Hi all,
First time posting here. I've been messing with the 18 watt Marshall variations for the last year or so, and when my PT fried last month I decided to build what they call the Trex at the 18 watt site. This is a mixture of a normal channel 18 watt and a loose interpretation of the express. Here is the shematic: http://mhuss.com/18watt/schematics/18wTRex.pdf
The amp I built is set up to run both octal and noval output tubes. After building it, and hearing it, I'm really intrigued by the express(ish) channel so I'm thinking I want to go more in that direction with the build, and I have a couple questions.
I can see that the Express power supply is a bit more elegant than the one I have. How much effect does the power supply have on the overall tone of the amp?
It wouldn't be that hard to 86 my current power supply and build the correct one, but is it worth it?
Well I guess that'sit for now.
Thanks,
Tim
Noob Express Q's
Moderators: pompeiisneaks, Colossal
Re: Noob Express Q's
The power supply has a very big influence on the sound of the amp. It affects headroom, transient response, hum and several other more subtle things.
The main difference between the TW and TRex is that the TW power supply is much stiffer. It provides a harder attack and less compression than the TRex power supply.
Some differences between the power supplies.
1. The TRex is cathode biased and the TW is fixed bias. Fixed bias gives a stiffer transient response. Cathode bias gives more compression ("squish").
2. The TRex uses a tube rectifier and the TW a solid state rectifier. Again, solid state rectifiers have a stiffer response. A tube rectifier has some "sag", i.e. voltage drops a bit on transients when pushed hard.
3. The TW has one extra filtering stage, supplying the first two gain stages. The TRex has all preamp gain stages supplied from a single filtering stage. Better filtering means less hum.
4. The TW uses a big wirewound 25W resistor for the screen supply filtering. This supposedly works to some extent like a small choke, so the filtering is better than with a regular metal film or carbon resistor.
If you want to replace the tube rectifier with silicon diodes, you need to take into account that the B+ voltage will go up something like 30-50V. Make sure your filtering caps can handle this. If you keep cathode bias, you may have to increase the bias resistor.
With fixed bias and higher voltage, output power will increase. Make sure your OT can handle it.
The main difference between the TW and TRex is that the TW power supply is much stiffer. It provides a harder attack and less compression than the TRex power supply.
Some differences between the power supplies.
1. The TRex is cathode biased and the TW is fixed bias. Fixed bias gives a stiffer transient response. Cathode bias gives more compression ("squish").
2. The TRex uses a tube rectifier and the TW a solid state rectifier. Again, solid state rectifiers have a stiffer response. A tube rectifier has some "sag", i.e. voltage drops a bit on transients when pushed hard.
3. The TW has one extra filtering stage, supplying the first two gain stages. The TRex has all preamp gain stages supplied from a single filtering stage. Better filtering means less hum.
4. The TW uses a big wirewound 25W resistor for the screen supply filtering. This supposedly works to some extent like a small choke, so the filtering is better than with a regular metal film or carbon resistor.
If you want to replace the tube rectifier with silicon diodes, you need to take into account that the B+ voltage will go up something like 30-50V. Make sure your filtering caps can handle this. If you keep cathode bias, you may have to increase the bias resistor.
With fixed bias and higher voltage, output power will increase. Make sure your OT can handle it.
Re: Noob Express Q's
okay, sounds like I have something to do this weekend. I think I have everything needed to do this. I failed to mention, I allready have silicone diode rectification, so that's no problem.
My PT has a 55v tap for fixed bias. Will the express bias circuit get me in the ballpark with this voltage?
Thanks
Tim
My PT has a 55v tap for fixed bias. Will the express bias circuit get me in the ballpark with this voltage?
Thanks
Tim