50W Plexi clone - V1 tube hum

[meierm01]

Hi -
I recently built a 50W 1987 Plexi clone from a mostly Triode kit.
Also added a Lar/Mar PPIMV. Amp sounds amazing, but after a couple of hours of play time, the V1 preamp tube started to hum quite loudly - even with no input connected. Otherwise, the amp sounded exactly the same.
I replaced the tube with another and the hum went away - but only for another few hours of play time (much longer, though, this time).
Seems too far fetched that it would be two bad tubes (both new Mullards) - any ideas?

[M Fowler]

Did you do a voltage check of your amp?

[meierm01]

I did check the voltages on V1 and compared them to a target chart from a similar Metro kit and they were all pretty close.
Not really sure what to look for...

[Phil_S]

I have read the new fake Mullards are so-so on quality, so maybe you actually do have 2 duds. I would look for a cheap Sovtek 12AX7, reputed to be dull of tone but also lowest noise floor.

Don't be bashful -- here for $9.99 (includes shipping) you probably can't beat it.
http://www.ebay.com/itm/Strong-SOVTEK-G ... 19e0cdc80f

...or you can try the 12AX7WA, available at Guitar Center (if you have one nearby) for the same price.

Once you establish it isn't the amp, you are then free to do some tube rolling.

[M Fowler]

JJ ECC803 gold pin tube for low noise with high gain.

I would check the wiring of V1 carefully and make sure no wire tips are touching to other pins or chassis.

[Cameron]

Hi -
I recently built a 50W 1987 Plexi clone from a mostly Triode kit.
Also added a Lar/Mar PPIMV. Amp sounds amazing, but after a couple of hours of play time, the V1 preamp tube started to hum quite loudly - even with no input connected. Otherwise, the amp sounded exactly the same.
I replaced the tube with another and the hum went away - but only for another few hours of play time (much longer, though, this time).
Seems too far fetched that it would be two bad tubes (both new Mullards) - any ideas?

Tubes suck these days so you need to do this.....

[Structo]

So float the voltage at 70vdc?

Since Cameron suggested that, maybe the cathode follower exceeds the heater/cathode voltage.

I have heard that the RI Tung Sol tubes do not like that role in Marshalls.

[M Fowler]

Cameron, why would anyone need all that in a plexi clone, not enough high gain to worry about noise, this is a very stable circuit. Not like were building a CCV100.

Mark

[Reeltarded]

We should be!

I vote shitty tubes. There is nothing left to blame.

[martin manning]

If it was the cathode follower tube (V2) that failed I'd say get one with higher Vh-k max, or elevate the filament circuit. SInce it's V1, my money is on two dud tubes.

[Cameron]

Cameron, why would anyone need all that in a plexi clone, not enough high gain to worry about noise, this is a very stable circuit. Not like were building a CCV100.

Mark

Tubes these days suck......even for a low gain amp + its 3 parts...... not all that much at all.

[Cameron]

Cameron, why would anyone need all that in a plexi clone, not enough high gain to worry about noise, this is a very stable circuit. Not like were building a CCV100.

Mark

Tubes these days suck......even for a low gain amp + its 3 parts...... not all that much at all.

Also........What is a CCV100......is that the name of the so called amp made by the three douchebags under a name they don't own. I called it(or Chris called it) the CCV. Never made a 50 watt CCV.......New....real ones..... coming soon.....never a down payment or any of that bullshit ever again!!

[renshen1957]

Hi -
I recently built a 50W 1987 Plexi clone from a mostly Triode kit.
Also added a Lar/Mar PPIMV. Amp sounds amazing, but after a couple of hours of play time, the V1 preamp tube started to hum quite loudly - even with no input connected. Otherwise, the amp sounded exactly the same.
I replaced the tube with another and the hum went away - but only for another few hours of play time (much longer, though, this time).
Seems too far fetched that it would be two bad tubes (both new Mullards) - any ideas?

Tubes suck these days so you need to do this.....

Hi Cameron,

I agree with you on the DC stand-off voltage method. Easier and cheaper than DC filaments, and tubes aren't were they were, unless NOS, and even these are a bit picked over.

I use DC stand-off in Tweed Amps in fact every amp, an amp doesn't have to be high gain to benefit.

Best regards,

Steve

[renshen1957]

Hi -
I recently built a 50W 1987 Plexi clone from a mostly Triode kit.
Also added a Lar/Mar PPIMV. Amp sounds amazing, but after a couple of hours of play time, the V1 preamp tube started to hum quite loudly - even with no input connected. Otherwise, the amp sounded exactly the same.
I replaced the tube with another and the hum went away - but only for another few hours of play time (much longer, though, this time).
Seems too far fetched that it would be two bad tubes (both new Mullards) - any ideas?

Hi,

You could have more than one issue.

Did you check if the input jack is tight and mechanically sound? Is the contact making connection to the tip when the jack is out?

Did you use shielded wire to V1 from the jack?

How is the amp grounded? Random and multiple, Star Ground, Galactic Ground (multiple stars, Kevin O'Connor The Ultimate Tone volume 3)?

Do you have a reliable, known working pair of tubes from another amp to substitute?

Did you try a different guitar chord?

Is your amp on the same circuit as a Fluorescent Light?

Did you try the amp on other outlets on a different circuit in your house. Did the hum go away?

Does it return when you go back to the original outlet?

If so so another piece of equipment, improper wiring, or a Fluorescent Light with the old style ballast could be causing a ground loop.

Best regards

Steve

[Guus]

Hi Meiermm01,

Could be that the hum problem is caused by the (new) Mullard.
Put in an other brand tube to find out.
Why?
A few mounths ago, I bought 4x 12AX7 Mullard because I thought, this brand was in the old day's a fine tube. So, who knows these new ones may have been made according the exact spec's and are of the same quality.

But....., for me very disappointing, I had bad luck. All 4 of them had from the start a badly hum, probably caused by the heater.
Putting in an old Philips ECC83 and the hum problem was gone.

Furtheron, I had a the same nasty experience with new Mullard EL84. I bought 2 of them, when buying the 12AX7. I thoought, Wow new Mullards!
They lasted for only ~1 mounth, then I had to swap them because of noise problems.
I know every brand can have a bad one in a serie.
But I wasn't lucky when buying them.

Guss