View Full Version : 6J1 Pre-amp modifications (hum removal)
Firebottle
21-02-2017, 11:52
As bought from here:
http://www.ebay.co.uk/itm/291786271575?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
https://www.qiannipicture.com/pic/uploadfile2/UploadFile/P0/SKU377287/B9829B56C6869E9177C61697CDCFCF9392CD48CE03CB9E73CD C7D296469CCE43D26666C6B133D29336C69DD216F473739313 CB33C9CE269BCC56A0CD.jpg
As standard there is a low level hum present, this is cured by fitting an extra capacitor. You need a 470uF 35V electrolytic, the NEGATIVE lead is connected to the junction of two 47 ohm resistors on the top of the board as shown below:
http://i601.photobucket.com/albums/tt97/Paramotorpilot/6J1%20buffer/P1030658.jpg
The POSITIVE lead is wrapped around the side of the board and connected to the 0V or 'earth' connection of the input sockets:
http://i601.photobucket.com/albums/tt97/Paramotorpilot/6J1%20buffer/P1030659.jpg
Modification to covert to a buffer stage.
The circuit of the board is this:
https://www.circuitlab.com/circuit/9vnehk/screenshot/1024x768/
...although the 100K output resistors are actually connected to 0V not -28V as shown.
To alter the function from a gain pre-amp to a zero gain buffer stage the circuit is altered so:
http://i601.photobucket.com/albums/tt97/Paramotorpilot/6J1%20buffer/P1030667.jpg
Three new components need to be added to give the correct valve biasing. 2x 150K resistors and a 1uF decoupling capacitor.
The two 4.7K anode load resistors are removed from the centre of the board and replaced with wire links:
http://i601.photobucket.com/albums/tt97/Paramotorpilot/6J1%20buffer/P1030660.jpg
Replace the two 220 ohm cathode resistors with the two 4.7K resistors just removed.
The two 1uF output capacitors have the front leads lifted from the board (closest to the volume control), then extended and connected to pin 2 of each valve, shown below:
http://i601.photobucket.com/albums/tt97/Paramotorpilot/6J1%20buffer/P1030662.jpg
The extra biasing components are fitted to the front of the board once the two 470K grid resistors also have their front leads lifted from the pcb.
The connecting points can be seen in this photo:
http://i601.photobucket.com/albums/tt97/Paramotorpilot/6J1%20buffer/P1030661.jpg
Job done :)
Stryder5
21-02-2017, 12:11
Cool, waaay over my head:):mental:
I appreciate you guys that do this "electronic stuff" , you may have well written this in Egyptian glyphs.:scratch:
See you Friday...lol
Gary
Mine are somewhere in Hong Kong right now. Thanks for the guide!
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Bigman80
21-02-2017, 14:09
Lmao, I have tried a couple of times to follow that but I still don't know what's going on haha.
I shall endeavour to understand.
Looking forward to hearing it !!!
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Ali Tait
21-02-2017, 14:51
How does it sound Alan?
He did a much better job of explaining it than I could. My diagrams look like a foreign language to everyone but me, and I don't use the right language when I'm explaining it. I learned how to do most of it by trial and error before I could even understand the math behind it.
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Bigman80
21-02-2017, 15:09
He did a much better job of explaining it than I could. My diagrams look like a foreign language to everyone but me, and I don't use the right language when I'm explaining it. I learned how to do most of it by trial and error before I could even understand the math behind it.
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Lol, I'm starting to understand it a bit more now.
What's that silver block next to the volume control ?
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Looks like a ceramic capacitor
Firebottle
21-02-2017, 16:08
It's a polycarbonate cap, it's the 1uF decoupling capacitor for the 2x 150K bias voltage divider. The lower lead is connected to the nearest 0V ground point.
Ali it sounds very neutral, I shall be trying it on the end of a lesser quality CDP later to see if it takes any of the digital glare away.
Is there any advantage in removing the 'blingy' blue LEDs?
Is there any advantage in removing the 'blingy' blue LEDs?
I think they're inside the valves.
Firebottle
22-02-2017, 07:20
None whatsoever Barry, they are under each valve and not in the audio circuit.
Looks less blingy now it's boxed up and only the top of the valves show :)
Thansk for posting the mods and schematics Alan. Is that silly led get-at-able with a sharpie? if it is i may well make like a rolling stone and paint it black. :)
walpurgis
22-02-2017, 11:23
Ali it sounds very neutral, I shall be trying it on the end of a lesser quality CDP later to see if it takes any of the digital glare away.
Tell us how it sounds on your CD player Alan.
Funny thing. I hadn't been following this, but coincidentally did have my eye on one on ebay. I'm tempted to get one and mod it as described. I have a nice power supply that suits.
RothwellAudio
22-02-2017, 11:50
I've just looked at the ebay listing...
HOW MUCH!!!!
A couple of valve bases with p+p would cost more than that from a UK supplier! I'm gobsmacked.
I was going to say that I didn't really see the point of this thread. That type of PCB construction is great for getting a circuit as compact as possible and it keeps production costs down for commercial products, but if you start modding the circuit or the board it loses those advantages and you might as well have hard-wired the circuit from scratch.
However, having seen the price I can see the attraction. How can they make any profit on that?
Thats what you get for making parts in huge bulk and a cheap labour force. Buying these things, even to strip for parts is a decent way to run a build business
Bigman80
22-02-2017, 12:07
Personally, I am very grateful they make them so cheap as it's gonna save me a lot of money to try something new. Alan knows his stuff so I'm very grateful I live so close to him too lol.
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"However, having seen the price I can see the attraction".
That was it for me too, £15 ish with the power supply ive ordered? Why the heck not. I've never heard Valves at all in audio, only guitar amps, so it seems a great way to dip a toe in.
I just ordered the boards but I already have all of the components. I'm not sure if I have those particular transistors, but I'm sure I have some that will work. I've probably got a few weeks to find some if I don't before those things make it here from China.
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RothwellAudio
22-02-2017, 14:50
Those transistors haven't got much voltage across them and aren't carrying a lot of current so almost anything will do as an alternative.
I'm sure I've got some lying around that will work. I just looked, and you can order 100 of both kinds from China for $3.50. I should order one of the full kits, and build a second with the components I have and see if there is any difference.
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Bigman80
22-02-2017, 15:11
I'm sure I've got some lying around that will work. I just looked, and you can order 100 of both kinds from China for $3.50. I should order one of the full kits, and build a second with the components I have and see if there is any difference.
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I like that idea.
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I like that idea.
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I can get the kit in a week and have that finished before the boards get here most likely. I never know how long stuff is going to take to show up from China. It could be a week or a month.
Bigman80
22-02-2017, 16:47
I can get the kit in a week and have that finished before the boards get here most likely. I never know how long stuff is going to take to show up from China. It could be a week or a month.
Keep us posted, might be a little money spinner in it for you and Alan. You never know !!!!
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I will. I'm doing it more out of curiosity than anything, not for money. Tinkering with different stuff is usually what gives me new ideas to try. This might keep me from destroying every valve reel to reel I come across.
If with a few component's changes it's a pre. Then its a buffer, can it be easily changed into a phono pre? electrical trickery is not my strong point but I've seen the same valves in the little bear I think. Or is that a totally different circuit? Just wondering.
Firebottle
22-02-2017, 18:10
Tell us how it sounds on your CD player Alan.
Well I'm afraid this might not be what folk want to hear. The buffer is SO neutral that I cannot tell a difference with it being in circuit or not.
I have tried a couple of lower ranking CD players, Philips CD-610-II and CD-713.
Well I'm afraid this might not be what folk want to hear. The buffer is SO neutral that I cannot tell a difference with it being in circuit or not.
I have tried a couple of lower ranking CD players, Philips CD-610-II and CD-713.
Is that what you hoped for or not?
If with a few component's changes it's a pre. Then its a buffer, can it be easily changed into a phono pre? electrical trickery is not my strong point but I've seen the same valves in the little bear I think. Or is that a totally different circuit? Just wondering.
It's quite a bit different. It could be done but you'd be better off starting with a kit designed to be a phono pre. There are plenty of those too.
walpurgis
22-02-2017, 18:31
Well I'm afraid this might not be what folk want to hear. The buffer is SO neutral that I cannot tell a difference with it being in circuit or not.
Not entirely surprising Alan. I had more or less the same result with the Chinese Yaqin valve buffer stage.
The Musical Fidelity X10-D I use is a different story though. That's the first model, the cylindical unit. It works! It adds a lucidity and ambience. False? Maybe, but the effect is rather good.
Bigman80
22-02-2017, 18:32
Well I'm afraid this might not be what folk want to hear. The buffer is SO neutral that I cannot tell a difference with it being in circuit or not.
I have tried a couple of lower ranking CD players, Philips CD-610-II and CD-713.
Oh 🤔 daft question, are you running in your Firebottle gear ?
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Firebottle
22-02-2017, 18:53
Yes Oliver, the rest of the system is very neutral/transparent.
I've tried the buffer with the Firebottle phono section and the aforementioned CDP's as input, feeding either the line stage of the KIN then Monoblocks, or the latest EL84 SE amp then into the ESL57's.
All sounds rather spiffing old chap :yesbruv:
If you have a CD player that you don't care about you can take out the output transistors, and use that to bump it back up to line level. That's how I usually do it. I've never heard much difference unless you add a little gain.
Mine uses the mu-vista 6112 tubes and its impedences are 470k Ohm input, and 33 Ohm output if thats any help. Certainly adds a touch of life to some things
Mine is made from parts scavenged out of the mic preamp from an Ampex 960. It's not technically a buffer. I don't remember the impedences. I'll have to pull it out of the CD player and check it.
Bigman80
22-02-2017, 20:30
Yes Oliver, the rest of the system is very neutral/transparent.
I've tried the buffer with the Firebottle phono section and the aforementioned CDP's as input, feeding either the line stage of the KIN then Monoblocks, or the latest EL84 SE amp then into the ESL57's.
All sounds rather spiffing old chap :yesbruv:
Oooo exciting!!!!
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Bigman80
22-02-2017, 20:33
Yes Oliver, the rest of the system is very neutral/transparent.
I've tried the buffer with the Firebottle phono section and the aforementioned CDP's as input, feeding either the line stage of the KIN then Monoblocks, or the latest EL84 SE amp then into the ESL57's.
All sounds rather spiffing old chap :yesbruv:
Would you notice the effect as much if its it already on a valve based system ?
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probably more noticable between valve pre and ss power
Bigman80
22-02-2017, 20:39
probably more noticable between valve pre and ss power
I was thinking that. In the signal path of a CDP to SS AMP would show its effects more so.
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Bigman80
22-02-2017, 20:40
Good thing is it isn't having a detrimental effect on Alan's gear
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There's only one way to find out, but I would think it would be more noticable going into a solid state pre-amp than a tube based one.
Firebottle
05-03-2017, 12:53
Having tried this now in various guises; gain stage - triode and pentode operation, buffer (cathode follower) operation, and finding very clean and transparent performance, I have added a couple more valves to try and give some 'tubiness' to the sound.
I have fitted a couple of 6418 miniature pentodes, wired as triode gain stages before the 6J1's in buffer configuration.
Looking at the spec sheet for the 6418 they show quite a lot of harmonic distortion, but listening through it gives a very satisfactory sound at typical CD levels.
http://i601.photobucket.com/albums/tt97/Paramotorpilot/6J1%20buffer/P1030677.jpg
I think it's possibly what you wanted now Oliver :D
Bigman80
05-03-2017, 13:19
Having tried this now in various guises; gain stage - triode and pentode operation, buffer (cathode follower) operation, and finding very clean and transparent performance, I have added a couple more valves to try and give some 'tubiness' to the sound.
I have fitted a couple of 6418 miniature pentodes, wired as triode gain stages before the 6J1's in buffer configuration.
Looking at the spec sheet for the 6418 they show quite a lot of harmonic distortion, but listening through it gives a very satisfactory sound at typical CD levels.
http://i601.photobucket.com/albums/tt97/Paramotorpilot/6J1%20buffer/P1030677.jpg
I think it's possibly what you wanted now Oliver :D
Lol, we can test it tuesday ! I don't know how you think of these things
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Cool. I'm still waiting on mine to show up. The transformer is here, but that's the last piece I need. I should just do it point to point.
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Posted this on the Wam 5 years ago. I am amazed the ebay link works now. It was about £50 then I am sure. High end then:D It broke very quickly unfortunately. One channel went.
http://www.ebay.co.uk/itm/Valve-Class-A-Tube-Headphone-Amplifier-preamp-audio-amp-Russian-EH-6922-Tube-/120926027421?pt=LH_DefaultDomain_0&hash=item1c27c0be9d
Put this in front of the A500, with the volume pots on the A500 reduced, ignoring all the stuff about the distortion the Behringer pots introduce. Then rip out the Russian 6922 and put a Tesla gold-pin E88CC from 1972 in the headphone amp. If you ask, I'll sell you the Tesla, as I have one spare.
The Tesla gives mad but good imaging in that precise headphone amp - dunno why but they do. I know cos I have done it and the results are surprisingly entertaining. I really enjoyed it feeding my little M&Ds. If the imaging gets a bit too insane, shove a Mullard 6922 in it, which I can also supply.
Just ordered me a quad of the Teslas for the EAR preamp. Be interesting to see how they work in it.
Obviously you need to use the headphone jack output into the Behringer, in case you were wondering. And don't blame me if the headphone amp stops working as mine eventually did.
Trust me the results are REALLY nice - much better than you would expect. Much, much better.
I don't think I could mount a Mullard on a PCB. That seems like sacrilege.
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They sent me the wrong boards. This looks like a SRPP driver board. It should serve the same purpose. A wall wart isn't going to drive this one.
https://uploads.tapatalk-cdn.com/20170307/d13c5354312e3831f09cbfc7496dbc2a.jpg
https://uploads.tapatalk-cdn.com/20170307/a2556e9507c199719ea42a795da93aac.jpg
Firebottle
07-03-2017, 06:44
It's not is it.
Unlucky.
I ordered one full kit and some of the boards. Maybe the full kit will be right.
This one doesn't look bad, but the transformer won't be cheap. I'll have to try and find a surplus one.
I got the full kit today. The components look decent. It's definitely got more than $10 worth of parts in the bag.
I got mine today. Have put a couple of hours on it and it sounds pretty decent.......far more decent than the paltry price tag (and fear of counterfeit parts) would suggest. I need to do the "hum" mod (although mine is making a sort of low level fizz) than hum.
Question for Alan : Am I right to assume that this board uses a voltage doubler?
Firebottle
18-03-2017, 18:53
Yes Rob, it uses two voltage doublers to generate plus and minus 30V, so any hum component on the output is going to be at 50Hz, not the 100Hz from a full wave rectifier.
:cool:
Yes Rob, it uses two voltage doublers to generate plus and minus 30V, so any hum component on the output is going to be at 50Hz, not the 100Hz from a full wave rectifier.
:cool:
Aha! I like my Saturday nights to have some FIZZZZZZ. Thanks Alan. Will do the "Hummmm/Fizzzzzz" mod soon.
I did the "hum" mod, but the fizz has been replaced with a..........hummmm hm? 50/60hz probably. I have disconnected one side of the 470uf cap (as I hot glued the cap to the board and might need to re-connect it - ?) Oddly the fizz has all but gone and there is only a very feint hum left. The cap has been left connected by it's NEG leg to the join of the 47r resistors only.
Firebottle
20-03-2017, 16:10
Sounds a bit odd. You should have plus and minus about 30V from the voltage doublers if everything is working properly.
Is it pretty close to 12V AC you are putting in? If it is 13V or more then the caps could be overrated.
Sounds a bit odd. You should have plus and minus about 30V from the voltage doublers if everything is working properly.
Is it pretty close to 12V AC you are putting in? If it is 13V or more then the caps could be overrated.
I measured the PS (a musical fidelity wall wart for an X-10D) and it was just over 13vac which I thought would be ok?
Would it be worth changing the cap? It had been used before, but I am not aware that it was duff.
Sorry to ask a daft question, but what is the difference, in this case, between the preamp version and the buffer version?
Thanks,
The Preamp has gain, the buffer has no gain.
Hi, thanks; so what's the point of a buffer?
R
Hi, thanks; so what's the point of a buffer?
R
Impedence mismatching
OK; thanks Grant.
How does one know if one has impedance mis-matching?
Richard.
Impedence mismatching
Ideally you want a very high input impedence and a very low output impedence. It is often mentioned in the specs or can be worked out via math. Sticking a buffer in guarentees this. Alas a poor buffer circuit may also add a rair bit of distortion so you have to be careful there too. If you try one and hear no difference then you dont likely need it. It can be inserted between any 2 components, although most will use it impedence wise between pre and power.
It will, so im told by some give a valvelike experience to solid state like a cd player. I dont really see it quite that way, but that is how i use mine..ie between cd and amp. Not sure why i like it but it sounds more real to me
Arkless Electronics
24-03-2017, 15:18
Bonky. Are you still wishing to try the Arkless phono stage which you put your name down for? I have PM'ed you but had no reply.
Hi, apologies, I didn't pick-up any PM. My fault.
Yes please ...but I'm on holiday at present and won't be back until mid-April. Perhaps after then?
Many thanks,
Richard.
Bonky. Are you still wishing to try the Arkless phono stage which you put your name down for? I have PM'ed you but had no reply.
Arkless Electronics
24-03-2017, 16:39
Hi, apologies, I didn't pick-up any PM. My fault.
Yes please ...but I'm on holiday at present and won't be back until mid-April. Perhaps after then?
Many thanks,
Richard.
OK no problem just pm me when you are ready to try it and if it's still in circulation by then you can try it.
I did the "hum" mod, but the fizz has been replaced with a..........hummmm hm? 50/60hz probably. I have disconnected one side of the 470uf cap (as I hot glued the cap to the board and might need to re-connect it - ?) Oddly the fizz has all but gone and there is only a very feint hum left. The cap has been left connected by it's NEG leg to the join of the 47r resistors only.
Actually the hum is noticeable without any music playing.
Finally got round to boxing this up as I was fed up with it skating across the table when I touched it. Being a cheapskate I used an old Sure amp box. Not too shabby IMO of course.
http://i73.photobucket.com/albums/i239/saxonsex/WP_20170505_06_56_33_Pro_zpsgnibjvgr.jpg
Alan, I am now using it with another power amp and the hum has gone (I have also removed the extra cap) Amazed at how completely silent both the pre and a valve power amp are.
Firebottle
05-05-2017, 10:08
Excellent :thumbsup:
http://i870.photobucket.com/albums/ab261/empire2600/DSCF2699.jpg Hello been meaning to post this for ages and ages finally got around to it, All working well, I made a bit of a mess drilling the top of the alu case with not a good enough drill set, but hey ho, am using this after my dac befor the amp, a Technics su-8011, im not great at describing sound but an certainly enjoying it, I have now switched to an old Rotel amp i got 2 days ago which has a 15db pad switch, It seems to all work together better then the Technics Top end now has a really nice clarity to it, things like chimes and bells you can really hear the tone of them rather then just knowing they are there. Thanks for posting all the info about this firebottle, and everyone else who chipped in.
Yatsushiro
14-07-2017, 18:13
As bought from here:
http://www.ebay.co.uk/itm/291786271575?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
https://www.qiannipicture.com/pic/uploadfile2/UploadFile/P0/SKU377287/B9829B56C6869E9177C61697CDCFCF9392CD48CE03CB9E73CD C7D296469CCE43D26666C6B133D29336C69DD216F473739313 CB33C9CE269BCC56A0CD.jpg
As standard there is a low level hum present, this is cured by fitting an extra capacitor. You need a 470uF 35V electrolytic, the NEGATIVE lead is connected to the junction of two 47 ohm resistors on the top of the board as shown below:
http://i601.photobucket.com/albums/tt97/Paramotorpilot/6J1%20buffer/P1030658.jpg
The POSITIVE lead is wrapped around the side of the board and connected to the 0V or 'earth' connection of the input sockets:
http://i601.photobucket.com/albums/tt97/Paramotorpilot/6J1%20buffer/P1030659.jpg
I've just started using one of these, and have the low level hum. Any chance you can email me the pictures of how to connect each end of the 470uF cap? (tdr250 at gmail.com)?
Thanks
Yatsushiro
17-07-2017, 18:32
Thanks for your help Alan:)
Jonesy_sa
28-10-2017, 09:45
As bought from here:
http://www.ebay.co.uk/itm/291786271575?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
Job done :)
I have had one of these for sometime and only getting around to use it now
Would it be possible to have the pictures re-uploaded for myself and others?
How much gain does the standard circuit provide?
Thank you
Jonesy_sa
31-10-2017, 04:43
Is 1amp enough current for these? Will providing more current pose a problem?
I have a few 12.6v Transformers to choose from.
Hi everyone, is it possible to upload photos? Because those that are there are not seen and cannot be opened.
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