|
Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
|
Thread Tools |
13th Jun 2021, 8:10 am | #21 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
|
Re: Surprise today
I am using this unit with my ‘ 1928 kit radio ‘ which was discussed at length back in 2013/14 but is no longer on here. I had it working quite well from this unit but some instability on LW and some undiagnosed fault with the aerial coil on MW. It got plenty of stations. I have just noticed that I had all the -negatives and GB + connected together to the earth which can’t be right.
There is only one connector on the radio marked LT - which feeds the on /off switch. More to come...isn’t this going to affect the PSU? Last edited by Pete_kaye; 13th Jun 2021 at 8:22 am. |
13th Jun 2021, 8:26 am | #22 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
|
Re: Surprise today
And the circuit I drew at the time. This ties both the HT AND LT NEGS TOGETHER. Won’t this affect the working of the PSU?
Last edited by Pete_kaye; 13th Jun 2021 at 8:48 am. |
13th Jun 2021, 9:09 am | #23 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
|
Re: Surprise today
Hi Pete, this was the standard connection for most sets (think separate batteries).
If your PSU has HT & Lt supplies and the neg lines are commoned, there is no problem, LT + was normally the only switched line, not HT. GB supplies are negative of common, so GB+ is connected to LT/HT neg. Do not run the set or even change GB taps with the set powered as iy can cause excessive currents and damage valves and I/V transformers Ed |
13th Jun 2021, 9:14 am | #24 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
|
Re: Surprise today
Thanks Ed.
|
13th Jun 2021, 10:37 am | #25 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
|
Re: Surprise today
What concerned me was that the PSU negatives are not common, as the circuit diagram.
|
13th Jun 2021, 12:13 pm | #26 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
|
Re: Surprise today
The filament and HT/GB supplies in the circuit given do have separate 0V but since they're fed from independent floating transformer secondaries, linking them to gether at the set (or at the PSU) will not cause any problems.
Not linking them anywhere might well do so!
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
21st Jun 2021, 7:31 am | #27 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
|
Re: Surprise today
A new question. I have used a 4 ohm speaker with a small output transformer of 320 ohm primary and 0.5 secondary. The output valve is PM 252 which is listed as needing anode load of 1900 ohms. Would this be resistance only or including impedance of the primary? The sound is loud and harsh. I may be damaging the valve. What do you think.
|
21st Jun 2021, 7:53 am | #28 |
Dekatron
Join Date: Aug 2007
Location: W.Butterwick, near Doncaster UK.
Posts: 8,935
|
Re: Surprise today
Not too convinced re valve damage but possibly need one a bit nearer correct impedance for quality I would think.
__________________
G8JET BVWS Archivist and Member V.M.A.R.S |
21st Jun 2021, 8:45 am | #29 |
Octode
Join Date: Aug 2003
Location: Welwyn Garden City, Herts. UK.
Posts: 1,906
|
Re: Surprise today
The 350 ohms figure is only the DC resistance of the transformer you are using. you can measure the impedance. have you tried a 0.01 capacitor across the primary?
|
21st Jun 2021, 9:20 am | #30 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
|
Re: Surprise today
What effect would a cap across primary have ?I will try it later.
|
21st Jun 2021, 9:45 am | #31 |
Octode
Join Date: Aug 2003
Location: Welwyn Garden City, Herts. UK.
Posts: 1,906
|
Re: Surprise today
The cap will reduce the harshness.
|
21st Jun 2021, 10:14 am | #32 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
|
Re: Surprise today
Yes it made a very slight difference.
|
21st Jun 2021, 10:34 am | #33 |
Moderator
Join Date: Dec 2010
Location: Exeter, Devon and Poole, Dorset UK.
Posts: 6,874
|
Re: Surprise today
The reaction isn't set too high is it?
This sort of set does sound horrible when they're on the brink of oscillating. I believe this might be the set under discussion. https://www.vintage-radio.net/forum/...ad.php?t=97229 Cheers Mike T
__________________
Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk Last edited by Cobaltblue; 21st Jun 2021 at 10:37 am. Reason: Added Link |
21st Jun 2021, 10:44 am | #34 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: Surprise today
Quote:
Another way you can tell that 1,900 Ohms is the anode impedance from the valve data is to divide the amplification factor by the mutual conductance, the amplification factor is given as 7 and the mutual conductance is given as 3.7mA/V.....7/0.0037 = 1,892 (Ohms) which is near enough. Lawrence. Last edited by ms660; 21st Jun 2021 at 11:06 am. Reason: extra info |
|
21st Jun 2021, 11:45 am | #35 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
|
Re: Surprise today
And remember that the load impedance seen by the output valve anode is essentially the nominal speaker impedance times the square of the output transformer turns ratio. The actual impedance of the output transformer primary on its own (mostly Henries plus a dash of Ohms) mainly only affects the LF roll off point of the system.
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |