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Headphone Amp Kit using 5670 + 6080 tubes - suggestions
Headphone Amp Kit using 5670 + 6080 tubes - suggestions
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Old 22nd May 2020, 05:56 PM   #1
evonimos is offline evonimos
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Default Headphone Amp Kit using 5670 + 6080 tubes - suggestions

Hello All,


I'm populating a PCB I bought from the auction site, advertised as:
"5670/ 6N3 + 6080/ 6N5P Stereo Tube Headphone Amplifier Heisang Amp Circuit PCB"


Link here:

5670/ 6N3 + 6080/ 6N5P Stereo Tube Headphone Amplifier Heisang Amp Circuit PCB | eBay


PCB came with the following schematic:
Headphone Amp Kit using 5670 + 6080 tubes - suggestions-pcb-jpg


It seems to be an SRPP configuration with a cathode follower.
Filaments are all AC powered.



I 'm particularly puzzled by the 6.3 (1A) supply.

Instead of just feeding the 5670 filaments it keeps going to a couple 100Ω resistors and then it connects to ground and on to the B+ supply via a 100KΩ resistor. I'm no expert here so please help explain what's the purpose of all that (if any)? I plan running DC supply to the heaters but I'm wondering if I need to change something in the process.


Also, any suggestions towards improving this design are very welcome.
It is a cheap little kit but I'm going to give it a try.


I think Douk sells it as Little Bear P7 in a completed form.
Will add some pictures later.


Thank you all for reading.
Attached Images
File Type: jpg PCB.jpg (131.2 KB, 136 views)

Last edited by evonimos; 22nd May 2020 at 06:05 PM.
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Old 23rd May 2020, 12:44 AM   #2
PRR is offline PRR  United States
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Headphone Amp Kit using 5670 + 6080 tubes - suggestions
> connects to ground and on to the B+ supply via a 100KΩ resistor.

Tell the whole story. "connects to ground" via a 53kXX 47k resistor, and 100k to +240VDC. Assuming the transformer and heaters are fully insulated, the heater line will sit at +80V (and vibrate +/-3V each side of that).

Why? If there is minor heater-cathode leakage, a DC bias tends to reduce the leakage. In this case, the upper small triode cathode sits at +120V, which is a lot for the heater insulation; the 80V bias reduces the stress.
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Old 23rd May 2020, 12:49 PM   #3
evonimos is offline evonimos
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Quote:
Originally Posted by PRR View Post
> connects to ground and on to the B+ supply via a 100KΩ resistor.

Tell the whole story. "connects to ground" via a 53kXX 47k resistor, and 100k to +240VDC. Assuming the transformer and heaters are fully insulated, the heater line will sit at +80V (and vibrate +/-3V each side of that).

Why? If there is minor heater-cathode leakage, a DC bias tends to reduce the leakage. In this case, the upper small triode cathode sits at +120V, which is a lot for the heater insulation; the 80V bias reduces the stress.

Thanks you very much for your help in understanding this.


If I were to power the filaments with DC, should I change anything to thhat part of the circuit?

I was thinking of omitting those couple of 100Ω resistors in order to completely isolate that circuit.


But if I understand correctly, that could put more stress to the heater-cathode isuation and risk leakage?


Here are a couple of pictures of the actual PCB.




Click the image to open in full size.
Click the image to open in full size.

Last edited by evonimos; 23rd May 2020 at 12:54 PM.
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Old 23rd May 2020, 01:48 PM   #4
duncan2 is online now duncan2  United Kingdom
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The 6080 "rings a bell " with me not in audio but old Tektronix oscilloscopes I repaired .
Consistently the 6080,s in them developed heater/cathode shorts ,if I was you I would only buy a high quality US version.


I was able to remove the shorts using a professional CRT rejuvenator which pulsed the current to stop stripping of the cathode surface .
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Old 23rd May 2020, 02:29 PM   #5
evonimos is offline evonimos
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Quote:
Originally Posted by duncan2 View Post
The 6080 "rings a bell " with me not in audio but old Tektronix oscilloscopes I repaired .
Yeah.


Reading about that tube, It seems that it was pretty commonly used in several non-audio applications like the one you mention.
What about the 6AS7G one?
Is it less prone to that kind of issue?


Actually, I got a couple of those 6080. They are the only ones I have for this kit at the moment..
One is a Sylvania (USA) which seems to be pretty common and the other one is a 6080WA by a rebranding company in the name of Chelmer (i think it used to be a UK company). It looks extremely similar to the Sylvania one, so it might actually be a rebranded one.
Both of them are used and the problem is I have no equipment or way to test these.

I haven't put them on the PCB for a test drive yet...

Actually the Sylvania one seems to ring and rattle a bit when I tap it lighlty with my finger. Both do, but the Chelmer seems to do it less.

I might post a video (with sound) so you can tell me your opinions on that.

Last edited by evonimos; 23rd May 2020 at 02:32 PM.
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Old 23rd May 2020, 04:13 PM   #6
duncan2 is online now duncan2  United Kingdom
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Yes the 6AS7G by GEC /RCA are a superior make than the stock 6080 never came across the same fault in them .


As a matter of fact in my original valve (tube ) equivalent books the 6AS7/G is vastly superior spec.wise -
Higher anode/plate voltage,
Higher gain ,
Higher current =135MA.


There are also some "SQ" (special quality ) versions--used by governments/military.


Be aware a common fault I found in old audio valves even back in the 60,s was tapping the valves produced a noise at the output when caried out when powered up .
Special rubber cushioned bases existed then to help cure it in ,50,s/60,s

rock and roll groups guitar amplifiers.


An original 6AS7 by either of those two manufacturers might be pricy.


Yes I know Chelmer used to deal with them and yes rebranded now.
Companies House ( UK business check ) state they dissolved in -14 June -2018 (not in liquidation )
previous name-GOIMAGE Ltd.
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Old Yesterday, 09:33 PM   #7
evonimos is offline evonimos
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I have been using this amp for a few hours now and really like it.
I think it is a keeper.
Will write back in a few days, for a more detailed opinion.


Headphone Amp Kit using 5670 + 6080 tubes - suggestions-ht-jpg

Headphone Amp Kit using 5670 + 6080 tubes - suggestions-ht2-jpg
Quote:
Originally Posted by duncan2 View Post
Yes the 6AS7G by GEC /RCA are a superior make than the stock 6080 never came across the same fault in them .
I will definitely look around for a 6AS7G.
A pic of the two tubes.
Headphone Amp Kit using 5670 + 6080 tubes - suggestions-tub-jpg
Attached Images
File Type: jpg HT.jpg (309.4 KB, 13 views)
File Type: jpg HT2.jpg (299.9 KB, 14 views)
File Type: jpg tub.jpg (279.1 KB, 14 views)
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Old Yesterday, 10:03 PM   #8
duncan2 is online now duncan2  United Kingdom
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If you check out the information on the 6080 you will find out why they develop grid/heater/cathode shorts .


The grid/cathode spacing is very close, closer than a normal tube
they get very hot and that causes expansion of the cathode.


Have a look at the superior -6AS7G, its actually designed for single ended audio use -MUCH superior construction .
I have had a 6080 in bits and can vouch for this.


As suspected original RCA/GEC -6AS7G,s are expensive !


6AS7G @ The Valve Museum
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