A tube amplifier is a high-voltage device. Its plate supply runs at 300 to 450 volts — higher in some designs, and higher still cold and unloaded — and its filter capacitors hold that charge for a long time after the amp is switched off and unplugged. This is the standard safety practice every amp manual and repair text sets out — read it before a chassis is ever opened.
The danger is not only the wall socket. It is the energy the amplifier deliberately stores, and keeps storing after the power is gone.
B+ is a lethal voltage. A guitar amp rectifies the power transformer's high-voltage winding up to its plate supply — its B+ — and runs the output tubes from it. In the circuits documented here that rail sits roughly between 300 V and 450 V in normal operation — and some run higher: a 360-0-360 transformer (the JTM45's) can push its reservoir near 500 V idling unloaded, and any supply reads highest when the amp is cold and drawing no current. Mains-frequency current at those potentials, across the body, is far more than enough to stop a heart; it is the current that kills, and a few tens of milliamps is enough.
Filter capacitors store the charge — and hold it. To smooth the rectified supply into steady DC, the amp charges a bank of electrolytic filter capacitors to the full B+. Switch off and the tubes stop drawing, but the capacitors stay charged: a 16 µF capacitor at 450 V holds more than a joule, and with no load to drain it, it can sit at a lethal voltage for many minutes — sometimes hours. A capacitor is, by design, a device for holding a charge, and it does its job long after the amp is dark.
A pilot lamp proves nothing. The lamp goes out and the tubes stop glowing while the reservoir cap is still at hundreds of volts. Some amps fit a bleeder resistor across the first filter cap to drain it slowly, but many do not, and a bleeder can be the wrong value, disconnected, or failed open. Nothing you can see from outside tells you a capacitor is safe. Only a meter, on that capacitor, does.
This is the routine amp technicians run every time a chassis is opened. The idea is simple: bleed each capacitor's charge away through a resistor, then prove with a meter that it is gone — before any part of you enters the chassis. Hover, tap, or focus a step to trace it on the diagram.
Hover, tap, or focus a step to trace it on the diagram.
The procedure is the one given in every reputable amp-repair text and service manual. Clear public write-ups include Rob Robinette's Tube Amp Safety and Merlin Blencowe's The Valve Wizard power-supply notes; the technical library at Aiken Amplification covers the same ground.
Discharging the caps clears the stored charge. These four rules cover the rest — the habits that keep a live-chassis mistake from becoming a fatal one.
Keep one hand in a pocket or behind your back when a circuit may be live, so a shock cannot cross your chest and stop your heart. One hand in, one hand out.
Have someone within earshot who knows where the mains disconnect is and how to break you free of a live circuit without becoming a second casualty.
A bleeder resistor may be absent, wrong, or failed. Treat every filter capacitor as holding full B+ until your own meter, on that cap, proves otherwise.
A meter you trust — rated CAT II 600 V or better, with sound probes — is the only thing that tells you a node is dead. Prove the reading before you reach in.
On the meter: a digital multimeter used near B+ should be rated for the job — CAT II at 600 V or better, with intact leads and probes — and the range set above the expected voltage before the probes touch anything. A meter you have not proven on a known voltage is not evidence a node is dead. The oxblood pins on the tube pinout diagrams mark exactly the points that sit at B+ in service.
These are the high-voltage filter capacitors listed in the corpus, generated straight from each amp's parts list — 38 caps across 10 circuits. Every one of them charges to its node's B+ in service and holds it after power-off. This is the bank the procedure above clears, node by node.
| Ref | Capacitor | Node | In circuit |
|---|---|---|---|
| C5 | 8 µF · 450 V | Filter, B+1 (reservoir) | 5E1 Tweed Champ-style |
| C6 | 8 µF · 450 V | Filter, B+2 | 5E1 Tweed Champ-style |
| C7 | 8 µF · 450 V | Filter, B+3 | 5E1 Tweed Champ-style |
| C11 | 16 µF · 450 V | Filter, B+1 | 5E3 Tweed Deluxe-style |
| C12 | 16 µF · 450 V | Filter, B+2 | 5E3 Tweed Deluxe-style |
| C13 | 16 µF · 450 V | Filter, B+3 | 5E3 Tweed Deluxe-style |
| C5 | 16 µF · 450 V | Filter, B+1 | 5F1 Tweed Champ-style |
| C7 | 8 µF · 450 V | Filter, B+2 | 5F1 Tweed Champ-style |
| C8 | 8 µF · 450 V | Filter, B+3 | 5F1 Tweed Champ-style |
| C10 | 16 µF · 450 V | Filter, B+3 | 5F10 Tweed Harvard-style |
| C8 | 16 µF · 450 V | Filter, B+1 | 5F10 Tweed Harvard-style |
| C9 | 16 µF · 450 V | Filter, B+2 | 5F10 Tweed Harvard-style |
| C10 | 8 µF · 450 V | Filter, B+3 | 5F2-A Tweed Princeton-style |
| C7 | 16 µF · 450 V | Filter, B+1 | 5F2-A Tweed Princeton-style |
| C8 | 16 µF · 450 V | Filter, B+1 (paralleled) | 5F2-A Tweed Princeton-style |
| C9 | 8 µF · 450 V | Filter, B+2 | 5F2-A Tweed Princeton-style |
| C11 | 16 µF · 450 V | Filter, B+1 (reservoir) | 5F4 Tweed Super-style |
| C12 | 16 µF · 450 V | Filter, B+2 | 5F4 Tweed Super-style |
| C13 | 16 µF · 450 V | Filter, B+3 | 5F4 Tweed Super-style |
| C14 | 8 µF · 450 V | Filter, B+4 | 5F4 Tweed Super-style |
| C11 | 20 µF · 600 V | Filter, B+1 | 5F6-A Tweed Bassman-style |
| C12 | 20 µF · 600 V | Filter, B+2 | 5F6-A Tweed Bassman-style |
| C13 | 20 µF · 600 V | Filter, B+3 | 5F6-A Tweed Bassman-style |
| C14 | 8 µF · 450 V | Filter, B+4 | 5F6-A Tweed Bassman-style |
| C10 | 16 µF · 450 V (×2) | Reservoir filter (post-standby) | AB763 Blackface Deluxe Reverb-style |
| C11 | 16 µF · 450 V | Filter, node B (screens/+415) | AB763 Blackface Deluxe Reverb-style |
| C12 | 16 µF · 450 V | Filter, node C (+325 PI/preamp) | AB763 Blackface Deluxe Reverb-style |
| C13 | 16 µF · 450 V | Filter, node D (input-stage rail) | AB763 Blackface Deluxe Reverb-style |
| C11 | 32 µF · 500 V | Reservoir filter, B+1 | JTM45 British lead-style |
| C12 | 16 µF · 500 V | Filter, B+2 | JTM45 British lead-style |
| C13 | 16 µF · 500 V | Filter, B+3 | JTM45 British lead-style |
| C14 | 16 µF · 450 V | Filter, B+4 | JTM45 British lead-style |
| C17 | 50 µF · 500 V | Reservoir filter (B+1) | M1987 Plexi lead 50-style |
| C18 | 80 µF · 500 V | Reservoir filter (B+1) | M1987 Plexi lead 50-style |
| C19 | 50 µF · 500 V | Screen-node filter | M1987 Plexi lead 50-style |
| C20 | 50 µF · 500 V | Phase-inverter-supply filter | M1987 Plexi lead 50-style |
| C21 | 50 µF · 350 V | Second-stage-supply filter | M1987 Plexi lead 50-style |
| C22 | 50 µF · 350 V | Input-stage-supply filter | M1987 Plexi lead 50-style |
Values are read from the published parts lists; cathode-bypass and bias-supply electrolytics, which sit at far lower voltages, are left out so the table shows only the caps that hold a dangerous charge.
One era-specific part deserves its own warning: the small capacitor wired from the AC line to the chassis on amps built for two-wire, ungrounded mains. Technicians call it the death cap, and the standard advice is unambiguous — remove it.
A shorted or leaky cap, or a ground switch left on the hot leg, puts the full line voltage on the chassis. Anyone touching the chassis and ground completes the circuit.
The chassis is tied straight to the mains safety earth. Hum is handled by the ground, not a cap. No death cap, no ground switch.
What it is. On amps built for two-wire mains — the era before a third, earth conductor was standard — there was no safety ground. To quiet hum, the designer wired a small capacitor, typically 0.05 µF at 600 V, from one leg of the AC line to the chassis, often through a two-position ground or polarity switch that chose which line conductor the cap connected to. The switch let the player flip the plug's effective orientation to find the quieter setting.
Why it was there. With no earth ground, the cap gave the chassis a high-impedance AC path to the grounded (neutral) side of the line, shunting hum and RF away. On the two-prong wiring of the day it was a reasonable, common trick, and it appears across tweed-era amplifiers.
Why it is unsafe on modern mains. A two-wire plug can go into the socket either way, so there is no guarantee which prong is neutral. If the plug or the switch connects the cap — and through it the chassis — to the hot leg, the chassis is coupled to the live line. Worse, these capacitors age and fail shorted: a failed death cap ties the chassis directly to a line conductor at full mains potential. The chassis, the guitar's strings through the amp ground, and a connected microphone all become live, and a person touching any of them and a grounded surface completes the circuit. This arrangement has caused electrocutions, which is how it earned its name.
The fix everyone agrees on. Remove the death cap and the ground switch entirely, and replace the two-wire cord with a modern three-wire grounded cord: line and neutral to the fuse, switch, and power transformer as before, and the third (green) earth conductor bonded directly to the chassis at a solid ground point. The chassis is then held at earth potential by the mains ground, not floated through a capacitor, and any fault trips the breaker instead of energising the chassis. Adding a proper earth is not enough on its own — the cap and switch come out. This is the standard, universally recommended conversion for any vintage amp returned to service.
The era cap appears on the AC-line network of these circuits in the archive — drawn as period annotation, never as a part to keep:
| Capacitor | As drawn | Circuit |
|---|---|---|
| 0.05 µF · 600 V | Across the AC line at the fuse | 5E1 Tweed Champ-style |
| 0.05 µF · 600 V | AC-line filter caps (×2, annotation only) | 5F10 Tweed Harvard-style |
| 0.05 µF · 600 V | Across the AC line | 5F2-A Tweed Princeton-style |
| 0.05 µF · 600 V (×2) | AC-line filter caps (annotation only) | 5F4 Tweed Super-style |
On the death cap and the three-wire conversion, see Rob Robinette's Tube Amp Safety. The period arrangement is visible on the factory drawings this archive cites, among them the Champ 5E1 and other tweed schematics archived at EL34World.
This page collects the safety practice common to the amp-repair literature and the original service manuals. The references below are linked, never rehosted; the manufacturer manuals of the era sit on the reference shelf.
This primer describes standard practice; it is not a substitute for hands-on training. High-voltage work carries real risk of injury and death, and the responsibility for working safely rests with the person holding the probes. If any part of the routine above is unfamiliar, do not open a powered or recently-powered amplifier — learn the practice first, ideally alongside someone who already works on these circuits.