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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 16th January 2020, 03:07 PM   #1
OldHector is offline OldHector
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Default SSE lacking brightness

I am in the final leg with my SSE build. It has moved from breadboard to an enclosure I built, and now I have been focusing on refining the sound so that I can down tools, and say that I have finished.

The top is some sort of heavy duty baking tray I rescued from recycling, and I like the rounded lines that are repeated with the base and toroids.

The bass is excellent, but it is lacking some elelement of brightness. I had been using a bluetooth source with Youtube, but I tested today with a CD player and good quality recording, and there is no doubt that there is something lacking in the upper ranges. For instance, Funeral for a Friend, after the initial synthesizer refrain, the band plays and there is a high hat, which is there, but sounding a bit muffled and not part of the sound stage.

It is a bit frustrating not having all the test tools, or a reference system to compare with. My SPP amp is not here, otherwise I could compare with that, but I remember that that had the 'wow' factor, which the SSE is currently lacking.

I am using UL with CFB. I have tried swapping tubes around. Currently have new JJ EL34s, but before had some old Telefunken EL34s liberated from a guitar amp. The driver is a Telefunken ECC81, but I have also tried a CV4024 (which does make the bass more dominant). While tube swapping, I managed to pick up an ECC83 without noticing (reduced gain and poorer sound), but it goes to show how you have to be careful and be organised when swapping tubes. Could have been something with a different pin out.

I have a DPDT switch for speaker impedance between the OPTs (where hopefully it will not get used during running), and I am pleased with that solution. I will be adding a 300ohm 5W resistor from the line to ground for each speaker connection to protect against no speakers. I am thinking this should be as standard as adding a fuse to the power supply - are there any nay-sayers?

I am pleased with the layout. The signal wiring is short and down one side, the power down the other, and the outputs at the back. Only downside is the proximity of the tubes to one of the OPTs, but I may well mount them vertically to get round that. (There is a perforated base to be fitted).

As I am sure most hobbyists will agree, the hard part is keeping the momentum going after the breadboard is delivering the goods, so that there is something that is safe and presentable. I am hoping that I will be able to sell it eventually for the cost of the materials, since it is hard to counter the 'what do you need another one for?' questioning of my better half. At least I am fortunate that she is appreciative of the detailed sound, and she is a music fan.
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Old 16th January 2020, 05:23 PM   #2
cogitech is offline cogitech  Canada
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SSE lacking brightness
I have read somewhere very recently (and it might have been George who wrote it, but I can't remember now) that particularly large/heavy OPTs can sometimes subdue the "air" and low level details in low powered amps. The statement seemed to indicate that OPT size needs to be reasonably matched with the overall power of the amplifier.

I have no idea if that concept applies to your specific amp build.
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Old 16th January 2020, 06:00 PM   #3
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Perhaps the treble notes are lack in the CD itself, they have some high frequency cut because of the low sampling rate.
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Old 16th January 2020, 09:57 PM   #4
OldHector is offline OldHector
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Hi guys, thanks for the suggestions. I’ve come across poor CD reproductions, particularly in the early days, but there is something that is subduing the performance. I suppose one option is to try triode mode, but that’ll be a bit of surgery for the weekend.
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Old 17th January 2020, 09:16 PM   #5
rmyauck is offline rmyauck  Canada
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What are your Output Trans as I see they aren't the usual ones used , bias resistors value, key voltages?
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Old 21st January 2020, 12:14 PM   #6
OldHector is offline OldHector
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I am using a Toroidy OPT, the details are attached. I am fairly sure that is not the weak link. Bias resistor is 560ohm, and B+ is 407v. Voltage drop is around 28v, so dissipation is 20W and OK.

I have noticed that when I disconnect one input the sound does not disappear in that channel, but remains at around 25% of the volume. Is that crossover distortion, or is it a consequence of using a single tube as a driver for both EL34s? If it is a problem, that is pointing to an issue with the wiring, so I will recheck that and make sure the RCA plugs are insulated properly.

The problem is going to be something that is non-standard. In my case that is:
  • I have used a DPDT switch to switch between 4ohm and 8om taps. Could that be the source of crossover distortion?
  • There is a silicon bridge added to the 5V tap on the transformer, to get a source of DC for the green power-on LED. Maybe tat was not a good choice, since the 5U4GB is directly heated, so that tap is elevated?

The final potential source of the lack of brightness is the speakers. They are Audio Pro Blue Diamond speakers, and seem OK, but as mentioned before, I don't have a proper reference system so it is hard to be sure.
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Last edited by OldHector; 21st January 2020 at 12:43 PM.
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Old 21st January 2020, 08:26 PM   #7
Tubelab_com is online now Tubelab_com  United States
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Quote:
I disconnect one input the sound does not disappear in that channel, but remains at around 25% of the volume.
That is a clue that there may be an issue with your input wiring. There should be near zero sound from the channel with no input even if the other is cranked full tilt.

Quote:
I am using a Toroidy OPT, the details are attached. I am fairly sure that is not the weak link.
I hear good things about these, so I ordered a pair of their BIGGEST SE OPT's for a 40+ WPC UNSET. I would have them already, but the distributor messed up the order and they are still in Poland. maybe next week.

Quote:
There is a silicon bridge added to the 5V tap on the transformer, to get a source of DC for the green power-on LED. Maybe tat was not a good choice, since the 5U4GB is directly heated, so that tap is elevated?
That should not affect the sound, but it will put your LED at B+ level with respect to ground. Really bad stuff could happen if it ever shorted to ground or the chassis. I would use the 6.3 volt winding instead. A short here won't blow anything.
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Old 22nd January 2020, 11:05 AM   #8
OldHector is offline OldHector
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Hi George, as ever, thank you for best after-sales support, bar none!

In my business, when we report IT support issues, sometimes we say the issue was 'the crank in front on the keyboard', and that was the case here. Suffice to say, I made a basic error connecting the wires to the OPTs (swapping the 4/8 line wires left and right), and it was no wonder that the stereo imaging was less than stellar :-/

The wires are now correct, and the wow-factor has been restored. And some ...

Now I just need to deal with the power-on-indicator-of-death.

All the best! Richard
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