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Cheap 300B project
Cheap 300B project
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Old 3rd April 2018, 01:45 AM   #1
2stuffed is offline 2stuffed  Australia
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Cheap 300B project
Default Cheap 300B project

Hi everyone, I just finished a amp using a cheap Chinese PCB kit. It works ok but hasn't got much volume or bass until I turn it up to max on the pot.I'm not skilled enough to work out whats wrong .I have no readable schematic, just one from the description online. I've attached a pic , maybe someone might be able to help.

thanks
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Old 3rd April 2018, 02:41 AM   #2
cspirou is offline cspirou  United States
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I've been interested in these eBay kits for awhile. What speakers and transformers are you using?? 300b amps can only put out 8 watts max, so if you have low sensitivity speakers you won't be hearing much.
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Old 3rd April 2018, 03:04 AM   #3
2stuffed is offline 2stuffed  Australia
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Cheap 300B project
Thanks for your reply. I'm using BEZ 3.3k output transformers good for 25watts, heavy iron. My speakers Alpair 10p MLTL . I made a KT66 triode amp with about 4 watts and that drives them easily, so 7 or 8 watts should be good.
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Old 3rd April 2018, 04:15 AM   #4
rotaspec is offline rotaspec  New Zealand
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You will need an active preamp to drive that to full power.
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Old 3rd April 2018, 04:33 AM   #5
2stuffed is offline 2stuffed  Australia
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Cheap 300B project
I thought the 6sn7,s would be for the preamp. But if not, I'll have to make a preamp.
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Old 3rd April 2018, 05:17 AM   #6
rotaspec is offline rotaspec  New Zealand
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The 6SN7 are drivers for the 300B, but in these circuits they will not provide enough gain to get full power from even a hot CD source. You will need about 4v RMS to drive that circuit to full power. CD is generally 2v max, and often lower, so some extra preamp gain is needed.
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Old 3rd April 2018, 07:17 AM   #7
2stuffed is offline 2stuffed  Australia
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Cheap 300B project
Thanks for your help, looks like a new project to build.
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Old 3rd April 2018, 08:59 AM   #8
waltube is offline waltube  Italy
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Hi
how much is the HT voltage? and thevalue cathode resistor of both sections of 6SN7
Maybe is possible to swap with 6SL7 that has much gain.

Walter
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Old 3rd April 2018, 09:49 AM   #9
azazello is offline azazello  Bulgaria
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Your gain of preamp is about 30. Or you have about 30 V signal after presamp. You need 6SL7 that provide You gain about 70-80 for enough driving. You have bias on the 300B about -70 V. Note, that gain of pre amp, practicaly, must be the same apr /about 70-80!/.
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Old 3rd April 2018, 11:23 AM   #10
2stuffed is offline 2stuffed  Australia
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Cheap 300B project
I measured the voltage . It's 235v on pin 2 and 108v on pin 5 on the left side and 245v and 118 v on the right side? The cathode resistors are 1k1 on both. I have some 6sl7,s if this would work.
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