PPIMV and its effect on NFB

[Natec]

Why is it that the NFB loop doesn't do much with a PPIMV installed?

[neikeel]

The negative feedback is taken from the output transformer and then injected back into the phase inverter.

If you are driving the phase inverter (by cranking the preamp, but not the power amp, you get less signal going back into the PI for a given output. The effect of the presence control is diminished too, for the same reason.

[danman]

Like Neikeel said, since you are cutting the signal down to the power amp with the ppimv, you lose nfb voltage. I have read that some guys are making the nfb adjustable with the use of a pot, which will allow you to dial back in the nfb. I need to give that a try sometime to see if it helps.

[Natec]

So , by adding in more feedback when the PPIMV is turned down it might compensate? I hope so. I need to tighten up the low end more. I have a 18 watt TMB and I have a 50 uf bypass cap on the el84's already. Is it safe to go lower ? Say a 20 uf.?

[darkbluemurder]

So , by adding in more feedback when the PPIMV is turned down it might compensate? I hope so.

Only to a point. When the feedback loop collapsed because it does not receive sufficient signal to feed back, adding more feedback will not help.

I need to tighten up the low end more. I have a 18 watt TMB and I have a 50 uf bypass cap on the el84's already. Is it safe to go lower ? Say a 20 uf.?

Yes, that mod would be safe.

A better way to tighten up the low end would be to reduce the coupling caps or the cathode bypass caps in the preamp or the coupling cap into the PI.

Cheers Stephan

[Natec]

Would I notice much difference changing my cathode bypass caps from 1uf to .68uf? Just looking for more bass tightening mods

[neikeel]

Would I notice much difference changing my cathode bypass caps from 1uf to .68uf? Just looking for more bass tightening mods

No you would not, decreasing the PI output couplers might

What value are they now?

[Natec]

both .01uf

[Natec]

I'm kinda wondering if this Mercury OT is letting more bass through? (Axiom 018rs-l )

or should I look into changing my can cap from 32/32 to 50/50 or higher?

[Carbia]

Modern transformers use to deliver more bass than the old ones, at least on my experience.

[danman]

I'm not well versed in the 18 watt circuit but I do know that the EL84's do not have the lows that the 6L6 and EL34 tubes have. I owned an Avatar 18 watt for a short while and I will say that the right speakers and cab played a huge role in the amp's low end response. What are you running your amp through? It would also help if you could give us a rundown on all of the coupling cap and bypass cap values in the preamp section.

[Natec]

For some reason the forum won't let me post any pics? I get the message " sorry, the board attachment quota has been reached"

[neikeel]

For some reason the forum won't let me post any pics? I get the message " sorry, the board attachment quota has been reached"

Use a free host like Photobucket and you can post as many pics as you want.

[Natec]

http://s1055.photobucket.com/user/Natec ... sort=3&o=1

This is the only way I seem to be able to show a picture?