Output rating on Tube Data Sheets RCA 6L6

[6l6gmike]

Background info: I am refreshing my Garage/Autoshop amp. I have tube amps and preamps everywhere. I am running russian spec 6l6g on it and they are driven by a 6N7 long tail that is driven by a single triode voltage stage, the whole thing is similar to a Mullard 5-20. Anyways I want a little more oomph from it. I was going to rebias it according to the RCA manual suggestion. Right now it puts out something weak and disappointing (I don't recall what it really does and I haven't tested it yet) Output trans are 8.6k 21 watt Edcor and I do have a fixed bias tap on the power trans.

On to the questions.

• Do you think (if my driver can handle it) I get the rated 26.5 watts shown below using the numbers they posted?
• Also are those wattage figures in RMS?
• Is this wishful thinking in the datasheet?

Thank you.
Mike

 

[Hi-FiGuy]

You would get 26W at those values, but the noise is 2% which is a bit high for a hi-fi amplifier. Your amp uses feedback to bring the gain and power down a bit to calm down that noise. You could aim for the voltages and the 8500 ohms on the transformer you may get just about 21W out with low noise. Is your amp in triode mode now? That would explain the low power.

 

[RWood]

6L6GC will allow for a higher plate dissipation, maybe give you a little more juice. But then you'd have to change your AK handle

 

[BinaryMike]

If we overlook possible differences in the Russian product, those datasheets are aimed at engineers with precious little tolerance for the sort of bushwa and propaganda commonly fed to consumers. The power output figures are real, but you're going to lose perhaps 10~15% in the OPT. The bigger problem here is your OPT primary impedance. There's no getting around the fact that it must be reduced in order to squeeze more audio power out of a 400VDC source. OTOH, if you can increase B+, then you might have to look for different power tubes.

 

[6l6gmike]

Since the trans are only rated for 21 watts I think rca specs would be ideal. I want to say I was only getting like 15watts out of them when I built it 3 years ago. I will be painting a car this summer so I figure some extra watts won't hurt to overcome the 5hp air compressor and power tools. I also want to put fix bias in it and will be using Dave G's Bias and Balance schematic.

RWood nice. yea I seem to use the 6l6g's in almost everything I build. I like them because they are cheap and several tube factories still make them so incase my stash ever runs out i know I can locate them again.

BinaryMike. I have 4,8, and 16 ohm taps on the trans so I could use the 16ohm one with 8 ohm speakers and then I would have a 4.3k impedance. But I am liminted to 400vdc on the plates.

I wonder what the ideal impedance would be for output with only 400vdc. I might have to learn how to plot on those curve charts.

 

[6l6gmike]

Yea I started this post as more of a hypothetical question about the data sheets for use in one of my amps. I was just making sure that those datasheets weren't full of fluff which I found out they are not and should be close to real life. The rest of the details I will work out and if I need some help I will post it here. Thanks.

EDIT:
I will build it and reply back with the output measurements and a schematic in case somebody in the future wants to build a project like this one.

 

[RWood]

maybe we should bark up another tree at the same time....what kind of speaker efficiency are you getting?

 

[BinaryMike]

I wonder what the ideal impedance would be for output with only 400vdc. I might have to learn how to plot on those curve charts.

I looked into an option that might be interesting if you don't mind pushing that OPT out of its comfort zone. According to RCA's curves with grid drive up to +15V, this tube can pull 300mA of anode current at 65V with G2 at 250V. Running on 400V of B+, that nets 48W into 4300 ohms in class AB2. Distortion wouldn't get worse because those curves are evenly spaced in the positive grid region. MOSFET source followers would make it easy to develop the necessary grid drive current, but you need an additional 15V of peak grid drive voltage in both directions. Is your driver circuit up to the job?

Edit: I see a class AB2 alignment listed in the datasheet that yields 47W into 3800 ohms with B+ of 360V. It calls for +13.5V peak at the grids and produces no more distortion than the listed class AB1 alignments.

 

[gadget73]

Wonder if you could use a cathode follower stage between the phase inverter and the output tube to do the same sort of thing. Depending on the current load required, you might have to bump the driver up to something more commonly considered to be an output tube.

The old usual method of creating AB2 drive was to use an interstage transformer to create the current needed.

 

[BinaryMike]

Cathode followers and interstage transformers were both commonly used in class AB2 amp designs of old, but I see no reason to prefer those solutions when MOSFETs work so well (although I have used BJTs in a Sziklai pair for the same purpose). If you like the CF approach, then you will enjoy reading an extremely interesting article by Joel Julie in the March 1955 Audio Magazine. Look it up on the Internet Archive.

 

[GordonW]

From experience, if you optimize everything for 400v and 8.6K outputs, you're going to top out at about 22w undistorted power (no sign of clipping), in pentode mode. I got about 20w from my "military" amp with 380v plates and 9K outputs, and I usually get the same from my Magnavox amp conversions using AMP132/142/150 output transformers on about 390-400v.

It took 440v and 6.2K outputs, in fixed bias, to get closer to 30w. My custom integrated amp with the Scott 340 output transformers were set up that way, would do 28.5w/ch before clipping, both channels driven simultaneously, and about 31w per channel, each individually driven.

In ultralinear, the power will be very slightly lower for the same B+ voltage, than the pentode mode amps I built. So, even though your amp has slightly higher B+ than my first two examples, that's why I'm saying maybe 20w max... but it will be a VERY GOOD 20w.

One note- with such high primary impedance- it will do very little to run the amp in Class AB2 mode. It already will come close to pulling the plate all the way as far down to close to zero voltage, before you reach zero grid volts. If you had a 5K or 4K primary, then it would do a lot of good... but not with twice that primary impedance...

EDIT: Oh, I just noticed- your OPTs are only rated at 21w. I usually want to leave some "headroom" with the Edcot OPTs. A 21w OPT, I would on expect maybe 17w before distortion just starts to set in (about 20% loss)...

Regards,
Gordon.

 

[6l6gmike]

So to answer some questions. The garage speakers are some old 12" 3 way with new drivers. They should be around 92 spl.
The transformers were purchased to build a vintage amp based on the Acrosound catalog with 6l6 and the to280 output trans. http://triodeel.com/ac6l6280.gif I gave up on that project to build a little better front end hence a triode into a ltp inverter.
The goal of this rebuild is to try to use the transformers to their full potential and also have good tube life. Its nice and warm today in the frozen north so I am going to be working in the garage this weekend but I will work on a schematic at night. Will post it when I have it done. Thanks for the inputs
Mike

 

[whoaru99]

And, even if you went from say 15W to 26W, it's a difference of just over than 2dB. That's not much difference just in terms of potential SPL increase...although I guess it may depend on what you were looking at/reason for wanting the change.

 

[tcdriver]

... your OPTs are only rated at 21w. I usually want to leave some "headroom" with the Edcot OPTs. A 21w OPT, I would on expect maybe 17w before distortion just starts to set in (about 20% loss)...

I think that is most telling. It would seem that without changing the output transformers, there will be little chance of a significant power increase. Also, even if you did increase the power from 20 to 25W, that would only be about a 1dB increase in sound level, hardly worth the effort.

Edit: While I was typing my response whoaru99 made essentially the same point.

 

[Tom Bavis]

The transformers will gain an octave of bass response and loose an octave on the high end if you use them as 4300 Ohms. My experience is that the Hammonds don't have an octave of treble to spare, but I don't know about Edcor. And resistive losses will be higher, so transformer efficiency will drop from maybe 94% to 76%. (Hammond measurements) They WILL be good for more than 21W (what they're rated at 8600 Ohms) but about a quarter will be lost as heat. And while we're at it, is the power supply up to twice the peak current? Voltage sag under power will reduce the max output as well. Tube data sheet figures assume a regulated power supply.

 

[6l6gmike]

So curiosity got the best of me this morning so I quickly ran some tests. The amp puts out 10watts before clipping. The driver stage puts out 46v p-p before it gets really ugly. Amp is wired UL with 300 vdc on the main. I was able to lower the voltage by going right from the rectifier in to a choke for my b plus. I remember running a lower voltage to be nice on the grids. Ok. That's all for now. Will post new schematic after I have it done.

 

[maxhifi]

Amp is wired UL with 300 vdc on the main. I was able to lower the voltage by going right from the rectifier in to a choke for my b plus. I remember running a lower voltage to be nice on the grids. Ok. That's all for now. Will post new schematic after I have it done.

This is confusing. Are we looking at the RCA manual operating conditions in pentode, with 400V on the plate and expecting it will make the same power in ultralinear with less plate voltage?

Ultralinear will make less power than pure pentode operation, and the lower plate voltage will make it even less.

I think if you really want more power, don't use the ultralinear taps and change the driver circuit accordingly.

But in your situation the more obvious and easier solution is sensitive speakers. I bet a pair of sansui kabuki speakers would kick ass in the garage with that amp.

 

[gadget73]

Only 10 before clipping? That sounds low for a 6L6 family tube, though depending on circuit particulars it could be all it can do. Combination of low plate voltage and a non-ideal transformer match might be the whole story here.

 

[6l6gmike]

Sorry for the confusion. 300vdc was on the plates on current build. I know the power trans is really huge but I don't recall the specs right now so voltage sag shouldn't be to bad. I plan to run the outputs in pentode mode. Ok I think I answered everybody's questions.


This whole project is more of a lets see if I can get to 20plus watts with the RCA datasheet as my guideline.

 

[gadget73]

Cathode bias or fixed? If its cathode you'll lose some power and effective plate voltage there.