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Old 9th May 2009, 02:47 PM   #1
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Default Changing 300B Tubelab SE to accomidate 50 triodes

Well, I haven't completed the build, but have the parts to build a 300B Tubelab SE. I have emailed George but haven't received a response, but I also wanted to get other's input to this idea.

What changes would be needed to accomidate 50 type triodes ina Tubelab SE? I know these can handle much more voltage than 300B's so the B+ caps voltage rating would need to increase. I haven't purchased any transformers for it, output or power so I can get the right one for the job. I'm not sure what kind of load 50's like in an output transformer. Also what kind of sound do these produce? I want to do this as I have received a pair of Shuguang 50's for free. I'd rather spend the money I would have spent on the 300B's to make these work in it. Any ideas are welcome. Thanks!
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Old 10th May 2009, 01:33 PM   #2
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I do not have any 50 tubes so I have never tried them. When my usual tube vendor finally decides to carry the SG-50 I may get a pair if the price hasn't risen too much.

When I was designing the Tubelab SE I chose the Sharp regulator for the filaments because it provided the best hum rejection with a low dropout voltage. It however will NOT work at the 7.5 volts required for the type 50. It is specified for 5.5 volts maximum, but I tried it at 7.5 volts anyway, and it fried. At that time there were no current production 50 tubes and NOS tubes were far beyond my price range, so this was not considered an issue. If you intend to use a 50 in a Tubelab SE board, you will need to use an external 7.5 volt filament supply for the output tubes.

Many people operate 50's in the 450 volt range which would require upgrading the voltage rating in the B+ and B- supplies. The resistor values in the bias supply divider will need adjusting too.
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Old 10th May 2009, 02:34 PM   #3
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I would tread carefully with regards to plate voltage ratings on a current production 50, they aren't really capable of much higher voltage operation than a properly manufactured 300B.

The 50 will produce roughly 3dB less output power than the 300B and is normally run in the 55 - 60 mA range. Personally I would not go much over 400V.

Have you also considered availability issues in the future when you have to replace those 50s? Selection is likely to be limited to a few expensive boutique vendors such as TJ.
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Old 12th May 2009, 02:53 AM   #4
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Actually I haven't thought about the long run (one of my big faults) but I was able to find a vendor who has a steady stock of 50 triodes on ebay for around $150 for a pair. This is waaaaaaay cheaper than i've seen. They look to be the same ones i have.

As far as the sound of these triodes, how do they compare to a 300B or a 45? If it's going to be a step down soundwise, i'll just let them go on eBay and continue on the 300B route.
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Old 12th May 2009, 02:56 AM   #5
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Forgot to post the auction I was talking about in case anyone is interested.
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