The coupling cap at input was initially after the potentiometer. That's usually what I do for my amp builds. The coupling cap keeps the little bit of DC from the grid away from the pot. The DC can eventually cause scratchiness in the pot. That being said, I have a stepped attenuator in...

Nah, I just had a couple of good 4.7uF caps lying around. If you have good low ESR electrolytic, then 0.47uF is likely all you'll need. I listened to a bunch of different value caps and couldn't really tell a difference, so I kept the 4.7uFs in there as I didn't have any other applications for...

That's the stepped attenuator that I used. It worked great. Well balanced, no appreciable noise. I used that attenuator because I had previously swapped the cheapo pot out, and it fit the original noard. Without a PCB, there's no constraint on the pot that one can use.

The filament voltages were right around 6.3 volts, but the mains voltage was 123V at my house. Undervolting filaments definitely reduces tube life. I saw a graph a while back showing that undervolting is worse than overvolting, but I can't find it again. As for delaying the high voltage...

As far as hum reduction goes, elevating the cathode of the EF95, the voltage amplifier, is more important. However, elevating the cathode by only 5 volts or so is kind of silly as it won't achieve the desired goal. Not elevating the upper triode of the 6N6 is probably ok, as the voltage is...

I wonder if the power ground is separate from signal ground. I didn't check on the PCB before I trashed it. The case is anodized, so it's going to be insulated. I found a picture of an unmodified amp (cribbed it from the the original LD MkII thread). I added the legends so you know where to...

Yep. From the factory, the heater center tap for the 6AK5 tubes are elevated above ground. It was only 5V or so iirc. The heater elevation reduces leakage from the heaters to cathode, and reduces hum. The 5 volt was a bit too low. Usually you want about 30V above the heater potential.

The Little Dot 1+ is a totally different animal than the Mk II, and I have no experience with it. I don't fool around with solid state stuff much, so I can't comment on it based on the pictures. Sorry.

There are 2 different 6.3V windings in the transformer, but they supply the EF95 and the 6N6P separately, so the left and right channels should get the same voltage.

Try swapping tubes between the channels. First do the EF95 and then the 6N6P. Other than the tubes and the volume pot, there shouldn't be anything else in the circuit to cause the imbalance.

I used a 100k SMD stepped attenuator. Search "DACT stepped attenuator" on e--y. I think the original value is 10k.

I built this for my kid in college. I trust him not to do anything stupid, but not his friends, who may plug who knows what into the input. I remember my college days and all the stupid things I did. It's insurance. :smile_phones:

With a total current draw of about 36mA, I'm not sure how much benefit a beefier power supply will provide. But just for fun, I ran it through the Duncan PSU designer and came up with this: The 6X4 rectifier is electrically the same as the Chinese 6Z4, but different pin-out. The 220 ohm...

Word of warning:. Even though this is a little amp, there are some lethal voltages, especially with the trio of the 330uF caps. Make sure you read up on the basics and safety of tube amps before trying anything. Never probe around with your fingers when the amp is on. When it's off, be sure...

It was definitely for fun. I didn't have the original to do a direct comparison. I can't go by memory, because it has been a few years since I had listened to the original amp, and I didn't bother to fire it up before I tore it apart. The main cost of an amp build is in the transformers...

Doh! I cleaned up my Mouser project list last week, and deleted a bunch of them, this one included. However, all the components are in the schematic above. Starting with the power supply, the 2 180 Ohm resistors are Ohmite 45F180E, wirewound, rated at 5 W. The filtering caps are United...

Without the 220k NFB resistor, you will have an additional 3.4dB of gain, at a cost of slightly decreased bandwidth, slight increase in harmonic distortion, and a more tubey sound.

Nope. I'm not paying that kind of money for basically no difference in sound. The output section is a White cathode follower, basically there to match the output impedance to headphones. The 6H30 (or 6N30, if you will) tube earned it's reputation as a driver for power amplifiers, a completely...

This is an excellent resource for soldering specifications: http://www.sal.wisc.edu/docs/Soldering%20Basics.pdf

All done. I revised the negative feedback scheme with a set 220k resistor and switched in 330k and 100k, for paralleled resistance of 132k and 68.8k. With only the 220k resistor there is an attenuation of -3.4dB, with 132K, -5.4dB, and 68.8k, -9.2dB. I have nicely matched Voskhod 6J1P-EV...

Well, there's the input cap, followed by 2 RC filters before power is taken off. The total capacitance is 990uF. For a low powered tube amp, this should be plenty. I'll figure out the ripple voltage after I finish the amp and measure the power draw. I decided to add back in the variable...

I forgot to mention, with pins 2 and 7 of the input stage tied together, the following list of tubes can be swapped in: 6AJ5, 6BH6, 6AK5(5654), 6AG5(6186), 6AU6, 6BC5, 6CB6, 6DE6, 6AH6, 6DK6, and 6EW6 (listed in order of gain from low to high). There are probably other tubes that can work, but...

I got my Little Dot Mk II several years ago. After playing around with it for a while, and totally screwing up the PCB with lifted pads and such ('cause I didn't know what I was doing), I put it away on the shelf. In the meantime, I got into building tube guitar amplifiers, then tube hi-fi...

Cribbed the specs from Topping: Looks pretty good. Less than half the power of a Jotunheim, though, and the headphone output impedance at 20 ohm may not play well with my M1060's, but should be fine with the HD6xx. At $399, it's the price of a base Jotunheim, but with a balanced DSD DAC...

Yep, got an email from topping that the DX7 is up on their site. http://www.tpdz.net/en/products/dx7/index.htm The op amps are all surface mount, so swapping them out is going to be a pain. Gonna wait to get one from Amazon.