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PostPosted: 14 Mar 2018, 15:44 
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Joined: 14 Aug 2017, 11:03
Posts: 8
Location: Spain
Hi All,

An update on my reproduction of oddwatt and a question. You may remember I was having red plating, since then I have removed the SMPS for the heaters, it was a big source of noise. Now the heaters of the power tubes are fed with AC. For the 6SL7 I have added a small 10W transformer and a LM317 to power the heaters with DC. The result is a big reduction in noise. I also switch the EL34 to JJ KT77, nonetheless I still get red plating with the JJ at 62mA. That doesn’t bug me at all, at 50mA it sounds great.

Now the question, I used some nice FT-3 220nF capacitor for the interstage connection, but for the slave valves I used 220 nF orange drops on the asumption that they are not so important for quality, is this right or I should install the same capacitor in both places?.


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PostPosted: 14 Mar 2018, 17:14 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, The reason to use identical capacitors is to make the impedance vs frequency loading on the grids as close to equal as is reasonable. Different capacitors will tend to color the sound each tube delivers and in doing so may add distortion. I would guess in many cases it would not be audibly obvious...but it can be measured. What is the B+ voltage on the tubes? At 62 ma (cathode current) you should not be getting red anodes. Howver there have been a lot of either bogus or just plain out of spec tubes floating around the past year or two and that could be the cause.

Good listening
Bruce

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PostPosted: 15 Mar 2018, 13:54 
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Joined: 14 Aug 2017, 11:03
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Location: Spain
Not a good "cost optimization measure" then :), thanks Bruce.


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PostPosted: 17 Mar 2018, 13:07 
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Joined: 25 May 2016, 16:30
Posts: 56
Is there any difference in effective damping between class a class ab amps? Some of the ab valve amps seem to work better with Tannoys than the Oddwatts so I'm searching for an explanation


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PostPosted: 17 Mar 2018, 14:02 
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Joined: 24 Oct 2010, 07:05
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It depends... The effectiveness of AB class is obviously better and power will be greater, but the linearity should suffer in the same time. So "work better" seems highly subjective, depends on what you look for.


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PostPosted: 18 Mar 2018, 23:53 
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Joined: 03 Feb 2018, 00:47
Posts: 41
Location: Brisbane, Australia
A site over here has a matched pair of JJ EL34L valves. They are described as The EL34L is a special version of the EL34 designed by JJ. They describe it as a more powerful EL34 with more headroom and deeper lows. It is a little more aggressive than a normal EL34.

Does anybody have any experience with them? Will they be ok in the Oddblock Octal? (Patiently waiting on Edcor to deliver to Australia... :) )


Thanks,
Glenn.


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PostPosted: 19 Mar 2018, 06:42 
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Joined: 25 May 2016, 16:30
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'It depends... The effectiveness of AB class is obviously better and power will be greater, but the linearity should suffer in the same time. So "work better" seems highly subjective, depends on what you look for.'

OK, 'work better' is a little loose but so is 'The effectiveness of AB class is obviously better '.

To be more precise, the Oddwatt 120 monoblocks do not work well with some Tannoy dual concentric speakers. The bass is completely uncontrolled and the cone flaps presumably at its resonant frequency. Feedback improves the situation slightly but sound quality as always is worse compared to no feedback.

An Armstrong ST25 makes a better job of driving these speakers with much better bass control. If Bruce's estimate of damping factor is correct there's not much in it in terms of damping factors, so among the other main variables is the fact that the Armstrong is Class AB compared to the Oddblock's Class A operation. So, is it Class A compared to AB that explains this difference or what?
Thanks


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PostPosted: 19 Mar 2018, 07:22 
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Joined: 24 Oct 2010, 07:05
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Juancho wrote:
OK, 'work better' is a little loose but so is 'The effectiveness of AB class is obviously better '.
It means: with the same power elements (model of tubes for example, like KT120) of an amplifier, the same "topology" (like SE, PP, UL, TL, CT, Bridge...) with the same number of active elements - the AB-amplifier will have more power than A.
In most cases it means - better "damping factor". It partially answers your question:
Juancho wrote:
is it Class A compared to AB that explains this difference or what?
Partially - because of other factors (again - partially mentioned above, like topology and the number of elements) which may or may not be significant. Here in Russia I was not able to find any info on Armstrong ST25 and Tannoy has several lines of DC speakers from rather small to stunningly big so I cannot answer the question precisely.
You can connect the 8Ohm speakers to the 4Ohm tap and get almost twice better value of damping. This connection noticeable lowers the output power.


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PostPosted: 19 Apr 2018, 07:19 
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Joined: 02 Aug 2016, 05:26
Posts: 21
Location: France
Hi folks,
I've finally powered up my first Oddblock! Mostly good news - no bang, flash or smoke and (almost) no noise apart from music. However, all is not quite right with the power supply voltages - I'd really appreciate your help with this.

(I used the Unified Schematic dated May 6 2015 and AC for the heater supply. The only (known!) deviations from the schematic are: the PIO cap is 0.33uF rather than 0.22; I couldn't locate a 62k resistor so used 15k and 47k in series. Tubes are EL34 and 6n1p.)

Under load, the voltages read:
Wall socket supply: 229 VAC
From power transformer: 373 VAC
Main B+ 486 V
To SRPP 289 V (I have a temporary 47k AFTER this, giving 215 V for the 6n1p)
DC to heater AC supply 63 V

1) Can you suggest the best value and position for resistance to lower the main B+?

2) The 6n1p datasheet shows a max anode voltage of 250 V and heater-to-cathode voltage of 100 V peak. Any suggestions as to a 'sweet spot' for this tube? Again, is there a 'best' placement for resistance in this case?

3) Can you suggest an appropriate value for the heater DC and how best to achieve it?

I hope this makes sense - many thanks!


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PostPosted: 19 Apr 2018, 09:06 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Actually those voltages are fine. The target for B+ is the 425-450 range mostly to protect the filter caps. The tubes really don't care. Your B+ is a bit high, but as long as it is below 500 when operating it is fine. The voltages when using EL34s and similar tubes will be higher than when KT88s are used due to the lighter loading on the power supply. The SRPP can run at 300 volts if it is well controlled (ie. regulated) and the heater reference voltage is about 80 volts. At 215 the 63 volts is fine as the middle cathode will be at about 1/2 the B+ applied and quite safe. Likely ok even with the 289. One half of 289 is about 140 and with the 63 volts reference neither cathode or heater will see enough difference to cause a problem. The circuit was designed with 5751 / 12/6SL7 tubes that can withstand higher anode voltages. The one you used is not be as robust and should use about 200-215 v. Putting the 47K resistor between the last filter and the capacitor that is right at the tube anode would be my choice. The concerns in why I like to keep the voltages well below the max ratings (typically 200v) is because the individual sections of the dual triode can warm up with slightly different times. This can cause one to see an abnormally high voltage with respect to either the ground or B+. Plus the assumption is that the triodes are equally matched for gain. This is nearly never true and will cause an offset at the mid junction. It can exceed 25%. And finally as the tubes age the ability to withstand the 200 volts goes down.

If there is some sort of noise then probably it is related to a circuit grounding or layout. The amps are quiet. With the inputs shorted (thus no effects from the preamps or sources) the noise voltage at the outputs should be below 5 millivolts and typically at about 1 mv. Unless you have extraordinarily sensitive speakers it is not audible. Mine are 96db/w and there is none.

Good listening
Bruce

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