Analog Discovery 2 test results for various tube amps

[BmWr75]

Post #1 - Analog Ethos Legendarium 2A3 SET
Post #2 - Analog Ethos Exordium EL84 SE
Post #3 - Kegger H-P Oscillator 6AC7/6AV5 SE
Post #4 - South Florida Audio Society EL84 Push/Pull << test results are erroneous, new results using AD2 5V PSU here >>
Post #5 - Skunkie Designs 5751/EL34 SE
Post #6 - Skunkie Designs 2A3 SET amp
Post #7 - Reisong A12 - 12AX7/EL34 SE - BEFORE Skunkie Designs mods performed
Post #8 - Reisong A12 - 12AX7/EL34 SE - AFTER Skunkie Designs mods performed
Post #9 - Magnavox console pull EL84 SE
Post #10 - RCA console pull EL84 SE
Post #11 - Decware Zen Triode SEUFO2.1
Post #12 - Harmon Kardon Trio A224 integrated EL84 Push/Pull
Post #36 - Rogue Audio M120 monoblocks KT-88 push/pull/parallel
Post #44 - Allen Organ Model 20a monoblocks push/pull 6L6GC
Post #45 - Dynaco ST-70 push/pull running KT77s
Post #47 - Musical Paradise MP-301 MKII EL34 SET
Post #48 - Ming Da MC84-C 07 EL84 SET
Post #52 - 300B SET built by @THD+N
Post #53 - Dynaco ST35 push/pull EL84
Post #60 - Analog Discover 2 frequency response calibration result before/after comparison

New thread - Decware SE34I.5 amp

Analog Ethos Legendarium 2A3 SET amp

 

[BmWr75]

Analog Ethos Exordium EL84 SE amp

THD vs. Frequency sweep starts at 40 Hz because THD >25% at lower frequencies. Most amps start at 20 Hz.

 

[BmWr75]

Kegger H-P Oscillator 6AC7/6AV5 SE amp - https://audiokarma.org/forums/index.php?threads/hp-200cd-6av5-se-amp.376120/

 

[BmWr75]

South Florida Audio Society EL84 Push/Pull amp

This amp hits 2% THD at about 2.4 watts. disappointing for a P-P EL84 amp. The small OPTs may have something to do this this, looks like the original OPTs were kept.

Edit: these results are erroneous due to a measurement error issue caused by powering off a USB connection instead of an optional 5V PSU.

 

[BmWr75]

Skunkie Designs 5751/EL34 SE amp

 

[BmWr75]

Skunkie Designs 2A3 SET amp

 

[BmWr75]

Reisong A12 - 12AX7/EL34 SE amp - BEFORE Skunkie Designs mods performed

Could not get THD vs. Frequency or THD vs. Power to run - THD too high (>25% at least up to 100 Hz) in stock form. Tested with PSVane tubes that came with it. This amp measures terrible in stock form.

 

[BmWr75]

Reisong A12 - 12AX7/EL34 SE amp - AFTER Skunkie Designs mods performed

Tested with PSVane tubes that came in it.

 

[BmWr75]

Magnavox console pull EL84 SE amp - built by an AK member (forget the username right now)

THD vs. Frequency sweep starts at 50 Hz because THD >25% at lower frequencies. Most amps start at 20 Hz.

 

[BmWr75]

RCA console pull EL84 SE amp - restored by me

THD vs. Frequency sweep starts at 40 Hz because THD >25% at lower frequencies. Most amps start at 20 Hz.

 

[BmWr75]

Decware Zen Triode SEUFO2.1

This is the only EL84/6BQ5 based amp I have tested so far that can start the THD vs. Frequency sweep at 20 Hz. Most have to start at 40-50Hz to get the THD below 25% (an Analog Discovery 2 internal safety cut-off limit).

 

[BmWr75]

Harmon Kardon Trio A224 integrated (1959 vintage push/pull EL84)

Had to use the balance knob to get the Sine waves to match up. Ended up setting it to about 1:30 o'clock. Can't remember what EL84s are installed in this amp, but that might have something to do with the channel imbalance and other differences shown below. Also have no idea what is causing the weirdness on the THD vs. Power chart starting at about 3.5 watts, but it is coming from the Input Signal (green line). I've seen this strange behavior before testing other amps.

 

[Selmerdave]

Just a minor point, the "T" in SET stand for triode. I don't think any of the above amps apart from the Legendarium use triode or triode-connected outputs, they are either SEUL or SEP.

 

[BmWr75]

Just a minor point, the "T" in SET stand for triode. I don't think any of the above amps apart from the Legendarium use triode or triode-connected outputs, they are either SEUL or SEP.

You are correct. Will fix that error. Too much copy/paste going on.

 

[restorer-john]

Can you post a loop-back of the analyzer (generator out to analogue in), so we know what we are dealing with here? Or is that in another thread? What is its inherent flatness across the bandwidth, what is the measurement BW, any weighting? Isn't the gen a 14bit DAC AWG?

 

[BmWr75]

Can you post a loop-back of the analyzer (generator out to analogue in), so we know what we are dealing with here? Or is that in another thread? What is its inherent flatness across the bandwidth, what is the measurement BW, any weighting? Isn't the gen a 14bit DAC AWG?

Don't currently know if the loopback test can be done. Will read up on that and report back.

Here are the specs for the AD2: https://www.digikey.com/en/product-...nGHeSS844vH1cL0ZLfIOTy2t3pTMropRoCUlkQAvD_BwE

 

[prww]

Wondering what the ringing (squiggle) is on the square wave at the top of the rise is. I have seen this before with tube amps. Does it matter, and can it be minimized? Paul.

 

[BmWr75]

Wondering what the ringing (squiggle) is on the square wave at the top of the rise is. I have seen this before with tube amps. Does it matter, and can it be minimized? Paul

Are you asking about a specific amp shown above?

 

[prww]

Are you asking about a specific amp shown above?

They all seem to have it more or less except the 2A3 SET. The SFAS EL43 PP seems most prominent. Paul.

 

[BmWr75]

They all seem to have it more or less except the 2A3 SET. The SFAS EL43 PP seems most prominent. Paul.

Here is what Google AI says about overshoot and ringing (cannot vouch for the accuracy):

Ringing in a tube amp's square wave output is caused by instability from unwanted high-frequency oscillations. This happens when the fast-rising square wave interacts with the circuit's stray inductance and capacitance, creating a resonant circuit that overshoots and oscillates. Key causes include poor power supply filtering, improper grounding, and issues with feedback networks or component placement.

Causes of ringing

• Oscillations from fast signal changes: The sharp rise and fall of a square wave can excite the natural resonant frequencies of the circuit. This is due to the interaction of stray inductance and capacitance in the wiring, components, and layout of the amplifier.
• Inadequate power supply filtering: A noisy or poorly regulated power supply can introduce unwanted signals onto the power rails. If the bypass capacitors are ineffective or faulty, this noise can feed back into the amplifier's signal path, leading to instability.
• Poor grounding: Improper grounding can create ground loops or allow noise from the high-power output stages to feed back into the sensitive input stages, causing high-frequency oscillations.
• Instability in feedback loops: Tube amps often use negative feedback to control gain and reduce distortion. However, the feedback loop can become unstable at higher frequencies. This is often caused by a missing or incorrectly valued Miller compensation capacitor, which is designed to limit bandwidth and prevent oscillation.
• Component or layout issues: Poor component placement, long wires, or components too close to each other can create unintended capacitive and inductive coupling, forming parasitic resonant circuits.