Valve Jr with Tremolo

[ChrisM]

Hello

I have a problem that I cant figure out and you seem like a knowledgeably bunch of guys so I brought my question here.

I just finished adding a tremolo effect to my Valve Jr. I used the Vibro Champ style of trem circuit (nice and simple just one tube).

Anyways it works but when the trem is not on the intensity pots acts like a volume control. If I turn the pot all the way off (0 resistance) its normal but at 10 the sound cuts out. So if I want to switch the effect on and off I need to adjust the intensity pot.

Other than that the trem is working and sounds great...Just cant figure this little part out.

Here is the schem...ignore everything but the trem circuit, I have made a lot of changes to the rest of the circuit since I drew that schem and there are mistakes in it (not in the trem circuit though).
http://img356.imageshack.us/my.php?imag ... ibegs5.jpg

Thanks so much for help!

[Phil_S]

These things only do what you tell them to do.
1) Give us an up to date schematic that has been double checked against the circuit.
2) Maybe the pot is bad? Unlikely, I think.
3) Pictures?

[Andy Le Blanc]

that circuit uses a cathode follower as a buffer..... it will have a voltage
present that will raise the bias point of the tube thats its hooked to (cathode)
to the point of shut off....... its because the 50k pot goes to ground and acts
as a variable bias voltage...... go to the fender pages at schematic heaven...
old tremolux had nearly identical circuit but the follower was connected to the
cathode thru a 250k pot. hooked directly from the cathode of the follower to
the top of the bias resistor at the cathode of the tube receiving the trem. signal
a 100k resistor went from the follower cathode to the pots wiper making it
variable... it makes a resister divider thru the 250k pot and the bias resistor
that keeps the voltage of the follower from swamping the bias of the stage...

[ChrisM]

that circuit uses a cathode follower as a buffer..... it will have a voltage
present that will raise the bias point of the tube thats its hooked to (cathode)
to the point of shut off....... its because the 50k pot goes to ground and acts
as a variable bias voltage...... go to the fender pages at schematic heaven...
old tremolux had nearly identical circuit but the follower was connected to the
cathode thru a 250k pot. hooked directly from the cathode of the follower to
the top of the bias resistor at the cathode of the tube receiving the trem. signal
a 100k resistor went from the follower cathode to the pots wiper making it
variable... it makes a resister divider thru the 250k pot and the bias resistor
that keeps the voltage of the follower from swamping the bias of the stage...

Comparing the Vibro Champ AA764 schem http://www.fenderholic.com/schem/champ_ ... _schem.gif
And the Tremolux schem http://www.schematicheaven.com/fenderam ... _schem.pdf

I dont exactly understand how I would implement the Tremolux trem into my amp.

Would basicaly I change to a 250K pot then have one lug going to the oscillator, one lug going to the supply voltage and the wiper going to my per-amp's cathode?

I feel like a lot is missing.

[jjman]

I tried a high value cap to block the DC on the trem line of my VC. 2200u/35v. It succeeded in preventing the problem but also reduced the trem affect a little. I removed it but if your effect is strong enough you may be ok. Or use a larger value cap.

Disconnect the outside wire on the trem intensity pot that does NOT go to ground. Attached the + side of the new cap to that terminal. Attached the loose wire to the – side of the cap. DC leakage blocked.

I had about 22vdc on the + side after it charged. A cap this high, with a voltage over 35 would be physically large. No need to use a higher voltage one unless yours moves somewhat close to it’s rating.

[ChrisM]

I tried a high value cap to block the DC on the trem line of my VC. 2200u/35v. It succeeded in preventing the problem but also reduced the trem affect a little. I removed it but if your effect is strong enough you may be ok. Or use a larger value cap.

Disconnect the outside wire on the trem intensity pot that does NOT go to ground. Attached the + side of the new cap to that terminal. Attached the loose wire to the – side of the cap. DC leakage blocked.

I had about 22vdc on the + side after it charged. A cap this high, with a voltage over 35 would be physically large. No need to use a higher voltage one unless yours moves somewhat close to it’s rating.

Tried this with a 3300uF/35V cap and the problem still persists.

Is this normal for the Vibro Champ trem circuit to do this?

[ChrisM]

I tried a high value cap to block the DC on the trem line of my VC. 2200u/35v. It succeeded in preventing the problem but also reduced the trem affect a little. I removed it but if your effect is strong enough you may be ok. Or use a larger value cap.

Disconnect the outside wire on the trem intensity pot that does NOT go to ground. Attached the + side of the new cap to that terminal. Attached the loose wire to the – side of the cap. DC leakage blocked.

I had about 22vdc on the + side after it charged. A cap this high, with a voltage over 35 would be physically large. No need to use a higher voltage one unless yours moves somewhat close to it’s rating.

Tried this with a 3300uF/35V cap and the problem still persists.

Is this normal for the Vibro Champ trem circuit to do this?
?

[M Fowler]

ChristM

Are you still having troubles with this amp? I thought that the pot for this circuit used a reversed taper and that could be the problem.

Mark

[ChrisM]

Ya Mark I am still having troubles unfortunately.

I wouldn't think the pot's taper would be the issue. But anyways what pot would you suggest?

Whats that Bronco? Modified Vibro Champ...

[Jana]

could it be that you are using a 50k pot instead of a 25k that is in the champ circuit? That extra 25k on the pot could be allowing the voltage from the tremolo tube to "get the upper hand" on the bias voltage of the the tube with the 1k5 resistor. Maybe try jumping the 50k pot with a 47k resistor from the wiper to ground. It will change the taper but it will allow you to test this theory since at full intensity the max resistance across the pot will be about 24k.

[M Fowler]

OK your schematic an the others definitely show a 3M RA (reverse audio taper) pot for the speed and either a 250KL for intensity on the Tremolux and 25K RA on the Broncho and Vibro Champ and you noted a 50K RA.

Are you using RA pots reverse taper or just plain audio pots?

Mark

[M Fowler]

The Broncho amp was a beginner amp listed in Fender's 1972 catalog I have. I have never seen one or I would own it. It has 6 watts RMS, 8inch speaker, 3 tubes and seems to be about the same as the Virbro Champ listed in the same catalog and then there was also the Champ listed. I have had and blew up three champs before. I used to hook them up to 212 bottom and run one and sometimes two Electro Harmonix LPB1 boosters in the input. She would buck and squeal with all kinds of grit and grim just what I was looking back then late 1960s. I also would run one of these paralleled into a larger amp for gain, but found I could get better distortion and fuzz with the EH big muff pi and a LPB2 booster. We didn't have many pedal to choose from and I had only seen pictures of a Dallas Arbiter fuzz face or Foxx box in those days.

I recently made myself a new LPB2 booster and use it all the time gigging for clean boost on lead runs. Next I have been thinking about building the rangemaster for giggles.

Mark

[M Fowler]

I see Mojo amps has the proper reverse taper pots you need. No I do not work for anyone in the industry just trying to help you solve your problem.

http://www.mojomusicalsupply.com/products.asp?id=45000

Part Number: 9027000
Style/Type: CGE 3M RA Vibrato/speed

Part Number: 4922000
Style/Type: CGE 50K RA vibrato intensity

They also have 25K RA as well as the same value pots from different makers varying price levels.

Mark

[jjman]

I tried a high value cap to block the DC on the trem line of my VC. 2200u/35v. It succeeded in preventing the problem but also reduced the trem affect a little. I removed it but if your effect is strong enough you may be ok. Or use a larger value cap.

Disconnect the outside wire on the trem intensity pot that does NOT go to ground. Attached the + side of the new cap to that terminal. Attached the loose wire to the – side of the cap. DC leakage blocked.

I had about 22vdc on the + side after it charged. A cap this high, with a voltage over 35 would be physically large. No need to use a higher voltage one unless yours moves somewhat close to it’s rating.

Tried this with a 3300uF/35V cap and the problem still persists.

Is this normal for the Vibro Champ trem circuit to do this?

It's normal for the VC to reduce volume somewhat when you crank the Intensity control. However, the addition of the cap, in the correct location, should have eliminated all of the "problem." I suspect there may be a different issue or more than one issue.

Measure the cathode voltage of the triode you are modulating at different Trem-Intensity settings. Wait for it to stablize each time. Do not plug a guitar in. What are those numbers?

[M Fowler]

ChrisM

OK I am still interested if this amp ever got fixed?

Mark