Dansette major rectifier

[Chris55000]

The simulated figures I quoted in my previous post assume Ia=50mA and Ig2=5mA.

For 40mA anode current, the rectified H.T. simulation (half-wave) averages 165V and screen-grid 140V.

The o/p transformer primary will be typically 300-500 ohms primary d.c. resistance, which would drop about 15-20V at the normal anode-current of a UL84 valve, so if you're reading about 112V at pin 7 of the UL84, I'd expect the H.T. at the top end to be about 125V, which is a bit low for the transformer you've got, if the 145V rating of the secondary is correct!

Can you measure the a.c. voltage from the negative (anode) side of the rectifier to chassis using the 250V AC range of your meter and tell us what it is?

Re-running the simulation from the valve-voltages you quoted suggests you've only got about 4uF of effective reservoir capacitance left, so you need new electrolytics in your chassis! If you can't get another good can (I think there are specialist firms manufacturing special dual units again) three Rifa 105C axial capacitors, one 15uF 350V, one 33uF 350V and one 33uF 50V (for cathode bias) will do, but they are expensive unfortunately!

Chris Williams

[Station X]

Originally Posted by Chris55000

The o/p transformer primary will be typically 300-500 ohms primary d.c. resistance

Measured as 250R in post #39 if I'm reading it correctly.

[leslie5555]

Amazing help from all of you many thanks, it appears that I may have to replace the can contents so I shall look into that .
I can't do any more testing today because the ale has come out lol

[Station X]

FWIW I did lash up a similar circuit.

I used a Variac to feed a 32uF capacitor and 1N4001 diode in series. I adjusted the Variac to give an output of 147 VAC to match the reading given in post #34. The voltage across the capacitor was then 201VDC, close to the theoretical figure of 208VDC. The figure measured in post #43 was 198VDC.

I found that with a 4000R (5 x 20k in parallel) resistor connected across the capacitor, the current flowing through the resistor was 45mA. This caused the voltage across the cap to drop to 190VDC.

Of course I had no mains transformer secondary resitance or 100R resistor in series with the diode to consider.

Once the OP has replaced the electrolytic caps it'll be interesting to see what voltages he measures.

[leslie5555]

I need to order the required caps even though I do have the values in 'stock' but they are all axials and I would prefer to use radials so it will be a while till I can complete this so please keep this thread open I will defiantly come back and let all of you know how it turns out.
Les.

[Heatercathodeshort]

Just a thought. Surprisingly an O/C reservoir capacitor does not always introduce hum, just reduces the H.T. An O/C smoothing section will of course create deafening hum. This can be misleading. J.

[Herald1360]

That would be a consequence of the high anode impedance of a pentode making its anode current relatively unaffected by large variations in anode voltage. A property that's taken advantage of by feeding the anode directly from the rough dc at the reservoir without additional smoothing.

[leslie5555]

There is very little hum on this player the only symptom that made me check the ht in the first place was that it was very tinny the tone control works fine you can hear the difference from one end to the other but not what I have experienced before even with other Dansettes with such a cheapo amp.

[ms660]

Ripple versus output transformer explained in this video, watch from 1:09:31 in:

https://www.youtube.com/watch?v=niBGkqlh2XY

Lawrence.

['LIVEWIRE?']

I know I've mentioned this before, but Modern Dual Can Electrolytic Capacitors, usually made by F&T, are available from a number of sources including Cricklewood Electronics and www.askjanfirst.de Values such as 2 x 16 & 2 x32uf are included. These will usually be a 'drop-in' replacement for the original can. As to the third element in the original, which is the cathode bypass capacitor I replace this with a small radial 22 or 47uf 25v Electrolytic, wired across the resistor under the chassis.

[Herald1360]

Originally Posted by ms660

Ripple versus output transformer explained in this video, watch from 1:09:31 in:

https://www.youtube.com/watch?v=niBGkqlh2XY

Lawrence.

Interesting clip, but he's a bit woolly about the whole ripple rejection effect- yes the ripple voltage is common mode on the TX primary, but no explanation as to why? That's where the high Ra factor comes in. If the output valve is a triode with low Ra it's a different ball game.

[ms660]

Potential divider is how I see it.

I seem to remember that somewhere on the radiomuseum site is an explanation with calcs.

Lawrence.

[Chris55000]

Hi!

M.G. Scroggie dealt with this topic at length in his first book of discussion articles, "Second Thoughts On Radio Theory", and again in the follow-up book "Second Thoughts on Electronics Theory" - search "Negative Feedback and Hum" by "Cathode Ray" in the "Wireless World" section of the American Radio History site for these articles!

There is also a very useful Australian-originated idea for hum-cancellation using a Wheatstone-Bridge technique that avoids the need for a tapped-primary output transformer - I've posted this article a while back - a Forum search should locate it!

Chris Williams

[ms660]

I couldn't find the radiomuseum article I was thinking of, but basically a pot. divider formed with ra and RL so the ripple at the anode is the HT ripple times ra/ra+RL, the difference between being the voltage across the transformer that produces hum in the loudspeaker, eg: say HT ripple is 10 volts, transformer primary is 250 ohms, valve anode resistance is 23k that would give a voltage across the transformer primary of 110mV (approx.) so 110mV's worth of hum not 10 volts worth.

Lawrence.

[Herald1360]

I don't think the transformer primary resistance has any significant effect. The top of the potential divider is RL but that's the reflected impedance of the speaker- probably around 5k. So the ripple voltage on the primary will be RL/(RL+Ra). Reduced noticeably but not by quite as much.

10v would fall to about 1.5v. That's about 0.4mW output. Not brilliant but about -33dBW. Given the lack of much speaker response at 50Hz in the player, acceptable enough.

[ms660]

Yes, potential divider, apologies for the RL mixup.

Lawrence.

[leslie5555]

Have replaced the electrolytic caps and the HT is 138V DC not much difference than before.
There is only a very slight increase in the bass and there is more than enough volume.
I checked the old capacitors and the values are...
32Uf =39Uf
16Uf=24Uf
25Uf=43Uf

[Edward Huggins]

You have stuck with this long and hard and now appear to have a working player. Noting the modest HT, you seem to be getting an adequate volume, which is always a challenge on these. Noting your comment as to the Bass response, there is not that much that can be done in this department to any Dansette "Major". The high-output cartridges these use, trade off frequency response for high voltage poke. The amplifier could not be be more basic. And these do use very small output transformers which have a minimum of metal in them. The only other variable at your disposal is to fit a larger loudspeaker. With this type of Dansette, I have been able to change out the 7" x 4" speaker currently fitted, to a 8" x 5" 3 ohm unit with some modest improvement in the "high Bass" area. Maybe that's worth a try?

[leslie5555]

just trawling through some of my old threads and found this one.
It has been confirmed that the ht on this particular Amp should be around 150v dc
So this thread can be closed thank you.

['LIVEWIRE?']

I have recently been repairing a Dansette Major, and, for what it is, the performance is quite good, with plenty of volume and tone, although the control, as is usual, is only a treble cut device. I haven't measured either the output power or frequency response, so all I can say is that both are adequate. The original selenium rectifier is still producing the correct HT voltage. Cartridge is a BSR mono X5M type.