Tubelab SE Treble response - diyAudio
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Old 30th April 2016, 09:25 PM   #1
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Default Tubelab SE Treble response

After swapping my TSE (Sovetek 300b, Trancendar 20w OPT - TT-020-OT, several years on the clock) in and out of my system with another amp, I noticed that my trebble was severely lacking. I've always thought so, but the A/B comparison really highlighted the issue.
I checked the frequency response (QuantAsylum QA400) and it appeared mostly flat to 20khz, but in reality there is a large difference to other amps.

Looking for improvements, I found the following post by George:
"The grid stopper is normally used to prevent oscillation. It has a second function here. The grid stopper resistor determines the time constant under overload conditions since it is in series with the grid. It also forms a low pass filter pole with the tubes Miller capacitance so it affects the high frequency response of the amp. A big resistor (10K) will eliminate blocking distortion in most cases, but it will kill your treble response in triode mode. It works in pentode mode though."
at
Tubelab SE Coupling Capacitors

So, would playing with this resistor value (R33 and R34 on the schematic) be wise? If not, any other suggestions to increase the top end?

Thanks!

Last edited by jazbo8; 4th May 2016 at 04:19 PM. Reason: typo
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Old 30th April 2016, 11:47 PM   #2
wdecho is offline wdecho  United States
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I think not. I have just finished build the SE amp and it has a very good treble top end to my ears, not as good as say the Simple SE which I also have or some Class A SS amps. I think the treble is mostly a factor of the tube used than the circuit. A better 300B tube that is known to have better treble is probably what you need if this is what you are seeking. I built the SE because of the outstanding mids and vocals knowing that other amps are better at the lows and highs. To me music lives in the mids and I want outstanding mids over anything else. I do not think you will find any better mids with another amp over a 300B or other DHT. All of this is IMHO.
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Old 1st May 2016, 12:35 PM   #3
wdecho is offline wdecho  United States
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I thought I would add that I am using these,

Genalex - Gold Lion PX 300B

You can find them on Ebay at a good price in pairs from a reputable seller. I read all the reviews I could find on 300B's before purchasing and these were the best bang for buck that I could find. So far after a short period of listening with this amp I am enjoying the sound immensely. I now see the allure of the 300B tube. The treble has more bloom than the simple SE that uses JJ tubes. The speakers are absolutely not there listening to music.

Last edited by wdecho; 1st May 2016 at 12:40 PM.
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Old 3rd May 2016, 05:51 PM   #4
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Thanks, wdecho! I was hoping to stay away from new tubes (so very expensive), but I guess its inevitable.
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Old 4th May 2016, 12:53 PM   #5
wdecho is offline wdecho  United States
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I now have both the simple SE and the SE and there is a very big difference in the treble between the two amps. Not that one is better than the other but very different. This is with my brand of tubes. I love the very smooth sound of the treble with the SE, very musical but then the Simple SE is more bright and quick sounding and I can see how others might prefer this sound over the SE. It probably comes down to personal preference and the sound you are after.

I do not know if buying new tubes are going to get the treble you are seeking but all the reviewers that I have found that have compared the Soveltek to the Genalex prefer the Genalex, not that there is anything wrong with the Soveltek.

I always lean to the better mids and live with the highs and lows of the one with the better mids.
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Old 4th May 2016, 03:48 PM   #6
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Frequency response measurement would be helpful, if you've got em.
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Old 4th May 2016, 05:58 PM   #7
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Here is an older Freq. Resp. graph on ARTA (from before I had the QA400).
Click the image to open in full size.

I thought I had the QA charts uploaded, but I guess I don't. I will try and find them tonight.
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Old 4th May 2016, 06:26 PM   #8
w5jag is offline w5jag  United States
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Does that graph represent a single tone input at 1 KHz?

How did you like that software, btw?

Win W5JAG
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Old 4th May 2016, 07:01 PM   #9
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by acceler8 View Post
After swapping my TSE (Sovetek 300b, Trancendar 20w OPT - TT-020-OT, several years on the clock) in and out of my system with another amp, I noticed that my trebble was severely lacking. I've always thought so, but the A/B comparison really highlighted the issue.
I checked the frequency response (QuantAsylum QA400) and it appeared mostly flat to 20khz, but in reality there is a large difference to other amps.
There are a few things that can contribute to a "lacking treble". One is a bandwidth limitation, which I think is what you're trying to measure. The other could be that the "lacking treble" circuit actually has a cleaner transient response. Less ringing on the transient -> less "fizz", which could be perceived as a "lacking treble response". Finally, it could also be that your tube amp has a relatively high output impedance, hence, doesn't control the speaker driver as well. It may not excite the cone break-up modes the same way, which could smooth out high-Q resonances in the speaker+amp's combined frequency response. Again less fizz -> "lacking treble". With measurements it would likely be pretty straight forward to see what's going on.

Quote:
Originally Posted by acceler8 View Post
Looking for improvements, I found the following post by George:
"The grid stopper is normally used to prevent oscillation. It has a second function here. The grid stopper resistor determines the time constant under overload conditions since it is in series with the grid. It also forms a low pass filter pole with the tubes Miller capacitance so it affects the high frequency response of the amp. A big resistor (10K) will eliminate blocking distortion in most cases, but it will kill your treble response in triode mode.
Playing with the grid stopper is generally a bad idea. What George is trying to say is that if you go extreme in one direction (say 0 Ω), you're likely to get oscillation. If you go extreme in the other direction (say 10 kΩ), you're likely to kill the treble. A good value would be between these extremes. Typically you see resistors in the range of 100 Ω*to a few kΩ used for grid stoppers. The actual value depends on the tubes involved. I wouldn't mess with it unless you have the equipment needed to detect HF oscillation (spectrum analyzer or wideband oscilloscope).

You can estimate the lowpass function of the grid stopper by looking at the input capacitance of the tube in the data sheet and using the following equation:

f = 1/(2*pi*R*C)

R is the grid stopper, C is the tube's input capacitance, f is the cutoff (-3 dB) frequency). All SI units.

Quote:
Originally Posted by acceler8 View Post
Here is an older Freq. Resp. graph on ARTA (from before I had the QA400).
That's an FFT of the harmonic spectrum, not a frequency response plot. A frequency response plot shows the amplitude/gain as function of frequency. Here's an example.
Click the image to open in full size.

Tom
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Last edited by tomchr; 4th May 2016 at 07:03 PM.
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Old 5th May 2016, 05:50 PM   #10
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W5JAG,
Yes, that was a 1Khz tone (with 60Hz hum & harmonics). I liked Arta well enough (esp. b/c it was Free!) But the QA400 is much easier to test with.


TomChr,
You are correct; sorry I posted the wrong image and not the F.R. Here is a screen cap from the QA400:
Click the image to open in full size.

Unfortunatley, I cut the bottom off (with the X-Axis labels). If you look in the top left, you can see that the measurement range was 20-20Khz (with a Log Scale).

Quote:
Originally Posted by tomchr View Post
With measurements it would likely be pretty straight forward to see what's going on.
I do have REW and a EMC8000 but had never thought of comparing amps for some reason. Thanks for the idea!!!
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