100 watts with two 6L6GC tubes?!

[Axtman]

I am currently repairing a Univox 1221 tube guitar amp head. The amp "claims" to put out 100 watts with a pair of 6L6GC tubes, (Yeah right.) The schematic shows the plates at 660 volts yet the tube data states 500 volts max for 6L6GC tubes. I always thought that a pair of 6L6GC tubes could put out about 50 watts on a good day if you round up....and could put out about 75 watts with an ultra-linear transformer again on a good day and rounding up.

Is it even possible to get close to 100 watts with two 6L6GC tubes?

I have yet to measure the voltage but if the amp indeed puts 660 volts on the plates (and 320 volts on the screen) but I would assume that high a voltage would eat up the output tubes in a short while.

[Leo_Gnardo]

I am currently repairing a Univox 1221 tube guitar amp head. The amp "claims" to put out 100 watts with a pair of 6L6GC tubes, (Yeah right.) The schematic shows the plates at 660 volts yet the tube data states 500 volts max for 6L6GC tubes. I always thought that a pair of 6L6GC tubes could put out about 50 watts on a good day if you round up....and could put out about 75 watts with an ultra-linear transformer again on a good day and rounding up.

Is it even possible to get close to 100 watts with two 6L6GC tubes?

I have yet to measure the voltage but if the amp indeed puts 660 volts on the plates (and 320 volts on the screen) but I would assume that high a voltage would eat up the output tubes in a short while.

Kind of like the 85 miles per hour my '62 VW claimed to be able to do. Yeh, maybe. With a hurricane wind at its back, if it fell off a cliff. That 100W figure, yeh maybe with 50% distortion, certainly not at the clipping point. Guitar amp manufacturers made all kinds of outrageous claims. So did hi fi amp builders, until the FTC made them all toe the line in the early 70's. All that seems to have gone out the window last couple of years once again. After all are they going to send the FBI after some made-in-China outfit that claims to deliver 10,000 watts a channel? "Hands up Mistah Chang, we got you for fibbin' about your output power. You're going to Leavenworth for a long stretch." Nah, they got bigger fish to fry.

I think your estimates for what can be expected from a pair of 6L6's are more in line with reality.

FWIW there are some ham radio wags who manage to get lots of excess power from tubes that are set up in trick ways, for instance operating immersed in an oil bath. Our own seldom-seen Stan (KM6XZ) told about a hi fi power amp he made & displayed at a Japanese trade show that was set up that way.

[Justin Thomas]

Maybe with the right speakers, it could SOUND like 100W...

Sort of like AC30s having a reputation for going head-to-head with a Marshall stack. Efficiency, baby!

Justin

[Axtman]

What do you think about running the plates at 660 volts?!

[nsubulysses]

I am currently repairing a Univox 1221 tube guitar amp head. The amp "claims" to put out 100 watts with a pair of 6L6GC tubes, (Yeah right.) The schematic shows the plates at 660 volts yet the tube data states 500 volts max for 6L6GC tubes. I always thought that a pair of 6L6GC tubes could put out about 50 watts on a good day if you round up....and could put out about 75 watts with an ultra-linear transformer again on a good day and rounding up.

Is it even possible to get close to 100 watts with two 6L6GC tubes?

I have yet to measure the voltage but if the amp indeed puts 660 volts on the plates (and 320 volts on the screen) but I would assume that high a voltage would eat up the output tubes in a short while.

I could be wrong and someone please correct me if I am. but I don't think high voltage would "eat up the output tubes in a short while." I think they either arc over and short or they don't.

You can make a 75W amp with two EL34/6CA7 with a 580V plate voltage and screen voltage not far behind and EL34/6CA7 is a weaker tube than 6L6GC. Seems like it would be possible to make 100W amp with two 6L6GC

There are plenty of amps that have 4 power tubes with 660V plate and 350V screen and do 200W or so with 4xKT88/6550. 6L6GC would be more of a stretch but I bet it could be done if OT primary impedance was low-ish, like 1.6 or 1.7K. An amp I know personally with 4xKT88 and 650V/390V plate/screen does 220W with 1.9K OT.

SVT has same plate and screen voltages as the univox and does 300W at clipping so 100W per pair and the OT primary impdance is kind of high for a 6 power tube amp, 1.9K. It seems like you usually see 1.7 -2.2K for a 4 power tube amp.

maybe they have like 3.4K OT primary and it can be done. Just guessing and speculating.

I can't see the schematic you posted but here is a 100W 2x6L6GC amp -- https://irationaudio.com/2015/05/03/musicman-2100-rd/

[J M Fahey]

Basic calculation on back of envelope:

*suppose stiff +750V supply and saturation down to 50V so 700V peak swing

* suppose 220mA peak current with 400V on screens and very cold bias of 10mA so 210 mA peak swing

then possible peak power is 700*0.21=147W so possible RMS out "just reaching clipping" is half that: 73.5W

No way to go beyond that because you won´t have more emission from that cathode , I already calculated it for 0V grid voltahe, and 750V plate voltage means during *routine* drive into a relatively mild load plate will reach 1500V peaks, and overdriving amplifier into a reactive load such as a speaker will certainly go way beyond that.

Even if tube guts can stand that, sockets will arc, so you woukd need to move plate cotact to a cap on glass bulb ... what 1625 did.

MAYBE an AB2 class , always positive grid, cathode driven amp such as Musicman could reach 100W, not by increasing plate voltage but available current.

[Justin Thomas]

Regarding that "over-spec" plate voltage, quoth The Enzo:

Voltage doesn't kill tubes, watts kill tubes."

Or something like that. Bias is everything.

Justin

[Enzo]

Well, voltage doesn't make plates melt, current does. More importantly, current melts screen grid wires rather than the voltage.

[alexradium]

As long as screen voltage is less than 400v and primary impedance is adequately high,you can get 100w,there are published designs from the golden years that prove that,and all the BS you hear today from the supposed gurus is just smoke in your eyes.
Check out how Roger modjeski gets 40 watts from a couple of el84s.

[nsubulysses]

As long as screen voltage is less than 400v and primary impedance is adequately high,you can get 100w

What does this mean to have primary impedance "adequetely high?" I thought the higher the primary impedance the harder it is to get max power output.

When I looked up the SVT output transformer and saw it was 1.9K primary impedance I was surprised. Now you are making me wonder why it is so high like that and what is the reason it's chosen. I believe Fender typically uses 2K or 2.2K for most designs for a 4 power tube amp, and Marshalls in a lot of cases are 1.7K, so i was expecting less for 6 power tube amp.

[J M Fahey]

Power is voltage*current.

Tubes are current limited.

So only way to increase power is to increase voltage.

Higher voltage @ same current=higher impedance.

Tubes are not transistors (which can supply any current you want).

[Helmholtz]

I thought the higher the primary impedance the harder it is to get max power output.

There is an optimum primary impedance for max output power depending on tube type and operating conditions/limits. Lower or higher than optimum impedance is likely to reduce available output or exceed plate dissipation (at low impedance). Generally higher plate supply voltage increases the optimum plate impedance. In our case I wouldn't go lower than 8k.

You can play with different plate loads using this great tool:

http://bmamps.com/ivds.html