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I just completed this build and what I noticed right away with the 6L6 tube, it's not as loud as my original 5 Watt Champlifier which I built several years ago. I also noticed when turning the volume knob to just shy of full, it begins to cut out and distorts in a not very pleasant way. I tried swapping the 6L6 and 12AX7 (ECC83) but achieved the same result. Any ideas?

Gary V

Could be a lot of things. Please post clear photos of your wiring and component placement.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

It's also useful to measure the bias of the 6L6. Weber's Bias Calculator calculates it for you when you input how many tubes share the same cathode resistor (1), the (DC) voltage measured across cathode resistor (measure DC volts from pin 8 to ground), the actual resistance of the cathode resistor (should be 470 ohms but measure to check), the DC voltage measured between plate and cathode. (Actual Plate Voltage this is red lead on pin 3 and black lead on pin , then hit 'calculate'. Post your readings and the calculate plate dissipation and current.

http://www.tedweber.com/webervst/tubes1/calcbias.htm

Be careful when measuring the actual plate voltage - that is high DC voltage there, make sure you don't make contact with any metal and don't short the DMM leads to any other pins.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

Yikes, it's a really big photo in preview, but good detail I think. You will notice I added 4 and 16 Ohm 1/4" speaker taps on the side of the chassis.

If possible, shots of each tube socket and the input jack wiring might help.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

I can't quite see what is going on with the OT secondary taps. You said you added 4 and 16 ohm taps, but in the photo it looks like all 3 speaker jacks are daisy chained together. That would cause a lot of havoc. I would disconnect the 4 and 16 ohm taps for now while you're troubleshooting.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

OK, so I removed the 4 and 16 Ohm taps. It is certainly louder, but it still exhibits the breakup and distortion at near full volume.

The voltage from pin 8 to ground on the 6L6 is 28VDC, plate voltage is 357VDC.

The Weber calculator shows 75.6 mA bias. The plate dissipation is 26 watts.

The cathode resister measures at just slightly less than 470 Ohm (unpowered measurement).

Here's some better pictures of sockets. Again, yikes, these are monster photo's on my desktop, but the detail is quite good.

Wiring layout could be better, but I'm not seeing any red flags. Let's get DC voltages from the 12AX7 pins 1, 3, 6, and 8.

Also, try a 6V6 in V2 if you have a known good one.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

It appears when I completely removed the 4 and 16 Ohm speaker taps, things improved. I made a similar mod to my 5 Watt Champlifier without any issues. I guess I will need to figure out another way to achieve this feature.

If you can show me how you did that mod, I might be able to help.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

I connected each of the 4/8/16 Ohm speaker outputs to separate 1/4" jacks and ran a common white wire from the 22K resistor.

I think the solution is to use a 3 way rotary switch but the problem with this chassis is space. A 3 position toggle switch would be a better choice for size. I have always enjoyed mixing amps with multiple home-built speaker cabinet combinations hence my desire for a convenient interface.

Right - you need a switch. By running a common wire to each tap, you effectively connected all three taps together. That's a no-no.

FYI - I've run into this issue with the amp sort of cutting out when you get to full volume on small, single-ended amps before. I'm not sure exactly what it is, but in the past when I've had amps that did that, I felt like it was just overloading the output transformer or something.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

Since I removed the multiple tap configuration, the amp doesn't cut out a higher volumes and seems plenty loud. Clearly that modification resulted in lower power output and cutting out. Thanks for your help Morgan. This amp really does sound sweet. For now I will just play into an 8 Ohm load until I can get some parts together.

The 22k resistor is for negative feedback. If the original Champ amps were 4-ohm output, the nominally correct amount of feedback would be if the 22k resistor was connected to the 4-ohm tap regardless of which tap is being used for the speaker. If the resistor is connected to the 8-ohm tap, negative feedback would be increased by about 1.414 (sqrt of2), and the output stage gain would be reduced somewhat at the same time. For the 16-ohm tap, negative feedback would be doubled from the original Champ. Changing the tap for the resistor probably changes the sound of the amp some too, aside from the different speaker used. However, it's safe. If it sounds good it is good.

The foregoing is simply because impedance ratios in transformers go with the square of the turns ratio, and voltage goes directly with turns ratio. Because the amplifier uses voltage feedback, that's what we need to use for feedback calculations.

Oh boy, what have I done now? After playing this amp for several days, I turned it on last night and heard a loud hum with the volume control at the lowest setting. It then made a popping sound with smoke. On inspection, the 25 uF electrolytic cap, parallel to the 470 Ohm/5 Watt resistor had blown. I checked all my connections and could not see any problems. It worked great for several days. I don't have a spare replacement capacitor but before I do any replacement, I need to figure out what may have caused this failure.

I'd just follow the Marshall approach - connect the feedback resistor to the 16 ohm tap.

I would assume if the 25uF/25v cap off the cathode blew, you had more than 25v coming off the cathode. What voltages are you getting on the power tube pins without the power tube installed?

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*patience is a virtue*

Please do not PM me. email is prefered. keith@buildyourownclone.com

I'm not familiar with the Marshall approach - perhaps I need more explaining.

The only voltage I could detect was pin 3: 466 VDC, pin 4: 435 VDC.

The heater voltage measured fine.

That's very high. Was this after the cap blew? You had 28 and 357 volts before (which is probably why the cap blew).

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

Yes, the measurements were with the cap disconnected - it literally blew the end off.

The amp should run fine but sound a little different with the cap disconnected. If the 470r cathode resistor is still connected it may be damaged, or the tube may be shorted. Pull the 6L6 and measure the resistance of the 470r.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

It measures about 463 Ohm.

Are you suggesting I should reinstall the tubes and try it without the 25 uF cap? I'm concerned about the loud hum I heard right before it went out.

The hum was likely the cap exploding because cathode voltage was too high for it. The cap is there for audio purposes, it does not bias the tube. The resistor does that. You can run the amp without the cap just fine - it will just have less gain.

For now, I would remove the remnants of the cap, run to amp to see if it's stable, and record the plate and cathode voltages.

Then swap out the 6L6 for another on and record voltages again. This might tell us if a tube is bad.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

Some background reading on the subject of cathode bypass capacitors:

http://blog.audioworkshop.org/the-catho ... ZzcPCiGNhG

https://www.ampbooks.com/mobile/amplifi ... capacitor/

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“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

Thanks Morgan. I powered up the amp without the cap and here are the voltages:

pin 3: 387
pin 8: 28

Another brand 6L6 measured only 1 Volt less on the pin 8.

It looks like I need to replace the 25uF cap with a higher voltage version. Keith sent me to this website and I see a couple of options for cathode bypass cap replacements:

http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Capacitors&ORDER_ID=713709899

..about 3/4 down the page are Sprague and Nichicon 50 Volt rated caps. The Nichicon is 22uF which I presume would result in a treble boost, as noted in the documents posted by duhvoodooman (thank you).

Any advice would be much appreciated. Thanks.