push pull

I am beginning to put together a Marshall 1974x, better known as the Marshall 18W amp. This all tube powered amp will be built in a head cabinet that can be hooked up to an external speaker cabinet of choice. This 1974x circuit won’t be stock though, I am using the 1974x circuit as a platform to try a few modifications and circuit tweaks/ideas. The end goal of the build is to have a low power Marshall JTM/JMP/Plexi type amp. Lots of big fat clear clean tones and sweet harmonic distortion.

The chassis, eyelet board, power transformer, output transformer, faceplate and head cabinet are coming from the good folks at Trinity Amps. The rest of the parts I am sourcing myself.

[you know you want to read more…]

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TC-15

15 July 2010

The TC-15 is a Trinity Amps version of a 15W Matchless DC-30 clone. The Matchless DC-30 is a very popular 30W boutique amplifier that is modeled after various Vox “AC” amps. The DC-30 is well known as a classic point to point handwired amp that recreates many classic Vox tones in one package.

The TC-15 takes the DC-30 but scales the output down to 15W by using two EL84 output tubes instead of four. The TC-15 also uses an eyelet board to build the amp off of instead of the somewhat chaotic layout of the point to point DC-30.

With two uniquely voiced channels and plenty of controls/switches the TC-15 is a versatile amp. I had played a few stock TC-15s before deciding to build my own so I knew just how good they were and how many great British tones they could produce. With this in mind I decided to build my own TC-15 but with a few tweaks and modifications.

Parts:

  • 1W carbon film resistors for the signal path
  • 1/2W carbon composition resistors for the plates
  • 2W metal oxide and 5W cermet resistors for the power supply
  • SBE 6PS Orange Drop capacitors
  • Alpha potentiometers
  • Cliff jacks
  • Belton tube sockets
  • Mountain switches
  • PVC wire
  • Accutronics reverb tank
  • Hammond choke and reverb transformer
  • Trinity Amps designed Heyboer made power and output transformer

Build:

Below are some pictures of the build. I tried to keep things neat as there was a lot going on inside the chassis wiring wise.  Lead dress was important with so many tubes and high gain signals running throughout the amp.

The build went smoothly though and the amp worked first time.

Design:

The TC-15 is a 15W and runs two EL84 output tubes in Class AB1 push-pull configuration. The amp has two channels, Channel 1 and Channel 2. Channel 1 uses an EF86 pentode preamp tube. Channel 2 uses two 12AX7 preamp tubes. The phase inverter uses a 12AX7 configured as a long tail pair. Finally a GZ34/5AR4 tube rectifier tube is used to supply the high voltage to the amp.

Channel 1 using the EF86 pentode features a Volume control and a six way contour rotary switch. The contour switch selects different coupling capacitors coming off the EF86 to provide six unique sounds ranging from thin, bright to full. The contour switch is used for simplicity and also because the output impedance of the EF86 stage is too high to drive a conventional tone stack.

Channel 2 uses two 12AX7s and has controls for Volume, Treble and Bass. Channel 2 uses a parallel triode input stage followed by DC coupled cathode follower to drive the Vox style tonestack.

In addition to the controls listed above there is also a Cut control which affects both channels. There is also a Master Volume that affects both channels to, it is a push-pull switch so it is possible to disable it. On the back of the amp there is two mini toggle switches. The first mini toggle switch labeled Crunch/Much selects between 5KΩ and 8KΩ primary impedance on the output transformer. This allows different loads to be presented to the EL84 output tubes and changes the tonal and distortion characteristics depending how the switch is set. The second switch is labeled Boost and adds a cathode bypass cap to the unbypassed cathode resistor on one of the triodes on Channel 2. This provides a gain boost across all frequencies and makes the amp a bit fuller and livelier sounding.

Cascade:

On Channel 2 the input stage is a parallel triode configuration. The parallel triode configuration allows slightly more gain than a single triode but not as much gain as two cascaded triodes. The parallel triode configuration also offers better noise rejection (very important for an input stage) and a slightly bigger fatter tone.

I decided to use a 3PDT toggle switch and wire it for Parallel/Cascade switch. When in Parallel mode the amp is completely stock. When in Cascade mode the two triodes are no longer in parallel but rather cascaded in series. The result of Cascade mode is a ton more gain and bite. Putting Channel 2 in Cascade mode is basically like putting the amp in overdrive mode.

The switch wiring is a little tricky and it took a while to get right. I had to control the amount of gain and make sure the amp did not oscillate because of the high gain/signal levels. It is very neat though to be able to toggle between Parallel and Cascade modes and hear the differences.

Reverb:

I thought a cool addition to an already harmonically complex amp would be to add reverb. I decided to add the reverb to Channel 2. I decided on Channel 2 because then I would not have phasing issues with the reverb signal and dry signal canceling each other out. This would happen if I added reverb to Channel 1. That said if you bride the inputs while plugged into Channel 1 you can get reverb with no ill effects.

Since I only had one noval socket to add the reverb I was physically limited to what reverb circuits to try. After much research and design I settled on a single 12AX7 reverb. The reverb circuit uses a 12AX7 tube setup in the typical drive and recover configuration as many reverb circuits are. A small Fender style reverb transformer is used to couple the tube to an Accutronics medium delay medium dwell reverb tank.

After much circuit tweaking I ended up with a very nice reverb circuit. I added a control on the back of the amp for Reverb Level and I also added a footswitch for turning the reverb on and off. Unlike some reverb circuits that can rob the amp of its original dry tone this circuit does not. When engaged the reverb adds a nice spacey effect to the amp which adds a lot of depth, makes the amp really fun to play. The reverb is not super intense and over the top like some circuits (Blackface Fenders for example). It is also not thin and “spanky” sounding but instead offers  a bit more midrange content and sounds a bit fuller.

The reverb tank barely fit inside the head cabinet and the wiring is very “busy” around the reverb tube socket but overall it was a worthwhile mod. I personally really like reverb on my amps so having it added to this already great sounding amp was a big bonus for me.

 Power Supply:

The power supply is unique in that the standby switch is a three position switch. In the middle position the amp is in standby mode. When the switch is put in the up position the amp is put into play mode with a choke in the power supply circuit creating the screen voltage node. When the switch is put in the down position the amp is put into play mode with a resistor in the power supply circuit creating the screen voltage node. Most amps either use a choke or resistor to drop some voltage in the amp and create a screen voltage and subsequent supply nodes. This three way standby toggle switch allows the user to experiment with a choke vs. a resistor in the power supply.

The choke provides a little tighter low end as it does not drop as much voltage across it as the resistor. The resistor has a larger voltage drop across it compared to the choke so with the resistor the tone is a bit grittier and not as tight. There is also slightly less headroom using the resistor in the power supply.

Other Modifications:

Other modifications to the circuit include the addition of a trim pot to control the amount of mids on Channel 2. The mod is done by replacing the 10KΩ resistor in the tonestack with a 50KΩ trim pot.

There is also a 850Ω trim pot in series with the 720Ω tail resistor on the phase inverter to control the gain/drive of the stage. This trim pot allows one to set the headroom of the phase inverter and essentially how quickly the amp distorts.

I also made subtle tweaks throughout the amp to control brightness and low end. Vox style amps can sometimes be a bit bright and loose sounding so I wanted to make sure I had just the right amount of high end sparkle with a solid bottom end.

I did find on the EF86 based Channel 1 the sound was a bit tight and stiff sounding without much bloom at first. I found that lowering the screen voltage made a larger impact on the compression, feel and tightness of the channel. With the stock 2MΩ screen resistor the screen voltage was 85V. With a 1MΩ resistor the screen voltage rose to 119V. The increased screen voltage caused a more pleasing tone, not as stiff with more compression and feel to the tone.

 

Schematic:

More schematics coming soon!

tc-15-done-reverb-schemtc-15-cascade-mod-schematic

 

 

 

Results:

 

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