03-13-2026, 11:09 AM
Hi Ken,
Sorry for joining the conversation late but the impedance mismatch was likely your problem. BTW if you want to keep the wiring the same, a 1Q5 is a drop-in substitution for a 1A5, transformer matching notwithstanding. Although the 1A5 is a pentode and the 1Q5 is a beam power tube, the pinouts are the same. The 1A5 has a load impedance of about 25 KOhms. The 1Q5 and 3Q5 beam power tubes have a load impedance of about 8 KOhms, which is much closer to the transformer that you have. Using the 1Q5 or 3Q5 has the added bonus of a much greater power output. The 1Q5 and 3Q5 have an output of more than 250 mW, vs 110 mW for the 1A5.
Do verify your grid bias, as it is based on a resistor between B- and chassis ground and is current dependent.
Sorry for joining the conversation late but the impedance mismatch was likely your problem. BTW if you want to keep the wiring the same, a 1Q5 is a drop-in substitution for a 1A5, transformer matching notwithstanding. Although the 1A5 is a pentode and the 1Q5 is a beam power tube, the pinouts are the same. The 1A5 has a load impedance of about 25 KOhms. The 1Q5 and 3Q5 beam power tubes have a load impedance of about 8 KOhms, which is much closer to the transformer that you have. Using the 1Q5 or 3Q5 has the added bonus of a much greater power output. The 1Q5 and 3Q5 have an output of more than 250 mW, vs 110 mW for the 1A5.
Do verify your grid bias, as it is based on a resistor between B- and chassis ground and is current dependent.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Best Regards,
MrFixr55
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