Author Topic: Sunn scepter head 1969 trem problem  (Read 4402 times)

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Offline Baddog

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Re: Sunn scepter head 1969 trem problem
« Reply #28 on: May 06, 2011, 04:04:06 pm »
It seems like this amp should be louder for being a 60watt amp. Hmmmmm  Maybe my hearing is going or something... It sounds pretty decent and stays relatively clean dimed ... How can I determain the watts it putting out?

Thanks again
BD

Offline Soundmasterg

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Re: Sunn scepter head 1969 trem problem
« Reply #29 on: May 07, 2011, 01:30:12 am »
The voltage jumping around thing is probably an oscillation. I had that in a project amp in the past...I never did solve it on that amp as I decided to do a new seperate preamp chassis design and never got around to it. But the voltages in one section of the preamp would jump by 100v for no apparent reason. The first two cap sections should have at least a 600v rating...the third one is fine with 500v. That is why I series connect caps for the first and second section...because two 350v caps will give a 700v rating....if you do it, don't forget the parallel resistors with each cap or your caps will blow. Your wall voltage is pretty low there....112v....here it is usually 120-125v, which makes the AC voltages much more of course. My 1970 Sonic 1 (200S) is often at 560v B+, and a customer's Spectrum 2 was at 520v.

The Morse code sounding thing is probably your cell phone...I've had that happen too. Tube circuitry being high impedance is very sensitive to things like that.

As far as checking the output power.....do you have a scope, signal generator, and dummy load? If so then it is easy. You input a 1KHz signal about 200 mv (.2v) with the amp on a dummy load. Monitor the output with the scope and get the signal with the highest amplitude with the trace just barely clipping, and then back up from there until it doesn't clip anymore. Measure the AC RMS voltage at the output, square it, then divide by the impedance, or in this case resistance. (Helps to have a true RMS meter, but without that you can still get a relatively accurate idea) You can use a speaker instead of a dummy load but it isn't as accurate since it's impedance changes with frequency and a resistor's does not. The tone controls should be at zero also. Anyway, say you have 28v AC RMS and 8 ohms.....so 28 times 28 = 768/8 = 96 watts RMS. Most of the Sunns that used two KT88's or 6550's will be around 70 watts RMS and a peak power much louder....thats with NFB engaged. They tend to break up around 4-5 and the distortion character is harsh and hard compared to your typical Fender, but they have a TON of bottom and top end. Some people like them, others don't. I fall in the camp of I don't like Sunn tube amps much for guitar, but love them for bass.

Greg