... humming machinery is fine, amplifiers not so much ...





overview


The M-125 is incredibly good value in today's insane HiFi Jewelry marketplace. It's not pretty or child safe, but is sonically very good. Reviewed by Glenn Young and enthused user comments in the Dynaco Tube Forum*, AudioAsylum, diyAudio, etc. The amp is very versatile with both pent/triode modes, and is sonically configurable with a variety of tube combinations. Since my Citation II I've had a soft spot for KT‑88 amps and owned several. In pentode mode, the M-125 travels a middle ground between the PrimaLuna Prologue 5 [PL5] and NuPrime ST-10. It's not as recessed as the PL5 nor as forward as the ST-10. IMO, I'd adjudge the amp a perfect match for the ≈4Ω TC-50s when paired with Kimber BiFocal XL in my system and ROOM!

As always, YMMV.

Mr. Latino was very responsive to pre-purchase email questions about the amplifiers.


This missive is intended for the competent DIY enthusiast. If you purchased built amps, by all means have at it if you feel competent.

Lethal V

WARNING: Lethal Voltages!!
- Proceed ENTIRELY AT YOUR OWN RISK -


Click thumbnails for a larger image.
ESC or Click Upper Right 'X' to close.

 Arrival 

pimple Switch

Two heavy boxes arrived, one with an ominous pimple. The assembled amps were shipped in the kit box, too small for such heavy objects, permitting only woefully inadequate packing. Upon removal, the left amp rattled. The power switch toggle had been forced into the switch, destroying it. The impact force was enough to cross-thread and deform the mounting nuts, making removal particularly difficult.

Sample Sample Wiring

One of the deciding factors to purchase factory assembled M-125s was the quality of the wiring as shown images on Tubes4HiFi and The Dynaco Tube Audio Forum. The wiring delivered is not quite up to the others. Additionally, driver board and tube complement may be somewhat arbitrary. The M-125 web page, which appears to be well out of date, stated “The recommended tube complement is ... 2 X 12AU7 or 12BH7 driver tubes”. Driver board revision v16, [Why did v16 drop GND plane on v15?] w U.S. Sylvania-made NEW National 5963, a data tube similar to 12AU7, received March 2019. Opinions vary as to 5963 suitability as an audio tube, but like everything audio, opinions are only valid in the system under test. From a construction standpoint, US made tubes are insuperabile. In the M-125, they are excellent wrt PL Chinese 12AU7.

In response to a query regarding the 5963 substitution, Mr. Latino replied “In the last 9 years running the M-125's with 5963 driver tubes...” which some might construe Bait and Switch recommending one tube and delivering another.

BuckTrans

Interestingly, the 12AU7/5963 requires ½ the heater current of the 12BH7. Tube heater supplies are nominally 6.3vac ± 10%, with ±5% providing longer tube life. At 117v line voltage, with 5963/12AU7 tubes & 2xKT−88 heater voltage is 104% of nominal. v1/3 are 106% of nominal. v2/4 heater supplies appears designed for 12BH7 drivers with 2x KT−120s. If line voltage is anywhere near U.S. standard 120v, a bucking transformer is required with standard [2x 12AU7/5963 & 4x KT-88/6550] tube complement. Schematic, mechanical and parts list to build the pictured Bucking Transformer. Optionally, but not optimally, a variac may be used. Beware of cheap Chinese products recommended by Messrs. Latino & Mottram.

Other tube combinations will increase the heater imbalance, possibly reducing SNR. TWO tube, half-power operation requires further line voltage reduction to keep heater supplies within 10% of 6.3v nominal with standard tube complement.

From the Tubes4HiFi website: Sensitivity ....................... 1 volt in for 125 watts out with no mention of load, but into 8Ω 1v = 48w or a gain of 19.6 / 25.85db. About 1.6v is required for 125w, somewhat beyond most -10db consumer gear.

It's as if the amp spec was never verified. Any claims for power consumption, tube life, SNR are likely to be inaccurate and unverified.

Kit builders estimate about 10 hours to build an amp. There are less than 100 solder connections. An experienced builder making multiple sets with pre-cut lengths should easily complete a pair in about 14 hours. The factory cost to build is $817 or about $50/hr, a not unreasonable rate for an experienced technician, plus about $100 for shipping packaging.

IF ONLY!

CAVEAT EMPTOR.

Hummmzzz...

After repairing the broken switch, connecting up the amps, powering them on, I was greeted with a buzzy hum!?!?!? The hum, audible from 1m, did not diminish with only the input shorted amps connected to the TC-50s, not particularly efficient @ 86dbV.

Mr. Latino stated “My impressions are that you are "over analyzing" the amp and its circuitry when maybe you should just be listening to the music.” Right. On amps with less than 55db SNR!

The system is minimal: CD6006 −> LC1 −> XO1 −> Amp. LC1 & XO1 are self-designed passives. See Last Hurrah. AC power is dedicated circuit w 10ft run to AC panel. I have multiple amps and ALL are dead quiet. The only Earthed component is/are the power amp[s].

As a bit of background, my company, Studio City Sound Corp, designed and installed recording studio wiring in Los Angeles. With monitor level on S T U N, all one could ever hear is a slight thermal rush. Buzz? Humm? _ N E V E R !! _ Ditto my HiFi for the past ½ century. We designed scoring panels for film consoles, The Vocal Splicer, distribution amps and other tidbits. I was head of Analog Design @ AMS-Neve, perhaps the world's premier recording console manufacturer, from 1996 to 1998. HiFi grounding is child's play compared to a 96 input analog console, 2x24 track, multiple 2T recorders, hundreds of microphone inputs, outboard channels, tri-amped main monitors, 3 sets of small monitors, multiple cue systems interfaced across multiple control rooms and studios. I consulted w Monster Cable, Ibanez, Alesis, Yamaha and others wrt the sonic quality of their products. I also designed satellite based system monitors for ocean going freighters, ferries, tugs and large yachts. Bad grounding at sea can be fatal to both personnel and ship.

After several fruitless “It's not the amps” emails, I declined Mr. Latino's offer to return the amps.


surely you're joking, Mr. Latino?

Assembly Faults

Hand-built amps are not always pretty. These, however are just plain UGH-Lee! It is appalling how poorly the amps were assembled by Ben/Benjamin “I take pride in my soldering and quality work is of utmost importance to me” Woloss, Tubes4hifi and VTA Amp Tech.

Amateurish. Slipshod! Unsafe!! Garbage!!!


ShortLeads GndLug Solder Nutz
WireJoin BadCap pinch pinch

I would have been much further ahead had I returned them. However, they do sound very good and seemed to have potential once quieted.

I do like a good challenge:


  • Left Amplifier Faults
    • • AB-Q board linked to driver with short crossed leads making service impossible
    • • AB-Q R20-C6 lead across trace on solder mask, wire not in hole
    • • fuse lead hanging wire, cold solder joint
    • • Earth Safety cold solder joint
    • • dry joint on Green Heater CT
    • • nicked 6.3VAC power wires separated from AB-Q board leaving stripped end in hole when de-soldered
    • • spliced GND lead on supplementary cap module
    • • AuthentiCap loose, not well secured by tabs and prone to accidental chassis short
    • • output transformer v2/v4 500VDC blue lead pinched between transformer and case

  • Right Amplifier Faults
    • • power transformer bolts not tightened, barely finger tight, 2 nuts not down to chassis
    • • VERY POOR Earth Safety ground crimp
    • • GND 'star' bolt barely snug - multiple GND lugs should have shorting jumper
    • • fuse Holder not tight, about ⅓ turn loose
    • • ESL cap solder to fwd GND lug, leaving long uninsulated lead exposed which could touch GND on AuthentiCap
    • • AB-Q 6.3vac GND lead nicked and broke when de-soldered to fix polarity
    • • v2 bias lead nicked, broke on AB-Q removal
    • • dry joint on NFB lead which showed up after about 150 hours
    • • dry joint on r14 robbing v6 of current
    • • R22-C8 lead to ABQ board fell off

  • Both Amplifier Faults
    • • AuthentiCap GND cold joint
    • • dull grey solder joints indicative of low-temperature/dirty iron and/or movement during cool
    • • insufficiently flowed solder 'tacks'
    • • inconsistent assembly: some leads wrapped around terminals, some pass through, some just tack soldered
    • • inconsistent wire lengths, twist, connections and solder joints
    • • bias measurement jacks not snugged - required ½ turn to tighten
    • • speaker binding posts insufficiently tight. Rotated when finger tightened. Required ¼ to ½ turn to tighten.
    • • spliced AB-Q leads
    • • inconsistent and wrong heater wiring for use in 2 tube ½ power mode
    • • melted insulation
    • • overly long wire strips leaving uninsulated 500VDC B+ unnecessarily exposed
    • • uncleaned driver board flux blobs
    • • speaker Black nuts stripped, plastic rotates and will not tighten on spade
    • • teflon insulated wire for AB-Q. Teflon is very difficult to strip w/o special stripping tool.
          That several leads broke indicates incompetence/wrong tool
    • • 20k & 30k resistors swapped starving v6 and applying 300vdc to v7 rated 250v MAX

Mr. Latino did not respond to email reporting assembly faults.


Design Faults

    • • Power Tube Schematic REVERSES C6 & C8
    • • Multi-point GND:
      • ⏚ AuthentiCap
      • ⏚ soldertab 1
      • ⏚ soldertab 2
      • ⏚ Earth Safety ground
      • ⏚ driver board mount
      • ⏚ driver board GND loop
    • • phase splitter CCS location causes L/R imbalance with 2 amps closely spaced due to lack of temperature compensation - mounting underside would improve stability
    • • choke wiring
    • • DC wiring is cheap PVC, 600v 150°C Silicone preferred
    • • AC BIAS wiring run next to DC
    • • unterminated tube rectifier heater winding
    • • 120VAC power wiring adjacent to DC
    • • long audio input lead run through choke field
    • • long feedback lead run through choke field
    • • feedback and input leads run parallel
    • • mismatched power transformer polarity
    • • ESL capacitor location
    • • v1&3 tubes should be mirrored and HT leads run between pairs for best CMRR
    • • speaker jack mount inadequate for heavy cables, jacks rotate
    • • power tube sockets should use flat head screws or be atop the chassis to preclude KT‑88 rocking
    • • non-standard 16mm speaker terminals spacing - 3 crammed into space for 2 - precluding better quality WBT or similar
    • • binding post mount allows jacks to rotate
    • • Mode Switch contact rating overkill for required current. Performance may degrade from non-use
    • • LM334 is temperature sensitive. With amps side by side, Left most amp reaches design voltage sooner. In a cool room, Right amp may never. Space amps adequately to ensure design voltage.

how bad can it be?


LeftStock LeftStock

Per Mr. Latino, pre-purchase [February 2019]:
“I had someone test the M-125's about 9 years ago. He came up with a -95 dB signal to noise ratio. I do not know how that was 'weighted'. Typically with good input driver tubes, you have to put your ears on the grille cloth to hear any hum/noise with speakers of average efficiency.”


All measurements made on 4Ω tap. Unweighted SNR ≅ 55db L / 51db R. Measurements made with shorting plug, amps connected to 4Ω 100W load resistors, both amps plugged into same grounded outlet.

Either amp alone was somewhat quieter. Not quite ear on the grille cloth quiet, but better.
With amps connected to source, hum/noise almost tolerable at seating position 4m distant.


Jigs

LineSyncAdpt BuildStand

Line Sync Adapter: Gives a 100:1 reduction for use with scopes with reduced input range and/or do not have Line Sync. DO NOT use this before determining the power socket polarity is correct. A mis-wired socket or power strip would put line voltage on the ground connection. If the voltmeter does not measure ≈1.2vac S T O P !! and sort out the fault.
This jig should only be used for initial transformer and heater polarity determination. Once polarities are determined, use one of the heater voltages referenced to Audio GND.

Build Stand: A pair of stands make working much easier. Heights are ex-pads. [From similar on The Dynaco Tube Audio Forum]


Power Transformer Polarity

TransChoke TransPolarity SyncLeft SyncRight
PTPolarity

First thing to check is the power transformer polarity. Since the Audio GND is tied to Earth Safety, if the transformers are inverted, induced fields are inverted and may increase noise. I've been checking polarities since the '70's and it often pays dividends to correct.
Polarity can be detected by means of a choke placed next to the power transformer and the choke output measured with a scope synced to the line frequency. Choke lead polarity is arbitrary, but marking it keeps measurements consistent. Measure only one amp powered at a time. As can be seen, the power transformers were inverted relative to one another. Choose the less noisy polarity and mark the power transformer leads, fixing as necessary.


The above drawing shows induced current flow with transformers with inverted and correct polarity.
The lower block shows the effect of a correct Single Point Audio GND. As built, with the 9 point Audio GND there is resistance between each gain block. Ground currents shift the reference. With a true single point ground, the relative voltage at each gain stage does not shift.

Heater Wiring

PwrTubeHtrPolarity

The build instructions correctly advise to invert heater polarities between v1 & v3 and v2 & v4 sockets. No mention is made of heater polarity for the driver and AB-Q installation.
A feature mentioned on the website: “NOTE - The amp may also be operated with just TWO OUTPUT TUBES - one of v1/v3 output tube and one of v2/v4 output tube.”
Given that there is no provision for determining output heater polarities, arbitrary polarities on any tube pair could compromise SNR. If half power operation is planned, ensure heater polarities are the same for socket pairs v1/2 and v3/4.


The above image shows the Heater Polarity concept for a Push-Pull [PP] amplifier. The inverter stage splits the signal into inverted polarities. If Heater Polarity is inverted, it functions the same as the signal and creates a current in the Output Transformer. If Heater Polarity is the same, no signal is created in the Output Transformer.
NOTE: For pairs of tubes, e.g. 4 output tubes, heater polarity should be inverted between the tubes. Tubes in mechanically the same location, e.g. v1/v2 on the M-125 should have the same heater polarity.
For most tubes, improvement is slight as their construction accounts for heater fields. In an all out design, I'd use an external DC heater supply.


Heater Polarity Chart

HeaterPolarity


P is in-phase with line, N is out.
Polarity is best determined before the transformer heater wiring is connected.
The right amp v2/v4 heaters were in-phase when they should be out, contributing to the as built 4db SNR reduction.
It was determined by inspection that Modified polarities produce the most benign noise signature in the M-125s I received. Other Revisions might require other choices.

I do wish Tubes4Hi/VTA were less circumspect in publishing revision changes.
With 16 M-125 revisions in 10 years, it's almost Amp of the Month Club!


DC Wiring

DCwiringStock WiringOrig

Prototype at left looked reasonable. Wiring as built very poorly implemented. AC input tie-wrapped to GND and DC. Choke input twisted with output contaminating the output with the pulsed DC input.
Chokes are not counter placed to cancel fields.
Too much back and forth to the capacitors unnecessarily increasing lead radiation and induction lengths.


DCwiringMod

Mount the chokes with leads inboard. Wire from the rectifier-out/AuthentiCap terminal to the rear choke leads. Wire the choke output direct to the output transformer RED lead along with a piece of black A/C lead long enough to reach the Supplementary Cap Module [SCM]. Wire the linked AuthentiCap & terminals and the added black lead to the SCM input. Wire the ESL film cap across the input and output of the SCM. Space may be tight with the Pent/Tri switch installed.


Miscellaneous Wiring

RectHeater ACWiring AudioInput

Rectifier Heater: With a Ted Webber WS1-T solid state rectifier, rectifier heater wiring is unused. An unterminated winding is a potential noise source. If using a solid state rectifier, remove from socket, add a ≈5k/1W to terminate the winding and mount off to the side to remove 120Hz pulsed 500VDC into the winding.

AC Input: With externally controlled systems, a power switch is superfluous. It's never a good idea to run 120VAC leads in close proximity to DC or signal wiring. Hard wire power transformer to IEC socket and route away from all other wiring. If a power switch is required, mount it on the rear panel. This will cut hum substantially.

Audio Input: Without the power switch, there a convenient hole on the front panel for the Audio Input jack. In addition to short leads, the audio input no longer passes through the choke fields and maximizes distance from the AC power fields.


GroundLifts/CheaterPlugs are DANGEROUS, Irresponsible and in some locales, ILLEGAL!!

Single Point Ground


MultiGnd

Although the M-125s are MonoBlocks, functionally they become one amp when connected to a common source due to their unbalanced inputs. With the Earth Safety ground connected to Audio GND, Ground Loops are almost a certainty.
Rod Elliott has a very good page on Earthing and Ground Loops in particular. The bottom line is that there should be only one Audio GND point. FULL STOP! The Earthed chassis should function as a Faraday cage only and ne'er the twain shall meet.

As designed, the M-125 has a 9 point ground. Never good.


CapInsulation CapCutout AuthentiCapInsulatorBottom AuthentiCapInsulator

AuthentiCap: Other than cachet, why use it? Add clearance for GND tabs. Insulate it from the chassis. Mount it securely with a clamp! Bend the ground lugs inward and add a shorting link across two, connected to the Audio GND Star terminal strip. Wire transformer Audio GND to this link.

GND Star Point: Replace the ground lugs with an insulated terminal strip similar to the one used for the triode resistors. Add a buss through the lugs. Link to the AuthentiCap GND and solder all ground leads here.

BIAS Wiring: Remove BIAS lead from wire loom and run down middle of power tubes. The pulsed ½ wave rectified DC has no business loomed with Audio DC.


DriverMountHdwr

Driver Board Mount: Replace the 4/40 steel hardware with #6 [3.5mm] nylon screws and stand offs. If using the AB-Q board, mounting holes need drilling to fit.
McMaster-Carr #6 parts w AB-Q installed: 92319A658 • 94735A729 • 94735A732 • 92319A610




DriverGNDtrace

Driver Board GND Loop: Verified M-125 Driver v16 revision ONLY. The driver board ground runs around the outside of the board, the very definition of a loop. From inspection, it appears the loop to the right of the vertical trace is for bias and the left for audio. Cut the loop to reduce the effect of the shared bias/Audio GND. Be certain and careful IF doing this revision!!
Other versions likely have a similar fault and the cut point may be different.


making it safe


BridgeMount

Per the Loop Breaker schematic on Rod Elliott's page, add a 50A [10x device 5A fuse] diode bridge linking Audio GND to chassis/EarthSafety ground. Cap provides RF sink. 10Ω/5W resistor is somewhat arbitrary, with values from 10 to 100 ohms generally regarded as satisfactory.
Amazon: 2x 50A diode bridge Parts Express: 10R resistor • 100nF capacitor.

Space is tight, so it is necessary to enlarge the right-rear output transformer chassis hole slighly to 11/64" [4.5mm]. A #8-1" bolt is required. Link Audio GND at the heater ground buss. Tie the Earth Safety link at the same bolt as the AC input Earth Safety.


things are looking up


LeftMod LeftMod

Unweighted SNR ≅78db L/R. Still not quite the 95db someone claimed to have measured. The waveform however is a dead giveaway for the half-wave bias. Shame on you, Messrs. Mottram & Latino. A full-wave bias would reduce current pulses by 50% and double the frequency decreasing the noise by 12db w bias LPF mod below.


An additional benefit would be obtained having a separate bias center-tapped winding rather than half-wave tied to the HiV CT [Audio GND]. Maybe later...


be careful, your bias is showing


BiasV4 BiasV4_Post

Left image is v4 bias, a classic ½-wave.
Holger @ Erhard Audio, U.S.A. agent of the AB-Q AutoBias, was kind enough to tell me that replacing R39 on the driver w 100Ω would allow enough current for the AB-Q board and a little more LP filtering. Move the bias lead to the C18/R39 junction.
Right image LP filtered bias is 13db better - but not as good as a full-wave regulated DC supply. Maybe later...


AB-Q_BIASmod

Note that some schematics are incorrect w C17 & C18 RefDes. The R39 end farthest from D1 is the correct connection point. DO NOT use the GRAY arrow connection.

Ignore shown feedback wire routing. Run it midway between the chokes, angling over to the side, connecting at chassis side of driver board. Tack in place.

AutoBias is EXCELLENT! All tubes hold idle @ -0.500v -0.001v/+0.000 and do not drift. With steady state signal applied, they hold precisely. A much better AutoBias implementation than on the PL5. AutoBias was not measured as built, only on the corrected build.


it doesn't get any better than this


LeftModBias LeftModBias

Unweighted SNR ≅93db L/R. I have to put an ear ON the grille cloth at the woofer to hear anything. Additionally, notice how little hash is present. Absolutely NOTHING can be heard from the tweeter, a vast improvement!



Mr. Latino did not respond to a friendly offer to tell him how the amps had been quietened.

ieLogical Revision

ModLeft Completed ModRight BiasSelector
ModSch BPmountOrig BindingPost M-125_BP_Mount

Since the amps are now Pentode only, a DP ON-OFF-ON switch was added in place of the Triode resistor terminal strip to allow v1/2 or v3/4 bias verification. Power Schematic - w AuthentiCap symbols, SCM and ESL values and AB-Q connections - as modified from original.

I got fed up with the Binding Posts and mount that would not hold. Replaced with Vampire BP-HEX from Parts Connexion. Mount is milled nylon, but could possibly be 3D printed. These amps remind me of a Fiat 131 mirafiori I owned in the 70's. Once I got the crappy original parts off, it was a great car!

"Audio Zealots"† will understand that the background is now inky black. Delicate sounds shimmer, decay goes longer, impact is more stunning.

Very Nice Amps, Bob & Roy. Truly!



... Original page ended here ...



banned!


Banned

Jan 4, 2020: created a DTAF topic
    "teaching the VTA M-125 to sing"

3 lines:

  • my VTA built M-125 amps were noisy
  • it is possible to make them almost 40db quieter
  • a link to this page

4 images showing the noise as delivered and as quietened



Jan 5, 2020: Not wholly expected, banned from DTAF. Topics & posts deleted. I guess Smilin' Big Bob wasn't amused.

* - One has to wonder about DTAF comment validity when accounts are banned for anything critical and unrelated topics deleted. Subsequent to publication, many Tubes4HiFi product owners have written confirming a lack of transparency and support from Messrs. Latino and Mottram.

† - Mr. Latino referred to me as an Audio Zealot because I didn't like a PoS Chinese Variac he recommended. In reply, I detailed numerous faults by whom I consider competent, not fan-boy "it lights up", Amazon users. Posts deleted.


who are those guys?


Pre-purchase Mr. Latino stated:
“The sonics of the amp will be the same with the AutoBias board. You won't really hear any difference in sound quality with the AutoBias board installed. I personally have never compared the two (manual bias and AutoBias) MYSELF but others tell me that the sound is the same. The AutoBias board will give you greater convenience.”

7-January-2020:

All AutoBias products and references scrubbed from Tubes4Hifi.

Mr. Latino states on diyAudio in VTA M-125 mono block kit. on diyAudio:

AutoBias Tubes4HiFi

“Ian's amps have the "autobias boards" that are sold by an independent distributor. Ian requested that they be added to his wired amp. The -95 dB signal to noise measurement for the M-125 was done 10 years before the autobias boards were ever created. When you remove the old bias pots and "piggyback" a second board on top the driver board and start adding wires here and there to connect the autobias board, I am sure that the amp's residual noise level will increase somewhat.“

This is utterly false. The product is branded Tubes4HiFi. I found the AB-Q boards on Tubes4HiFi and DTAF, their propaganda website. Tubes4HiFi / VTA were building my amps. The original wiring is a rats nest of wires here and there specified by someone who never gave routing a 2nd thought! See DC Wiring above.

In response to a pre-purchase query about the AutoBias boards, Mr. Latino stated:
“The cost for just me buying the two autobias boards for the two M-125's are $180. To install them would be $100 for the two boards ($50 per each amp) - Total > An extra $280 for the cost of the two boards + installation. The cost is high. It takes time after installation of the board to adjust the boards for the proper bias”

Yeah, right. All of 30 seconds per amp to adjust level in a shop with a 6.3vac supply.

In response to to a query about R39 specific to AB-Q, Mr. Latino stated:
“I can tell you that on the M-125's R39 can be either a 1K resistor or a jumper wire. I use a 1K resistor but a jumper wire will work OK also. What R39 does is that it sets the "range" over which the bias may be adjusted.”

Mr. Latino exhibits complete ignorance on something as simple as a voltage divider and LPF relative to the AB-Q install. VTA's instructions are at fault as proven by the 15db noise reduction when the AB-Q is properly installed. See be careful, your bias is showing above.

Why the complete turnabout from the 6 pages extolling AB-Q products on DTAF AutoBias board on Roy's website?

I'm beginning to think Tubes4HiFi / VTA may be good at copying 1950s designs when 117VAC did not have a Earth Safety connection, but incapable of designs for US Standard Earth Safety Grounded [since 1959] 120VAC [since 1967]. Their designs may work in some, but not, all systems. They may be completely out of their depth in solving issues from bad instructions and design.     “Easy? Help — Hard? Ignore”

Mr. Latino stated in the above referenced diyAudio thread:
“I am now not recommending the addition of the autobias boards to any of the VTA amp kits or wired amps.”
Based on experience to date, a Tubes4HiFi / VTA non-recommendation may be a ringing endorsement.

Adding Insult to Injury: DTAF users are not informed that an account is banned. User messages to banned users cannot be replied on DTAF. Users may think the banned user is giving them the shine.
Messrs. Latino & Mottram: Truth will out!
<Soto voce: Should these guys decide to go into politics, their integrity is well on its way.>


See Erhard Audio AutoBias Modules for all the reasons why AutoBias is preferred. These are my 9th tube amplifiers, 2 x M-125 counting as one. The last two are AutoBias and, perhaps due to my advancing age, I'd not buy an amplifier without it!
The AutoBias boards now have updates to make them more bulletproof to get around some of the Tubes4HiFi flaws.

ieLogical has no affiliation with Erhard Audio or AudioAmp.eu and all information here is presented as a public service to victims of Tubes4HiFi / VTA mistakes and/or non-support.


the project that wouldn't die


ResLeft ResRight NoFB R22-C8

Things started getting weird in February 2020. I was primarily interested in Tri-Amping the Eminent Technology LFT-8b, never listening closely, but something was clearly not quite right. Channel imbalance, strange distortion. Just as it was getting too obvious to ignore:

CoViD-19, a page to cut through the crap.

Finally getting the amps on the bench, I find the 20k & 30k resistors are swapped on the driver board. A mere 30% over voltage and current on v7. Think Big Bob will send me new tubes? No? Didn't think so either.
The NFB lead on the right amp is a dry joint and failed. Instead of Negative it became NO Feedback.
Right Amp: R14 also had a dry joint dropping v6 power. R22-C8 lead popped off upon ABQ removal.

Nothing to do but tear this garbage to bits and rework it all like I should have done last year!

Without a doubt Mr. Bennihaha Woloss is the absolute worst technician klutz I've come across in 50+ years in HiFi.

and hopefully last, the left input jack has an intermittent short!!! Ever since day one, the left amp would fail to play. Swapping cables and tubes appeared to fix, but problem always re-appeared.


OCD takes a holiday - NOT!

AB-Qwiring BiasCct BiasModComp BiasModInstall
First image is recommended AB-Q connection and wires to remove.
The AB-Q board handles Bias. There is ZERO need for any Bias on the driver board.
Power tube current is returned to the AB-Q board. AB-Q 0vA should be returned to 0vA star.
Bias current requirement is very low, ≈5mA. A simple RC filter shown in the 2nd image above reduces Bias noise substantially.
3rd image above compares Bias noise of factory, first 100R mod and the new filter. A 26db improvement over factory!
SNR is now >100db unweighted wrt full power.

RED plated! now I'm pissed

So after fixing the driver board, the bloody amps red plate and oscillate at anything over ≈40W! Ken Galer had similar problems with his custom build. See M-125 Build on DTAF. The amps are works of art.

Since the M-125 is basically a rip of the Dynaco 120 / Mk VI, I went back to the originals to see what Latino & co. had screwed up. See Dynaco Transformers for the original 120 and Dynaco Mark VI Manual. Both the 120 & Mk VI have a load [680ohm;/120 - 620Ω/MkVI] across the output transformer 16Ω tap, plate & g2 snubber resistors, and local feedback from the -phase U/L g2 tap nested inside the global from the OPT 16Ω tap.