SKYLINER build - inspired by ODS #124 and #060

[Stephen1966]

Some pictures of the completed head and cab.

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[Stephen1966]

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[Stephen1966]

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[Stephen1966]

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[bepone]

great! what kind of cab design is that? you have added some "wings" from back plate to inside?

[Stephen1966]

great! what kind of cab design is that? you have added some "wings" from back plate to inside?

Thanks, it's my own design with a rear-facing horn which can be removed and replaced with a more standard dipole back as found in normal MrD open-back designs.

[Richard1001]

It looks a lot like the Barefaced guitar speaker cabs:

AVD-Woofered.jpg

[Stephen1966]

It looks a lot like the Barefaced guitar speaker cabs:

AVD-Woofered.jpg

It does, doesn't it?

[Stephen1966]

I've completed the biasing now. I had to swap tubes 8 and 9 to get the balance and using both the transformer resistance method (measuring the voltage drop across the OT) and the cathode biasing method I got figures that corresponded to about 70% PD. Oddly enough, I still don't see a huge difference between the cathode biasing current and the current I calculated at the plates - in the area 1mA. And I only got this when counted the cathode resistors as 1ohm. I had been counting the actual resistance measured in the cathode lines (between 1.14 ohm-1.2ohm) but this put my cathode biasing figures for current much below the plate currents I was finding. It's only when I counted the cathode resistors as 1 ohm that the methods started to correspond. After running it for a couple of hours now, it all seems good, no red-plating or distortion. Swapping the tubes also put each side of the transformers output within 1mA.

With the amp biased, and warmed up I next set out to adjust the PI trimmer using Ayan Gill and Martin Manning's method mentioned in https://ampgarage.com/forum/viewtopic.p ... 89#p416389

If I copy the method here you will see how I interpreted it...

1. Tie together the cathodes of the power tubes that are in phase with each other (i.e., the tubes that have their plates connected to the same B+ wire coming off the OT's primary).

SAM_7834.JPG

2. Connect a digital multimeter set to DC Volts (yes, DC!) to each pair of cathodes, and set the scale to millivolts. You will be measuring the voltage difference between the two pairs of cathodes.
Before I could do this though I had to calibrate the signal generator... (I've only got the one DMM)

SAM_7832.JPG

3. Inject a 1 Vrms 100 Hz signal into the amp. I keep the amp in clean mode with the preamp volume turned down low and the tone controls where I normally set them (it will not matter much where you set them anyway, all I try to do is to not overdrive anything).

4. With a load connected to the speaker jack, set the output level (master volume) to the point where the amp dissipates about as much power as you think it dissipates when you normally use the amp. My rule of thumb is that at gigs I may be running at 50W tops, so into an 8-ohm load I will have a 20 Vrms output (P = V x V/R, so V = SQRT (PxR) = SQRT (50 x 8.) = 20). If you play your amp at lower volumes you may want to shoot for say 20W, which translates into 12.7V, etc. The output level will change the results/readings.

I aimed for about 50W as well and so was looking for about 20Vrms, and to get there I had volume at about 40% and Master at about 60% with the PAB engaged.

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With the signal sorted out out, I was then able to go back to step 2 and get into step 5

5. The multimeter's reading should be on the order of a few millivolts. Turn the PI trimmer until the reading is 0 mV; 0 may go by quickly, so look for the reading to shift from + to - or vice versa. When 0 is reached, each pair of cathodes is at the same voltage with respect to ground and the two sides are therefore balanced. If the reading doesn't cross over, try another tube in the PI slot; it could also mean that your power tubes are not matched.

The initial readings with the DMM were not very comprehensible but I finally got it to shift (more or less equally) + and - about 2 mV. 0 mV didn't appear. So with that more or less equal swing from positive to negative I plugged in the cab (for the first time - yeah!) and proceeded to rattle the windows and literally shake the earth under my feet. The speaker is brand new so I needed to play loud and percussive for an hour or two to start breaking it in. The amp has a massive range of tones and my guitar too, so settling on one was difficult but with pickups in series and the amp in clean mode I set about playing single notes, listening for the bloom.

The attack is tight with no apparent SAG - "bloom" is hard to characterise though. Playing single notes on the second string, I was able to find a point on the PI trimmer where the upper partials became clearly audible over the fundamental D, Eb and E. This was just a fraction off from the balanced setting I had made with the DMM. The DMM setting gave upper partials on the D and E but not the Eb. After the tweak, playing up and down the neck, I could identify the upper partials across the range and the top notes (the dusty end) are really penetrating but still have that sense of a full tone.

I started to get into the OD trimmer setting as well and there, after I dialled it back I was able to get a nice crunchy rock vibe across the bottom strings. Muddy at first, it wasn't possible to make out the notes across the strings but after dialling it back a bit, I could hear a good delineation of notes in intervals and licks. Standing in the right place with the volume shaking stuff off the shelves, there are also moments of feedback, it isn't a big feature though. All the examples of real Dumbles in recordings I've heard haven't fed back.

I'm kinda at a place where I am happy with the sound but what do you think? Is there something I'm missing, am I interpreting "bloom" correctly, am I looking at the right kind of response from the OD? I know everyone has their own tastes but I'm happy to hear what you think, even if I might think differently.

[Stephen1966]

chasing the "bloom" here, I found this rather extreme example



But then referenced back to this



There's a ton of reverb on the Peter Green but this how I envision the bloom we speak of.

Fact is, I just can't seem to get it from the amp... close, yes, but no cigar yet. Before I go any further I should mention that I am expecting delivery of a balanced 12AX7 for the PI any day now so this might all be a non-issue.

No, the problem I am seeing is of contradictory statements, which may or may not have any real significant impact on argument.

I developed my schematic from the 1.6 (1988) version of the #124 which led (I believe) to the 1984 version 1.7. I also had a copy of odourboy's 1.7 version. I didn't pay a lot of attention to the PI in the planning, taking it as a given that the description that the non-feedback side of the PI was approximately 10 volts lower than the feedback side, which is what we see in the schematics at the head of the thread.

https://ampgarage.com/forum/viewtopic.php?f=5&t=5719

Trouble is, I find the situation is reversed when I measure the voltage off the plates of the PI and this agrees with what Martin said, and what was shown in the schematic by odourboy.

The input (inverting) side, pin 6 in your schematic, should have the lower plate load resistance and higher voltage.

https://ampgarage.com/forum/viewtopic.p ... 64#p145164

I'm not sure this is such a big deal since opinions seem to differ about which side should the higher and whether it makes any real difference but as context, here are a couple of screenshots of the different schematics.

ODS 124 phase inverter version 1.jpg

Odourboy's ODS 124 phase inverter version 1.jpg

I'm planning to go in with the scope to see if I can make some visual sense of what I seem unable to hear in the equivalence of the signal going to the grids of the power tubes. I am thinking the probes should go right on the (V8/V9 power tubes' on opposing sides of the OT) grid pins (after the coupling caps from the PI), I don't want to blow my scope with high DC voltage in the order of 300V.

Still if any of the major players here could chime on with their thinking on the voltage disparity shown in their observations and the schematics that would set me on the right path I believe.

[dbharris]

Do you have 100x probes? I would think you need those to do that safely.

-Dan

[Richard1001]

I set the PI of one of my amps with a scope. One probe on each primary transformer winding (X10 probe and scope on AC range). Load resistor on the output and a sine wave on the input of the power amp / effect return.
I added channel A and B together and then set the trimmer for the lowest displayed signal. The PI is balanced if both sides cancel each other out.

(Check your scope specs for maximum DC blocking. and prevent a ground loop)

[Stephen1966]

Thank you Dan/Richard, my scope has a maximum input voltage of 400V (ac + dc). I will use the DMM to check voltage before applying the probes but I think we are operating in a safe zone when applying the probes after the coupling caps from PI. This is first, just to check the signal going to the power tubes grids. When you say "primary transformer winding", Richard, do you mean the output transformer's secondary winding? [Edit: my brain is mush today - of course, it's the primary side of the transformer!] If so, I think the probes' attenuation of x10 would only be seeing a voltage of around 40VDC with the AC on top. I'm just thinking it might be just as easy to see the signal at the grids of the power tubes instead and so the voltage would be the AC coming off the PI plates plus the DC bias supply which is in the order, IIRC of -60VDC max. I think it may be possible to get very similar results either way, but as previously pointed out elsewhere, it doesn't take account in variations between the power tubes' gain - reading the signal from the grids. And, for a safe (and established) method that does take the variations into account, the Ampeg SVT method is looking at the signal from the cathodes. I understand, it is only really to get us into the ball park area and that final, incremental tuning is probably best done by ear.

After you had set the PI trimmer on your amp with the scope used at the power tube plates, did you have to tweak it much? That's the real question I suppose. Seems there are always more ways than one to skin a cat

[Richard1001]

When you say "primary transformer winding", Richard, do you mean the output transformer's secondary winding?

Brilliant question

No, the primary output transformer winding is connected to the plates of the powertubes.

The signals on both sides are 180 degrees out of phase. Adding both signals together, they will cancel each other out. When both signals are equal (in balance) the result is zero. In practice it will never be exactly zero. So look for the lowest signal.

The trimmer sets the AC balance for the power amp, not only the PI. For the measurement you need to let the amp work at about 1/2 power output. So for example for a 100W amp you need 20 Volt RMS over a 8 ohm load. On the primary side there will be a high AC voltage so you need two x10 probes. Set the scope channels to AC, this will block the DC supply voltage which is of no interest.


If you have bias resistors on the cathodes of the powertubes you could also use the DC voltmeter methode. But with 4 tubes and bias resistors for each tube, you should connect the cathodes / measuringpoints of both tube pairs together.

I don't have bias resistors in my amp so i use a scope. I did not need to tweak it further, but i did set it twice. First when finishing the amp, and again after playing the amp for a few months. It is not as critical as some believe imo.

[Stephen1966]

I'm sorry Richard, I really am an idiot - I didn't mean to make you cry but I'm glad you see the funny side. I corrected myself and hoped you noticed it before replying but I was a bit late out of the gate.

It's starting to make sense as I understand the long-tail pair better. Because of the common tail resistors (the long tail), the non-inverting [correction: inverting] (input) side of the PI tube is lower in gain than the inverting [correction: non-inverting] (feedback) side thus the need for dissimilar fixed resistors to equalise the gain from both anodes. Result, the voltage needs to be higher on the non-inverting [correction: inverting], input side of the tube because if the internal resistance of the tube is a pseudo constant [edit: the long tail, specifically], the increased voltage, increases the current and thereby increases the gain. Bringing both sides of the tube into a balance. The trimmer is matching this to any imbalance in the power tubes as well so reading the signal at the grids of the power tubes wouldn't give the full picture, and for that you have to go to the plates (V8 and V9 just happen to be on the primary blue and brown ends of the OT in my case) or the cathodes of the power tubes which takes in the addition of the screen grid currents as well.

If I inject the signal at the power amp in, bypassing the preamp, I guess it is possible to look at the load voltage of the output (about 20Vrms as we said before) to find that 50% output. I guess it is also possible to look for the point where the sine wave starts to clip and then dial it back a tad, using that as the operating point. The first, expressed elsewhere, offers us the option of setting the amp at what we consider nominal operating volumes and lower voltages at the output if we normally use the amp quieter. The second option, looking for clipping, given the amps ability to stay clean might result in a very hot optimised signal which we might never approach in working conditions.

I do have the cathode resistors in my build and above, you will see that I can tie the cathodes together just by connecting the opposite pairs of bias outputs, it's electrically equivalent to tying the cathodes together directly at the resistors. I would be curious to see if we could connect the probes to the cathode pairs and then add the signals together in the scope to achieve a ~0V trace. Similar to the way you describe adding the signals from the plates. A bit also like the SVT method for Harmonic Distortion Meter. I'm not clear whether the signals for the scope would be in DC or AC in the cathode method though, but given the low voltages and currents there, it would be safe to flip between and see which yields the best results. AC I guess.