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6DJ8 (ECC88) Tube Headphone Amp Project
http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4161
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Author:  WillyKirsch [ 08 Jun 2018, 17:57 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

BIGslim wrote:
I will have to trade up for better head phones after everything else is built.

Thinking about incorporating the ODAC into the same case. This will be in my cubicle at work, so I don't want to go all out on a DAC. Are there better USB DACs for less out there?

http://www.jdslabs.com/products/39/odac-objectivedac/


Hi BigSlim, finally did you put the ODAC on the HeadAmp? i'm on the same path and it's good to hear that someone has an experience on it!!

Regards!

Author:  WillyKirsch [ 18 Jun 2018, 21:31 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hey all!! finally a my HeadAmp saw the first light!! and sounds very nice!! But....

The hum is there... fortunately the 80 or 90% it's picked up by the tubes from the OPT's I just put an aluminum plate in between the OPT and the tubes(with no electrical contact) and almost all hum disappeared... (i haven't installed the PT and OPT's shielding yet (the supplier failed me...)

for the 10-20% remaining, I have some doubts about to where to start...

I defined 4 main GND routes:

1- input stage RCA to Volume pot then to input pin (7) on the tube (but not linked it there)
2- heaters GND.
3- tubes and OPTs primaries (HV return)
4- output stage, from opt's secondaries to impedance selector to phone Jack.

Apart of the inputs that is a shielded cable, all the rest of the wiring was made win AWG 24 cable and twisted pairs

All four GNDs mate at one point and there goes thru the X2 capacitor and the 120 Ohm resistance to the line ground.
There is an EMI filter in the line input.

There is just the chassis standing on 3 screws doubling as feet...

Any Idea to start to tweaking to get rid off the Hum?

Additionally, I measured the B+ voltage at the center tap of the OPT's and during the heating curve of the heaters (bout 20 seconds) the B+ voltage rises to 260+V!! then when tube start to drain current, the voltage drops to a nicer 140Vcc (I had to change the first R1 in the PSU from 500 to 1300 ohm to get that) I know that we already discussed about B+ delay and standby switches but, my concern is that there are 1uF 250V capacitors and in every start are facing 260+V for a while, this will not degrade the dielectric material? finally it will not be blown away??? Also, the tubes are not getting too much stress at every start up? if the tubes are fine, I can change the votage rating of the 1uF capacitors and sleep like a baby!!

Thank you!!

Author:  muskyhuntr [ 19 Jun 2018, 04:41 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Those output transformers are way too close to your power transformer. Most, if not all of the hum is being picked up there. They should be located as far from it as possible. Also aluminum is not a very good shield.

Me

Author:  gofar99 [ 19 Jun 2018, 16:22 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi, My suggestion is to fix the shields and then see what is left. I would avoid modding anything else as it may not be needed or may not be effective. Aluminum is a poor shield for transformer emi. Steel, iron and sometimes Mu metal are much better.

Good listening
Bruce

Author:  WillyKirsch [ 19 Jun 2018, 16:58 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Thank you guys!! I understand that aluminum it not as good as iron or steel, the shielding for the three transformers will be steel, the main chassis is aluminum in a try to avoid magnetic interaction in between transformers...

One curious discovery, if I place an aluminum plate in between the tubes and OPTs as I mentioned before, the hum almost disappear, but if I move the same plate in between the PT and OPTs, it increases by a factor of two !!

well I will take the Bruce's advice (less work also!!) I will install the shielding and see how much hum is there..

Another fact is that its not picked up by the input stage I think, because it not changes despite the volume pot position, the hum level its constant...

Thanks again and I'll post news as I get the shielding

Author:  WillyKirsch [ 19 Jun 2018, 17:01 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Bruce, what do you think about the hi voltage during warming up?

Thank you!!

Author:  gofar99 [ 19 Jun 2018, 20:21 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi, IMO pretty harmless. I use 350 volt caps mostly, but 250s should handle the few seconds without issue. If they can't then I would get a different brand as most whether they say so or not are good for at least 25% over.

Good listening
Bruce

Author:  WillyKirsch [ 20 Jun 2018, 10:24 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Thank You Bruce!!!

Author:  Bradley [ 21 Jun 2018, 07:52 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

I finally finished my build a few weeks back but have been too busy to post any updates here. Will have some photos soon.

@WillyKirsch As Bruce notes, any name brand 250V cap should handle a bit more than its rating for a few seconds, no problem. That said, Nichicon PW are available in 315V, which are considerably smaller than their 350V siblings (at least for 220uF). I'm using them for C3 in my modified 6CG7 build with 220V B+, since the warm-up is near 300V for about ten seconds.

Author:  WillyKirsch [ 29 Jun 2018, 20:49 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Bradley wrote:
I finally finished my build a few weeks back but have been too busy to post any updates here. Will have some photos soon.

@WillyKirsch As Bruce notes, any name brand 250V cap should handle a bit more than its rating for a few seconds, no problem. That said, Nichicon PW are available in 315V, which are considerably smaller than their 350V siblings (at least for 220uF). I'm using them for C3 in my modified 6CG7 build with 220V B+, since the warm-up is near 300V for about ten seconds.



Thank You for your comments Bradley!!

I'm still waiting for the casing... finally I dropped the order for the previous supplier, and move to one pretty expensive but I will have may boxes in three weeks...

In the meantime I'm starting with the wooden frame, to assemble all together when the cases arrives, then I 'll post final pictures!!!

By now I'm listening Tidal thru my ODAC RevB + Bruce's HeadAmp and I'm delighted!!!

Regards!!

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