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Cheap 300B project

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2stuffed

Member
Joined 2016
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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rotaspec

Member
Joined 2007
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.
 
azazello

azazello

Member
Joined 2008
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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L

LJT

Member
Joined 2015
waltube said:
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
Click to expand...

I have experimented quite a lot lately with a very similar configuration.

There is a Music Angle 300B amp that use a SRPP configuration for the driver and an input stage based on a standard Common Cathode tube (ECC90 / 3A5). Unfortunately that input stage is hampered with issues related to noise and microphonics and I had to remove it, leaving only the driver with samme basic configuration as yours.

The stock MA amp has 6SL7 as SRPP driver. Using this 6SL7 (with 2K7 cathode resistors) gave a nice enough sound with reasonable sensitivity. To my ears using 6SN7 instead of 6SL7 gave an improvement in sound quality, although a preamplifier capable of delivering high current and high voltage swing into amp is required.

Using an AudioGD preamp with "discrete op-amp" capable of 10V p/p swing means turning the volume control up to 3 o'clock positions with 84dB sensitive speakers. Bass reproduction is then fully OK and as expected from the Seas magnesium Bookshelf speakers used.
Using a Emotive preamp, there was a total lack of bass/lower-end frequencies. Voltage swing from Emotive was not sufficient for full volume.

I believe that the lack of lower register reproduction is due to insufficient current drive to overcome the SRPP drivers Miller-capasitance.

Conclution: As I see it there are 3 options (in order of compexity)
- Use your circuit as is, but make sure that your pre can deliver lots of voltage and current.
- You may be happy with 6SL7 driver and possible change of cathode resistors.
- Introduce an input stage to your amplifier.

You may want to seek out this link: Music Angel 300B

You will find description of a 300B amp using 6SN7 as SRPP driver and input stage. He is using an input stage based on 6H1P due to space restrictions. It is my understaning that this tube may be substituted for a 6SN7.

Lars
 
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