6P13S SE cheap amp: improvement suggestions

[6A3sUMMER]

azazello,

Referring to your amplifier in Post # 29, . . .

Very nice looking amplifier!
Simple, Clean, and nothing in the way of being able to 'show off' the tubes.

Did you use any negative feedback?
What kind(s)?

What is the Primary impedance of your output transformers?

What is the driver tube type?

I hate to admit it, I might have to try building a single ended Pentode Amplifier again (squashing my pride again).

Thanks!

 

[azazello]

Thanks! No FB, OT - 4.7 kohm, Driver tube 12AD7-NEC. The sound is very close to 2A3. /It is PCB for EL34 from ebay and I modified for 6BQ6 and 12AD7/

 

[zintolo]

Hi azazello,

looking at the layout you have, the kit seems this one:
EL34 bordo amplificatore Single ended tubo kit scheda di amplificazione|Amplificatore| - AliExpress

May I ask you the working point of your system? or some more details you would like to share?

Seems intriguing!

 

[azazello]

Yes, this kit.

 

[zintolo]

Thanks, that kit seems superior in terms of ripple reduction due to the mosfet on PS, and the cost is very low. PCB tracks seem quite weak, even considering that the photo used for the promo has one ring broken on preamp socket.

 

[azazello]

You can cut sec. grid output tube and connected it to B+ cross 33 kohm /for 6P31S, 6BQ6/. +U sec.grid must be around 160-180V.

 

[zintolo]

Some of those kits are incredibly cheap, so am following with interest how it pans out for you.

They are indeed. This kit is intriguing because you get everything except the PT for less than 90 €. The other kit costs like 15 € and you are free to chose the OPTs you want, so let's say you don't waste money in OPTs you'll then probably upgrade.

 

[zintolo]

You can cut sec. grid output tube and connected it to B+ cross 33 kohm /for 6P31S, 6BQ6/. +U sec.grid must be around 160-180V.

Thank you azazello!

 

[zintolo]

Yesterday I was checking for different tubes and I thought to approach the amp in another way: use the SE amp as a "preamp" for a class D amp.

The first gain control used to set the kind of harmonic content of the amp.
Load the SE amp with resistors and feed the class D amp with this signal through a volume pot to have whatever power is needed at higher efficiency.

Is it something you guys would suggest?
At that point it's just a matter of tone, not power, so the output tubes can be set to triode mode also not to overload the output transformers.

Thanks in advance.

 

[RockyKing]

I had tested both 6P12P and 6P13P SE with Pentode mode and Triode mode. There are very high distortion appears when output power over 3W at Pentode mode. Triode mode is much better. and the maximum output power at load side is about 4.2W in my test.

 

[zintolo]

Update on this topic:
I've receipt the kit including tubes and transformers.

 

[zintolo]

Here the datasheet of the preamp tube: https://frank.pocnet.net/sheets/095/6/6N2.pdf

Looking at the curves of the output tube ( https://frank.pocnet.net/sheets/095/6/6P13P.pdf ) am I correct saying that I need around 60V on screens to hit g1=0 with a=50V with Ra=10k4 and working point at 300V 40mA?

I would like to connect a 8 Ohm speaker to the 4 Ohm tap, in order to get around 3 Wrms that should be handled by those small transformers.

A bit more can be obtained by working at 250V and 50mA with Ra=5k2 and hitting g1=0 at 50V and around 90 mA, but I think the output transformers would suffer.

In any case, the suggested voltage for the secondary voltage of the power supply seems a bit high to me (230 to 310 Vac means 310 to 420Vdc).