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.

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

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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

 

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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 -  #1  (May 2013) 
		
		
 M125 review by a Newbie  (May 2015) 
	
		
					
	

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