Bassman - no headroom

[lowell]

Hi Rick,
yes thanks for your help, however i posted the correct schematic which is the AA270 and it has the 500pf across the anode resistor, I changed the feedback resistor on the driver to 220k w/ little improvement. any other ideas? I've checked and rechecked everything! Could there be something in the power supply? I checked for AC and there is VERY little, around .004VAC on the preamp C+. Being that the pin voltages are correct for that tube that I isolated and tested the signal out of, the only culprit could be the filter cap for that right? That cap test for little to no leakage, should I try replacing it anyway? Is a leakage test a SURE way to test caps or could there still be something wrong w/ one if it passes a leakage test?

[Rick Erickson]

You're trying to troubleshoot this amp without an oscilloscope and signal generator?

[lowell]

Rick,
No, I do have an oscilloscope and a "quick and dirty test oscillator" that I built from geofex site. I however am not trained in using the o-scope. Could you provide me w/ some pointers on what to connect and what to look for etc...??

[stokes]

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?

I already pointed out that the oscillation was most likely in your first stage.Again,check that first cap for ac voltage,as well as the other tips pointed out by everyone.But concentrate on that first stage,that is where it is originating.

[lowell]

stokes,
i posted that the ac on the first filter cap is next to nill. I used my multimeter on ac volts setting. is there something I'm missing? i also tested that cap for leakage and it tested ok. I have measured all ground connections w/ the meter too and all seems good i.e. .1-.2ohms. Is a leakage test and ac meter test sufficient or is there a more thorough test. Should I replace it and see?

[stokes]

Sorry,I missed that part.Where are you measuring the ac?You say C+,I assume you mean B+?Are you checking at the cap on the first preamp tube?Are you checking it while it is oscillating?The oscillation is definately in that first stage,that is why it is distorting there.If you are sure that cap is clean of ac voltage,you could try moving those 68k input resistors and mounting them right on the tube sockets,clip the lead on the 68k very short,so the body of the resistor is very close to the socket pin.Then connect the input with a sheilded wire.Make sure the sheild is only grounded at the input side.

[lowell]

i measured for ac on top of the anode resistor for the first gain stage. Is that the right spot? Also, the oscillation only happens when the guitar is plugged in, amp volume around 8+ and guitar volume at 8+...at least the "audible" osciallation is only then. I thought that this was a parasitic thing that I couldn't hear...? If the inputs have nothing plugged then there are shorted to ground through the jack. Could there still be oscillation if the input is grounded? Should I measure for ac on first stage filter cap while the guitar is in and creating the oscillation? I will try that and test again.

[lowell]

Ok all, I tried another cab and the problem is gone!! I then tried the original cab I've been using that was giving me problems w/ another amp. The cab is 8ohms. It works great w/ the other amp which is an 8ohm output. The Bassman is a 4 ohm out right? So now running into a different cab at 4ohms the problem is totally gone! I would not think that 8 vs. 4ohms would create such a drastic problem... weird. Anyone had this before?

[stokes]

i measured for ac on top of the anode resistor for the first gain stage. Is that the right spot? Also, the oscillation only happens when the guitar is plugged in, amp volume around 8+ and guitar volume at 8+...at least the "audible" osciallation is only then. I thought that this was a parasitic thing that I couldn't hear...? If the inputs have nothing plugged then there are shorted to ground through the jack. Could there still be oscillation if the input is grounded? Should I measure for ac on first stage filter cap while the guitar is in and creating the oscillation? I will try that and test again.

The ac should be measured at the cap in question,while the oscillation is happening.In an earlier post,I thought you said it happened with the volume at zero,no?If occurring with the volume at zero,it would indicate a power supply problem.Some parasitics are audible,some arent.Your cure doesnt make sense,all indications you have given point to the first stage,specifically,but not only the distortion appearing at the first stage.Is it possible that the 8ohm load is somehow changing the frequency of the oscillation,or masking it so you dont hear it now?Is it in fact that you dont hear it,but it is still there?Do you now have normal headroom?You also described earlier that you had the problem with the cap from the PI to the power tubes disconnected,so I dont see how the 8ohm load would change things.

[lowell]

stokes,
they way i discovered the cab issue was that I decided to work on another amp, pulled that chassis, only to find this amp had the same oscillation problem...?? this threw up a red flag to me that it wasn't the amp itself that was the problem. then i swapped the 4ohm cab in and it was great, lots of headroom and NO audible oscillation.... don't know if this answers your questions, but I'm confused as to why it fixed it too. I had reconnected all components (PI coupling cap) before this test and was running the amp in normal operation.

[lowell]

ok this is what might've happened. the cab i use to test most of my amps is a combo amp. i have a jack w/ alligator clips clipped to the speaker leads. I generally leave the combo amp connected to the speaker while performing these tests. Could it be that this wiring of the cab/speaker was causing problems? I'm still getting it happen right now, just for s*&ts and giggles... and it ONLY happens when plugging into combo amp 8ohm speaker through alligator clipped jack. Like I said the combo amp sounds great through this speaker, and the external "test" amp sounds great through it's own cab...??? Also fyi the combo amp is a solid state so there is no "parallel" output transformer issues.

[Rick Erickson]

Also fyi the combo amp is a solid state so there is no "parallel" output transformer issues.

No, just direct coupled output transistors....
You are very lucky that the solid state amp still works.
I would not recommend repeating this experiment, unless you want to become versed in solid state amp repair.

RE

[lowell]

yeah, it still works... thankfully. on another note, is there any harm in running a head on another tube combo amp's speaker via speaker clips with the combo's OT still connected to the speaker, but the amp not connected to power or turned on?

[Rick Erickson]

The outputs of two amps should never be directly connected together in the way you describe. You are forcing the working amplifier to drive not only the speaker(s) but the secondary winding of the combo amp's output transformer. The Primary side of this transformer will look like an open circuit looking for a discharge path. This can arc a winding, a tube, or a tube socket. You should always disconnect the speakers from the unused amp before connecting them to another amp.

RE

[lowell]

ok good to know thanks!