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Very soft bass - scratching my head

38.112.26.42

Posted by single_ended on February 28, 2007 at 12:42:32

Here's the circuit

Input stage

WE 102L (@40K plate resistance, 1.1ma, 150V plate): 82K plate resistor, 100uf BG N coupling cap, MQ hi-nickel grid choke, 1.8K bias resistor bypassed with 1000uf BG

Driver stage

WE 101L (@5K plate resistance, 10ma, 160V plate): 250H plate choke, 68uf BG N coupling cap, MQ RIT-jr 5K 1:1 interstage, 1.3K bias resistor bypassed with 2000uf BG N

Output stage

WE275A (running at 50ma, 250V plate voltage): Tamura F5003, 1.21K bias resistor bypassed with 47uf BG VK

Power supply

Dual 6CJ3 choke input power supply. BG WKZ PS capacitors.

I understand that the Tamura F5003 has a reputation of weak bass, but in this amp the bass is so far off (rolling off in the bass really early) that I am scratching my head wondering if one of the earlier stages is also contributing to the sonics.

I have tried various capacitor swapping in the input and driver stages so I hope someone can spot a design mistake or suggest a circuit change to boost the mid/lower bass region.

A side question, do tubes that are almost gone have weak bass?

Much appreciated!

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