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Old tube guitar amp repair question
Old 2nd March 2014
  #1
Lives for gear
 
3 Reviews written
๐ŸŽง 5 years
Old tube guitar amp repair question

Hey
I've got this strange old amp, a lifco model 630-2. Nice old amp, sounds great now that it's been recapped and re tubed and added a 3 prong plug.
There is one problem remaining, whenever the treble control is pushed past 8, the input distorts and there is an additional oscillation noise coming thru the speaker. While interesting, it's definitely not desired!

Any idea as to the cause? I'm thinking maybe the cap that's connected to the treble pot is going south? It would be c7 510pf on the schematic, link provided.
http://web.ncf.ca/ac151/LIFCO630_2_Draft03.pdf
Old 2nd March 2014
  #2
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Richard Crowley's Avatar
 
๐ŸŽง 10 years
The symptoms indicate high-frequency (maybe even ultra-sonic) oscillation (feedback) with extra high-frequency gain (i.e. the treble control turned-up).
Did you replace ALL the caps? Did you accidentally get one of the caps replaced with the wrong value?
Did the wiring (particularly WHERE the wires are) get disturbed? (to cause increased accidental coupling between the input part of the circuit and the output)
Did the tubes get replaced with the same types?
Old 2nd March 2014
  #3
Deleted User
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Quote:
Originally Posted by enginefire โžก๏ธ
There is one problem remaining, whenever the treble control is pushed past 8, the input distorts and there is an additional oscillation noise coming thru the speaker.
You have a parasitic oscillation. It can occur when an output coupling cap is too close to the grid of the previous stage. Check the new caps you put in to make sure they are not in close proximity to one of the previous grids. Bend them into a position where they are isolated from other wiring in the circuit. Keep the new caps at least a half inch away from adjacent wiring. And don't let one coupling cap physically touch another.

If that doesn't work, sometimes you can resolve the problem by reversing the offending coupling cap, causing the shield of the cap to reverse electrical sides of the circuit. Usually you want the shield (i.e., outer most foil layer of the cap) to be connected to the low impedance side of the circuit, and the inner conductor of the cap to be connected to the high impedance side of the circuit. Why? Because a high impedance circuit is subject to picking up stray signals thru small stray capacitance, meaning high impedance circuits need more shielding.

Finally, if all fails, install 1k ohm resistors directly on each grid in each preamp stage. Install a 1500-5000 ohm resistor on each grid of each output tube. These are called grid stoppers and increase the resistance for low level parasitic signals thereby killing them without significantly effecting the audio signal.
Old 2nd March 2014
  #4
Lives for gear
 
3 Reviews written
๐ŸŽง 5 years
I changed out the caps a while back, maybe close to a year ago, before this project got side lined. I had my tech give it the once over quickly to ensure that everything looked safe, he gave it the green light.

Do I remember if I changed all the caps? No, but I probably did, I'm thorough that way. I only replaced EL caps.

It was doing this before I re tubed it, which I did yesterday. Retubing fixed every other problem I was having with this amp (also swapping the ax7 for an at7 really helped the reverb channel).

I never noticed that it had no grid stoppers! Next time I get a chance I will check re the capacitor placements.
Old 2nd March 2014 | Show parent
  #5
Deleted User
Guest
Quote:
Originally Posted by enginefire โžก๏ธ
I never noticed that it had no grid stoppers! Next time I get a chance I will check re the capacitor placements.
I've run into this problem before. Move the caps around a bit as a first attempt before adding grid stoppers. That works in most cases. Most amp manufacturers put grid stoppers on the output tubes and not the preamp tubes. So at least add them to the output tubes because it's very unusual not to have them there. Grid stoppers need to soldered directly to the tube socket with as little lead as possible to minimize pickup after the resistor. And if all else fails, add them to the preamp tubes as well.
Old 3rd March 2014
  #6
Lives for gear
 
3 Reviews written
๐ŸŽง 5 years
I'll check my parts bin, I'm pretty sure I have some 1k 3watt resistors I can use as grid stoppers for the power tubes. Might be a while before I do this though so don't be discouraged if I bring no news on this for a week or so!
Old 3rd March 2014
  #7
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Richard Crowley's Avatar
 
๐ŸŽง 10 years
There is virtually zero current (= 0 power) flowing there. You don't even need 1/4 watt resistors. In fact, physically larger components just create bigger "antennas" and may increase the tendency to oscillate.
Old 9th April 2014
  #8
Here for the gear
 
๐ŸŽง 5 years
I'm the one who drafted the referenced schematic.

Most of it was validated against what seemed to be an unmodified Model 630 - but some other areas (esp. the interfaces with the reverb, which don't exist on the 630) were educated guesses as the unit I had was heavily modified prior to my ownership.

If you do have a Model 630-2 on hand, it would certainly be appreciated if you could fill in some of the blanks from the existing schematic (and I'd then update the on-line copy so that others would have access too).

The 'iffy' bits which could benefit from a review/check are:

- the interconnection of components in the treble and bass control sections

- the value of components in the treble and bass sections

- the value and placement of the c17 and c18 caps (currently shown as "?pf")

- the interconnection and component values of the items in the reverb channel (most of this was missing from the unit I had - so components and values were best guesses).

- the details of where the reverb footswitch control connects into the reverb cct

My direct e-mail is included in the schematic ([email protected])

Thank you,

David Clarke
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