50W Plexi clone - V1 tube hum

[Firestorm]

Some tubes, especially some new ones, suffer from heater induced hum in the cathode. In a Plexi, the 0.68uF cap isn't big enough to couple 60Hz to ground, so the DC elevation of the heater supply suggested here may be your best bet to avoid endless tube rolling every time you have to change V1. Also protects the CF.

[renshen1957]

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

Hi Guss,

I will go slightly off topic, and not to hijack the thread,

I am not surprized with your experience. Similar things have been said about all the tube manufacturers recent offerings.

I have found good tubes and bad tubes from current tube manufacturers, at least those made to a price point.

New Sensor, et al, are building to a price point (cheap) for the guitar market and the dollar (or whatever) is slipping. A tube that costs $15.00 in 2003 would cost 19.04 (27% cumulative inflation). So to keep the tube at this price for the market sector, something has to be cut to reduce cost.

I have good luck with the Sovtek 12AX7LPS tubes which are 12 years old, and like the sound of recent EH12AX7, but neither tubes have the life expectancy of 2 1/2 NOS power tubes.

Too many posts have comment on the blatant marketing trick of purchasing a famous, if long ago defunct, brand name. Groove Tubes Chinese made 12AX7Ms claimed to capture the sound, and were good tubes with excellent examples.

The New Sensor New Mullard doesn't resemble the old Mullards in construction or to my eye or ear. The RI Mullard resembles the new Genalex/Gold Lion 12ax7. Both come from the same factory Xpo-pul. Essentially these are tricked out Sovtek LPS, just repackaged and in the GL case with pretty gold printing.

Maybe Mike M. just purchases the tubes in bulk tests and grades. The Sovtek 5751 is the lower gain tubes, probably culled from the the Sovtek LPS runs. It's not a 5751, just an anemic sample from the LPS family. I don't see double or triple micas, a higher filament draw (350 mA), or anything that is outstanding for the original industrial tube.

Tubes within the same batch will sound different, and different batches sound different (hot batch or cold batch). Some had said there is enough of a difference between the LPS, Gold Lion, Mullards sonically, and have speculated that different materials or chemicals where used in the construction. I don't buy it.

I would rather pay for a Used Old Stock Japanese Mullard, at least these tubes were made on Mullard toolings.

End of hijack.

As others and I have said, get some known (if possible tested used old stock) and test the amp. I keep tubes for this very purpose.

Best regards,

Steve