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Reduce power Randall RG80?

Started by Fretts, December 19, 2008, 10:08:37 PM

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Fretts

I always liked these amps.  Currently, (electricity joke) I have two of them.
They sound fine but too loud.  I have taken one simple step that helped but I want to take it further. The amp is set up for 8ohms (80W) or optionally 4ohms (100W)
I'm a longtime tube guy and only dabble in SS, but I'm getting more into it these days.
How can I reduce the maximum power capability of the power amps section?  I don't know what to focus on.
Can I limit the current, can I reduce the rail voltage?  Do I have to swap out the power transformer entirely?  I'd like to get it down to around 20-30 watts max.
Can anybody point me in the right direction?
Thanx

armstrom

You could swap in the next highest impedance speaker. If the amp has an 8 ohm put in a 16 ohm. Assuming everything else stays the same that will cut your output wattage roughly in half. You could also just put in a low efficiency speaker assuming what you have is high efficiency.

The "best" solution though would be to add an attenuator in front of the speaker. Then you could make the amp as loud or as quiet as you want without swapping parts. The MASS series by Ted Weber look interesting: http://www.tedweber.com/atten.htm


teemuk

Uh, don't get this wrong I'm not trying to sound blunt but how about just using the volume control? There shouldn't be much power amp distortion going on in these things – all of it is mainly generated in the preamp. Most SS amps are not like those certain tube amps that need to be cranked to power amp distortion.

Even if you reduce the power from about 80W to about 20W (higher impedance speaker swap is the easiest way for this indeed) it's still only a 6 dB cut, which approximately decreases the volume only by half (which is not that much really) but at the same time "smears" the sound at higher volume levels because the power amplifier will clip earlier. In my experience, amplifiers having their output power reduced - and nothing else - do not really seem to get any quieter in a practical situation - they just have less headroom and clip earlier. Mostly you'll dial the amp to a volume level you're comfortable with anyway so it's the same output power going to the speaker regardless. If you want to make it quieter, turn down the volume. You could also get that 6 dB reduction much easier by switching to speakers that are less sensitive. Even in this case the apparent headroom will be limited because in order to get to those volume levels you're comfortable with you need to crank the amp harder. That's about 4 times harder when the efficiency reduction is 6dB, which can be pretty stressfull for the circuit. These reduction things only work if you really, really need them. I at least wouldn't do any permanent modifications to the amp.

May I ask is it a question of "un-sensitive" volume level control, or what's the reason why you wish to reduce the power? Maybe there's another solution...

Either way, your amp will sound different than it does now if you reduce the overall volume level or the overall output power.

It will even sound different just because the reduction of volume because our ears reacts differently to louder and quieter signals. This is why the amps often get dialed to about the same volume level. Whenever you turn down the volume from this point on, the amp will begin to sound thinner. Hence you might likely find it more efficient to use the volume control plus an EQ in the FX loop to boost the bass and high frequencies when the overall volume is turned down. (Google equal loudness contours unless you're familiar with the concept already).

daffe

I also own a RG-80 and at some point I wanted to find a way to cut the power somehow.

The main reason behind that is/was to use a vintage voiced, low-wattage speaker, because I prefer the way they sound and feel.  Of course, I could just put a 30 watt G12H speaker in there and make sure not to dime the amp (which would be unbearable anyway) but I just have a problem using an underspec'ed speaker.

The RG-80 is really a sleeper.  I doesn't make me miss any tube amp, and I had a couple nice vintage Fenders. 

Fretts

#4
I already have installed a 16ohm Celestion that is a mule/hybrid thing (It was a G12T-75 reconed using parts intended for a 16 ohmGreenback).  It did help some.
This amp really does sound different when cranking; there is feedback off the speaker that is used to modify the impedance/damping of the power amp.  Personally, I can hear it, so I want to get that same effect with less earsplitting volume.

I lack basic familiarity with SS power amp topologies.  This unit uses a pair of 2n6254 power Q's, cascaded apparently, per rail.  It has an elaborate biasing scheme I don't understand very well, but it does not look like there is any quick n dirty way to operate the existing amp to produce less power.  Technically, is the only real way to reduce the rail voltage?  Can I artificially limit the available current by modding the power supply and introducing intentional current limiting there?

phatt

Quote from: Fretts on December 22, 2008, 08:18:25 PM
there is feedback off the speaker that is used to modify the impedance/damping of the power amp.  Personally, I can hear it, so I want to get that same effect with less earsplitting volume.

I lack basic familiarity with SS power amp topologies.  This unit uses a pair of 2n6254 power Q's, cascaded apparently, per rail.  It has an elaborate biasing scheme I don't understand very well, but it does not look like there is any quick n dirty way to operate the existing amp to produce less power.  Technically, is the only real way to reduce the rail voltage?  Can I artificially limit the available current by modding the power supply and introducing intentional current limiting there?
Hi Fretts,
              If I follow you correctly, then That Feedback Resistor is likely a very small value [ie less than 1 ohm] Large Wattage wire wound.  You can "Increase" the value of that R and it will turn down the Wattage somewhat.
If it is say .1R, or .22R go up to One ohm and try it. [Make sure you understand the circuit first!!]
The idea is called a "Defined impeadence" setup ,,trying to emulate a non zero imp to sound more like valves,, Yeah works kind of OK but IMO it does not do that much.
Depending on the circuit and more Ïmportantly the speaker used,, turning down the voltage rails won't work well as some poweramp circuits are designed to work at specified voltage. you could stuff up the Amp.

Note also for "daffe"
I personally would take a long look at speakers as use of Big name Drivers with SState Amps is kinda Daffed [no pun intended] if you really understand whats goin on. They are mostly designed for tube Amps remember and I'm sorry but SState Amps are vastly different in the way they work with speakers.
Tube Amps of old had limited bandwidth especially under full output and hence NEED the extra peak at say 2/3khZ but with SState it's a whole different story. 90% of SState Power sections have a dead flat freq response at any volume level,  so you really need to be cutting not boosting those freq otherwise they get painfully harsh at high SPL.
I just fixed the harshness of a Tradsman 60 [tech21] by changing the speaker from the over sensitive "Special design wank" to a much lower SPL driver. [note also lots cheaper]
Now the Amps tone makes sense, It was stupidly harsh and the guy wanted to get rid of it but I think I may have changed his mind :)
Cheers,  Phil Nambour Australia.