Clipped the .002uf cap on bassman 10 and got major distortion

[sleepingAwake]

So I thought I would see if I noticed a difference for the better if I removed the cap going from pin 1 to ground. I actually play bass through my amp.

It's a ceramic cap valued at .002uf it connects pin 1 to pin 8 which is grounded. Pin 5 has a 1500ohm resistor going to Pin 1.

I thought it was there just to kill parasitic oscillation on the silver face amps and expected it to be going from Pin 5 to ground.
When I removed them my amp sounded farty and had some strange overtones and distortion.
Threw them back in and it's back to normal.

Do I actually have some oscillation that might be to high a freq to hear or is that the wrong cap to be removing?

[sleepingAwake]

Also when I did this I measured the bias voltage across the 1ohm resistor from pin 8 to ground as .31mv on each tube.
I know thats a little on the cold side.

With the caps back in place it went up to .32mv on each tube. I still have the bias balance circuit in place and plan on going to a "true" bias adjust soon.

[Enzo]

.32mv or 32mv? 32mv is 0.032 volt. 0.32mv is 0.00032 volt. 500 volts on the tube and 32mv across 1 ohm, meaning 32ma current yields 16 watts dissipation. A little cool, but perfectly reasonable.


Yes, that cap stops parasitics, and when you removed it you probably let them back in.

It is a brute force method of killing parasitics though, and there are other ways to tame them less obtrusively. But does the amp sound OK with them?

[sleepingAwake]

32mv so yeah a little cold

I think I made a dumb mistake when I disconnected the cap by not providing a path to ground to the grid resistor when I removed the cap.

It's just that all other silver face schematics show the cap going from pin 5 to ground. The bassman 10 schematic has it going from the other side of the grid resistor (pin 1) to ground.

The amp sounds ok but still has a little bit of breakup on the initial attack of a plucked note.
I'd like it a little tighter.

[pontiacpete]

I think I made a dumb mistake when I disconnected the cap by not providing a path to ground to the grid resistor when I removed the cap.

I don't think you want the grid resister to go to ground, that will kill the signal, no?

[sleepingAwake]

Your exactly right pontiacpete.
I was just looking at the schematic.

I'm still confused then.
I realize the silver face amps were more prone to oscillation due to lead dress issues but mine is a 72 and looks pretty good compared to some later examples.

I think parasitic oscillation of frequencies beyond on hearing range can cause the amp to be running at full tilt and sound distorted when an instrument is played but I'm not sure.
I really did not expect it to sound so bad when I removed those caps. I swore I broke something when I did it.
Luckily putting them back got me right back where I started.

[sleepingAwake]

My bass has active pickups.
Perhaps the overall gain is to high and it's blocking distortion.
Maybe I could eliminate the cathode bypass cap on V2? I replaced it with a 4.7uf instead of 5uf when I recapped the entire amp. I believe this would lower the signal the power tubes will see.

Or maybe going to a true bias adjustment and upping the bias to 35ma would help some?

I know I'm just spitballin and asking a lot of questions and making mistakes.
I'm still learning. I just finished reading Tube amp guitar essentials yesterday. Read it cover to cover in one day.

[pontiacpete]

If makes you feel any better I have on many occasions just removed those caps on SF fenders and never had a problem.
do you have the original bias cap in there?

[sleepingAwake]

If makes you feel any better I have on many occasions just removed those caps on SF fenders and never had a problem.
do you have the original bias cap in there?

Nope. I replaced all electrolytics with either FT or sprague using the closet available values.
I just played again and it sounds good but I can't help feeling there is a slight amount of looseness/distortion to initial attack still. At least compared to my ampeg BA115.

I probably do the bias conversion later today since it's too hot to go out and work on the car.

[sleepingAwake]

Comparing a Silverface Super Reverb schematic to Bassman 10 points out the difference in placement of the oscillation caps.
Anyone care to postulate what sort of difference it would make and perhaps explain why it sounds like crap on a Bassman 10 when you remove it?

http://ampwares.com/schematics/super_reverb_aa270.pdf

http://www.ampwares.com/schematics/Bassman_10_alt.jpg

[Bruce / Mission Amps]

32mv so yeah a little cold

I think I made a dumb mistake when I disconnected the cap by not providing a path to ground to the grid resistor when I removed the cap.

It's just that all other silver face schematics show the cap going from pin 5 to ground. The bassman 10 schematic has it going from the other side of the grid resistor (pin 1) to ground.

The amp sounds ok but still has a little bit of breakup on the initial attack of a plucked note.
I'd like it a little tighter.

Hmmm... 32ma, well, IMHO, that is not really cold. Cold would be like 15ma-20ma.
This whole thing about hotter idle current and 60% or 70% rule is a NET driven idea based on someone else's subjective idea of good guitar tone.
If you are playing bass then you should stick with your ears and figure out what the idle current is with respect to the plate voltage of the power tubes.
I think electric bass sounds best with the power tubes idle at around 50%-55% of plate dissipation rating... but that is me and you are you.
We don't know what your musical tastes are... you might love ugly-nasty bass distortion but active pickups on bass guitars in most vintage amps like a well used 50 watt Bassman 10.... is a mistake unless you really have the gain set very low... it is way too easy to overdrive one or more of the preamp stages with low frequency energy. YUK!

[sleepingAwake]

Hmmm... 32ma, well, IMHO, that is not really cold. Cold would be like 15ma-20ma.
This whole thing about hotter idle current and 60% or 70% rule is a NET driven idea based on someone else's subjective idea of good guitar tone.
If you are playing bass then you should stick with your ears and figure out what the idle current is with respect to the plate voltage of the power tubes.
I think electric bass sounds best with the power tubes idle at around 50%-55% of plate dissipation rating... but that is me and you are you.
We don't know what your musical tastes are... you might love ugly-nasty bass distortion but active pickups on bass guitars in most vintage amps like a well used 50 watt Bassman 10.... is a mistake unless you really have the gain set very low... it is way too easy to overdrive one or more of the preamp stages with low frequency energy. YUK!

Yes I typically turn down the volume output on my bass. Its a Cirrus with an 18v active system.
I noticed today that the Studio channel has a lot more low freq output than the Bass. I was surprised by this as I've always used the Bass channel. It seems as those the entire Studio channel has more gain than the Bass channel.
I can actually feel air moving when using the Studio channel. Not so with the Bass channel.

I like a nice clean pop with a lot of overtones and harmonics. I'm not a fan of distorted bass.

[pdf64]

'Comparing a Silverface Super Reverb schematic to Bassman 10 points out the difference in placement of the oscillation caps.
Anyone care to postulate what sort of difference it would make and perhaps explain why it sounds like crap on a Bassman 10 when you remove it?'
My feeling is that the cap placement on the Bassman 10 is probably better, in that putting the caps on the tube grid terminal creates additional antennae.

'explain why it sounds like crap on a Bassman 10 when you remove it'
The most likely reason is that the amp is oscillating at an ultrasonic frequency.
An oscilloscope is very beneficial to resolve the issues causing this.

'I can't help feeling there is a slight amount of looseness/distortion to initial attack still'
Are you using a 2 ohm cab with it?
The reservoir cap values could be increased, in order to give a more solid attack; at least 2 x 100uF, up to 2 x 220uF should be ok.
Pete.

[sleepingAwake]

'Comparing a Silverface Super Reverb schematic to Bassman 10 points out the difference in placement of the oscillation caps.
Anyone care to postulate what sort of difference it would make and perhaps explain why it sounds like crap on a Bassman 10 when you remove it?'
My feeling is that the cap placement on the Bassman 10 is probably better, in that putting the caps on the tube grid terminal creates additional antennae.

'explain why it sounds like crap on a Bassman 10 when you remove it'
The most likely reason is that the amp is oscillating at an ultrasonic frequency.
An oscilloscope is very beneficial to resolve the issues causing this.

'I can't help feeling there is a slight amount of looseness/distortion to initial attack still'
Are you using a 2 ohm cab with it?
The reservoir cap values could be increased, in order to give a more solid attack; at least 2 x 100uF, up to 2 x 220uF should be ok.
Pete.

The head is in the original combo cab. I replaced the stock speakers with 4 8ohm Eminence speakers.
Measuring the resistance on the the speaker jack I get 11.8 ohms. I believe the working load should drop down closer to 8ohm.

I swapped out the 15k resistor on the bias balance pot today as the old one had drifted to 16k. Not much but enough to limit the bias to 30ma (I got a different reading yesterday??)
With a fresh 15k resistor I get a nice 34.5ma

I just noticed something else. On the Bass channel the "Bass" knob has hardly any effect on the tone. You have to strain to tell the difference between 1 and 10 on the knob.
Maybe that might explain why the Studio channel has so much lower "boom". Like I said the studio channel actually pushes the air around you. The bass channel not at all.

[pdf64]

This schematic http://www.webphix.com/schematic%20h...n_10_schem.pdf indicates that the original speakers were 32 ohms each, wired in parallel to achieve a total of 8 ohms.
How did you connect your 8 ohm speakers?