Crate V5 mods... anyone?

[jim p]

Look at post #168 this takes the op amp out for an all tube amp.

[rjohnj]

Look at post #168 this takes the op amp out for an all tube amp.

I've completed all my mods and the amp sounds great - better than anything else in it's class, except for one thing..... when I have the amp and guitar cranked and hit the E-string or power chord (especially on the neck pickup) the amp gets really 'farty'.
I've messed with the bias and measured all my voltages and if I keep it in the 'healthy' range it still gets farty. I've tried running it really hot and it seems to help but then the power tube red-plates. The standard 330 ohm cathode bias resistor actually seems to be about right to keep the amp within the recommended limits - it's even at the upper end of acceptable.Though I'm currently a touch hotter than that to help with the fartiness - 14v plate dissipation. Any recommendations on how I can solve this or do I just need to live with it?

Ryan

[jim p]

Just to check on your plate power dissipation calculations. You should have determined the cathode current then subtracted the screen grid current to obtain the plate current. Then the voltage across the tube which is plate voltage minus the cathode voltage is the voltage across the plate times the plate current for the power dissipation of the tube. The ball park number screen grid current is 4mA but there is a screen grid resistor you can measure the voltage across to get an exact number if you want.

No answer on the farty sound I would try driving another speaker if one is available to use.

[rjohnj]

Just to check on your plate power dissipation calculations. You should have determined the cathode current then subtracted the screen grid current to obtain the plate current. Then the voltage across the tube which is plate voltage minus the cathode voltage is the voltage across the plate times the plate current for the power dissipation of the tube. The ball park number screen grid current is 4mA but there is a screen grid resistor you can measure the voltage across to get an exact number if you want.

No answer on the farty sound I would try driving another speaker if one is available to use.

without rechecking, i think, I had something like 319 volts between plate and cathode (taken from pin 7 to 3), 11 volts for the cathode resistor. That's with 250 ohms resistance. (screen is estimation).
then i plugged my numbers into the tool below to get my plate dissipation (and to save me doing the sums)
http://www.webervst.com/tubes1/calcbias.htm
I might try plugging through my 1x12 cab and see what happens.

[jim p]

It looks like the Weber calculator allows just 2mA for the screen maybe just being conservative. Going by the 11 volts on the cathode if you had a 270 ohm resistor and the voltage stays at 11 volts your cathode current would be 41ma so would be just about bang on 12 watts on the plate ( 319 volts times 38mA = 12.1 watts)

[rjohnj]

It looks like the Weber calculator allows just 2mA for the screen maybe just being conservative. Going by the 11 volts on the cathode if you had a 270 ohm resistor and the voltage stays at 11 volts your cathode current would be 41ma so would be just about bang on 12 watts on the plate ( 319 volts times 38mA = 12.1 watts)

yeah that's pretty much what I got just pushing the boundaries slightly with 250 ohms to help with the sound a little.
Not sure why I have to run it so hot to get rid of the 'farting' though. 100ohm resistor sounded good but just ran the tube too hot.

[SciHi]

...get rid of the 'farting' ...

I have found that the v1a cathode bypass cap and/or first coupling cap control the flabby bass. For HBs I have used either 0.68 or 1 uF cathode bypass caps, you can go a little higher with single coils, ~2uF. Or use a real small coupling cap, like the vj fender/voxy mod value, 0.0022uF, my v5 has a 0.0047uF coupling cap. I am using 10K R1, added a master volume with 6.8K grid resistor after the mv, and did the screen resistor mod to the el84. My tone stack sucks lots of gain so the amp is very clean and very tight bass but still retains the thick tone. I also tried adding the nfb but it was too thin with my setup. My speaker is also a thick cone Celestion Tube 10 (G10E-30), and doesn't breakup even playing a bass through the amp. I suggest you try to figure out if it is the amp or speaker that is causing the "farting."

Rock_Mumbles posted this on his tech site "zaphod_phil Quote:
I think the moral of the story is to either use a smallish cathode cap (below 100uF) and bias fairly hot with a low cathode resistor value, or use a very large cathode cap (1000uF to 2200uF) and bias cooler with a larger resistor value. In between values will usually sound woofy."

found here:
http://www.18watt.com/modules.php?na...r=asc&start=15

So maybe as you change the el84 cathode resistor value, you need to change the cap value?