My first power amp

[JORGETronic]

Hi,
I have started my first tube amp from the scratch:
Just one concern about the circuit….
I would like to ask you about the cathode resistor of the EF86, I found it too high , 100K, reviewing the data from various manufacturers this tube wiring as triode usually has a Cathode resistor of about 2K2, in fact if you see the voltage of the capacitor that is bypassing cathode resistor is rated @ 25V, having a resistor a value of 100K and a current of about 1mA (100V across Rk) the capacitor will explote.

Please your comments about this point and please add as much recommendations as you want, I have all the parts right now, the chassis is finished and I will start the wiring this week end..

Thanks in advance to all of you!!!

[rdf]

It's a typo, I suspect the correct value was probably 100 ohms. Somewhere online is a Svetlana application note on using a triode-mode EF86 in a low distortion line stage. It would make a great starting point for your application.

Edit - The somewhat similar Mullard 3-3 uses between 82 and 150 ohms on the EF86 cathode depending on speaker and global NFB.

[rdf]

Ooops, sorry JORGETronic. Make that 1000 ohms instead of 100 ohms. I forgot the Mullard uses the EF86 in starvation mode. The line stage I meant is shown below.

[JORGETronic]

Thanks for your help in my first amp RDF!!!
Yes I found the same when you mention it, i guess I will try with 2k2 first as this is the value recommended in a couple od data sheets form diferent vendors.....
has this amp a classic tube sound?

[rdf]

No problem . I haven't built that exact circuit but for a while ran something close with a trioded E180F as a driver. It sounded very nice. The results of a simulation with SE Amp CAD under those conditions suggest you can expect about 1 watt at 2% harmonic distortion (almost pure 2nd) from the EL84. I'm not sure what you meant by classic tube sound but it will sound good. My only suggestion is to buy a few extra caps when ordering parts. I'm not sure what the original designer had in mind but the interstage coupling cap and the EL84's cathode cap are pretty small. Both time constants fall around 20 Hz. If you're using quality output transformers consider making them bigger.

[Klimon]

Hi Jorge,

There's been a little talk about the combination ef86 triode / triode-strapped el84 on the decware forum and the conclusion is that the ef86 is a fantastic sounding driver tube for el84, even better than the acclaimed e88cc. A few notes about your circuit though (to be honest I'ld leave it altogether): the output tubes run probably at a voltage of around 225-240V depending on the primary resistance of your opts; that's very low: 280-300V should be better (200ohms cathode resistor or 100ohms, 220-1000µF with a shared cathode resistor = the output stage of the decware zen = gives wider soundstage). Without doing the maths it's also obvious that the ef86 runs at a much too low voltage, better to aim for 270-280V (1,3k cathode resistor, 100µF). For best sound swap the ef86 for it's high quality equivalent e80f, it's as cheap as a normal ef86.

If you're interested in building an el84 single-ended I'ld recommend to take a look at decware's pages where you can find the schematics to some of their amps (el84/ecc88 set & el84/ecc83 single-ended pentode).

The bonavolta schematic you're planning to build will - when using 1k cathode resistor instead of 100k as stated - certainly work (although it's full of obvious flaws) but if your power transformer has some extra voltage just go for an ef86/el84 like the one I described above or take a look at the Decware website.

Also don't rule out el84 pentode, you'll have more power, better attack, highs and lows but miss the midrange forwardness and lushness of the triode-strapped el84. The triode-strapped babies + bill's triode strapping method (see decware forum / t-linespeakers) might be the ideal solution, I'm going that road with my e80cc / triode-strapped el84.

Lastly the choice of output tubes is very important (as with all tube-amps), try to find some Mullards (sometimes rebranded as Philips, Valvo) or Matsushita. This makes the difference between a boring sounding versus a magical tap-your-feet amp...

Good luck,

Simon

[JORGETronic]

Hi Simon
Thans for taking the time to comment, the PSU is almost done, then I will be implemmenting one by one your recommendations, I will start more conservative, as thye original first, this is my first shot!!!!
saludos

[JORGETronic]

I have finished the amp. there is a terrible oscillation in the el84, i could see sparks from the plate pin to the earth bus soldered in the center of the socket.
The only changes to the circuit were: el84 cathode cap is 1000uF, the same with the ef86, the Rk of the ef86 is 2,2K......
I did not connect the shield of the imput tube, It is necesary?, if yes how? connected to catode?

any suggestios?

[JORGETronic]

Hi,
the amp is up and running, just a motor boating noise from time to time....
I would like to get opinions about this implementation, my first implementation..

[JORGETronic]

here you can see the ground star do you see any problem there, I have a very strong noise from time to time and I was wondering if Ihave a grounding issue