807 generic

The story goes that the 807 was the type that inspired the nickname 'bottle' for valves because they were so large.

Certainly they were the first choice for any sort of power application, radio or audio, for many years.

Rows of 807's were the mainstay of transmitters and modulators during and after WW2 and were common in PA amplifiers. A number of homebrewer used these in guitar amps but by the mid 1970's they were being used as targets in a shooting gallery at the Geelong (Melbourne) Hamfest.

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Maximum ratings:

Plate diss. 30W
Plate voltage 750V
Screen diss. 3.5W
Screen voltage 300V
Full power freq 60MHz
Heater voltage 6.3VAC
Heater current 0.9A
Cin 12pF
Cgp 0.2pF
Cout 7pF

Class AB₂ push-pull:

Plate voltage 750V
Screen voltage 300V
G₁ voltage -32V
Plate current 60/240mA
Screen current 5/10mA
G₁-to-G₁ drive 92Vpk
Drive power 0.2W
R_L p-p 6.95k
Power out 120W

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The three circuits shown below appear to be from Ferguson Transformers as it was not uncommon for transformer manufacturers to supply designs that used their components.

These designs differ from later practice by using a driver transformer rather than a phase-splitter stage. Naturally two transformers in the signal path produced limited bandwidth, extra weight, and expense.

One important point not shown in these circuits was the need for 'stopper' resistors hard on each anode (47 ohm) and control grid (1k) terminal to prevent radio frequency oscillation which these were particularly prone to.

807 Modulator (and CRO)

Here is a photograph of a circuit for a 50 watt modulator for Ham radio service. (sorry it's a bit fuzzy).
Source: Joe (amp_mangler)

Some notable features set this apart from a guitar amp:

• The preamp is a pentode for a lower level microphone input (far left)
• No tone controls
• Line level input jack (after pentode stage)
• Two triodes as a power driver
• Driver transformer-coupled to output pair
• The power supply isn't shown, bias is applied to the driver tranny center tap
• Anode current is metered (right middle)
• In-built CRO for modulation monitoring

The power supply shown serves the CRT (lower left). This will be quite a small tube like a 913 or 1CP1 with a 1-inch screen. Note that the CRT heater/cathode is at full HT voltage. The minimal input circuit to the Y plates on the CRT are high-level RF coming from the transmitter, and requires no amplification.

The output transformer, or modulation transformer as is is called in this case, differs mainly in that the secondary winding is much higher impedance than the eight ohms typical of a guitar amp, several k-ohms, and therefore useless for much else.

DogZilla

I don't know how he lifts this, but here's an example of where you can go with 807's. Not Australian and totally over the top, but worth studying as a good set of examples in valve design.

DogZilla.

Lounge Toy

For a nice local 807 build, take a look at Marty Stone's “lounge toy”.

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