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6L6's in an SVT Classic?

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devinmasters

Sep 18, 2011
10
0
#1
Was just looking through my old stuff and I came across a box of old tubes from my svt. I realized that they were Mesa 6L6 GC tubes.

Has anyone heard of replacing their 6550's or KT88's with 6L6's? And what would that do as far as your tone, or wattage?

Thanks.
 
#5
devinmasters said:
Yeah they were definitely in the svt. I guess it was modded. I'm just wondering why someone would do that?
Click to expand...

The pinout is the same, and they bias in about the same range, though their max plate dissipation is a bit less. It is doable and a economical alternative if you are in a bind. No drastic mods required, just a rebias by someone who knows what they are doing. As previously noted, power output will be somewhat less. If the mesas were the old Philips/Sylvania built, they were about the stoutest 6L6s made and would be up to the task. Music Man ran 6L6's at close to 700 volts, so they can easily handle the voltage in an SVT as long as the plate current is adjusted accordingly.
 
#6
Hi.

devinmasters said:
Yeah they were definitely in the svt. I guess it was modded. I'm just wondering why someone would do that?
Click to expand...

Ok then, while there's a few particular 6L6 tubes that are up to the task in a specific circuit, 99% of the time they'd burn in about a second.

As for why, there can be plenty of reasons. As there always is with amp modding ;).

Assuming the unlikely event that the head is not modded, reducing the high cost of retubing isn't one of them though, the special 6L6 varieties as any special tubes are almost without exception expensive as hell. Matched sextets of 6L6 are hard to come by as well AFAIK.

If the head was modded, it was probably either because of tone, or break up characteristics of the output tubes. The reduced load on the OT would virtually eliminate the OT's saturation and distortion characteristics though, but the amount of clean headroom on lower frequencies would also be increased.

The easiest way to find out if there's a mod is to measure the anode voltage, KEEPING THE SAFETY IN MIND AT ALL TIMES. The 6L6 should be somewhere in the 400V-500V region, 6550 somewhere in the 600V-700V region.

The "best" way would be to take the head to a tech for a once-over, especially if it's been a while since the amp was serviced the last time.

Regards
Sam
 
#7
Voltage alone does not burn up tubes. Tubes burn up when the maximum plate dissipation is exceeded. This is measured in watts. Music Man HD150 ran a quad of 6L6GC @ 700 volts. Plate voltage on a vintage stock SVT is ~600V. Limit plate current to 25 or 30 ma, recheck plate voltage (it will vary depending on current drawn) and you should be well within 60% of maximum plate dissipation recommended for 6L6GCs in class AB push/pull amplifiers. I doubt the amp was modded beyond a rebias. This sub was done frequently back in the day by people on a tight budget. Phillips/Sylvania 6L6GC's were excellent tubes, but run of the mill pricewise in their time. JJ's are equally robust today and are not "expensive as hell", either. sextets are readily available from Triodelectronics, Tube Depot or Lord Valve. I'm not saying I recommend this sub, but it is doable.
 
#10
Fender Deluxe Reverbs run a pair of 6V6GC at 430V, yet they are only rated for 350V.
Musical instrument amp designers routinely exceed tube manufacturers recommendations.
The ratings are based on continuous duty cycles, guitar/bass amp duty cycles are somewhat less.
I'll repeat, I don't recommend this mod, but I have seen it done without fireworks.
I'm curious about the tubes that were in the amp. How many hours are on them and are they still good? The proof of the pudding is in the eating, as they say. Maybe devinmasters could take them to a tube tester and report back. A sextet of GT 6L6GCs could be put to use elsewhere. If they were mine I'd want to know if they were good. If they were bad, I would think they would have been pitched already.
 
#12
BassmanPaul said:
Leo Fender was well known to run tubes in his amps above their design specs. Back in the day he could get away with it. Today not so much. Dropping a 6L6GC into a slot slated for a 6550 or a KT88 will kill that tube in short order.
Click to expand...

Are you speaking from experience or engaging in speculation?

Offshore tube manufacturing has come a long way since the day of Ruby firecrackers. You haven't seen the tests run on JJs. People are running their 6V6GCs in Fender Twins at 475 volts. The 6L6GCs are proportionately robust. If I were going to attempt this sub with any confidence, that would be my tube of choice.
I do heartily recommend the JJ 6L6GC for 7027 equipped Ampeg amps like the V4B. They can be subbed without rewiring the sockets. Ampeg built them using the pins common to both tubes. This may not be the case with amps from other manufacturers.

The Sovtek Mig 50 runs @ 560Vp and came stock with the ersatz "5881" Russian 6NC3-E found in many mass produced 6L6 amps of the 90's. These are being retubed with JJ and EH 6L6GCs without issue.
 
#13
I ran the amp for a few months with the 6l6's in it. And the guy who had it before me was a touring musician. He had the tubes put in the amp by the guitar tech of one of the bands he was on tour with. I don't know if the amp was modded to do this, I just know it was done and it worked until one of the tubes became microphonic. I would imagine that there were quite a lot of hours on the tubes.

Not sure why I didn't throw them away, but finding them sparked my interest in the subject.
 

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