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Fender Bassman 5F6-A vs Marshall JTM45
Circuit differences, stage by stage, from Gridleak’s verified redraws of both schematics: 10 differences, 37 the same.
What’s different, and why it matters
On the schematic these two are close. Both feed two channels into a shared second gain stage and a cathode follower, then a treble/bass/middle stack with the same 56 kΩ slope resistor, a long-tail-pair phase inverter with a presence control, and a GZ34 rectifier. What changes is the output tubes (two 5881s in the Bassman, two KT66s in the JTM45), the first-stage tube (12AY7 against ECC83), the bright cap (100 pF against 500 pF) and where the feedback is taken from.
- The Bassman 5F6-A runs two 5881s and the JTM45 runs two KT66s. 5881s (a 6L6-type tube) are usually described as round, with a firm low end; KT66s are usually described as full and smooth, with a softer breakup.
What they share: a long-tail pair phase inverter, the same tone-stack layout with near-identical values, a GZ34 tube rectifier and a cathode follower.
At a glance
| Fender Bassman 5F6-A | Marshall JTM45 |
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| Years | 1958–1960 | 1962–1966 |
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| Origin | USA | UK |
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| Rated power | — | 45 W |
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| Channels | 2 | 2 |
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| Preamp tubes | 2 × 12AX7 + 1 × 12AY7 | 3 × ECC83 |
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| Power tubes | 2 × 5881 | 2 × KT66 |
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| Rectifier | Tube (GZ34) | Tube (GZ34) |
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| Phase inverter | Long-tail pair | Long-tail pair |
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| Master volume | none | none |
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| Effects loop | no | no |
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| Reverb | no | no |
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| Notable players | — | Eric Clapton |
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Stage by stage
Inputs and channels
| Fender Bassman 5F6-A | Marshall JTM45 |
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| Inputs | 4 | 4 |
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| Channel switching | front panel only / none | front panel only / none |
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| Voicing / power switches | none | none |
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Preamp
| Fender Bassman 5F6-A | Marshall JTM45 |
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| Preamp tubes | 2 × 12AX7 + 1 × 12AY7 | 3 × ECC83 |
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| Triode gain stages (all channels) | 3 | 3 |
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| Most gain stages in series | 2 | 2 |
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| Gain stage 1 | 12AY7 · plate 100 kΩ · cathode 820 Ω, 250 µF bypass (shared) (×2: Bright channel, Normal channel) | ECC83 · plate 100 kΩ · cathode 820 Ω, 250 µF bypass (shared) (×2: Ch. 2 first stage, Ch. 1 first stage) |
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| Gain stage 2 | 12AX7 · plate 100 kΩ · cathode 820 Ω, unbypassed | ECC83 · plate 100 kΩ · cathode 820 Ω, unbypassed |
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| Cold-biased stage (clipper) | none | none |
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| Cathode follower | yes | yes |
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| Bright cap(s) | 100 pF | 500 pF |
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| Coupling caps (preamp) | 0.02 µF | 0.02 µF |
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| Diode clipping | no | no |
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Tone stack
| Fender Bassman 5F6-A | Marshall JTM45 |
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| Type | Treble / bass / middle stack with a slope resistor | Treble / bass / middle stack with a slope resistor |
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| Position | after gain stage 2, driven by a cathode follower; no gain stages after it | after gain stage 2, driven by a cathode follower; no gain stages after it |
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| Slope resistor | 56 kΩ | 56 kΩ |
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| Treble cap | 250 pF | 270 pF |
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| Bass cap | 0.02 µF | 0.02 µF |
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| Middle cap | 0.02 µF | 0.02 µF |
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| Pots | treble 250 kΩ · bass 1 MΩ · middle 25 kΩ | treble 250 kΩ · bass 1 MΩ · middle 25 kΩ |
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| Master volume | none | none |
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Effects
| Fender Bassman 5F6-A | Marshall JTM45 |
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| Effects loop | no | no |
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| Reverb | no | no |
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| Tremolo / vibrato | no | no |
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Phase inverter
| Fender Bassman 5F6-A | Marshall JTM45 |
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| Type | Long-tail pair | Long-tail pair |
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| Tube | 12AX7 | ECC83 |
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| Plate loads | 82 kΩ / 100 kΩ | 82 kΩ / 100 kΩ |
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| Tail / cathode load | 10 kΩ | 10 kΩ |
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| Cathode bias resistor | 470 Ω | 470 Ω |
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| Input coupling cap | 0.02 µF | 0.02 µF |
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| Coupling to power tubes | 0.1 µF | 0.1 µF |
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Feedback
| Fender Bassman 5F6-A | Marshall JTM45 |
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| Negative feedback | yes | yes |
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| Feedback resistor | 27 kΩ | 27 kΩ |
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| Feedback taken from | speaker output | 16 Ω tap |
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| Presence | yes (5 kΩ pot, 0.1 µF cap) | yes (5 kΩ pot, 0.1 µF cap) |
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| Resonance / depth | no | no |
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Power amp
| Fender Bassman 5F6-A | Marshall JTM45 |
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| Power tubes | 2 × 5881 | 2 × KT66 |
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| Configuration | Push-pull | Push-pull |
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| Bias | Fixed bias | Fixed bias |
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| Screen resistors | 470 Ω | 470 Ω |
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| Grid stoppers | none | none |
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Power supply
| Fender Bassman 5F6-A | Marshall JTM45 |
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| Rectifier | Tube (GZ34) | Tube (GZ34) |
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| First filter (reservoir) | 40 µF (2 × 20 µF) | 32 µF |
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| Filter nodes, in order | 40 µF (2 × 20 µF) → 20 µF → 20 µF → 8 µF | 32 µF → 32 µF → 16 µF → 16 µF |
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| Filter choke | yes | yes (20 H) |
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| Highest DC voltage printed on the drawing | 432 V | not printed |
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| Printed bias voltage | -48 V | not printed |
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Presets and schematics
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