• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

New project, Damn Good 300B amp.

I haven't noticed any distortion beyond what I measured and posted on my website (about halfway down here: Damn Good 300B (DG300B)). Without measurements I can't really comment on what you're experiencing.

An incorrectly wired OPT will certainly cause distortion. You'll need an OPT that'll handle about 800 V peak-to-peak swing and that can handle the 85 mA bias current. It'll need to load the 300B with 5 kΩ and have a primary inductance beyond 10 H. A 15 W transformer should be enough.

Tom
 
As described I have confidence in the design and after being through the driver board I am also confident that there is no errors made regarding resistors caps and so on. I have also reheated all solder points to eleminate any soldering errors.

I will have the amp up running by tomorrow again to take some measurments but since I don't own an oscilloscope I have no possibility to "see" the signals.....:(:(
I am also trying to borrow a pair of verified 300b tubes to eliminate the fact that mine can be faulty but I highly doubt it, but just to be shure.

First time I have come over papers about my Shuguang Treasure 300b-Z tubes was now in the weekend searching the internet and since I have no clue at all how to interpret those papers, maybe I could get some professional help understanding what they say.

shuguang treasure tube spec 300B-Z-vacuum tube-0010.jpg shuguang treasure tube spec 300B-Z-vacuum tube-008.jpg

It can be that I am way off when it come to what voltages and Bias I am running but before I took the amp apart I was running B+ at 410V and Bias was set to 85 mA. I will check tomorrow if this is still valid and then I also have a question, mostly directed to Tom but maybe anyone else have been buildning this Damn Good 300b and have the answer.

If I want to adjust the Bias when B+ is set to 410V I can't go below 83 mA cause there is no margin on the trim pots. The Bias depend on R12, R13 and R14 in the schematics and if I want to be able to go lower on the Bias, which resistor should be changed to get the required adjustment??
I suppose that it should be R11 in the schematics but do I change for a higher or lower resistance??
 
The B+ should be 400 V not 410 V. I doubt that makes a dramatic difference, but it could impact your ability to adjust to the correct bias point. I'm able to get down near 55-60 mA in my builds and consider that plenty of margin for an 85 mA target. I use JJ tubes.

The Shugang tube data sheet says "70 mA max for fixed bias", so stay below 70 mA. I would dial the B+ down as far as it goes and set the tube at 65 mA to start. That won't give you much output power, but you should at least get 4-5 W before clipping. I would expect it to sound good at low volume settings.

Many bias the 300B at 360 V, 60 mA for a low power option. I'd do that for older 300B tubes. The more modern ones can handle higher voltages.

Which output transformer are you using?

Tom
 
Are you sure is well wired for SE 90mA?

Yes, I have talked directly to Per Lundahl here in Sweden and we concluded that the OPT are just the ones that I should use and yes, I have wired them for SE, not PP according to page 4 in your link.

Now I have been down putting things together again and everything is working fine, all measurments are, if not spot on, just close to what Tom have described in his build manual.

As Tom suggested I have now wound down B+ to 375 Volt and adjusted Bias to 700 mV corresponding to 70 mA. When I connect my CD player directly to the amp I have still distortion and as an alternative I connected my Android phone directly to the amp and..... Ahhh, sweet music without any kind of distortion.

Yes, I don't get the same level of music (as Tom described earlier) but it is absolutely free from distortion and I also seem to get a wider sound stage when the music is playing without tendency to distortion.

I measured the AC signal from my phone and I get on average barely 0.1 Volt AC on the signal out while I have approximatly 0.9 Volt AC signal from the CD player so it seem to be something spooky with my signal into the amp. The amp should manage over 3 Volt without clipping so I don't know what is wrong.

So now is the question, what to do?? Where do I start searching and what can be changed. I have 220 nF caps on input, should I start messing with these?
 
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At the moment I am trying out in the basement where I run the amp without pre amp but later I will run it with a Aleph P1.7 in the living room. I also have better speakers there with sensitivity around 98 dB so I will still be able to enjoy music even if I can't access more than 3-4 Watt.

Though I would like to fix this cause according to build manual I should get approximately 8 Watt and that would be awsome.
 
I manage with 3-4 Watt but since my listening room is fairly big I could use the extra watts.
I usually sit about 4-4.5 meters away from the speakers and the speakers are parted about 3.8 meters.

Question, I have information that maybe I could use a passive pre amp to just get the signal level down a little. Is this a way to go??
 
You can't expect from 300B 8 watt with distortion free.

I measure 10 W at 3 % THD, 50 Hz on my DG300B. 10 W @ 1.5 % THD (1 kHz). 1 W @ 0.2% THD (1 kHz) if I recall correctly. You're right that some don't get more than a few watt at 5% THD, but that's not the case with the DG300B.

Tom

Question, I have information that maybe I could use a passive pre amp to just get the signal level down a little. Is this a way to go??

Passive preamp (aka volume pot). I'd try that between the CD player and the amp. If the sound is distorted at all volume settings, I bet you have an issue with the grounding between the CD player and the amp.

Usually people have the opposite problem. They need a preamp with gain to get enough power out of the DG300B as it is a low gain amp.

Which input tube are you using? Which biasing scheme? How many LEDs are installed?

With the input transformers installed and E88CC tubes for the inputs, you'll need 3.9 V RMS to drive the amp to clipping. That's with the input transformers. With capacitive coupling, you'll need 2.8 V RMS. Most CD players deliver 2 V RMS.

Tom
 
I bet you have an issue with the grounding between the CD player and the amp.

Which input tube are you using? Which biasing scheme? How many LEDs are installed?

Tom

I doubt the grounding problem since it occure both when running two different CD players or connected to the DAC. On the other hand the wall plugs aren't grounded if that would affect things.

I am using the 6N6P and I have checked so that I am using jumpers in the right place. I can also mention that I have three sets of 6N6P tubes and all of them behave the same, so my thought is that either I have a problem with the 300B tubes (clipping, broken or whatever) or it has to do with how the signal is entering the driver pcb via the input tubes. My problem is that I have too little knowledge about electric sircuits to know which way to go from here.

At the moment I have three green LED:s per channel (as per BOM) but on the other hand I never measured them before installation so in principle I don't know exactly how many volts goes that way.

Now when you mention the input tubes, is it possible that clipping is occuring in the input tubes and that the 300B only amplify the clipping (if you understand my idea (seem like a reasonably possible idea also to me))??

If so should I try other LED:s (I have red and yellow at home) or eventually try removing/adding to the LED:s?? I know from some Salas PSU:s that biasing with LED:s can sometimes be tricky and require rather much fiddeling between colours and sometimes changing one red for another red because you want that last decimal to be correct, to get the exact voltage you are aming for.
 
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Without schematic and measurement voltage points the only person can help you is Tom. Always you can send back the PCB to Tom to check if it's OK.

I measure 10 W at 3 % THD, 50 Hz on my DG300B. 10 W @ 1.5 % THD (1 kHz). 1 W @ 0.2% THD (1 kHz) if I recall correctly. You're right that some don't get more than a few watt at 5% THD, but that's not the case with the DG300B.

Tom

Nice figures:)
 
I am using the 6N6P and I have checked so that I am using jumpers in the right place.

You're more than welcome to email me a picture of your board if you'd like me to double-check for you. Take my user name here and add @neurochrome.com for my email address.

At the moment I have three green LED:s per channel (as per BOM) but on the other hand I never measured them before installation so in principle I don't know exactly how many volts goes that way.

Three LEDs is correct for that tube. You should have 5.5-6 V from pin 8 of the 6N6P to ground on both input tubes. You can find this voltage on the cathode of the footprints of D2 and D7 as well.

I'd also measure the following voltages:
Input tube, pin 1 (85 V).
Input tube, pin 3 (-80 V - this depends a lot on the 300B tube, so don't be alarmed if it's off by a bit).
Input tube, pin 6 (180 V)

Figure ±10% tolerance on these values. Maybe a bit more. You'll need to measure after the amp has warmed up fully. Be careful with the DMM probes as a probe slip can be devastating (for both you and the amp).

Make sure R8 = R22 = 470 kΩ. See the notes on the schematic on page 33, 34 of the DG300B doc.

Now when you mention the input tubes, is it possible that clipping is occuring in the input tubes and that the 300B only amplify the clipping (if you understand my idea (seem like a reasonably possible idea also to me))??

Yep. That's entirely possible.

If so should I try other LED:s (I have red and yellow at home) or eventually try removing/adding to the LED:s??

I suggest measuring the voltage across the LEDs first.

I know from some Salas PSU:s that biasing with LED:s can sometimes be tricky and require rather much fiddeling between colours and sometimes changing one red for another red because you want that last decimal to be correct, to get the exact voltage you are aming for.

The 6N6P is not that selective about the LEDs, but you do want the voltage to be about right. If you've used the HLMP-3507 I specified, you shouldn't have to mess with the LEDs. It should be plug-n-play.

Tom
 
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Three LEDs is correct for that tube. You should have 5.5-6 V from pin 8 of the 6N6P to ground on both input tubes. You can find this voltage on the cathode of the footprints of D2 and D7 as well.

I'd also measure the following voltages:
Input tube, pin 1 (85 V).
Input tube, pin 3 (-80 V - this depends a lot on the 300B tube, so don't be alarmed if it's off by a bit).
Input tube, pin 6 (180 V)

Figure ±10% tolerance on these values. Maybe a bit more. You'll need to measure after the amp has warmed up fully. Be careful with the DMM probes as a probe slip can be devastating (for both you and the amp).

Make sure R8 = R22 = 470 kΩ. See the notes on the schematic on page 33, 34 of the DG300B doc.
Tom

Since I have picked up a some info here on the forum about being thorough and careful when it come to high voltage I did some preparing by sharpening the points on my DMM and I also applied some shrink tube on them to be on the safe side.

20180907_173714.jpg

Ok, after some little pause with my Damn Good 300B I have finally been making the asked measuring.

R8 and R22 are 470 kOhms, Check

Pin 8 is measuring 6.33 Volts
Pin 3 is measuring -74.8 Volts
Pin 6 is measuring 168.8 and 170.4 Volts

So far, so good but, and here come the big but(t)
Pin1 is measuring -4.85 Volts.....???:eek::eek:
well only 90 volts wrong in the wrong direction so what now....???:confused:

By the way Tom, you have mail with some extra pictures
 

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Pin 8 is measuring 6.33 Volts
Pin 3 is measuring -74.8 Volts
Pin 6 is measuring 168.8 and 170.4 Volts

Those are all normal.

So far, so good but, and here come the big but(t)
Pin1 is measuring -4.85 Volts.....???:eek::eek:
well only 90 volts wrong in the wrong direction so what now....???:confused:

By the way Tom, you have mail with some extra pictures

In your email you mentioned that R17 (100 Ω) measures 100 Ω. Good. Do you have 90-91 V on each end of R17? If not, your issue is around D11, R29, and D12.

If you do have 90-91 V on each end of R17, measure the drain voltage on Q3. That's the centre pin (which is also connected to the metal back of the device). Be very careful here as a probe slip can be fatal (both for you and the amp). You should see 400 V at the drain of Q3. Then measure the source voltage of Q3. That's pin 3 (pin 1 is marked with a square pad; the pad for pin 3 is round). You should have 85 V.
If all this checks out but you don't have 85 V at pin one of the driver tube socket, you have a bad connection on the board.
If you have 90 V on the gate (pin 1) of Q3 but not 85 V on the source, Q3 is dead.

I'd reflow the solder joints to pin 1 on the driver tubes and to Q1/Q3.

The symptoms you describe would occur if D12 is reversed. I'd double-check that you have it installed correctly. If D12 is reversed, you'll have around 0.7-1 V on the gate of Q3.

Tom
 
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Done and I am happy!

After some.... no, a few or...... rather, some many emails between Tom and me things are done.:cool:

Turned out to be a diode that was wrongly delivered from Digi Key and with my lack of experience in this kind of builds (at the time, I have learned a lot after that but apperantly not enough to resolve this problem) that had killed the Mosfets feeding the anode of the input tubes so they have never worked as intended.

Now the diode and the Mosfets are swapped for new, good ones and the Damn Good 300B is nowdays sounding...... Damn Good!!!

I was dissapointed cause it did not sound as good as my Hiraga Le Monstre (not original yet but will be, in time) but that I can assure, now it does and in many ways, far better.

The amp has a resolution of those small details that is really amazing and the musicality of the amp is.... complete. The room and the sound stage is soooo present and you can really see the musicians sitting/standing in front of you. You know when you get goose bumps just from listening to music.....:nod::nod:

Yes it could have a little more punch in the lowest region but still it produce some really nice LF when it come to acustic instruments and a cello or contrabass sound very present and real. Mid- and HF are just amazing and listening to Norah Jones or Diana Krall...... is pure magic!

As said, a Damn Good amp and now I can and will enjoy it to the fullest cause I can't hear any tendency to the kind of distortion that I experienced earlier, it is just wow-music!

Thank you for all your help and a special thnx to Tom who have put up with me and my trial and errors, now I will be occupied listening to music for real!:D:D
Thank you again!