A foray into the world of glowy stuff.

We all start somewhere
Post Reply
chris661
Shed dweller
Posts: 2559
Joined: Mon Feb 22, 2010 7:29 am
Location: Sheffield

#1 A foray into the world of glowy stuff.

Post by chris661 »

So, in a moment of... Insanity... I bought a 1950s radio.

Some pictures here,
http://www.vintage-radio.com/recent-rep ... 242-2.html

Circuit here,
http://www.murphy-radio.co.uk/diagrams/A242.jpg

All works apart from the FM receiving bit. Plus, the gramophone input on the back (Dad reckons a ceramic cartridge input) suits my iPod nicely 8)

Also tried my guitar multi-fx into said input. Turn everything all the way up, and some crunchy (loud!) valvey sounds come forth. The treble cut gives a very smooth sound, too.

Anyway. Thinking of replacing the output valve (6P1), and work out which valve does the FM bit. One of them (in a metal tube - heatsink? its on the left, nearest the camera http://vintageradio.me.uk/valve/a242/mu ... hassis.jpg) isn't glowing, so that's my prime suspect.

Well, whaddya think? :D

Chris
Top
User avatar
pre65
Amstrad Tower of Power
Posts: 21400
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#2

Post by pre65 »

Can you read the number on the duff valve ?
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
Top
chris661
Shed dweller
Posts: 2559
Joined: Mon Feb 22, 2010 7:29 am
Location: Sheffield

#3

Post by chris661 »

That'd be a 6D2, Phil.

Dual diode from what I can tell. The guy on the first link had a similar problem (non-glowyness), and said that wiggling the valve helped. So that's my first port of call in the morning.

Not 100% of this though - inference from the other guy's problem with the FM bit, linked to a non-glowy valve of my own (same symptom as his), and my own FM bit isn't working.

Will post more when I've worked out how to get the metal sleeve off the valve in question.

Chris

Edit - unfortunately, I can't seem to find a 6P1 on eBay with an octal base. The valves in mine are Mazda, dunno if that helps.
Top
chris661
Shed dweller
Posts: 2559
Joined: Mon Feb 22, 2010 7:29 am
Location: Sheffield

#4

Post by chris661 »

The plot thickens.

The valve at fault (now I've figured out how to remove the metal sleeve) is an EC92.
Looks brand new. As do the PSU caps. Looks like someone else has had a play with this before I have. Everything else looks original enough.

The EC92 feels loose in its socket (compared to the others), and still refuses to glow.

The valve itself is at the left on the circuit diagram. Can't see any link with the FM bit, myself. Can someone better at reading these things see what they think?

Anyway, contemplating seeing if I can remove the chassis (minus the radio stuff), and putting it into some kind of guitar amp. With a 3-way tone switch. 8)

Chris

Edit, Mike - on http://vintageradio.me.uk/valve/a242/mu ... hassis.jpg
it's the valve at the far left that's refusing to go. The one in the metal sleeve (on mine, the one further up also has a metal sleeve but there you go).
Last edited by chris661 on Mon Jul 04, 2011 3:46 pm, edited 1 time in total.
Top
User avatar
Mike H
Amstrad Tower of Power
Posts: 20189
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:
Contact Mike H

#5

Post by Mike H »

Looking at diagram I would say nearest match for the output should be a 6V6-GT, same pin numbers, should be a straight swap.

Next step on the slippery slope is give it a better o/p transformer, as the original ones tend to be small and weedy :D

Note that C76 could be leaky as most old caps often are, if so it puts extra positive bias on V7 causing too much anode current and premature failure (anode P-dis exceeded), common fault with old radios.

In photo I presume 6D2 is the one to the left of "2047 AS" written on the chassis?

Double-diode with separate cathodes, it's the FM audio detector so that'd be why nuffink's coming out. List of equivalents here: http://tdsl.duncanamps.com/show.php?des=6D2

I notice EB91 there.

Or, bung a pair of germanium signal diodes onto the valveholder tags.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
Top
chris661
Shed dweller
Posts: 2559
Joined: Mon Feb 22, 2010 7:29 am
Location: Sheffield

#6

Post by chris661 »

Thinking of using the sockets and things to make a low powered valve guitar amp.

Any thoughts/opinions on this? It'd mean the likkle output transformer stays :)
Top
chris661
Shed dweller
Posts: 2559
Joined: Mon Feb 22, 2010 7:29 am
Location: Sheffield

#7

Post by chris661 »

So, measured the voltages around the output valve.
Working to this,
http://www.murphy-radio.co.uk/diagrams/A242.jpg

Pin 3 had 227v
4 had 188v
5 had 2.81v
8 had 8.9v
HT is at 240V (late DIET)

Unless I've read the numbers on the base wrong.

Chris

Edit - just realised my next post will be number 1000.
Last edited by chris661 on Thu Jul 07, 2011 6:08 pm, edited 2 times in total.
Top
User avatar
Mike H
Amstrad Tower of Power
Posts: 20189
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:
Contact Mike H

#8

Post by Mike H »

What mods will that involve? Vintage restorers may be horrified :D
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
Top
chris661
Shed dweller
Posts: 2559
Joined: Mon Feb 22, 2010 7:29 am
Location: Sheffield

#9

Post by chris661 »

The mods would probably be a simple as ripping out everything I won't be using (variable capacitors, radio valves, stuff like that), using the existing sockets and PSU for a ECC83 based pre-amp, putting it all in a different box with a bigger speaker (10" minimum)...

Think I'll keep the variable treble cut circuit. That's a nice touch. Feels much more meaningful to use than these modern tone controls... :roll:

As I said, in it's current state it's not worth a whole lot.

WOAH. Post #1000. :wav:
Top
User avatar
Mike H
Amstrad Tower of Power
Posts: 20189
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:
Contact Mike H

#10

Post by Mike H »

Image   Image   Image
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
Top
chris661
Shed dweller
Posts: 2559
Joined: Mon Feb 22, 2010 7:29 am
Location: Sheffield

#11

Post by chris661 »

Anyone any ideas what the circuit that looks like it might be a feedback circuit is? It's on the transformer output.

Seems odd that it'd feed into the volume control again.

Dad reckons short out the 68ohm resistor to ground, and feedback will disappear. Any thoughts on this? Guitar amps tend to use little if any feedback, but I don't know how much this is providing, or if its safe to cut it out.

Chris
Top
User avatar
Mike H
Amstrad Tower of Power
Posts: 20189
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:
Contact Mike H

#12

Post by Mike H »

Could be NFB, if the transformer windings are in the phase as shown! I.e. the speaker "live" o/p is inverted compared to the input.

Can't remember what's the deal with these but does mean it's like an inverting amplifier where the vol control is an input resistor, maybe it helps flatten the freq. response a bit or something.

Yes I think simplest is short out the 68R. :D

BTW if you're taking the radio stuff parts off the chassis will you be keeping them?
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
Top
User avatar
Mike H
Amstrad Tower of Power
Posts: 20189
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:
Contact Mike H

#13

Post by Mike H »

BTW(2) anyone else notice the treble boost for Medium Wave & Long Wave? 47pF + 680k across the vol pot and wiper, so the A.M. treble sounds a bit more like F.M. :lol:
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
Top
chris661
Shed dweller
Posts: 2559
Joined: Mon Feb 22, 2010 7:29 am
Location: Sheffield

#14

Post by chris661 »

Mike H wrote:Could be NFB, if the transformer windings are in the phase as shown! I.e. the speaker "live" o/p is inverted compared to the input.

Can't remember what's the deal with these but does mean it's like an inverting amplifier where the vol control is an input resistor, maybe it helps flatten the freq. response a bit or something.

Yes I think simplest is short out the 68R. :D

BTW if you're taking the radio stuff parts off the chassis will you be keeping them?
Thanks, Mike. Think I'll solder a switch across it, easy to compare then.

I'm not sure what I'll be keeping. The variable capacitors look cool, but from what I can tell, there's 3 of them.
Apart from that, the radio valves will probably appear on sale around here at some point. I really don't know how much these are worth, but won't be asking a lot, if anything at all.
The sliding tuning bits might (slim chance) be of use to someone, probably give those away.

So, here's the plan.
http://www.voxamps.com/uk/modernclassic/ac4tv/
How do I make it so the output level is switchable? (some switchable resistors in series with the speaker?)

Cheers,
Chris
Top
Bizzie
Old Hand
Posts: 500
Joined: Mon Dec 17, 2007 1:22 pm
Location: North Lincolnshire

#15

Post by Bizzie »

You could take a look at this site Chris, this amp has 3 power setting using reduced voltage for 2 of them might give you some ideas.

http://www.ampmaker.com/store/Double-Si ... ifier.html

construction manual here

http://www.privateeramps.com/infocentre ... um.php?f=1
Top
Post Reply

Return to “Beginners”