6AQ5/6005 12AX7 Amp

[briansune]

Here is the new mini amp for a small speaker. Feedback is welcome, learn for each others!
Schematic is welcome to share too~

 

[Fitero]

Very nicely put together amplifier briansune.

I also like that it's discrete, muted styling.

 

[briansune]

Very nicely put together amplifier briansune.

I also like that it's discrete, muted styling.

Thanks, got those tubes from an old store GE 6AQ5 & RCA 6x4

 

[nikon f]

Can you post the schematics?

 

[briansune]

No problem, but it will take me fews day cause i am so busy, and really sorry for missing the reply!

 

[MrCurwen]

What's the reason for the 100 ohm resistor below input stage RC bias system? It looks like it's an input node for gNFB, left unused.

 

[briansune]

You are right the 100 ohm is used for NFB, but only if the amp turns out to hum or for some case e.g. 16ohm 32ohm speaker.

I always make a NFB path in case it is needed.

 

[briansune]

Dear peoples,

After changing a new speaker i found out there is an obvious 60Hz hum.
Not sure why.

Can any one helps?

I replace the output tube with a pentode configuration.

Thanks

 

[Palustris]

I would suggest you re-evaluate your grounding scheme. It looks like the input RCA jacks are grounded to the chassis rather than the ground buss; they should be isolated from the chassis. I also don't see a connection from the ground buss to the chassis ground. It appears that only the power transformer is connected to the chassis ground.

 

[kodabmx]

Also keep in mind the PSRR of a typical SE stage is 6db... virtually any PS ripple will appear at the output. I've built many amps using the chassis as ground without hum, but I use aluminum and DC heaters.

 

[briansune]

I would suggest you re-evaluate your grounding scheme. It looks like the input RCA jacks are grounded to the chassis rather than the ground buss; they should be isolated from the chassis. I also don't see a connection from the ground buss to the chassis ground. It appears that only the power transformer is connected to the chassis ground.

I recall my memory that the chassis do isolated from the signal ground. If so what is the highest possibility that might cause such issue?

Thanks

Also keep in mind the PSRR of a typical SE stage is 6db... virtually any PS ripple will appear at the output. I've built many amps using the chassis as ground without hum, but I use aluminum and DC heaters.

I do leave a feedback resistor for future hum solution, do i really need to add feedback resistor to suppress the hum? (Not really wanna add FB resistor to tube AMP).

Thanks

 

[kodabmx]

Short the input to ground. Does the hum go away? if so it's a loop, if not it's power supply hum.

 

[briansune]

Short the input to ground. Does the hum go away? if so it's a loop, if not it's power supply hum.

OK, after your suggestion, i am quite sure it is a power supply hum.

R-C tank issues?

And thanks for the analysis method.

 

[briansune]

Short the input to ground. Does the hum go away? if so it's a loop, if not it's power supply hum.

Problem still questionable.
Just do some research and not sure does the hum freq. tells the tales?

From video "Two Types of Tube Amplifier Hum and How to Determine the Source" - YouTube

What do you think?

 

[kodabmx]

60/50Hz usually a loop, 100/120Hz power hum.

 

[briansune]

60/50Hz usually a loop, 100/120Hz power hum.

Would it possible that i do have bad tubes as video mentioned.

My transformer do have a heater with 3.15 * 2 center tapped.
In general the center tapped should leave it unconnected.
Should i also ground the center tap to common ground or not?

Or it won't affect the noise?

 

[kodabmx]

You can ground the centre tap and see if that helps. Most people do it like that, I use DC. Also sometimes the hum can be induced from the transformer being too close to the circuitry/tubes. I've switched to SMPS power for most of my designs, but on those where there is an EI core transformer I usually build a separate unit, and use a cable to connect them of about 1 meter. It could be the tubes, but it probably isn't. Some tubes have bad hum from AC heaters no matter what. Others are dead quiet. Some heaters are spiral wound, that helps mitigate the problem.

 

[briansune]

You can ground the centre tap and see if that helps. Most people do it like that, I use DC. Also sometimes the hum can be induced from the transformer being too close to the circuitry/tubes. I've switched to SMPS power for most of my designs, but on those where there is an EI core transformer I usually build a separate unit, and use a cable to connect them of about 1 meter. It could be the tubes, but it probably isn't. Some tubes have bad hum from AC heaters no matter what. Others are dead quiet. Some heaters are spiral wound, that helps mitigate the problem.

Really thanks for the details sharing. =]

I would like to generalize some of the information after some measurement.
(1) - After power up the Amp. the hum exist, by probing some of power rail indeed it is a 50-55Hz hum.
(2) - I try to measure different voltage points i.e. B+ B+' output of the pre-amp etc. voltage off the expected schematic. (Voltage get lower than expected)
(3) - Try to adjust the volume, the hum do have relationship after the volume is turned into half or above.
(4) - Without inserting other tubes just the rectifier tubes, the B+ do have 300V


Base on these info. not sure where i start my modification.