Fender Bassman 50 bias high

[vintagekiki]

Yes, but it only gives you the sum of all power tube plate currents.

Just as idle current measure by principle "tube-by-tube" using bias test adapter, or temporary resistor (sensor) montage in cathode, so as same principle (tube-by-tube) measure idle current using temporary resistor (sensor) montage on center tap OT primary.


Bias test adapter

[Helmholtz]

Just as idle current measure by principle "tube-by-tube" using bias test adapter, or temporary resistor (sensor) montage in cathode, so as same principle (tube-by-tube) measure idle current using temporary resistor (sensor) montage on center tap OT primary.

While your translation is almost impossibe to understand, I think you mean to only insert one power tube at a time.

Yes that would work here, though I expect some (small) effect of increased screen voltage.

It would not work with a cathode biased amp.

I still think the OT DCR method is quite useful if done carefully. I.e. measuring DCR while the OT is warm and shorting the secondary during measurements.

[vintagekiki]

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[KCman]

I didn’t get time with the amp yet. However, I checked into my tester and found it isn’t working correctly. The bias probe is probably fine. It’s one that puts your meter in series and measures current.

time for a new meter. It still works for resistance though. But the voltage is reading high and I bet the amperage is off as well. I tried new batteries. Didn’t work.

I’ll post the results. Thanks

[Leo_Gnardo]

measuring DCR while the OT is warm and shorting the secondary during measurements.

Definitely an important factor when one wants an accurate measurement. Resistance of the copper windings noticeably increases from room temperature after an amp has been in operation say 30 minutes or more.

Some techs are in a rush - can't wait for a proper warmup. "Boss wants me to fix a dozen amps today or else I'm fired!" And others aren't much interested in accuracy, close enough will have to do.

Of all the repair techs in this area, I think I'm the only one who bothers to check bias current. Some local musicians have gone to the expense of buying their own bias probe and the trouble of learning how to use it. Over the course of the last 40 years - when bias probes became available - I've encountered a grand total of . . . 3. One of them since moved to the LA/Hollywood area and has become a go-to amp tech there besides being a busy session player.

[vintagekiki]

Resistance of the copper windings noticeably increases from room temperature after an amp has been in operation say 30 minutes or more.

Therefore, measuring or setting idle current is unreliable.

[Helmholtz]

Please be aware that your translating machine often produces ridiculous and incomprehensible texts. Others have told you before.
Otherwise you'd never know because it might work ok for you, i.e. from English into your native language.
I would never use something like that.

No sense defending a bad tool.

Also some of your comments sound rather insulting and I wonder if this is intended .

[vintagekiki]

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[Leo_Gnardo]

Also some of your comments sound rather insulting and I wonder if this is intended .

I wouldn't take it personally. And I also have trouble sometimes figuring out what comes out of VK's translator. I have to give VK a lot of credit, he's always trying to help. And he's contributed a great number of schematics & manuals, many rare & obscure. Must have an enormous library.

Helmholtz & VK, I have respect for both of you.

One question remains of our friend VK. To clear up the mystery, and help us understand better, what is your native language?

OK, on with the show. I'm glad to see our OP has checked in after several days of commentary (we haven't scared him away) and I'm interested in seeing how the case of this Bassman 50 develops.

[Helmholtz]

Definitely an important factor when one wants an accurate measurement. Resistance of the copper windings noticeably increases from room temperature after an amp has been in operation say 30 minutes or more.

That's why I mentioned OT temperature.

An OT may reach equilibrium operating temperature after 30 minutes or so and it takes at least the same time to cool down to room temperature after turn-off.
So you have a few minutes to measure correct DCRs

[Leo_Gnardo]

That's why I mentioned OT temperature.

An OT may reach equilibrium operating temperature after 30 minutes or so and it takes about the same time to cool down to room temperature after turn-off.
So you have a few minutes to measure correct DCRs

I'm with you 100%. Rare that I'm in a rush. Letting amps warm up properly is standard practice here.

Even after I "think" I have an amp sorted out, I'll let amps run in another room for hours, just to make sure there's no long term bugs. And sometimes there are. In one difficult case I ran an old brown Fender Concert in my bedroom. Sure enough at 3 AM it started making a racket just like its owner claimed it did. Early start to the work day. But I did witness the problem when nobody else had the patience to observe long term, and I was able to solve it. Bad preamp tube, naturally. Just took an extra long time to warm up and sing its annoying song - shhhhhhhqueeeeeek!

[KCman]

Finally got back to the amp and all is well. Plate v was only 476v. The bias current was a bit over 70% so I lowered it quite a bit. Sounds good. Super quiet. It was the meter giving high readings. (After fixing the initial fuse blowing problem)