Yes, measuring the DC voltage across R65 is an excellent strategy for verifying oscillator activity and amplitude across all bands — and it is practically mandatory for Band 6.
Monitoring a DC voltage drop associated with the 6SA7 oscillator gives you a reliable, relative proxy for the RF oscillation amplitude without having to directly tap the tuned RF circuit. When the 6SA7 oscillates, the average grid bias goes negative, which in turn alters the average DC current flowing through the tube. Measuring the resulting voltage changes across a network resistor like R65 lets you "see" the RF activity safely.
On the lower bands, the 6SA7 oscillator often has enough vigor that it might survive a direct RF probe. But on Band 6, the oscillator is operating at high frequencies where it is much more fragile and sensitive to stray capacitance.
A standard DMM or oscilloscope probe introduces capacitance (usually 10–20 pF with the leads). If you touch a high-impedance RF point in the oscillator circuit on Band 6, you will almost certainly:
* **Detune the oscillator:** Shifting the frequency far out of the IF passband. * **Kill the oscillation entirely:** The added capacitance can damp the circuit so much that it stops running altogether while the probe is attached.
By measuring a DC voltage instead, you circumvent the probe loading issue completely.
### A Quick Caveat on "Isolation"
You noted that R65 seems to have ample isolation. Just to ensure your measurement remains entirely invisible to the RF circuit, double-check where you are attaching the probe:
* **If R65 is decoupled/bypassed to ground:** (For example, if it's acting as a plate or screen supply dropping resistor with a bypass capacitor on the tube side). In this case, you are perfectly isolated. Probing here will read the DC current change when the tube oscillates without affecting the RF at all. * **If R65 is the grid-leak resistor:** The "hot" end connected to the 6SA7 grid is *not* isolated from the RF. Probing the grid side directly will still introduce capacitance and likely kill Band 6. **The fix:** If you need to measure the DC grid-leak voltage directly, temporarily attach a **1 Megohm or 2.2 Megohm isolation resistor** to the very tip of your positive probe, and touch the free end of *that* resistor to the circuit. This high-value resistor acts as an RF choke, allowing the DC bias to pass to your meter while blocking your probe's capacitance from loading the tuned circuit.
Since the DC voltage will vary proportionally with the strength of the oscillation, watching this voltage as you sweep the tuning dial is a fantastic way to confirm that the 6SA7 is maintaining a steady amplitude across the entirety of Band 6 without dropping out.
Still missing the target. This answer doesn't indicate an understanding of where R65 is. See the next one.
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Last modified: Monday, 27 Jul 2026