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Old 10th July 2012, 03:33 PM   #1
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Default Looking for High voltage double triode

Dear All, I am looking for a high voltage (500v on plate), medium mu, double triode as a second stage after a 6sl7 based PI.

It should be a 6.3v heater model but 12.6v will do. Preferably octal socket but then again it is not a determining factor.

I need to replace a 6sn7 stagee with 520v power supply voltage in a driver board for my current gu81m project. the second stage is coupled with mosfet follower to achieve AB2 operation for the gu81.

Any help would be very appreciated.
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Old 10th July 2012, 03:38 PM   #2
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6BL7GTA or 6BX7. Same pinout as a 6SN7 with a 500 volt plate rating.
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Old 10th July 2012, 05:30 PM   #3
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Thank you very much!

A very neat tube. Hopefully it will prove an acceptable replacement.
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Old 10th July 2012, 06:08 PM   #4
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I have a driver board based on the design that came from the cooperative effort in this thread:

6L6GC AB2 Amp

The schematic is in post #559 or post #460

I was using a 6SL7 input tube and a 6SN7 driver tube. I wanted to experiment with screen driven sweep tubes so I plugged in a 6BL7GTA and cranked the power supply up to 650 volts. I adjusted the CCS in the tail to get the plate voltages back to about 400 volts. Everything worked just fine. In a moment of stupidity, I plugged in a 6SN7......it worked too. The plate voltage is around 400 and the cathode is about 150 so nothing is being violated.

Note everything is from 2 year old memories so I could be off on the voltages, but the 6SN7's worked from a 650 volt supply.
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Old 10th July 2012, 06:20 PM   #5
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thank you for your post, as always informative and helpful!

the problem is not the anode voltage of the 6sn7 but rather the fact that I made a stupid mistake and bought chinese 6sn7 which after countless attempts and hours of testing will not show any resemblance to typical 6sn7 datasheet curves.

There are a number of alternatives like the russian 6N8S but in order to have a clean 400vpp signal to driver the gu81m tubes I need a tube which can take high plate voltages happily. I just bought 2x NIB 6BL7gta for 10 euros which should be just perfect!

Current will be provided by toshiba mosfets.

I managed o get 355vpp signal out of 6sl7 and 6sn7 but 400vpp is the target lem8r set for his design.

Once I get the board ready I'll finally start the gu81m with the 1700va OPT. Can't wait!
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Old 10th July 2012, 07:08 PM   #6
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I did some tests a while back and had a 6SN7 giving about 1% distortion at 560Vpk-pk. I ran them at 6mA and 400V with an 800V supply, CCS loaded. These were new production (Russian) Tung-Sol tubes. A good 6SN7 should be able to do what you want. I'm building an amp that requires this much drive but decided to go with 841s instead since I wanted less distortion and more eye candy.

I wanted to try 6BL7 so that I could run them a bit hotter and maybe get less distortion but haven't gotten around to it yet. That's my current plan for the next amp I am building. Judging by the datasheet curves, I expect to see less distortion from the 6BL7 than the 6SN7 for these very large swings.
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Old 10th July 2012, 11:41 PM   #7
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I used 6BX7 as a differential pair with pentode CCS to drive 813s. The problem with 6BX7 is finding an example whose halves match; I was fortunate back in the glory days of stocks of valves, test gear, employment and credit cards: I had a good stock of 6BX7 and also an Avo 163 valve tester. Incidentally I had only four 6BL7s; they were branded Pinnacle and were excellent - no heater/cathode problems and well matched internally. I daresay that they were Russian production. The best 6BX7s I had were Japanese; I forget the brand.

Paul
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Old 10th July 2012, 11:47 PM   #8
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Another idea: would you consider a pair of nine-pins in place of one octal? I ask because I think a good candidate would be 6S4A; a cheap powerful triode, 8W dissipation, rated at 550V, mu of 16 and with a decent Ra (about 4.5k from memory). Remember that 6BX7 and 6BL7 have greedy heaters (1.5A) and Morgan Jones says (3rd Edition) that 6BX7 is very variable as to distortion performance.

P.
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Old 11th July 2012, 04:52 AM   #9
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@7M7 thankyou for you suggestion. I don't mind changing socket but there are two limitations:

1. I have 5 holes in the chassis for a) extra gain stage b) 2x phase splitters c) second stage tubes

2. 2x 6S4A would require 1.2A of heater current. By using the single 3A 6.3v line and "stealing" some 12.6v voltage (in series) from the gu81m I can cover most tube demands.

However I will keep the suggestion in mind. For now what I know is that the golden dragon stuff is not reliable. The 6sl7 biases according to specs but the 6sn7 is really a can of worms.
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