DIY Audio Projects Forum

Just finished building my amp! I had a couple mistakes starting up, but debugged and got it working.

However I fear there's some major mistake somewhere... I'm reading 280V at both B+ locations with the amp fully powered (not for long, I'm afraid of running it...). I've checked power supply wiring and all seems correct, but I'm not sure where to look next. It does amplify and music does come out of the phones, but it seems very distorted especially at higher volumes.

Where might I look or check? No idea where to start...

Hi, there is supposed to be a fairly large resistor bringing the voltage for the tube anodes down to about 135-150. Measure the voltage on one of them and see if it is OK. On start up and until the tubes conduct (about 30-45 seconds) it will be higher. If there is distortion then there is something else at fault. You didn't say waht phones you are using and the impedance and power needs of them. It could be a mismatch.

Good listening
Bruce

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Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Barring any major wiring faults, it sounds like maybe your tubes aren’t drawing current. You should be able to run the amp without damaging it if the caps are rated higher than 280V, so make sure the wiring is correct and then power it on. Check the voltages at the cathodes (pins 3+8) to make sure your ccs is on. It should be a couple volts I believe.

Thanks -- I'm away at the moment but triple-checked the wiring, and all appeared correct.

The headphone impedance is a good lead—I was testing with cheap headphones of unknown impedance. They are likely very low. Would that impact the voltages seen as it's effectively the load?

I'll check the CCS next, and use the 75Ω phones I have now that I am at least confident they're not going to be damaged, since everything else appears to be working fine and the 'music is flowing' all things considered.

Cheers and thank you.

Ah yes I did assume you were measuring voltage at the center tap of the transformer primary (pin 2 of the WSM Edcor I believe). That should be ~135 as Bruce says, but the voltages earlier in the supply do get quite high, such as coming off the rectifier diodes.

Cheap headphones - probably 32 ohm - would probably be too low for even the 150 ohm center tapped Edcor transformer, so it makes sense that you’d get distortion at high volumes. But since they’re on the secondary side of the xformer (thankfully!) they won’t affect the DC operating point of the circuit.

Basically, everything might be ok. But yes if the voltage is high at the tube plates, it means they aren’t drawing any current

Tried on my 80Ω beyers; almost exactly the same result.

I am measuring voltage at the center tap of the output transformer primary, that's where it's 260V. It's 272V right after the rectifier on the high voltage tap of the PT, which seems right according to the schematic. The voltage across pin 8/3 to ground is 7.3V on one side and 8V on the other, which seems high from the "a couple volts" you noted.

Just tried 2 more sets of tubes, same issue.

Just to make sure I'm measuring correctly, the B+ is measured from the point where it hits that output transformer pin 2 to ground, correct? That's where I'm seeing 260V.

The weirdest part is that it sounds relatively normal... while it doesn't feel perfectly low distortion, it's close. But my meter is showing me 260V on those output transformers...

I'll keep tracing the circuit over and over

Hmm the voltages at both pin 3 and 8 should be the same actually, since they are tied together. That suggests a possible wiring error, or maybe the circuit was warming up as you moved the probe from one pin to the other

Since the voltage there is 7 volts, it could mean that the ccs is drawing minimal current. Check your wiring of the LM317: the pinout is ADJ - OUT - IN facing the "front" of the package with leads pointed down. With a 50-ohm resistor connecting ADJ and OUT, and the output taken at the ADJ terminal, the current through the device should be ~25mA

edit: and by "output" I mean the connection to ground. the "input" of the 317 should be the cathodes of the 6DJ8's

Aha. They are backwards Thank you!

(And by the different pin 3/8 voltages, I was referring to each tube)

Update: 154V on each. Close enough! And the distortion is clearly gone now, the amp sounds wonderful. Thank you so much for the help, I finally found where the pinout on the schematic said "Bottom View" and smacked my forehead Thanks again.

Thanks again for the help. Been listening to it nonstop for the last few days, making the music really enjoyable all over again.

I figured I should post some pictures and story of my build, as it's somewhat unique.

I built this all inside a very old microphone preamp enclosure from the 1940's from General Radio, made down the street from where I live now. I picked it up at a garage sale for $2, already missing some parts, then went looking for a good headphone amplifier design to build inside it, and found this one because I have about 8 6DJ8's in my tube collection.

Build went well. The challenge was fitting everything in the small case (about 7" x 5.5" x 2.75"). I was afraid there would be hum due to the proximity of all the transformers to one another, but in the finished amp it's barely noticeable, mostly only at power-up. Whew. While tightly packed, the whole thing is well grounded and isolated so I feel confident in it. And it's running great and stable.

Currently have it with Amperex Holland orange-globe 6DJ8's now and it sounds glorious. Thank you for the design and for the help, it was a ton of fun and I learned a lot in the process—my first end-to-end build of something new from the ground up, as opposed to a rebuild or restoration.

Full story in this sequence of pics: https://imgur.com/gallery/xiOGB1A

i'm about to order parts.

my Senheiser headphones are 23 ohms. So I found the Edcor output transformers that Bruce used and they are available with 15K/600K with 5.1 turn ratio which matches up exactly to my 23 ohm headphones. If I use that 15/600 transformers, will i need to change anything else?