Never mind. I do a better job by looking at different info on the net! Lenny

I don't know enough to tell you what triode makes the best regen detector but the audio amp stage is a plain old audio amp stage and there's more to it than simply looking at 'load impedance'. The triode you're using has only 1/3 the mu of a 12AT7 and the 6V6 has less than half the gm of a 50C5. The two combined simply have no where near the gain of the original circuit.

What output transformer are you using? The original looks like an 'AA5' type that produces a 2k5 plate load, for the 50C5, with 8 ohms on the secondary. Plugging in a 1k (per Heathkit's 'typical') headphone zooms that plate impedance to over 300k. That is not the 'plate load' seen in tube spec sheets, though. That number is for 'maximum power' and, at a given B+, higher plate load impedance means less maximum power produced.

I guess Lenny didn't like my post.

Would be good for him to share what he finds on the internet as that may help others here.

That said there's plenty of knowledgable people here willing to help which would save the OP the trouble of looking on the internet.

I am not very knowledgable about the radios that I am building, but "DAMN, IT`S FUN". I got the beast to work last night, and I couldn`t believe the number of ham radio operators out there. There were dozens. I switched my 6V6 for a 6SJ7, which can be used with 100 volts on the plate. The thing wouldn`t work with the 6V6, because, as I found out, it requires at least 180 volts on the plate to function properly. I am using two 6C5GT MM triodes instead of the 12AT7. I have a homemade power supply, 102 volts, and a seperate 6.3 volt filiment transformer. It works fine. My only problem is that the volume isn`t up to par. I can hear everything clearly. My regen works great. The volume control works great, but the volume is limited. Would a couple of hi mu triodes do a better job here? I see that a 6SF5GT has an amplification factor of 100, but a different pinout than my 6C5GT. It uses 100 volts on the plate, like my 6SJ7. I think that may be the way to go, if I feel like rewireing the sockets! I used the same values for caps and resistors as the GR-81 schematic. I am using 10K piezio crystal headphones, via Dave Schmarder`s article on building such phones. I believe the 6SJ7 has a 6K plate load. Would the piezio headphones, with 5K total value of resistance across them, work ok? Any comments welcome....Lenny

You're RIGHT, Lenny. It IS fun. But I have to let you know I have had some success with the 6V6 at low voltage, low as 9 volts on the plate driving a small speaker set up in the bottom of an old speaker horn through a line voltage transformer. Basically, it's a copy of KB7NRN's 3Q5 hikers two, version five (I think). I had two weak six volt batteries in series powering both the filaments in series AND providing the plate voltage. Yeah, my windows didn't rattle and the good china was safe, but I could hear everything local and then some. It started to get a lot better around 36 volts on the plate. I suspect these beam pentodes are a bit overlooked.

I am learning, too. I am just beginning to get beyond just following directions and starting to get some ideas of my own. These guys here and others have unknowingly taught me a LOT! (Thanks, guys).

Lindsey

We'd need more info on the circuit. I'd stay with a low mu tube for the regenerative
detector. Less squirrelly and easier to control.

Is the other triode being used for a RF amp or the first audio stage?

If it is being used for an RF amp, then I would suggest adding another triode
as an audio amplifier before your 6sj7 audio output tube. Or you could try
adding a 1:3 or 1:4 audio matching transformer to increase the audio
going to the audio stage's grid.

Pete

The circuit is the Heathkit GR-81, with their power supply discarded. I have a power unit of my own. I used two 6CS5 low mu triods and a 6v6. I then changed to a 6SJ7 in place of the 6V6. The radio seemed to work ok , but the volume was really low. I tried a 10k to 8 ohm audio transformer, but nothing new! I am using the 10K piezio crystal earphones, with no audio transformer. I picked up many SW broadcasts. Radio Havana, PRC, and whatever Alex Jones broadcasts on. I changed my tube lineup to a 6SF5 high mu triode RF amplifier, kept the 6C5gt as my detector, and kept the the 6SJ7 as my audio amplifier. So far, so good! My volume was to low, so I thought I would try 250 volts on the plates. I did, and I am still scratching my head. I thought that tayloring the cathode bias on the tubes for the 250 volts would do the trick. I was wrong. I guess that the 250 volts changed the value of all caps and resistors needed. I dont know! I will keep on endevoring to persevere! I try this and that, and eventually, I hope to get it going again. Lenny

I don't mean to beat a dead horse here but you're misinterpreting tube specs and how they work. Tubes do not "have" a plate load, that is the load one is driving from the plate (if doing it that way), and the number you keep referring to for the 6SJ7 is the tube's characteristic plate impedance, and in triode mode at that. Its plate impedance in pentode mode, as it would be wired in that circuit, is 700,000 Ohm and has nothing to do with what the best plate load would be.

The 6V6 does work at 100V and, in fact, will do three times the plate current of a 6SJ7. I gather you're looking at the "typical" characteristics, since one of them is at 180V, and concluding that is 'what it takes to work' but that is not so. Those curves down below on the datasheet tell you how the tube works and on the plate curves for multiple screen values you see it runs (0 grid) 30mA at 100V while, at the same 100 screen volts, the 6SJ7 does only 10mA. Further, the 6SJ7 gm (gain) is even lower than the 6V6.

In short, going to a 6SJ7 is working in the opposite direction as you need more gain, not less. In that vein, going to the 6GF5 for the first audio amp was a good move.

The 50C5 (12C5, 25C5, 35C5) was designed for 'high gain' (gm) at line voltages but if you need a 6.3V heater tube there are others. The 6Y6 and 6CU5 have about the same gm, the 6CW5 is better at 10,000, and should work with the same output transformer. The venerable 6BQ5 (and 6GK6) is 11,000 and the winner is a 6HB6 at 20,000.

Lastly, you can't use '10k headphones' as a 'plate load' because you can't get any idle current through 10k and lowering the resistor to allow plate current destroys gain. That's why they used a transformer in the first place: there is almost no resistance at DC, so it has no effect on idle current, but high impedance to audio, so you get gain. In short, you need an output transformer like the original circuit showed.

Thank you for the info. I am trying to learn enough to build my own radio, and one can gain more useful info, on a site like this, than trying to find that info in a book. Which book to pick up and look through for that particular info is the question. I will keep plugging away, and asking questions, and maybe I can make a radio. The set I constructed worked great. but the volume was down. Flipperhome: do you think the tube lineup of 6SF5, the 6c5, and the 6v6 tube would be the way to go, or would you reccomend the 6Y6 tube in place of the 6v6? I intend to stay with the octal tubes, because my old fingers work better soldering on the bigger tube sockets of the Octal tubes! I will run the 100 volt power supply, since you said that 100 volts would be adaquate. I have a 10K to 8 Ohm transformer, which I can use. Any info appreciated. Thanks again....Lenny

It depends on where you start from, electronic experience wise, and how deep you want to get into it. The easiest way is to buy and build a kit. Next is to copy an existing design, exactly as it is. That might seem like 'the same thing' but there are often subtleties to part selection and layout that aren't explicit or clear from simply a schematic so duplicating a 'DIY' project someone else has done, with details of how they did it, can help a lot.

If you start 'changing things' then you need to know basic electronics and how tubes (and/or transistors or whatever you're using, etc.) work. I know it's 'a lot' but you are going to have to read up some because the fact it's 'a lot' makes it impossible to get everything from chatting. To begin with, download an RCA 'Receiving Tube' manual from here http://www.tubebooks.org/tube_data.htm. They all have a brief section on how tubes work at the beginning, plus you get a lot of tube data.

Next, download Radiotron Designer's Handbook, Fourth Edition, (full text) from here http://www.tubebooks.org/technical_books_online.htm Now, don't pass out. I'm not suggesting you read it cover to cover but it's considered sort of the 'bible' of tube design and includes most things, including even R, C, and L basics. It's probably more than you think you want but it costs nothing to download. You'll also notice there are a ton of manuals there so download whatever else strikes your fancy.

As for the 6Y6, it looks to be rather similar to the 50C5 but capable of more power. Unfortunately the plate curves are done at different screen volts so I can't read it straight off the graphs but it might even bias up right, or close enough to right, with the same circuit values. Put a volt meter on the cathode side of the 220 ohm Rk (the term for a "cathode resistor" with "k" standing, in this case, for C[k]athode) and monitor volts as it warms up. If it stays under 14 volts then you're ok. If it starts to go over then kill power, increase Rk to 270 ohm and try again but, this time, since Rk is larger (this is where knowing ohms law. E=I*R, is needed) you'd check for no more than 17 volts.

Those numbers are not 'optimum' but should get you in the ballpark. And, btw, the gr-81 'power supply' (B+) value is 115V, not 100V. That's not a big deal but 115 would be a tad better than 100.

As for the transformer, impedance is one of those things I mentioned above that is usually missing from just a schematic and so it is in this case as well. However, the 'typical' transformer for a 12C5 was 2k5 primary impedance and we can 'guess' they likely used the typical value, which means your 10k transformer is too high an impedance. 2k/2k5 was the most common value for AA5 table radios (since a ton of them used 50C5s) so if you have a scrap radio you could get one there. Alternately, Radiodaze sells a replacement 2k job for a reasonable 13 bucks. http://www.radiodaze.com/category/258.aspx (2k vs 2k5, or vice versa, is close enough)

In a pinch you could use a 115VAC to 6.3VAC 1 amp (or larger) filament transformer with the 6.3VAC secondary as the 8 ohm speaker output and the 115VAC primary for the tube plate load as that should be 'in the ballpark' of around 2k5 impedance. Or a 12.6VCT transformer and use half the secondary, which is 6.3V. They don't make for 'great' audio transformers but should work reasonably well since AM isn't 'hi-fi' anyway.

Thanks for the info. I will try to buy the books online, since the Library charges 10 cents per pagefor printing!!!! Unfortunatilly, I reversed the leads on my 220/240 volt to 115 volt transformer on my power supply, using the 115 volts as primary, and now I have a secondary of 260 volts! How much difference would that make on the tubes? I know the cathode resistors would have to be changed, and the the grid bias could be taylored to what is needed. If I can get the tube lineup to be workable, I can do it! Will my tube selection be ok? I will stay Octal no matter what! Any input appreciated Lenny

Sorry, I didn't realize you were on a library computer. Maybe getting a thumb drive to download and store documents to would be a good idea.

The power supply reminds me of Young Frankenstein when Gene Wilder and Terri Garr discover the hidden passage behind a rotating bookcase: PUT... the... CANDLE ... BACK

Actually, the 6V6 would be ok... but we don't want a 6V6 and the 6Y6 is limited to 200V. The 50C5 is limited to 135V/117V(screen).

When copying a circuit it's best if you, uh, copy it so let's not go needlessly changing things until you get something that works right.

Well what do you know? Accidently, my GR-81 clone, using a 6C5, a 6SF5, and a 6V6 seems to operate pretty well with the standard GR-81 capacitor and resistor values. My power supply is 150 volts rather than the factory 115 volts. Will some Elmer, who owns a tube manual, take a look at the GR-81 schematic, which is online at 2 different sites, and see if they would change any values to accomomidate the 3 octal tubes listed above? Any comments or helpful hints welcome.....Lenny