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PostPosted: 27 Jan 2018, 11:32 
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Location: Prêles, Switzerland
cbueche wrote:
Bruce, thanks for your answer and sorry for being such a noob with tubes... ok, so I understand each channel of the PSU will see about 25mA flowing out, and because I don't want to dissipate more than 3.6W, it means I really want my B+ to be <145V, ok so far.

It's a bit too late in my timezone to power my circuit again to verify how much I get after the diode bridge, but your schema mention 270V so let's take it as a base for my calculations. Note I'm using the exact same power transformer (Edcor XPWR083A-120/240) wired for 240V.

If I apply ohm's law across the R-circuit from the 270V right after the diodes bridge down to the B+ outputs of the PSU, I get the following data :

- assuming a constant charge (at least during my testings) of 25 mA pulled by the CCS
- assuming each PSU channel outputs 25 mA, it means :
- each R2 sees 25V across it (1000 * 0.025)
- each R1 sees as well 25 V across it (500 * 0.05)
- so I get 3 times 25V drops across each R (R1, R1, twice R2)
- so the complete circuit "loose" about 75 V
- 270 - 75 is 195, about what I got in my measure of B+

So I'm not sure why your circuit is supposed to present a B+ of 135-150V with these load values... The 180V I got with my 6600 ohms load is more in line with my calculations. I'm lost... I suppose I will have to rise the R values here and there, but I would like to understand where I'm wrong.

Edit: if my calculation is correct and I replace both R1 with 1K, I get 125V drop, so 270-125 = 145V.

Kind regards,
Charles

P.S. electronic engineer with 28 years in IT doesn't help to understand tubes !


Bruce and others, here my long-due feedback:

The original tubes I bought were marked 6DJ8 / ECC88 on the box, but where in fact 6H23P. With those, I came to 145V B+ only with R2 = 1K8. The sound was good, but one of the tube exhibited tiny noise (crackling) in silences between music. I then looked at the tubes in more details and I found 6H23P markings on them... bad vendor. Not even sure if the operating points of the circuit is correct for them.

I then ordered JJ-Electronics 6DJ8 (silver pins) from a french vendor (audiophonics.fr) who delivered new tubes, marked ECC88. Well, let's try them... and here I am, my B+ drops to 127V !!!

Recalculation showed me I need R2 = 1K. Solder them and yep, B+ is now 145V after the initial spike, then slowly goes down to 143/142V. So Bruce my apologies for not trusting your circuit, I stand corrected !

I tested by listening to Ennio Morricone - Chi Mai and found out low-levels "clangs" on the left side... Thinking my left circuit was reverberating in some way... no, that's the song played correctly, I never heard the clangs in the last 35 years of this song. Unreal experience !

Now I'm not very happy with the look of my amps. View of the inside below. It works, but the top is an empty plate with two tubes and multiple screws. My son liked the sound but hated the look. I will move the amp to a metal enclosure, separating the PSU from the amp by a metal plate and some mu-metal sticked on it for good measure. Not completely sure the cooling will be as good, I have to think whether I let the tubes come up holes of the top or leave them inside with large venting on bottom and top of the box. Have to think some more nights to decide.

Attachment:
ampinwood.jpg


Building this first amp has bought me back the same pleasure as when I last did electronics in 1988 or so. Thanks to the community !


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PostPosted: 27 Jan 2018, 13:07 
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Location: Arizona, USA
Hi, I'm glad all is well. For a simplistic project it delivers amazing performance. A favorite of mine. You have just proved once more that tube type designations are not consistent. A 12AX7 from one company is not the same as one from another one. That is part of the reason I indicate brands of tubes used in the projects. Plus the equivalent types like ECC83, ECC803, ECC83S and so on are not exactly the same either. One company was saying that their 12AX7 was the the same as a 5751. Get real folks, the gain of the one is only 70% that of the other. They do share the same pin connections, but are not equals. In power tubes it is even worse. I know of two brands of KT88s that will go up in flames in the KT88 version of the Oddblocks. I am wary of any company that says their tubes are lower noise than others as well. I sent back a batch of 10 to a supplier because they were worse than standard ones from another company. It gets even muddier as a number of sellers are passing off seconds and rejects as quality new stock. NOS supplies can be even worse...there are many re-branded and fakes out there. ......Jumping off my soap box now.

Good listening
Bruce

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PostPosted: 28 Jan 2018, 06:21 
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Joined: 17 Mar 2016, 07:26
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Location: Prêles, Switzerland
Dear Bruce, community,

The amp sounds good, but being curious, I wanted to play with the FFT of my DSO (Siglent SDS 1072CML). Well, the FFT was not nice to look at, I then started hooking my sondes and look at the output with a 1KHz sinus as input. The output connected to my Sennheiser PX-200 (32 ohms).

The first screenshot shows a fairly ok (old Metex generator) 1kHz sinus input and the output being polluted with lots of noise. I managed to get rid of the 22 kHz noise by disconnecting a Raspberry cheap PSU.
Attachment:
1-sin.jpg

The 2nd screenshot is with grounded input. Same ugly noise on the outside.
Attachment:
2-22khz.jpg

Third pic, I scalled up to the limit of my DSO and found something in the range 10-50ns, with some 200KHz base signal on top :
Attachment:
3-hf.jpg

From the pics, I deduce that my 230V line power is severely polluted. Not surprised, given the number of switching PSU's all around the house, but I was supposing this noise would be filtered by the amp's PSU. I then connected the sonde to the B+ (my scope is supposed to accept 400V), but in AC coupling it shows no noise at all.

And the worse is, wanting to capture the B+ PSU noise, all the other issues are gone... I can't reproduce the issue anymore. 53 years-old junior puzzled :-)

My PSU is the original scheme of Bruce and I do have 1uF/200V capacitors on the B+ pin of the OPTs. Hints welcome !


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PostPosted: 28 Jan 2018, 09:02 
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Hi I believe you are on the right track. Any amplifying device can pick up and amplify noise. I have seen a number of cases similar to yours. The ground side of the AC mains is full of junk. Since an amplifying device senses the difference between ground and signal imput it has no clue if the stuff is signal or not and it gets amplified. The AC mains noise is being mixed with the input signal. What I have found needed in high noise conditions is to first have three wire AC mains power going to everything it can with noise filters on each input. I like the one piece IEC power entry style with combined fuse and line filter built in. Second (seems redundant) I run a separate chassis ground from each piece of equipment. The only exceptions I have in my own system are an OPPO Blue Ray player that only has a two wire mains cord and the final mono-block power amps that do have three wire power but I find they do not need the extra ground. You are correct though in that there are many sources of noise that have crept into our homes. I went through 4 external A to D external boxes to get one that would not containimate the signal going into the PC (most were powered by the PC, and its power supply is really full of crud). In my shop ...regardless of what I do, I have extensive grounding and use APC and Triplet power filters and still can not get the noise as low as I would like. The best I can get is about -100dbv. This is not good enough if you want to measure things that are quieter. There is a way to make the amp more immune to above band noise. Increase the 1K resistor in the grid to a higher value. This will form a RC filter when combined with the tube's internal capacitance. You could start with a value of about 10K and see what happens. If you increase it too much the actual response of the amp at high frequencies will be reduced. I would expect values above about 25 K or so to slightly reduce the gain as well.

Good listening
Bruce

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PostPosted: 28 Jan 2018, 13:35 
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Bruce thank you for your answer. As I will re-box my amp, it will be the opportunity to add a good input filter. I skipped those because they were expensive, but now I regret it. I will as well try to rise the grid resistor, maybe this week on evenings if times allows. It's great to have your help !


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PostPosted: 29 Jan 2018, 22:18 
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So I ended up slapping in 1.5KΩ resistors in the R2 position and it brought me down to 153V B+ on start up and after the amp warms up my guess is around 147-148V, which is in the right range.

Brought the amp to a meet in Philly and a lot of the guys were impressed. I sent a few to diyaudioprojects to check things out. ;)


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PostPosted: 09 Feb 2018, 18:16 
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I'm part way through my build on jbwebst's excellent PCBs ordered through Osh Park. Once the transformers come in and I get it running, I hope to share my BOM and some photos.

Bruce, I notice back on page 5 [http://diyaudioprojects.com/Forum/viewtopic.php?p=37461#p37461] you mention regarding speaker loads, "A possible variation would be to use something on the order of 6CG7 tubes The circuit changes would be few and they could deliver about 1 watt rather easily." I'm curious what circuit changes you had in mind? My intent is to run some favorite 6CG7 makes I enjoy in other 6DJ8-targeted designs, and I'm sure they will sound fine. If some minor modifications and the addition of a couple transformers will allow me to use some small desktop speakers, I'm all for it!

Thanks Bruce and everyone who has paved the way forward on this project.


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PostPosted: 10 Feb 2018, 16:57 
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Hi, My thoughts were to increase the voltage for 6CG7s to around 200v and juggle the idle current up a bit to see if I could get more output. The gain would be a little lower, but most sources can deliver 0.25 volts or more and an expected gain of perhaps 15 or so would be enough to get a significant output. I have not tried this as the 6CG7s are more costly that 6DJ8s and I really don't need the extra power. There is no particular reason to stick with only dual triodes except for simplicity. Any pairs of medium gain, medium current capability ones should work fine.

Good listening
Bruce

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PostPosted: 10 Feb 2018, 18:35 
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Bruce,

Thanks. It looks like most 6CG7 can handle a fair amount of voltage, 300+. So if I read you correctly, I should lower the values of R1 in the power supply? What value would you suggest as a starting point? I can do the math, but always appreciate a second set of eyes.

Back to the idle current, it seems with the 50 Ohm resistor configuring the LM317L there should be about 25 mA flowing. The GE and Tung-Sol 6CG7 datasheets suggest 20-22 mA for Class A operation, 70-77 mA peak. If I'm correct, a 39 Ohm resistor would yield about 32 mA. Does this seem too high?


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PostPosted: 11 Feb 2018, 08:49 
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Hi, That current level ought to be OK. Since the parameter we want to observe and not exceed is anode dissipation the voltage applied is what we should adjust. It seems according to RCA that to run that current you will actually need to set the voltage to under 215. Since there are a number of variable that are undefined (like the actual impedance of the transformer and efficiency of the filters) it is difficult to predict what the dropping resistor ought to be Taking a guess I would try something in the 750 ohm range. A two watt one would be OK , but my preference is a 5 watt one.

Good listening

Bruce

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