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
thanks
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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.
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.
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.
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
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
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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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.
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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At 240v seems low for a 300b. Most projects I've seen have the voltage around 340v
Hmm, what voltages do you get on the 300B?
6SN7 SRPP is a terrible choice for driving a 300B, but YGWYPF.
6SN7 SRPP is a terrible choice for driving a 300B, but YGWYPF.
You could get two of these:
1pc 12AX7 12AU7 to 6SN7 6SL7 Tube Adapter Converter Socket 9pin to 8pin Adaptor | eBay
Then you could use a 12at7 which may be just the right amount of gain here.
1pc 12AX7 12AU7 to 6SN7 6SL7 Tube Adapter Converter Socket 9pin to 8pin Adaptor | eBay
Then you could use a 12at7 which may be just the right amount of gain here.
A 12AT7 SRPP driver is a much better choice, but it's unlikely to work properly with the resistors currently used.
Yup, too bad the schematic is unreadable. Here is a schematic for a 12at7 driver => 300B. Scroll down the page you will find it. Audio Asylum Thread Printer
Please read the voltage on last cap of Power supply circuit.
The use of 6SL7 is reasonable with the bias current at about 5-7 mA.
Leave the 1k cathode resistor, put the 6SL7 and check the voltage on both cathode
Walter
The use of 6SL7 is reasonable with the bias current at about 5-7 mA.
Leave the 1k cathode resistor, put the 6SL7 and check the voltage on both cathode
Walter
I measured the voltage on the 300b, it's 403v at the anode ,333v anode to cathode, 69v cathode to earth and 418v B+. Is this ok?
I put some 6sl7,s in and it's still working so I measured the voltage from pin 3 to earth is 178 v and pin 6 to earth is 1 volt. Is this good?
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