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PostPosted: 21 Jan 2018, 09:21 
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Joined: 17 Jan 2018, 16:13
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Hi all,
thanks for quick answer, even if it was already posted few times in this thread, which I have finally gone through. Bruce, you're the most patient and helpful designer I have ever found on diy forums!
After reading this topis almost all is quite clear for my, however have one (hope) last concern regarding PT. I can order the one with sec output 6,3V. Question is whether I can use it to supply heaters directly in AC mode and then I assume all the heaters (EL84 and 5751) should be connected in parallel?
Please correct me if I'm not right.
Thanks :)


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PostPosted: 21 Jan 2018, 09:45 
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Hi, It can be done, but I recommend against it. I really hate hum and noise. The Groove is a high gain circuit that because of the RIAA equalization needed has 20 db more gain at low frequencies than at middle ones (and 20 db less at high ones as well). This means you get about 60 db of gain in the frequency range of 50/60HZ. The likelihood of increasing the hum level is high when near a power transformer. The Groove is very quiet when constructed properly, but unless you do extensive shielding it will not be as good when near the power transformer of the Poddwatt. I find that distance is the best cure for hum pick up other than extensive shielding. My Grooves are a number of feet away from the power amps. It also makes it easier to get them close to the turntables. Again this will help reduce hum and noise. Since the output is rather low impedance you can actually run fairly long cables from it to either your line stage or in this case the Poddwatt. With quality cables (build your own from Belden or Morgami cable) you can go about 10 feet without any significant signal degradation. As picky as I am about phonograph performance (I am told by my spouse that I'm obsessive) I find no difference between 1 meter and 3 meter cable lengths. I use Belden 8412 with both center conductors tied together and the shield as ground. I have tried using the center conductors as hot and cold with the shield connected only at one end. They functioned the same in my system. The alternate wire is Mogani Quad Neglex 2534. That works best with two conductors as signal hot and two as cold (tie the whites together and the blacks together) with the shield fastened at only one end. I usually get the wire in bulk from Markertek (on the web). Use quality (not big buck ones) connectors. Markertek has some gold/nickel RCAs that are well liked by audiophiles and they are rather inexpensive at under $2 each. I also use some from Parts Express that have twist on sleeves that cause them to clamp tightly to the female connectors. I forget exactly what they call them but they work fine as well and run about $6 each. It believe they were in the pro audio section.

Good listening
Bruce

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PostPosted: 21 Jan 2018, 12:38 
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Thank you for your recommendations!

I'm new to hifi audio and still learning the ropes for building tube amplifiers. For the sake of the idea.... what would be required for extensive shielding if they were within the same chassis? Also I'm trying to figure out how one would get enough current using the XPWR236 (Edcor PoddWatt transformer) that has 12V (6-0-6) at 2A. I'm afraid it couldn't handle the current needed for each fulfillment.


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PostPosted: 21 Jan 2018, 17:29 
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Hi, A case of knowing that Edcor transformers are seriously underrated. The 4 El84s will use up 1.8 of the 2amps and the other 5 tubes will use about 0.45 amps. My personal preference is a transformer with about 1-2 more amps. The B+ use of the phono section is not significant in this case (under 20ma). For shielding....the output trannie is going to have to be on the top of the chassis. Use only a steel chassis about 17X10 inches (bigger is better) I think might do. The power trannie at one end and the phono section at the other. The phono section tubes would be inside and the power tubes outside (a heat issue for them). Lots of venting over the preamp tubes is still needed though. I would use a steel divider inside the chassis between the preamp and output sections. Mu metal would be useful as an alternate. Tube shields on the preamp tubes as well. Those shields would connect to the signal ground and not the chassis. If you look at the photo...what you want to do is something like this inside the larger chassis. There are Mu metal shields(covered with black insulation) between the power supply and active circuitry. The power transformer is fully enclosed in a steel housing as well as copper shielding inside it. The actual input jacks are shielded as well in the black enclosure on the left of the photo. The preamp in the photo is quiet at around -90 dbv S/N.
Good listening
Bruce
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IMG_1520b.jpg


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PostPosted: 21 Jan 2018, 19:30 
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Great information! :D Thank you very much.

One more quick question if you could.... Now when you say Edcor transformers are underrated did you mean the published 2A for that specific transformer can be pushed a bit further? Or did you mean it clearly shouldn't be used to supply the filament for all of the tubes that would be used? (EL84 x4, 5751 x2 & 12AX7 x5 for the phono preamp).

I'll very likely stick with modular units as your writings recomend just for the sake of simplicity and trouble shooting but the idea of everything in one unit appeals to me strongly - so long as it isn't more trouble than it's worth.


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PostPosted: 26 Jan 2018, 14:34 
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Hi Guys,

I'm preparing BOM to order all parts for Podwatt. I'm aware, that while turning ON the amplifier, anode voltage should be switched on +/- 1 minute after switching on the heaters.
But the question is how it looks while turning the amp OFF? Does the sequence should be switched - anode voltage switched off as first and then after some seconds is safe to switch off the heaters?


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PostPosted: 26 Jan 2018, 16:37 
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Hi, Edcor transformers deliver more than their ratings as opposed to some other well know brands. I still prefer to not go past about 75% in any case.

Yes switch off the B+ before the heaters.

I only have delays built into the bigger amps. I don't bother in the EL84 ones. Since one of mine has about an enormous number of hours on it with the original tubes I guess it really doesn't matter. That one ran every day for around 7 years in my office for about 8 hours. It still plays fine. This brings up a few thoughts...if you don't kill the B+ before the heaters it may not matter as they are still hot and will conduct for several seconds. This effectively discharges the filter caps. Even if it didn't ...so what? Unless you are going to stick you fingers in there there is really no problem with them discharging slowly. On start up there is the thought that hitting the tubes with the B+ before they are conducting will in time strip the coatings off the cathodes. I have not experienced this ever. I concede it might be an issue with directly heated tubes or tubes running at really high (like over 1000 volts)voltages. My experience with DH tubes is that they start conducting nearly immediately. I'll let you all decide on that one. In some designs that use solid state power supplies that are not regulated it is possible to have rather high voltages on filter caps that are further down the stream that perhaps are not rated for the voltage. For example the third one in a filter stream that would only see 250 volts during operation when all the tubes are warmed up. On start up it could see the full B+ voltage which in many amps is around 450. If the capacitor is not rated highly enough it would fail. In that instance either a delay is needed or a capacitor rated for the higher voltage.

Good listening
Bruce

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PostPosted: 26 Jan 2018, 18:03 
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Hi Bruce,
Thanks a lot for detailed explanation.
I'm going to implement some simple circuit of automatic B+ voltage delay to let the heaters warm up, but you calmed me that it's not needed when amp is switched off - cathodes are still warm and additionally there is still some current in capacitors.
I will report the project progress once I receive all the ordered parts.

Cheers!


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PostPosted: 26 Jan 2018, 21:35 
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Hi, look at the delay circuit in the KT120 amps that uses a LM555 IC. If you already have the DC from the heater supply then it is easy to implement.

Good listening
Bruce
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PostPosted: 28 Jan 2018, 16:26 
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Hi Bruce,
Thanks for the scheme.
I'm collecting parts and found cheap soviet tube 6N2P, which is described as equvalent of 5751. Parametres look the same, but there is some small difference of the heater pins:
5751 has three heater pins, 6N2P only two (no mid-tap). Does it mean, that 5751 can have only half heater working (for one triode) and 6N2P only whole heater?
As Podwatt uses only 1/2 of 5751 each channel, does 6N2P can be used as well?
Please have a look on the attached when you have a while.
Thanks!


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