Photos and hand drawn diagram.

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These photos should work!

Well it turns out you shouldnt try and draw things with the kids around and in poor light!

This is a better drawn diagram with correct values. I have decided that the 1st valve should be a 6SN7.

Does this look like it might work now?? All comments welcome. This was something built by someone else and may not have been built correctly.

The radio chassis that connects to this is a complete write off and not all there so I am hoping to use this as a power amp and a normal preamp (read modern kit or if I feel like it a valve setup) attached.

There is space in the chassis for another valve and I suspect that it was once used or meant to have the rect valve.

I might put in an extra preamp stage if people think it will need it.

Image Link

AutoCad drawing with valve elements sent. May be clearer.

6SN7 / 6SL7:- one element pre amp, the other (Triodes) Phase splitter.

Would like to have seen voltage divider across series caps. HV television (Blue) types to get voltage rating.

6SN7 suggests 40's early 50's. Filter capacitor values consitent with Valve (5AR4, 5R4) not Silicon.

As there are no grid resistors look for a 5V winding and another winding for the "C" bias power for the grids.

Marc

I am having difficulty understanding what you mean by Grid resistors. Where should they be? I have triple checked my current diagram witch is here Image Link


There is no other winding coming from the transformer. There is a good chance when whoever put this thing together they cut them back into the transformer when they decided on the power supply so I am stuck with it in that state.

Without the basic understanding of the fundamentals of a thermionic valve, things can be difficult.

The heaters & filaments being hot will (in vacuum) throw off electrons. The first observed, but not understood example of this was lightbulbs going black as the ejecta (filament material) deposited on the slightly positive glass, of the bulb.

Edison tried to work out what was going on but it was not until around 1898 that Boyle described the electron. However, Edison's work did eventually cause the discovery of the Valve.

To remove the abstract concept consider the Valve as a gate valve on a water pipe. The water is flowing from the Cathode to the Plate. If there is no gate the water flow is unimpeeded. As the gate closes the flow of water is gradually cut off to the point of stopped or severely restricted..

The control grid works in a similar fashion. By forcing it more and more negative, a very small voltage causes sufficient repulsion to control, or even stop the electron flow. This is the Bias or "C" voltage and is used to set the operating point in a similar fashion as moving the gate, in or out.

The Valve amplifies because small fluctuations in the grid, cause far greater fluctuations in the current flowing from Cathode to Plate.

The controlling voltage appearing on the grid must be developed or there is no control. Originally this was done with "C" or bias Batteries, it is with some push pull circuits, done with a seperate power supply (must be seperate as it is reverse) or it is developed by the cathode resistor.

Where I am coming from, is that in order to develop a controlling voltage on the grids when there is a cathode resistor (in this case) the circuit must be complete between grid and ground, or the cathode.

As there were no resistors and no source of "C" voltage shown on the grids, in the original drawings; A resistor (or "C" supply)has to be there or the voltage cannot be developed, the valve will not amplify, as it will present as a short circuit & probably bend something.

Therefore, do not put the KT66's in and turn it on.

I will look at the new info as a seperate item.

Marc

Now that looks more like it has the potential to work. However, I still have issues with some of it.

As I said the diodes will give a higher B+. Allowing for mains fluctuations that probably is a 385 - 400V secondary.

I would consider the following:

With the caps in, the surge voltage will be considerably higher.

Modern caps may be ok but, one or both of the 24μF 350V in series will let go also, without a divider. Seen it happen.

I would prefer to see 470K resistors across them to make a 50:50 divider. That way you can use 350 or 450V Caps.
Use the HV TV resistors (blue?) or they will fail. This is the penalty with seriously high voltages.

Capacitors in series reduces their value.

The surge at start up still has the potental to blow the two underated 0.1 mfd coupling caps in series,Plate to the grid. Work on twice PIV.(which is probably why series) I would consider replacing them with 0.047 mfd (473) at not less than 1KV and that includes beefing up the 0.01 on the 6SN7's other plate.

Someone may let us know what they think the surge voltage will be? However adding two good HV electrolytics in series with divider will soon establish that.

Marc

Thanks Marc

I will have fun experimenting with my setup. I have big plans with this chassis. I just hope my KT66's work!

Is there a way to test them without a valve tester? Anyone in Canberra area got one who can test them for me???

I plan to strip the chassis, repaint it and put some nice stained wood sides and front on it, Bend up a mesh cover to cover the transformers and make it look like a brought one!

Same with the Radio/Pre amp that I am yet to tackle!

Well that what I hope to do anyway!

Just as a quick question.... With my power supply, Should I make a stand by setting to drop out the HT and allow the heaters to warm up since it is Solid State Rect?

My tester is an Emmission type. I have another Tech brand that was salvaged, however it has only one Rat chewed card.

I would possibly need something to compare them too. If you can spot an amp with 6L6, apparently KT66 will run in there.

In desperation I can probably find a way of testing them, but I do not like shipping these things around due to breakage risk.

Many of the sets with 5Y3's never had much problem with bleeding off the surge. I have a Philips Line Amp with 320V on the 6CM5's. It was built with Solid state rectifiers (OA210, 400PIV 5A) and no special circuits were used however it has negative feedback from the audio secondary.

This is also a trap for the unwary, as the caps on the valve tops are the anodes.

Marc

Just going bak to the question of building an AM receiver, R&H published what they called a Universal Radio in the late '40's - early 50's. This showed a circuit diagram with several different valves suggested for each position, along with component values to suit. I built such a radio with an X61M, 6U7G, 6Q7G, 6F6G & a 5Y3. It works very well, but as with all sets without an RF stage, needs a good aerial to keep hash to a minimum. Very good luck with this, it's a rewarding experience.
John.

Just found it, it's from January 1949. I can send you scans if you would like

Re hash.

I have found that if radios are devoid of a decoupling cap on the HT often tend to have more pronounced hash in the background. ( 0.047 -- 0.22 mfd is not unusual )

I have a lot of transmitters ranging from 400 - 1MW+ around me and a considerable amount of RF is riding on the 22KV power aerials (Rural --- miles of them) this invariably gets into the 240V lines. I have actually had to resort to placing Mains caps on the Workshop lines to try & get rid of it.

Worth a try if its a hassle.

Some of the compact flouro's are beyond silencing and I wonder who dropped the ball, and let them be sold?

Marc

Thanks guys.

I have a RT&H dvd. I have been slowly looking through them. I Must admit I missed looking at that one.

I am still working on the power amp and now I am wondering on how much Pre amp I need to add. I plan useing a preamp with Tone control (Base & Treb). I have found a few that look interesting.

Now... How much pre amp do I need to drive my above cct???

I have searched the net for a few designs and seem to like these


AMP cct Image Link
this one Image Link looks good for the tone stuff.
this one Image Link looks good to get the i/p to line level.

Do I need the 2nd one at all or could I just change the way that the first one is setup??

Depending on which way you go there would be likely sufficient drive from a 6AV6 [AF Triode double diode] (horrid little animal) or 6B8 (RFpentode double diode), when fed into the 6SN7.

Check "Franks Electron Tube pages" there may be info on the Valve data sheet?

Marc

I want to have a "AUX" type input so I can hook up a CD player and MP3 as well as the Radio when I build it. Thants why I was looking at having both stages. The "Preamp" Image Link on the web where I found this cct said it was for hooking up a CD player or other Line Level imput to the williamson amps.

I cant work out if I still need the preamp stage AND the tone control valve stage again for my setup.

I have the valves sitting in my large valve collection and am not just going to keep valves just ot have them sit in a box if I could get the use out of them.

MP3 Player would have sufficient drive, if CD is designed to drive headphones, that possibly will also. These may also have tone control.

The greater issue will be to impedence match. If you output to the valve from a CD player or MP3 player that has been designed for modern head phones (and not the drive itself).
Then it may have to feed in via a cathode follower, as the Valve is high impedance and the MP3 player, very likely, low impedance.

Its getting messy.

Marc