1st valve project - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

marknotts · 29th Jul 2008, 3:15 pm

Quote:

Originally Posted by Variometer

Curiosity - Have any of the replies been any help, or are you still searching?
Bob

Yes, very much so!
I'm actually on holiday at the moment but when I return I hope to make a start!
Thanks all.

Neil Breward · 30th Jul 2008, 1:22 pm

Mark - I'm in Keyworth, just outside Nottingham, so if you want any help or components please let me know!

Cheers,
Neil

marknotts · 2nd Aug 2008, 4:27 pm

Just had a thought, Does anybody know the resistance of an earpiece from an old style rotary dialling telephone?? Could this be used for the headphones

Variometer · 2nd Aug 2008, 6:57 pm

I don't think so - have just stuck the meter on one & it came out at 32 ohms. Best method is to use a small transfrormer & normal low impedance headphones as already suggested above.
Bob

Station X · 2nd Aug 2008, 7:09 pm

Would a crystal earpeice available from Maplin work, or is that way too high in impedance?

Variometer · 2nd Aug 2008, 7:40 pm

Probably too high for a valve if in the anode circuit, it would kill the anode current dead. Easeist option is small transformer that is known to work.
Bob

wireful3 · 3rd Aug 2008, 8:49 am

Another thought is to use a resistive load for the valve ( about 10K ) and then capacitive coupling from anode to the Crystal earphone and earth. The transformer is the more usual solution though.

Variometer · 3rd Aug 2008, 11:27 am

I have no doubt that would work, I have used the same method with normal HR headphones. I would say the main disadavantage when messing about with earpieces & experimental sets (even battery) is that if you connect things up wrong & send a high volume shriek straight into you ear via an earpiece, you might blow your eardrum out. Even with normal headphones, I tend to have them forward of the ears initially until I know it is going to work ok without a high-powered audio coming out of it!

Easiest & safest method is the transformer. You can pick up a suitable transformer on a car boot sale for 50p or so. They come inside those little portable cassette recorders PSUs that we all tend to accumulate over the years. There seems to be a bottomless pit of them around.
Bob

OErjan_S · 4th Aug 2008, 10:14 am

i have just tried the two telephone earpieces in series.
worked decently well, much better with a series 10k resistor and 0.01 cap to "hot" lead of phones best with an old 4W LM Ericsson PA transformer.
just some information as to some of these speakers, they are sometimes not direct magnetics and therefore these speakers are somewhat misleading when you measure the DC resistance, there is also a mechanical impedance matching built in (look for balanced armeture and soundpowered.

EDIT
I have entered a few links to elements with similar internal buildup.
the site http://www.crystalradio.net
has LOTS of info on crystalradios and phones (specially the soundpowered variety).
here is a decent explanation of the internals
http://www.crystalradio.net/soundpow...ts/index.shtml
look here for Some Impedance Measurements
http://www.crystalradio.net/soundpow...ce/index.shtml
here are the internals of some Swedish telephone speakers.
http://crystalradio.net/sweeden/
the British version is the RX 4T i think.

Magic Band · 2nd Sep 2008, 8:29 pm

Rather simple project, however great sound, a 1 valve AF-amp.:
(the 2nd halve of the 6EM7 is almost a 6B4G, first section almost a ECC81/12AT7)

http://img48.imageshack.us/img48/811...amp1xk7.th.jpg

30th Jul 2008, 1:22 pm	#22
Neil Breward Retired Dormant Member Join Date: Aug 2005 Location: Keyworth nr. Nottingham, UK Posts: 140	Re: 1st valve project Mark - I'm in Keyworth, just outside Nottingham, so if you want any help or components please let me know! Cheers, Neil

2nd Aug 2008, 4:27 pm	#23
marknotts Retired Dormant Member Join Date: Jul 2008 Location: nottingham Posts: 11	Re: 1st valve project Just had a thought, Does anybody know the resistance of an earpiece from an old style rotary dialling telephone?? Could this be used for the headphones

2nd Aug 2008, 6:57 pm	#24
Variometer Retired Dormant Member Join Date: May 2008 Location: Preston, Lancashire, UK. Posts: 428	Re: 1st valve project I don't think so - have just stuck the meter on one & it came out at 32 ohms. Best method is to use a small transfrormer & normal low impedance headphones as already suggested above. Bob

2nd Aug 2008, 7:09 pm	#25
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: 1st valve project Would a crystal earpeice available from Maplin work, or is that way too high in impedance? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

2nd Aug 2008, 7:40 pm	#26
Variometer Retired Dormant Member Join Date: May 2008 Location: Preston, Lancashire, UK. Posts: 428	Re: 1st valve project Probably too high for a valve if in the anode circuit, it would kill the anode current dead. Easeist option is small transformer that is known to work. Bob

3rd Aug 2008, 8:49 am	#27
wireful3 Retired Dormant Member Join Date: Sep 2003 Location: Bristol, UK. Posts: 808	Re: 1st valve project Another thought is to use a resistive load for the valve ( about 10K ) and then capacitive coupling from anode to the Crystal earphone and earth. The transformer is the more usual solution though.

3rd Aug 2008, 11:27 am	#28
Variometer Retired Dormant Member Join Date: May 2008 Location: Preston, Lancashire, UK. Posts: 428	Re: 1st valve project I have no doubt that would work, I have used the same method with normal HR headphones. I would say the main disadavantage when messing about with earpieces & experimental sets (even battery) is that if you connect things up wrong & send a high volume shriek straight into you ear via an earpiece, you might blow your eardrum out. Even with normal headphones, I tend to have them forward of the ears initially until I know it is going to work ok without a high-powered audio coming out of it! Easiest & safest method is the transformer. You can pick up a suitable transformer on a car boot sale for 50p or so. They come inside those little portable cassette recorders PSUs that we all tend to accumulate over the years. There seems to be a bottomless pit of them around. Bob

4th Aug 2008, 10:14 am	#29
OErjan_S Retired Dormant Member Join Date: Mar 2007 Location: Skelleftea, Sweden Posts: 250	Re: 1st valve project i have just tried the two telephone earpieces in series. worked decently well, much better with a series 10k resistor and 0.01 cap to "hot" lead of phones best with an old 4W LM Ericsson PA transformer. just some information as to some of these speakers, they are sometimes not direct magnetics and therefore these speakers are somewhat misleading when you measure the DC resistance, there is also a mechanical impedance matching built in (look for balanced armeture and soundpowered. EDIT I have entered a few links to elements with similar internal buildup. the site http://www.crystalradio.net has LOTS of info on crystalradios and phones (specially the soundpowered variety). here is a decent explanation of the internals http://www.crystalradio.net/soundpow...ts/index.shtml look here for Some Impedance Measurements http://www.crystalradio.net/soundpow...ce/index.shtml here are the internals of some Swedish telephone speakers. http://crystalradio.net/sweeden/ the British version is the RX 4T i think.

2nd Sep 2008, 8:29 pm	#30
Magic Band Retired Dormant Member Join Date: Sep 2008 Location: Arnhem, Netherlands Posts: 9	Re: 1st valve project Rather simple project, however great sound, a 1 valve AF-amp.: (the 2nd halve of the 6EM7 is almost a 6B4G, first section almost a ECC81/12AT7) http://img48.imageshack.us/img48/811...amp1xk7.th.jpg