Single-ended 6v6 diy; over-conducting - diyAudio
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Old 13th May 2013, 08:17 PM   #1
cancon is offline cancon  Canada
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Join Date: Mar 2011
Default Single-ended 6v6 diy; over-conducting

Schematic here!

I've got this home made amp that has been over-conducting and I'd like some general help for remedying this (main issue) as well as any insight as to tweaking the circuit for best operation. This is a sort of champ/garnet herzog mash-up. The preamp slot is meant to accept either 12AX7 or 12AU7 (or anything in between) depending on application. The main idea is for the 12AX7 to serve as herzog duty, or the 12AU7 for taming the circuit and allowing for a broader spectrum use as a preamp. (i.e. leslie preamp).

With the preamp tube out of circuit, the 6V6 conducts at 23mA (cathode+screen). Adding a 12AU7 yields 38mA. Assuming 305vdc plate voltage. The power transformer was slightly under-loaded, hence the zeners. Also the heaters are running slightly hot at 6.8vac. Any help would be appreciated.

Thanks!
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Old 13th May 2013, 10:01 PM   #2
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Location: was Chicago IL, now Long Beach CA
The Champ 5E1 and 5F1 had 22K feedback from the speaker to your pin8. I guess you want that 'no feedback' sound but maybe that's why it's working harder. Maybe the 5K primary on the output transformer is a bit low. Your master volume is REALLY final! Another volume at the grid input to the 6V6 might make it more versatile; personally I would have left out the tone control and added another volume control, added a 'bright' switch across the first volme pot, and some kind of tone control at the output transformer, either on the primary or on the secondary like part of a crossover.
I assume you want it a bit raunchy and are getting that?
Keep in mind I'm a guitar guy, not an EE.
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Old 14th May 2013, 12:49 AM   #3
Enzo is offline Enzo  United States
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The feedback line shouldn;t affect the DC current of the power tube, there is no direct connection, a cap isolates the power tube from the previous stage.

Make yourself a chart. B+ at the first cap, and X, Y, and Z with no tubes, then with power tube only then with both tubes. Don;t assume 305 or any other volts, you need to KNOW.

The 5k ohms of the OT primary is impedance, not resistance.
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Old 14th May 2013, 02:07 AM   #4
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Location: was Chicago IL, now Long Beach CA
Hm, yes I was confusing DC idle current with just louder AC without the feedback loop or into the lower AC transformer impedance...DOH! I'm no help, sorry. Plus his dummy load is conservative, so I'm way off and clueless.
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Old 14th May 2013, 03:28 AM   #5
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Location: Adelaide South Oz
cancon,
IF the unit has been built exactly to that schematic then having the preamp tube in or out will not make a rodents hindquarters difference to the 6V6 running current.
I suspect that you have a wiring problem, check the cathode wiring on both the 6V6 and preamp tube and their 0V return points.
Don't worry about the 6.8V on the heaters.
Cheers,
Ian
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Old 14th May 2013, 10:02 AM   #6
DF96 is offline DF96  England
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Is there oscillation when the 12AU7 is in place? That could increase the current in the output stage. Try adding grid stoppers, and perhaps improve supply rail decoupling so two stages aren't using the same feed - which can lead to feedback via the supply rail.
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Old 14th May 2013, 06:30 PM   #7
cancon is offline cancon  Canada
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Join Date: Mar 2011
Thanks everyone!

Enzo:

No tubes:
B+ 386
X,Y,Z: 370

6V6 only:
B+ 333
X,Y,Z&preamp plates: 315-310

Both tubes:
B+ 330
X: 312, 305 at 6V6 plate
Y: 305
12AU7:
Z: 255, plate 1: 40, plate 2: 80
12AX7:
Z: 280, plate 1: 130, plate 2: 190

The current through 6V6 cathode/screen is 38mA (12AU7) and 36mA (12AX7).


Ian: the cathodes appear to be wired correctly, all the bypass caps (neg) all have good continuity to ground.

DF96: I don't have a reference for what oscillation should sound like, but I think it is present: When the gain is turned up the circuit doesn't sound happy (squelch?)...you can also hear the signal vibrating through the circuitry/chassis.

What is your recommendation for grid-stoppers? 1.5k to both 6v6 input grid and 2nd triode? Do you have any suggestions for improving the supply rail?

Thanks everyone!
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Old 14th May 2013, 10:48 PM   #8
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Location: Adelaide South Oz
I think DF96 may be on the right track and the 6V6 is oscillating. The last SE 6V6 I built definitely oscillated without a grid stop. I fitted 15K grid stop to it, you need at least 10K in that 6V6 grid although I've seen people use anything up to 47K.
Cheers,
Ian
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Old 14th May 2013, 11:16 PM   #9
Enzo is offline Enzo  United States
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If you lack a scope to see oscillation, here is an alternative.

Google "RF probe" and see the results. There are variations, but they all boil down to a cap, a diode, and a resistor. A very simple thing to make and use in front of a meter. It allows a basic voltmeter to read levels of RF. If a seemingly silent amp measures several volts of RF, then indeed it must be oscillating.

What this little circuit is would be the "detector" stage of an AM radio.
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Old 14th May 2013, 11:45 PM   #10
cancon is offline cancon  Canada
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Thanks guys.
I've got a scope, what and where do I check?
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