Luix Dual Channel EL34 50W Power Amplifier

[luix]

Hi guys,

the pandemic event gives me the time to build again a tube power amplifier and I'd like to share it with you, since I was following this forum long time before my subscription.

In the attached rar file you can find a block diagram and everything is needed to build my amp (I think ), nothing special I've got to say, a dual channel with a classic JCM800 and a clean EF86 preamp designed by me, relay switched.

In the EF86 channel I've used a MOSFET to regulate the G2 voltage and compress the penthode curves, in this way the gain decrease and overdriving the channel will give a nice compression/saturation.
The EF86 stage have no catode bypass cap, for two reason:
1) I'd like to make a clean channel where part of the gain is given by a 12AU7 or 12AT7 thus maintaining the penthode brillance character.
2) using bypass capacitor this preamp becomes unstable, no matter how many damping capacitors you would add, I don't know why and I don't want to know because without this cap the sound is what I want to get

I had some problems with the power supply transformer, it has high leakage inductance and generate orrible spike noise in the audio frequency 100Hz modulated, I ended up to use SiC diode instead of classical 1N/UF5408 and a big RC snubber on the high voltage tap, this is reported on the schematic but if you don't have this problem the snubber can be omitted.
I will change the power supply transformer later.

Since I'm a big fan of PCB I've done also the power stage on it.
In the power stage PCB there is also the gain recovery and the cathode follower that enables me to use a 100k master potentiometer, this is a plus because the master volume is on the front panel and with low impedance it will not be affected by crosstalk.

The PCB comprend also a relè switching designed to switch between the two channels and to ON/OFF the loop, when one channel is not used the relative input is grounded.

This is the block diagram:

BLOCK_DIAGRAM.png

This is the result: (samples as soon as possible)

Front_1.jpg

Back_1.jpg

Thank you for the usefull information and topics on this forum, I'm glad to contribute to it.

Regards
luix

[roberto]

Ciao Luix,

as already told you, very well done!

[pompeiisneaks]

Very nice looking project! Well done.

I always love hearing sound demos.... if you can

~Phil

[xtian]

and, of course, we'd love to see inside!

[luix]

Very nice looking project! Well done.

I always love hearing sound demos.... if you can

~Phil

Of course, here are two audio samples, I'm not so good playing the guitar so I've used a recorded clean guitar track and a guitar impedance simulator, the amp is loaded with a real speaker and the record is taken with a SM57 at 20cm from the bottom right angle of the cone and an old Lexicon Alpha:

Clean EF86 channel with Bass = 5, High = 10,Gain = 9, Level = 10, G2 Comp = 0 (Vg2 = max) Sparkle = ON, Master = 3

EF86_Bass5_High10_Level10_G2Comp0_SparkleON.mp3

JCM800 channel with Bass =7, Mid = 9, High = 6, Gain = 10, Master = 4

JCM800_Gain10_Bassi7_Medi9_Alti6_Level5_Master4.mp3

Mod Edit: Gave links to the audio for inline playback.

[pompeiisneaks]

Sounds cool, thanks for the demo. It has an odd feel to the sound, a bit compressed? Maybe due to the impedance simulation or something similar. I think I still get the nice grindy tone to it.

~Phil

[luix]

Maybe because the MP3 or maybe because you know the recording method

[luix]

and, of course, we'd love to see inside!

The inside is still "man at work"

Due to the transformer problem I've done infinite modification to the PSU and choke board, I found that using an aluminium panel between the mains cable and the JCM800 preamp attenuate a lot the fizzing noise.
I should re-route the ground cables, I also found that connecting the circuit GND to the chassis on the JCM800 preamp lower a bit the noise so I've disconnected the green/yellow cable that you see fooling around without connection.

I also found that I love the sound of blue high voltage ceramic capacitor on tonestack

EL34 Power Amp Inside.JPG

[luix]

This is the same track with the clean track added and some reverb, effectively the clean track seems a bit compressed in some point, I don't know why?!?

[pompeiisneaks]

That definitely sounds different, more like I'd think it should sound... A bit in the middle did something, maybe very minor microphone clipping? At any rate, it sounds great. Love that tone.

~Phil

[luix]

No, the compression artifact is present also in the clean recording, the clean recording was taken with guitar directly in the instrument input of the Focusrite Scarlet.

Here you can hear the clean guitar recording, this version is amplified and was NOT the one used for driving the amp:

[pompeiisneaks]

Hmm maybe the recording software has some effects turned on by default? or maybe it's the position of hte mic?

I can definitely tell the tone sounds great, but it does have that odd compression... that seems 'not from an amp'..

~Phil

[luix]

Nope, the clean recording was taken directly guitar in audio card, it's seems that my guitar man has taken a "mute" chord, I don't know.

The clean sound was than passed trough a passive impedance simulator and taken to the amp.

The amp was recorded by mic (SM57) and audio card (lexicon alpha)

[TUBEDUDE]

How does it sound without the preprocessing, just the guitar, cord, amp?

[pompeiisneaks]

Yeah I'm curious too, you've got the classic mic, audio input etc to make it sound just right, and it sounds 'different' as I mentioned, a bit hard to explain but somewhat compressed maybe is the right term?

~Phil