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| [#1] Not the same VAA-120 6550 push-pull anymore… Not the same VAA-120 6550 push-pull anymore… Based on the net, the VAA-120 6550/KT88/KT100 60W push-pull main amplifier was produced by the Shenzhen Valve Audio Lab back in 1995. It was later marketed by the Monarchy Audio of California USA for around US$1000. I like the basic design of this amplifier very much, imo, it is the best mid-power push-pull design when compared with the traditional Williamson and the H.J. Leak. If you like pentode input stage then the Mullard 5-20 design could be you choice over this all-triode input and driver design. The VAA-120 is actually pretty much a copy of the Eico ST-70 (and later improved by Steve H. Lafferty). There are a couple of drawbacks on the VAA-120 circuit, 1) the phase inverter/driver stage (6SN7’s) heater-to-cathode voltage has been exceeded by a wide margin, the heaters should be lifted to at least +70V for the cathodes, I choose 100V. 2) the 12AX7 and 6SN7’s employs dc heater supply with large filter capacitors, the unregulated dc at power-on will stress the heaters a lot. I choose to use 6.3Vac heater supply lifted at +100Vdc as mentioned earlier. 3) the four individual fixed bias pots (one for each power tube) will prove to be a potential disaster for the inexperienced endusers because people tend to tweak every potentiometer insight. Without any meter indication, there will be more amplifiers like this to smoke and ‘eating’ power tubes like crazy. The sad thing is that usually people would not realize what they have done was so wrong and that the damage would be quite costly. I install a tuning and signal meter savaged from a vintage Marantz receiver. The tuning meter will show the DC balance condition of the power tube pair while the signal meter will indicate power tube bias. Btw, the bias current has been increased to 50mA for each 6550. I prefer tube rectifier such as the GZ34/5AR4 for this amplifier but there is no more real estate on the chassis.  |
tubepower 正式會員61.xxx.xxx.15 |
2017-05-15 21:27 | |
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| [#2] Not the same VAA-120 6550 push-pull anymore… produced by the Shenzhen Valve Audio Lab back in 1995. It was later marketed by the Monarchy Audio of California USA. 長篇大論.大極典呀. VAA係KIT. MP係廠製. 當年發燒光極力推. ＿ |
伏喱哩 正式會員 58.xxx.xxx.37 |
2017-05-15 22:04 |
| [#3] Not the same VAA-120 6550 push-pull anymore… here is the modified VAA-120 circuit fyi.  |
tubepower 正式會員 61.xxx.xxx.15 |
2017-05-15 22:16 |
| [#4] Not the same VAA-120 6550 push-pull anymore… #3 供B電濾波用的高壓電容，若串聯使用的話，應個別並聯上均壓電阻。 |
羅密歐 正式會員 219.xxx.xxx.216 |
2017-05-15 22:35 |
| [#5] Not the same VAA-120 6550 push-pull anymore… 多謝羅密歐師兄忠告。 我一向都深信高壓電容若串聯使用要用"均壓電阻"的說法，直至今年在討論群组學到原來各說紛紜，各深資電學者理論實踐兼備，還未能立定論。如果兩隻電容不同型號或不同廠，我還會用均壓電阻。 最後修改時間: 2017-05-15 23:07:16 |
tubepower 正式會員 61.xxx.xxx.15 |
2017-05-15 22:54 |
| [#6] Not the same VAA-120 6550 push-pull anymore… tubepower師兄，用上均壓電阻，除耗費一點點電流之外，又有何缺點呢？請指教，謝謝！ |
羅密歐 正式會員 202.xxx.xxx.10 |
2017-05-16 10:09 |
| [#7] Not the same VAA-120 6550 push-pull anymore… re: #6 - 羅密歐師兄，please check pm. |
tubepower 正式會員 61.xxx.xxx.15 |
2017-05-16 13:00 |
| [#8] Not the same VAA-120 6550 push-pull anymore… 謝tubepower兄提供的資料。 那老外的實驗，我並不看得很懂。 如果那兩個串聯的電容，在530V的電壓下，有12mA的電流流過(即漏電電流)，那它倆就要發出6.36W的熱量，那它倆還在演電容的角色嗎？分明就早已被局部擊穿，變成了一個發熱體而矣，與有沒有個別並聯上均壓電阻無關。 如我理解有誤，請指正，謝謝！  |
羅密歐 正式會員 219.xxx.xxx.216 |
2017-05-16 13:39 |
| [#9] Not the same VAA-120 6550 push-pull anymore… re: #8 - I read 12uA only in his text and on his cap leakage meter. |
tubepower 正式會員 61.xxx.xxx.15 |
2017-05-16 23:34 |
| [#10] Not the same VAA-120 6550 push-pull anymore… I got one from a friend that needs repair service. The 100 ohm resistor at one of the 6550 screen grid was burnt. I re-aligned the bias of the 6550 tubes from 40mA to 60mA idle current for better linearity which still with max power dissipation. The output transformer were upgraded with Dignity Audio permalloy core. Johnny  最後修改時間: 2026-04-02 10:17:34 |
johnnykmtang 會員 70.xxx.xxx.22 |
2026-04-02 10:15 |
| [#11] Not the same VAA-120 6550 push-pull anymore… Yes, this is NOT the same VAA-120. The picture shows the Franky output transformer made by Uncle Wong. Johnny  |
johnnykmtang 會員 70.xxx.xxx.22 |
2026-04-02 21:19 |
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