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EL34 DIY "centre-tapped" tube ??

jonathoule

Member
2016-01-14 1:11 pm
Hi everyone! I'm a big noob and I'm sorry

I've started building a Aiqin L02 EL34 from a kit, very standard tube amp, and I have an issue with the schematic.

One of the grid of the EL34 is "center tapped"... and from what I read it means its connected to halfway of the coil of the transformer...

I have no idea how to physically do that. I'm sure someone here knows

Here is the schematic and a picture of the transformer to tap

thanks a bunch!!
 

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ballpencil

Member
2013-11-15 4:44 am
Your transformer has no center-tap on the primary.
The instruction and schematic intended for an Ultra-Linear connection but your transformer does not support it as there is no center-tap. Unfortunately, this is a typical problem with cheap chinese kit.

So you can either connect the screen grid (pin 4)to B+ for pentode connection or to plate (pin 3 of EL34) for triode connection, via 100R to 1k resistor. See which one gets you the sound you like. I would suggest try triode connection first. Don't worry, tubes are more forgiving to incorrect wiring than solid states. They won't be forgiving for incorrect finger placement though :)

edit: B+ is where the blue wire goes, Plate is where the red wire goes.
 

jonathoule

Member
2016-01-14 1:11 pm
Ok! Wow thanks ! I'll keep the fingers away... or I'll try at least

My plan with this project was to get a cheap kit and as my readings go forward I would want to upgrade some parts.

Would finding a transformer with a center-tap be an improvement?

Thanks again, really appreciated
 

jonathoule

Member
2016-01-14 1:11 pm
Thanks!

I can't see the difference between both suggestions though...

But it seems like a really good idea, I have no reasonable reason but I feel like putting the grid screen straight in B+ is a bit intense.

I'll definitely try those.

Have you tried any of those configuration? Any opinion on the effect on sound?
 

Koonw

Member
2013-04-09 9:37 pm
There are some discussions on TubeCad, where it said ".. Ultra-Empathic circuits, as they force a rethink, which is amazingly rare in tube audio. The goal is to retain all the degenerative negative feedback from the output tube's plate that we can, thereby making the output stage Ultra-Empathic, rather than just a partial feedback design. But several readers have pointed out that by increasing the driver stage's cathode resistor (Rk) value, we can get much closer to Ultra"

No sure anyone has built this though, maybe for the experience?
 
it works 'halfway' and that is assuming the triode gets sufficient Vak (which means G2 potential will be much lower than the plate, maybe works for those "screen-drive" TV tubes).

'halfway' because it works only for the lower half of the swing where the triode cathode voltage remains lower that its plate.

on the upper half of the swing, the triode will get less and less Vak. worst case, Vak is so low, the triode will cutoff and G2 current supply comes to a halt. this is because the pentode plate can swing higher than B+ while the triode's plate is fixed at B+.

to make it work, put another triode where its plate connects to the pentode plate and drive the grid from the same RV1 tap. this triode takes care of the upper swing.. well at least i think so.. i may have overlooked something.

i simulated something similar but i use BJT. this ensures i can get pentode G2 potential close to its plate. as can be seen on the capture, it works.. i christened this method "Faux-UL" :D
 

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it works 'halfway' and that is assuming the triode gets sufficient Vak (which means G2 potential will be much lower than the plate, maybe works for those "screen-drive" TV tubes).

That's what the original design intent: UL performance from pents with restricted screen voltage specs, which would include the TV tubes and the 6L6-oids. The alternative was tertiary windings on the OPT that would allow for lNFB while keeping plate and screen PSA's separate. I had an OPT like that in the junk box that was originally intended for PP Class A 6L6s. I taped off the tertialries and used it instead with PP 6BQ6s, as the load resistance was right. The power rating was a bit on the thin side, so I'm not getting the design nominal 37W (30W OPT, and that's what I'm limited to without core saturation.)

i simulated something similar but i use BJT. this ensures i can get pentode G2 potential close to its plate. as can be seen on the capture, it works.. i christened this method "Faux-UL" :D

Another case where SS provides a better answer. Power MOSFET source followers are also good to go here.
 
Hi everyone, thanks for your answers, I must say I'm not getting everything you're saying but I'm working on it, and, in the meantime, I need another advice.

I completed the amp, voltages are a bit on the high side everywhere in the amp, since I'm in Canada 120v 60 Hz and the primary was intended for the US 110 50 hz. Like a good 20% higher than it should everywhere.

The volume nob pot that came with the kit was really bad quality and as I intended to use the amp as a power amp I bypassed the pot with parallel resistors on both canal. the pot was going up to 100k, I only had 70K resistors so that's what I used.

When turned on, the amp is first silent and after a few seconds, starts making a loud hum that can be modified in pitch and intensity by touching the 1/4 jack that goes in it. I could also hear the radio through the hum yesterday. The hum is identical in both canals. All lamps tungsten turn on, no sparks or flashes. All caps seem intact.

I checked each solder with the ohmmeter as I was building it. I don' know if that's a reliable way of checking solders... its the only way I could think of and everything seemed fine

Where would you look first for problems? We're two friends on the schematic, we reviewed the built many times but... as I mentioned earlier.. we're big newbies

Thanks everyone

jo
 
Apologies for numerous messages but after a couple adjustments, there is music (!!!) and it sounds pretty good actually. :D

But, remains a low pitch buzz in both canals that disappears when I disconnect the source from the amp... the source is a DAC Atoll, that has never caused any issues like that before, could it be grounding issues?

Thanks everyone, can't believe it's actually working :wiz:
 
Yes there is a feedback loop so it does matter..
I rechecked the schematic pdf and i stand by my comment: red goes to plate, blue goes to B+
I don't understand chinese so maybe the transformer label says it's the other way around..
Pretty sure you're right. The Chinese says "B: Blue - 3.5K - P: Red"

I take that to mean B = B+ = power; P = Plate

And 8 ohms: Green, 4 ohms: Yellow, 0: Black

A quick google translate from English to Chinese of the colour names shows the same characters, even when you have trouble to read them. :)
 
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I confirm, I checked the schematic and compared the symbols on the primary with the secondaries and it is blue = B+ = power and red to plate.

It sounds great that way btw, thanks Ballpencil!

Only a 60 hz hum remains on the left channel, I double checked the grounds, the solders seem ok with the ohmmeter.

It becomes higher pitch when I disconnect the source... any ideas?
 
Did you twist the wire coming from the input RCA terminals tightly? or use screened cable? The high impedance of tube grid makes it susceptible to picking up hum and noise from the surroundings. If the wires have been twisted tightly, try shorting the input RCA terminal to ground. If the hum disappears, try moving the 70k pot substitute resistor to the input RCA terminals.