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First time with hi-fi, need some advise with amp desing

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Helo to all!

I been reading this forum for some time, and I always wanted to build myself hi-fi equipment, and now I have an little opportunity

I have experience designing instrument tube amplifiers and electrical math background.

I been investigating about hi-fi tube amplifiers in the net, but is too extense!, and because Im new to the hi-hi word, I cant cover it all... I have not experience with hi-fi before...

I not have the means to listen the power amplifier topologies (class A, AB, triode vs triode connected pentode etc... in the hi-fi camp) to develop my own tastes, so I'm relying on what I have been reading in the net.

Here I need your advise as veterans of tube hi-fi world...

Here goes my proyect:

Speakers: Fostex FE126En 4" Full Range speaker with a folded horn design that I found in the Madisound speaker store page (The Madisound Speaker Store)

They are rated at 15W, 93 dB and 83Hz-25kHz range.

For the amplifier, Im thinking in 8-10 W of power, single ended, triode connected pentode (because of budget), and these very price-attractive Edcor 15W SE output transformers (they seems to have overall good price-quality reviews)

As you noticed I can spend tons of money (and because I have to pay shipping taxes for almost every component here in the corner of the world), if some of you think that is better to save my money instead of building cheap stuff I will apreciate advise anyway, thats why Im asking advice :eek:

Im new to this, so Im not looking for a super fi-hi amplifier with ultra fast dinamic response etcs... just a pleasant listening experience :)

what do you think?

Thanks in advance from Chile

Joined 2009
Paid Member
Hey... Welcome to the forum.

With a 93 dB efficient speaker, you'll probably get around 100 dB SPL with a stereo pair in a regular living room with 1 W input to each speaker. Figure +3 dB because you have two speakers, and +5~6 dB for the room gain.

So you don't need a whole lot of power to drive these speakers to ear splitting volumes. Almost any Single-Ended triode amp should work. Like the 4P1L SE or a sweep tube Spud like my 6LU8 Spud (see my website). A Tubelab SE or SSE would be other good candidates (see the Vendors->Tubelab forum).

Many thanks Tom, you are right about the power rating, that let me thinking about some ~2W amplifier options, I'm starting to get tempted with some 2A3 tube SE amplifier designs. :)

Thanks for the circuit input, but its seem a little complex with the soild-state assist, the idea is to pick a simple circuit and learn the basics of hi-fi audio design.

I have been looking for output transformers for 2A3 tubes on ebay, in the US$300 range, I got a couple of options and I will like to know out of your experience, what of those will be the best choise? (thinking for 2A3 tubes)

I have foud several James brand OT:




And these reddish transformers with weird looking, but I wasnt able to find references or reviews on the net:

Valab AM3.5-16-SE Tetra- C Amorphous Core Output Transformer

And the edcor transformers:

CXSE series 25W

I'm very confused on OT´s performance by brand, if I'm going to build an amplifier I would like to get good transformers to start with :)

Thanks in advance for your help

Here's another recommendation for a 4P1L PSE amp. The parts are very cheap - just get a dozen 4P1L to start with. They are very reliable and measure quite close together. Add your Edcor OPT and a Hammond 126C, which isn't expensive, and you have an amplifier which is as good as or better than 300b SETs. It is all in filament bias, as you will see from the schematic. What this means is a separate DC supply for each tube, so a few small cheap filament transformers. It's best to use Rod Coleman's regulators but you can also use LM1085s in current mode. Very cheap and does the job - you need to heatsink them, though. So the filaments need more attention. But the results are great - no coupling capacitors and no cathode bypass capacitors. Very pure sound.


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You might want to have a look at my design suggestions using the 6CB5a triode strapped. I published several schematics on my blog. Check these articles:

VinylSavor: 6CB5A

I am in contact with someone in Chile who is building a 6CB5A amp based on the transformer coupled schematic. Maybe he is not living too far from you and I can bring you two in contact

Best regards

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