@plasticvonaband

Yep, but are you sure that 50mv per tube is not too hot longevity-wise for a tube rated at 22/23 watts compared to a 6l6gc's 30 watts?    Cause that is what JAN Philips 6l6wgb's are rated at, 22-23 watts.    65% of max dissipation rating gives up some sweet tones fo sho.   The manual for HRM amps uses that 35-40mv setting advice for getting that 65%-ish zone for 6l6gc's...The wgb series of 6l6 are an older generation in the evolution of 6l6 types.  I don't doubt for a minute how great they sound, but dang, that is a VERY hot bias to run those tubes of yours at.  I have some of those tubes and yes, they sound great, big bottom, lower mids, and detailed highs, but watch how quickly those side getter flashings erode at hot, high settings!  Been there, done that, and got the t-shirt...

And if you are in a dark room after they are on 5-10 minutes and see one or both of those tube's plates glowing even a bit of red, you are at or beyond those particular tubes' max ratings in your bias setting.   If I had your amp, I'd start at 28-30 mv, play through it at the lowest setting and bump up 2-3 mv at a time, playing through your amp at each setting.  You will come to a zone where you get the goods, and I'm sure it will happen at less than 50mv per tube.  Look at your manual.  My HRM manual sez 35-40mv for approximately 65-ish% max plate dissipation for a 6l6gc.  23 watts is close to 4/5 of 30 watts.  So 4/5 of 40mv is 32mv.  You might find a more than decent tone within that or up to 5-6mv higher and your tubes will live much longer than at 50mv apiece... 

I have replaced those GE 6l6gc's today in my HRM with some vintage Sylvania 6l6gc's biased at 38-ish mv apiece, and if I'm correctly informed, HAD used those tubes in many of his original builds, they sound huuuuuge in my amp. 

Peace.

Just got my 50w HRM and for a couple of the first hours stuck in a Telefunken smooth plate in V1 and PI, with a Philips/Mullard 12ax7a/7025a in V2.  Vintage GE 6l6's running 34-37mv plate current.  I was stunned at the range of usable tones provided by the amp at a wide range of volumes.  Played it through a semi closed back old Peavey 2/12 Mace combo cab that I modded, filled with JBL g125's in series for 16 ohms.  Fat, full, rich.  Woooooooeeeeeeeeeee! 

Winged C el34's tend to be a bit on the bright side anyway.  I know that their 6l6gc's also can also be pretty bright for a 6l6 type.  A slight adjustment to the hotter side of biasing can help in both instances. 

I've been busy on fleabay stocking up on VOS power tubes for my incoming 50w HRM head, a pair of GE 6l6gc's, a pair of what appear to be early Sylvania (Conn labeled) 6l6gc's, a slightly used pair of TAD 6l6wgc's, and a couple pairs of Siemens el34's.  Starting to Jones a bit...        

I like Wyatt's post here. it is very well done and is accurate with regard to tube amp theory. 

One thing that I found by a happy accident is that many modern build guitar amps use plate voltages that are above or at the maximum ratings for the power tubes.  A few amp builders are using power transformers that offer lower secondary (high voltage tap) voltages than what the majority use.   This is in regard to 6v6's and 6l6's.   There is a wonderfully different tone palette available by doing this.

I found this out purely by accident from purchasing a wonderful sounding amp through eBay, a Siegmund Midnight Blues (Blues Breaker) head.  It is offered from the builder tube rectified, also supplied with a proprietary SS rec adapter in three different voltage ratings for the main power transformer and also a choice of at least three different styles of output transformer in style of windings and capacity for current.  Mine was built with the lowest voltage PT option and has the larger, but not largest sized Radiospares type OT.  This type of OT was only used in the very few first Marshall amps due to its excessive cost.  Even when using only a pair of 6f6g's, the amp is surprisingly loud, and wonderful sounding. 

Being a vintage tube collector with literally hundreds of vintage preamp and power tubes in my stash, I found that I was able to run the older versions of 6v6 and particularly 6l6 power tubes, and the tones I got from doing that were jaw droppingly gorgeous.  Since the amp uses a fixed bias pot and bias balance pot in it's build, I was able to try tubes ranging in output from 6f6g all the way to 6550 in power tubes, and the output sockets had pins 1 and 8 connected to run el34's and kt66's as well.

My point here is that if a lower voltage PT is used, then the user gets to try the earlier versions of 6l6.  There is an increase in power over 6v6, but not as much as the jump in output to the later build 6l6gc types available today, and frankly the old glass just sounds better, in some cases MUCH better.  Currently I'm running a Bendix Red Bank 5r4 "tater masher" in the rec socket, and a pair of British vintage Emitron (Genelex type) kt66's in the power tube sockets, and the tones are beano-ish to say the least. 

So if this 5x3 were built with a PT having say, 275- 300v secondary taps and in sufficient current capacity, with all other things considered the use of these older 6l6 types, particularly the real 5881's, (Sovteks excluded) 6l6wgb mil stock, or 6l6gb's  should result in a wonderful sounding build.  Output will fall within the range that the OP wanted.  Old style tubes for an old style amp platform!  These older tubes are available on eBay for less than horribly expensive prices, and even though you might have to get  more than just a pair within a chosen type to put together a nicely balanced pair, the results are very much worth the effort.  And of course with pins 1 and 8 connected on the power tube sockets, this platform should be able to run kt66's or el34's as long as sufficient heater current is available from the PT.   Perhaps a few component values in the power section could be tweaked to optimize the circuit for el34's.     

Yes.

Saaaawwweeeeeeet!

Only way to find out is to just do it and listen!  Again, thanks to all who have posted here, some good stuff is happening!

Chances are significant that the Brimar would be more pleasing to many ears than any JJ.

Excellent, Brent! 

I have some of those JAN Philips 6l6wgb's.  They sound great. 

Baldwin labeled Japanese 12ax7 longplates???  Are you sure that they were Japanese?   There were very few Japanese made longplate 12ax7's, ever.  Perhaps you are thinking about Raytheons or Sylvanias, they made the longplate Baldwin labeled organ tubes.  The only Japanese Baldwins I have ever seen have gray short plates, with blue printing, and yes, they are very smooth and musical.

Your  preference for Chinese 12ax7's for the PI is interesting, why is that?

And thanks again, very much. for your post!

I especially like ToneControl's idea about closing up the back of your Pitbull.  Leave at least a one inch slot for air to enter and waft up towards towards the hot tubes, and of course leave room for that hot air to escape.  You might be pleased at the bang for the buck results.  If it doesn't work to your satisfaction, well, it didn't cost much money to try it out...

Honestly, your last thread input is tough for me to answer.   But here is my best try...

I had major success in the studio helping a friend out.  The improvement in sound quality using a Mass into the board compared to using a Shure sm58 in front of the speaker cab was more than substantial.  YMMV with other mics, though.  The miced setup in this instance did not accurately reproduce the speaker tone at all.  What works for me might not work for you, though.  The review comparing the Weber and the hotplate says it pretty well.

I enjoy the changes in tone that using the Weber MASS brings.  In my own use practicing or playing out I crank the amp of choice's master up just enough to get the basic tonal character I'm looking for and attenuate with the MASS just enough to make the volume manageable.   Most changes in tone due to the MASS are compensated for in the main amp's tone stack fairly easily, so no tone is lost for me if I squash the volume down a lot.  I don't have a way of measuring numerically how many decibels the reduction in volume is. 

If what comes in and out of the sound board sounds good, then it sounds good, period.

Peace. 

Have not owned a hotplate, but the Weber unit offers more than just attenuation, much better feature set.  Rather than just a network or resistors, the Weber uses an actual speaker motor to provide some of that (flyback voltage) feedback to the OT like a real speaker, mine has footswitch to turn attenuation on/off, headphone jack, TRS balanced out, hi or lo rate of attenuation, 4/8/16 ohm impedance matching, treble boost switch (more like presence) 0, +3, or +6db, plus a full tone stack (volume treb, mid, bass) controlling only the line out signal.   That gives the user better control over his tone through the mixer, no more micing ever.  About the only option I didn't get with mine was the rackmount ears.  I get some amazing "cranked up" amp tones at small room regular gig volume levels.   Better sustain and girth in the tone for sure.   No loss of treble unless wanted due to treb boost switch.

Still sticking by my previous post.  Have never seen any ill effects from flyback voltages EVER with low OT to high speaker mismatched impedance, and have spent lots of hours doing just that where the speakers had very big magnets and flattened ribbon wire VC's, and the amp was also powerful.  Think that the "flyback hypothesis" is exaggerated in it's ill effect from my experiences.  Maybe if the amp was dimed it would be slightly more of an issue, but in my experience that has not been a problem. 

With that said, yes, I'd much rather run a matched impedance setup and use an attenuator instead to reduce volume of a cranked up amp, by far.  It's true that you will lose surprisingly little volume with lower value impedance OT going into higher impedance speaker(s).  A lot depends on the speakers used.  The previously mentioned Weber MASS is available in different wattage capacities, MASS wattage rating should be double the wattage rating of your tube amp.  Mine was worth every penny.  Tones are better, particularly dirty ones than without using it at a given reduced volume if properly used.  Overdoing the attenuation thang on a clean and dirty channel selectable 2 channel amp will cost you clean headroom on the clean channel, though.

The thing to watch for is too LOW of a load for the output transformer, not too high.  You are fine with 4 ohm setting going into that 16 ohm speaker. 

I have an old Fender 160PS vocal amp running a pair of 6550's that is rated for 160 watts rms into a 2.3 ohm load.  It's been modded in a couple of the six separate preamp sections for guitar use.  The speaker outs on the back are rated 5.6 ohms.  So when both speaker outs are parallelled the load drops to that 2.3 figure.  Using one speaker out I run it into a 2/15 semi closed back with JBL g135's in series for 16 ohms.  With that kind of loading, the amp is more compressed and easier to play.  Because those speakers are so efficient, that rig is still VERY loud. 

The best way to get great sounding breakup at lower volumes for me by FAR is my custom ordered Weber MASS attenuator.  Because I ordered it with a TRS balanced line out, I will never have to mic an amp again at a gig, the line out puts out a very hot signal to go to the PA board, and there is a full tone stack only for that line out with volume, treble, mid, and bass controls.  I can adjust those controls and send them to the board, which is something quite special.  I recently used it to help a friend of mine in the recording studio and the results were spectacular in a good way, and easy to setup and use.   www.tedweber.com 

Thank you!   Done to death???  Naaaah.  Not if we keep learning from each other...  Better to have more inputs...especially more on vintage power tubes.  Keep 'em coming!