300B GSG Mods

2nd harmonics for the masses

Re: 300B GSG Mods

Postby Shannon Parks » Wed Jan 21, 2015 5:30 pm

zman01 wrote:Shannon,

Since 300B tubes would need higher B+ vs 6B4G, what if a builder chooses with a transformer with higher secondaries i.e. 360-0-360 vs PA-774 which is 330-0-330? Will the 6AX5GT rectifier be able to get the B+ to acceptable levels (380v-390v)? Any changes in the BOM for this?


The 6AX5GT could handle the voltages. The question is what voltages would be present on the power supply caps at start up. Since the 6AX5GT is an indirectly heated tube (and slow), I think you might be OK with a stock build. But that is the first thing I would keep an eye on.

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Re: 300B GSG Mods

Postby zman01 » Thu Jan 22, 2015 10:57 am

Shannon Parks wrote:The 6AX5GT could handle the voltages. The question is what voltages would be present on the power supply caps at start up. Since the 6AX5GT is an indirectly heated tube (and slow), I think you might be OK with a stock build. But that is the first thing I would keep an eye on.

Shannon


500v power supply capacitors should be safer/ok then? The stock build lists 450v capacitors IIRC.
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Re: 300B GSG Mods

Postby Shannon Parks » Fri Jan 23, 2015 7:55 am

zman01 wrote:
Shannon Parks wrote:The 6AX5GT could handle the voltages. The question is what voltages would be present on the power supply caps at start up. Since the 6AX5GT is an indirectly heated tube (and slow), I think you might be OK with a stock build. But that is the first thing I would keep an eye on.

Shannon


500v power supply capacitors should be safer/ok then? The stock build lists 450v capacitors IIRC.


Correct, but 500V snap mount 25mm diameter caps are a rare thing. A bench test monitoring the startup voltages with a new transformer design is almost a must. You might be OK with the 450V depending on the 6AX5GT characteristics. A CL120 can help, too.

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Re: 300B GSG Mods

Postby zman01 » Sat Feb 21, 2015 11:45 pm

Shannon,

Rarity located:

http://www.digikey.com/product-detail/e ... ND/2545372

Now the power transformer I have in mind has 1 x 6.3V tap, 1 x 5V tap.

The 6.3V tap will be used for the 6AX5GT rectifier; what value of resistor do I need to consider in the circuit for the 5V tap? Understand the 300B mod helps drop the 6.3V of the PA774 to 5V required by the 300B, but in this case I am starting with 5V.
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Re: 300B GSG Mods

Postby Shannon Parks » Sun Feb 22, 2015 2:24 pm

zman01 wrote:The 6.3V tap will be used for the 6AX5GT rectifier; what value of resistor do I need to consider in the circuit for the 5V tap? Understand the 300B mod helps drop the 6.3V of the PA774 to 5V required by the 300B, but in this case I am starting with 5V.


This sub won't work for a couple of reasons, and changes the design. You need two separate filaments for the power tubes, and you need them 6.3VAC so you can get 5VDC.

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