Express Issues

[jkey04]

This is my first express style build. As you can see from the pics, its not an exact replica but pretty close. I added a LarMar MV, elevated the heaters above the sockets in the preamp, and there may be a few other differences here and there...

My issue is the amp is OVERLY bright. Even with the presence and treble all the way down its still too bright. What's weird to me is the Mid control affects the upper mids almost trebly mids and the treble controls affects the super high frequencies. To me, it seems their respective frequency bands are skewed. As to be expect with this circuit and that much treble, there's some oscillation issues at high gain settings. Does anything stand out to anyone from looking at the pics? At this time, there isn't a bright cap installed.

I have not yet disconnected the Neg Feedback. That high on my list of things to try. Then, i was just going to go through the build guide and look and lead dress pics to try to find the remedy. I do believe I have something wired incorrectly or maybe wrong component value (even though I measured everything before installing). Just thought I'd ask for some insight as i troubleshoot this thing.

[xtian]

When you have oscillation on a new build with NFB, the first thing to try is swapping the OT primaries. Give that a try and see if it cures the issue.

[jkey04]

Will do. I'll have to splice one side to get it to reach the other tube position if thats the cure. I'll use clip leads to test before I solder anything. Thanks.

[Littlewyan]

If it makes it easier you can try swapping the power valve grids.

Also when you say high gain settings what do you mean? As these amps will oscillate easily with the bottom cover off. Especially if you turn the volume up full.

[jkey04]

Good point. I'll just swap the grid wires.

By higher gain i just meant above 4 or 5 on the volume knob. When i do a "play" check i have been putting the bottom cover on. You are right, it run away in oscillation very easy without it!

[Colossal]

First things first to check that the OT primaries are correct. If after that, trouble still exists, be careful with proximity of the negative feedback lead and the output grid wires coming off the PPIMV to one another. A measure of stability with these amps is that they should be able to idle with all of the controls on 10 and the bottom plate cover removed. There should be no high pitched whistling and they should not need any coaxial/shielded wire except from the input jack to V1. You can use shielded on the output of the Volume pot to the second input grid, as you have done, but it is not critical if the amp is stable.

[RockinRocket]

First things first to check that the OT primaries are correct. If after that, trouble still exists, be careful with proximity of the negative feedback lead and the output grid wires coming off the PPIMV to one another. A measure of stability with these amps is that they should be able to idle with all of the controls on 10 and the bottom plate cover removed. There should be no high pitched whistling and they should not need any coaxial/shielded wire except from the input jack to V1. You can use shielded on the output of the Volume pot to the second input grid, as you have done, but it is not critical if the amp is stable.

Just to be clear no guitar cable plugged in? I hope to get mine started and done by spring.

[jkey04]

Ok, so I swapped the grid wires on the EL34s and that only made things way worse so I swapped them back. The amp is just just so stinkin bright. To even get a somewhat usable tone the treble and mid knobs must be barely on and the presence pretty off. Even then it's not the great of a tone. And remember all this is still with no bright cap installed.

I plan to mess with the negative feedback this evening and possibly try a few other mods to tame the harsh treble.

Any other thoughts?

On a good note, the amp seems pretty stable. I can put all knobs on 10 and get the gain knob up to 9 before it starts wanting to oscillate.

[Littlewyan]

Theres no need to mod it from the original circuit. Something may be up. Its a naturally bright amp but not so that it hurts your ears. Do all of the tone controls work? What speakers are you using and what guitar? Are all of the pots the correct value? Make sure the volume, treble and bass pots are Log Taper.

Its about as stable as my Express. You can turn every control to 10 on mine except the Presence or Treble. If either or those goes above 8 with everything else on full then it screams. Or sometimes it fools you by staying quiet but then suddenly it will screech! Scares the hell out of me every time!

[jkey04]

Thanks for the thoughts Littlewyan. The speaker cab is a Dr Z Z-Best 2x12 with V30/H30 speakers, and the guitar is a R7 Les Paul with OX4 low wind humbuckers.

As I dug into the tone stack further over the weekend, I noticed I had installed a 250kA treble pot. I swapped it for a 1MA pot, but I was disappointed to find that there wasn't a huge difference. So...I began to tweak some more.

What seemed to do the trick was a 0.0047uF snubber cap across the plate resistor of V1b. This tamed the harsh treble and allowed for the 500pf bright cap to be usable. It also seemed to tighten the lows and make the mid-range creamier and focused...if that makes any sense. It really sounds great with the Les Paul now. I'm sure that if i had a different guitar and/or speakers this mod may not be needed??

[Littlewyan]

I've never played through a V30 or a H30 but I've they're both very bright speakers. Especially if they're not broken in. Is the H30 the anniversary model?

Also what valves are you using?

[jkey04]

Yes, the H30 is the anniversary model. You are correct; they are bright speakers. I have a H75 creamback that I have yet to play the express through. I currently do not have any "M" series Celestions or clones there of.

The tubes are TungSol RI EL34's, V1 is a old GE 12ax7 pulled out of an old Fender amp, V2 and V3 are new production Mullard and TungSol but i cant remember right now what tube is in which position. I'd have to check later.

[Littlewyan]

The anniversaries are especially bright speakers. Be interesting to see what you think when you install the H75. Which speaker are you replacing?

[jkey04]

The H75 is in a cab by itself so I won't be replacing either speaker in the Z-best cab.

I'd love to try a 2x12 greenback and gold combo...

[Littlewyan]

Ah yes I've heard about Greenbacks being mixed with Alnico speakers. I could never imagine how that would sound but apparently its pretty awesome.