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Old 4th Oct 2012, 10:31 pm   #21
gezza123
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Default Re: AR88D sound output question please

Hi, G6Tanuki. RE. post.
More of interest to me is the whole issue of rectifiers for the AR88: oruginally it was fitted with a 5Y3GT which had a directly-heated 'tape' filament. When this failed, the replacement 1960s-era NOS Mullard 5Y3GT was an indirectly-heated tubular-cathode job.

Will this push the B+ rail too high? who knows?
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I have now discovered that the rectifier in my set is a (5u4g)that has a 3.0A heater, so you may well have hit the nail on the head. IE: extra ht/Amps, so do these sets have a wide percentage for variable Ht.
But I have also noticed that there is no overheating from anywhere in the set and it has been running for hours on end for a long periods of time every day, and I love it.
-------------------------------------------------------------------------
So back to my first post, and as Colin said "If it ain't broke..."
My question is that my (AR-88D) has got a 6v6gt/g in it, and there has been no modifications to the output stage, so is this ok to be used with the above transformer No. K901573-501, or do I need to get a 6k6gt valve.
I know that the two valves have different anode loadings but this does not seem the affect the working of the output.
The set is working very well with all sorts of stations being received,and excellent sound. your comments please.
Thanks again gents for your all your feedback.( Excellent).Gezza123
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Old 5th Oct 2012, 12:35 pm   #22
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Default Re: AR88D sound output question please

Why not just check the anode current of the 6V6 through the TX primary? If this is less than about 35mA then it's no more stress on the TX primary than a 6K6 would have been. If it's higher, maybe add a bit of cathode resistance to bias it back some (probably easier than finding some more -ve volts for the grid).
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Old 6th Oct 2012, 12:51 pm   #23
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Default Re: AR88D sound output question please

Hi, Chris
Just put some new batteries in the meter, I then disconnected the out transformer and measured the resistance, @ 1000 ohm on pins 3 + 4.

I then measured the anode current @ 30 milliamp, the Ht varies between
278v and 282v when useing the gain control.

I have found this now, so think it may be ok, thanks for all the input Guy's, Gezza123
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Old 6th Oct 2012, 1:45 pm   #24
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Default Re: AR88D sound output question please

Thats very interesting Gezza123.

That table of voltages are different from the one in my AR88D manual and a better match to the anode and screen voltages actually measured today on my 6K6 output valve.

My output transformer is marked 901666-501 and has a primary resistance of 542 Ohms. The cathode current of my 6K6 is 30mA. Anode current is about 27mA.

The second choke was faulty when I bought my AR88 and I have fitted a 500R resistor in its place to get it going. I thought the voltages in my radio were too high but it now seems they are OK.

Many thanks,
Brendan
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Old 6th Oct 2012, 2:20 pm   #25
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Default Re: AR88D sound output question please

Quote:
Originally Posted by G6Tanuki View Post
My angst is rather more related to the greater emission / lower effective series-resistance of the indirectly-heated cathodes - which will cause a higher average operating B+ voltage which could over-stress things that were not really designed to handle an extra 10% B+.
I don't think it's true to state that indirectly heated cathodes are necessarily capable of greater emission than directly heated ones. Thinking logically, the opposite is more likely to be true. Having the emissive oxide diirectly coated onto the filament will mean that it is able to reach a higher temperature for a given amount of heater power and thus provide more efficient emission.
I suspect that the problem of excessive HT surge voltages due to directly heated rectifiers was well known even in the early '30s. The reason that it took some time for higher power indirectly heated rectifiers to become available was that it was difficult to make such a valve with reasonable heater power requirements using earlier processes.

John
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Old 6th Oct 2012, 5:45 pm   #26
G6Tanuki
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Default Re: AR88D sound output question please

Quote:
Originally Posted by jjl View Post
I don't think it's true to state that indirectly heated cathodes are necessarily capable of greater emission than directly heated ones. Thinking logically, the opposite is more likely to be true. Having the emissive oxide diirectly coated onto the filament will mean that it is able to reach a higher temperature for a given amount of heater power and thus provide more efficient emission.
John
You may well be right. somehow I mentally associate sensibly sized i.h. valves as having greater cathode-emission-per-watt than d.h. valves. I know that the original 4-pin-base '80' rectifier - which was the direct ancestor of many of the 5[x|y|z]-series of octal-based rectifiers - was originally d.h. but variants were produced by some manufacturers as i.h. with the heater and cathode strapped internally.

Whether this was to provide slower rise of HT and less stress on the smoothing electrolytics (remember that the lots of turns needed to get enough magnetic flux in the energized-speaker-smoothing-choke used in pre-WWII radios often had quite a high resistance and the voltage-drop meant you needed a higher initial HT voltage: using an i.h. rectifier along with i.h. valves would mean the valves started drawing current as the rectifier came up to emission, so avoiding the 'voltage overshoot' an i.h. rectifier could cause) or whether there was some other benefit, i don't know.

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Old 8th Oct 2012, 6:37 pm   #27
Leon Crampin
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Default Re: AR88D sound output question please

I have never ever seen an indirectly heated type 80 rectifier. The octal equivalents to the type 80 are 5Y3 and 5Y4 - both directly heated.

An indirectly heated rectifier is of course kinder to the smoothing capacitors and is in many ways preferable, but a perusal of the ratings will show that for a given rated output current, a directly heated rectifier will have a higher peak current rating than its indirectly heated counterpart.

This means that for a given output, either a lower value of source impedance is allowed (transformer winding resistance + reflected primary resistance) or a larger reservoir capacitor may be used. Either improves regulation.

Leon.
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Old 9th Oct 2012, 7:00 pm   #28
G6Tanuki
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Default Re: AR88D sound output question please

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Originally Posted by Leon Crampin View Post
I have never ever seen an indirectly heated type 80 rectifier. Leon.
The i.h 80 was the 80A/80K/80S depending on manufacturer.

See:

http://www.radiomuseum.org/tubes/tube_80s.html

and

http://www.radiomuseum.org/tubes/tube_80k.html

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Old 10th Oct 2012, 5:55 pm   #29
Leon Crampin
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Default Re: AR88D sound output question please

Indeed, but not an "80".

Leon.
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