5E3 Type Circuit with 6L6's, best OT impedance?

[makepeace]

Hi guys,

Preamble: I am a beginner in this field so I do not claim to know anything, and would welcome any info, be it in laymans terms, or technical. Either would help me learn! Also don't really need discouragement in terms of what I want to do, as I've read about it all over the internet (ie, if you do this its not going to sound like 5E3 so you may as well build something else).Thanks!

So I've built a 5E3 using the standard arrangement except for subbing the "normal" volume control with a Blackface type tonestack and putting a choke between the screen and plate HVs. The PT I was using wasn't appropriate, and I've finally taken the plunge and ordered a PT that will supply the "correct" voltages and currents. Its tapped for 610 and 710HV's and is capable of supplying big heater currents so will hopefully provide some room for tinkering.

The next step is ordering an OT with lots of iron (relatively speaking) and appropriate primaries.

The primary goal here is to be able to run 6L6 type tubes (specifically the Tung-Sol 5881, but possibly other things like KT66's and other 6L6's) in triode connection (possibly pentode later), and still be able to swap out for 6V6's (don't mind if i have to flick switches and swap plugs and stuff to change OT, PT secondary voltages (might also use the zener trick) and cathode resistors).

What my main problem is at this stage is deciding on an OT that has an appropriate combination/versatility of primaries, and also to ascertain what the "right" value of the OT primaries would be for 6V6's and 6L6's respectively with this type of setup.

From what I've read, this can be quite a painful topic, and that there are quite a few variables involved.

So, my two scenarios:

6V6's in push-pull, cathode biased triodes (relatively low plate V)
6L6's in push-pull, cathode biased triode connected (screen to plate) (relatively high plate V)

5Y3 rectifier, so theoretically, my B+ will have a span of 345 to 401 VDC (305x1.13= 345VDC to 355x1.13= 401VDC)
This is of course without the use of zeners to drop voltage, so we could probably call the range 300V to 400V.
I am also open to swapping out for a beefier rectifier, but not SS.

So far what I've gathered is that the "accepted" primary impedance for the 5E3 circuit layout is 8Kohms, and I've found that the most similar circuit to the 5E3 with 6L6's is the rare 1st production narrow-panel Pro (5E5) which has an OT with primary impedance 6.6Kohms (people say that these things are rough, but mainly due to the 15" on them). I've also read that anything between 4Kohms and 8Kohms will work for this setup, but thats not really good enough for me.

I've been looking into the OTs from Musical Power Supplies, and have been referred to the OT42PP, which has the capability to be run as 4200ohms into 2,4,8,16 ohms or 8400ohms into 4,8,16,32 ohms.

I guess what I'm getting at, is how do you determine what the value of your OT primary SHOULD be, theoretically, based on tube specifications and what you put into them re voltage etc, ie what the output impedance of your power stage is (and I know that it depends, but obviously there would be an average value based on the range of operation). In the JJ 6V6S datasheet it states that the Ra-a for a PP pair is 10Kohms. Why doesn't the 5E3 use a 10Kohms - 8ohms OT?

Would appreciate any comments!

Jono

[pdf64]

Don't agonise about this.
See the GE datasheets, tubes work fine over a wide range of load impedances, p9 of http://www.mif.pg.gda.pl/homepages/f...93/6/6L6GC.pdf
Try the triode arrangement but don't put too much effort into it; my experience is that it just doesn't sound like a guitar amp.
The thoughtful folks at Mercury Magnetics make an OT that may suit your requirements -

FTDO-59M Tweed -- '59 -- Dual 8Ω taps (one optimized for at 5k for use with two 6L6s
the other optimized at 8k for use with two 6V6s) -- grain-oriented soft iron laminations

only $165.

Why doesn't the 5E3 use a 10Kohms - 8ohms OT?

I think that era of amp designs acknowlededged IP to RCA, ie they were basically the suggested circuits that the tube manufacturer's published.
I can't quickly find any 50s 6V6 datasheets, but the GE one from 63 recommends a load of 8k http://www.mif.pg.gda.pl/homepages/f...93/6/6V6GT.pdf
with 10k for lower B+, lower output operation.
Pete.

[makepeace]

Don't agonise about this.

ahhhh.. but where's the fun in that?

Thanks for the reply Pete. That transformer does sound pretty applicable, but high on the price scale though! Although, undoubtedly the quality will be amazing. I think I won't really have trouble getting what I want though, when I know what I want, even if it has to be custom wound. Pretty much anything is possible these days with the internet.

Would really love to have a comprehensive understanding of the whole output impedance story.

So you would suggest against triode application of 6L6's? Any particular reasons?

Edit: Woops. Have made a major stuff-up.. confusing triodes for tetrodes here.. my 5E3 has the 6V6's connected as tetrodes and the 6L6's will also be connected as tetrodes. mi scuzzi.

[txstrat]

Would really love to have a comprehensive understanding of the whole output impedance story.

The transformer is wound with a fixed ratio which matches the high impedance of the tubes to the low impedance of the speaker.
For instance the Hammond 1760F has a primary of 4000 ohms and three speaker wires of 4, 8 and 16 ohms.

This ratio means, if you put a pair of tubes with a raa (plate to plate load resistance) of 4000 ohms to the primary you'll get either 4, 8 or 16 ohms at the appropriate secondary.

It's not critical to match the load resistance exactly - it just changes (due to the ratio of the OT) the secondary impedance.
It's sufficient if you can get "close".
A 6L6GC can have a raa from 3800 ohms to 6000 ohms depending - among others - on the plate voltage. You can get those "specs" from the tube data sheets. (see data sheet).

Mismatch of the primary will result in a different (mismatched) secondary. (That's why you set the speaker switch to a lower value when pulling two power tubes of a 4 power tube amp).

If you put a pair of 6V6 (which have roughly a raa of 8000 to 10000 ohms) to the above mentioned OT instead, the secondary will change to (roughly) 8, 16, and 32 ohms at the 4, 8 and 16 ohms taps respectively.

[MWJB]

Early/mid Tweed era 6L6 amps often used a higher primary Z than late tweed/tolex amps. Between 6K & 7K would be ball park and work fine with 6V6...if you weren't bothered about esoterics then a Fender Tweed Blues Deluxe OT would be perfect.

If you build the amp to only run with a 5Y3 rectifier, when you switch from 6V6 to 6L6/KT66 your power will not rise, it may even drop. You will need to bump up the B+ to exploit the power of the bigger tubes. So I would opt for a bias that gaave good performance with 6L6 and a GZ34, maybe will work fine too with 5Y3 & 6V6?

Wire the power tube sockets as you would for a EL34 (pin 1 & 8 joined, 1K 5W screen grid resistors).

It's not a "painful topic"..unless you make in one ;-)

[makepeace]

Thanks txstrat. I understand the transformer turns ratio story and what size load the tubes see the speaker as etc etc

What I don't understand:

Why you shouldn't try and match speaker to power stage (as close as practically possible)
-surely unless there is some desirable quality that you are missing out on, you would aim for maximum efficiency?

How to determine output impedance of power stage from tube datasheets
-surely its not as simple as using your bias voltage and the plate voltage to determine your plate current and then R=V/I?

[MWJB]

If you were aiming for maximum efficiency your amp would be fixed bias running 510-525vdc on the plates, LTPI, SS rectifier, multiple speaker array...it wouldn't be a 5E3/5E5 style amp, which you would build because you liked the tone. Tone is why you choose a particular OT primary, few guitar amps ar built to be the pinnacle of efficiency..."enough" power and efficiency with a tone that is pleasing is what many builders actually aim for.

6L6 amps have been built with OT Primaries from 2.5K up to 8K, anything between these points can be considered a "match".

[makepeace]

Probably should have made it more clear, but I meant efficiency of power transfer between load and source at the end of the chain. I understand that the circuit is not necessarily designed to be efficient, but to sound good, and thats fine.

Tone is why you choose a particular OT primary, few guitar amps ar built to be the pinnacle of efficiency..."enough" power and efficiency with a tone that is pleasing is what many builders actually aim for.

Ok, so following from that.. how do you choose your particular OT primary for "good" tone? Surely you can (and it makes sense to) narrow it down to less than 44% diversion from the mean? I mean that's huge.

[MWJB]

Good tone is subjective, you listen. If you don't know what you want - which is liberating as you can choose anything in the workable range & be "right" ;-) -, then hedge your bets & buy a 3.6K-4K OT with a 4ohm & 8ohm speaker tap, run it on the 8 ohm tap with 6L6/KT66 etc, plug the 8ohm speaker into the 4ohm jack for 6V6s (for a 7.2K-8K primary)...then try it like this with 6L6s & see what you think.

Diversion from the mean is more like +100%/-40%.

You seem to be looking for a single, definitive answer, there isn't one beyond what your ears tell you. As long as you have this "tunnel vision" you will struggle because it applies to every aspect of your amp's design (power tube plate voltage, preamp voltage, etc.) Have faith, bite the bullet & build...if it sounds right to you then enjoy, or fine tune once it's up & running and you have a datum to work from.

[Chuck H]

What MWJB said... One thing to remember is that you want it to sound good. All exacting tolerances aside, a lot of happy accidents in tone have occured due to the low stringency of design criteria where tube guitar amps are concearned. The bottom line is how it sounds.

I say just use a 6.6k OT and be done with it. Fine for all the tubes you mentioned at the voltages you mentioned. And near always proves a good tonal choice. Just ask Ken Fischer re the Express. Well, he's dead but you know what I mean. It will also serve to greatly simplify your design by not needing to switch OT parameters when switching tubes. Also, FWIW and IMHE most OT's seem to sound their best at a particular primary Z. Meaning that an OT that sounds good with 6V6 tubes at 8k will often sound best with 6L6 tubes at 8k and changing the impedance to 4k for the 6L6's may not sound as good. I've even found that to my ears changing speaker loads can effect an OT's tone. As in changing from an 8ohm loadwith a single 8ohm speaker to a 4ohm load and two of that same speaker. Secondary adjusted for the 4 ohm load. Probably some interactive difference in the reflected inductance in the OT itself, dunno, but it can be heard over and above the difference of one vs. two speakers.

The goal is still tone. You probably won't be using this amp for hi fi reproduction so exacting tolerances and ideals give way to broad safe ranges and how they affect the amplifier as an instrument. BUT, it's a black art with sooo many variables in tubes, voltages, impedances, speakers, nature of the winding and laminate materials that a happy accident is best you can hope for. You can develope an intrinsic feel for some things with experience and you can clone well known designs when well reproduced parts are available. But usually the game is "Build, listen, adjust, repeat."

[makepeace]

Hmmm. I'll let it slide for the moment, but I think you're wrong about there being no definitive answer at all. There must be a theoretical answer at least, on which the practical and "real" (if you will) answer will be based.

As for subjectivity, I think that the majority of people will agree on what sounds good and what sounds bad. That's controlled by chemicals released into your body caused by nervous stimuli, which everyone has in common. There will obviously be a range of what sounds good, and that's where subjectivity comes in. You still need a theoretical starting point, which with a bit of modelling would hopefully be somewhere in the middle of that range.

And anyway, I think there needs to be a balance of "tunnel vision" as you call it, and a feeling for what sounds better than average. The latter is already there, and is definitely subjective and will develop on its own. The former is the aspect that can be controlled and can be developed by questioning and learning.

[makepeace]

thanks for that insight Chuck. I think that I'm going to go with a 4K primary into 2,4,8,16, which would be 8K into 4,8,16,32. and then use a rotary switch for the secondary impedances. going to have two jacks anyway, one for an external cab.

[MWJB]

" I think that the majority of people will agree on what sounds good and what sounds bad." The majority of people won't give a rat's ass what your OT primary Z is, 2.5K/4K/8K? They're all right, they have all been used in successful 6L6 designs. You will, only by comparing them in listening tests, determine your preference.

As Chuck says, never underestimate the "happy accident". Many manufacturers based OT choice on what they could get plentiful supplies of, at a favourable price. Primary Z just had to be in the range that didn't cause failure under warranty.

[makepeace]

Great. So now you've taken what I've said completely out of context. I hope that the fact that you've completely missed the point is down to a mere misunderstanding. I'm not going to waste any more of your time. Thank you to everyone who has had something constructive to say.

[pdf64]

The precise load value of the OT is not significant. Really.
Check out speaker impedance against frequency, see grey line http://www.eminence.com/pdf/Legend_GB128.pdf
It varies MASSIVELY. Play a walking bass line; the tubes will be driving a load that probably varies over a decade range, but the tone stays consistant (even the sound level stays basically the same).
My experience of flipping the output tap switch on my 6L6 p-p amp (AB763 based but no NFB) between 8 and 4 ohms, with an 8 ohm speaker, is that the tone is cleaner / punchier on 8 ohms (therefore 4k primary), more harmonicy on 4 ohms (8k primary). Similar to, but as extreme as, flipping the pickup switch on a guitar (the switch was flipped when the amp was quiet).