5F10 Tweed Harvard-style · 1958–1961 · 10 W

The tweed line's fixed-bias oddball: a roughly 10-watt student amp built around a 6AT6, a 12AX7, a fixed-biased 6V6GT pair and a 5Y3GT rectifier. Where almost every other tweed Fender cathode-biases its output tubes, the Harvard runs a proper negative-bias supply — the same idea as the bigger Bassman and the later brownface amps, shrunk into a one-knob-tone practice combo. It also opens with a 6AT6, a twin-diode/high-mu triode borrowed from the radio-and-television parts bin (only the triode section is used), rather than the usual 12AY7/12AX7 front end. Produced through the late tweed years.

Circuit walkthrough (short form)

Three inputs (each a 68 kΩ stopper, grid grounded through the switched jacks) → 6AT6 first stage (100 kΩ plate load, 1.5 kΩ cathode with a 25 µF bypass) → 0.02 µF coupler → 1 MΩ volume (with a 500 pF bright cap) and a 1 MΩ tone control (0.005 µF) → 12AX7 driver (100 kΩ plate, 1.5 kΩ unbypassed cathode) → 0.02 µF → 12AX7 cathodyne phase inverter (56 kΩ plate load, 1.5 kΩ + 56 kΩ cathode stack, 1 MΩ grid leak returned to the junction) → 0.1 µF couplers → 6V6GT pair, fixed-biased at −21 V through 220 kΩ grid leaks with 1.5 kΩ stoppers → output transformer → speaker. A 56 kΩ negative-feedback resistor runs from the speaker back to the driver's cathode.

Power: HT winding → 5Y3GT → +305 V plates (16 µF) → 470 Ω → +302 V screens → 22 kΩ → +250 V preamp (16 µF). The bias supply is a selenium rectifier off an HT tap, filtered by 6.8 kΩ / 56 kΩ and two 25 µF cans, giving the −21 V grid line.

Fixed bias in a tweed

Two details set the Harvard apart from its cathode-biased tweed siblings. First, the 6V6 grids are held at a fixed −21 V from the selenium bias supply rather than lifted by a shared cathode resistor, so the output tubes idle warm and give up a little of the tweed "sag and bloom" for a firmer, louder response. Second, the phase inverter is a cathodyne: the 12AX7's cathode runs through 1.5 kΩ to a junction and then 56 kΩ to ground, with the 1 MΩ grid leak returned to that junction so the stage self-biases — simulation puts the cathode pin at 47.4 V and the junction at 46.1 V, a clean −1.3 V grid-to-cathode. The plate (56 kΩ from the +250 V rail) sits at 197 V, balancing the two drive signals to the output pair.

Verification — against the printed factory chart

The F-EF drawing prints a full voltage chart, and simulation matches it across the board: the +302 V and +250 V rails land within 2 %, and every compared tube pin is within 8.4 % (the chart's own convention is ±20 %). The driver cathode is the one node left informational — it is unbypassed and loaded at DC by the 56 kΩ feedback resistor returning from the speaker, and the anchor-fit 12AX7 model runs a little light there, so simulation reads about 1.2 V against the printed 1.5 V. The 6AT6 uses a purpose-built, public-domain model fitted to its RCA datasheet (triode section only); the two diode units play no part in the amplifier and are left unmodeled.