FAQ
TL;DR: EL84 output tubes can run at 220–240 °C, and “working time at this temperature is unlimited” [Elektroda, fzyga43, #14606349; Elektrola, aksakal, #14606691]. Focus on plate current ≤ 12 W, swap only bad electrolytics, and an ECC85 often revives FM.
Why it matters: Correct triage prevents needless parts swaps and protects rare vintage components.
Quick Facts
• EL84 max anode dissipation: 12 W at 250 V [Philips Datasheet].
• Loewe Opta Apollo VHF range: 87–100 MHz [Elektroda, aksakal, post #14608318]
• Filter electrolytics: Typ. 2×50 µF/350 V, reform time 2–4 h daily for 3–5 days [Elektroda, aksakal, post #14606855]
• New ECC85 tube price: ≈ 8–15 PLN [Elektroda, dostojny.kocur, post #14609032]
• Safe chassis surface temp: < 50 °C to avoid user burns [IEC 60065].
1. Why do the EL84 tubes feel dangerously hot?
Power pentodes dissipate up to 12 W each, heating the glass to 220–240 °C, which is within spec [Elektroda, fzyga43, post #14606349] Touching them causes instant pain, but the tubes are operating normally.
2. How can I check if my EL84s run within safe limits without a tube tester?
Measure voltage at pin 3 (anode) and pin 7 (cathode). Calculate current: I = V/Rk. Ensure Pa = Va × I ≤ 12 W [Philips Datasheet]. A basic multimeter suffices [Elektroda, aksakal, post #14606691]
3. One EL84 glows brighter—is that a fault?
Slight heater brightness mismatch is common. Worry only if plate glows red or cathode resistor values differ by >10 % [Elektroda, dostojny.kocur, post #14606277]
4. Should I replace all capacitors in this radio?
No. Replacing every cap detunes RF filters and may “waste the radio” [Elektroda, aksakal, post #14606728] Swap only failed electrolytics or leaky coupling caps.
5. Which capacitors are critical to change first?
Priority parts: 1) Filter electrolytics that cause hum; 2) Paper coupling caps feeding EL84 grids—leakage here can push plate current above 70 mA, risking transformer failure [Elektroda, piotrekjjj, post #14607682]
6. What is capacitor reforming and how do I do it?
Reforming slowly repolarizes electrolytics. 3-step method:
- Power radio 2-4 h daily.
- Repeat for 3–5 days.
- Monitor hum reduction.
[Elektroda, aksakal, post #14606855]
7. How much will new parts cost?
Approximate Polish prices: ECC85 ≈ 8–15 PLN [Elektroda, 14609032]; two 50 µF electrolytics ≈ 10 PLN each (TME list); premium EL84 pair ≈ 120 PLN. Labour varies; hobbyists budget 50–100 PLN total.
8. Why does the set hum with volume at zero?
Residual hum indicates tired filter electrolytics. If hum is inaudible beyond 0.5 m distance, levels are acceptable [Elektroda, aksakal, post #14607910]
9. How can I improve FM reception?
Connect a 1.5–2 m wire to the small-spacing antenna socket (FM). Move it away from concrete walls for +6 dB gain [Elektroda, aksakal, post #14608457]
10. Which antenna socket is which?
Narrow twin-lead = VHF/FM; wide twin-lead = AM bands (LW/MW/SW) [Elektroda, kris8888, post #14608852]
11. Can I substitute an ECC81 for an ECC85?
Both share pinout, but ECC81 may draw 150 mA heater vs 300 mA for ECC85 and has lower gm. Use ECC85 to meet gain spec [Elektroda, aksakal, post #14609202]
12. Do new tubes require bias adjustment in the Apollo?
No manual bias. The set uses cathode resistors that self-bias; simply insert matched tubes [Service Manual].
