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Old 4th March 2004, 04:38 AM   #11
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Join Date: Mar 2004
Location: Wichita,KS
I was already using the 8ohm tap, had bias set to 35ma ect.. I was sitting there looking at it (you know, open the hood and stare at it. Maybe it will work ) When I saw two things. One: I had 150K instead of 1.5K going to ground on the 12ax7 Two: Right beside me was my Peavey 260 solid state power amp, I found a way to test the preamp sections away from the power tubes. I started going from one gain stage to the next into the Power amp. Found the problem in stage 3. The only way I found to fix it was to insert a 1Mohm resistor from the screen to ground(A trick I copied from my Peavey Bravo 112 tube amp. I know, to much Peavey stuff), cleans right up. Now if I can just get rid of the noise at idle. The power section is not bad, the pre section is very noisey. The 6av6 is very microphonic, Is this normal for this tube or should I look for a new one? I may have a mullard one of these around Thanks again for your help.
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Old 4th March 2004, 05:45 PM   #12
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Location: Boston, MA
I had 150K instead of 1.5K going to ground on the 12ax7

Yep, that would account for a pretty ugly sound.

Found the problem in stage 3. The only way I found to fix it was to insert a 1Mohm resistor from the screen to ground(A trick I copied from my Peavey Bravo 112 tube amp.

I'm confused here, is stage 3 the second half of the 12AX7 preamp? Did you mean to say grid instead of screen?

The 6av6 is very microphonic, Is this normal for this tube or should I look for a new one?

This tube was common as detector / AVC in the "All American 5" type AM radio. This is the final stage before the power tube. While the triode curves are similar to 12AX7, I don't believe the heater winding is specifically wound for low hum operation like the 7025/12AX7. Thus, this tube may not be suitable as the first gain stage of a high gain preamp like yours. You may find a good one if you sort through 10 of them. Or maybe not.

I would personally not use the 6AV6. Ideally you'd put a 9 pin socket in there and use a good old 12A_7. If you must keep the 7 pin socket you could use a 6AU6 pentode wired as a triode. This tube tends to be microphonic too, but not too bad wired as a triode.

Another option for the first gain stage is the FET/MOSFET cascode.
see http://www.blueguitar.org/new/articl...er/hi_v_ss.pdf
This is considered heresy among tube afficianados, but works extremely well if you want a simple, low distortion, low noise, non-microphonic gain stage that requires no filament current and easily integrates into the high voltage world of tubes.

The power section is not bad, the pre section is very noisey.

What kind of noise are we talking about here? 60hz hum? 120hz buzz? Hiss?

Regards,
Mike D.
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Old 5th March 2004, 12:11 AM   #13
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Join Date: Mar 2004
Location: Wichita,KS
Mike, Stage three is the second half of 12ax7 and yes I meant to say grid. The noise is 60hz and hiss. It is from the 6av6. I may put it as the last stage to keep from amplifing it's noise in the latter stages. I thought about replacing it with a 12ax7 but the amp is steel. Don't know if it would be too much trouble. As far as the mosfet thing, Thanks but no thanks, I'm one of the people you mentioned
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Old 5th March 2004, 07:20 AM   #14
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Location: Boston, MA
[color= blue]I thought about replacing it with a 12ax7 but the amp is steel. Don't know if it would be too much trouble. [/color]
It's not too much trouble. about 5 minutes with a rounded file. Faster with a Dremel or Rotozip with grinding stone. Faster still with a drill and a uni-bit.
[color= blue]As far as the mosfet thing, Thanks but no thanks, I'm one of the people you mentioned [/color]
That's cool, I used to feel the same way. But I have found mosfets very handy for:
1. reverb driver (driving a standard reverb transformer)
2. reverb recovery
3. tremolo oscillator
4. "cathode" follower (source follower) after 1st stage of AB763 style preamp to drive tone stack with low impedence. (secret weapon for fender amps! can change tone stack to 5F6A, Marshall Plexi, or Vox top boost style)
5. 1st gain stage when no chassis space available for extra tube. This stage is normally not overdriven, it exists mainly to get the guitar signal out of the noise floor. And even if it is overdriven it does not hard clip like an opamp. It rounds off the peaks in an asymmetrical manner. Not exactly like a tube but nowhere near as harsh as an opamp. Not too many opamps can output 150volts peak-to-peak either!
If I can use tubes, that's my first choice, if not I use mosfets.
No one I know can tell the difference in these applications.

Regards,
Mike D.
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Old 21st April 2004, 09:31 PM   #15
rljones is offline rljones  United States
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Join Date: Mar 2001
Location: california
Tim,

Since I don't know what your power supply topology looks like, I'll describe a few possiblities for your hum and noise problem.

Hum and noise in tube amps are frequently due to a few sources: bad filtering caps, AC heater supply, ground loops and meager B+ filtering. The last is the easiest to treat: add more capacitance to the supply going to the output transformers. Additionally, you can add a choke immediately before the first capacitor off the B+ regulator. The first possibility may mean replacing any suspiciously old caps to see if the hum disappears.

The heater supply can have its hum contribution reduced by floating it off ground up to around 30 to 40VDC, or you can switch it to a DC heater supply using a solid state bridge rectifier. Before you do anything, however, you might make sure all heater wiring (if AC) is via twisted pair that is not running parallel to any signal wires. The signal wires should be close to the chassis and the heater wires away from the chassis in mid air (some manufactures run these wires vice versa: heater wires against chassis, signal in mid air). If the two groups should cross, try to cross them at 90 degrees.

If you choose to float the heater AC, you need to make sure no heater supply is referenced to ground. Next, you need to create the reference voltage. For example, using a 220K in series with a 22K, the 220K goes to B+ (that supplies the output transformer) and the opposite side of the 22K goes to ground (and can be bypassed with a 100 uF/100V cap). The junction between the two (voltage divider) will supply about 40VDC if your B+ is 450VDC. If your heater has a center tap connect this to the 40VDC supply point. If not, then run a 100R from one heater rail to the 40VDC point and another 100R from the other heater rail to the same 40VDC point; this creates an artificial center tap and floats the heater supply off ground. Again, make sure no heater rail is left going to ground. You might also wish to run the 12AX7 in 6V heater mode, if not already: short pins 4 to 5 together and run heater to the 4,5 short and pin 9, and then run all tube heaters in parallel.

If these suggestions don't help, then you'll need to evaluate the chassis for ground loops, which can be a bit difficult to isolate.
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