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This is a 125 watt monoblock amp IN KIT FORM that uses four output tubes in a parallel push pull output circuit. The amp is built on a 10 inch by 12 inch 16 gauge brushed stainless steel chassis and uses a modified VTA Mark III driver circuit. The recommended tube complement is 4 X 6650 output tubes, 2 X 12AU7, and a 5U4 rectifier tube. Any of these tubes can be used . . . 6550, KT88, KT90, KT100 or KT120. 12BH7 can be substituted for 12AU7. NOTE - A tube set is not included with this amp KIT. On the front face is an on/off switch and three bias test pins. One test pin is used in biasing the two left output tubes and one test pin is used in biasing the two right output tubes. The bottom center test pin is a ground pin. The rear face contains one RCA input jack, a fuse holder, an IEC AC connector, and a pair of output binding posts. The output binding posts may be set up for 4, 8 or 16 ohm speakers. The M125 amps now come standard with capacitor upgrades (output caps and 100uF 400v power supply caps). The triode/ultralinear switch is now an OPTIONAL addition (since most people don't use them). Note - Pricing $2000 for a PAIR, not including tubes. Taking deposits now You will need a PAIR of M-125 monoblock amps for a two channel stereo system. PA -125M power transformer 4.50" W X 3.75" D X 3.875" H Weight 13.5 pounds 415-0-415 secondaries @ 450 milliamps Two 6.3 VAC @ 5 amp heater lines One 5 VAC @ 5 amp rectifier line One 55 VAC bias line A-125M output transformer 4.625" H X 3.875" W X 4.25" D Weight 11.5 pounds Primary impedance 2200 ohms Secondary impedance 4, 8 and 16 ohms List of parts. 1 - 16 gauge heavy duty brushed stainless steel chassis 1 - VTA driver board and parts set with upgraded power supply and main coupling caps included 1 - PA-125M power transformer 1 - A-125M output transformer 5 - Celanex octal tube sockets 4 - ground lugs 1 - 550 volt rated quad cap 2 - C24-X chokes 1 - SCM (supplementary cap module) 1 - set of ESL bypass capacitors 1 - fuse post and 5 amp fuse 1 - 15 amp rated SPST on/off switch 3 - bias test pins 1 - Bulgin IEC connector 1 - 14 gauge 15 amp rated 6 foot IEC AC power cord 1 - 3 lug terminal strip 1 - 2 lug terminal strip 2 - 10 ohm 5 watt bias resistors 4 - 1000 ohm 1 watt resistors 4 - 100 ohm 1 watt resistors 1 - 20 foot coil of 20 gauge tinned solid core copper wire 1 - gold plated RCA input connector 1 - pair of gold plated output binding posts 1 - amp hardware set with all stainless steel screws NOTE - The amp may also be operated with just TWO OUTPUT TUBES - one left side output tube and one right side output tube. The amp just needs to be rebiased when switching to two output tube mode. With just two output tubes, the amp will have about 65 watts available. Or order with two KT120 tubes, increased bias will put out 100 watts with just two tubes, longer rectifier life ! Specifications for the VTA M-125 amp KIT ... Power output (4 X KT88) 130 watts from 17 Hz to 32 KHz at less than 1% THD in ultralinear - 70 watts in triode Power output (2 X KT88) 70 watts from 17 Hz to 34 KHz at less than 1% THD in ultralinear - 40 watts in triode IMD .................................. < .38% (19 Khz and 20 Khz) at rated power Frequency response ....... 10 Hz to 35 KHz +/- .1 dB at 1 watt Power bandwidth ............ 17 Hz to 37 KHz +/- .3 dB at 125 watts Sensitivity ....................... 1 volt in for 125 watts out Feedback .........................10 dB Damping factor ................14 Input impedance .............. 270,000 ohms Hum and noise ................ > 95 dB below 125 watt output The M-125 does not invert phase Dimensions - 10.0" wide X 12.0" deep X 7.10" high Weight (each) - 34 pounds > shipping weight 38 pounds
|pair of M-125 KITs
with NO TUBES
$2000 + shipping
|WIRED PAIR OF M-125 amps
with NO TUBES
$2800 + shipping
TUBE SETS - you can buy your tubes from any vendor you want,
or directly from here for your convenience
for four Sovtek 6550WE tubes and two Tungsol 6189/12AU7,
$375 total for EACH amp.
CLICK on the "add to cart" button TWICE
for a PAIR of M125 tube sets
& hold an amplifier
|220/240 volt power transformer for the M125 amp kits (add $120 USD for two)
for use in Europe, Australia and other 220/240 volt areas of the world.
8-12 weeks needed to get these.
Email me for more information on the 220/240 volt availability.
This is a "time delay relay" board" for tube amps and preamps.
The board is 1 7/8 inches wide by 2 1/2 inches long. It takes about 1/2 hour to assemble and is powered by a 6.3 volt AC line taken off V2. When the board is powered up a RED LED lights up to show that the board has power. About 17 seconds after power is applied, a GREEN LED comes on, and the relay kicks in to apply high voltage to your amp. Another nice thing is that if you are in an area that has quick ON/OFF/ON power interruptions the board will automatically reset itself. In other words, if your power goes OFF and then ON again a split second later the board will automatically reset itself and no high voltage will be applied for another 17 seconds.
This is only a delay board - you still need a solid state or tube rectifier to plug into the rectifer socket. If you use a GZ34 tube rectifier IMHO you can get by without it. If you use a solid state rectifier like the Weber WZ68, which only has a 3 to 5 second delay, I think it is helpful. It would also be helpful if you have the quick ON/OFF/ON power interruptions.
This board is to be used ONLY for solid state rectifiers. If used with a TUBE rectifier, the TDR can cause problems with the normal slow warm up of the tube rectifier and may damage a tube rectifier. If you intend to use a TUBE rectifier ONLY with your amp, you do not need the TDR. If you do use a tube rectifier with the TDR, you should BYPASS the TDR. Instructions are included with the TDR on how to bypass the TDR.
This board is a relay switch that will not apply the high voltage power to the amplifier until after the tube filaments are warmed up, the board has a 17 second delay and that can be changed by changing one resistor on the board.
This is a very heavy duty industrial relay used for 270vac 3 phase and rated at 8 amps, with 1500v of isolation between the contacts. Similar delay circuits I evaluated for tube amp use had 240v contacts with 1000v of isolation and were physically larger, this is a great addition to your tube amp. It can run on 6vAC or 12vDC.
|ADD a TDR KIT to your kit amp purchase $35||ADD a TDR to your WIRED amp purchase $60||
(free shipping in USA only)
Triode/pentode switching kit |
This kit may be added to any amp to allow your amp to operate
in either the normal pentode ultralinear mode, or switch to sweet sounding triode mode.
In triode mode the power is about half as much, and the bass and treble are rolled off
MOST people much prefer the normal ultralinear mode, but triode can sound better on certain vocals or instrumentals
Comes with all parts and instructions. $30 for a pair with FREE First Class mail delivery in the USA.
1. Are there any advantages to the 16 gauge stainless chassis?
Besides the obvious fact that this chassis will never rust, a stainless chassis is non-magnetic and cannot hold a magnetic field.
Amps built on stainless steel chassis tend to be quieter than amps built on standard steel chassis.
The 16 gauge chassis weighs about 1.25 pounds more than if it was made of 18 gauge steel.
The 1.25 pound extra mass better supports the heavy transformers and also acts as a somewhat larger heat sink to help the two transformers run a little cooler.
2. How does the VTA M-125 compare in sound quality to your other two kits - the ST-70 and ST-120?
3. What is this 4 output tube or 2 output tube option?
4. Could a single VTA M-125 be used as a guitar amp?
5. Is the VTA M-125 as easy to assemble as your ST-70 and ST-120 amp kits ?
7. Is the VTA M-125 amp available in a 220/240 volt version for 240 volt countries and can you ship one to me in (any country with 240 volt AC line voltage)?
send us email at:
|M125 build by a first time kit builder (June 2011)|
|M125 completed - review #1 (May 2013)|
|M125 review by a Newbie (May 2015)|
check your bias !
one for $14 free shipping two for $22 free shipping