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the VTA M-125 tube amplifier KIT


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.
6SN7 driver boards are available as an option.
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 (Mundorf output caps and 100uF 400v power supply caps).
The triode/ultralinear switch is now OPTIONAL.

Note - Pricing  $2100 for a PAIR, not including tubes.  Taking deposits now
Single amps are available, just send an email. 

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

"Shipping costs will be invoiced separately - ~68 pounds (boxed up weight for two kits) by UPS ground"

pair of M-125 KITs
$2100 + shipping

$3000 + 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
make a $200 deposit to pre-order
& 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.

Time Delay Relay

This is a "time delay relay" board" for tube amps and preamps.
The time delay allows the tube filament heaters to come up before any high voltage is applied. The time delay is nominally about 17 seconds but may be adjusted for more or less delay at the discretion of the user. There are complete instructions for the assembly and installation of the board. In a stock ST-70 the board replaces lugs 5, 6 and 7 of the 7 lug terminal strip. In a VTA ST-70/ST-120 tube amp kit, the VTA-TDB replaces the 3 lug terminal strip. The VTA-TDB may also be easily retrofitted to any VTA ST-70/ST-120 tube amp kit.

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
purchase a TDR KIT separately
(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.

triode-pentode switching added to your wired amplifier order $60 for a pair


M-125 FAQs
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?
The M-125 uses the same basic circuit as the ST-70 and ST-120 and sounds very similar to my other amp kits at lower volume levels.
The M-125 will, however, play about 3 dB louder than the ST-120 and about 6 dB louder than the ST-70.
Slight differences are that the M-125 has a somewhat wider bandwidth and a little less gain.

3. What is this 4 output tube or 2 output tube option?
The circuit is arranged so that the amp may be operated with either two left side and two right side output tubes OR just one left side and one right side output tube.
For two output tube operation it doesn't matter which left side tube socket or which right side tube socket is used as long as one of each are used.
The amp must be rebiased when you switch from 4 output tube operation to 2 output tube operation.
The recommended bias setting with four output tubes is 1.00 volts DC FOR EACH SIDE which provides a bias current (using a 10 ohm bias resistor)
of 50 milliamps for each of the four output tubes. For two tube operation the recommended bias setting is .500 volts DC per each side
which also provides a bias current of 50 milliamps for each of the two output tubes.

4. Could a single VTA M-125 be used as a guitar amp?
The amp has a standard RCA input and is designed to be used for home audio. The input jack could be easily modified by any kit builder to accept an electric guitar input.

5. Is the VTA M-125 as easy to assemble as your ST-70 and ST-120 amp kits ?
Actually the M-125 is even EASIER to assemble than either the ST-70 or ST-120 amp kits. The chassis is about the same size as the ST-70/ST-120
but there is only the wiring for ONE channel instead of two channels. As with the ST-70 and ST-120 amp kits, the assembly manual is written in detail for the beginner.
The color pictorial photograph included with the amp kit shows the location and termination point of every wire.

6. Are there any options for the M-125 amp kit ?
No - The amp comes with power supply upgrade caps and upgraded main coupling capacitors as standard equipment.

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)?
The M-125 amps in kit or wired form are now available in 220/240 volt.
The 220/240 volt transformers are special order and can take 8-12 weeks to get in stock. Email me for more details


send us email at: for any questions about the VTA M-125 amp KIT...

Classic style M125
New style M125 available upon request

bottom view of M125 showing wiring

YouTube construction videos

M125 build part 1 M125 build part 2 M125 build part 3 M125 build part 4 M125 build part 5 M125 build part 6 M125 build part 7 M125 build part 8

amplifier parts - transformers & chassis

review of the M125 amp on Audio Asylum

M125 build by a first time kit builder (July 2011)
M125 completed -
(May 2013)
M125 review by a Newbie (May 2015)

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Digital Voltmeter

check your bias !
one for $14 free shipping
two for $22 free shipping


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 tubes 4 hifi
 "the path of least resistance is through a vacuum"