86-Special ready with noise problem

[Tillydog]

In case it might help I did a short clip of the buzz problem.

That almost sounds mechanical to me - are you sure it isn't your speaker cab?

Failing that, I think it's oscillation (I think there's a phaser-y quality to it in places).

[lord preset]

In case it might help I did a short clip of the buzz problem.

That almost sounds mechanical to me - are you sure it isn't your speaker cab?

Failing that, I think it's oscillation (I think there's a phaser-y quality to it in places).

Yeah, when it 1st happened I thought someone had put a screwdriver through the speaker cone. But I've tried different speakers/cabs and it's still there. And yes it does sound a bit phaser-y.

thx

[lord preset]

OK, I've been working though this and went back through the bypass tests.

1st test: Bypass v1b as per 10thtx's troubleshooting schematic.

Results: Sounds good. Clean tone with vol, bright, mid, treble, bass and PAB working as expected.

2nd test: Bypass v1a as per 10thtx's troubleshooting schematic. Note that I took the troubleshooting schematic literally - i.e. in addition to bypassing v1a, I completely bypassed the Master/OD section.

Results: Buzzy distortion. It's buzzy both with and without LNFB in the circuit.


At this point I started swapping components and making connections PTP off the board to see if there was a bad part or bad wiring in the circuit around V1b. New .047 cap, 150K resistor, 5uf bypass cap and even a new tube socket. No dice.

Then I started looking at the LTPI circuit and noticed that for the 1st test, the bypass entered at the .02 cap, whereas for the 2nd test it entered at the 270K resistor. So I tried the 2nd test with the bypass entering the LTPI at the .02 cap and it's gainy and overdriven but NOT buzzy.

As an additional test I went back to the 1st test (bypassing V1b) but this time connected the bypass at the 270K resistor rather than the .02 cap. I got very low volume but if turned up enough the buzz was there.

As a final test I connected V1a and V1b directly to the LTPI at the .02 cap and its even more gainy but not buzzy. Connect to the 270K resistor and the buzz is back.

So having the 270K resistor in front of the .02 cap is associated with the buzz. I replaced it with no change in behavior.

Does this point to anything? I assume the 270K resistor is there for a reason but it's presence/absence in the circuit as tested does make a difference.

[10thTx]

I'd just remove it completely if it's causing some buzz.

IF you need it to reduce gain, than put a 220k after the master volume prior to the switch ............ and/OR after the EF86 OD stage into the switch.

After removing it, try the EF86 gain stage in the circuit. How does it sound? Please let us know. Thanks for keeping us posted.

With respect, 10thtx

[lord preset]

Well hot damn. It's fine now that the 270K resistor is gone. I had half disassembled the amp in the process of testing but I got it all back in place. Everything works. The OD is good - not all that Dumble-like but still smooth and good.

I'm just so happy it works that I may not be that objective about the tone. I'll give it a few days and see if I want to tweak anything. Next up - goofy robot cabinet. Should be ready for my brother in time for XMAS.

Thanks to Mat for the plan and 10thtx for all the invaluable assistance. It's amazing what you can do with a little help from folks on the Great Interwebs. I will post a clip after the amp is set up properly in it's cabinet.

Thanks!

[10thTx]

The OD is good - not all that Dumble-like but still smooth and good.

Hooray! Congrats on getting it working and persevering to success!

I hope you will try a 5751 (at some point) in the V1 position to see if you like that.

With respect, 10thtx

[lord preset]

The OD is good - not all that Dumble-like but still smooth and good.

Hooray! Congrats on getting it working and persevering to success!

I hope you will try a 5751 (at some point) in the V1 position to see if you like that.

With respect, 10thtx

OK, short honeymoon

There are good tones to be had but it still needs work:

1 The gain definitely needs to be knocked down - on the master volume anything past about 4 gets nasty and the OD level has to be way down to be workable. I tried putting the 220K between the master and the OD/Clean switch and I get exactly same the nasty buzz that I had before. I continue to by mystified by this and need to find some other way to reduce overall gain.

2. The OD Tone does not have a useful range. It has to stay all the way down or close to down for the OD to not be shrill. I tried subbing higher and lower value caps for the .0022 cap to ground off the OD Tone pot with no success. Presumably that was not the place to tweak.

3. I am not entirely happy with the PAB switch on this. It doesn't seen to thicken the tone the way the PAB does on my D'lite. It's still useful when in OD mode but much less so on the clean channel. I was looking at an old thread on alternative PAB circuits (https://tubeamparchive.com/viewtopic.ph ... omparisons) and may try either the d'lite or 124/183 circuit, although it was unclear from the thread how the different options compare in terms of tone.

I don't have a 5751 to try but I'm going to get one. I have a 12at7 in V1 now which does reduce gain but apparently a 5751 would be better tone wise in that spot. I also picked up some of Mat's ideas in this thread to increase OD drive and smoothness. I swapped the 1.8K cathode resistor for a 1.2K and the 220pf cap across the 82K plate resistor for a 470pf. The net effect seems to be subtle but positive.


This is still a cool little amp as is, but it's a bit to hard to dial in and I'd like it to be more user friendly for my brother.

[10thTx]

More suggestions for you.

I use a 12AY7 in my TweedBluezMeister and in my D'Mars ODS amps which are D-inspired.

Try rewiring the tone stack as shown which is what I use on my amps and I find it works great for me.

You can always rewire the EF86 for a 5879 if needed also. The 5879 is a great tube with less gain than the EF86. I've used it quite a bit and really like it.

With respect,10thtx

[lord preset]

Well I said I was going to house this amp in a goofy robot cabinet and I did.

All ready for Xmas. As far as the circuit goes I ended up add a resistor right before pin 2 of the PI that knocked down the gain a bit without inducing any nasty sounds. I decided that I was over thinking the rest and left the PAB circuit and everything else as is. Now that it's in the cab I'm really pleased with the sound overall. There are some very nice cleans and OD tones at reasonable volumes. It does take tweaking to get each one but this amp isn't foot-switchable, and my brother doesn't play out, so it's really not an issue.

I'll get a clip up in the next few days.

[Reeltarded]

I LOVE IT!!!



Needs a spark spitting thing that is foot switchable... haha

[10thTx]

Man, that is wild! Very creative. Thanks for sharing the photos.

With respect, 10thtx

[Structo]

Cool!

Do the eyes light up?

[lord preset]

Cool!

Do the eyes light up?

Indeed they do.

[Colossal]

Brilliant! Great job and very creative.

[rogb]

That's crazee man, nice job, Jetson style