Fixed to Cathode Bias On The Fly

[Justin Thomas]

Sticking a pentode somewhere upnfront usually doesn't hurt, either!

Justin

[Mike Sulzer]

Agree. Another option, which might be worth a try, is to remove the cathode bypass cap on cathode-biased EL84s. This introduces local negative feedback around the tube and means you need 2 or 3 times more input swing on the EL84 grids to reach clipping. It makes them sound more like 'big bottle' tubes, if that's what you want.

EDIT: Re-reading, I think OP is trying to do the opposite of this!

If this is an AB output stage, then removing the bypass C has one effect for the A operation (cathode current changes cancel and so they are effectively connected to ground) than for the B operation (the on tube looks into the cathode resistor). Thus, the gain difference between A and B is substantial.

[Malcolm Irving]

If this is an AB output stage, then removing the bypass C has one effect for the A operation (cathode current changes cancel and so they are effectively connected to ground) than for the B operation (the on tube looks into the cathode resistor). Thus, the gain difference between A and B is substantial.

Yes, this is true. It's greatly exaggerating the usual gain change that you get in the transition from class A to class B (sometimes called 'gm doubling').

I think it's one of the 'nicer' sounding distortions. It also gives a kind of compression - relatively higher output when you play soft and relatively less output when you play more aggressively. (Just my opinion! Others can and will disagree. )

[SoulFetish]

Yes, this is true. It's greatly exaggerating the usual gain change that you get in the transition from class A to class B (sometimes called 'gm doubling').

I think it's one of the 'nicer' sounding distortions. It also gives a kind of compression - relatively higher output when you play soft and relatively less output when you play more aggressively. (Just my opinion! Others can and will disagree. )

Totally agree with this as far as EL84s go. The last amp i built used a bias switching arrangement, where i switched between cathode bias wiith an unbypassed shared resistor (or CBUSR. ), a pretty good emulation of fixed bias, and the ability to dial in a mix of the two. The circuit worked great but, I realized that i actually liked the CBUSR quite a bit more than the fixed bias sound. It seem to be the consensus for people i played it for as well. So I ditched switchin arrangement in favor of the cathode bias. This style of bias in combination with a higher P-P load impedance has a dynamic compression effect when driving the grids which really sweetens up the sound and "touch" of EL84s. At least to my taste.

[Malcolm Irving]

You can also have separate un-bypassed cathode resistors for the ‘push’ and ‘pull’ sides, or even one for each individual EL84 (if there are 4). In this case the local negative feedback applies during class A operation as well as class B.

[chunkitup]

Wow! I turn away from my phone for a day to celebrate the holiday and the thread explodes. Chuck, here is the schematic and if you can't read it I'll try to do better pics but this is how I build my amps. First the schematic and then map out the board and this is one of my busier amps.
And I have to laugh when you say "it's basically a SLO preamp" which it is not but I have been around long enough to know what you gurus need as far as info to help me get what I'm looking for. It's two totally separate channels and I'll include a pic of the innards to date to show you I have plenty of filtering. To explain the crunch channel feel with the SE84 was magical. It reacted really well to how hard you attack the strings and it had the right amount of compression and attack. Sounded so buttery awesome on the neck pick up then switch in the bridge and it sang out with great sustain. Sorry I've been watching Almost Famous and I want to say the guitar sound was "Incendiary "!!! Lol anyway here it is.

[Chuck H]

Very conscientious build My most complex design might have been as treacherous

I'm a little confused though... We've been discussing a single ended amp and this schem is clearly a push/pull.?. If it's basically the amp shown, but the feed to the PI is the feed to the power tube for the single ended version then please clarify that. If anything different is happening in the single ended version I will need to know about it before I can assess much.

[SoulFetish]

I usually run relatively low PI plate voltage on my el84 designs to limit swing and allow the preamp and/or PI to get into clipping only shortly after the power tubes. This is more similar to the type of performance people have come to expect from guitar amps with other tube types. But I do use quite a bit of pre PI attenuation in my personal amp, which is a proto for most of my other el84 designs that ever came later. In fact I'm working on a refinement of that design right now and getting the right balance of PI drive/clipping relative to enough drive at the power tube grids to get a little bias shift due to blocking (cap charging) and just really dialing the whole thing in... Well, I'm a little compulsive and insane as a result. But I'm taking my time and, obviously, I sort of like it.

This is an important point you bring up here, Chuck! The high input sensitivity of an EL84 can lead to a lot of unwanted distortion problems under heavy overdrive conditions. I personally think an important part of managing bias excursion and blocking is in the phase inverter design. The type of tube you choose for your phase inverter will have a big effect on the relationships between input sensitivity, gain, balance, and available p-p output voltage swing. Particularly in a Long Tail Pair.

[Chuck H]

Indeed! And I've considered using different tube types for the PI (other than the standard 12ax7) but then I find that, in the case of the LTP there is also a particular distortion contribution relative to design and tube type. There's also the aesthetic of using a "standard" tube in that position. I do occasionally sell an amp and part of my MO in design is to secure future use. So my designs need to use currently made and readily available tubes. To that end I don't even mess with at7's, ay7's or au7's since I know that, WRT available tubes for guitar amps, the ax7 will surely be the last to fall So trimming the operating voltage a bit allows me to keep a similar design (and response) at a somewhat lower output swing and still use the most available tube. I've read Merlin's whole thing on PI design and I like it a lot. I just think that idealizing has to include those particular distortions our ears are accustomed to as well as future maintenance.

The "unwanted distortion" problem is MUCH smaller with el84's, as I noted in post #10. el84's have always proven very forgiving with overdriving the beejeesus outta them. I have a lot more trouble with other tube types when it comes to clipping and grid drive levels. In fact on my last two builds, that weren't el84 amps, I had to limit grid drive and modify the HV rail to mitigate excess screen dissipation because I tried to push the power tube/tubes like I've always done with my el84 designs. A guy can only melt so many screen grids before learning his lesson

[chunkitup]

Very conscientious build My most complex design might have been as treacherous

I'm a little confused though... We've been discussing a single ended amp and this schem is clearly a push/pull.?. If it's basically the amp shown, but the feed to the PI is the feed to the power tube for the single ended version then please clarify that. If anything different is happening in the single ended version I will need to know about it before I can assess much.

Hey Chuck here is the SE84 amp that my friend wants as the crunch channel. Yes the convo has turned into a SE84 amp but my original question was could I use a cathode biased PP 5881 PA to help get the mojo of the original SE PA. You said it would not that the EL84 is unique.
Edit: yes I took the exact preamp that feeds the SE PA and it is now feeding the PP PA. Perhaps there is where a change needs to happen. I also was wondering about adding a EL84 as the fourth gain stage and then feed the 5881 PA?

[chunkitup]

Here's a pic of the innards. Not much room left over.

[SoulFetish]

Well, it only looks like one question still remains:
What is the story with these? why are they under your amplifier? and what the in the world do they say?
(okay, 3 questions...)



..something about all the fishies in the sea

[chunkitup]

Well, it only looks like one question still remains:
What is the story with these? why are they under your amplifier? and what the in the world do they say?
(okay, 3 questions...)

[ATTACH=CONFIG]43997[/ATTACH]

..something about all the fishies in the sea

OMG!!! The new girlfriend put all these little notes over my work bench. That chewed up toy was in the hamster cage. So freaking funny you keyed in on those notes. Maybe that is her way of saying come away from your amps and give me what I need. Ha!!

[Chuck H]

I also was wondering about adding a EL84 as the fourth gain stage and then feed the 5881 PA?

That would be like the Herzog that Justin referenced earlier. An amp made by Garnet decades ago that used a single ended el84 amp to feed a bigger tube amp in one chassis. The only hitch is that the el84 would behave like you were running the amp into an attenuator rather the dynamic load of a speaker. This isn't something that bothers me, but some players like to complain about the sound of their amps with an attenuator. YMMV.

Because your design has the TS directly feeding either the power tube or the PI there's no obvious late stage voltage division to mess with. There IS a voltage divider at the stage behind the cathode follower circuit, but reducing attenuation there would change the preamp distortion properties.

Looking at the overall design I think the big difference in tone between the SE el84 version and the PP 6v6 version would be that the SE el84 clips more asymmetrically than the LTP PI or the PP power tubes. The asymmetry pronounces the 2nd harmonic more and, to my ears, sounds more hairy (in a good way). You probably have just as much clipping in the PP amp actually. But it's more of a true square wave and that can sound flat and bland sometimes. There are a couple of things you can try. It would be REALLY convenient if you still have or can get your hands on the PP 6v6 amp for experimenting...

Increase PI voltages
Increase coupling cap size
increase power tube grid loads
Heat the power tube bias as much as practical

These things would greatly increase the likelihood of grid conduction and that should skew the duty cycle giving a more similar tone to the SE clipping. But with the addition of some crossover distortion that may or may not contribute something good or bad depending on some hardware particulars (the OT and speaker) and your own tonal sensibilities.

I'm actually working on the production model of a proto PP design right now where the proto sounds fabulous and exhibits some degree of uneven duty cycle but the production example doesn't sound nearly as good overdriven and lacks the asymmetry. There's definitely something to this.

[chunkitup]

I want to "Like" what you wrote but I don't see an icon anywhere to do that. You are always so informative and you say it in a way I can understand.
I think what it's coming down to is me and my buddy sitting in his practice space cranking the channel and changing what needs to be changed and making it work. Actually I did finish a Jose type amp WITH the zener clippers and he was ok with it. I could do that although I found that amp a finicky wench. Squishing the living crap out of the signal takes away a lot of things you can do before the master volume in that circuit. Oh I just remembered I have a triode not being used and I always loved the Bogner xtc vibe and I think that would be good for a hard driving blues sound. Hmm!!?