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6SN7-2A3 SE Amp - James Transformers
6SN7-2A3 SE Amp - James Transformers
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Old 23rd March 2010, 04:26 PM   #11
kevinkr is offline kevinkr  United States
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6SN7-2A3 SE Amp - James Transformers
Do you have scope and some sort of signal generator?

I would measure the output voltage from 20Hz - 20kHz at the plate of both the 6SN7 and the 2A3 before buying new OPTs. Check that there is not appreciable audio across each bypass cap at higher frequencies.

What sort of cathode bypass cap are you using on the 2A3?

Not likely to account for all of your FR problems, but I have run into problems with some motor run types that were somewhat inductive, paralleling them with a smaller film cap helped a lot.

For some hum cancelling if needed you could take a 22uF cap directly from the cathode of the 2A3 to the B+ side of the OPT. This will also help the HF response provided that the cap used is a reasonably good one. Leave the 100uF cap in place. (Western Electric type connection)

It probably is the output transformers, but it would be best to make absolutely sure that something else isn't amiss before spending your limited funds.
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Old 23rd March 2010, 04:39 PM   #12
christian.lenci is offline christian.lenci  Italy
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Yes I have a scope and a test cd with 0dB sine waves from 5 Hz to 22 KHz.

The tests you have suggested would definitely work in identifying the guilty part. Anyway, all the parts are new; all cathode bypass capacitors are Elna Cerafines.

I'll let you know the results.

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Old 23rd March 2010, 08:28 PM   #13
christian.lenci is offline christian.lenci  Italy
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I did some measurements as kevinkr suggested. The response on the plate of the 6SN7 is flat up to 20 KHz.

Anyway, on the plate of the 2A3, I measure the same HF roll-off I get at the outputs. How can I tell if this is due to the final stage or the OPTs?
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Old 23rd March 2010, 10:02 PM   #14
zigzagflux is offline zigzagflux  United States
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I had the same problem with a 300B amplifier. If the signal at the grids of the 2A3 is strong at 20kHz, the load impedance of the OPT is correct for the tube, and the tube is biased strong enough, then it is almost certainly the OPT.

At the end of the day, it was the OPT. Swapping out with a Lundahl solved it.
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Old 23rd March 2010, 11:23 PM   #15
kevinkr is offline kevinkr  United States
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6SN7-2A3 SE Amp - James Transformers
Quote:
Originally Posted by zigzagflux View Post
I had the same problem with a 300B amplifier. If the signal at the grids of the 2A3 is strong at 20kHz, the load impedance of the OPT is correct for the tube, and the tube is biased strong enough, then it is almost certainly the OPT.

At the end of the day, it was the OPT. Swapping out with a Lundahl solved it.
I'd have to agree. Checking that there is no audio across the cathode bias network is the only other thing you should do before ordering better transformers. I am a fan of the cerafine and would expect its performance to be quite good even at 20kHz - so cap performance is probably ruled out.

I would save your money for a bit and buy an even better transformer. I would also talk to Bartolucci and see what they have available at a reasonable price - imho something comparably priced from them almost has to be better than the comparably priced James with the shipping costs factored in. And they make very good transformers.
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Old 23rd March 2010, 11:32 PM   #16
christian.lenci is offline christian.lenci  Italy
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I can confirm there is no audio across cathode bypass caps, just some millivolts at very low (100 Hz or lower) frequency, as the reactance of the cap increases.

I also measured the response directly at the grids of the 2A3s. Absolutely flat up to 20 KHz.

I understand James transformers are nothing fancy. But I'm fine if they are free of such major frequency limitations, until I can afford very good OPTs. I would do a switch now, because as it is now, the amplifier is too much rolled-off and I really can't enjoy it at all.
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Old 24th March 2010, 03:51 PM   #17
christian.lenci is offline christian.lenci  Italy
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I did some more measurements.

I used a 8 ohm resistive load at the output instead of my loudspeakers (B&W DM602). The roll-off at 20KHz is only 0.7dB under the response in the pass-band.

How is this possible?
Is it normal with DHT SE amps to expect such a heavy HF roll-off with the speakers connected (-3dB @ 8-9 KHz)?
Or are my OPTs not able to drive my speakers correctly?
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Old 24th March 2010, 04:07 PM   #18
kevinkr is offline kevinkr  United States
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6SN7-2A3 SE Amp - James Transformers
Neither actually, it is an indication that your speaker system has a strongly falling impedance at high frequencies and hence is not a great load for a small SE amplifier. Replacing the transformers actually won't fix this issue - you need to determine what the issue is with the speakers. (Probably impedance flatness with frequency) Do the transformers have a lower impedance tap you could try with these speakers as that may help at the expense of rather less output power.

Complex cross-overs are a problem with small SE amps as there is very limited current drive capability. Same can generally be said of low impedance speaker systems as well. Note that there are always exceptions and the above are generalizations.

I suspect the B&W speaker is not a good match to your current amplifier preference - finding an impedance plot over frequency should confirm this. Ideally you either want a large efficient 2 way speaker system with simple x-o, preferably first order or an efficient FR single driver system.
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Old 24th March 2010, 04:10 PM   #19
Ty_Bower is offline Ty_Bower  United States
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Have you ever generated any impedance plots for your speakers? It's fairly common for them to have significant peaks, especially around port resonances and crossover frequencies. I do not recall seeing any feedback in your amplifier design, so high impedance peaks in the load would likely result in measurable changes in the power delivery.

Click the image to open in full size.
http://i69.photobucket.com/albums/i4...usIIleftPR.gif
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Old 24th March 2010, 04:18 PM   #20
kevinkr is offline kevinkr  United States
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6SN7-2A3 SE Amp - James Transformers
The minimum impedance of the DM602 is listed as 4.3 ohms with a nominal impedance of 8 ohms, under FTC parlance here in the USA I believe this would/should probably be considered a 6 ohm speaker.

If you have a 4 ohm tap I would give this a try, otherwise replacing the OPTs with ones that do offer a 4 ohm tap would probably help somewhat. This reduces the effective source impedance of the amplifier to the speaker, and reflects a higher impedance back to the plate of the 2A3. Without knowing the impedance curve of the speaker system it is hard to know how distortion and power output will be effected - one or both may be improved or not.

The efficiency is on the low side for a small SE as well.

You might be as well off getting your hands on some inexpensive FR drivers and building something to go specifically with this amplifier.

Manufacturer Specs:

Frequency Response: 52Hz - 20kHz 3dB on reference axis
Frequency Range: -6dB at 43Hz and 30kHz
Sensitivity: 90dB spl (2.83V 1m)
Nominal Impedance: 8 ohms (minimum 4.3 ohms)
Power Handling: 25 - 120W into 8 ohms on unclipped program


From here: B&W DM602 Series 2 Loudspeakers
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