Bias shifts after playing

[osing]

Howdy,


I built a Tweed Bassman 5F6-A style amp with several switchable gain mods. After biasing the Tung Sol Reissue 5881 tubes using bias probes, I left the probes plugged in and played the amp at full volume and noticed that after a few minutes of playing relatively hard,when I stopped playing, the bias current had shifted. On the one tube (V4), the current went up, and on the other (V5) is decreased. The initial readings were


V4 plate: 428V
V4 idle current: 36mA
V5 plate: 424V
V5 idle current: 32mA


The 5881 tubes are rated at 23W so the plate dissipation above can be calculated to be 0.67 for V4 and 0.59 for V5.


After playing at full volume, the values immediately after stopping were:



V4 plate: 420V
V4 idle current: 48mA
V5 plate: 419V
V5 idle current: 26mA


Plate dissipation can be calculated to be 0.88 for V4 and 0.47 for V5.


After a short while of not playing (a few minutes at most), the bias values went back to where they started.


I started wondering if the screen were drawing too much current, so I measured the voltages across the 470 Ohm screen resistors at idle to be ~0.7V which would equate to a screen dissipation at idle of 0.7/470*428 = 0.64W (the screen voltage and plate voltage are very close, so I am using the plate voltage reading here). I tried measuring the screen voltage drop while playing continuous chords and saw voltages around 10V or so. I don't really know how to determine if at these voltages there is too much screen dissipation, because I assume the screen voltages at pins 4 are dropping while I am playing. Assuming the screen voltage at worst case was still around 428V, the 10V voltage drop across the 470 Ohm screen resistor would indicate a 9W or so screen dissipation (10/470*428), which I doubt is realistic as I assume the tubes would be failing quickly. I should point out that I have 6.5W screen resistors and they are not hot to the touch immediately after playing.


I tested the coupling caps from the PI for DC leakage, but there did not seem to be any.


I have swapped the tube positions and still got bias drift, but I believe both bias currents in that case actually increased versus going opposite directions.


I tried 1K screen resistors, and still see the bias drift.


Playing at lower volumes does not seem to cause a drift, at least not after playing just a few minutes.


Anyway, very long story short, is this bias shift an issue, or is this completely natural? On one hand, I am thinking it is heat related and that it is not a big deal since the bias does come back after playing has stopped for a short while, but on the other I wonder if there is something wrong with my bias supply and whether I am causing damage.


One final note - I tried swapping in a brand new pair of 5881 Tung Sol reissue tubes, but after playing less than a minute started I getting some strange sounds. I looked over and happened to notice a blue spark near the silver coated end of  one of the tubes and quickly shut the amp off. I am hoping this is just a coincidence and that it was a bad tube, but I cannot say for sure.


Thoughts anyone?

[eleventeen]

The first set of output tubes you installed, were they brand new? It's certainly common for brand new tubes to "drift" and "settle in" during the first say 100 hours of their life out of the box before they reach a condition of how much emission and how much current will flow with this voltage here and that voltage there, that they will return to over and over again in operation. That would be my guess. Maybe your line volts rose 5 volts and then fell back during the time you happened to be cranking the amp.

[xm52]

When setting the bias, you need to let it warm up in playing mode, standby off, to the point where it reaches a steady state. In other words, it has warmed up completely and component values are no longer changing with temperature. Set the bias roughly and then play it for a while or let the amp sit idle for at least half an hour with the standby off. Then fine tune the bias setting.


If the line power is drifting, it will affect the bias in most amps. You might start off at 120VAC and an hour later it can be 122VAC. When setting the bias, it's a good idea to measure the wall voltage and keep it as a reference.

[tubeswell]

Also, after having been played for a while, the tubes heat up and the bias changes anyway. Which is why its a good idea to bias them on the coolish side if its a regular gigging amp

[osing]

The first set of tubes were not quite brand new - I had played them in the amp for a while, but probably less than 100 hours. I have been trying to troubleshoot this issue for a couple of weeks now and have been able to see similar drifting while playing on many different days, so I doubt it is the line voltage into the house that's causing it. However, I will check the line voltage for reference before and after just in case. Regarding the drifting of new tubes, do they tend to drift toward a different bias altogether, or are you saying they drift while you play but then come back again to the initial bias setting at least for the first 100 hours or so of playing?


If the bias tends to shift as the tubes heat up, perhaps this is indeed what is happening since it seems to occur somewhat consistently as I play loudly and then check the bias again immediately. After playing, the tube surfaces at the middle of the glass are at 250F or so when I measure with a thermocouple and my meter, but I don't know how reliable that reading is nor what a typical tube glass surface temperature is when under heavy load. As long as the bias comes back down again within a couple of minutes of the amp not being played (but still in play mode with standby off), would you think that I should not need to worry?

[eleventeen]

"Regarding the drifting of new tubes, do they tend to drift toward a different bias altogether...."


No, I am saying that if you had a regulated rock solid bias supply and plate supply, with brand new tubes there would be some subtle changes in the operating characteristics of the tube as it went from box to amp over the first some-number of hours of use (that I am not pretending to be able to predict or quantify) as a teeny bit of gas boiled out of the internal metal parts and the electron emission from the cathode changed a bit.


I don't really think you have to worry about this, even the worst case is not beyond the tubes ratings, is it? It's odd, though, how one tube goes one way and the other the other way, I'll grant you that.

[Paul1453]

My recent 6V6 Plexi build was the 1st time I built a fixed bias amp,
so I don't have a lot of experience in this area.

But because I was new at it I, did pay close attention to what occurred in my case.

Here are some of the things I noticed.

If the two tubes were generally closely matched, within 10%, as measured shortly after warming up.
After playing these tubes for a few minutes, they actually closed the gap of performance.

Let's use some numbers to better illustrate what I saw.
1st tube initial reading .355 / 2nd tube initial reading .337
After some playing together readings became .353 & .352
They both came together almost exactly.

Now I swapped in an unknown performer for one of my .352 tubes.
No adjustment of the bias pot made.
Checked the unknown.  Let's say I got .280 on this one.
The .352 tube would now read say .362
After playing for a minute or two, the .352 tube would go to say .367
and the .280 tube would remain at about that level.
If I put the other .352 tube back in for the lower performer, and play for a minute,
both of these tubes would go back to their .352 - .353 levels.

OK, so now I took out the other .352 tube and put in an unknown.
Got a real low performer this time, say .180
The .352 tube would now read like .372
and after some playing would increase even more to say .380
while the lower performer would not increase much, if any at all.

Took the poor tube out and put the other .352 tube in.
The just reading .380 tube would now read like .365
and after a minute or two of playing both had returned to their .352 levels.

So I went on to test almost all my 6V6 tubes this way.
Because I have a basic tube tester, tubes that gave me the same readings on it,
could be significantly (33% or more) different when tested in this bias circuit.

So after a while of testing my tubes, I could make an educated guess on an unknown tube from the other tubes reading.
If my .352 tube's reading rose after putting in an unknown, then the unknown tube was likely weaker than this tube.
In every case, when then checking the unknown tubes reading it was always weaker.
The more the .352 tube rose when initially paired with an unknown,
the weaker that unknown tube was when checked.

After going through my stock of 6V6's, and then putting both of the .352 tubes back in.
After a short amount of playing they both settled back at their .352 readings.

Interesting, for a newbie to this bias stuff like me.   

I haven't tried it yet, but these results have given me some questions I want to test.

If I take two closer matched weaklings, and put them in there, will I get the same type of results?
The two weaker tubes will also tend to bridge the performance gap and meet closer to the middle after warmed up playing??? 

Can a tube that say read .250 with a .352 testing tube,
If it is matched with a .280 tube can it now rise more, so that after playing both tubes read say .265 or even .280???   

[HotBluePlates]

... I built a Tweed Bassman 5F6-A style amp with several switchable gain mods. After biasing the Tung Sol Reissue 5881 tubes using bias probes, I left the probes plugged in and played the amp at full volume and noticed that after a few minutes of playing relatively hard,when I stopped playing, the bias current had shifted. ...

How did the amp sound?

If it sounded fine, you probably would never have known if you hadn't left the bias probe in. This kind of stuff is normal, and as other said, new(er) tubes have more drift until they're burned-in enough to settle down. No set time on how much is "burned-in enough". And even after burn-in, there will still be drift, just less.

Unless it is severe (35mA become 65mA, or 32mA become 10mA), it's probably not worth bothering about.

And your tubes will read a shifted current while you're actually playing, because even when measuring d.c. you'll see a change in the average current level (because of power output & distortion).

As long as your bias voltage on pin 5 is bobbling around unsteady, there's not much you can or should do.

[osing]

The amp indeed sounds great, and I think you're right in that I would not have noticed anything if the bias probes were not left connected.


I have a new set of 6L6GC tubes, so I plugged those into my other amp, a Super Reverb, biased them, and played at max volume for 10-15 minutes. I did not see any idle bias shift at all once I stopped playing, or at least nothing more than a milliamp or two. I then plugged those 6L6GC tubes into the Tweed Bassman amp and repeated the experiment. I have a fairly high voltage PT, so I have five 10V 5W zener diodes that I can switch in between the PT CT and ground to lower B+ and get closer to the voltages on the 5F6-A schematic. A second bias pot is included in high voltage mode so that proper bias can be achieved in both high and low voltage modes. With the 6L6GCs in high voltage mode (plates at 470V or so), there was very little idle bias shift after I stopped playing. In low voltage mode (plates at 420V or so), both tubes' idle bias drifted down a few mA, but no more than 5mA or so from what I could tell. It seems odd to me that there was more bias shift at the lower voltage, but since the bias settled back up again after a couple of minutes, and since the shift was not huge, it might not be worth worrying about. I might try repeating this and playing for a longer period to see if the drift goes beyond 5mA or so.


I am getting a replacement set of 5881 tubes since one of the new ones I had hoped to put in the amp as a comparison to the other 5881s was dead on arrival. I will be curious to see how they behave. These Tung-Sol 5881 reissues are only rated for 400V, so I don't know if perhaps there is more bias shift in these vs. the 6L6GCs because they are operating at or above their spec. Once I get them, I will try them out and reply here with what I see. It sounds like I don't have too much to worry about except perhaps maybe I risk shortening tube life somewhat. Then again, I won't always be playing with the volume maxed out.


I do have a question about the zeners, though, but I will start another thread on those. They are getting very hot, yet I don't see them running outside of spec in terms of wattage.

Paul - your observations seem pretty interesting. Just for clarity, the values you are referencing, are those voltages or current?

[Paul1453]

I only had 10 ohm resistors for my cathodes.

Those were mostly actual voltage readings.

With 10 ohms you shift the readings you get with a 1 ohm resistor over one place.

Basically my .352 VDC indicates 35.2 mA of current, with the screen current included in that.

So, if I have the Wattage calculation correct I get 325V x 32mA = @ 10 Watts output.
Actually, I'm pretty sure that is not the correct way to calculate output Wattage.
But, I think that even though it is wrong, it is in the ballpark.   

A rather conservative bias setting.

These are old vintage Bell & Howell and RCA tubes which still test at or above NOS values.
They sound great, and I am babying them to make sure they stay that way.