Oscillating point to point EQP-1A clone

[MrMegaEli]

Hey all,
I built a p-t-p pultec eqp1a. It worked well, but the grayhill switches didn’t seem so good to me, so i changed the low & high frequency switches to old ones.

I’ve heard that its possible to make a pultec oscillate so you can test it, and that seems to be the case. (I took the opportunity to go through it and the filter seems to work). With no audio running through it if I boost at 20 Hz, I get a 20Hz tone, boost at 60Hz, I get a 60Hz tone, etc etc

Any ideas on what causes oscillation in pultecs? It wasn't oscillating before, but than I put in the new switches.

Eli

 

[CJ]

grayhills are supposed to be good, what was the dis satisfaction?


positive feedback causes osc.

so phase got swapped somehow,

which circuit are you using,

try flipping the wires that come off the switch, that is, the top and bottom, leave all the taps

 

[lassoharp]

Try reversing the leads on your OT primary.  That's a common cause of oscillation in those as the phase has to be correct since the NFB is taken from the OT sec.  Does it sound like a dentist drill or a mad swarm of yellow jackets?

 

[MrMegaEli]

grayhills are supposed to be good, what was the dis satisfaction?


positive feedback causes osc.

so phase got swapped somehow,

which circuit are you using,

try flipping the wires that come off the switch, that is, the top and bottom, leave all the taps

the grayhills snapped pins! I took them apart, must of not put them back together correctly, so I got the old ones.

Leave all taps on the switches? Do you mean commons on the poles?

Try reversing the leads on your OT primary.  That's a common cause of oscillation in those as the phase has to be correct since the NFB is taken from the OT sec.  Does it sound like a dentist drill or a mad swarm of yellow jackets?

Doesn’t sound like either, sounds like a synth! I’m assuming its the switches, since there was no oscillation before I switched the switches.

 

[CJ]

at this point, we need a circuit diagram.

 

[MrMegaEli]

1

 

[MrMegaEli]

2

 

[Jeremy.Starseed]

My Rock Ruepel tube compressor oscillates wildly when the inputs are unconnected. It sounds really cool  :

 

[MrMegaEli]

both inputs are connected. PS the rockrupels have maybe the coolest VU meters ever!

 

[CJ]

no wonder, thats a pat moreford schemo

:-X :-\ :'(

we have better circuit diagrams now, check gyraf site,

we need the whole enchillada, line amp could be causing problems , neg feedback, etc

 

[lassoharp]

no wonder, thats a pat moreford schemo

;D    . . . been there.  You got it.

 

[lassoharp]

both inputs are connected.

Just to make sure - have you tried disconnecting the passive section from the interstage primary - just to isolate the amp section and make sure it's stable?

If you rewired the replacement switches the same as the grayhills it might point to some odd situation with a faulty switch.  I can't think of what that might be but as far as I've read you've got:

Grayhills = stable circuit

Replaced Grayhills with exact wiring = oscillation

Another possibility is a bad tube.  I've seen those 12_ _ 's oscillate in guitar amps and it was apparently just the tube.

 

[MrMegaEli]

The only way the amp would cause problems would be a bad tube, I tested the amp section alone awhile ago. (with some help though). Here’s something else I noticed, with all controls set flat, the more I boost low frequencies, the frequency seems to change a bit, even though I’m leaving the freq. select @ the same place