＞#5841,

如果用家分不出不同訊號線的聲音差別的話（不同LCR參數）的話，那就不必考慮這個問題。

但如果有所感覺，例如每一檔都覺得音質有所分别的時候, 就需要深入研究這種resistor impedance bridging的基本要求，即訊號源和接收端amplifier的impedance, 嚴格來說這種resistor attenuator基於分佈電容電抗等問題會産生frequency dependent的參數，在專業的應用上即是産生一個或以上的poles, 那就需要用zeros去補償，有興趣可問AI, 甚麼是pole zero correction, 基本上是希望抵消所有對頻率有關的參數，下圖是簡化了的解説和一塊PCB用SMD去bond 電阻和pole zero correction components 的FRA trending curve，可以看出每一檔在音频範圍内還是有所差別。



最後修改時間: 2026-07-15 07:56:15

图

用Shallco 銀接點Volume制同Riken RMG 电阻

走咁多綫，唔知用咗呢部無源前級好咗定差咗

Hi LaLaLam,

Here's the comment by AI Gemini:- most practical amplifier designs, you can expect the 300B's total effective input capacitance to sit between 65pF and 83oF to its high. The 300B requires a driver stage capable of delivering sufficient current to cleanly charge and discharge this capacitance at high audio frequencies without causing high-frequency roll-off or slew-rate limiting.

The 5842/417A is an Ideal MatchThe main struggle with driving a 300B is finding a tube that has high voltage gain (to swing ) and a low output impedance (to drive the 300B's large Miller capacitance). Usually, you get one or the other. The 5842 gives you both massive Transconductance: The 5842 has an incredibly high transconductance of around $25,000. When you drop that into an RC-coupled circuit, your actual source impedance driving the 300B grid drops significantly, easily wrestling the 300B's Miller capacitance all the way past the audio spectrum ＞100Khz. Healthy Voltage Gain of 43. In a real circuit, you can easily achieve an operating voltage gain of 25 to 30. By using CCS or inductive load, the full gain of 43 will be achieved.

最後修改時間: 2026-07-14 22:53:26

del

最後修改時間: 2026-07-14 21:52:36

Actually, EL34 can be drived more easily than the 300b. This happened on their difference of interelectrode capacitance, especially the plate-grid capacitance ”so called C-miller”. EL34 got a low Cag1 (1pF) as the capacitance is blocked by g2/g3. However, for 300b, there is only one grid, the Cag is over 20pF including tube socket as measured by an impedance analyzer (shown below pic). Also, the Cag value and its Q-factor changed a lot when apply different filament voltage. Therefore, 300b needs to have a powerful driver stage to get rid of the miller effect and perhaps can yield a better performance.

Hi Bambino,

I use two 27 ohm resistors across the filament pins. The cathode current is going through the center tap of the resistors. I believe this should be OK.

it's matching tubes by comparison at different operating points. in either cases, it uses a single operating point as reference. it's not matching by comparing characteristic curves. although not ideal, it's very easy to implement.

what matter is how you measure grid voltage. voltage across grid and filament mid-point, vs voltage across grid and filament negative end......it makes a big difference.

Both LInLai and JYL would set up the power supply with 300V B+ and -60V for the grid. Then, it takes the reading of the plate current. The matched tubes would have the same plate current.

I did it a bit differently. I set the B+ to 350V. Adjusted the negative voltage to the grid for 60mA plate current. Then, I take the grid voltage as my reference for matching. With this method, I can get the tubes matched in my amp. Yeah.

By the way, you can test your 300B to see if it is the getting weak with the same method.

Curve tracer is great to determine to optimal operating point. Of course, it does tell us the conditon of the tube.

However, it is rather difficult and expensive to buy or make one.

Curve Tracer DIY：

https://www.bartola.co.uk/valves/tag/tube-curve-tracer/page/2/

Simple test fixture for 300B

I just made this for checking the condition of the 300B. I set the Plate Voltage to 350Vdc and measure the negative bias voltage which allow 60mA through the tube. It shows that many of 300B that I have are way out of spec. Ah oh............Too bad.

It's the real 夢幻魔球. My question is :- how would you know the condition of the 300B's that you own???



最後修改時間: 2026-07-01 18:27:14

833A 與 GM100 會不會像WE212一樣因為高壓而發放幅射的?

If I were forced to explain the difference using an analogy, I would compare amplifiers to hearts.

A small EL34 integrated amplifier is like a mouse's heart. Fast. Energetic. Perfectly capable of sustaining a small organism.

A good 845 amplifier is like the heart of a human being. Larger and more powerful, it moves a much greater volume while remaining calm and controlled.

The 833A is the heart of a whale, capable of moving an extraordinary amount of material with a single effortless contraction.
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When 833A is placed next to GM100, 833A is like a baby whale heart at best.



最後修改時間: 2026-06-28 20:10:46

WAVAC 833a: Overkill for Most Systems, Magic for the Best

https://forum.audiogon.com/discussions/wavac-833a-overkill-for-most-systems-magic-for-the-best

嘩100磅重....each

日昇音響 YS ENCORE 845 mono block演示

https://fb.watch/HEv-lascKT/



最後修改時間: 2026-06-10 10:09:29

Fisher 亦用原子粒整流 好多老友捧場

#5821

Jimmy

原子粒整流改成膽整流可能抗干擾有幫助. 我自己冇AB比較過,吾敢亂講. 但有吾少廠機用原子粒整流都吾問題. 例如Marantz 7 和 Audio Research SP-3A. 佢地連水缐都冇,一樣係咁. 真正問題未必係線路. 可以留意下零件安裝位置和走線路徑.

玩磁環只是二十年前初玩hifi的小實測. 當時SSP有得賣. 套係電源線的接頭後面. 我吾太喜歡玩一些gadgets. 原因係可能對某方面音效有些改善,但總會帶來side effect. 我知有好多人鐘意玩一些小玩意,但我就沒有興趣了.

最後修改時間: 2026-05-22 13:27:21