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Old 25th Jul 2021, 10:06 pm   #1
cathoderay57
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Default 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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Old 25th Jul 2021, 10:21 pm   #2
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Default 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

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Old 26th Jul 2021, 3:23 pm   #3
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Default 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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Old 26th Jul 2021, 3:48 pm   #4
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Default 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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Old 26th Jul 2021, 8:57 pm   #5
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Default Re: Barker 88 Residual Hum

Quote:
Originally Posted by Gabe001 View Post
It would be interesting to know what the experience of others with the same set has been.
. Agreed. Anybody else got one and willing to crank the volume right up with no signal and report on hum level? Cheers, Jerry
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Old 26th Jul 2021, 10:39 pm   #6
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Default 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?
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Old 27th Jul 2021, 8:17 am   #7
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Default 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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Old 27th Jul 2021, 8:45 am   #8
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Default Re: Barker 88 Residual Hum

Quote:
Originally Posted by Gabe001 View Post
It would be interesting to know what the experience of others with the same set has been.
I have one of these which works well. Remember that there are quite a few variations of these sets so checking one against another might not give the same results. Mine has a 6J7 triode as a phase splitter as well as a 6Q7GT AF amp. I couldn't find an exact circuit for mine so traced it out instead.

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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Old 27th Jul 2021, 9:24 am   #9
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Default Re: Barker 88 Residual Hum

Quote:
Originally Posted by cathoderay57 View Post
Agreed. Anybody else got one and willing to crank the volume right up with no signal and report on hum level? Cheers, Jerry
I've never run mine with no signal and the volume up full. Personally I would expect quite a high level of hum when doing that. It's not a Hi Fi unit and when used as a domestic radio or even a gram amplifier, I doubt anyone would run it at full volume. 8 watts is LOUD! What if you short the gram input to chassis or even load it with a typical cartridge or, say, a 1 megohm dummy load. I would expect unloaded gram sockets to cause some hum.

Under normal listening conditions, the hum level should be acceptable.
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Old 27th Jul 2021, 10:10 am   #10
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Default 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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Old 27th Jul 2021, 4:45 pm   #11
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Default 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

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Old 27th Jul 2021, 10:35 pm   #12
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Default 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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Old 27th Jul 2021, 11:19 pm   #13
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Default 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
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Old 28th Jul 2021, 7:56 am   #14
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Default 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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Old 28th Jul 2021, 7:58 am   #15
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Default 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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Old 28th Jul 2021, 7:38 pm   #16
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Default 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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Old 28th Jul 2021, 8:02 pm   #17
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Default 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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Old 28th Jul 2021, 8:12 pm   #18
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Default 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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