How can this work? Amp takes any octal tubes, no problem

[waldner]

The Morgan RCA35 demonstrated at the following link and described by Mr. Morgan as "class A, cathode biased and can take any octal pair of power tubes, just swap them in."  https://www.youtube.com/watch?v=aQJRf7mpL0Q&ab_channel=Sweetwater


I understand that cathode biased means non-adjustable will bias itself, but how can an amp design be made to work just fine with things as different as 6V6 vs EL34 vs 6L6 vs 6550, etc...  I always understood that it's critical to have the optimal power and output transformer for the type of power tubes you are designing the amp for and their respective power output and requirements?


Another even more curious example is the Victoria Regal that is said to be single ended, class A and can take any octal ranging from a single 6V6 at 5 watts to a pair of EL34's putting out 35 watts.  Here's a link:  https://victoriaamp.com/product/regal/


Help me understand what is going on with these flexible designs.  Thanks!

[PRR]

If you pay for 6550, you expect 6550 power.

_IF_ you relax that expectation, you can get 6V6 power out of any larger tube.

> "single 6V6 at 5 watts to a pair of EL34's putting out 35 watts"

Well, that seems to have switching, whether owner or computer I do not know.

[HotBluePlates]

...  I always understood that it's critical to have the optimal power and output transformer for the type of power tubes ...

The folks who emphasize "critical" (typically on guitar amp forums) seem to know little about practical amp-building.

As long as the plate eventually connects to a "positive voltage," the cathode eventually connects to a "less-positive voltage," and the grid eventually connects to an "even-less-positive voltage" then an amp will work despite some very-poorly-selected circuit values.

We've had guys on this forum salvage random parts, sea them together in an objectively goofy circuit configuration and, " the amp works well."  It's only by comparison to a more-optimized amp/circuit that the deficiencies are apparent.

... Victoria Regal that is said to be single ended, class A and can take any octal ranging from a single 6V6 at 5 watts to a pair of EL34's putting out 35 watts. ...

A different link, so you can see what's in the amp.

It's just a single output transformer, and Victoria claims each has "its own independent transformer winding."  So this is parallel single-ended.

You only get "5 watts" if you yank both power tubes and install a single 6V6 into one of the sockets.  Presumably you get more like "8-10w" if you put a 6V6 in each output tube socket.

They do say "35w" for two EL34s installed, but that must be counting distortion because two clean EL34s in Class A should only be able to manage 24-25w.  Either way, your ear won't notice much change from "24w" to "35w".

[PRR]

....They do say "35w" for two EL34s installed, but that must be counting distortion because two clean EL34s in Class A should only be able to manage 24-25w.

Philips too claims 35W in "AB", meaning cathode bias. The rise of current with signal is enough to foil the 2*Pdiss simplification but not enough to ruin the THD.

https://frank.pocnet.net/sheets/030/e/EL34.pdf

[HotBluePlates]

....They do say "35w" for two EL34s installed, but that must be counting distortion because two clean EL34s in Class A should only be able to manage 24-25w.

Philips too claims 35W in "AB", meaning cathode bias. ...

Except the Regal's selling point is that it is single-ended.  It does say, "The Regal is designed around a custom bifilar output transformer that is essentially two discrete transformers driving a common speaker load."  I'm not sure of a way for 2x EL34 in some form of parallel-single-ended to get an honest 35w.   

[waldner]

Well, I couldn't resist and my curiosity got the best of me.  I bought a used Regal II that was available near me.  It's a pretty damn good sounding amp!  So far I've just tried running the two 5881s that came with it along with a 5U4 rectifier and then swapped in a single vintage 6V6.  Both sounded great.  The two 5881s had more headroom and were louder but the single 6V6 gets really good dirt earlier at more reasonable levels.  I can't wait to play out with this one to see how it sounds live.  I've got quite a collection of tubes to try, so this is the perfect amp for me to experiment with... weird combinations like a 6V6 and EL34.  Different rectifiers...5Y3 vs 5AR4.

I've always been interested in trying a Regal ever since I saw Terry Frank (now deceased) play a gig through one at a local bar.  He was getting great sound- better and different than most.  I think much of that is the single-ended sound, though this amp is less grainy and crude sounding than what I've experienced with Fender Champs.  It does have a little higher noise floor though than any of my push pull amps.

The reason I bought it is that I couldn't build one.  I've built the majority of my own amps in the last 12 years, but this one is a unique single-ended design with its "bifilar" output transformer said to behave like two independent transformers in one package, each output tube having it's own winding which then drive a common speaker.  I wish there was a schematic for this so those of you who understand circuits way better than me could analyze and explain what's going on in more detail.   

[DummyLoad]

35W out of pair of EL34 in parallel SE? Maybe for a short while until the cherry-red plates collapse and/or the glass melts. 

You'd be good to get 22W out a pair in parallel with 10% or more THD running them at 90% of rated Pdiss with optimum load.

--Pete

[HotBluePlates]

... this one is a unique single-ended design with its "bifilar" output transformer ...  I wish there was a schematic for this so those of you who understand circuits way better than me could analyze and explain what's going on in more detail.   

Schematic not needed.  We can see from photos that each output use socket has a 270Ω cathode resistor.  And we know that a "bifilar" transformer has 2 primary windings formed by laying the wire of each winding side-by-side, and building up the turns together.  That creates tight-coupling between the windings along the entirety of their length.


What would be needed is to apply a small voltage one a winding (primary or secondary) and measuring the volts at each of the other windings to establish the turns-ratio among them.  From there, using a scope to monitor the cathode current of each output tube and the secondary voltage across a known-load, while applying a known drive-signal to the output tube grids.  Rinse & repeat with different tube combinations to understand how the 2 primary sections interact, and the actual power output delivered by each output tube set.