Randall RH100 final output convertion

[Sir. Witchback]

This is my first post on this forum, I just bought a new Randall R412XLT100 Cabinet that has Four Celestion G12H-100 speakers that are rated at 16 Ohms. My RH100 head does not have quite the power to drive this cab hard enough. I have been pondering on the thought of changing out the finals to that of the same in a Randall RH200. Im not sure if this will work. I have been searching the web for schematics on both amp heads, and havent had any luck. Im thinking they are about the same, with the exception of the Pre-driver, Bias, and final transitors. But I think the Pre-driver would be the same in both amps, with a slightly higher Bias voltage. Or not. Does anyone know the Final transitor part No. that are used in the RH200? Are these amps the same w/ exeption of the entire final end? And help would be nice to have. I personaly love my RH100, and would like to get out of replacing it for a larger amp, or just buying a stand-alone for boost.

[Steve Conner]

Power amps are usually designed as a balanced whole. All of the parts are "only just enough" for the advertised power, so they're all fully utilized and the maker gets maximum watts per dollar.

Therefore, if you want to up the power, you end up just having to replace everything including the power transformer.

One thing that might be worth looking into is impedance matching. Check the back of the amp to see if it's rated to drive 4 ohms. If so, then you're only getting one-quarter of the power it's capable of, with your 16 ohm cabinet, and you should rewire the cab for 4 ohms.

If the amp is only rated down to 8 ohms then you're out of luck. You could theoretically get it down to 4 by doubling up the final transistors, but then you'd just risk burning out the transformer, which is the most expensive part of the amp.

If the Randall has a Line Out jack, maybe you should just hook that up to a 1kW PA amp or something. :-O

[Sir. Witchback]

The Randall RH100 head is 4 to 8 Ohm stable. Randall's R412XLT100 has (4) Celestion G12H-100 speakers that are rated at 16 ohms each. So In Mono Two speaker's parallel impedance is 8 Ohms, and Stereo Four speaker's parallel impedance is 4 Ohms. I also have another Randall RH100 Half stack, that I switched 2 Two speaker's out of the the 4X12 cab that are Celestion 80's witch are 8 Ohm. So now I have Two 4X12 Speaker cabs that have a Mono Two speaker's parallel impedance of 5.333 Ohms, so lets just say 6. And if I where to run them in Stereo Four speaker's parallel impedance would be 2.667 Ohms, and would most likely kill my amp(s) But running the 6 Ohm load rather then the 8, has gave me a tad more gain. But I am hoping to change my speaker's back the way they where and just have adequate power to run the Four 16 Ohm cab with one head. The lacking of funds is what I was hoping to avoid. But , the RH100 does have a line out and I could put a secondary larger "PA" amp in line, but that isent exactly what I was wanting to do.

Do the RH100, and RH200 have different transormers? I would think both amps are very simular in electrical design. But I have no problem with changing it out is I would have to. I have a good bit of leftover parts laying around from past rf amplifiers I have built.

[Dave Curtis, dB AudioTech]

Originally Posted by Sir. Witchback

The Randall RH100 head is 4 to 8 Ohm stable. Randall's R412XLT100 has (4) Celestion G12H-100 speakers that are rated at 16 ohms each. So In Mono Two speaker's parallel impedance is 8 Ohms, and Stereo Four speaker's parallel impedance is 4 Ohms. I also have another Randall RH100 Half stack, that I switched 2 Two speaker's out of the the 4X12 cab that are Celestion 80's witch are 8 Ohm. So now I have Two 4X12 Speaker cabs that have a Mono Two speaker's parallel impedance of 5.333 Ohms, so lets just say 6. And if I where to run them in Stereo Four speaker's parallel impedance would be 2.667 Ohms, and would most likely kill my amp(s) But running the 6 Ohm load rather then the 8, has gave me a tad more gain. But I am hoping to change my speaker's back the way they where and just have adequate power to run the Four 16 Ohm cab with one head. The lacking of funds is what I was hoping to avoid. But , the RH100 does have a line out and I could put a secondary larger "PA" amp in line, but that isent exactly what I was wanting to do.

I'd rewire the 16 ohm cab for 4 ohms (all in parallel).

Originally Posted by Sir. Witchback

Do the RH100, and RH200 have different transormers? I would think both amps are very simular in electrical design. But I have no problem with changing it out is I would have to. I have a good bit of leftover parts laying around from past rf amplifiers I have built.

I just had an RH200SC in here; didn't take pix (dammit). It has two seperate outputs, and I'm just guessing that the 100 has one. So, even if this doesn't apply, here's an FYI: The OP 'zistors are 2SC5198 and 2SA1941 (1 ea. per side) with a 2SC4793 & a 2SA1837 also bolted to the heatsink on the left & right sides. There's also a 2SC1815 face-down in heatsink compound there, too.

DC