I was wrong about RA100 Power Soak being resistive

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Triple Recto
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I was wrong about RA100 Power Soak being resistive

Post by bandit2013 » Sat Mar 10, 2018 11:34 am

It has been a while since I removed the RA chassis so I totally forgot about the chokes used in the power attenuator circuit. Duh! :|

It is so easy to assume that the RA100 power soak feature is resistive since it has the large resistor in an aluminum cage with a small fan blowing on it. :oops: Once you look in further it has a pair of chokes sitting behind the resistor. The TC-100 is very similar as it too has the two chokes but the difference being, the resistor is inside the amp chassis and not exposed like it is on the RA100. In other words, the RA has a reactive power soak just like the TC-100. I feel like an idiot. :roll: Oh well. I never stated I was accurate. :shock:

The difference between the two is how the power soak is labeled and where the controls are positioned. There is reason for the different labeling applied to the RA compared to the TC. RA is labeled in db where the TC is labeled in equivalent power setting. The reason for using dB vs power level is the RA can be run with two tubes in 50W mode where as the TC is all tubes always active.

Both TC and RA use a bank of relays to switch the level of attenuation onto the power tubes. In essence they are both the same circuit. Also neither of the amps have a negative feedback loop from the speakers like most amps have as a presence control. Yes, the TC series has a presence control but it is defined in the manual as not being a negative feedback control. It is, however a high pass filter control to increase the higher frequencies above hearing level that aids in shaping the distorted signal (increases the range of higer order harmonic frequencies and if set too high could lend to an icy experience, ouch. I have yet to experience the ice so at least that is under control). Sure the PTs are different. RA has a lower voltage than the TC amp.

I am so tempted to compare the two amps gain stage and see what the differences are for each triode circuit for CH2 and CH3 (vintage Lo and vintage Hi). Sice the RA shares its tone and gain controls there is one part that is different and that is the pot for the gain setting, it is a dual pot. I am sort of curious to learn of the differences between the two. I am thinking about making some mods on the RA100 combo. Actually I am going to be looking for mods if there were any as this amp was purchased used and there is one mod that is obvious, It has two input jacks, one on the front and an added one on the back with no label. Yes, the footswitch control is also on both front and and back. Also I am not confusing the extra jack for the Reverb kill switch as that is there too.

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