Troubleshoot help for vibrato type problem

[PHOENIXJAZZ]

Hi,
I've rebuilt an old Bogen CHB10a and it works fine with one problem!
Sound is nice, just what I want/expect but when I turn the volume up past about 8 or so I get a vibrato type sound out of it.
I have not run into this before and I am hoping someone with more experience might know where to look.
I've attached the Schematic and a very small mp3 (zipped) file so you can hear what's going on.
I replaced all components with new or NOS for some of the foil caps. Everything was on value when I put it in.

Thanks in advance for any help or suggestions.
Chris

[Brion55]

Possible power supply decoupling problem?

[M Gregg]

Hi,

A few ideas.

Check value of B+ rail when Warble happens does it "dip".

What is S2? Does the problem change if switch positions are changed?

Also what happens if tone control is changed?
Check is C7 cap faulty?

Can you post PSU schematic?


Regards
M. Gregg

[dcgillespie]

Since the schematic says it's modified, it makes it harder to know where to start -- but it sounds like a classic case of inadequate decoupling in the power supply, so that feedback occurs back to an earlier stage when the gain is advanced beyond a certain point. I would use two separate isolated B+ points for the first two stages to confirm if this is what's happening.

Dave

[Wavebourn]

It looks like feedback through power supply, but may be HF oscillation of the 1'st stage as well. Try 68K grid leak resistor.
Add PS filter capacitance in parallel to existing one and check if frequency of vibrato goes lower, or if it stops.

[PHOENIXJAZZ]

Thanks All.
I'll take a look in the places you suggest and let you know how it turns out.

[PHOENIXJAZZ]

Thanks for the help folks,
Replacing the PS filter Caps one at a time fixed most of the problem. Adding the 68k Grid Leak resistor cleaned up the rest but did make it more sensitive to picking up radio signals!
Looks like I have more reading to do.........

Thanks again.

Chris

[Wavebourn]

Originally Posted by PHOENIXJAZZ

Adding the 68k Grid Leak resistor cleaned up the rest but did make it more sensitive to picking up radio signals!

That means, your input stage must be shielded.

Try to cover it by grounded foil to check if it helps, then construct a shield made from thin copper, or even from tuna can metal if you can't find copper.

[trobbins]

You're running grid leak bias on first stage. That may be quite temperamental. I suggest temporarily changing to a standard cathode bias configuration (eg. 1.5k or 2k2 bypassed with 25uF 16V) to see if that is the major influence.

Also use say a 68k grid stopper placed on the input valve base. You may also need to use a shorting input jack if you don't have a low impedance pickup connected, or a phono output stage connected.

As per post above, insert an RC decoupling of the power supply rail from PI stage to input stage.

Ciao, Tim

[Wavebourn]

My bad; I meant grid stopper 68K, not a grid leak resistor.