Photo Discussion

[billfort]

Originally posted by Sch3mat1c


Hey Bill, why'd you use stainless and mild? Mild rusts like a nut if you don't give it a little maintanance. (Hept'AU7 is breaking out in red spots, darned humidity..)

Tim

I used stainless & mild because I wanted a simple, decent top plate finish and a solid, damped assembly. The top plate is 16ga. polished steel that I punched on an NC turret press with a plastic film in place for protection - just peeled off the plastic and done. The mild steel is actually 14ga. wipe coat galvanized material (no rust worry) that forms a sandwich with a 1/8" thick cork core. The main reason I picked these materials (aside from ascetics) is that I have easy (cheap) access to them at work.

Billfort

[Bas Horneman]

Sjef..do you have a schematic of your pp pentode amp?

Cheers,
Bas

[planet10]

Sjef...

< http://www.diyaudio.com/forums/showt...733#post234733>

You say you amp uses EL34s... the tubes in the picture seem to me more like EL84s?

dave

[Sjef]

No it doesn't use EL34's. I was saying that I have compared it against a couple of EL34 amps and this small baby was clearly the winner. Before this one I've build a EL34 amp wich was not better than my transistor amp so I was a bit dissapointed in tubes at that time. This one was an eye opener however. Single ended triode sounds better in my opinion but this little thing made me get hooked on tubes.

Like I told in the other topic, I realy don't know what makes this thing that good. No audiophile components are used. I even tried building a very same one with "better" components but that one didn't sound half as good as this one. The only advantage was that it didn't turn that hot. I think the army makes pretty good stuff, such a shame that they don't make more music with it.

The amp uses the ECL82 / 6BM8 tubes. In fact this is the amp of wich I mailed you the schematics some time ago.

[planet10]

Originally posted by Sjef
The amp uses the ECL82 / 6BM8 tubes. In fact this is the amp of wich I mailed you the schematics some time ago.

Cool... i've just come into some suitable iron (an ELL80 PP -- can't afford to keep the ELL80s). Is the attached the one?

dave

[Sjef]

Yep, that's the one.

I see I've forgot to mention the value of the upper resistor of the phase inverter. It's 100K too, but you would probably allready figured that out by yourself. It's a standard cathodyne phase splitter.

I still don't understand the input circuit. In theory this thing doesn't work !!!!! The cathode of the input tube is almost grounded ??!! But in practice it works, very strange. It's a schematic wich was publiced somewhere early sixties in a dutch magazine called "Radio Bulletin". They called it the "Eccelent". The power and output transformers are from an old "Trio" stereo amp (now known as Kenwood) wich I found in the street by the garbage.

[Sjef]

Yep, that's the one.

I see I've forgot to mention the value of the upper resistor of the phase inverter. It's 100K too, but you would probably allready figured that out by yourself. It's a standard cathodyne phase splitter

[fdegrove]

Hi Sjef,

Seems you're posting in stereo....errrr, well, almost.

I still don't understand the input circuit. In theory this thing doesn't work !!!!!

Some tubes are linear even with Vg=0 and can be drive slightly into positive grid.

I'm not familiar enough with the ECL82 to tell you what's going on but if you have the curves of the triode section you maybe able to figure it out.

Cheers,

[Sch3mat1c]

I would presume it has little to do with linearity about Vg=0V, and everything to do with having a cap-coupled driving stage. Often used in RF for class C stages: grid current has to go somewhere in this circuit, and it flows through the grid leak, back into the cathode, developing a negative voltage across the grid leak, biasing the grid negative. As a result, bias is always equal to the peak voltage of the input signal.

With a large grid leak, grid current will be very low, thus loading the driving stage very little. If it were direct or transformer coupled (such that would provide a stronger DC path to ground) it would pull significantly more grid current, causing distortion if the driver has a high Zo.

Tim

[fdegrove]

Hi,

Often used in RF for class C stages: grid current has to go somewhere in this circuit, and it flows through the grid leak, back into the cathode, developing a negative voltage across the grid leak, biasing the grid negative. As a result, bias is always equal to the peak voltage of the input signal.

Yep, that's how I figured it too.

With a large grid leak, grid current will be very low, thus loading the driving stage very little. If it were direct or transformer coupled (such that would provide a stronger DC path to ground) it would pull significantly more grid current, causing distortion if the driver has a high Zo.

In this case, within the NFB loop, that arrangement wouldn't fly, it's the NFB that allows for correct gridbias in this case.

Not really my cup of tea technically but pretty clever nonetheless.

Seems we agree, Zener Tim,