Gary Mattis
Active Member
I am about to rebuild a 400 , with 2nd. Edition, one with Full wave diode rectified. I see mods on here that I think would change the amps tone over just a few watts of power ? , The amp in question has no issues and is a pristine rebuild. Power voltages match SKU prior to the rebuild. This amp has not been hurt from what I can see so far. I have not run amp only ohm and voltage tested it without the output tubes, I do not like using a variac for testing with the old caps. My questions are these ? Is tube deficiency and years of the old caps the real problem with these amps ? Not really a problem with the Fisher design ?. A Failure to meet output spec. Seems like a common vintage tube amp problem in all vintage rebuilds to me with 80 % to 90 % NOS tubes. I would assume no one uses the new russian or china tubes. Next is why go to a harsher sounding fixed bias for just a few watts ? , And why would you use modern solid state MOSFETs ? Doesn't that further suppress the tone ? , a few more volts of negative bias I could see, but not with MOSFETs. Does a half or third voltage regulation on the screen do anything to stop plate surges or heater surges that effect screen performance ?. It seems to me a regulator tube like what Bogen uses would stabalize without tone loss and solid state compressions. Also other types of resistors seem like a more rationale approach dealing with a hurt amp and tube deficiencies. I'm not a fisher expert but in the last 4 years I have built and many stereo and mono amps from scratch with very good results. And fixed all the problem amps I have come across. I'm here mostly to sound off the board for the owners benefit. He thinks I am un-teachable because I tend to want to trust Fishers design more then I do internet forms. My experience has been it is better to stay as close as you can to the design with stereo amps. Alteration usually bring tone and other problems to the table that you have to repair either now or later , their designs have been field and lab tested already. Bias pots and better resistor's I have no problems with. But I think tone will suffer for a few watts and a little bit lower distortion level and to me tone is why you buy a tube amp. All of your hard diagnoatic work and your comments are able to teach me, I'm here like you are to gain knowledge and a better understanding of tube audio and I have studied the RCA tube data for the fishers output tubes already. Are we really smarter today then they were back then about tube audio ? I do scope amps that I work on and I try to match my output patterns as close as possible to tube data pattern's for that output tube being used. Louder and lower distortion is not great tone in itself , linear output is what makes good tone, it is always a compromise to some extent unless it is ultra linear OT. More gain in gives more gain out , tweeking the pre amp seems where any mods should take place to me if more output is desired, just real good tubes can do that !. The short box plate inverters fisher uses would be a good place to start at !. If the PT is hurt and below specs , shouldn't you just replace it ? ! . PT have a temp rating and usually are supposed to run kind of hot to create a desired tone and resistance at the core temperatures. Unless you smell enamel burning it is most likely not even a issue !. Tube amps and woman all have their own personalities , no two will act the same ! . Have a Great Day !
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