PY 81 as power rectifiers?

[Amie]

Terry/kalee20.

Trying to decouple the bottom of the grid resistor at the divider causes LF instability....

The Grundig method seems the most repeatable, but causes most noise....

Thanks,
Amie.

[Kalee20]

OK. I'm not completely surprised, but for this sort of circuit, the devil is in the detail.

You can't beat lots of (good quality) μF's from the cathode pin, to 0V.

Keep us posted on trials this weekend, looking forward to photos (even if it is birds-nest standard at the mo!)

[Amie]

Hi.
I'm sitting with Gran this weekend, so the project will be gathering dust....and I'm not letting anyone see that skywired lashup!

But....I have been in the process of making, and assembling an aluminium chassis for the set (with help from our next door neighbour he has folded the sections for me), I've piloted and punched it where I want the main components to go, in the picture you see the filament transformer, the audio output transformer (from Kalee20,) and the ECL80 in position, but not yet fixed down, underneath the chassis where the filament transformer is, is a C core smoothing choke. The filament transformer is in fact a 0-6 0-6 6W type, but with the secondary in parallel and feeding 3×300mA heaters in parallel, it loads down to just a smidge under 6.3volts AC.
   

It's being made as compact as I can and will be run from an isolated supply. When finished it will be capable of being used straight off the plug (I've dad's old pal Don on standby to have the final say on SAFETY).

I would like to admit I'm going to relegate the PY81 idea to the notebook, mainly because I found that the transformer I was going to use was an autotransformer and what I assumed was a "secondary" was in fact in series with what was marked "primary" so the top of the "secondary" could be at more than 500V above neutral ....

I would like to get the audio stage to fly with the cathode bias idea as it seems the most long term stable, in that when a badly worn valve is swapped in for the NOS one, the anode values are still similar, unlike the fixed bias pentode/contact bias triode type....

I think this could now be renamed the "completely off topic thread" or something more suitable to advertise the diverse discussions!

Sorry!
Amie.

[Kalee20]

Looks good.

One tip: wire up your mains transformer, to valve heaters. Wire up your output transformer to your loudspeaker. Switch on.

You may notice a hum in the speaker. It's because there is a small AC field from the mains transformer, which the output transformer picks up. If so, move the transformers around a bit, for minimum hum. You may find turning one you can find a zero position. Only then, mark and drill the positions for the fixing holes.

Some radios do seem to have transformers at weird angles, as though someone had just thrown them down at random, rather than in a nice neat layout. I rather suspect it was for minimum-hum reasons.

[Amie]

Looks good.

One tip: wire up your mains transformer, to valve heaters. Wire up your output transformer to your loudspeaker. Switch on.

You may notice a hum in the speaker. It's because there is a small AC field from the mains transformer, which the output transformer picks up. If so, move the transformers around a bit, for minimum hum. You may find turning one you can find a zero position. Only then, mark and drill the positions for the fixing holes.

Thanks lots!
I was concerned about hum from the transformer and choke field, so was prepared for the chassis to be a "practice" one. Uncle Don said similar to you, but to screw the inductors to a board and connect the lashup to them to experiment.

It didn't occur to me to run the heater transformer and just wire the speaker to the op transformer for the same test.....
'Spose the chassis I've made may work out OK....

I was also going to earth the chassis metal work, and provide an 0V buss, (for safety more than anything else), will this be likely to cause problems?
Thanks,
Amie.

[Kalee20]

Guess how I learn that myself, about the transformer positioning? 

I was making a compact chassis myself!! 

Earthing... Earthing the chassis and having an (isolated) 0V bus will work, but it's not so convenient as having the whole chassis 0V.

The little mains transformer you have is heaters only, how are you getting HT? As your PY81 is dumped. I get the idea you are rectifying direct from mains. It's quite possible (I have a completely open chassis, connected direct to mains neutral, in my workshop). However, safety is important, and an isolating transformer for HT is highly desirable. If you can find one of those, you are sorted; if not, I will do a bit of digging around for you.

[Amie]

While I was tinkering I was going to run it from the isolated supply transformer, when its done, and boxed then it was going to be what Don called a widow maker design.... I don't like the idea of it being live chassis unless its necessary, but it will be compact, and I'm not going to have earphone sockets or non isolated aerial connections.

I'm not dumping the idea of the py81 just storing it for use later, as I said in a previous post the transformer I thought was suitable turned out to be nothing of the sort, you cant be too careful....

[Kalee20]

I'm not dumping the idea of the py81 just storing it for use later, as I said in a previous post the transformer I thought was suitable turned out to be nothing of the sort, you cant be too careful....

Yes I picked up on that earlier, it was an auto-transformer. If you can sort the heater for your future project, it will do a good job.

I don't like the idea of it being live chassis unless its necessary, but it will be compact, and I'm not going to have earphone sockets or non isolated aerial connections.

I don't like the idea either, safety apart, a live chassis is a pain when it comes to connecting to test equipment, indeed, connecting to ANY other equipment. I may be able to help - I have a project in the 'day job' which is a one-off, but will need a low-power isolated supply and for quickness I'll be using an off-the-shelf small toroidal transformer, modified. If so I can get an extra one modified for you. Will send you a PM.

[Amie]

Thanks.

For the benefit of our readers

I still consider this to be a prototype, I will run it on my isolated 230 AC supply while I tinker, if the project gets a serious remake then I may apprieciate a hand with a grow up HT supply....when I know what's needed.

Amie.

[Kalee20]

For the benefit of our readers

I still consider this to be a prototype, I will run it on my isolated 230 AC supply while I tinker...

The plot is still unfolding!     Readers will be looking forward to next instalment   ... with deluxe, limited, collector's edition in the future.

All makes good sense so far, have fun Amie!