Odd-ball amplifier ideas that actually work. All suggestions comments welcome!

[imaradiostar]

Actually, I remember reading about tube circuits without a B+ rail that worked off the tiny charge imparted by the electrons being boiled off the cathode. Not exactly sure where I read it though... I'll try to find it, might make for an interesting read!

Pretty sure that's grid leak bias. Common on some older stuff.

Generally that requires a blocking cap and is subject to severe blocking distortion when overdriven. Probably not the way to go with your circuit.

There's nothing wrong with using multiple supplies stacked as needed to achieve direct coupled operation. This was pretty common in lots of scientific circuits (scopes, wide band amps and preamps, etc). You can still use resistors at each grid for RF stopping and to give pleasing overdrive characteristics in a guitar circuit.

You can do similar things with standard 300 to 400 volt supplies if you desire. There is a chapter in Merlin's book that talks about it. He shows a design for a simple 12au7 two gain stage circuit that's direct coupled. I've been meaning to build it and listen to it but hadn't had a chance. I figure being direct coupled it'll make a decent bass overdrive in a bass preamp.

JT

[Austin]

There are simpler ways to reduce blocking. Direct coupling is not that hard, but look at your circuit. This approach requires a whole extra supply to lift the otherwise conventional power tube stage up 400 volts from the floor.

Okay to reduce blocking distortion using a high value resistor in series with the signal would do it. Is that really a good way to do it though?

Simplest method is the interstage transformer but I don't have one of those and good ones are expensive. But really, how much simpler can it possibly get than this? I started with magnavox amp with a power supply for 2 el84 and one 12ax7 with 5y3 rectifier tube and wanted to see what I could do with it. The solid state diodes made the power supply b+ much higher, I used the center tap to split it into two rails. Very simple! I got 100 1n4007 diodes for $1.89 with free shipping on ebay so you can't beat that.

I want to use the unused octal slot for this 6sn7 I have somehow. I have a 6BL7 that I can use but they draw twice the heater current..

[exclamationmark]

I should have clarified; my comment was directed towards SGM's design with no apparent power source.

[Enzo]

Is a resistor a good way to do it? A bazillion Fenders have them, a gazillion Marshalls, and a host of others.

I surely don't want to make you think experimenting is not useful. Just pointing out if the whole purpose was simply the blocking, it was a roundabout way to get there.

This isn't all that complicated, other than the dual power supplies. COmpared to a resistor, maybe a bit more complex. But we wind up with 400v on the plate of the 12AX7, and though there is only 400v ACROSS the EL84, it still has 800v to ground on parts. Is the transformer safe at 800v to frame? The EL84 cathode is at 400v. I don't see the heater circuit, but unless you have the heaters elevated, you are far exceding the 100v maximum heater to cathode voltage rating of the tube.

[Austin]

Is a resistor a good way to do it? A bazillion Fenders have them, a gazillion Marshalls, and a host of others.

I surely don't want to make you think experimenting is not useful. Just pointing out if the whole purpose was simply the blocking, it was a roundabout way to get there.

This isn't all that complicated, other than the dual power supplies. COmpared to a resistor, maybe a bit more complex. But we wind up with 400v on the plate of the 12AX7, and though there is only 400v ACROSS the EL84, it still has 800v to ground on parts. Is the transformer safe at 800v to frame? The EL84 cathode is at 400v. I don't see the heater circuit, but unless you have the heaters elevated, you are far exceding the 100v maximum heater to cathode voltage rating of the tube.

Thanks, I am experimenting and have alot to learn. I don't understand what you mean by "though there is only 400v ACROSS the EL84, it still has 800v to ground on parts." since nothing is referenced to ground except the input jack, the 12ax7 cathode and one wire of both output transformers secondary.

And the heaters are simply floating now. They were originally referenced to ground with two 100 ohm resistors. What should I do with them?
I could run the 12ax7 off of the 5 volt tap and reference it with a 100 ohm resistor to ground and refernce the regular 6.3 heater to the the 400 volt center tap I spose. 5v is kinda low for heater that should be on 6.3 but it does work just fine, I already tried it.

If I use a high value resistor in series with the signal it will prevent blocking distortion by limiting current which is fine for a capacitor coupled amp because you can't get grid current with caps anyway.
However I want to build an a2 type amp, I think the music peaks should be able to push the grids positive if I crank it up really loud and that would require that current be possible. Like class A that can get class a2 if I turn the volume up. So since I don't have interstage transformers, I will need a cathode follower driver then right? A nice beefy one too...

[Enzo]

Your power supply is grounded, your output tranformer secondary is grounded. Your input jack may be wher the circuit connects to chassis, but is there continuity between that jack and the ground symbols on this drawing? If so, ther is your 800v. And even if your circuit isn;t connected to chasis at all (and how weird would that be?), your circuit common would still be 800v away from those elements.

[Austin]

Yeah I should do a little better job detailing on my next drawing.

[teemuk]

Not super oddball at all, and I'm not overly convinced of its overall usefullness, but the way to power the fist gain stage of the power amp is pretty clever in this:
http://www.prowessamplifiers.com/sch...ages/sg100.jpg

[J M Fahey]

And the heaters are simply floating now. They were originally referenced to ground with two 100 ohm resistors. What should I do with them?

Let me understand this because your drawing is not clear: are the pentode and the 12AX7 filaments fed from the same filament winding?
Yes, I know you lifted the 100 ohm balancing resistor and it's now floating, that part is clear.
Thanks.

[J M Fahey]

@ teemuk.
Very interesting, thanks for posting.
Agree with you that with the exact same 2 triodes they could have had a conventional long tail PI or at least a cathodyne.
In this case (as I see it, may be mistaken), both triodes form a mixed circuit.
On one side they are some form of a cascode, both being in series and the upper one receiving signal from the lower one and at the same time becoming an active load for it.
The top one also behaves somewhat like a cathodyne, because its cathode drives the lower output tube, and its plate the upper one.
I really don't understand how both signals are the same or even close by.
Quirky amp.
I am old enough to remember these when they were new.
They came in cool cabinets, somewhat similar to Anvil cases, but disappeared soon, apparently nobody liked them very much.
Do you have any pictures of them? Or old ads.
Thanks again.

[Austin]

Let me understand this because your drawing is not clear: are the pentode and the 12AX7 filaments fed from the same filament winding?
Yes, I know you lifted the 100 ohm balancing resistor and it's now floating, that part is clear.
Thanks.

Yes at the moment both the 12ax7 and the two el84s are fed from the same winding.

[loudthud]

One odd ball circuit you see in HiFi sometimes is screen grid drive of power tubes. KOC did this in one of his books. I seem to remember he used a push pull output transformer in reverse to drive the screens. Since the screens were near zero volts at idle, the power tubes didn't conduct much until the signal drove the screens to several hundred volts. Cathode and grid were grounded. See if you can find some examples and do some reading before you try it.

Here is one link: Screen Driven, DC Coupled, Single-ended EL509 Amp

[Austin]

One odd ball circuit you see in HiFi sometimes is screen grid drive of power tubes. KOC did this in one of his books. I seem to remember he used a push pull output transformer in reverse to drive the screens. Since the screens were near zero volts at idle, the power tubes didn't conduct much until the signal drove the screens to several hundred volts. Cathode and grid were grounded. See if you can find some examples and do some reading before you try it.

Here is one link: Screen Driven, DC Coupled, Single-ended EL509 Amp

I forgot about this one, but I already tried it. In my obsession to direct couple everything I connnected the 12ax7 output directly to the screen of the el84. It worked but sounded bad, thin and distorted, supposedly you are to ground the grid or actually use positive bias and it takes a preamp tube that can source current not a wimpy 12ax7 and I did neither. But I just wanted to try it... Sweep tubes are supposed to be the best for it? I dunno but I think it would work with any tube. I would like to try it again sometime though using a better driver now that I feel like I know a little more about it. Sposta convert a pentode into a low mu triode...

I like that link!

[exclamationmark]

There's also cathode drive (or grounded grid operation). It was popular with RF tubes for several reasons, none of which are advantageous to musical amplifiers. That being said, I drove a pre-amp stage with a cathode follower (it needs to be driven with a low impedance source) and it had a peculiar overdrive character - it needed a huge coupling cap though - near 1 uf. I'd assume the difference would be because of the grid not drawing current... or something. I actually biased the cathode follower to a low voltage and used a low voltage poly cap. Might be good for an input stage.

[Austin]

There's also cathode drive (or grounded grid operation). It was popular with RF tubes for several reasons, none of which are advantageous to musical amplifiers. That being said, I drove a pre-amp stage with a cathode follower (it needs to be driven with a low impedance source) and it had a peculiar overdrive character - it needed a huge coupling cap though - near 1 uf. I'd assume the difference would be because of the grid not drawing current... or something. I actually biased the cathode follower to a low voltage and used a low voltage poly cap. Might be good for an input stage.

I am interested in seeing how grounded grid works for audio or guitar too. The top tube of cascode is grounded grid, it is the only way I could figure out how to do a grounded grid output stage without a coupling transformer but it seemed to work fine. If the overdrive sounded interesting enough it could be built into a pedal maybe...