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Author Topic: AM Broadcast Receiver Reconditioning  (Read 23090 times)
G3RZP
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« Reply #15 on: August 11, 2013, 01:20:43 PM »

You should differentiate between electromagnetic and electrostatic speakers. Electrostatic speakers were relatively rare e.g. the Quad ones,  while electromagnetic were common in the 1930s BC receivers.

I presume you are referring to electromagnetic speakers.

The drive requirements are completely different.
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K0OD
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« Reply #16 on: August 11, 2013, 02:25:17 PM »

What's the condition of the cone?

Small tears and holes can be easily fixed. I just repaired a 1937 Zenith speaker by backing the damaged portion of the cone with a strong paper towel material saturated with Elmers.  Sounds great now. I think I found a video on re-coning on Youtube.

PM speakers came into use in the late 1940s when stronger alnico type magnets were invented. 
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WB6DGN
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« Reply #17 on: August 11, 2013, 09:58:02 PM »

Quote
What's the condition of the cone?

I'll have to pull the speaker out of the radio to know for sure but, based on the condition of the grille cloth, its probably pretty bad.  I can see some large tears where the cloth is missing but don't know if they're repairable or not.
Just after I started on this, my dog got very sick so I had to set it aside.  I'll know more in a week or so.
Tom
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WB6DGN
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« Reply #18 on: August 11, 2013, 10:20:34 PM »

Quote
You should differentiate between electromagnetic and electrostatic speakers.

I didn't realize there was a difference.  The only term I've ever heard used for those (from over 50 years ago) was electrostatic; so I just parroted the term.  This radio has four wires from the speaker to the radio chassis, two from the output transformer mounted on the speaker and two from the vicinity of the magnet coil.  Since I haven't pulled the chassis yet, I can't confirm where they go but I did look at the audio output tube which is a 6F6 (one I had not seen used before this).  I'm familiar with 6K6, 6V6 and 6L6 (naturally) but 6F6 is new to me so I don't know where it fits in output ranking.  I do know that the output transformer is quite a bit larger than I would have expected for just an 8" speaker.  Not sure if that means anything or not, though.  I'll know more when I get back to it in a week or so.
Tom
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AC5UP
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« Reply #19 on: August 12, 2013, 01:18:42 AM »

An electrostatic speaker works like the opposite of a ribbon microphone........ A thin membrane of metalized film held taut and suspended between two charged plates. Typical application is a tweeter as a larger version capable of modest bass needs a fairly large power supply. Compared to a dynamic speaker they're very inefficient and would be a poor choice for a vintage console.

An electromagnetic speaker uses a magnetic winding on a soft iron core in place of a permanent magnet assembly. Back then magnets weren't as strong nor as durable as today and it wasn't unusual for smaller PM speakers to go bad as the magnet was slowly demagnetized by the voice coil.
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G3RZP
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« Reply #20 on: August 12, 2013, 03:29:41 AM »

The Quad company in the UK produced some HiFi electrostatic speakers - about the size of a heating radiator. But electrostatic speakers weren't common.

The use of the electromagnet for the speaker also allowed saving on the cost of a filter choke....later on, the drive for cost reduction led to using an R-C filter for the B+. Because of the high plate resistance of pentodes, you could have pretty ripply B+ on the plate if you bypassed the screen well.

The 6F6 is a power pentode, rather than a beam tetrode like the 6V6, 6L6 etc. It has a slightly higher plate resistance and lower plate dissipation and transconductance than a 6V6. It is the octal based equivalent of the older 42 tube.
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W9GB
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« Reply #21 on: August 12, 2013, 05:07:35 AM »

Quote from: WB6DGN
I'm familiar with 6K6, 6V6 and 6L6 (naturally) but 6F6 is new to me so I don't know where it fits in output ranking.
List of vacuum tubes (6 volt filaments) - Wikipedia
Nice quick reference, then you can pull specification sheet from RCA/GE Tube Reference
http://en.wikipedia.org/wiki/List_of_vacuum_tubes#6_volt_heater.2Ffilament_tubes

6F6, KT63 Power Pentode. Octal base version of type 42.
Moderate power output rating—9 watts max. (Single-ended Class A circuit); 11 watts max. (Push-pull Class A circuit); 19 watts max. (push-pull Class AB2 circuit). Available in metal (numbered "6F6"), shouldered glass ("6F6-G"), and cylindrical glass ("6F6-GT"). Sometimes used as a transformer-coupled audio driver for types 6L6-GC and 807 when those tubes were used in Class AB2 or Class B amplifiers. Also used as a Class C oscillator/amplifier in transmitters.
« Last Edit: August 12, 2013, 05:10:37 AM by W9GB » Logged
K0OD
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« Reply #22 on: August 12, 2013, 06:44:29 AM »

Quote
based on the condition of the grille cloth, its probably pretty bad.

Grille cloth was probably the first thing to go in radios of that vintage. There was a reason old people used to smell like moth balls! Modern replacement grille cloth is available from restoration websites.

The cone might be better than you think. But cones too could be attacked by bugs, and they dry out from the heat of vacuum tubes. The patched original speaker might suffice for occasional demonstration listening. Depends on the level of restoration you want.

Most old console radios are common. There are plenty on Craigslists. I wouldn't put much into restoring it unless it has sentimental value. The eye tube adds value. BTW, does it have shortwave, as most of the better ones had in the late 1930s?
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KE3WD
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« Reply #23 on: August 12, 2013, 07:02:13 AM »


I'll have to pull the speaker out of the radio to know for sure but, based on the condition of the grille cloth, its probably pretty bad.  I can see some large tears where the cloth is missing but don't know if they're repairable or not.
Just after I started on this, my dog got very sick so I had to set it aside.  I'll know more in a week or so.
Tom

Don't assume anything then until you get to look behind the grille cloth and examine the actual speaker cone, I've seen plenty of examples where the cloth was deteriorated but the speaker cone itself was just fine. 


73
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G3RZP
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« Reply #24 on: August 12, 2013, 12:44:53 PM »

I am not sure about KT63 being a 6F6. 'KT stood for 'kinkless tetrode': there were close equivalents to 6K7 and 6J7 - KTW63 and KTZ 63, all from the Marconi - Osram valve company - MOV. But they were all tetrodes although arranged to exhibit pentode characteristics

The guy who would know is SK - G6JP - while the only other guy of that era around is G2BUP, and he dealt with tx tubes.

I have a feeling that the 'KT' approach was to get around RCA patents on beam tetrodes...
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KD0REQ
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« Reply #25 on: August 20, 2013, 11:38:29 AM »

the Brits actually invented beam-power tetrodes in '35 or '36, but put their tube on the bench.  RCA got wind of the patents, and the 6L6 was introduced in 1936 to a firestorm of demand and demonstration circuits.  GEC/MOV (M-O-U-S-E) then brought out the KT66 a year or so later, after the market was primed.  the interrelationships of the British and European tube makers have always been a mystery to me, but light reading indicates the smell of cartel.

as you could say of Radio Corporation of America...
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W1BR
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Posts: 4189




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« Reply #26 on: August 20, 2013, 01:36:00 PM »




99% percent of vintage radios use Class A stages, even when running P-P. There is no need to worry about reserve energy in the power supply.

Some guys have used pass regulators with a pass transistor to replace defective field coils. I will note that replacing the original
speaker with a PM replacement will ruin the collector value to large extend for more of the more desirable consumer sets.

Pete
« Last Edit: August 20, 2013, 01:42:00 PM by K1ZJH » Logged
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