6SN7-2A3 SE Amp - James Transformers

[christian.lenci]

Hello everyone,
The attached schematic shows the SE amplifier I have built. Sorry if I drew it in a hurry.

Since the circuit is very simple, I used good quality parts (Cerafine and Audyn Cap Plus caps, Kiwame Resistors, a GE JAN black plates 6SN7, Golden Dragon 2A3 tubes, etc), but I bought very cheap OPTs for a first run.

I did some measurements and while the results in the bass region are very satisfactory on paper and in listening tests, the treble are quite heavily rolled-off. My measurements (I did them with the speaker connected to the amp, not a simple resistive load) indicate a -3dB point in the 8-9KHz region, that is very low. It is not a dip, since the response doesn't rise again, going higher in frequency. I think this depends on the output trannies.

Since I am on a budget, I was thinking about upgrading them with James 6112HS transformers. Will these trannies help me getting a more linear response or is it not worth it? I have read praising reviews about them.

Feel free to suggest any possible advice about my schematic.

Thank you!

[nicoch58]

the 6123 are better too more bass

[kmtang]

LUNDAHL 1623 60mA is having super duper bass, warm and thick mids and sweety highs.

My friend just built one with 56 driver direct coupled to 2A3 and Lundahl's 1623 60mA amorphous OPT. He reported the sound is F******g good with comparing with his 2A6/2A3 with James 6123 OPT.


Johnny

[christian.lenci]

Thank you for your help!

The 6123 and the Lundahl are beyond my budget, for now.

I just wanted to know if the James 6112HS units (or something else in their price range, 150$ a pair) are good enough to avoid such major frequency response limitations.


Thanks again!

[nicoch58]

I have the ll1623 120ma (not AM) sound very good

the 6112 is good, the 6123 is better on bass, but I dont know if the difference is big.....

an used ones ?

[Ty_Bower]

Originally Posted by christian.lenci

but I bought very cheap OPTs for a first run.

Just how cheap were they? Exactly what did you use?

[yagoolar]

It may be of your interest - I once built SE amp with Russian 6C4C (almost equiv. to 6B4G). It was used for a couple of years. After I had built SY's Red Light District amp, I decided to do the same trick with the former amp - I replaced RC network in output triode with red LED matrices. The improvement in bass was astonishing - it became a new amp. If you are interested - let me know.

Total workload, calculation + soldering, was 3 hrs.

[christian.lenci]

Originally Posted by Ty_Bower

Just how cheap were they? Exactly what did you use?

They were handmade here in Italy by a local maker. They were 40$ each.

[Ty_Bower]

I'm no expert, but I don't see anything obviously wrong with your circuit. I'm a little surprised the bandwidth of the amplifier is poor. I would hope that even the most inexperienced transformer winder could have supplied something that would perform a little better than what you are seeing.

Has your winder made output transformers before? Did he make any attempt to interleave the primary and secondary windings? It's not a "split bobbin" type, is it? (primary wound on the lower half of the bobbin, completely separate from the secondary which is wound on the upper half).

[christian.lenci]

Originally Posted by Ty_Bower

I'm no expert, but I don't see anything obviously wrong with your circuit. I'm a little surprised the bandwidth of the amplifier is poor.

The circuit itself is performing very well indeed. I have the 6SN7 halves correctly biased at 6mA, the 2A3s at 60mA, 50V cathode bias (250V A-K),
~2mV 50Hz ripple at the speaker.

Waveforms are correct, the output power is 3W before clipping in the center of the band, transformer saturation effects take place only under 70 Hz, from 1.5/2W upwards.

Excluding this roll-off, it's very lush sounding, vocals and guitars are beautiful.

So I would think I have to blame the OPTs for that.

Originally Posted by Ty_Bower

I would hope that even the most inexperienced transformer winder could have supplied something that would perform a little better than what you are seeing.Has your winder made output transformers before? Did he make any attempt to interleave the primary and secondary windings? It's not a "split bobbin" type, is it? (primary wound on the lower half of the bobbin, completely separate from the secondary which is wound on the upper half).

Making transformers is their main activity. I don't know if it's a split bobbin type. I accepted the fact they were cheap OPT, and I was prepared for somewhat weak bass or not very sparkling highs. In the end, bass is quite satisfactory, but the high-end roll-off is definitely unacceptable.