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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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25th Jul 2021, 10:06 pm | #1 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
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Barker 88 Residual Hum
Hi folks. I've got a little bit of residual hum on the Barker 88 that I have just restored. The hum disappears at min volume and rises gradually as I turn it up. The problem appears to be pick-up by the unscreened glass 6Q7G(T) - I have tried both G and GT variants and both hum. I could change the valveholder for one with a screening can but for a simpler solution I'd like to try a 6Q7 metal valve and see if that does the trick. I've posted a wanted Ad. The set has been recapped and since the hum is volume control dependent it would appear not to be the fault of the HT smoothers, which are original but reformed satisfactorily (<1mA leakage). Any other ideas welcome.
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25th Jul 2021, 10:21 pm | #2 |
Moderator
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Re: Barker 88 Residual Hum
Whilst you waiting for a metal valve is it worth wrapping a strip of bacofoil around the existing valve wrap a few turns of wire to hold it in place and make a connection then connect the free end to the chassis under a screw head or something.
Only take a few mins to try Cheers Mike T
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26th Jul 2021, 3:23 pm | #3 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
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Re: Barker 88 Residual Hum
Hi Mike, thanks for the tip. It did reduce the hum, but only marginally. I'm thinking it's maybe a live-with factor resulting from the wiring layout. There is quite a long run of coax from the volume control to the gram socket and thence to the 6Q7G(T) top cap. I've got all the components in that area with shortest possible leads and as close to chassis as I can. Cheers, Jerry
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26th Jul 2021, 3:48 pm | #4 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,611
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Re: Barker 88 Residual Hum
It would be interesting to know what the experience of others with the same set has been.
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26th Jul 2021, 8:57 pm | #5 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
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Re: Barker 88 Residual Hum
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26th Jul 2021, 10:39 pm | #6 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,152
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Re: Barker 88 Residual Hum
Could the hum be coming from the cable attached to the top end of the volume control?
Is that screened? |
27th Jul 2021, 8:17 am | #7 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
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Re: Barker 88 Residual Hum
The cable to the volume control consists of 2 cores inside a shielded braid. The braid is earthed at several points on the chassis and so is well bonded to earth. The remote end of the cable is anchored to the gram sockets at the rear of the chassis, one cable going to a resistor connected to the detector circuit and the other cable to the AF coupling capacitor to the triode grid via a second screened wire. Cheers, Jerry
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27th Jul 2021, 8:45 am | #8 | |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,548
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Re: Barker 88 Residual Hum
Quote:
I don't remember the hum level being particularly high but there is always going to be some residual hum anyway.
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27th Jul 2021, 9:24 am | #9 | |
Dekatron
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Re: Barker 88 Residual Hum
Quote:
Under normal listening conditions, the hum level should be acceptable.
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27th Jul 2021, 10:10 am | #10 |
Octode
Join Date: Oct 2019
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Re: Barker 88 Residual Hum
Just a thought following the comment by sideband. If you're using a BT adapter connected to the gram input of the radio, hum should improve when it is switched on as opposed to turned off. This has been my experience anyway. I never understood why until I read the above.
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27th Jul 2021, 4:45 pm | #11 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
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Re: Barker 88 Residual Hum
OK, thanks. I'll report back again when I get the 6Q7. Re: circuit, this one at the RadioMuseum is pretty close for the intermediate model https://www.radiomuseum.org/r/barker...?language_id=2 and is different from the Trader Sheet available here (so I'm not breaking any Forum rules). Cheers, Jerry
Last edited by cathoderay57; 27th Jul 2021 at 4:54 pm. |
27th Jul 2021, 10:35 pm | #12 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
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Re: Barker 88 Residual Hum
As a matter of interest, I went into the mancave this evening and powered the Barker 88. With no aerial connected and full volume, yes there is a variable hum and it's quite loud but not excessive....reminiscent of a high-gain amplifier with an open or 'floating' input. My version of the 88 doesn't have a switched Gram input so I assume the the gram input is directly connected across the volume control. Next I sought out some suitable wander plugs to fit the sockets. I then found my Jason tuner and made a lead from that to the Barker gram sockets. As soon as I connected the tuner, the hum level dropped. Powering the tuner produced a very reasonable output....it was far too loud towards full volume and much more reasonable from about a quarter volume. Certainly no annoying hum and at zero volume only the normal residual hum of a typical valve radio which was masked at even quite low volume.
What this little test did show is that the output stage of the Barker is far from Hi Fi..I doubt the treble response extends much above about 8Khz. Despite this the output stage is more than adequate for the radio and in fact sounds very good on a good AM broadcast. The output from the Jason is about 500mV and is excellent quality although only mono of course. In short, cathoderay, you are probably getting the best from the amplifier as it stands. Cheers
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27th Jul 2021, 11:19 pm | #13 |
Heptode
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Location: St Albans, Hertfordshire, UK.
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Re: Barker 88 Residual Hum
Hi Jerry is it a early or late model, I have a early model but need to sorce a op for it
Mark |
28th Jul 2021, 7:56 am | #14 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
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Re: Barker 88 Residual Hum
Hi Mark, mine is the intermediate model with 6K8G, 6K7G, 6Q7G, 6J5G, 6V6G (x2), U50, 6U5G. This transformer will work https://www.ebay.co.uk/sch/i.html?_f...9A10F&_sacat=0 . I know it says for EL84 but the turns ratio is OK for a pair of 6V6 or 6F6. Alternatively you could ask Ed Dinning for a rewind - he did a fine job on mine. Cheers, Jerry
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28th Jul 2021, 7:58 am | #15 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
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Re: Barker 88 Residual Hum
Thank you Sideband - I'll fit the 6Q7 when I get it, then enjoy it as it is Cheers, Jerry
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28th Jul 2021, 7:38 pm | #16 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
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Re: Barker 88 Residual Hum
Hi folks, got the metal 6Q7 today and inserted it. Great news - the hum level even at max volume dropped by over 70%. Many thanks to Steve Bell for supplying the valve. Jerry
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28th Jul 2021, 8:02 pm | #17 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,611
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Re: Barker 88 Residual Hum
You must be very pleased with that Jerry, nice job identifying the cause of the hum.
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28th Jul 2021, 8:12 pm | #18 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
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Re: Barker 88 Residual Hum
Interesting. The metal ones must have been more expensive. I do have one radio somewhere that has the 6Q7 screened with a can. Maybe some 6Q7's were better than others.
Great result!
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