The Cary 805RS is only equipped with 8K primary OPT, with installing the 211 the impedance ratio would be just 2, below the lowest tolerance ratio of 2.5.

In this case 211 will abnormally generate a lot of 2nd Harmonics that could be misinterpreted as bass. Your friend should have used 4 ohm connections if he plans to use 211.

最後修改時間: 2025-03-27 21:53:59

Hi Eric,

I agree with you that the 845 is superior than the 211. My friend who owns the Cary CAD805-RS which can use either the 211 or the 845. He said the 211 is better because it has stronger bass. I explained to him that the 211 simply has higher distortion which makes him feel the bass is stronger and the mid is thicker. I get him to put the 845 tube in his amp and keep him patiently to listen to the music with 845. I said to him he shall listen carefully with the clarity and details instead of those bommy bass.

After listened to the 845 for some time, he realized he was wrong.

845 美國胆係7,80年代已經炒起，D美國同歐洲人加埋日本仔及韓國人唔係傻，知道乜嘢胆係靚声，係7,8,90年代佢地大手筆買哂2A3, 300B, PX25, 845, 211就知

我記得90年代一枝全新有原裝盒美國制211賣90美元，而一枝全新有原裝盒美國制845係1100美元，時至今日一枝用過二手有原裝盒美国845已係過3000美元

而有些人話845冇211靚声(例如大陸老曹)，因為佢地都搞唔掂個＞400V P to P 推動級，845可以話係直熱三極胆之中对推動級要求最高

真正用過高班推動級嘅發燒友我睇到個個prefer 845 over 211




最後修改時間: 2025-03-27 01:43:59

Among 211, 845, 838, 805 big bottle tubes, 845 is the first choice for Audio amplifier because of its relatively low plate resistance.

Take a closer look at the normal practice for Triode 845 plate load ranging from 6K to 12K (depending on how high the plate voltage). The nowadays renowned high quality output transformer normally would not exceed 80 Henry. 80 Henry is only 10K XL, well，which is only 5 times of 845 plate resistance (1700 ohm) PLUS the DCR of Output Transformer primary winding(roughly 300 ohm). 5 times of Triode plate resistance can expect a reasonably good linearity. And of course the higher the load reactance the better linearity, nevertheless 5 times of Triode plate resistance load is always chosen for high quality SET amp.

Then take a look of 211, having the plate resistance of 3600 ohm, 10K XL can only be 2.5 times of 211 plate resistance PLUS Output Transformer primary winding, 2.5 times for Triode load is marginal that it can expose to loosen bases response and more non linearity encountered

No need to mention 805 as it has ＞10K plate resistance, 805 was primarily invented for RF application. 805 used as Audio amplifier MUST use loop NFB......

To utilize the Direct Heated Triode still rely on loop NFB? Rather use Tetrodes or Pentodes like KT150 or EL34 that can make the Output more wattage, efficiency can be 200%of 805. So why use 805 when loop NFB is finally required ?


最後修改時間: 2025-03-27 00:27:11

Hi Eric, thanks for you good info.

The 805 plate resistance is approx 12K which I found from the diyaudio.com forum.

No wonder the 805 power amp would require deep NFB to lower the damping factor.

Oh dear.

Johnny

Hi Johnny,


For Audio use, 805 was naturally born to use loop feedback due to its very high internal resistance 10K and the high amplification factor of 50

Never seen any 805 udio amp without loop feedback so far

最後修改時間: 2025-03-26 09:32:47

I have been curious how is the performance of the 805 tube Single-Ended Amp. The 805 is basically working in A2 mode mostly, i.e., it is bias with positive voltage of 10~15Vdc between the grid and cathode.

The circuit I have seen is using the 300B cathode follower to drive the 805. I tried to simulate the circuit and found the 300B needs 3 different power supplies for such application. Voltages includes the B+, B-, and the negative bias at grid.

One simple approach is using the Inter-stage transformer in series of a positive DC voltage bias for the 805. The driving tube can be 45 or 2A3.

Wondering if someone could tell us how you like the sound from a 805 SE amp?

Johnny

Cary呢隻interstage 設計有失販地方，用300B 接interstage 又同時用coupling cap load 隻220K, 佢隻interstage頻率响應係咪唔掂？

而實際上interstage 比輸出牛難做好多係事實，interstage 初及次級都是幾千圈，电容量很難搞，要interstage 性能好最好使用貴價permalloy 或Cobalt Based Amorphous 或Nano Crystalline





最後修改時間: 2025-03-23 13:29:14

Traced the CARY CAD-805RS 211/845 Mono Power Amplifier circuit. Here's the schematic I would share with you.
Note: The two 6SN7GTB tubes' filament share the same power supply with the 300B. Therefore, the 6SN7GTB filament is at ground potential.

NFB or Non-NFB

With NFB, the THD figure can be reduced significantly. However, it will creat wide spread high frequency harmonics. Here's one example I ran the simulation of the CARY CAD-805RS power amp to show the difference.

Johnny

Hi WilliamsonAudio

已搵到noise的原因來自條ｒｃａ線及未完成的diy前級；
RCA短路或斷開，ｍｖ都接近０或至0.6 左右。
ｍｈ４燈絲冇接地，直接４ｖａｃ



壞的那條ＲＣＡ度不到mv, 我用手機測的



最後修改時間: 2025-03-18 16:26:39

Hi Karas,

先把後級看成一個unit， 如果把"後級"RCA 座，短路(輸入短路)。輸出噪聲是多少？又或者是否正常？

另外假設你 MH4 的燈絲有接地。(因為沒有完整電路圖，所以這一點不能確定)

頂 搞搞下，原來係條rca線問題
一頭冇駁 一頭駁amp 就噪，點解呢？
他過條線又冇short，線又冇傷

我重新焊過條line input部份 做回100k input，同執執ground loop breaker, 無左噪聲了


johnnykmtang

Yes, this is the board I bought. I asking the shop what status in "0 and mute".

Found this TVC relay control board

https://www.aliexpress.com/item/1005005647465831.html#nav-specification

Dear Karas,
When the volume control is at 0 or Mute, the RCA output should be shorted. I think it is the problem is with the connection at the relay control board.
I don't have any information of the board. Therefore, I have no idea how to fix the problem.

3 Hum 應該是來自前面（前級）。因為short RCA 沒有。

關於前級，可能我會講到有d亂，而我都係第一次接觸TVC
1. 新前級係Khozmo 新TVC relay式遙控，有volume 0同mute，用佢聽歌係全段都冇問題既，mute的時候 阻值接近0
係甩 rca線時會有噪

2. 之後我又買左日本24級finemet core Tvc，夾淘仔24級relay板去用，個24段都係無問題，
個問題又係0及mute時，會發生甩rca線咁有噪，
發現呢塊板mute的狀態是正負斷開當mute


所以rca short埋係正常定唔正常？

WilliamsonAudio

1. 可以排除由第一級的柵電阻感應。 100K 是可以。電學原理是電阻越大，noise 越高。也容易pick up noise.
原來如此

2. 可以排除燈絲入，因為有接地和VR 可調
我昨日才發現圖的mh4燈絲都有vr, 這個我沒裝 不是直熱裝vr 才有用？

兩款網屏都有，但始終ac/hl最正

Hi johnnykmtang
Thank you for the simulation.
The actual b+ doesn’t very high. Around 390-400v for px25.
The R4 replaced a choke load. May I revise the value in drawing later?

MH4 最早期是網屏，師兄有否試過?