltage down drastically and that the draw was much too high.

 

[maybeoneday]

he didn’t say 90% he has 90 mA. Way too high.


your cathode resistor is too low. 268.8 needs to be at least 470 ohms per the schematic.

too low of bias will result in too high of current which will drop your B+ and drop your volume.

Yes you are right, instead of the cathode resistor I put the OT primary resistance.
Now it is correct but it is way lower than the one I got, which is 92mA
I should have a bias resistor around 620ohm to be in range, am I correct?

 

[Wally]

What does the amp sound like? Assess that, then move the bias resistance up in steps so that you can experience what each incremental change does for the sonics. Do the math at each step. What you like is what you like, but as I noted it is common to find Champs, VCs, and Bronco amp running that much current…and more.
If one has a supply of 6V6 singles, imho, it is best to change the tube while keeping the bias resistance at 470ohms. I have done an experiment with about ten NOS RCA 6V6GTs. I found plate dissipation numbers from 87-147%.

 

[maybeoneday]

What does the amp sound like? Assess that, then move the bias resistance up in steps so that you can experience what each incremental change does for the sonics. Do the math at each step. What you like is what you like, but as I noted it is common to find Champs, VCs, and Bronco amp running that much current…and more.
If one has a supply of 6V6 singles, imho, it is best to change the tube while keeping the bias resistance at 470ohms. I have done an experiment with about ten NOS RCA 6V6GTs. I found plate dissipation numbers from 87-147%.

ok, I will start trying the 6V6GT from my deluxe reverb and take it from there

 

[peteb]

I should have a bias resistor around 620ohm to be in range, am I correct?

your 18.3 W is in range for these amps.

your plate voltage is in the 380’s?

20 W is not unheard of.

16-17W is where I like to find mine.

A 620R cathode resistor would work well.

 

[maybeoneday]

your 18.3 W is in range for these amps.

your plate voltage is in the 380’s?

20 W is not unheard of.

16-17W is where I like to find mine.

A 620R cathode resistor would work well.

The main problem is that when I measure the current between B+ and pin3 I have 92mA, this is what doesn't add up.
I have also did the mats and still gives me 92mA
(B+VDC - Pin3 Voltage) / OT primary resistance = current
(318.9VDC - 294VDC) / 268.8ohm = 0.092A (around 29w plate dissipation)

Edit: I have also tried 2 EH 6V6GT and the best it goes is 86mA. There must be something that drains so much current.

 

[SnidelyWhiplash]

I think I helped cause the failure by resting the chassis on a table with the tubes and filter cap can bearing all of the weight.

why would I do this?


because I saw Gerald Weber do it.

Gerald Weber??? I think that's your first mistake.

 

[peteb]

The main problem is that when I measure the current between B+ and pin3 I have 92mA, this is what doesn't add up.
I have also did the mats and still gives me 92mA
(B+VDC - Pin3 Voltage) / OT primary resistance = current
(318.9VDC - 294VDC) / 268.8ohm = 0.092A (around 29w plate dissipation)

this appears to be wrong. Possibly done wrong. I doubt that you are measuring 92 mA of plate current. does the plate red plate. I think it would have to at 92 mA.


i Think this method is more reliable.

POST 69: cathode resistor 463.8 ohms, cathode current 53.7 mA. I assume the program used Vc = 25.1 V = .0537*467.8

these are all valid numbers.


cathode current = plate current plus screen current.

the screen current is 10% or less of the cathode current. The plate current is the majority of the cathode current, 90-95% of it.

the plate current MUST be less than the cathode current.

your plate current is incorrect. It is closer to the 50 mA that the program says.

 

[peteb]

The main problem is that when I measure the current between B+ and pin3 I have 92mA, this is what doesn't add up.
I have also did the mats and still gives me 92mA
(B+VDC - Pin3 Voltage) / OT primary resistance = current
(318.9VDC - 294VDC) / 268.8ohm = 0.092A (around 29w plate dissipation)

What is right and what is wrong about these numbers?

268.8 ohm is correct.

you want about a 12 volt VDC drop across the primary for 40-50 mA.

You have a 25 volt drop across the primary. That is consistent with double the expected plate current. Your plate and b+ voltages are about 100 volts too low. This too is consistent with excessive plate current.

however, all of the plate current has to first flow thru the cathode. Current can not be created out of no where. The plate current has to be less than the cathode current unless the grid is conducting current or the screen is in reverse conduction. Not likely since you have tried multiple tubes. You could measure the resistance to ground from the grid, pin 5.


the output transformer plate current measurement and calculation method is very accurate.

I like to measure directly between the plate and the b+, but it should not matter. Subtract the cathode voltage from the plate voltage when doing the power calculation. (Even though the current is the really important number to get) The bias is really between the cathode and the grid. You don’t really know the bias voltage unless you know both DC voltages and subtract.

do the calculations yourself and don’t rely on the program.


best of luck!

 

[maybeoneday]

this appears to be wrong. Possibly done wrong. I doubt that you are measuring 92 mA of plate current. does the plate red plate. I think it would have to at 92 mA.


i Think this method is more reliable.

POST 69: cathode resistor 463.8 ohms, cathode current 53.7 mA. I assume the program used Vc = 25.1 V = .0537*467.8

these are all valid numbers.


cathode current = plate current plus screen current.

the screen current is 10% or less of the cathode current. The plate current is the majority of the cathode current, 90-95% of it.

the plate current MUST be less than the cathode current.

your plate current is incorrect. It is closer to the 50 mA that the program says.

92mA come putting red probe on B+ and black on pin 3, set the multimeter to Ampere, choose the correct input jacks on the multimeter. Switch on the amp and let it warm up, there come the reading.
I followed letter by letter 2 methods form Rob Robinette web site and indeed the 92mA come up from both of them.
Do you have another method I could follow?

I attach also a pic from the reading

 

[moonlighter]

Guess I've missed a bit here...
And maybe I'm glossing over it, but the new 3W cathode resistor you put in is 470R?
It should not be any less than that.
If it is to spec, measure your cathode voltage, and we'll draw a load line to maybe see what's happening here.
The thing about cathode biasing is that it self-regulates. Plate is drawing high current? Cathode resistor drops more voltage and bias goes more negative (cooling the dissipation).
The stock 470R resistor should cover most tube-to-tube variances without issue.
Depending on what your cathode voltage is, I would start directing my attention to the OT, because DC resistance is one thing, but if the 6V6 isn't seeing an AC load for a given reason we're going to have to change how we're looking at this.

 

[maybeoneday]

Guess I've missed a bit here...
And maybe I'm glossing over it, but the new 3W cathode resistor you put in is 470R?
It should not be any less than that.
If it is to spec, measure your cathode voltage, and we'll draw a load line to maybe see what's happening here.
The thing about cathode biasing is that it self-regulates. Plate is drawing high current? Cathode resistor drops more voltage and bias goes more negative (cooling the dissipation).
The stock 470R resistor should cover most tube-to-tube variances without issue.
Depending on what your cathode voltage is, I would start directing my attention to the OT, because DC resistance is one thing, but if the 6V6 isn't seeing an AC load for a given reason we're going to have to change how we're looking at this.

This is the reading of the voltage across the bias cathode, 47.81VDC.
From pin 5 of the 6V6 to Ground there is unlimited resistance.
The bias resistor measures 463.2ohm

 

[moonlighter]

View attachment 1050647
This is the reading of the voltage across the bias cathode, 47.81VDC.
From pin 5 of the 6V6 to Ground there is unlimited resistance.
The bias resistor measures 463.2ohm

Okay, so based on that info we're drawing about 96ma between the plate and screen grid, just as you calculated before... that 47V is more than double what we should be seeing (21-22V).
That cathode resistance is plenty good enough, so we'll stop worrying about that.
Pin 5 to ground should measure 220K. If that is reading open, we do have an issue because the tube has no grid leak. This could very well be the problem. Without a grid leak, electrons pool on the grid and increase the bias (in power tubes this is dangerously drastic). The 6V6 datasheet specifies a maximum of 500k, and Fender used a safe 220k.