V1 Microphonic.

[Tinkerer]

After much success and huge rants about the first Express inspired amp I built for a good friend, I decided to build one for myself.
My first build worked perfectly from day one, but mine...well, it's a better microphone than a Shure SM58! I've traced the problem to V1, so I used a variety of different brand tubes in that position (all 12AX7s) to no avail.
I've pushed and prodded wires around with a chopstick, but still own a microphone. I've thought about using isolating 'O' rings under the tube socket, but figure there has to be a simple answer as I haven't read about using them on any search on the forum.
Any help would be much appreciated.

Thanks in advance,
Tinkerer

[M Fowler]

Try replace that 500pf cap on the board for V1.

[Tinkerer]

Tried replacing the 500pF cap and it made no difference. Still open to suggestions.

Tinkerer

[M Fowler]

Use shielded wire from V1 to volume pot, keep the wires pulled away from the volume pot and or use stranded wire rather than solid core here if not using shielded wire.

Also a 33k to 15k grid resistor on pin 7 V1.

[fishy]

Maybe post a pic of the amp around V1. Lead dress is critical for the microphonics.

[KellyBass]

Slight changes around V1 make HUGE differences. Wire length, type, lead dress can all add up to make, like you say, a very efficient mic. What type of 500pf cap are you using? I haven't had much luck with silver mica's in this position. A polystyrene in this spot solved at least 1/2 of my microphonic problems. The rest was lead dress, etc.

[WhopperPlate]

Are you using known working tubes? Tubes are always #1 suspect. I wouldn't underestimate v2's microphonic potential either, so try swapping tubes there as well.

Also, the v1b grid wire if positioned right can cause crazy feedback. I used a piece of stranded 20 guage wire and it seemed the resonance frequency of the thick insulation was extremely susceptible to chassis vibration. If I set it right it was howling like I flipped the OT primaries.

[C Moore]

It has been said already but....shileded cable and did you try a 10k-68k grid resistor?
Do you have a 5751 you can try in V1?
Good Luck

[PlinytheWelder]

To keep mine from being microphonic, I used a Teflon socket for V1and put some of those silicone o-rings around the tube to damp it out a bit.
In general. short plate tube are less microphonic than long plate ones. I have a pile of NOS tubes that are not usable in V1 because of that. Right now I have a Tung-Sol RI in V1 on mine and had to go through a few to find a good one...
Keeping the amp off of the speaker can help, too...

[populartsl]

Use shielded wire from V1 to volume pot, keep the wires pulled away from the volume pot and or use stranded wire rather than solid core here if not using shielded wire.

Also a 33k to 15k grid resistor on pin 7 V1.

+1 on this. Mine was howling like a banshee until i did this. I didn't have to put a grid resistor though, it was alright without.

Cheers Karl

[Tinkerer]

Thanks for all the input people, it's much appreciated.
The volume pot to V1 is shielded cable, and I've tried a 15k snubber resistor on pin 7 which made no difference.
The tubes I've substituted are all new and know good as I've tried them in other amps. I've tried 3 Tung Sol reissues, 4 Svetlana, 4 JJ, 2 EH and 5 Shuguang. Also tried a NOS 5751, NOS JAN Phillips 12AT7 and NOS JAN GE 12AT7.
I tested V2 by tapping the tube and it is quiet as is the PI, but V1 comes through the speaker when tapped as does the chassis around it. The 2 X 0.022uF caps in the tone stack are also microphonic, but only right in the middle. As you move away from the center position they get progressively quieter. I've used PS6 Orange Drops in the amp, so I swapped them out for Mallory 150s and Sozo but it made no difference so I re-installed the PS6s.
I would love to try polystyrene in lieu of the Silver Mica, but nobody in Australia (that I can find) carries them, and, unless I put a huge stock order in to the U.S. to justify the expense, it's going to cost me big $$$ to bring in 6 X 500pF caps, so I can't try that option as much as I'd like to.
I've rewired the socket as per the first amp I built...inspired by the express build guide which is posted on this forum, which by the way is a work of art, and although taming the microphonics somewhat, they still persist.
I'll post photos when my daughter brings back my camera. I'd use my phone, but it refuses to take photos and insists on taking video footage!

Tinkerer

[redshark]

Just a reminder, you want to control microphonics not to do nasty stuff but you still want it to be like that. Microphonics makes the express what it is...or at least contribute partially to its performance.

[WhopperPlate]

What frequencies are most susceptible to inducing microphonics?

[passfan]

Have you damped the chassis ? The tone also changes when you drop it in a cabinet. Is it still microphonic in a cabinet ?

[fishy]

Loose turrets or eyelets will cause issues too. Easy to overlook these.