EL84 amp counterexamples, please

[eschertron]

The amplifier schematics that I've seen for a pair of EL84s in a PP configuration all have a long-tail pair PI. (Vox AC15, Marshall 18W and 2022 models)
From Merlin's preamp book I sussed that a cathodyne PI would generate plenty of signal for EL84s. Is this true? Can someone point me to a manufacturer or amp schematic that shows a cathodyne PI in this application?

[Enzo]

PV Classic 20

PV Classic 30 has four power tubes, but I don't see why that matters.

PV Classic 50, same story

PV Bravo

Gibson did many. A lot of them used 6V6s, but if you can drive a 6V6, you ought to be able to drive EL84s.

A quick tour of the manual, these all use EL84:
Maestro Stereo amp.

GA8

GA18T

GA78

GA79

Falcon

Hawk

EA7

EA35

[eschertron]

Gibson did many. A lot of them used 6V6s, but if you can drive a 6V6, you ought to be able to drive EL84s.

Thanks, Enzo! From what I read, the gain of the EL84 and the 6V6 seemed the about the same - so I came to that conclusion. All the fender schematics I looked at were (of course) 6_6 tubes and cathodyne PI. Thought maybe it was an American vs. British thing. I will gratefully look up those models you mentioned.

[Chuck H]

First... Even if 6V6 and EL84 tubes are capable of the same gain, that doesn't make them circuit compatible. 6V6's need A LOT more drive voltage than EL84's to achieve that gain. That's why Enzo indicated that an amp that uses a cathodyne for 6V6's should be able to drive EL84's. You couldn't make that statement the other way around without more consideration.

Second... How to drive your EL84's depends on your goals! I use a LTP to get the result "I" want. Which is to drive the piss out of the power tubes and also get useful clipping from the PI stage. Cathodyne PI's aren't every ones tastes as far as clipping goes. As far as text book "what is needed to get max output from EL84/6BQ5 tubes" I would say that a cathodyne is ideal. But, if your after an overdriven tone the game changes. Maybe a cathodyne will give you what you want. Maybe not.

[Gingertube]

One of the things I found out from correspondence with KOC is that he uses cathodyne splitters exclusively for his product range even though his TUT series books are full of differential PIs.

The Cathodyne tends to be used for smaller bottles but has been used for 6L6 etc(see examples above). Its a matter of having enough rail voltage to get enough signal swing.
I'm interested in this as I'm doing a new PP 6V6 Amp with a cathodyne at the moment.
One issue seems to be Merlins claim that a large grid stop is required on the cathodyne triode so as to not get a "nasty?" overdrive sound. When I discussed this with Kevin he said he'd did use a grid stop but only a few K and had never found it necessary to use large (100's of K grid stops).

The other issue seems to be the ability of the cathodyne to overdrive the output tubes. The output impedance of the cathodyne lines is MUCH smaller than that of a differential PI and it will behave differently when driven or driving hard.

The output impedance of the Anode and Cathode drive lines is approximately 1/gm (thats ONLY 600 Ohms or so even for a 12AX7) SO LONG AS THE LOAD IMPEDANCES ARE MATCHED.

As soon as you drive the output hard enough such that one tube cuts off then that is no longer true.

Here is the summary from a post I made on another forum (with my maths analysis) - I've left the maths out here.

For equal values of anode and cathode loads Zout at both anode and cathode is approximately = 1/gm
As the loads become unbalanced:
Zout(anode) increases from 1/gm toward RL
Zout(cathode) increases from 1/gm by a maximum factor of RL/mu

That is why the HIFI boffins have been recommending the use of low mu tubes for concertina splitters when driving anything other than a pure Class A output stage.

And it is why (In my opinion) the concertina splitter for guitar amps sound better with a HIGH mu tube to emphasize the even order harmonic distortion which result when the drive lines go out of balance at the Class A to Class B transition. Note also that when a tube cuts off then there is no gain and Miller Capacitance goes away which will also affect the High Frequency load balance.

It seems to me that the concertina behaviour would be susceptible to manipulation by changing the output tube grid stops (minimum load).

I have prototype running 6SL7 concertina (56K Anode and Cathode loads) to drive a pair 6V6G (Vintage ST Shape) with 10K grid stops.

For EL84 I would suggest sticking to 12AX7.

Please let me know what you think about the grid stop on the concertina triode.

Cheers,
Ian

[Chuck H]

The Valve Wizard

[loudthud]

Add PV ValveKing 50-112 to the list. Cathodyne driving a pair of 6L6 with a Texture control.

[eschertron]

Thanks for all the replies! I have been poring over Merlin's Preamp book, which is what led me to ask for examples of EL84s and a cathodyne. I want to see some drawings, hear about different designs, and hopefully actually HEAR some amps using these designs. Looking forward to my homework

Ian, you raise the same concerns that Merlin does in his book. I will keep in mind the things he says are 'the cure' for the unbalanced load problems but I will definitely experiment with circuits and component values. I'll post my findings.

Chuck, I am anxious to hear some of these topologies in action. My idea for this build is a riff off of the AC15, and I was surprised to find the LTP in it. I *do* want a high-gain type amp with a ferocious amount of treble, so wanted to pack as many stages as I could before the PI - hence the first choice of cathodyne. As Ian says, cathodynes tend not to play well with overdriven power tubes. Is that why you said what you did about 'driving the piss' out of the power tubes? Or is there more to the tonal differences between cathodyne and LTP than that?

LT, I will look up the texture control. Sounds interesting!

[EFK]

If you're after gain, and you want to drive the power tubes hard as well, why not use a Supro-type paraphase? No shortage of dirt and drive there.

[Chuck H]

True. You can't get more gain from a PI than a paraphase. But it's overkill for EL84's. With a typical guitar preamp, a paraphase PI could easily provide too much swing to me useful. But if you had only one gain stage... Like a two channel amp that only needs two 12**7 tubes! It's a fun idea.

eschertron, yes. Drive capability and the affects of grid loading are why I mentioned the LTP. I'm going out of my range saying this, but I think the LTP is more graceful with grid loading than the cathodyne. But there are some tricks that I've used and refined with LTP's that, according to other threads, work with cathodyne inverters too for minimizing problems with grid loading. I'd suggest building with the notion that you may change from a one triode inverter to a two triode inverter. That way you can make alterations and sample tones for yourself and decide what's right for you.

[soundmasterg]

I've messed around in a project amp with a floating paraphase, a cathodyne, and a LTP. The first two sounded pretty good after tweaking, with the cathodyne having espeically nice cleans, but the LTP had the overdriven tone I was looking for. I think it is as Chuck mentioned...the LTP can give useful phase inverter distortion in addition to power tube distortion, and it seems to do that better than the other designs. That said, I plan to mess around with the cathodyne more in the future.

Greg

[Gingertube]

To clarify something I said above - the cathodynes low output impedance means that it can drive plenty of grid current into the output tubes. While it is doing this the anode and cathode loads become unbalanced and the output impedances begin to rise. That was the point I was trying to make in saying that the cathodynes behaviour could be changed by manipulating the size of the grid stops on the output tubes, thus effectively "playing" with the minimum load impedance on the cathodyne.

I suspect that some of the attraction of the cathodyne is exactly the sound it achieves due to its 'warts', certainly (for guitar) it seems to sound better when those "warts" are emphasized by using a high mu triode, rather than minimized by using a low mu triode.
A bit of "lateral" thinking here - if the above opinion is correct then it begs the question - Anyone ever seen a pentode cathodyne? How far can the "warts" be emphasized before it turns to custard?

Cheers,
Ian

[eschertron]

I've done some looking and some youtube listening, so I've decided to go ahead with the design of an 18 watter. The Peavey Classic 20 (thanks for the tip, Enzo!) drives its EL84s with a pretty traditional cathodyne arrangement, so I will try that first, keeping a stage available for a switch to an LTP, per Chuck H's suggestion. I'll bread-board this like my first build, and try the options that I can think of... the important thing in my mind is that I've found working examples (starting points) for the designs I want to try.

The Classic 20 has a very "Marshall" 3-stage preamp, and with a boost switch certainly tries to maximize the preamp gain. I'll go with something different, planning on trying out a cascode pair for one stage. I'm still thinking about a build that might be reminiscent of an AC15, and so the "pentode sound" somewhere in the preamp might do it for me.

[printer2]

I've done some looking and some youtube listening, so I've decided to go ahead with the design of an 18 watter. The Peavey Classic 20 (thanks for the tip, Enzo!) drives its EL84s with a pretty traditional cathodyne arrangement, so I will try that first, keeping a stage available for a switch to an LTP, per Chuck H's suggestion. I'll bread-board this like my first build, and try the options that I can think of... the important thing in my mind is that I've found working examples (starting points) for the designs I want to try.

The Classic 20 has a very "Marshall" 3-stage preamp, and with a boost switch certainly tries to maximize the preamp gain. I'll go with something different, planning on trying out a cascode pair for one stage. I'm still thinking about a build that might be reminiscent of an AC15, and so the "pentode sound" somewhere in the preamp might do it for me.

Just for the heck of it the PI section might be of interest in seeing a cathodyne and a LTP using the same triodes.

[eschertron]

Just for the heck of it the PI section might be of interest in seeing a cathodyne and a LTP using the same triodes.

Very cool with the switching and the PPIMV. It took me a minute to see that the crossed wires on either side of S3-B were actually connected to the cathode strings and not just crossing over. Otherwise I couldn't see how the cathodyne would work.

edit: I like the switchable gain stage too. Have you built this? Have you published any pics or soundclips?