Running a 2W guitar tube amp line out (speaker load) into seperate tube power amp

[dwrockdoctor]

Hi there,

I have a question for the experts here... I built a 2W guitar tube amp that is based off an old orange OR120 layout but using an ECC99 tube for the output section, effectively making an OR2. It works fine and gives me a bit of the vibe of an OR120 at speaking volumes. I also wired up 10W ceramic 4 ohm resistor from the speaker output so I can have a loaded line out to run DI to my computer interface for recording with a speaker simulator plug in which also works fine.

What I tried also adding today was a slave out as per the old OR120 schematics online that means I can run the preamp to the power amp or effects return of a larger amp for more power when I need it (I modded a power amp in on a Vox NT15H Night Train head, straight into the screen of the 12AX7 concertina phase inverter). For whatever reason that doesn't work properly, I just get weird pumping sounds when I tried that.

On a whim I tried plugging from the loaded line out of the 2W head into the power amp in of the Night Train and voila, I had sound! Benefit of this setup is a can all the power amp breakup I like from the 2W amp and control volume with the Night Train power amp master. Plenty of power and punch but also very bright to the point that I had to turn the treble almost all the way off on the 2W amp.

My two questions, is it safe to run it this way?

And should I be impedance matching between the 4 ohm loaded line out going into the power amp of the Night Train and if so, what is the best way to achieve this? (Maybe thats why it's super bright due to an impedance mismatch?). Would a reamp box or DI do the job of matching up impedance?

Thanks for reading!

[eschertron]

Haha! You and Randy Bachman are of the same mind!
this link may shed some light on the historical developments, including impedance matching or filtering. IIRC, the Herzog was plugged straight into the input of the bigger amp.

[Helmholtz]

Hi there,

I have a question for the experts here... I built a 2W guitar tube amp that is based off an old orange OR120 layout but using an ECC99 tube for the output section, effectively making an OR2. It works fine and gives me a bit of the vibe of an OR120 at speaking volumes. I also wired up 10W ceramic 4 ohm resistor from the speaker output so I can have a loaded line out to run DI to my computer interface for recording with a speaker simulator plug in which also works fine.

2W into a 4R load means a voltage of about 3Vrms. That should be safe when connecting to the other amp.

What I tried also adding today was a slave out as per the old OR120 schematics online that means I can run the preamp to the power amp or effects return of a larger amp for more power when I need it (I modded a power amp in on a Vox NT15H Night Train head, straight into the screen of the 12AX7 concertina phase inverter). For whatever reason that doesn't work properly, I just get weird pumping sounds when I tried that.

The PI input grid sits on a DC voltage (maybe 50V to 70V with a Concertina).
This means your signal must be AC coupled using a say 10n coupling cap.

And should I be impedance matching between the 4 ohm loaded line out going into the power amp of the Night Train and if so, what is the best way to achieve this? (Maybe thats why it's super bright due to an impedance mismatch?). Would a reamp box or DI do the job of matching up impedance?

No impedance matching needed here.


For a better analysis I need to see schematics of everything

[Tom Phillips]

Some things to consider when you want to drive another amp or a mixing board with a signal from the output of an amp. You already covered the need to keep a load on the output if you don't need to hear the sound from the first amp.
1) Attenuate the signal so you don't overdrive the input in the second device. A 50mV to 300mV maximum level is a good starting point. This can be done with a simple resistive attenuation circuit. You can see examples in many available schematics.
2) If you are driving a mixing board with the signal then you need to add some frequency shaping. Primarily you need to knock down the high end to simulate the natural roll off of the guitar amp speaker. Otherwise, the result of the putting the raw signal directly into the mixing board will be an overly bright harsh sound.

I have mentioned just the basics. There are discussions about refining the process that go on for months, years and hundreds of posts. They cover things such as a reactive load on the first amp to more closely simulate the speaker and more sophisticated frequency shaping of any signal going directly to a mixing board.

My advise is to start with a simple resistive attenuator as you will find on modern Fender amp schematics and determine if you like the result.

[Tom Phillips]

Attached is a schematic that shows the simple resistive attenuator used in the Fender Princeton II amp. With the values shown it attenuates the output signal by a factor of about 9.

[Helmholtz]

When connecting to the power amp of the Night Train attenuation might not be necessary with a 3Vrms signal.

As said, need to see the schematic.

[dwrockdoctor]

Thanks for all the replies!

In the bottom right hand corner of the first attached picture you can see the slave out coming after 100k resistor from FAC control of the orange. That's where I tapped out of the preamp.

The second picture the power amp in on the night train is going straight into the grid of V4A.

Let us know if you need full schematics of both. I'll try to add a 10n coupling cap and see if that helps for when I connect the slave out to the power amp in that I added.

When I connected the loaded line out of the OR it was to the power amp in of the night train, would that still need filtering considering it's going to a guitar speaker? Like I mentioned, it was super bright unless I knocked the treble to almost all the way off. With treble knocked back it basically sounded like a maxed out OR sound at manageable volume using the night train master volume but it would be nice to not have to turn the treble all the way off.

[Helmholtz]

Let us know if you need full schematics of both.

Yes, full schematics please.

[dwrockdoctor]

Here you go Thanks!