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SVT-CL: Mixing Power Tubes

#61
1) If you have a spare replace it with the spare (and even know about it). If no backup amp and no spare, pull a tube from the opposite "side".
2) Push pull circuits have a positive going and a negative going signal going to however many tubes you "line up" pair, quad, sextet, octet.....
3) Balanced overall gain.
4) Equally, schools of thought say just the bad tube (unless you are the seller matched sets) if the failure was early in the expected service life. If you can also be reasonably sure the remaining tubes (valves) are good.

Now let us get the darn kids off the lawn.
 
#62
BassmanPaul said:
This thread seems to be getting out of hand. For the record in any of the tube SVTs there is no such thing as a pair. bias is adjusted, in most, by adjusting one side and then the other. If a single tube goes down you need to match a tube to the others on that side and adjust the bias appropriately. If you cannot match then you NEED to replace the triple on that side and redo the bias to suit.

Also for the record I went to engineer school when tubes were current technology and transistors were a science that would never last!! :)
Click to expand...

You must have had some of the same professors? I think one blamed you for his retirement? JK ;)
 
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#63
To go a bit further, if I may. In a "fixed bias" tube amp with two output tubes and one bias control a matched pair works best. Two bias pots removes the need for matching. Four output tubes and one bias pot a matched quad is required. Two pots and two sets of matched pair works. Four and pretty much any four tubes of the same type work fine.

Six and you need triples if there are are two bias pots. One and it's a matched sextet. Six pots and you no longer need to match. Note that there is no choice available for pairs matched or otherwise!
 
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#69
To help understand the tube layout, here is the power tube section of the SVT-CL. There is a separate bias adjustment for each trio. Each trio functions as a group, There is a cathode resistor for each tube in the power amp as part of with the LED based bias circuit.

The best way to match power tubes is at the working plate voltage, matching transconductance and cathode current between each tube. When you see two numbers on a matched tube, they usually relate to these two parameters. The SVT-VR has a balance adjustment for each trio, the CL doesn't. The balance allows you to minimize THD caused by unbalanced levels in the push and pull trios. So if the two trios are not matched, this can be compensated for by adjusting the balance. A nice feature to have.

 
#70
Hey all. I think i understand what ipopes is saying. Correct me if I'm wrong but he is agreeing with you all that the thing is in triplets that are push pull. Where the confusion is coming in is from wording and math. I'm admittedly not a tech but i have fixed a few amps and modded an amp or two, and by that i mean literally just a few. But what he is saying is that when you go buy tubes from your local music mega mart they all have little stickers on them that have numbers written on them that should indicate the bias voltage. If you started out with brand x tubes and they all came with a sticker that had a number of 50 written on it then each side of the amp should have an average bias of 50. When 1 tube dies you need to replace it, but you can only find brand y tubes that have number 45 written on them. Now who knows what the numbers mean and how they compare to each other but if you took out the bad tube and replaced it and also took 1 tube from the other side of the amp then both sides should now have 3 tubes that add up to 145. Now this assumes a lot, but it would let you get the bias for the whole side right even if the individual tubes were way off. I know that is a very simplistic version but i think that is what ipopes was trying to say.
 
#72
Beans schematic made me think of something to mention. If one tube blows, when you try the replacement make sure the new tube has the "blue" electron glow when the amp is set to operate. If not the tube may have burned out a screen resistor or cathode resistor (on the circuit with individual cathode resistors).
 
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#73
iiipopes said:
Well, I am not perfect, but I know you need to get the pairs together.
http://www.talkbass.com/threads/is-a-matched-sextet-for-svt-necessary.531377/
Bassman Paul sums it up: "Matching in pairs, as long as one tube from each of the pairs is on opposite sides of the OPT balances the current on each side of the OPT, BUT one pair will do more work that the next." This is exactly the point I am making: matching pairs.

1) This whole thread is what to do when one tube blows to get by until new tubes can be purchased.
2) Push-pull amplifiers work with pairs of tubes tied together, one pushes, the other pulls. To get more power, add pairs like is done on an SVT.
3) Balancing the current draw is what we are trying to achieve
4) Yes, replacing all tubes at once is optimal, but not usually economically possible, so a pair at a time replaced, knowing that the pairs may load slightly differently.

Now, I'm done. I've given all the resources I can find about push-pull amplifiers working in pairs, one push, one pull. All the push tubes are tied together to a common bias and all the pull tubes are tied together for a common bias.

Everyone on this forum needs to have a fundamental course on how push-pull amplifiers work, if they are going to continue to discuss it. I did all those years ago, so if I forget a detail here and there, so be it. The fundamentals are as they are.

When your SVT tubes blow up because you overloaded one side with the higher gain tubes, and did not balance them as three pairs, remember I warned you.
Click to expand...



Would it be more correct to say that a push pull amplifier works on 2 equally matched groups of tubes which may contain multiple matched tubes. Thes groups are biased individually as groups and because of this each group should be matched as a whole. This matching could be achieved by creating the groups from 1 or more matched pairs of tubes and using 1 tube from each matched pair in each group.

ie: pair a, pair b, pair c.


Group1, Group2,
A1. A2
B1. B2
C1. C2

If the pairs are matched then the groups should match and if one tube died then removing the matching tube from the other group and replacing both tubes with a new matched pair should result in the 2 groups matching again.
 
#75
TheREALRandyMos said:
Would it be more correct to say that a push pull amplifier works on 2 equally matched groups of tubes which may contain multiple matched tubes. Thes groups are biased individually as groups and because of this each group should be matched as a whole. This matching could be achieved by creating the groups from 1 or more matched (insert individual, not pairs).

.


Group1, Group2,
A1. A2
B1. B2
C1. C2

If the pairs are matched then the groups should match and if one tube died then removing the matching tube from the other group and replacing both tubes with a new matched pair should result in the 2 groups matching again.
Click to expand...

Corrections made.
 
#76
If you had four pairs of tubes, two that match at 39ma and two that match at 41ma you could use three of the 39ma on one "side" or triad and three 41ma on the other. The mis-match would be corrected by adjusting the bias points of the two dis-simular triads of tubes. You would also have two new unused spares as you bought pairs for a six tube amp. ;)
 
#77
I am admitting that the numbers are mostly meaningless. They should let you know that 2 tubes mstch each other. I think i am now confused though. But if someone can answer this question i will be ok. If i was buying tubes for an svt do i need to make sure to match side a to side b? Or is it only important to match the tubes within side a to reach other and repeat the same for side b?
 
#78
TheREALRandyMos said:
I am admitting that the numbers are mostly meaningless. They should let you know that 2 tubes mstch each other. I think i am now confused though. But if someone can answer this question i will be ok. If i was buying tubes for an svt do i need to make sure to match side a to side b? Or is it only important to match the tubes within side a to reach other and repeat the same for side b?
Click to expand...

You make it easy on yourself an buy a matched sextet of tubes. Unless you want to spend more for two matched quads or an octet and have new spares. Most find a sextet of tubes more than enough budget strain.
95% of the time (or more) the new spares will no longer "match" anymore as the in use tubes have "worn".
 
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