harmonic vibrato questions

[uki]

Does the tube frequency(range) matter for the vibrato to work?
Or could any 12ax7 tube work with the vibrato, is there any restrictions on tube specs ?

I did gather the following info about frequency from the vibrato, at the intensity wiper, 3.5Hz to 9.3Hz,
intensity at max read 3.5Hz and with the speed at max 9.3Hz.

Thanks in advance!

[shooter]

I believe the only real "concern" is getting the Osc to jump-start and stay running.
the freq range you posted is about the range i'm used to seeing

[uki]

Thanks shooter !

I forgot to mention it is the Fender 6G8 vibrato.
Somehow I got the vibrato working after removing and testing components and refluxing solders!!!   

It is working with a 12ax7 but it is very weak and there is hum, intensity at 9-10 it is there, but when turn it to 8 it is gone. Btw the speed is 3M pot, couldn't find 4M.

With the lesser gain tube(12au7) which the hum disappear the vibrato also disappear. So I'm at the crossroad here, With this new info it is probably possible to fix it, what can be done?

Thanks in advance!

[sluckey]

That oscillator circuit requires a strong 12AX7 to work correctly.

[uki]

That oscillator circuit requires a strong 12AX7 to work correctly.

Well i tried only one of the old tubes on V5, probably I should try others.
But still there is the hum problem...
Which modern tubes are more reliable and are strong enough for the vibrato?

[sluckey]

Any good 12AX7 will work. There are six 7025s (12AX7) in that amp. Use the tube from the normal channel or any of the others.

[uki]

Any good 12AX7 will work. There are six 7025s (12AX7) in that amp. Use the tube from the normal channel or any of the others.

I got 3 other tubes other than the JJs that were first used in the amp, 2 Sovtek and one old Miniwatt(sounds really good!).

All of those 3 tubes the vibrato does work but with same results, with speed at zero and intensity at past eight, only then the effect become noticeable.

Since this is the first time I ever heard this harmonic vibrato its unknown to me how it should behave, but to me the speed seen to be to fast at zero, something like 140BPM.

With any of those tubes the hum comes live, its a 120hz frequency(i may be wrong).

[sluckey]

140BPM is 2.3Hz. That's about as slow as that circuit can do without a lot of tweaking. The speed range is typically 3 to 10Hz.

Is this a real factory built 6G8 or did you build a clone? If a clone, did you use a 10M-RA pot as called for by the original schematic? The intensity pot issue could just be a matter of the correct pot taper.

You can make the tremolo stronger with a wider speed range if you replace the 4.7K||25µF that's connected to V3 pin 3 with a red LED (cathode to ground).

[shooter]

its a 120hz frequency

verify your shorting jack is shorting
120hz typically indicates a sensitive circuit (your trem Osc) is in close proximity to "dirty power", or large signals

[uki]

Is this a real factory built 6G8 or did you build a clone? If a clone, did you use a 10M-RA pot as called for by the original schematic? The intensity pot issue could just be a matter of the correct pot taper.

This is a clone, the schematic(attached) says 4M-RA, when I build the amp(2015) the only one i could find was 3M-RA from Hoffman, this:

CTS amp pots
The 50KRA and 3MRA pots are used in Fender tremolo and vibrato circuits.

You can make the tremolo stronger with a wider speed range if you replace the 4.7K||25µF that's connected to V3 pin 3 with a red LED (cathode to ground).

Very interesting ! Any red LED? Does size matter? I got 3mm and 5mm.

its a 120hz frequency

verify your shorting jack is shorting
120hz typically indicates a sensitive circuit (your trem Osc) is in close proximity to "dirty power", or large signals

Going to look into that. But I'm a bit lost with the shorting jack is shorting 

Just for reference, here more info about the amp:
https://el34world.com/Forum/index.php?topic=19371.0

[sluckey]

The intensity pot must be 10M-RA. Is that what you have?

Use the 5mm red LED.

https://www.tedweber.com/wp106ra/

[uki]

The intensity pot must be 10M-RA. Is that what you have?

I got 3MRA on speed and 10MRA on intensity. What RA does ?

Sorry I wasnt precise. Edited.

[sluckey]

RA is Reverse Audio taper. So, you are using a 3M-RA for the Speed and another 3M-RA for the Intensity. I suggest you use a 10M-RA for the Intensity in accordance with the schematic. Hoffman used to sell them back before he semi-retired. I already posted a link to Weber for the pot. You may be able to find the 10M-RA somewhere else.

[shooter]

the schematic shows the input jack as a shorting jack (when the guitar plug is NOT inserted, the jack will short tip to ring) that helps stop stray signals from getting in while nothing is plugged in.

[kagliostro]

In absence of a specified pot, sometime is possibile to find a solution

not always, but .... read here

http://www.geofex.com/article_folders/potsecrets/potscret.htm

Franco

[pdf64]

In absence of a specified pot, sometime is possibile to find a solution

not always, but .... read here

http://www.geofex.com/article_folders/potsecrets/potscret.htm

Franco

That can work if the change in load impedance (as the pot is adjusted over its range) on the preceding stage can be tolerated.
But if, as here, the potentiometer is being used as a variable resistor (as opposed to a potential divider), it’s not applicable.

[uki]

So, you are using a 3M-RA for the Speed and another 3M-RA for the Intensity.

AH nono, only the speed is 3M. The intensity pot is 10M, it was probably bough from Hoffman or Weber years ago.

@shooter , jack tips are shorting to ground

[PRR]

Pots are a vanishing technology. However there is some slack in this circuit, and a 3Meg pot *may* work for the Intensity if you change other parts.

There is a 10Meg fixed to 10Meg pot divider; change to 3Meg fixed to 3Meg pot divider. Or round-down to 2.2:3 to give a little more drive. That makes more load on the cathode follower but it is far from its limit. Change 470k to 220k (or tack another 470k directly across the existing 470k). Juggle the cap values for the lower impedance.

[uki]

You can make the tremolo stronger with a wider speed range if you replace the 4.7K||25µF that's connected to V3 pin 3 with a red LED (cathode to ground).

I did not had the chance yet to fiddle with the amp and try the LED, but I will soon.

3 questions

1 - What is the trick with the LED?

2 - What happens if the cap value is changed, up or down?

V3 circuit
3 - A friend suggested lowering the 10M resistor, and PRR did point that way as well, but since the intensity pot is 10M, how low can that 10M resistor go? If that is possible.

Thanks

[PRR]

> how low can that 10M resistor go?

Until the cathode follower strains. That's why I changed a part there also.

We could make it a 1k pot, with a 6080 to drive it, but that's not needed. Apparently you can get 3Meg pots and what is there can be modified that much easily.

[shooter]

1 - What is the trick with the LED?2 - What happens if the cap value is changed, up or down?

The led replaces BOTH the cathode Resistor AND cap


the LED "sets" the tubes bias, typically .6 to 1.2VDC (biased hot)
I believe it also shifts the frequency knee to DC where the cap/R would limit the knee above the OSC freq