72 plexi vol 1 comes on to loud

[redmojo]

 I have a problem with volume #1 it comes on all at once much louder than volume #2 .
It also has a mild hiss. I have replaced the pot twice also replaced inputs. Resister values in the preamp section seem to be close enough. swapped tubes but still cant control low volume in channel 1.
Any ideas?
Thanks
Bill

[Jennings]

Going from the description only that seems quite normal to me for that amp...I just clip the little cap on the vol pot itself as I don't like that vol jump, or the high treble at low vol it promotes.  Try that and see if it cures what you're describing to some extent. 

[plexi50]

Do you have a pic of the pot and the wiring?

[redmojo]

pic might not show much because i took a mod out of this amp and replaced both wires with purple
even looking right at it i have to wiggle the wires to see that they come up threw the correct holes.
the middle wire goes to 470k
    right wire goes to .0022 cap
    left wire goes to ground.
I have tried it with and with out the bright cap its still the same.
could a bad solder on the tube socket do this?
this is not normal the channel does not realy respond until the knob goes to about 2  way louder than it should. pic attaced

[plexi50]

What is the value of the cathode resistor and or bypass cap if any? Have you tested your preamp tubes (hiss)

[Jennings]

A lot of modern pots do have a taper that seems to give a bit of an all or nothing response, just like you describe.  You could experiment with a NOS pot if you have one to compare...or see whether another manufacturer's pot gives you a taper that suits you better?  That's if you can't find anything wired wrong etc.

[redmojo]

2.7 k = 2.618 (cathode)
100k = 100 k (plate)
1meg jack = 1.034meg
pot is 1meg audio
470 into next stage =.480k
waiting for a chance to shotgun the solders and clean the tube socket

[plexi50]

The only thing i can think of is you have a bad volume pot. Ive seen resistors with the wrong color code on them but never a pot so far. Is the amp in your face at volume level 2? Is the low impedance input lower in volume?

[redmojo]

I would say it is 10 or 15 watts in you face not full blast.
Much worse than  the other channel. The inputs are wired correctly. I did not know
there was a low impedance input, it was never clear to me how the jacks work with that
1 meg resister. Anyway there does not appear to be a low impedance input on channel 1.
I pulled the bright cap out but that did not help much. Is there any way i can change a resister value somewhere to get this channel under control?
Thanks to All  for your ideas.

voltage on v1
pin 1=163
pin 2=0
pin 3=1.04
pin 6=219
pin 7=0
pin 8=1.9

[sluckey]

It's possible that your ground wire for the pot is not zero ohms. Put a gator clip lead between the ground lugs of the two volume pots. Does that help? Also, if the other two pot wires are reversed, the pot may behave odd. Your description says they are connected properly. Double check.

Has the amp always behaved like this? My TRRI does. Is the amp a factory build?

[redmojo]

Ill try the jumper soon and check the other pot.
Yes the amp is a factory build but was modded (i think some kind of a 800 mod). I took the mod out
and I think I have it back to factory specs but obviously something is not working correctly yet. I

[sluckey]

Yes the amp is a factory build but was modded (i think some kind of a 800 mod). I took the mod out and I think I have it back to factory specs but obviously something is not working correctly yet.

Ah, probably the cascade mod? I think the mod is probably not reversed back to stock yet. My only suggestion is to wire it exactly like a 1987 schematic. Shouldn't be too difficult.

[redmojo]

After checking this amp- vol 1,2 ,treb. 100ks, tube socket, 2.7k, 820 ohm ,64ks inputs and 1 meg several times.
I am convinced that it is wired correctly. the info about the low impedance input helped. pot starts at 90-95
db in the low input, 95-100 in the high input. still seems like it jumps right in but maybe that is the nature of the beast. I would post pics but i don't Know how it would help because looking right at it you still have to move the wires to see where they come up threw the board. I am tempted to put ppimv in, I am happy with
the one in my 1987. I here they come in different configurations no what one is said to be the best?
Thanks for the help.
Bill

[Willabe]

quote author=redmojo link=topic=13187.msg123173#msg123173 date=1328749114]
 I have a problem with volume #1 it comes on all at once much louder than volume #2 .[/quote]

Are you sure vol. pot #1 is an audio (logarithmic) taper pot and not a linear taper pot? Even if the markings on the pot says its an audio taper, I'd put a meter on it and see what it reads at 50% rotation. The factory could have mixed up the the resistive track in their parts bins. It's happened before.

That would explain the problem.

I'd switch vol. pot #1 with vol. pot #2 to see if it flip/ flops the problem. Then it's the pots taper.


                   Brad      

 

[Jennings]

A master vol will help in as way, as then between the MV knob and the original vol knob you can balance the amp's vol at low levels.  Have the MV level low, and when you turn the amp on and slowly start turning the original vol knob, the master will be keeping the amp's output low and prevent the in your face hit.