Any interest in seeing some Dumble Tweed Deluxe photos?

[martin manning]

The 0.1u coupling cap/220k resistor junctions are the same circuit nodes as pin 6's, and probably easier to access. Measure AC voltage to ground on each side and see if you can get the readings to match by adjusting the trimmer.

[Charlie Wilson]

Charlie

So what I did was I injected a signal into the amp and then checked AC voltages at the power tubes pin 6. In order to get the tubes balanced , I had to turn the potentiometer completely counterclockwise , Out of travel point, that put one tube at 29.04 and 29.6. There was no discernible difference in tone at this setting. I also rechecked the ohm reading on the potentiometer

Delmar, looks to me like you got it pretty close. I am curious though, that AC voltage seems pretty high. What is your volume set at? You don't really want the amp distorting when you measure this.
CW

[martin manning]

That's a good point, CW. I think keeping the PI output down to around 20 Vrms would be a good place to be.

[Delmar Evans]

I was plugged into the bright channel volume and tone both set to 6. However the tone coming in was from my cell and volume may have been way up on the input So you want to keep ac around 20 vac? Does keeping the voltage writer on their improve accuracy in setting the potentiometer?

[Charlie Wilson]

Hmmm, I'm not sure if a signal from a cell phone is ideal. I don't know if it could put out 1 volt or so of signal. Your balance setting will be more accurate if the amp is not clipping and I imagine it is at 6 on the volume control.
CW

[Delmar Evans]

Using a signal generator app on the cell. Can pick various frequencies and the output on the cell is not the same as coming out of the cell speaker. It is a stereo out and is very quicker clean signal. I know when I turn the volume up on the cell, the vac goes up on v3 and v4 substantially. I will try 1/3 volume on the amp and slowly bring up volume on cell to get 20 vac

[Delmar Evans]

Final adjustment. Set controls on amp to how I play. Put 1000 hz tone into amp. While measuring V3 and v4 input grids for vac, I adjusted the volume on the input to around 20 volts. Then fine tuned the pot. Got the voltage delta to .65 vac. That was the closest I could get. Using a matched set of JJ’s in v3 and v4. Then measured the drop across the 1 ohm cathode resistors from V3 to v4. Came in at .011 mv. Attempted to turn the pot. Lock to lock on the pot the voltage changed max of .002 mv

[martin manning]

Let me make sure I understand:

Put 1000 hz tone into amp. While measuring V3 and v4 input grids for vac, I adjusted the volume on the input to around 20 volts. Then fine tuned the pot. Got the voltage delta to .65 vac. That was the closest I could get.

These measurements were from the grids to ground, and the closest you could get was one grid 0.65 VAC higher than the other, correct?

This was with 100k or 110k plate load on the PI?

Using a matched set of JJ’s in v3 and v4.

This measurement won't be affected by the power tubes, but anyway a matched pair is desirable.

Then measured the drop across the 1 ohm cathode resistors from V3 to v4. Came in at .011 mv. Attempted to turn the pot. Lock to lock on the pot the voltage changed max of .002 mv.

Do you mean the voltage difference from cathode to cathode on the power tubes was initially 11 mV, and you could move that over a range of 2 mV, say 10 to 12, or 11 to 13?
Without going to larger current sense resistors (10Ω), that voltage will be very low. Placing a 1Ω from the bottom of the cathode resistor to ground, and looking at the positive peaks on a scope seems like the better method.

[Delmar Evans]

Yes. On the grids to ground measurements

The cathode measurements were cathode to cathode
With a dvom and a 110k plate load resistor and the original 1 meg at the PI. The range on meter was set to mv range
So the measurement was .011 mv and the entire sweep of the potentiometer only varied .002 mv. So from .002 to .004 mv which would make the reading from .011 to max .015 mv

[Charlie Wilson]

Delmar, so the question I have is, were you able to get a balanced output before with the 1 meg grid leak and now with the 10 meg are not? The more I think about what the LNFB is doing the more the 10 meg makes sense. Martin, I have been reading up on the split load invertor, so basically the plate load(100kish) is split in half. Half goes on the plate and half goes on the cathode(Fender design 56k) so the same signal current that flows through the plate flows through the cathode also. Dumble doubled the Fender value, I would assume, to get closer to unity gain. If you add up the resistance of the cathode you get 103.7K and the plate is 110k. So it is a bit unequal and I guess you suggest the 100k to get the plate and cathode resistance closer in resistance. My source material(Tales from the Tone Lounge) also states that a split load invertor is a terrible way to try and balance the signal since the actual impedance characteristics of the plate and cathode are different. So is a balanced output even possible with Fender's or Dumble's circuit or is the trimmer(and LNFB) possibly a way of smoothing things out so that is is more pleasing sonically? I don't have a Tweedle Dee right now to play around with but I do remember that I could hear a difference in the way the amp sounded by adjusting the trimmer in the first replica I built. Finally, has anyone who has built this circuit been able to get a dead on output balance by adjusting the trimmer?
CW

[Delmar Evans]

Delmar, so the question I have is, were you able to get a balanced output before with the 1 meg grid leak and now with the 10 meg are not?

This whole conversation started because I was putting V3 and V4 at a 6 1/2 volt imbalance. This was checking grid to ground. I never tried to get them equal prior to this because all the threads that I read everyone was talking about a 6.5 V differential. So after making all of these changes i.e. 10 meg grid leak resistor and adding the one ohm resistors to V3 and V4, I’m finding that the phase inverter potentiometer does very little In this circuit. So I put it back To stock with the exception of the one ohm resistors. Once back in stock I followed another suggestion by using 1000 Hz had approximately 1 V input and trying to balance the grids RV three and V4. I got them within .65 of each other and that was the closest I could get. When I did that I then did the voltage drop across the cathodes When I did that I then did the voltage drop across the cathodes That is where I measured .011 mV. So I tried to adjust the potentiometer looking at the voltage drop across the cathode to get it closer to zero. I never got any closer than .011 mV drop. So at this point I’m not even sure what that phase inverter Potentiometer does
After these adjustments I then checked the ohm reading across the potentiometer bottom leg was 8.85k. Top leg was .997 k.

[Charlie Wilson]

Delmar, are you referencing posts like this for the 6.5v thing:

Another method: set the trimmer for a 6.5 volt difference (I like the input side as the higher plate). Set the amp clean (no boost , no mid boost, no overdrive, no bright) with all the appropriate controls at noon. Use a neutral sounding guitar on both pickups with it's volume(s) and tone(s) on 10. Play these single notes on the 3rd,4th,5th frets of the B string - a D, E flat E. Let each ring out before playing the next one (no finger vibrato please). In an ideal world they should all bloom equally. If the E blooms but not the D move the trimmer to make the input side lower by a half a volt. Fine tune to taste by ear. Most of the time the 6.5V is near perfect, have seen some amps do this better at a 3 volt difference but it's less common in my experience.

This was posted by Redplate Henry and applies to setting the PI DC plate voltages of an ODS. It has nothing to do with balancing between a plate and cathode in a split load phase invertor.
CW

[sluckey]

That is where I measured .011 mV.

You keep referring to .011mV throughout this whole conversation. I don't know of any DMM that can produce that reading. My Fluke 87V only has a resolution of .1mV. IOW, it can measure 20.1mV, or 100.9mV, but anything less than .1mV cannot be measured by my meter. Do you realize that .011mV is only 11µV (11 microvolts)? So, what are you really measuring? Can you show a pic of your meter showing a reading of .011mV?

[ijedouglas]

Delmar, so the question I have is, were you able to get a balanced output before with the 1 meg grid leak and now with the 10 meg are not? The more I think about what the LNFB is doing the more the 10 meg makes sense. Martin, I have been reading up on the split load invertor, so basically the plate load(100kish) is split in half. Half goes on the plate and half goes on the cathode(Fender design 56k) so the same signal current that flows through the plate flows through the cathode also. Dumble doubled the Fender value, I would assume, to get closer to unity gain. If you add up the resistance of the cathode you get 103.7K and the plate is 110k. So it is a bit unequal and I guess you suggest the 100k to get the plate and cathode resistance closer in resistance. My source material(Tales from the Tone Lounge) also states that a split load invertor is a terrible way to try and balance the signal since the actual impedance characteristics of the plate and cathode are different. So is a balanced output even possible with Fender's or Dumble's circuit or is the trimmer(and LNFB) possibly a way of smoothing things out so that is is more pleasing sonically? I don't have a Tweedle Dee right now to play around with but I do remember that I could hear a difference in the way the amp sounded by adjusting the trimmer in the first replica I built. Finally, has anyone who has built this circuit been able to get a dead on output balance by adjusting the trimmer?
CW

Hey Charlie,

I can swing by with my amp if you want to try a couple of things?

[Charlie Wilson]

Ok so I found a picture of the grid leak resistor in my replica, same camera and same Xicon 1/2 carbon film that Dumble used and I now say there is zero chance that it is 1 meg.
CW