Capacitor Materials in HF amplifiers

 For the Heathkit SB-200 (572B tubes) and SB-220 (3-500Z tubes), the critical capacitor positions are the RF input/output pi-network tuning and the grid bypass capacitors (200 pF in the SB-200, which are specifically chosen to resonate with the tubes' internal grid leads near ~70 MHz for VHF parasitic suppression). 

 This makes dielectric choice far more consequential than in a typical circuit.

Heathkit SB-200 SB-220 amplifier capacitor dielectric comparison

Ceramic Dielectric Comparison for These Amps

Dielectric	Class	Stability	Loss / Q	Suitability in SB-200 / SB-220
C0G / NP0	1	±30 ppm/°C	Very low (tan δ ≈ 0.0015), high Q	Only recommended ceramic. Used at 1–2 kV rating for RF tuning and grid bypass.  Preserves the grid-lead resonance and pi-network Q.
C0H	1	±60 ppm/°C	Very low	Technically fine but rarely stocked; no practical advantage over C0G in these amps.
P100, N33, N75	1	Compensating	Low	Designed for temperature-compensation networks, not general RF bypass. Not a substitute.
X7R	2	±15%	Moderate, piezoelectric (microphonic)	Not suitable for RF tuning or grid bypass.  Capacitance drifts with temperature and DC bias, shifting the grid-lead resonance and detuning the pi-networks. Acceptable only for non-critical power-supply bypass.
X5R	2	±15%	Moderate	Same limitations as X7R, narrower temp range. Not suitable for RF-critical positions.
X8R	2	±15%	Moderate	Higher density, same drift problems. Not suitable.
Y5V	3	+22% / −82%	High loss	Unsuitable. Large capacitance swing would detune the RF networks and could destabilize the grid-lead resonance.
Z5U	3	+22% / −56%	High loss	Unsuitable for the same reasons.

Practical Guidance

• RF tuning (pi-networks) and grid bypass: Use C0G/NP0 ceramic rated 1–2 kV (or the original silver mica at 500 V, which is the stock choice).  The SB-220 literature explicitly recommends "1000 V ceramic, NP0 type" for these positions. Do not substitute X7R, Y5V, or any Class 2/3 dielectric here — the capacitance drift will shift the parasitic-suppression resonance and degrade SWR and output. 

• Power-supply bypass (filter caps, filament bypass): X7R or even Y5V would work electrically, but these positions in the SB-200/SB-220 are traditionally filled with electrolytic or paper caps, not ceramic, so it's rarely a question in practice. 

• Grid bypass value must stay at 200 pF (SB-200): The value is not arbitrary — it's tuned to the 572B's internal grid-lead inductance. Changing the dielectric changes the effective capacitance under operating conditions (especially for Class 2 types), which can break the ~70 MHz series resonance and allow VHF parasitics.