R1155 Power supply question

[CesarLG]

Hello!
I am building it up from a diagram I found on this forum published by Radio_Dave (thanks!).
I see pin 5 is sharing 2 wires... one is the audio wire. Is it fine?

thank you!

[Radio_Dave]

Yikes, No that's not right... Sorry! I think the audio in should be pin 6.

Regards
David

[CesarLG]

That is what I though
Thank you!

I am still building it

Do you think this transformer and choke are suitable for the psu? I am wondering how to wire the transformer properly. I am a noob!

[Herald1360]

The TX and choke look fine, though the HT current draw may be marginal to keep the choke current from being discontinuous. Any less than about 50mA could be dodgy. On load at 50mA or more the dc voltage should be about right.

The main problem with using silicon rectifiers here will be a voltage overshoot to some 320V before the valves in the set warm up and draw current. You would do better to use a half thermionic bridge- use something like an EZ80 for the "top" half of the bridge- its h-k insulation rating will be fine to use on the same LT supply as the rest of the set, though a separate winding would be nicer.

[CesarLG]

Thank you for your valuable input.
I did a test connecting the HT section and the 220K resistor volatilized. It was a 0.25W

Meanwhile I will try to upgrade the circuit with your commentaries

Thank you again for your answers

[M0FYA Andy]

To reduce the current drawn, remove the three DF valves (unless you want to use the DF function!). This is totally safe, and was a standard 'mod' in service.

Andy

[G6Tanuki]

Why bother with the choke? They're big and bulky and - while appropriate in the days when capacitance was expensive and thermionic rectifiers needed protection from high peak currents - modern semiconductor rectifiers handle peak currents with ease and these days capacitance is cheap.

I heard it suggested (by the late Pat Hawker no-less) that choke-input filters can actually reduce the reliability of semiconductor rectifiers because the collapsing field generates nasty back-EMFs.

[trh01uk]

Why bother with the choke? Because choke input smoothing gives a nice stable HT line, without a need to add a regulator. If you just have a capacitor, then the voltage will go from the peak output of the rectifier down to whatever level the load reduces it to. Furthermore the choke provides a significant reduction in ripple.

The question is of course whether the R1155 can cope with an unstable HT supply? The stage that will definitely suffer from a variable HT supply is the local oscillator. Most oscillators will move significantly with changes in HT. Ripple on the HT is also likely to modulate the oscillator, and that may then put a measurable hum on all received signals. Whether it is audible will depend mostly on what audio system is used - a small speaker in a box may well deliver little of the actual hum output of course.

Receivers that have a stabiliser fitted on the local oscillator are usually free of the problem I am describing of course. The R1155 is not one of those.

Richard

[M0FYA Andy]

New chokes are available at sensible prices from ebay seller big.daddy!
Made by Triad, so probably from China, but I can't complain about their quality, nor the service offered by the seller.
He doesn't offer many types at the moment, so probably worth an email if he isn't listing what you want.

Andy

[CesarLG]

I have tested the HT power line and it delivers 308v... Way too much. I am working on it to reduce the voltage. A friend is helping me out with the circuit. This is the approach: adding two resistors (2K5 and 4K) and two caps

[Herald1360]

Was that 300 odd volts on load or off load? Off load the choke input filter will still let the capacitor charge up to near peak mains voltage. With "only" 7H, you need a load of around 50mA to allow the circuit to work properly.

My earlier comments related to this need for a minimum HT load for the circuit to deliver what would be given in a design note as its output voltage. My comments were based on a SPICE simulation of the circuit with a 50mA dc load (a stab at likely HT current for a comms set with no output stage).

Choke input filters for varying loads like a class (A)B p-p amplifier or a linear amp for SSB use a "swinging" choke with a high inductance at low current which falls away as dc current rises (careful design of the saturation characteristics of the laminated iron core) which is smaller and cheaper than a high current rated high inductance plain choke.

[CesarLG]

It was without load. So you think that once there is a load the voltage would be under 300? I get your point saying that the tubes receive too much before they are warmed up. Do you think that with the new components added to the circuit I can reduce the peak and make it less painful for the tubes?

I am trying to keep this as simple as I can, I can't afford buying more things for the already payed transformer and choque... So I would like to keep this..

Thank you!

[Herald1360]

The new circuit arrangement will not really help- it's now a capacitor input filter where the first capacitor will sit near the ac peak voltage all the time and you're relying on the voltage drop of the 2.5k resistor to lose the excess and get hot in the process. It will still deliver overvoltage until the load comes on with the valve heaters' warm up. The choke input arrangement is definitely more elegant and will deliver the right sort of voltage on load without the resistor losses.

If all the decoupling and any other capacitors in the set exposed to HT can stand say 350V, the transient overvoltage whilst waiting for the valves to warm up should not cause a problem.

The half valve bridge I suggested would bring up the HT slowly as the rectifier heaters warm up at the same sort of rate as the rest of the valves and should eliminate most of the transient overvoltage. Otherwise your easiest solution would be a new HT TX with about 150-160V secondary and go for the capacitor input filter. With that 300uF capacitor the voltage will rise rapidly to about the right level and won't sag much when the load comes up.

You're somewhere between a rock and a hard place- you need to do something- either spend money on extra parts or possibly a lot of time and expletives on uprating capacitors in the set- one which I know from my own experience is a pig to work on!

[CesarLG]

I have been trying different resistor values. Finally I found that with a 1000k (5W) series resitor and a 120k (1W) resistor (the one in the end of the Ht section) I get 240V with no load. Once connected to the R1155 I get 170 volts. Is this a good sign? I get no sound but the magic eye is on!

I still have to find out if the speaker amp works or maybe there is something wrong on the R1155

[ms660]

Aim for 220 volts DC with a 60mA current draw by a load resistor and it shouldn't be far out.

Lawrence.

[Ed_Dinning]

Hi, classic way to get speaker level audio is to put an output valve in the PSU box, 6V6 was the usual valve of choice, but many other types will do.
The audio feed is in the power connector to the set.

Ed

[Herald1360]

Playing around with resistors can get you the right voltage either on load or off load but not both and won't solve the surge overvoltage problem. The only easy way to get both is a lower voltage transformer secondary and a capacitor input filter with a biggish capacitor.

[CesarLG]

At this point I feel a little disappointed by the diagram.
I just ordered the transformer in order to fulfill the diagram requirements. Also the choke

It has been over 70€ in components

[Herald1360]

I see what you mean, now- the first diagram was someone else's take on a PSU/Speaker amplifier unit for the R1155. Presumably they used it without any problems in which case it ought to work for you, too. Its turn on behaviour is what it is though, which may not have been considered.

As it stands, my take on it as a result of some simulation runs is that it should work OK, as long as the short term HT overvoltage at switch-on doesn't cause any problems. It probably won't but if it does you might need to replace a few awkward to access capacitors and resistors. It's highly unlikely to do anything seriously nasty to the set. If the choke input filter had been implemented using indirectly heated (slow warm up) valve rectification it would have been a nice design- better than a capacitor input filter one for several reasons technically, though no more suitable for its ultimate end use unless it was deliberately done to use an existing 240V secondary transformer to give a 210V or so DC supply.

If the R1155 has been recapped with modern parts, I'd have no qualms about using it.

[G6Tanuki]

Originally Posted by Ed_Dinning

Hi, classic way to get speaker level audio is to put an output valve in the PSU box, 6V6 was the usual valve of choice, but many other types will do.
The audio feed is in the power connector to the set.

On the 1155s I played-around with half-a-century ago the usual thing was to excise the useless DF-circuit valves and stick a decent output-stage in the space freed-up. A 6V6 for audio, often a 150-Volt purple-glow-regulator for the local-oscillator/BFO, and a product-detector [twin-triode, or a 6K8] could easily be incorporated.

The 'odd' intermediate-frequency made installing a Q-multiplier to sharpen-up the IF a bit problematic.