SVT-CL: Mixing Power Tubes

[beans-on-toast]

Some KT88’s have larger coke bottle glass envelopes. Make sure that the ones that you buy fit in the amp leaving sufficient space for air circulation. Tube clamps can also be a consideration. The multi-spring type can rattle against the glass. A damping tube can help but it can all be a lot to contend with.

 

[bigtone23]

hi jimmy i have a question.i have an svt and i need tubes because the one tube is out.the tubes are 6 or 7 years old .here in greece i cant find 6 matched tubes.can i take two sets of four and put them in each triplet and have spares the other two?is it good?thanks

If I owned an SVT (or any amp with 6 power tubes-haven't owned one since the the early 80s when I had a Peavey Mace 6 x 6L6GC), I'd most certainly buy a matched octet just for that spare pair that can be dropped in at midnight on a Saturday, in between sets, when a tech is not available nor do I have time to get out the bias probe.
I buy matched quartets for my 2 tube amps for that same reason.

 

[dimitris rigas]

If I owned an SVT (or any amp with 6 power tubes-haven't owned one since the the early 80s when I had a Peavey Mace 6 x 6L6GC), I'd most certainly buy a matched octet just for that spare pair that can be dropped in at midnight on a Saturday, in between sets, when a tech is not available nor do I have time to get out the bias probe.
I buy matched quartets for my 2 tube amps for that same reason.

i can only find 2 quartets each one matched ,not all the 8 tubes matched,so i asked if i could use them for each triplet and bias them properly

 

[agedhorse]

I agree with Jimmy, no need to second guess the designer. He's a VERY smart guy and knew what he was doing, I was lucky to work with him for a short time.

 

[beans-on-toast]

i can only find 2 quartets each one matched ,not all the 8 tubes matched,so i asked if i could use them for each triplet and bias them properly

The VR has a balance pot that allows you to compensate for unbalanced sides, the CL doesn’t. Not balancing each side can cause hum.

Find a reseller that has what you need, a matched sextet, in stock.

 

[dimitris rigas]

The VR has a balance pot that allows you to compensate for unbalanced sides, the CL doesn’t. Not balancing each side can cause hum.

Find a reseller that has what you need, a matched sextet, in stock.

i thought that this would not be a problem...so the only thing i can do is 6 matched tubes?

 

[beans-on-toast]

i thought that this would not be a problem...so the only thing i can do is 6 matched tubes?

It would be best to have six matched power tubes for the CL.

 

[dimitris rigas]

It would be best to have six matched power tubes for the CL.

yes i understand but in case i cannot find 6 matched tubes i am digging to buy 2 sets of four,keeping the 2 for spair

 

[agedhorse]

yes i understand but in case i cannot find 6 matched tubes i am digging to buy 2 sets of four,keeping the 2 for spair

Then you will need to accept the limitations that an unbalanced side may produce.

 

[beans-on-toast]

yes i understand but in case i cannot find 6 matched tubes i am digging to buy 2 sets of four,keeping the 2 for spair

How well this will work depends on how closely matched the two sets of three are. Otherwise one tube on each side will be drawing more or less current than the other two.

With mail order, you can find six matched tubes from a good reputable reseller.

 

[dimitris rigas]

How well this will work depends on how closely matched the two sets of three are. Otherwise one tube on each side will be drawing more or less current than the other two.

With mail order, you can find six matched tubes from a good reputable reseller.

can you suggest someone in europe?thanks

 

[JimmyM]

The VR has a balance pot that allows you to compensate for unbalanced sides, the CL doesn’t. Not balancing each side can cause hum.

Find a reseller that has what you need, a matched sextet, in stock.

Then you will need to accept the limitations that an unbalanced side may produce.

You learn something new every day! So biasing each side to spec in the CL, which does have two bias pots and lights, won't balance them out and keep hum at bay? In real life as opposed to on paper, is this a real problem? I have to ask because I have found that there are certain things that some folks feel is quite important but don't really add up to much in real life. Now yes, I've heard noise in my 69 SVT that the hum balance pot fixes, and it's not pleasant, but how much of that is entirely tube related and how much of it is just because it was tremendously out of whack once I restored it? I really haven't had to touch it since then and I have changed out power tubes since restoring it, often without biasing

 

[rickdog]

It's been a while since I had to bias my CL, but as I recall each bias pot has a small range within which bias is "in spec" (green light); and it's possible to tweak the bias on the two sides to minimize hum while keeping them both within this range.

That's balance at idle. If the two sets of three tubes aren't perfectly matched, you may get no hum, but there will be some difference in the gain of the two sets. This will cause some asymmetrical distortion when the amp is driven hard. You might detect that with the right measurements, but I really doubt you'll hear it.

Bottom line, in my opinion: this ain't a hifi, it's a bass amp. If the LEDs say the bias is good, crank it up and rock out, it'll be fine!

 

[Wasnex]

How well this will work depends on how closely matched the two sets of three are. Otherwise one tube on each side will be drawing more or less current than the other two.

With mail order, you can find six matched tubes from a good reputable reseller.

He's talking about buying two matched quads. From this he will pull two matched triplets and have a spare for each triplet.

IMHO, this is a good strategy, especially if a bias probes is used to select the best three tubes out of each quad. I do agree the two quartets should be relatively close in their matching.

The only concern I have is he mentioned buying EHX6550s. Every EHX 6550 or KT88 I have ever had has failed in very short order. Sixteen tubes total in three different amps; pretty much a 100% failure rate with very light usage. Multiple failure modes: some flashed over, some developed low emissions, some stopped conducting completely, some last the ability to hold bias.

 

[dimitris rigas]

He's talking about buying two matched quads. From this he will pull two matched triplets and have a spare for each triplet.

IMHO, this is a good strategy, especially if a bias probes is used to select the best three tubes out of each quad. I do agree the two quartets should be relatively close in their matching.

The only concern I have is he mentioned buying EHX6550s. Every EHX 6550 or KT88 I have ever had has failed in very short order. Sixteen tubes total in three different amps; pretty much a 100% failure rate with very light usage. Multiple failure modes: some flashed over, some developed low emissions, some stopped conducting completely, some last the ability to hold bias.

oh thats very disapointing.i am thinking about the sovteks 6550 too,i hear good things about the reliability.its a pitty because i just bought my svt used here in greece(very rare find),i sold my ampeg he410 cab(8 ohm),and bought the hlf4x10 4 ohm (sounds way much better) and after 2 sessions my tubes failed..they were the original jj6550 came from the ampeg factory 2012..

 

[Wasnex]

oh thats very disapointing.i am thinking about the sovteks 6550 too,i hear good things about the reliability.its a pitty because i just bought my svt used here in greece(very rare find),i sold my ampeg he410 cab(8 ohm),and bought the hlf4x10 4 ohm (sounds way much better) and after 2 sessions my tubes failed..they were the original jj6550 came from the ampeg factory 2012..

I haven't used the Sovtek 6550WE personally, but it does seem to have a reputation for being more reliable than other New Sensor brands. I believe the Sovtek is a legacy model that was in production before New Sensor came into existence.

I have an 87 SVT running the original Philips 6550s. I will probably try a set of Sovteks when/if the Philips wear out. I do believe that it's important to buy output tubes from a reputable vendor and pay for burn in services. Burn in basically involves operating the tubes at real world voltages for 24-48 hours. There is fairly high rate of infant mortality with new tubes, and many of these are caught/eliminated during burn in. Also the burn in activates the tube getters to clear the glass envelope of any gas that was remaining after manufacturing. This stabilizes the tubes operating characteristics so it will hold matching better.

 

[DavidEdenAria]

He's talking about buying two matched quads. From this he will pull two matched triplets and have a spare for each triplet.

IMHO, this is a good strategy, especially if a bias probes is used to select the best three tubes out of each quad. I do agree the two quartets should be relatively close in their matching.

The only concern I have is he mentioned buying EHX6550s. Every EHX 6550 or KT88 I have ever had has failed in very short order. Sixteen tubes total in three different amps; pretty much a 100% failure rate with very light usage. Multiple failure modes: some flashed over, some developed low emissions, some stopped conducting completely, some last the ability to hold bias.

Were these tubes burned in at operating volts by the vendor and then matched?

Thats a shame and seems unusual but something tells me these tubes were not screened properly.....man, that is scary.

 

[coreyfyfe]

can you suggest someone in europe?thanks

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These guys are in Germany. I've had good luck with their branded tubes in a number of amps as well, though I haven't tried their 6550. They do sell a matched sextet.

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[DavidEdenAria]

I haven't used the Sovtek 6550WE personally, but it does seem to have a reputation for being more reliable than other New Sensor brands. I believe the Sovtek is a legacy model that was in production before New Sensor came into existence.

I have an 87 SVT running the original Philips 6550s. I will probably try a set of Sovteks when/if the Philips wear out. I do believe that it's important to buy output tubes from a reputable vendor and pay for burn in services. Burn in basically involves operating the tubes at real world voltages for 24-48 hours. There is fairly high rate of infant mortality with new tubes, and many of these are caught/eliminated during burn in. Also the burn in activates the tube getters to clear the glass envelope of any gas that was remaining after manufacturing. This stabilizes the tubes operating characteristics so it will hold matching better.

Do we really think these 'rejects' get destroyed? I wonder how many get re-sold.

I am down to maybe 7 or 8 USA 6550 that test strong....GE, Sylvania and a few very rare real Tung Sol 6550....the 'Ei' brand was another with a bad reputation in power tubes though the 12ax7 can sound great but often go microphonic (a sort of Telefunken copy).

 

[DavidEdenAria]

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These guys are in Germany. I've had good luck with their branded tubes in a number of amps as well, though I haven't tried their 6550. They do sell a matched sextet.

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From what ive gathered the 'TAD' brand has a good reputation but im far from an expert and only know what ive read over the years from consumers.