revisit of and old discussion. Class B amplification

[drgonzonm]

About a year ago, Mats started a topic using 6n7 tubes, a tube which was designed as a class B amplifier.  Part of the discussion in that thread, was most class b amps use interstage transformers for PI's 

Did a lot of searching, and found an all tube amp in an audiophile site that used 12au7 as a PI.  here's a link to that site.  http://www.tube-amps.net/EA_Hashimoto_EL34_PP_01.htm

It looks like the p/t is providing the current required for the class b1 service. 
There are some interesting things, I thought I saw.
the PI is wired dc coupled. (reduced hum?) change to AC coupled for guitar amp?
The output is 70w, on el34's (phillips state these tubes are good for 100w at 800v)

Interestingly, this amp or one of its brothers/sisters was modified for KT88's (bridge rectification to half wave), ultralinear output. 

[PRR]

> http://www.tube-amps.net/EA_Hashimoto_EL34_PP_01.htm 

AB1, not B2. 30mA bias current is far from ideal "zero"; and there's obviously no grid current.

Not direct-coupled.

> p/t is providing the current required for the class b1 service

Yeah; where else would we get current? (485V battery anyone?)

The larger Marshalls will deliver 70W from a pair of EL34. (Without the extreme conditions used for that 100W trick.)

[PRR]

The 12AU7 is direct coupled to the EF86 which drives it, yes. But signals here are not extreme, so it's not directly important. (What it does is minimize one bass-cut. When you have a chain of bass-cuts and try to apply large NFB around the chain, it motorboats.)

12AU7 is not direct-coupled to the EL34, which is where we might care.

> how do you wire an el34 as a class B amplifier?

Same as any other device. Idle current is zero. Each tube conducts half the waveform.

In class A, idle current is half the peak current. Each tube conducts the whole waveform.

Here we see "30mA/125mA", which suggests that idle current is 1/4 of peak current (or 1/8 if that "125mA" is a DC meter). That is, IMHO, well up into AB operation.

Few audio amps run class B. It is very hard to do (a tube won't go to ZERO current, and it is hard to get a very-low current, and to then get more current on soft signals). It is an Extreme Case. However when we do attempt such an extreme case, we usually *also* try another extreme trick: run the control grids positive on signal peaks. Class B2. We get more plate current. We also get control grid current, only over part of the cycle (often a small part).

That's where we want transformer-coupled or direct-coupled. If a coupling capacitor must flow current one way, but not the other, it will gain DC charge. In this case, in the direction of lower tube current, until we no longer get control grid current. "Blocking distortion." A class 2 output stage needs a transformer drive, or a class 1 direct-coupled drive.

> Phillips rates this tube at 100w for a pair at 800 volts on the anode.

A Toyota pickup can tow the Space Shuttle. They are both show-off stunts. Toyota was a bit more honest about it.

[PRR]

> My farm tractor will move a space shuttle

Only if it is fairly heavy for a 35HP (or 4WD). The limit is traction. As I figure it, there needs to be 5K-6K lb of weight on the driving tires, which need to be street-type (and on clean pavement).

Your farm tractor may well be over 6,000 pounds but much of that is on the front. When plowing, with hitch at the right height, some (to all (or more)) of that is transferred to the rear tires. That can happen on pavement, but tends to be fairly sudden, thus dangerous.

The horsepower is, of course, almost unimportant.

Gearing matters; but most 2WD cars are geared very close to wheel-spin in 1st gear. My mild Honda will chirp its tires on almost any stop-and-turn. I doubt road-mode 4WD is geared to wheel-spin; most folks are not ready for that much GO and they want 4WD to NOT wheel-spin. However a HD 4WD also has a low-range. Some are 2:1, my plow-truck's is 2.7:1. I figure 2,400lb pull in road-mode and well over 5,000lb pull in low-range.

Weight on drive wheels is key. ALSO rolling-resistance of the shuttle dollies, and road slope.

The rolling resistance is probably quite low (as fraction of shuttle weight). In their normal uses (houses and utility transformers), rolling resistance is a drag and a waste of costly fuel. And if speed gets up much higher, heat in heavily loaded tires. Still and all, it has to be down around 1% to get drawbar pull down to 3,850 lb. If we assume coefficient of traction as nice/round 1.0, then we need 3,850 pounds on the drive wheels. (My plow-truck's tires are old and hard, probably traction nearer 0.7, so at 6K pounds weight I can only get 4K2 pounds pull.)

The Toyota truck was about 5,700. Clearly a HD model, surely with 4WD, and possibly some ballast.

Actually rolling resistance is higher after standing still a while, and drops once rolling. Most RR data is "rolling", because that affects economy which is usually important. In this case they must also have checked starting RR after a stop.

That's assuming dead-level road. That area of LA is flat. My road is nearly flat, but really a 2% grade. It would take over 7,800 pounds pull to keep the SS rolling up my road, zero to keep it rolling down to the jiffy-mart.

But the actual course was a bridge over a sunken freeway. Bridges are usually high at mid-span so rain (it does rain in SoCal) will run to the ends where it can be handled. Can the Toyota take that grade?

But another thing. The run started well back from the bridge. All else equal, the road to the bridge sometimes falls, again so rain runs to the bridge abutment where there's already heavy drainage for the span (and so neighbors don't say the bridge sheds onto their land). So the total profile is W shape. And the Toyota just-might have started on a slight downhill slope. A 1% slope would negate a 1% rolling-resistance, and an under-geared lawn-tractor might have eventually got the SS rolling. The slope on the approach is probably well under 1%, but still probably in favor of getting things rolling.

The Toyota's retail tow-rating is closer to 10K than 385K pounds. Why the difference? That's 10K at 55MPH and with reasonable stability and STOPPING power (possibly with trailer brakes helping). I doubt the SS job peaked at 2MPH, and the hitch used gave the tiny truck considerable leverage on SS direction.

My main concern would still be stopping. Yes the Toyota could slow the rig from 1MPH to a standstill eventually. But what if a child runs into its path? What if the hitch breaks? My guess is that the *dollies* have brakes, to assist the tractor, for parking, and to slow a runaway situation. With 385K pounds on dolly wheels, it should be possible to stop right-quick (or somewhat less to avoid throwing the SS off its blocking).

BTW, this was not only a stunt. That bridge could carry the shuttle but not the oversize truck-tractor at the same time. They'd either have to brace the bridge (closing the freeway for days) or switch to a smaller truck just for this crossing. (I'm sure the hauling company had their medium truck hiding in an alley in-case the Toyota flubbed-out.)

[PRR]

> The space shuttle tires are high pressure tires, 340psi on the rear and 300 psi

This was the move through Los Angeles. The SS did not roll on its own wheels; see the publicity photos and the movie clip I linked. The landing gear was up, they pulled it on things like you see when they move a house, or a 250K pound utility transformer. "Truck tires", but small diameter, and probably super-extra thick fabric. Rated for very low speed and possible low mileage, they can do things they can't do on highway tires. Possibly rated over 10K pounds each, 5X what my truck's large doughnuts are rated. Probably a lot of air in them, though maybe limited to 100PSI because garage compressors don't go a lot higher.

These undercarrages probably have some braking system too. Certainly a parking brake: moving a house, sometimes you have to take the tractor off the front and come around to the rear, and you don't want the house to roll away. Also because the house weighs more than the tractor, which is OK for gentle pulls and slow-downs but won't give MAXimum braking in emergency unless you employ the weight of the house for traction.

Yeah, softer tires, brush the sand off the street, and limited-slip aint bad but shouldn't do much if both sides have 99% equal traction. Front-rear lock is important; that's normal in my old 4WD but I just remembered that many snazzy-new 4WDs have a center differential which is better on hard turns but worse for a straight pull unless weight distribution is exact 50:50.

[mat janssen]

At my webpage (www.amplifiers-with-valves.nl) you can seen how a class B amplifier is made with 4 x EL34.