Self split push pull el84

allesz · Post by **allesz** » 10 Mar 2015, 10:36

Hallo mates, it's been a wile since I posted something new: as you can see my latest efforts have been in the tube amp fields, rather in stompboxes.

This is a self split push pull I build following some treads on tdpri.com (and also from the info I got on ax84.com) about the magnatone melodier 109 (wich used 6V6 power tubes btw).

The sound is really sweet: some people says this kind of output retains some of the richness of single ended amps, and I think they are right. For loud and clean chords a regular push pull (maybe fixed bias) amp is probably better and more powerfull, but with single notes, crunchy or full out distorted, the richness of the sound is great: it really sings.

I don't have the money and parts right now (and I don't really need so much power) but I suspect that an amp with a couple of EL34 or 6L6 would be just great in terms of power and tone.

The schematic is just a magnatone modified one; I omitted the diode bridge rectifier.

allesz · Post by **allesz** » 24 Mar 2015, 15:50

A small update after some rehearsals and live testing: I swapped the 10uF cathode bypass cap on the second triode with a 100nF one, the sound was too farty in the low end.

Sunday I had a rehearsal with my band, and I plugged the amp in a 2x12 cab with two celestion vintage 30 (great speakers me think), the sound was really good and the volume was right in a big band (drums, bass, sax and a keyboard player); it only started to crunch at 4 on the volume pot; so today I substituted the two 470K resistors on the power tubes grid with a couple of 220K (a popular mod on the 18 watt marshall clones) in order to get a little more clean headroom. I will test this mod soon at high volumes, at home the amp sounds just the same, but I can open it only to 1 or two before it gets annoying.

Another mod to increase the clean headroom would be to remove one (or both) cathode bypass caps; I am reluctant to do this because: a) I really like the tone right now. b) it seems that those caps are good in order to cut hum and noise (never expected this

). Maybe I can use a smaller cap on V1, but this will change the tone of the amp, or not?

allesz · Post by **allesz** » 21 Apr 2015, 16:40

The last mod I swear!

I modded the tone control to get more clean headrom and tonal range; it's a BMP tone control as you can see. I tweaked it a little by ear and by the duncan tone calculator. Really happy of the tones I get.

The amp now is a lot cleaner, it starts to distort at seven on the volume pot. Tested it at a couple of gigs, both unmiked, with a 1x12 cab and a marshall 4x12 one. The results were really good and the volume was enough to keep up with bass, drums, another guitar player, sax and electric piano

; it was a small place btw.

HTH · Post by **HTH** » 29 Apr 2015, 12:21

Sweet design. I've never played around with self-split yet, but it piques my curiosity as its a less-efficient PI than the ubiquitous LTP.

allesz · Post by **allesz** » 06 May 2015, 08:29

It's really nice sounding.
After you try it you will find LTP quite stiff in comparison: this amp was a rebuiild of a cathode biased LTP amp.
I also have a stock jet city jca20H (LTP, fixed bias, cathode follower before tone controls, master volume, positive feedback... a very different amp) and it's a lot louder than this one (even the cleans are quite loud with my stock telecaster); but I think this is due to fixed bias: the LTP cathode biased amp (a fame tonmeister btw) was not much louder (if any) from this.

allesz · Post by **allesz** » 28 Jun 2016, 13:24

This amp sounds very good and I use it for almost all the rehearsals and (few) gigs I did. I also have some lower and higher power options (vht sp6 and jet cith 20 head), but I use them sparingly right now; and I am building the board for an incoming 6L6 version right now. It will take months.

Wile building I had an idea to fight some hum/ripple from the circuit (not that I find the noise too much, but of course keeping it as low as possible can be triky with this phase splitting topology): put an electrolitic cap to ground where the two 4,7 K resistors in series, the ones that feed the screens of the pentodes, meets. You have just added another filtering stage (I used a 22uF - 450V cap), and the noise has gone down quite a bit.

Wile building the new board I have a big question about the value of those two screen grid resistors: the magnatone uses only one 4,7K resistor, with 6V6; I used two 4,7K resistors with the el84.... will this value will be good even for the 6L6?

btw I plan to put a small grid stopper in front of the second triode, and decrease (probably at 1,5K) the grid stoppers on the pentodes. The rest of the preamp will stay the same; even that probably the voltage going to the plates of the triodes will be higher...

clintrubber · Post by **clintrubber** » 28 Jun 2016, 18:45

Nice thread & thanks for sharing those experiences! No idea how those items you mention w.r.t. the 6L6 would turn out, more pondering/experimenting...

allesz wrote:It's really nice sounding.
After you try it you will find LTP quite stiff in comparison: this amp was a rebuiild of a cathode biased LTP amp.
I also have a stock jet city jca20H (LTP, fixed bias, cathode follower before tone controls, master volume, positive feedback... a very different amp) and it's a lot louder than this one (even the cleans are quite loud with my stock telecaster); but I think this is due to fixed bias: the LTP cathode biased amp (a fame tonmeister btw) was not much louder (if any) from this.

So the Fame Tonmeister amp was the donor for the Self split push pull amp ? As in: using PT & OT from the Fame ?

Thanks

allesz · Post by **allesz** » 28 Jun 2016, 20:41

Yes, I recycled everithing I could.
It was not bad, but I wanted to build something new.
No regrets, but I am lusting over a 25/30 2x6L6 version.... I may rebuild again the tonemeister, but I fear regrets and, mostly, I wonder if bigger transformers will fit

clintrubber · Post by **clintrubber** » 28 Jun 2016, 22:18

Hadn't heard of the Tonmeister before, but I like the idea of it. Saw some pics. Can well imagine larger iron wouldn't fit, and the tube-compartiment looked small, so even if the iron wiuld fit, would those larger 6L6 tubes do ?

I figure you've also considered a quad of EL84 ? (but would still require larger iron)

Curious to your 6L6 experiments, please keep us posted.

Thanks