First try at designing an amp

[Dunc]

So I thought I'd follow the valve wizard tutorials on small signal pentode and single ended designs.
Years back I ended up with a 5k transformer, some random tubes and a power transformer that puts out 230v and 6.3V; so I thought I'd start from there.
The tubes are Chinese 6j1 (x2) and a Chinese 6P1 (x1). After a bit of research (took bloomin ages) I found the equivalent tubes (ef95 and 6AQ5) and got there data sheets.
I'm hoping I'm right on the equivalents as the Chinese datasheets were terrible!
Anyway I was hoping you experienced designers might cast an eye on my attempt and tell me if I'm making a complete mess of it or not?
I've not got as far as the power supply  element yet so my schematic sketch only has voltages written on.
Cheers
Duncan

[shaun]

I can offer general feedback on a few points. First, I have found the VW's info to be presented in a somewhat abstract manner, in that it often doesn't seem to match real-world examples. So while he is the esteemed wizard and knows a hellavalot more than me, I recommend you find some schematics online that use 6AQ5s as power tubes, and the same for EF95 preamp tubes.  The 6AQ5 makes a good little power tube, but I don't know much about the EF95. In any case, look at tried and tested schematics to see the values those designers were going for, and you'll get a ball park figure - it takes a lot of research, but it's a good way to learn.

More specifically, your B+ voltage on the 6AQ5 is rather low; I'd want at least 200 volts on the plate itself. The cathode resistor says 100. Presumable that means 100 ohms, which may be too low - you won't know until you need to bias it, but again, google "singled ended 6AQ5 schematics" and take note of the values they've used.

As a last thought, if this is your first design, you will find a 12ax7 much more user friendly in V1 than a pentode preamp tube.

I'm attaching a couple of schematics to look at. Hope they help.

[thetragichero]

I'm excited to see your results as i have a few ef95/5654 pulled from an old signal generator that i might experiment with next. so far have had good results with 12au6 (I'm all about pentode goodness without paying the cost for ef86!)

[sluckey]

I have a little 5654 and 6AQ5 amp that's very similar to what you are doing. May be of some interest. Schematic at bottom of page...

     http://sluckeyamps.com/smoky/smoky.htm

[thetragichero]

*yoink!*
thanks steve! i use similar values in a 12au6 preamp push/pull build i finally got together and love the simplicity of just a single preamp tube

[tubeswell]

If you've got a 500k pot for volume, you don't need another 1m grid leak in parallel on the 6AQ5 - because the pot can act as the grid leak path on its own. (You can, of course, keep the 1M if you want, but keeping it decreases the input impedance on the 6AQ5, loading the signal down a bit more. But you might like that, or not. Just saying)

[Dunc]

Thanks for the responses everyone. Much appreciated.
I certainly  agree that the VW site offers information in a way that you have to read around it. It took many, many readthroughs to get it.

Some great little circuits being shared here and again thanks. Maybe I'll consider a more standard 12ax7 preamp. Although I'd have to buy a couple.
Sluckeys toaster amp is quite the looker! I love the story of your amp / toaster.
Circuit wise it looks similar to what I'm after, a little too loud maybe at 4 Watts and I'd struggle to get the b+ required from my transformer.
I'm particularly interested in the "gain" control on the cathode of the preamp.
Could you help me understand what's going on there and how I could implement something similar?
Also I notice the b+ for the preamp tube is higher than I thought I could use on my tube.
I thought I was pushing it with my preamp gain, obviously I'm being a little conservative. If I up my gain I could afford to put some tone control in?
I know my b+ is quite low but I'm aiming for 1 to 2 Watts. Does this low b+ cause other problems / difficulties?
I'll also remove the 1 Meg as suggested by tubes well. Thanks, I've saved a few pence!

[pdf64]

A 230VAC power transformer (PT) secondary winding will provide over 300VDC, ie due to the reservoir cap charging up to near the Vpeak of the (full wave bridge FWB) rectified AC waveform.
Loaded by a real circuit, both AC and DC voltages will reduce to some degree.

Hence the 125V HT supply to your amp looks a bit weird.
Maybe the winding is 115-0-115? In which case it would provide maybe 160VDC with a biphase rectifier and a reservoir cap, unloaded.

[sluckey]

Could you help me understand what's going on there and how I could implement something similar?

Adding a bypass cap to the cathode will increase the gain of that stage. The bypass cap increases gain because it bypasses/decreases the amount of degenerative feedback that's inherent in a cathode biased stage. Adding the cap via a pot allows you to vary the gain by varying the amount of degenerative feedback. This is a true gain control, not just a voltage divider like a typical volume or gain pot.

Easy to put this in your circuit. First connect your screen cap directly to ground. Then replace your cathode resistor with my cap/pot/resistor as shown in my schematic.

[Dunc]

Regarding my transformer. I've obviously got way more voltage than I'd need for my circuit from this tranny.
I was hoping to design something around 1 - 2 Watts. It seems this transformer won't do that, but would be OK for Sluckys toaster amp?

I did think that the gain control on that circuit was a proper gain control. Now I understand how. Thanks.

[sluckey]

Regarding my transformer. I've obviously got way more voltage than I'd need for my circuit from this tranny.

All you told us was that your PT puts out 230V. If that secondary has a center tap (230 w/CT, same as 115-0-115) then it can produce 162vdc unloaded using a conventional full wave rectifier with cap input.

[tubeswell]

Regarding my transformer. I've obviously got way more voltage than I'd need for my circuit from this tranny.

All you told us was that your PT puts out 230V. If that secondary has a center tap (230 w/CT, same as 115-0-115) then it can produce 162vdc unloaded using a conventional full wave rectifier with cap input.

Or 324VDC if a 2-phase voltage doubler is used (@Dunc The humdinger isn't necessary for this - just a bit of finessing)

[Dunc]

Sorry I should have said, but I have no centre tap.
That doubler looks interesting though.

[tubeswell]

Sorry I should have said, but I have no centre tap.
That doubler looks interesting though.

You could run a 1/2-wave voltage doubler (but regulation will not be as good, i.e. more sag)


Or add a boost/buck PT (configured for boost)



Or get a higher voltage PT

[kagliostro]

May be something you can give a try





it depends on the current that is available at the Power Transformer and the current you need for your circuit

Franco

[pdf64]

...Or 324VDC if a 2-phase voltage doubler is used...

How’s that a voltage doubler 
It seems to look and function exactly as a full wave bridge, so why not refer to it as such?
If any load current flowed via the CT, there would be reason to analyse and refer to it differently, but I can’t see that any would.

[PRR]

And his PT is *higher* voltage than his amp plan, so why we posting doublers??

[tubeswell]

...Or 324VDC if a 2-phase voltage doubler is used...

How’s that a voltage doubler 
It seems to look and function exactly as a full wave bridge, so why not refer to it as such?
If any load current flowed via the CT, there would be reason to analyse and refer to it differently, but I can’t see that any would.

...Or 324VDC if a 2-phase voltage doubler is used...

How’s that a voltage doubler    It seems to look and function exactly as a full wave bridge, so why not refer to it as such?If any load current flowed via the CT, there would be reason to analyse and refer to it differently, but I can’t see that any would.

 The CT is tied to the junction between the reservoir caps - Power supplies in some Marshall and later Fender amps used this.

[pdf64]

...The CT is tied to the junction between the reservoir caps...

My thinking is that connection is trivial / immaterial, as no load current flows via the CT, and the connection can be made or broken with no change in circuit operation or performance.

Hence it seems a rather different scenario to the apparently same split / bipolar supply used to supply an actual split / bipolar circuit. With that, load current flows via the CT, and the CT connection is necessary for circuit function.

As I see it, the benefit of using a CT to Marshall / Fender was that it saved on the necessity for balancing resistors across the reservoir caps.
Though some models used the CT output for a half HT - 1/4 power option, in which case the 'split supply' description would be appropriate.

But then it would seem more appropriate to describe it as a voltage halver, rather than doubler 
I don't see how the 'two phase / half wave voltage doubler' description is arrived at?
Especially as it could lead to confusion with rather different 'half wave voltage doubler circuits', eg

[tubeswell]

I don't see how the 'two phase / half wave voltage doubler' description is arrived at?

The 2-phase 'doubler' has a +ve 2-phase rectifier and a -ve 2-phase rectifier, and placement of the CT at the junction of the filter caps with the ground on the 'bottom' of the most negative cap, results in the products of these two sets of rectification summing up positively. I attached a schematic explaining what I meant in case the nomenclature was too much of a misnomer. It's no biggie and I'm not going to get hung up about it.


My suggestion for a 1/2 wave voltage doubler was an alternate suggestion from the 2-phase 'voltage doubler'. As you have already noted, these are 2 different things. I suggested that after the OP said there was no CT. (Prior to that, various replies were talking about 115-0-115, so I assumed that was what was at hand)