Plexi 50 Presence circuit has feedback &nbs

[Geezer]

  Hoffman Amplifiers
    > Tube Amp Building - Tweaks - Repairs
        > Plexi 50 Presence circuit has feedback      
 
<< Prev Topic | Next Topic >>  
Author  Comment  
Tiny E
Junior tube assistant
Posts: 1
(2/13/04 1:02 pm)
 Plexi 50 Presence circuit has feedback
--------------------------------------------------------------------------------
 Good afternoon,

It has been a long time since you guys helped me with my Hoffman AC30 project, (which is still cadillackin' by the way).

I came accross a Sovtek Midget 50 head and decided to build a Hoffman Plexi board for it, which I have completed. After I got everything shoe-horned into the sub-sized chassis, everything works as advertised except for the Presence circuit.

When I hook the wire off of the 47K resistor to the speaker side of the 8 ohm tap, I get herendous feedback. I changed the cap on the 5KL pot, and verified the resistance of the 47K on the board. Tone is great as long as I don't try to hook up the presence feed.

Has anyone got any "feedback" about this problem? I realize I could just leave it off, but you know how we are...if it's got a place to hook up the wire, we needs to hook up the wire.

I appreciate your thoughts on this.

Tiny E
 
ganzonimx
Hey get your own solder
Posts: 373
(2/13/04 1:06 pm)
 Re: Plexi 50 Presence circuit has feedback
--------------------------------------------------------------------------------
 Seem like you get positive feedback instead of negative feedback. Have you tried to invert the primaries of the OT?

Cris
 
Tiny E
Junior tube assistant
Posts: 2
(2/13/04 1:14 pm)
 Plexi50 feedback
--------------------------------------------------------------------------------
 I have seen that in other posts, but I don't follow? I have a red center tap on the input of my OT, and 2 black wires. The resistance between red and either black is 70 ohms, 140 ohm black to black. I have one black wire on pin3 of V4, and one on pin 3 of V5 (both power tubes).

When you say invert, do you mean switch the 2 black wires with each other, or do you mean swap the red center tap with one of the black wires? Thanks.

Tiny E
 
ganzonimx
Hey get your own solder
Posts: 375
(2/13/04 1:38 pm)
 Re: Plexi50 feedback
--------------------------------------------------------------------------------
 Swap the black wires on pin 3 of the power tubes, the red wire (center tap) has to stay werw it is.

Cris
 
GroundhogKen
Forum Moderator
Posts: 2263
(2/13/04 2:04 pm)
 Re: Plexi50 feedback
--------------------------------------------------------------------------------
 Yes, swap the two black wires which connect to pin 3 of the power tubes. This will reverse the polarity of the speaker signal and you will no longer get loud, screeching out of control feedback.

Ken
 
Tiny E
Junior tube assistant
Posts: 3
(2/13/04 8:50 pm)
 Plexi Squeal squelched
--------------------------------------------------------------------------------
 Thanks guys for the quick response. I don't entirely follow the theory behind the polarity squeal factor, but it certainly knocked it out.

Now I'm just trying to go literally inch by inch and make sure all the AC wires have as much twist and segregation as humanly possible in what may possibly be the smallest Amp head ever made. This is serious close quarters.

If you're up for another question... since this is the first EL34 system I've ever heard, what do you consider acceptable levels of noise. I have distinguishable 60 cycle, but not obnoxious, and occasional whishing sounds from the input side, again, not terrible, but definitely more noticeable than my AC30...

Thanks again for the advice.

Tiny E
 
ganzonimx
Hey get your own solder
Posts: 379
(2/13/04 9:11 pm)
 Re: Plexi Squeal squelched
--------------------------------------------------------------------------------
 The Mig 50 seams to have the AC heater winding for V2 - V5 that is grounded at one side and a DC heater from a separate winding for the first tube.

How is your heater circuit? Do you still use the DC heater system? and how did you reference the AC heater to ground? like the mig ore with two 100 ohm resistors? You might have a look to Doug's layout in the library for the reference.

Cris
 
Tiny E
Junior tube assistant
Posts: 4
(2/13/04 10:28 pm)
 Plexi Squeal squelched
--------------------------------------------------------------------------------
 I used the transformers in place, using the tap off of the 2 resistors for the heater circuit. I don't know how much difference it would actually make in this thing running the heaters either way.

No offense to anyone's lineage, but the components I utilized that were originally Sovtek, will not go down in history as being the highest quality components.

The next time I run accross a Sovtek anything as a potential mod opportunity, I will probably pass.

Tiny E
 
 
 << Prev Topic | Next Topic >>

 

 Topic Commands
 Click to receive email notification of replies
  
jump to:  Tube Amp Building - Tweaks - Repairs (New)New Product Suggestions (New)Praises - Complaints - Suggestions (New)Buy - Sell- Trade (New)Archives (New)    
 

- Hoffman Amplifiers - Tube Amp Building - Tweaks - Repairs - Hoffman Amplifiers -



Powered By ezboard® Ver. 7.32
Copyright ©1999-2005 ezboard, Inc.