Bassman - no headroom

[lowell]

i have retubed, and recapped bypass and coupling caps, (not the tone caps) this amp. The bias is 35ma on the 2 6L6. All voltages on the tube pins of preamp and power tubes are correct per schematic. I even performed all output transformer tests per the geofex article and the OT seems great. This amp is dirty at 2 on the volume. when i put the volume at 6 the amps produces a high chirping/beating oscillation noise. FYI this is into an 8ohm speaker. It does this in both channels. FYI the wires from the inputs to the grid of first tubes are NOT shielded, I know the amp could benefit noise-wise from shielded wires here, but will that fix the headroom issue? I wouldn't think so. Anyone experience this? Oh and this amp has a variable mains voltage selector which cuts out the power completely if it's jiggled hard... just fyi.

[lowell]

ok i'm a bit confused about one aspect of OT testing. what ohmage should the secondary on this amp measure? I get 4.5ohms w/ secondary disconnected at speaker jack. Is this good/bad?

[52 Bill]

when i put the volume at 6 the amps produces a high chirping/beating oscillation noise.

Perhaps this is the reason for your power loss, distortion, etc.
The amp may have an ultrasonic oscillation that you can't hear, but is causing the amp to distort in the audible frequency range. Try scoping the output signal to look for an oscillation.

As for your other question, if you are reading the secondary of the output transformer only, I would think that normal range of these transformers is closer to 1 ohm dc resistance or less. What reading do you get when you short your meter leads together?

[lowell]

Bill,
thanks for help... the meter leads shorted measure 4.3ohms... I should have checked that more accurately. So this gives a OT secondary resistance of .2ohms. What can we gather from this?

[52 Bill]

So this gives a OT secondary resistance of .2ohms. What can we gather from this?

This is, in my opinion, a normal resistance reading for the secondary winding.

Was the amp doing what you describe before you did the re-cap?

[lowell]

yes it was doing this before. i will check out the sine wave o-scope and report back.

[lowell]

Ok,
I've got the o-scope on it and there is definitely a sine wave showing up at ALL volume levels. The o-scope setting is at 20mv and 5us and the wave is at a stand-still with 2 peaks showing across the screen. With ALL knobs at 0, there is the wave. If only the Normal channel volume is increased, the sine wave disappears around 3 and reappears at 9. If the Treble on the Normal channel is maxed out, the wave exists throughout the Volume sweep however it changes form during the 3-9 part of the rotation. What does this tell us?

[stokes]

As Bill pointed out,you have a high frequency oscillation problem which would definately cause the problems you have.The fact that the volume and tone controls affect it indicate it is originating at a point prior to the volume control.The fact that it is there with the knobs at zero could indicate it is coming from the power supply.Check the 8uf cap in the B+ rail on the first preamp tube for ac voltage.You should see only .0XX vac here.

[Jag]

Other than recapping it, have there been any other changes?

[lowell]

I added a cathode/fixed bias switch, however the switch is currently in the stock fixed bias mode and I've double checked it mutiple times to make sure I didn't screw it up.

Ok otherwise I've noticed that the oscillation isn't there unless I have something plugged into the input jack. The volume on the guitar also must be turned up a bit. I have swapped cords and guitars w/ no difference just to be sure. I measured the VAC on the tops of all the anode resistors in the preamp w/ nothing over 10mvac. The meter starts out higher and quickly drops to around that figure. I made sure the meter was on AC setting...??

[52 Bill]

These kinds of oscillations are sometimes caused by bad grounding connections, bad lead dress and even by bad/drifted components.

Try tightening up all mechanical grounds; input/output jacks, pots and even the transformer hardware. Try moving grid wires around, to see if you can change the oscillation. Maybe shielding the pre-amp grid leads will help.

Try and isolate the source by noting which controls change or cause the oscillation to change either frequency or amplitude.

[Rick Erickson]

Look at the AA-371 Schematic and note the 500pF cap across the first stage plate resistor in the Normal channel. Add this capacitor to your amp. I'll bet you removed the local feedback networks on the power tube sockets. (a good thing) This jumped the gain & noise of the amp up around 3-6 db. You can compensate this by changing the 470K feedback resistor on the third (summing) stage to 220K, which will allow you to turn up the volume control a little higher before the amp maxes out. The 500pf cap will eliminate an oscillation caused by a bad layout design, where the output of the second stage of the Normal preamp crosses over the first stage en-route to the summung (3rd) stage, whose output then crosses over the same point again en-route to the PI. Not a good layout, especially if you want to jack the gain up.

RE
Aviator Audio
Edmonds, WA
www.aviatoraudio.com
http://www.myspace.com/rick_erickson

[lowell]

I have tested the grounds on all pots and cathode connections. I added the shielded wiring to the first preamp tubes grids with not much improvement. Here is the exact schematic of the version of this amp as well. When I move the guitar when it's plugged in the amp will go CRAZY if the pickups are close to it, just fyi.

If anyone thinks these are fishy let me know... FYI here are the pin voltages:

Tube 1:
1- 211
2- 0
3- 1.6
6- 225
7- 0
8- 1.6

Tube 2:
6- 266
7- 0
8- 1.9

Tube 3:
1- 210
2- 0
3- 1.8
6- 262
7- 0
8- 2.01

PI:
1- 309VDC
2- 97VDC
3- 122VDC
6- 291
7- 98
8- 123

[lowell]

i just disconnected the phase inverter coupling cap so I could test the preamp circuit by itself and not harm the OT.

I am using a capacitor probe on the 1st preamp tube. The signal out of this tube on the plate is very distorted when I turn up the volume. I'm thinking that if this signal is distorted then there's no hope of clean signal later on. why would this be so distorted in the FIRST stage?

[Rick Erickson]

[why would this be so distorted in the FIRST stage?]

Did you read my post from yesterday? Oscillation can do strange things to a signal. I even gave you the cure, if indeed this is your problem.

RE