Working on Bugera 6260 amp - Question about Tube Grade...

[DrGonz78]

I am about to set check the bias on a Bugera 6260 that has 6L6GC output tubes. My question is in regard to how to check the grade rating for these tubes. They are the original tubes and they just say Bugera 6L6GC Premium Selected. Here is a picture...

Google Image Result for http://www.avalive.com/pimages/pimage_176576.jpg

I just need to know which grade to select for suggested bias setting, as labeled on this chart. Probably a simply question but I appreciate anyone's time to help figure this out, thanks.

[Jazz P Bass]

Have you pulled a tube & looked at the bottom of the plastic 'keying pin'?
There may be a designator stamped there.
If you did not change the tubes then the bias should not have to be changed.

[DrGonz78]

Thanks Jazz Yeah I had actually looked at the bottom of the tube last night and must have been in blind mode. Just looked right now and there is a stamped letter D right there the whole time, lol.

They recommend checking the bias every six months, because the vibrations of the amp can cause the silly little potentiometer to move. Silly design but they make it easy to set the bias on the amp via RCA terminal.

[g1]

Now that you have set the bias voltage, it may be worthwhile to find out what kind of idle current that voltage setting translates to.
Then you will have a ballpark figure for biasing non-factory tubes of any brand.

As a side note, it's shocking to see the range of bias voltages (10 volts range) for different grades of 6L6, which I presume are all from the same manufacturer. Just one more example of the poor state of QC for modern tubes.

[Enzo]

Or consider this: what if you never knew there was a chart and graded tubes? What if you just approached it like ANY other tube amp? There is absolutely nothing unique about this amp. It works like any other, and it biases like any other. WHy try to change the simple bias procedure you already know to accommodate their tube marketing scale?

[DrGonz78]

As always you all have the greatest points here and I appreciate them very much. It makes me want to open the amp back up and take some measurements... However in this case I had to replace almost all the caged nuts and it is a nightmare taking this chassis out again. I originally had to fix the melted molex heater connection problem. That is really the main point of view stopping me from opening the sucker up again, and the fact that it is already sold.

If my friend wants to change the tubes to something else then I will go that direction later. As I understand the bias is being set by measuring the vDC from the grids, in the way Bugera is using the RCA terminal. I will post anything else related to this if he wants the tubes changed to something else down the road. Sorry for the let down there as it sounds like a cope out. It would be really great if I had it opened up right now and could contribute more information about this amp for others online. Maybe somebody here has done this already to this amp and can add the related information?

Enzo I wish I would have just did the bias setting while I had it opened too!! lol. Right now I just want this amp gone really. Setting the bias was really an after thought once the amp was put all back together. Never changed the tubes and am doing it only for common courtesy as it says to check it somewhat frequently.

[DrGonz78]

Just talked a bit with my friend and possibly we might go the route of testing the idle current on the amp. I will need some help if I go this direction and I still hate the idea of opening up this amp again. The cage nuts are a real problem as I have to really thread them into the cages if one breaks. My replacements nut cages are not ideal but they are stronger than that crap from China!! Really nobody has documented this before??

Edit: I am talking to my friend about documenting how grid voltage relates to the idle current on this amp. I am sure this can be done! I have no schematic so I might need some help! Not to sound silly but what do I measure on the amp to check the idle current? I want all online to know more about this amp and any amp that I touch! I might just need some help

[Enzo]

The test points are exactly the same as the ones on Peavey amps. All they do is monitor the bias supply voltage.

You can map grid voltage versus tube current, but it will not apply to all tubes. It will only relate if you have predictable tubes. Since Bugera sells graded tubes, they did the work already and made the chart you already have. Unless you will always use graded tubes from some consistent supplier, then all we will really ever know is that -50v will be cooler than -40v.

Isn;t there enough room in the cab for a probe socket under the tubes?

My advice? Set the damned thing in the middle and close it up.

[km6xz]

I have been seeing a number of these coming in since opening a part time shop. I found them to be better than their reputation for build but no matter what the original problem is reported to be, every one has had terribly microphonic power tubes. The ones I have seen to have high gain but just sound unusable, due to microphonics that appear to me to be from very loose or broken supports inside the tube. I get a lot of Chinese tubes and this is not typical at all.
Do the owner a big favor and change the tubes before he has to discover it does not sound right. I have been replacing the stock tubes with similar gain Chinese or if they want to pay more and desire EH, I put those in. None have come back due to power tube problems.

When adjusting bias, setting to a specific current is not solving the two separate demands of the power tube idle current setting: 1: to set a survivable idle current and 2, to set one that does not have high cross over notch distortion unless fulling into clipping. A balance between those two settings will both cause tubes to last and the customer to not complain about the unpleasant intermodulation distortion that even a small crossover notch can induce. Biasing for that compromise requires a scope and low distortion generate at the least. If you have a spectrum analyzer, monitor that and you will see the distortion products that are not harmonically related to the tone suddenly pop up, increasing plate current just a smidge is usually enough to drop them by a factor 10 instantly.

What? No $25,000 spectrum analyzer?
Got a sound card? Then, download this: Visual Analyzer. You will need to build a divider to fit the signal level into the input range of unbalanced sound cards before measuring a power amp 15-30 volts RMS output. The tone generator is very low distortion with sine, sweep and assignable waveforms. If you calibrate the sound card, the measurement readings for volts, db, frequency, distortion, signal to noise ratio etc will be very accurate. With a good audio interface card, 24 bit/96, the band width is 48Khz, and DR is well over 110db with accurate distortion measurements down below 0.0003% Thd. In carefully shielded tests, I could get readings that matched my best Sound Technology digital analyzer, to 0.00008%
Not bad for the total sum of "FREE"

With all the search for tone and talk of magic tubes and caps, the most overlooked and free determination whether a tube type sounds good is whether it is biased optimally. There is a TREMENDOUS difference between one set for published spec and one that is set right for the actual signal performance of those tubes in this circuit. That is pretty much ignored, with people going by rules of thumb for concrete values that can be measured with a voltmeter alone. Set up a two tone test, cancel out the two fundamentals and you will clearly see the big difference in distortion products when varying over a few milliamps of plate current. Setting it with a dynamic signal instead of idle and only using idle current with no signal to cross check whether the optimum bias is providing a safe idle current.

[Jazz P Bass]

You have got to get a 'Kill-A-Watt' meter.
By monitoring the AC line current you can amass all sorts of data.
As an example: In parallel output tube heater amps, you can measure the idle current of output tubes, one tube at a time.
That info along with the maximum clean output voltage amplitude is all that is required to 'grade' a tube.
It is interesting to note that specific tubes that idle at a low mains current also output a smaller voltage. (and vice verse: higher current = higher output voltage).

[km6xz]

I have a current monitor in-line with the variac for full time monitoring if my variac does not have an accurate current meter. Every amp that(or anything) that gets plugged has the current noted in the repair database. I am not sure if I could even try to use a variac without current monitoring. Back home in storage I have Staco IC1010 metered variacs but they were not suitable for here since they are 120 volts. Great big meters side by side, for current and voltage. I did not have a way to get metered variacs here so I got a cheap Chinese 1600 VA voltage metered variac. The meter is small and not finely enough graduated on its scale. I found some very cheap digital panel meters in China which are 4 digit, one for up to 600 volts and the other AC current using a supplied external shunt good for 30 amps at 230volts. These meters are on eBay also for $6 each. Those are lit so I can see them across the room. But for more resolution I got a AC clamp-on current meter which is very accurate and fine resolution.
Running an amp up to 230 volts, and noting the current, before even measuring anything, the idle current reading reveals if all the tubes are lit, or if there are problems with excess or too little current for that model based on the accumulated database of models tested and logged dating back 25 years. I prefer analog meters so I can spot trends easier but the digital meters are cheaper and have higher resolution. You can never have too much relevant data.
I had a rehearsal studio bring an Ampeg amp and I went through it and it measured and sounded great. He gets to his 15 room rehearsal studio and complains that it sounds terrible. I check it again and it tests and sounds great with no change in distortion from the database. He called right away saying I do not know what I am doing because it never sounded so bad with distortion riding on every clean note. I travel all the way across the city to check it out in operation and and before turning it off standby, I checked the cabinet and the clamp ammeter and notices way too little current being drawn. Tested the line voltage and found it 196 volts instead of 230 nominal. His industrial park location was drawing power from a private substation transform a few hundred meters away. I checked the sockets in a building closer to the transformer and found it OK. Traced the problem to one corroded aluminum overhead cable termination that was easy to find, it was the one with a burning smell..

Check mains and verify the settings you use are compatible with the user's source. When he got the mains connection repaired, all the other rooms sounded better also. Such a simple obvious problem overlooked. I should have concluded that just from the symptoms described to me. Meter it, log it, don't assume.