Fender Bassman 70 buzz

[Leo_Gnardo]

New matched pair of tubes + Tube matching pot made the buzz go away...

The old tubes were impossible to match with the tube matching pot. Is this because they were too far off from each other perhaps?

Good progress. It's possible one of the old tubes wasn't conducting at all, totally dead. Now I'm concerned why the bias balance pot doesn't seem to function as expected. I would find out why, and see to that first.

I do think it's a good idea to have an overall bias control as you proposed. I've done lots of them, and usually wind up with an 8K2 or 10K fixed resistor in series with 5K or 10K preset (trimpot for USAns).

[JAelec]

I'm sorry but what do you mean by bias balance pot not functioning as expected? It works well with the new matched pair I could easily dial away all the buzz... and if one of the old tubes were dead as you proposed, then the pot wouldn't be to blame. I'm probably missing something...

[eschertron]

The hum balance pot does nothing, no difference in pot position.

Now I'm concerned why the bias balance pot doesn't seem to function as expected. I would find out why, and see to that first.

I'm sorry but what do you mean by bias balance pot not functioning as expected? It works well..

..We are confused, that's all..

[g1]

The 'hum balance' pot and 'output matching' pot are 2 different things. From what I understand, the 'matching' pot works but the 'hum balance' does not.

[Helmholtz]

.. but the 'hum balance' does not.

It might only show some effect with an input tube having poor heater to cathode insulation.

[JAelec]

Checked the bias with transformer method

V5
Resitance plate to center tap: 44.4 Ohms
Vdrop: 1.9 V
Plate voltage: 469.6
Plate current: 0,043 A
Plate dissipation: 20,1 W
% of max: 67 %



V6
Resitance plate to center tap: 53.3 Ohms
Vdrop: 2.3 V
Plate voltage: 471.3
Plate current: 0.043 A
Plate dissipation: 20.3 W
% of max: 68%


I recently learned that fenders are usually biased in th 50-60% span. So this amp is running a bit hotter then factory. The 33k resistor on the tube matching pot has been replaced by. someone to a 22k. Will being at 67-68% of max dissipation be bad for tube life? And will it make the amp sound too cold?

Thanks for your input!

[g1]

Yes it will reduce the tube life when compared to lower idle current. The hotter idle current will not make the amp sound 'colder', rather the opposite.
I would recommend changing that resistor back to stock value.

[JAelec]

Yes it will reduce the tube life when compared to lower idle current. The hotter idle current will not make the amp sound 'colder', rather the opposite.
I would recommend changing that resistor back to stock value.

A hotter idle current increases clean headroom, so there is less likelyhood of clipping/compression… Since clipping and compression increase sustain and overtones and is percieved as warmth one could argue that the amp sounds colder with a hot bias current.


Perhaps I’m missing something.

[Helmholtz]

A hotter idle current increases clean headroom, so there is less likelyhood of clipping/compression… Since clipping and compression increase sustain and overtones and is percieved as warmth one could argue that the amp sounds colder with a hot bias current.
Perhaps I’m missing something.

Clean output before clipping essentially stays the same.
Hotter bias increases power tube gain so it takes less grid drive for full output. This could be interpreted as less clean "headroom".
Also hot bias increases compression.

[g1]

A hotter idle current increases clean headroom, so there is less likelyhood of clipping/compression

I'm not sure where you are getting your information, but some of it seems opposite of tube operation. If you provide sources we could see if there is some misunderstanding, or just plain wrong information.
Like Helmholtz said, hotter bias increases tube gain, which gets you to power tube breakup sooner. It does not increase headroom. Also starts compressing sooner.
As far as headroom at the power tube grids, colder bias (more negative voltage) allows more peak signal before grid clipping.

[JAelec]

I got the info from an "Uncle Doug" video...

"Biasing Single-Ended Amplifier Output Tubes: A Basic Explanation and Demonstration"
https://www.youtube.com/watch?v=L3rRk3eSTnA

at 4:13 he explains about how hot and cold bias effect headroom...

Thanks for taking you time.... I would really like to understand this.



May I also ask another question in parallel...

I'm having a bit of a hard time with the buzz in this Bassman amp still.

Until the power tubes have warmed up there is SEVERE buzzing for quite a while, the amp sounds like it is completely broken...then it fades away as the tubes have warmed and is almost gone but not dead silent as I would like it to be.

I have the matched set of tubes installed... I adjust the tube matching until the buzz is gone... but I find it comes back after restarting the amp. It's not loud but noticeable, like the bias pot has been nudged a tiny bit somehow. So I adjust the tube matching once more. After restart I have to adjust again... I feel that I can't trust the amp to stay completly quiet. I feel that the balance of the bias circuit or the tubes is very delicate and the pot has to be spot on to for the buzz to dissapear.

Perhaps I do need to change a filter cap after all to lower the buzz that the power tubes have to deal with.

[Helmholtz]

I got the info from an "Uncle Doug" video...

"Biasing Single-Ended Amplifier Output Tubes: A Basic Explanation and Demonstration"
https://www.youtube.com/watch?v=L3rRk3eSTnA

at 4:13 he explains about how hot and cold bias effect headroom...

He's simply wrong. Doesn't even explain where the borderlines come from and why they would change.
In reality the borderlines are given by supply voltage which at large output doesn't depend on bias.

[g1]

I have said this before here, but Uncle Doug is not a reliable source of information. Some times he is correct, but other times he is very wrong. It is a bit disheartening that he has become such a widely attributed source. (ex. https://music-electronics-forum.com/...544#post951544)
He did go back and note his error in the video description: "My comments regarding "hot bias = lots of headroom" are wrong. Hot bias = Early Breakup. Cold bias = Lots of Headroom"

As far as the buzzing, all adjustments should be made once the amp is fully warmed up. The matching may not be valid as the tubes warm up, but should get there fairly soon. If there is horrible buzzing while the tubes warm up, there is probably some other fault. Maybe a slightly leaky coupling cap from the PI could do that.

[The Dude]

The buzzing could also be a defective filter cap that starts working as it slowly charges. Try temporarily tacking a cap across each filter cap to see if one might be causing the hum.
As far as bias effecting headroom, it's B.S as has been stated. You need to understand what tube bias accomplishes. Bias sets up the idling condition of the tubes and decides how much signal is needed to get the tubes into a certain area of conduction. If tubes are biased at cutoff, it will take more signal to get them conducting than it will if they are already biased on. If biased too low, you will get crossover distortion because tubes are off near the crossover point. If biased too hot, tube life is unnecessarily shortened. None of this has anything to do with maximum power output or "clean headroom". As Helmholtz stated, it is primarily supply voltage that decides max output or "headroom".

[Helmholtz]

Actually the increase in "headroom" with cold bias is rather small, maybe 20% (or 1.6dB).
As a player I mainly notice a lack of "headroom" with amps not providing enough power/SPL for the band context (drummer, room size, open air, etc.)
But power output doesn't change with bias.

Nevertheless a little hotter bias can sound better with some amps/tubes due to increased compression and other involved effects.