18W Plexi with EL84s?

[mcamelo]

Hi, all.

First time poster and second time amp builder (just completed of my first 5E3 build ).

I'm looking to build a Marshall-style amp next. 18W would be preferred for my use (home practice + small-sized parties). I've considered the Stout TMB but maybe a Plexi would be more representative of the classic Marshall sound? The Plexi 6v6 seems to fit the bill but I am wondering if I should go the extra mile and build it to use EL84s instead. I've watched comparison videos and the difference is subjective and not huge, but if the work involved to adapt the circuit is trivial, I would be willing to give it a try. So my two questions are:

1: Are EL34s closer to EL84s than they are to 6V6s?
2: What are the modifications required to the Plexi 6v6 circuit to use EL84s?

Thank you.

-Marcelo

[JB]

It's been done - the AX84 November:
https://ax84.com/archive/ax84.com/index.php/oldprojectse71c.html?project_id=november
See also Sluckey's November build:
https://sluckeyamps.com/november/november.htm

[Bieworm]

Hi, all.

First time poster and second time amp builder (just completed of my first 5E3 build ).

I'm looking to build a Marshall-style amp next. 18W would be preferred for my use (home practice + small-sized parties). I've considered the Stout TMB but maybe a Plexi would be more representative of the classic Marshall sound? The Plexi 6v6 seems to fit the bill but I am wondering if I should go the extra mile and build it to use EL84s instead. I've watched comparison videos and the difference is subjective and not huge, but if the work involved to adapt the circuit is trivial, I would be willing to give it a try. So my two questions are:

1: Are EL34s closer to EL84s than they are to 6V6s?
2: What are the modifications required to the Plexi 6v6 circuit to use EL84s?

Thank you.

-Marcelo

I have built 2 already.1 classic 18W with tremolo, and 1 tremolo tmb.  The latter is marshall heaven..  in fact, I'm building a second one right now, but with reverb. At 18watt.com there is lots of information about those amps. Mostly el84 based..

[mcamelo]

So it's not looking like a completely foolish and vain pursuit :-)

I will look closely at how the schematics differ between the 6v6 and EL84 variaties and comeback with more questions. 

Thank you for the pointers so far.

[sluckey]

If I were doing this I'd ditch the fixed bias and use simple cathode bias on the EL84s.

[bmccowan]

If I were doing this I'd ditch the fixed bias and use simple cathode bias on the EL84s.

Steve - this reminds me of something I've wondered about for a while. Almost all amp builders, DIY and pro, seem to prefer cathode bias for EL84s. But folks are split on big bottle tubes. Is there something intrinsic to EL84s that limits the sonic differences between the two schemes. Any amps I've built with 84s I have used cathode bias - but just the blind following the sighted. I have built many 6V6 amps both ways and can notice a significant difference in character. I just wonder what is behind this?

[sluckey]

Wish I could answer that. I've only built one fixed bias EL84 amp, but about a half dozen cat biased EL84 amps. I prefer the sound of my cat biased amps, but that may have nothing to do with the EL84s. Probably just my copy cats. 

[shooter]

never built fixed with '84s
have fixed a handful, mostly the PA circuit fails, including bias part.
I have no "scientific" reason, my suspicion is the ease at which the 84's can be driven.
feed it to much to fast and it "over-reacts", where an 88 would be, "ya, ya, i'll get there"

[bmccowan]

I remember reading an interview with Richard Goodsell who got his start building Super 17 EL84 amps, originally from Hammond organ A0-35s (Doc Z was not alone.) He was asked why his amps were cathode biased, and his answer was, "why would anyone do anything else?" Shooter, I suspect you are on the right track there, as they sure can be easily pushed over the edge. I've come to not like El-84 guitar amps much, but love EL-84s in hi-fi amps.

[mcamelo]

If I were doing this I'd ditch the fixed bias and use simple cathode bias on the EL84s.

That would simplify the build somewhat and the responses in this thread suggest that any possible sonic difference is not worth the trouble, so I will definitely consider that option. Again, the main motivation behind this build is to have an 18 watter that gets me as close as possible to the quintessential Marshall tone (within reasonable cost, skills and effort requirements), so I'd love to hear everyone experience in that regards.

I've been looking at the November schematic and comparing that to the Stout's, both sporting EL84 power sections (fixed vs. cathode biased, respectively) and the difference in operating voltages seem quite large (296 vs 340 and 295 vs 348 for plate and screen, respectively). I would imagine that would have quite a dramatic difference in tone, no? Also interesting that the Plexi 6v6 schematic lists voltages that are higher still, despite being fixed biased. I would love to understand the reason for those differences and also the impact in sonic characteristics.

Thank you again.

[mcamelo]

Is there something intrinsic to EL84s that limits the sonic differences between the two schemes. I have built many 6V6 amps both ways and can notice a significant difference in character. I just wonder what is behind this?

Found this in an older thread:

"EL84s aren't used with fixed bias often because they need less bias voltage... the effects of cathode biasing are reduced"

https://el34world.com/Forum/index.php?topic=23238.0

[sluckey]

If fixed bias EL84s is desirable you may want to look at Hoffman's Blues Junior and Pro junior. Would be easy to put a Plexi preamp in front of either of those fixed bias power amps.

[shooter]

love EL-84s in hi-fi amps.

Agree, i did 2 guitar and 2 audio, I got the parts for the audio builds from the guitar builds 

[d95err]

I’ve built a few EL84 amps, incuding one ”plexi”-ish build. Never really bonded with any of them and keep rebuilding them over and over. Some kind of harshness there I just can’t dial out. Could be my limited skills though.

Next time I build a smaller Marshall-esque amp I will go with 6V6, or even the low-voltage EL34 route.

[shaun]

I can't respond directly to your two questions, but I built something along those lines with help from Sluckey and others in this forum. I was very pleased with the results, and maybe it'll be of some help.

It's close to a Stout. I used EL84s, but it has less gain in the preamp. I still get a nice break-up at an appropriate point though, so I'm a happy camper. I'm posting the schematics I worked off. They are scrawled on, but that was part of the process; I really should re-draw them into a nice schematic, but at least they'll give you a good idea of how things went, with voltages when using EL84s. Specs say EL84 should be around 250 plate volts, but they can handle considerably more. Mine sound great:).

I used the original transformers out of an old Gulbransen organ amp, so I went with an EZ81 rectifier which was part of the power system already, but I had to tamp it down with a power-dropping resistor. Anyway, hope this helps your decision making.

[mcamelo]

Thank you, everyone, for the input so far.

I've run into several posts in gear forums describing EL84s as harsh and a few saying how they prefer 6V6s in low wattage Plexi clones. In other words, I've found no evidence to support my initial premise that an EL84 would be closer to an EL34 than a 6V6 other than in naming. With that in mind, I would be inclined to stick with the Plexi 6V6 circuit and remove quite a bit of risk (though also a bit of learning).

But, with my top priority being to gett as close to the cranked Plexi tone as I can without the police knocking on my door, I'm now asking myself if I should go with the Plexi 50 circuit, instead, and rely on a reactive attenuator to bring the volume down to sane levels.

Do you think that would that get me closer to my sonic goal versus a downsized power amp?

I'm halfway through building an M2 attenuator (https://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/), so should soon be able to hear how much tone it sucks.

[shooter]

without the police knocking on my door

that's 2019, doesn't happen any more, unless you're eating turkey 

[brewdude]

I’ve been wondering if a 6K6GT would be a good choice for a lower power pentode in a plexi style circuit. One issue might be lower rated maximum voltages—plates 315v, screens 285v

[shooter]

fwiw;
here's the thread on my version of a "plexi'd" Silvertone 1482 using 6K6
I re-rolled it for 6V6
Neither version got to real crunch, the 6K6 did break up better at lower loud for a house with ppl.




https://el34world.com/Forum/index.php?topic=25104.msg271638#msg271638

[AmberB]

Weirdly enough, I thought that the EL-84 was the standard output tube for the 18 watt amps...

[sluckey]

Don't know what standard is any more when people change tubes like changing underwear, but EL84 was in the very first Marshall 1958, 1973, and 1974 18Watt amps that all the various clones are based (or inspired) on.

[Beezerboy]

the Hoffman 6V6 Plexi is one of the sweetest amps I've ever played through.   it's way good

[JB]

Weirdly enough, I thought that the EL-84 was the standard output tube for the 18 watt amps...

Yes but the OP is asking about making a Plexi with EL84's instead of EL34's.  The JTM45/5F6a derived Marshall Plexi is a very different amp (topology, sound) to their WEM Dominator based 18W series.
If you want an '84 based Plexi it's all there in the November build doc's. 

EL84's can sound a bit harsh when driven hard, maybe more so in fixed bias.  Not unusal to see conjunctive filters across the OT primary to counter that, e.g. in the fixed bias Fender Pro Junior or cathode biased Dr Z Carmen Ghia.

[vampwizzard]

Bo Ramsey moved off tweed amps to boutique el84 amps in 2009.. Carr Hammerhead, Xits X10, Matchless Lightning. They sound pretty damn good. He dimes the volume and controls volume at the guitar and by pick attack.  Carr made the Hammerhead with both el84s and el34s, cathode biased. May be worth hunting down examples of each for sound reference.

a WGS ET65 12 inch speaker and el84s sound pretty DAMN good to me.

[bmccowan]

I like Bo's playing and tone. But I have a hard time thinking its the Marshall tone that is being sought out here. But then I have not heard all of Bo's stuff. And for playing at home, you better have a big home, if you are going to dime the amp and control volume only at the guitar. If I was attempting to do what Mcamelo was seeking, I'd follow the preamp schematic of the Marshall he likes and dial everything back for 6V6 power tubes. I'd be ending up with some version of the Plexi 6V6. I've built one and use JJ 6V6s in it. Its a proven and great sounding circuit. A good attenuator will allow it to sing at home. Not claiming that those 6V6s nail the EL-34 sound, but they are closer that EL-84s IMO.

[mcamelo]

And for playing at home, you better have a big home, if you are going to dime the amp (...) I'd follow the preamp schematic of the Marshall he likes and dial everything back for 6V6 power tubes. I'd be ending up with some version of the Plexi 6V6. I've built one and use JJ 6V6s in it. Its a proven and great sounding circuit. A good attenuator will allow it to sing at home. Not claiming that those 6V6s nail the EL-34 sound, but they are closer that EL-84s IMO.

Real story: last evening I pulled the trigger on parts for a Plexi 50 build on Hoffman's store and set down to play my recently built 5e3, alongside the Egnater Rebel 20, plugged into my just built M2 attenuator. After one hour of playing at decent volumes (the missus was out and about), with and without the attenuator, I decided that 50W would just be too loud to be practical. Sent Doug and email cancelling the order and replaced it with a Plexi 6v6 kit before going to bad. Woke up to bmccowan's post above.

I'm in piece with my decision :-)

Thank you all for your insights.