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setting bias using test point - tube V's are different

ohmslaw

Member
Messages
208
I just set the bias in my Super Sonic, using the test points on the PC board. Fender recommends 32 mV at each of the two test points when replacing the power tubes. I'm getting different readings on the two replacement tubes and settled by splitting the difference, one is at 30 mV and the other at 34 mV. As there is no way to adjust the tubes individually, is this correct and is it common to see a so-called "matched pair" of tubes be off by a few mV?

Fender also mentioned checking the power supply voltage at TP9, for a voltage of -54.5 volts. Why?

Thanks for any help.
 

donnyjaguar

Member
Messages
4,199
This sounds about right. If you use a matched set of tubes the readings will be identical. TP9 is the supply for the bias and checking this first will confirm that this part of the circuit is working properly. Failure modes here are generally catastrophic so you'd measure 0V or AC if something were wrong.
 

ohmslaw

Member
Messages
208
There was a recent discussion on this very subject. https://www.thegearpage.net/board/index.php?threads/500575&highlight=tube+matching The general concensus was that it's OK if they're off by a couple mV. The concept of "matched" was another story, as different sellers use different methods to "match" the tubes. Best to find a vendor who does more precise matching.

Mike
Well, that answers that question. "Matched Pair" can mean something quite different from one supplier to the next! Thank you all for your help!
 

rockon1

Platinum Supporting Member
Messages
13,197
No way to idividually set the tubes. 4 ma aint no thang! BTW- I mentioned it on the other thread but I'll say it again. Be careful on those test points . I slipped and blew out a couple of resitors on my SS using those test points. I recommend a probe-its safer for the amp! lol! Bob
 

59Vampire

Platinum Supporting Member
Messages
4,439
Well, that answers that question. "Matched Pair" can mean something quite different from one supplier to the next! Thank you all for your help!


in my opinion, there is only one way you can call a set of tubes matched. And that is when the current draw on the cathode is the same or within a specified percentage. Ebay morons call their tubes matched if the mutual conductance is similar:horse

There are many trains of thought on this subject and since I have an opinion( you know the saying) I thought I would add my 4 cents.

First, most new product amps are designed to have the tubes matched within a certain percentage. There is a number of reasons why, but QC is a main concern. Mass production needs reliability and a big difference in current may cause a huge problem down the road. It is easier for manufacturers to work and design with known parameters.

Not being there, I cant really know for sure, I would think that when tube guitar amps were being built in the early 50s, and 60s, tubes were just taken right off the shelf. A particular type was spec'ed in and away you go. On the opposite end, my Dad swears up and down that when he was young he built an EL84 based Heath amplifier with mullards. He claims that Heath said they were matched. But what the heck does that mean? Heclaims because it was audio, it was a better build.

Now, I would not use a tube that is 36ma and one that is 22ma in the same amp. However, in my experieince and purely my opinion, with in 20% is reasonable. I prefer randomness with my tone. I prefer unbalanced phase inverters. It is too sterile when things are perfectly this and perfectly that.

OK im done
 

ohmslaw

Member
Messages
208
No way to idividually set the tubes. 4 ma aint no thang! BTW- I mentioned it on the other thread but I'll say it again. Be careful on those test points . I slipped and blew out a couple of resitors on my SS using those test points. I recommend a probe-its safer for the amp! lol! Bob
Heh heh...I just did exactly what you described. Brand new SuperSonic combo, less than a week old. Directly under one of the TPs is a copper trace on the PC board. I was stupidly using a test probe with a very sharp tip and as I rested it on the resistor lead, the probe penetrated the very thin coating over the trace, and boom! the trace melted, the fuse blew, and I was just a little annoyed. I was gonna take the amp to Fender, figuring the whole PC board needed replacing, and just suck it up and pay. But my buddy said to take a close look and maybe I could just jumper the trace, and sure enough, it was a simple repair. An inch of thin insulate wire soldered point-to-point did the trick. Of course, the 5 year warranty is now void.

Lesson is don't screw around in these amps unless you are prepared to suffer the consequences, which could be ruining the amp or being seriously injured or killed. Fortunately I am back in business but it could have been much worse.

Anyway, yes, you all are right about the voltage difference. 5 mV is nothing. However I did test a set of tubes from ARS in Van Nuys, CA and they were perfectly matched, to within one mVDC. If you can get tubes from these guys, I recommend it. Unfortunately I could not get the tubes voltage reading into Fender spec...the most I could get with the max bias adjustment was 24 mVDC and Fender wants to see 32 mVDC. The guy at ARS said to get a tube with a higher rating number (their rating) and try it again. Apparently Fender is not providing a lot of adjustment range on the bias control which limits the tube choices.

If I understand correctly, running these tubes at 24 mV is going to result in the tube running too hot and result in a lot more headroom and much later breakup than running in the high 30's. Is this right? Is there a danger in this?

Thanks for all this info...looking forward to more answers!
 
Last edited:

ohmslaw

Member
Messages
208
Not sure why there's so much emphasis on tube "matching" in guitar amps. Ever measured primary resistance in an output transformer? Ever seen the center tap *exactly* in the center? Usually, the output circuit is "off center" to some degree anyway.
Yep...Fender says the values for each test point can vary as much as 25 percent, so if you're looking for 32 mV you can run it from it 24-40 according to the schematic notes. Add up those variations over the whole amp and that's a lot of variation. It all gets smoothed out in the wash, which is the beautiful thing about these tube amps.
 

Structo

Member
Messages
9,556
On my Brown Note D'Lite 44 (6L6) amp, I installed separate bias adjustment pots.

While all this does is allow me to keep the bias current close to the same on both tubes, I'm sure there is enough difference in the tube characteristics to give it enough imbalance for good harmonic tone.

I did this mainly because I had planned on trying NOS tubes that weren't necessarily a matched pair.
But after trying a few different expensive power tubes as well as some NOS 12ax7's, I didn't find the cost to tone ratio good enough to warrant the expenditure for the NOS tubes.
At least for me.
Maybe if this had been a vintage blackface I would have noticed an improvement in tone.

I do have a nice Sylvania 12ax7 in my clean channel spot that sounds nice but I only paid $30 for that one.

I like the new Winged =C= 6L6's I have in there now. Mucho good for a good price.
 

rick13

Silver Supporting Member
Messages
770
I understood that a mismatch of 5-6 mv was no big deal, in fact that small mismatch was sonically better.
Rick
 

rjgtr

Member
Messages
28
The other thing to keep in mind is that a new set will bias differently than a set of tubes that's 1 month, 6 months, etc. old. You'll want to go back and rebias the amp after the tubes settle in. After that amount of time, I've never found a matched pair (within 2ms) stay matched. I just look for a set that stays within 6ms.

And I agree that perfectly matched tubes are only important when you want as clean and hifi a tone as possible. In distortion the matching usually is out the window anyway.
 

ohmslaw

Member
Messages
208
The other thing to keep in mind is that a new set will bias differently than a set of tubes that's 1 month, 6 months, etc. old. You'll want to go back and rebias the amp after the tubes settle in. After that amount of time, I've never found a matched pair (within 2ms) stay matched. I just look for a set that stays within 6ms.

And I agree that perfectly matched tubes are only important when you want as clean and hifi a tone as possible. In distortion the matching usually is out the window anyway.
Makes sense!:BEER
 

rockon1

Platinum Supporting Member
Messages
13,197
If I understand correctly, running these tubes at 24 mV is going to result in the tube running too hot and result in a lot more headroom and much later breakup than running in the high 30's. Is this right? Is there a danger in this?

Thanks for all this info...looking forward to more answers!
No 24 mv is way cold actually. 24mv across the 1 ohm resistor is 24ma. With a PV of around 470 thats less than 40% idle dissapation. Heck 33ma is only slightly above 50% idle dissapation. That said they seem to sound better at 33ma than hotter at 60% or 38 ma. Bob
 




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