EL84 amp counterexamples, please

[paggerman]

My first ever build was the Lamington. It's a design based on low cost readily available components. It uses two 30v power transformers in parallel and a voltage quadrupler to get the HT. Power transformers are expensive and quite hard to get in Australia. The OPT is a 100v line transformer. It uses 2*12AX7 and 2*EL84. The second 12AX7 is used for tone stack recovery and a cathodyne PI

Valve Heaven - DIY Amp

I still use this amp regularly.

Cheers

[eschertron]

Paggerman, thanks for the link! Interesting design philosophy; I must admit the scarcity of parts in Australia, along with the poisonous arthropods and the way deadly jellyfish, make me glad that I don't have to live under such harsh conditions!

So do you overdrive the PI and power tubes much when you play? Can you describe the sound you get, please?

[paggerman]

No, I don't overdrive. My playing is inadequate and I wouldn't want to inflict it on the neighbours (too much). . I also have a Champ based build using a Russian 6n2p and a NOS RCA 6V6-GT which sounds brillant even at low levels - its my preferred daily player. I just like building the amps for fun. I'm currently building a 5F6-A based amp. I'll probably never get to play it at home.

[printer2]

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?

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?

I did a different circuit with a switched PI that did cathodyne and paraphase. I thought I also had a LTP position but I missed one connection and only had a 4P switch. Since then I thought this one up. I have parts on a board for it and a chassis with some sockets in it but I have not got around to completing it. Another builder used the circuit but had a couple different values. He has had a bit of trouble with oscillation at higher gains, I can see needing to be careful with your layout. I never had a problem with my cathodyne/paraphase amp but then again it was not high gain, basically a Fender Harvard with cathode bias. I liked the paraphase better at edge of breakup as it gave a thicker sound but going beyond that it started to fall apart. Mind you, it all depends on how you dial in the inverted triode. Had a pot on it to change how much signal it got.

Switchable gain stage, not sure I understand. I thought I said to ignore the front end, it was on another site I guess. The bottom triode is wrong, I was changing the circuit and this is an unfinished version. I never noticed it but did not want to throw too much out there when I just wanted to point out the PI. The working schematic I have is a little more involved.



The PI is the same, the back end not much different than what others have done, the real odd stuff happens out front. I picked up some nifty switches where the center position has one switch going up and the other down, as shown in the diagrams. This allows you to have one high/low input jacks and send the signal to one input ot the other or in the middle position to both. The same switch allowed me to change the bias on the input so an 820 ohm resistor is shared between the inputs (use a 12AX7 in the V1 position and gain dropped down without cathode bypass to simulate a lower gain tube) middle is 820R plus 22uF bypass, the last position is 829R plus 22uF on one input and 2.7k plus 0.68uF on the other.

Now ignore all the above funny stuff we have the a regular 5E3 input. But why the four pole two position (actually has a center position but you do not want to go there) switch around the volume and tone control and from the volume of the other channel? Glad you asked. While I think the switched PI thing is pretty cool (spent the longest time getting from five pole to four) I do not see it getting around much. But the switches around the volume tone pots change their operation from the funky 5E3 controls into a regular voltage divider volume pot and tone control.



And the implementation on a 5E3 amp.



But getting back to the first schematic that was incomplete and was just suppose to be a simpler version of the next schematic, (Oh I am really going to like this part.), If you have the switches in one position you have a 5E3 front end and with the PI switch in cathodyne, pretty much the Fender amp. Now if you switch the PI to LTP and switch the volume tone switches (drum roll please), you have a 18 Watt Lite. Well ignoring the fact that you are using the same output tube from one of the pair. But you know with another pair of sockets run in parallel and a couple of parts and a 8k OT there is nothing stopping you from popping out some 6V6's and in some EL84's. Not the kind of thing you can do on the fly on stage but if you want to have both versions of those amps at home all it will cost you is a little more room and maybe $30 in parts.

Now it would be real interesting how much of the sound of the two amps are due to the output tubes and how much due to the rest of the circuit. And not to say you can not use the 5E3 front end with a LTP. Almost have a 5G9 Tremolux (without the fixed bias and the Tremolo). Or have a cathodyne PI and regular voltage divider volume controls (Oh yeah, almost forgot the mixing resistors). Or how about this with the EL84's? (Given the second set of sockets)


So yeah I have been doing some thinking. Got lot of other quirky ideas that I grafted onto other amps. One little nagging thing though maybe someone here can answer me, some real smart cookies on this site. The 18 Watt light has the two parallel triodes up front and it gives a gain boost. I have the cathodes shared but the plate resistor separated and the outputs combined after the caps. My thinking is I do not get the gain boost this way and I will actually have to switch on the high voltage side of the cap. Can someone confirm that for me?

Sorry for the long hijack. But you did ask.


Come to think of it, I could have just said, no the amp is not finished yet.

[eschertron]

Come to think of it, I could have just said, no the amp is not finished yet.

LOL! No, that's great!

[Dave H]

The 18 Watt light has the two parallel triodes up front and it gives a gain boost. I have the cathodes shared but the plate resistor separated and the outputs combined after the caps. My thinking is I do not get the gain boost this way and I will actually have to switch on the high voltage side of the cap. Can someone confirm that for me?

The parallel triodes in the 18W give hardly any gain boost compared to a ‘normal’ single triode stage but compared to the single triode input of the 18W the gain difference is about 6dB (double). This is because the single triode input of the 18W has to drive into the plate of the unused triode which reduces its gain. A ‘normal’ single triode gain stage only has to drive its 100k plate load and the vol pot.

If you use two coupling caps and common the plates after the caps (volume pot side) without mixing resistors the gain will be the same as the 18W in parallel mode but it will change the frequency response, with mixing resistors the gain will be reduced.

[printer2]

The parallel triodes in the 18W give hardly any gain boost compared to a ‘normal’ single triode stage but compared to the single triode input of the 18W the gain difference is about 6dB (double). This is because the single triode input of the 18W has to drive into the plate of the unused triode which reduces its gain. A ‘normal’ single triode gain stage only has to drive its 100k plate load and the vol pot.

If you use two coupling caps and common the plates after the caps (volume pot side) without mixing resistors the gain will be the same as the 18W in parallel mode but it will change the frequency response, with mixing resistors the gain will be reduced.

Hate to say it but I am confused. Will have to read this over a few times.

LOL! No, that's great!

Normally I get a 'Huh?' when I post my schematics elsewhere. Probably get more of a response by finishing them and trying them out.

[Dave H]

Hate to say it but I am confused. Will have to read this over a few times.

Sorry about that, tell me what's confusing and I'll rewrite it.

[printer2]

Sorry about that, tell me what's confusing and I'll rewrite it.

Been up most of last night and running on little sleep. Hoping tonight will be better. Today has been a write off for me.

[Ken Moon]

Here's one called the DaisyCutter by Randy Fay (Phaez Amps) that he posted on AX84, and he has some clips of it on Youtube:

[printer2]

The parallel triodes in the 18W give hardly any gain boost compared to a ‘normal’ single triode stage but compared to the single triode input of the 18W the gain difference is about 6dB (double). This is because the single triode input of the 18W has to drive into the plate of the unused triode which reduces its gain. A ‘normal’ single triode gain stage only has to drive its 100k plate load and the vol pot.

If you use two coupling caps and common the plates after the caps (volume pot side) without mixing resistors the gain will be the same as the 18W in parallel mode but it will change the frequency response, with mixing resistors the gain will be reduced.

Little late, reread it right now. Makes perfect sense. Lack of sleep is an evil thing. Thanks.

[eschertron]

Here's one called the DaisyCutter by Randy Fay (Phaez Amps) that he posted on AX84, and he has some clips of it on Youtube:
[ATTACH=CONFIG]24687[/ATTACH]

Thanks for turning me on to Fay's stuff. Some interesting youtube videos there!

[eschertron]

I've just noticed the 57 twin has a pair of 6L6s driven by a cathodyne PI. And from the rudimentary (bar-napkin) math I did the PI should start to clip right after the output tubes. So does this amp break the rules? Or is it the prime example of cathodyne PI distortion vs LTP PI distortion?

[Dave H]

Is that the 5E8-A? The PI only has a 300V supply. It could clip before the 6L6s. Perhaps that's why they changed the PI to LTP?

[Chuck H]

Yup. I don't know if the 5e8a is an example of a preferred overdrive Twin. Not the design usually copied by cloners and boo-teekers.