BEYOND Push Pull Parallel (Tubes)

Yes. The Ampeg SVT and the Fender 400 both use six. They're just push-pull ( Class AB ) with more tubes in parallel.

-- Les Cargill

Four (2x2) or six (2x3) EL34 pentodes was pretty standard way in push pull configuration for generating the "magical" 100 W in guitar amplifiers with 400-450 V anode voltage.

I have also seen schematic diagrams, but never worked with amplifiers using six power triodes in a totem pole configuration driving a 16 ohm loudspeaker without an output transformer. The layout was quite similar to current NPN/PNP complementary (emitter follower) stage, with three triodes in parallel in the same position as the upper NPN emitter follower and three triodes in parallel in the same position which is currently occupied by the PNP emitter follower. Triodes have lower internal resistance than pentodes and putting several triodes in parallel further reduced the resistance and thus, the amplifier could drive "standard" 16 ohm loudspeakers directly.

While two EL86 pentodes could be used in totem pole configuration without output transformers, special loudspeakers were required with an impedance about 1000 ohms.

The 1946 BBC 'Disk Recording Equipment Type D' used six AL/60 valves in parallel push-pull in the recording amplifier (Type DRA/4). With an HT of 365V, this gave the 75 to 100VA needed to drive the inductive cutterhead with the sufficient current for full low frequency modulation and the enough voltage to take the response up to 10 Kc/s with recording characteristic boost and radius compensation.

In the late 1950s, Wireless World published a design which used about eight paralleled pentodes each side. The layout was a strange breadboard affair with bakelite surface-mounting valveholders.

Just curious. Many old tube amplifiers, especially guitar and PA amps had four audio output tubes, such as four 6L6 tubes. Was there ever an amplifier designed with MORE tubes for the output stage? For example, using EIGHT 6L6 tubes? I'm not sure what that would be called? (push pull parallel xxxxxx)?

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Of course. Many older high power(> 200W) bass guitar amps used more than 4. I've seen an old Mesa that had 12 power tubes.

Wasn't there a direct drive valve affair that used LOTS of valves in // ?

But still with a vast mismatch betwen amp and speaker, hence it was seldom used.

NT

RCA used 16 parallel 6146 as the video amplifier to modulate a 4CX250 in their TTU-25 series TV transmitters.

It's sometimes more economical to use multiple small devices, rather than have to find some single big device to do the job.

With semiconductors, it can often be advantageous to use several cheap transistors and run them gently, rather than push a big expensive one to its limits. If there is a lot of dissipation, the thermal paths from multiple chips to the heatsink are parallelled, which allows a smaller heatsink that can safely be run hotter.

In the case of valves, there was often a big jump in price between a 'consumer' output pentode and the next size up, which fell into the 'industrial' class. For guitar amplifiers, which were, in essence, 'consumer' equipment, it made sense to stick to consumer valves, even if it meant using more of them.

It also made the power supplies cheaper because bigger valves usually needed higher voltages, which meant special rectifiers and paper smoothing capacitors. A smoothing system with paper capacitors worked out more economical if the capacitors could be relatively small and the extra smoothing was provided by chokes. The chokes made the equipment bigger and heavier- and the higher voltages made it very dangerous for semi-skilled 'roadies' to service (and more spectacular when it went wrong).

Yes, but this is an odd one. A 6146 should be good for 100W, or so, IIRC. A

I located a schematic on the web for a Fender 400 with 6 output tubes. But 12, that's insane..... Damn, I got to find a schematic for this beast. Any idea of the model number?

Thanks

It was a distributed amplifer, and flat from DC to 5 MHz.

Not a lot of choices for a DC to 5 MHz amp in 1950.

I've always been in awe of distributed amplifier design. The theory is straightforward enough, but what happens if you build one and it goes unstable? Where do you begin to sort it out?

Les Cargill Inscribed thus:

I belive some of the old "Trix" cinema amplifiers used push/pull triples.

They are, with a network between each tube. What I thought was interesting was the output tubes. Water cooled power tetrodes at 12.5 or 25 KW across the entire UHF TV band.

One tube at a time. it would be easier to troubleshoot, than trying to match the 16 6146 that are needed for proper operation.

How does one water cool a tube without the glass shattering?

snipped-for-privacy@myplace.com wrote in news: snipped-for-privacy@4ax.com:

Use metal and ceramics? And make the anode (almost)ground, using the coils as water feed and make the cathode negative supply voltage.

That's just one solution.

Grasshopper, not all tubes are small and made of glass. Some are stainless steel and need 30 GPM (or more) of cool water flowing through them to keep them from burning up.

Down at the pub?