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OUT OF STOCK, you may make a deposit for June delivery

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.
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 for June shipments.  See below.

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) from zip code 44024 by UPS ground"

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,
$350 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
available in JUNE 2022

Time Delay Relay

This is a "time delay relay" board" for tube amps and preamps.
This board is now INCLUDED with the M125 amplifiers.
A relay switch will apply the high voltage power to the amplifier after a 17 second delay

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.
$30

 
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.
 


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. We normally have the 220/240 volt transformers in stock. Email me for more details

 


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

   
classic style above left - - new style above right



bottom view of M125 showing wiring

YouTube construction videos

(they were all removed by YouTube ? )

review of the M125 amp on Audio Asylum

M125 build by a first time kit builder (June 2011)
M125 completed - review #1 (May 2013)
M125 review by a Newbie (May 2015)

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Digital Voltmeter


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 "the path of least resistance is through a vacuum"