It seems that combination is possible. How about bias? The question is why- just to be good for lower volume gigs and small cabs under 200w.
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Vintage SVT with 6L6GC?
- Thread starter boroman
- Start date
Too high a voltage for a 6L6GC. EL34 could handle it but would have to be biased accordingly and the impedance of the output transformer would probably not be correct.
Plate voltage and impedance match would be issues. There may be some 6l6's that would survive, but all the biasing and so on would have to be done. Reissue V4B or the V4BH do a quad of 6L6's and might be less scary. A vintage V4B with 7027A's would be great too.
SVT is around 665pv, V4B - 590pv... it's not the biggest issue. V4Bs can handle almost all current 6L6GCs (or the way round
Try some 6l6GCs out at 665V and let me know how long they last! You're just asking for a failure at that point. Plus you would have to bias 6L6s cooler in an SVT since they have a lower power rating than a 6550 which will raise the plate voltage even higher since the plate supply is not regulated and it sags with increased current. How much? I don't know but I would guess 20V or so. Not recommended.
okcrum
in your chest
Actually, it would work (plate resistances are close enough) but you'd have to manually set the bias current to about half what the 6550s use. I have a 4 holer 200w amp I use both types in without any other issues.
EDIT: and it wouldn't really be a 300 watt amp anymore, unless you liked melting EL34s to slag. It would be OK to about half power tho.
If I'm lugging those transformers around, I want the power that goes with them.
If you want a lower power SVT, sell it, get a V4B and keep the change.
If you want a lower power SVT, sell it, get a V4B and keep the change.
V4B in all incarnations are nice. Used ones still are somewhat rational. The middle incarnation V4BH has less cachet than it deserves and its preamp section is SVT.
Word to the last two posts. A V4B works fine with its smaller transformers and lighter weight and 4 6L6's (or 7027's if you have an old one, but new 7027's are basically 6L6's that require a rewiring of your sockets). But lug those SVT trannies to get half the wattage? No way! That's the mark of an amp tech with too much time on his/her hands
SVT is around 665pv, V4B - 590pv... it's not the biggest issue. V4Bs can handle almost all current 6L6GCs (or the way round
Is it the classic V4B's running at 590pv or the reissues?
Some modern production 6L6's can take high plate voltages - I've got a pair of recent JJ 7027A's (aka 6L6GC) running at 610pv.
Sorry - that's a bit off topic for this thread.
webmonster, the 70s v4b's were 590pv, the new reissue is 445pv and I don't know what was pv of the V4BH.
Maybe there's another way to lower the svt wattage - a low output 6550 tube. Which ones pump less than 50W?
Maybe there's another way to lower the svt wattage - a low output 6550 tube. Which ones pump less than 50W?
okcrum
in your chest
Word to the last two posts. A V4B works fine with its smaller transformers and lighter weight and 4 6L6's (or 7027's if you have an old one, but new 7027's are basically 6L6's that require a rewiring of your sockets). But lug those SVT trannies to get half the wattage? No way! That's the mark of an amp tech with too much time on his/her hands
The only reason I ever would have thought of that was because I had to.
I decided to run with 4 output tubes instead for a couple of small gigs.
How about a less-efficient speaker cabinet and turning the amp down? That's what I did when I had an SVT over twenty years ago.
I wouldn't pull tubes. Yes, on most multi-pair push-pull amps you can do that, especially on older Marshall guitar amps, because you can select the impedance match on the back of the amp to account for the different impedance of pulling the tubes. But the SVT is already at 4 ohms with a transformer tap on the other speaker jack to 2 ohms. So you can't do that.
Some models of 6CA7 tubes, which have similar innards, might work if the plates can handle the voltage and the bias can be adjusted, but that is not a good situation, either because of the internal impedance issues.
Finally, as above, need less power? V4B for classic Ampeg tone.
I wouldn't pull tubes. Yes, on most multi-pair push-pull amps you can do that, especially on older Marshall guitar amps, because you can select the impedance match on the back of the amp to account for the different impedance of pulling the tubes. But the SVT is already at 4 ohms with a transformer tap on the other speaker jack to 2 ohms. So you can't do that.
Some models of 6CA7 tubes, which have similar innards, might work if the plates can handle the voltage and the bias can be adjusted, but that is not a good situation, either because of the internal impedance issues.
Finally, as above, need less power? V4B for classic Ampeg tone.
The early to mid 70's V-4 (no distortion models) ran 540 volts plate voltage. The late 70's V4 with distortion and four 7027's ran 545 on the plates-- but ran 590 on the V2 model with only two 7027's.
The V4BH shows 460 volts on the plates.
How to run 6L6GC tubes in an SVT is not so straight forward. If it was, people would have been doing it over the last 45 years.
My SVT has 675VDC on the plates with 117VAC, at 120VAC that goes up to 695VDC. This matches what is on the schematic. With a 125VAC line voltage, which isn't uncommon in some areas, this goes up to 724VDC. Yikes!
The power supply voltages will change depending on the load, so the number of tubes and type of tubes will affect the numbers. But the plate voltage and class of operation aren't the only considerations when operating a tube. Other factors such as screen voltage, heater supply, maximum plate dissipation, load resistance, power supply impedance and how it reacts to demand are all part of the balancing act.
Some amps that are designed to run a 6L6GC can work well with 6550A's in them. The 6550A won't break a sweat and will last a very long time.
However the amp is designed and configured, how it sounds is what's most important. If I wanted to run 6L6GC tubes with an SVT pre amp, I've buy the V4B reissue.
My SVT has 675VDC on the plates with 117VAC, at 120VAC that goes up to 695VDC. This matches what is on the schematic. With a 125VAC line voltage, which isn't uncommon in some areas, this goes up to 724VDC. Yikes!
The power supply voltages will change depending on the load, so the number of tubes and type of tubes will affect the numbers. But the plate voltage and class of operation aren't the only considerations when operating a tube. Other factors such as screen voltage, heater supply, maximum plate dissipation, load resistance, power supply impedance and how it reacts to demand are all part of the balancing act.
Some amps that are designed to run a 6L6GC can work well with 6550A's in them. The 6550A won't break a sweat and will last a very long time.
However the amp is designed and configured, how it sounds is what's most important. If I wanted to run 6L6GC tubes with an SVT pre amp, I've buy the V4B reissue.
okcrum
in your chest
Perhaps I was unclear. In my case I did not need less power. I had a broken tube, and gigs to do. Thanks for all the advice to use a V4B.
That was not in my gig budget.And despite this and all the other consensus that it wouldn't work, would hurt the tubes, output tranny, etc. it worked just fine.
In fact either approach (4 tubes or 6CA7/EL34) would have worked fine. I'm glad I don't define what's possible by what I read on TB.
I didn't say it wasn't possible. Of course it will work and sound will come out. But as I said before, that is well beyond the rating of a 6l6 and not to expect them to last a long time in an SVT.
Are you running modern production 6550A's? Do many of them like 700+volts?
I was thinking about this for my 7027A equipped Jansen Bassman. It runs 610V on the plates which a 6550A would handle more easily, but then the PT (and OT) is old and would the extra current tax it too much?
I run NOS GE 6550A's in my SVT. The line voltage in my area is 118VDC so I never get over 700V. What you have to keep in mind is that the data sheet specifications for a tube are intended to provide an optimal service life. Designers often exceed the specs. It doesn't necessarily mean that the tube will melt. It could mean that tube life will be shorter. How much shorter, if at all, depends on the tube design. Some tubes are rated conservatively, some aren't.
The 6550 heater draws 1.6A per tube. The 7027A draws 0.9A per tube. Your heater circuit would have to be capable of handling 0.7A of additional current per tube. If you amp has two tubes for example, that's an additional 1.4A that would be needed. Whether or not you amp can deliver this without taxing the power transformer depends on the design of your amp. If you had a model number for the power transformer you could look up the specs and see how much current the 6.3V winding can supply.
I looked at the Jansen Bassman 75 schematic and didn't see a transformer spec. They might be able to tell you at Jansen Link Removed. I take it that John Veldwijk is the technical guy to talk to. I would also ask them how they think the amp would perform with 6550 tubes installed. The designer knows what the output transformer specs are and what it is capable of. Knowing the impedance would help determine if the 6550's would be a good match for the output transformer.
