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Plate to Grid resistor ? why

B

brud

Active Member
Hello

I am new to the valve scene, have been doing alot of research over the past 18 months - now trying my hand at understanding a few things.

Can someone please explain to me why we would place a 100 ohm resistor between the Plate ( Pin 3) and Grid 2 (Pin 4) on a KT 66 tube .

I have purchased a couple of old Rogers Developments KT 66 amps without a schematic - does anyone have a methodical way ( or point me to an article) to backward engineer to a schematic.

Cheers
Bruce
Sydney Oz
 
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brud said:
Hello

I am new to the valve scene, have been doing alot of research over the past 18 months - now trying my hand at understanding a few things.

Can someone please explain to me why we would place a 100 ohm resistor between the Plate ( Pin 3) and Grid 2 (Pin 4) on a KT 66 tube .

I have purchased a couple of old Rogers Developments KT 66 amps without a schematic - does anyone have a methodical way ( or point me to an article) to backward engineer to a schematic.

Cheers
Bruce
Sydney Oz
Click to expand...
This is a triod connected tube, 100 ohm is between the plate and screen.
 
brud said:
without a schematic - does anyone have a methodical way ( or point me to an article) to backward engineer to a schematic.
Click to expand...

It might be helpful to remember that there is a good chance that whatever circuit you are trying to figure out is a common archetype used over and over. Examples are the Williamson, Mullard 5-20, or Dynaco ST70. There are variations of these, but what is common to each of these archetypes is that they are composed of predictable stages or blocks of components that relate to each other to provide a function.

These stages can be very generic and easy to figure out and easy to draw because you can follow a schematic of the archetype. Here's an example. The Williamson amplifier is certainly one of the most popular audio circuits ever developed so it is extremely common in commercial and hobby builds.

You see below the Williamson amp is composed of five discrete blocks of components that each serve a specific purpose. Each of these five blocks of components is common to many different amplifiers, so it makes sense to learn to draw these typical blocks and expect to see them in the amp you are attempting to draw.

So, starting with the yellow we have the power supply block. Every amplifier is going to have a variation of this one here: draw it in. Pink is the output stage. Every push pull amp is going to have a variation of this. Dog puke green is generally called a "long tailed pair" or differential amplifier; it's a common gain stage in amps. Blue is the phase inverter; this one's a split load inverter. The first stage is the most common triode gain stage there is.

Draw out these stages as practice and then look at the amp for which you need the schematic and look for the power supply components: fill them in to your drawing. Check out the output stage: is it similar to the Williamson? Discover each of the discrete blocks of your amplifier and put them together. Now that you know what you are looking for and how to draw them, the job should be easier.
williamson_blocks.jpg
 
Palustris, this should be saved up in the stickys. Very good explanation of the various stages. Only one thing, next time use a schematic that goes left to right. Who draws them that way? LOL

On Edit: Ok, I see that the stages are left to right, just never saw the PS all the way to the right. Should be bottom left as in most schematics. At a quick glance it confused me, but that's easy to do.

BillWojo
 
No power supply in that, its just a classic triode-wired Williamson amp schematic. Williamson amps have what looks to be an extra stage in them compared to other designs.
 
British schematics always seem to put the power supply at the far right. Layout-wise it works, but I prefer to keep the power supply separated.
 
Power supply at the far right is very common, standard in fact, in correctly drafted tube amp schematics. Not just a British thing.

Think about it- the first thing the power supply feeds is the output transformer. Then through a successive chain of dropping resistors and decoupling caps back to earlier stages in the amp. I still recall from my high-school electronics classes: when drawing an amplifier schematic, signal flow is left to right; power supply flow is from right to left. Rule of thumb.

Here's a ca.1950 example of a Simplex theater amp schematic from International Projector Corp. (IPC) in New Jersey. I own several of these. Note that solid-state diodes were a new thing, and they got the diode symbols backwards in the fixed-bias supply (!).
http://www.film-tech.com/warehouse/manuals/SIMPLEXAM1027.pdf

Note floating-paraphase input stage/phase splitter, and cathode-follower output tube drivers. The schematic flow is just like the Williamson posted above.

Peter
 
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US schematics tend to put the power supply at the bottom, with feeds going up to the rest of the circuit. Mains feed on the left, B+ out on the right, with feeds up and to the rest of the circuit. Not everything is drawn that way, but it seems to have become the convention for one reason or another. Wild guess, it may have been Riders manuals or one of the major radio makers from the 20s or 30s that started doing it that way and others just copied the trend.

It is sort of interesting looking at schematics from other places and the "same but different" way things are drawn.
 
gadget73, I think Heathkit and many US manufacturers followed the convention you describe. I've drawn many schematics that way, too. Lots of old "rules of thumb' have fallen by the wayside over the years.

Sorry this has wandered OT... my apologies to the OP. I'll stop now. :)
 
Rational35 said:
gadget73, I think Heathkit and many US manufacturers followed the convention you describe. I've drawn many schematics that way, too. Lots of old "rules of thumb' have fallen by the wayside over the years.

Sorry this has wandered OT... my apologies to the OP. I'll stop now. :)
Click to expand...

You say "off topic", I read "wow, this is really cool stuff to learn from the experience of others". It's all a matter of perspective, I guess.
 
The British are right about some things, wrong about others (IMHO).

If you cut a potato so as to produce chips, the resulting food should be called potato chips, if you slice it, and then fry it until crisp, the result is clearly potato crisps, not chips.

If you play a game that mostly involves kicking a ball with the foot, that game is logically called football. A game that uses something that is not shaped like a ball, and is mostly thrown with the hands, and seldom kicked, would logically and obviously not be called football.

However, Beer should be cold, and.... (to get to the point)...

Schematics should always show the signal flow from left to right. For a power supply, that means the mains input (note terminology choice) should be on the left, and the needed voltages should be on the right.

If the power supply is incidental to the operation of an audio amplifier, then the power supply should be below the main signal path of the amplifier, which should also, of course, be drawn left to right.

If I cover all but the extreme left of the drawing, I should see only phono jacks and a line plug. If I cover all but the extreme right, I should see speaker terminals (in the case of a power amp.)

The way the Williamson schematic shown above is drawn bothered me enough to keep this thread going OT. I apologize, but I needed the therapy.
 
linuxslate said:
If you cut a potato so as to produce chips, the resulting food should be called potato chips, if you slice it, and then fry it until crisp, the result is clearly potato crisps, not chips.
Click to expand...

and they should only involve the word "French" if the concept originated in France.

but then again you get hang-ons like the expression "B+" that came from the battery radio days, but nobody refers to the heater supply as "A" or the bias supply as "C".
 
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