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6B4G Integrated Mono

The amp is based on Ralph Power's simple IT-coupled PP amp. However, being a Fixed Bias Believer, I converted both stages to fixed bias. Output stage uses a conventional power supply for biasing; first stage, 6C45P into LL1160 (Alt-U wiring), is biased with an inline lithium button cell to 220..250V, 25..30mA. This is probably unsafe. 6C45P usually requires a low resistance grid-to-ground path (150K or less by the book) for stability. Here, it's up to 300K plus battery. As if it's not enough, Ua is 100V above rated maximum and Pa is close to maximum. So far, it's stable.

I wanted a more-or-less-portable, self-sufficient amp for a kitchen or a second home. This added a simple input selector (with optional stereo-to-mono mixing) and a shunt volume switch. Overall package size is 38*22*22 cm and only 12 kilos with Lundahl iron and 2 millimeter brass chassis - quite portable ...

Hum pot is really unnecessary and can be replaced with two 10..30 Ohm resistors. 1 Ohm resistor between hum pot wiper and ground is used as a shunt for current meter (it's actually 0.83 Ohms - value tied to meter's own scale). Transformer wiring: LL1660 - Alt-U, or Alt-U with all primaries in series for better bass. LL1623 - Alt-B. Bias battery is a lithium button cell with PCB leads. I tried soldering leadless button cells, ruined a few - no, thanks...

I also considered using gas regulators on the first stage, but eventually discarded the idea. First problem: fixed bias and fixed plate voltage don't necessarily result in optimal current. Second: traditional Russian gas regulators are rated for 5-30 mA current. That is, idle current through regulator must be set at around 20mA, allowing 10..15mA AC current swing (plus and minus). If the current through 6C45P exceeds idle plus 15mA, which is quite possible at low frequencies and high input level - regulator shuts down, and this is something I don't want to hear.


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