Dual 2061-ish, troubleshooting

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goldenGeek
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Dual 2061-ish, troubleshooting

Post by goldenGeek » Mon Nov 20, 2017 5:43 pm

I've spent a few days on putting together an amp for a friend. Its basically a 2061 with EL34 on relatively low voltage into an under-dimensioned output transformer (20W). Its got 2 channels, one is the lead channel from the 2061, the other is based the Swank Motee-mod (lead channel cascaded into bass channel), but its a totally separate channel. I didnt bother to type up the schematics, but I did some layout preparation (theres at least one error in the attached layout, the pots says LIN, but in reality it should be LOG). So far I disconnected the bright cap on the cascaded channel, it was way too ice-pick. But theres some issues going on, maybe someone can help? There seems to be some weird (subtile but hearable) tremolo-effect going on at some frequencies, especially open A-string. Theres also some weird harmonics going on in the background at, for instance, overtones at the 5th fret. It sounds rather disharmonic. Finally theres a loud hum (probably 50hz) for a while after turning it on, but it goes away after 5-10 minutes of playing. I'll also upload a gut-shot, my lead dressing is far from perfect, maybe someone can give some hints on improving the wiring?
Warhuus-testrender.png
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622CCC8F-33CD-4ACA-8AB9-515148AB4896.jpeg
5534E129-8C0B-4B4D-B4B6-9994C8FDBFC0.jpeg
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xtian
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Re: Dual 2061-ish, troubleshooting

Post by xtian » Mon Nov 20, 2017 8:17 pm

The symptoms you mention all sound like poor filtering. Those 50/50 cans look sufficient, but something weird is going on.
I build and repair tube amps. http://amps.monkeymatic.com

goldenGeek
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Re: Dual 2061-ish, troubleshooting

Post by goldenGeek » Mon Nov 20, 2017 9:36 pm

I did not actually check the cap cans before installing, they are all brand new, but maybe one or more sections are defective?

Another thing I got to think about - the cathode resistor on the output section - should I maybe up that quite a bit? I didnt actually change it from when the schematics was for EL84s, Its a 120R at the moment. Could this cause some of these problems also? I just tried to switch off the light in the room while plying and I noticed the tubes lighting up bluish when idle, when playing the blue colour goes away. Does that give any hints?

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xtian
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Re: Dual 2061-ish, troubleshooting

Post by xtian » Mon Nov 20, 2017 10:28 pm

goldenGeek wrote:
Mon Nov 20, 2017 9:36 pm
I did not actually check the cap cans before installing, they are all brand new, but maybe one or more sections are defective?
Easy test is to clip in (literally--you can use alligator clips) an additional 47uF in parallel with the existing reservoir caps to see if it help with the motorboating, ghost notes, and hum.
goldenGeek wrote:
Mon Nov 20, 2017 9:36 pm
Another thing I got to think about - the cathode resistor on the output section - should I maybe up that quite a bit? I didnt actually change it from when the schematics was for EL84s, Its a 120R at the moment. Could this cause some of these problems also? I just tried to switch off the light in the room while plying and I noticed the tubes lighting up bluish when idle, when playing the blue colour goes away. Does that give any hints?
Faint blue glow is normal. To answer your question about the cathode resistor value, you have to measure the idle plate dissipation. Do you know how to do that?
I build and repair tube amps. http://amps.monkeymatic.com

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Re: Dual 2061-ish, troubleshooting

Post by Stevem » Tue Nov 21, 2017 12:51 pm

There's only a hand full of things that will make for 50 or 60 hz hum, in no peticular order here they are, or atleast the ones I can recall off the top of my head!

Output tube matching no close enough.

Not enough filtering / bad filters / more than 2 gain stages on one filter node.

Heater wiring on output tubes not right.

Bad preamp tube.

Output and power transformers not faced right from each other and AC flux is coupling into the OT.

PT center tap no good or its chassis connection is no good.

If no center tap then 100 ohm balance resistors bad , or heater balance pot bad.

Output tubes biased too hot and plate(s) glowing some level of red.

Grounding layout not the best.

In regards to the grounding layout bussed grounds like on your control pots can be a issue to some level or not, but since you amp is simple without many ground points I would convert over to a star grounding layout.

If you pull your output driver tube does the hum go away?

In regards to your other issue you might try reversing the two output tube plate leads to see if the amp gets better , or if it screams like stuck pig and drives you out of the room , lol!

Your comment that the hum subsides after a bit would point me to the output stage.
Most times with good Amps just like it is with sex, it's best to just plug straight in!

goldenGeek
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Re: Dual 2061-ish, troubleshooting

Post by goldenGeek » Tue Nov 21, 2017 4:05 pm

Thank you :) Lots of good advice here.

First I will check the idle current and make the cathode bias okay.

Maybe I should implement another filter cap? Both preamp channels (3 gain stages in total) share the same filter node at the moment.
As for the star ground tip - should I run all ground points at separate cables to a my commond ground point? I have done this kind of bus-wiring on many amps before without any issues, but you never know.

I have checked some of the things you pointed out, but I'll go over it again. As for the hum that goes away after a while, its most certainly in the output section. If I roll off the master volume (which is just a simple pot cancelling out the signals) or pull the pi-tube the hum doesn't change.

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pompeiisneaks
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Re: Dual 2061-ish, troubleshooting

Post by pompeiisneaks » Tue Nov 21, 2017 4:16 pm

I may be off kilter, but I thought the only source of 50-60hz hum was either heaters or incoming power being too close to signal wires? Any other hum would be 100/120hz because it's been through the rectifier right?

I once watched one of Uncle Doug's videos where he lets you hear the difference and it's pretty obvious which is 60 and which is 120.

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Stevem
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Re: Dual 2061-ish, troubleshooting

Post by Stevem » Tue Nov 21, 2017 5:18 pm

Yes, sorry!
Anything connected up that is under D.C. Voltage after the recto would be making 100 or 120 hz hum, everything else I posted still stands.
Most times with good Amps just like it is with sex, it's best to just plug straight in!

Stevem
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Re: Dual 2061-ish, troubleshooting

Post by Stevem » Tue Nov 21, 2017 5:20 pm

Yes, sorry!
Anything connected up that is under D.C. Voltage after the recto would be making 100 or 120 hz hum, everything else I posted still stands.

Since you have 3 stages on one node I would add another filter section before your last stage with a 1K ohm dropping resistor before that new filter section.
Most times with good Amps just like it is with sex, it's best to just plug straight in!

goldenGeek
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Re: Dual 2061-ish, troubleshooting

Post by goldenGeek » Wed Nov 22, 2017 1:00 pm

I added another filter for channel2, I had a 22uf around that I used, and the tremolo effect is gone. The disharmonic ghost notes is still there. I also changed the cathode resistor to 270R. However, this design doesnt work really well, the two channels work fine by them selves, but theres a problem matching levels. So I re-drew the layout and made the 2nd channel more 2203/2204-ish. I also incorporated a few other parts from the 2204. Do you think this will work out better, so that I'll get some more drive from channel2? Originally most of the overdrive comes from the PI-tube. Oh, I also put the whole tube for channel 1 in parallel, almost like in the Matchless Spitfire. I've done this a few times before and it usually sounds a bit fuller. I will be offline for some days, but I'll post back and also add some soundclips eventually when I get around to incorporate the latest changes.
Warhuus_v2.png
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statorvane
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Re: Dual 2061-ish, troubleshooting

Post by statorvane » Sat Nov 25, 2017 12:44 pm

One thing I've never understood abut the 2061X (and the other Marshall 20 watt PA amps) is the power tube grid resistors are on the board; not attached to the tube pins. I'd try moving the 2K7 resistors directly to pin 5. On the bigger Marshalls, this is usually a 5K6 resistor.

Also, it is not clear to me if you are trying to operate both channels simultaneously. If you do, they are probably out of phase.

Good luck with this; a very interesting project.

Stevem
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Re: Dual 2061-ish, troubleshooting

Post by Stevem » Sun Nov 26, 2017 12:05 pm

I ment to post that question!
If both of your channels do not have the same number of gain statges then they are out of phase to each other and will sound like crap unless you like them to sound like the pickup out of phase position on a Strat, or other type guitar!
Most times with good Amps just like it is with sex, it's best to just plug straight in!

goldenGeek
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Re: Dual 2061-ish, troubleshooting

Post by goldenGeek » Sun Nov 26, 2017 12:46 pm

The two channels does not operate at the same time, so the phase reversal shouldnt be of any problem. As for the resistors, I drew up another resistor on the tubes, but Id might remove the 2k7 and replace the ones at the tube sockets as well with about 5k as you said.

goldenGeek
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Re: Dual 2061-ish, troubleshooting

Post by goldenGeek » Mon Dec 04, 2017 10:59 am

So I did some changes and its finally coming together as a good sounding amp. However, the hum is still present at times. I havent measured, but I think its 100hz hum. Its in the power amp section though, pullig the PI-tube ot turning down the master doesnt cure it. Last time i fired it up it was okay for a little while, but all of a sudden the hum came back. What can this be? I guess my ground-bus, that someone suggested, cant cause this since the hum is not present at alle times? There is a ground/shield wire coming from the power transformer. I soldered that to ground, but is it possible that theres a ground loop present there from the mounting screws? Im all out of stuff to try. Its a pain to troubleshoot since the hum is not constantly there.

Heres a picture of the amp so far:
sw-3.jpeg
Edit: Heres a sound clip: https://www.dropbox.com/s/192pnnbc2n7t8 ... 2.mp3?dl=0 Its just recorded with one mic in the middle of the room so its a bit boxy/roomy, but I hope you get the picture how it sounds. First section is channel B (JCM800-ish) then theres a section from channel A with gain and tone rolled off a bit. Finally theres a section with channel A with all controls at 11. I used a Gibson SG 1967, the speaker cab is an empty combo cab with a Marshall/Celestion Vintage 30. I'm quite happy with the sound so I must figure out this final hum issue and then I can ship it off to my friend :D
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goldenGeek
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Re: Dual 2061-ish, troubleshooting

Post by goldenGeek » Mon Dec 11, 2017 5:43 pm

Oookay, so I figured out the hum. I forgot to tie pin 1 and 8 together (edit: on the output tubes)... :oops: But that's not the worst thing - I did this error a few years ago as well :oops: I hope I have learned my lesson one last time.

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