Express Swirl

[Noval_novice]

Hey, guys. I recently put together an Express from parts obtained from RJ (thanks, RJ for the great service). The amp sounds pretty good so far with great clean-to-mean character. Unfortunately, I am experiencing some a distinct phaser effect on sustained notes, mostly when using a neck humbucker. It is present all the time, but perhaps a bit more pronounced when playing at higher volume, gain. I do not have access to a oscilloscope yet. My suspicion is this represents some degree of crossover distortion. Does anyone have any suggestions about how to approach this problem?

I should mention that my voltages on start-up were well within posted limits. Power tubes are Svetlana EL34 biased to 28 mA each. Tried multiple tubes in V1-3. My main cabinet is a THD 2x12 loaded with Weber Thames speakers, but I also tried a 2x12 with Scumbacks and it makes no difference (although I prefer the tone of the Thames speakers). Any help is appreciated

[M Fowler]

Shielded coax wire from wiper of volume pot to 820R grid resistor to V1 pin 2
will help.

[Noval_novice]

Thanks, Mark! I will try that.

[Noval_novice]

The grid stopper and shielded coax from volume to second gain stage may have helped a smidgeon.

[John_P_WI]

Have you tried lowering the neck pick up away from the strings?

[Noval_novice]

No, but the noise is present with any pickup -- bridge or neck, humbucker or single coil or p90. It is just more prominent with the neck pickup.

Tried a lot of glass in V1. An NOS 5751 seems best.

[guitarsnguns04]

Have you tried different power tubes?

[M Fowler]

Increase the output tube bias so they run slightly hotter and see if that makes a difference otherwise switch out power tubes to test.

[Noval_novice]

I tried increasing the bias on power tubes up to around 38 mA each without much change in the noise. I think I will try to make a recording to demonstrate what I'm hearing. It's a little frustrating because I have noticed various amounts of similar noise on several builds including an 18 watt marshall clone, a Liverpool clone and a Trinity Triwatt. Maybe my ears are just screwed up.

[M Fowler]

It could be your guitar or guitars, your speakers, line noise, etc.

Or maybe those steering diodes on the output tubes are screwing with you.

Mark

[fishy]

If you notice similar traits on other builds, this might prove difficult. The pickup / speaker issue made the most sense to me based on that comment.

Have you isolated the power - tried the amp somewhere else.

Is the amp shielded - bottom cover? Flourescent lighting can cause havoc.

Is the bias voltage stable?

Does the bright switch make a difference to the noise. Those styrene caps can get damaged easily. Maybe take that out of circuit and see if it makes a difference.

[Noval_novice]

Fishy,

I currently do not have the bottom plate on.

I have lots of guitars and have tried everyone of them, so I don't think that it is a guitar/pickup issue.

I will try the amp in another room of the house when I get home from work.

The bright switch does not affect the noise.

Thanks for all the suggestions!

[tubedogsmith]

A lot of the noise you're describing can come from the phase inverter. I'd try swapping tubes there. If that doesn't help upping your power supply cap there to 47uf may. I'm guessing you have a 47pf resistor across the PI plates. You can try bumping that to 100pf. All that said you can also try snubbers on the first preamp tube. If you want to try this start with something like a 5pf from plate to cathode, a little goes a long way. Just some ideas to try that have worked for me in the past.

[passfan]

If you're playing in a room with a fan running shut it off.

[Noval_novice]

Well, I think the 'problem' is solved. I replaced V3 with an NOS 5751 and upped the fizz cap on the PI to 500 pF, but the sound still keep coming through in my playing. Finally, I had a day alone at the house with neither my lovely wife nor my daughter at home, so I decided to let 'er rip. The swirl I was experiencing is only noticeable when using my homemade airbrake (sans rheostat) at the highest level. When not using the airbrake or only using it for the first few clicks of attenuation, the sound is minimal and really quite pleasant. I had expected a resistive attenuator to decrease the noise floor and perhaps shave off a little high end, but I did not expect this. Has anyone else had similar problems with the airbrake?

By the way, I want to express my gratitude to all those who took the time to reply to my distress call. Your suggestions are all appreciated! I will try to pay it forward when I have a little more experience under my belt.

--Christopher