Fisher 400 , mod, ? rebuild ? why mod ?

[Gary Mattis]

I am about to rebuild a 400 , with 2nd. Edition , one with Full wave diode rectified. I see mods on here that change the amp and tone over 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 the Fisher design ? Failure to meet output spec. Seems like a common vintage tube 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 fixed bias for a just few watts ? , Then 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 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 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 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 want to trust in 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. Have a great day ! . Are we really smarter today then they were back then about tube audio ?

 

[RS Steve]

Having a fleet, or should I say school of Fisher tube units, I have gone through the same thought process. More wattage is usually not what the goal is since the output tubes are the determining factor there. Most of the mods have to do with getting the unit to operate and last with todays wall voltage. Screen stability resistors added to keep tube arching from happening, and individual bias pots are things I like to have done. There are some that tinker with the original design to try and find better sound, but having two 400's myself, it's hard to improve on something that sounds so good to begin with. The Fisher forum on AK is where you should post this at, there are many great minds there for all things Fisher.

 

[gadget73]

Some of the modifications also deal with reducing distortion. You're just not going to get vastly more power from a tube amp that was halfway sensibly designed. The voltages and tubes involved put a pretty firm cap on that. Most of the rest of the mods are to improve durability of the amplifier and it's parts.

 

[Gary Mattis]

Some of the modifications also deal with reducing distortion. You're just not going to get vastly more power from a tube amp that was halfway sensibly designed. The voltages and tubes involved put a pretty firm cap on that. Most of the rest of the mods are to improve durability of the amplifier and it's parts.

 

[Gary Mattis]

I did not know Fisher 400s had any distortion or tone problems in their designs ? or reliability problems with good caps ? There are a lot of them still working without any real problems.This is the first time I have heard anything bad about a Fisher amplifier. A little fine tuning on the scope after the rebuild checking each stage and the voltages is about all the mod I think should be done to such a fine amplifier. Done After a good break in on the tubes and capacitors of about 30 to 50 hours of use.

 

[Gary Mattis]

Having a fleet, or should I say school of Fisher tube units, I have gone through the same thought process. More wattage is usually not what the goal is since the output tubes are the determining factor there. Most of the mods have to do with getting the unit to operate and last with todays wall voltage. Screen stability resistors added to keep tube arching from happening, and individual bias pots are things I like to have done. There are some that tinker with the original design to try and find better sound, but having two 400's myself, it's hard to improve on something that sounds so good to begin with. The Fisher forum on AK is where you should post this at, there are many great minds there for all things Fisher.

Is tube arcing really a problem ? , The amp I am working on has voltages right on spec with the higher line voltages today.

 

[larryderouin]

Fisher made quite a few design compromises in the 400. The change from 6.6K to 10K output transformers is probably the worst of the bunch. There is also the Phase inverter circuit that is somewhat strangled in the early version (<48001) which is made worse in the later version(>48001), that can be fixed with a resistor change or two. None of the reliability and stability mods that I do to any FISHER improves watts, but rather brings the unit up to a standard that is more in tune with todays electrical grid. And has NOT changed the Tone or Sound Quality of the unit.

Ok, using old stock output's is fine, but the 7868 suffers from the same caveman mentality of the 60's wattage wars with overspec grid returns, no screen stability, an almost criminally negligent biasing circuit with no way to adjust it in stock form, and a lack of even 1/2 way matching output tubes so they at least balance under real world conditions. Which is why customers were out purchasing output tubes every year or two. As for your quote about "crappy russian tubes", The EH7868's only problem is the novar pins which are just slightly larger than the original noval pins. Otherwise it is a quite capable tube. Running them for going on 10 years I've not had a bad tube, and run them constantly (10-12 hours a day 7 days a week in my 400). I've never had to replace one either due to wearing out.....yet!

Come on over to the FISHER Forum. There are quite a few threads detailing the whys and wherefore's of each and every "mod" that is done with technical explanations behind each one.

Some light reading for you. Start with the 1st one as this deals with tube arcing and why they do, and what to do about it. It is NOT 400 specific, but the solutions will work in the 400. All of these threads are by Dave Gillespie, who is acknowledged as a Master Electronics Engineer, and Tube GURU on AK. His testing methods are on par with the best of the old school testing labs if not better.

Techniques to Maximize Power Tube Life
http://www.tronola.com/html/maximize_tube_life.html

Brief comparison of 7591 vacuum tubes (as the 7868 is in the 7591 family this is included.)
http://www.tronola.com/html/7591a_tubes.html

FISHER 400 Output Transformers
http://www.audiokarma.org/forums/index.php?threads/fisher-400-output-transformers.508954/

Fisher 400 Output Transformers--------Revisited.
http://www.audiokarma.org/forums/index.php?threads/fisher-400-output-transformers-revisited.524133/

Improving the 400
http://www.audiokarma.org/forums/index.php?threads/improving-the-fisher-400.511867/

Modifying the 400 with EFB
http://www.audiokarma.org/forums/index.php?threads/modifying-the-fisher-400-with-efb.509090/

This is a sample of what can be done to the 400, to increase reliability, and stability WITHOUT impinging on Sound Quality.

Larry

 

[Gary Mattis]

Fisher made quite a few design compromises in the 400. The change from 6.6K to 10K output transformers is probably the worst of the bunch. There is also the Phase inverter circuit that is somewhat strangled in the early version (<48001) which is made worse in the later version(>48001), that can be fixed with a resistor change or two. None of the reliability and stability mods that I do to any FISHER improves watts, but rather brings the unit up to a standard that is more in tune with todays electrical grid. And has NOT changed the Tone or Sound Quality of the unit.

Ok, using old stock output's is fine, but the 7868 suffers from the same caveman mentality of the 60's wattage wars with overspec grid returns, no screen stability, an almost criminally negligent biasing circuit with no way to adjust it in stock form, and a lack of even 1/2 way matching output tubes so they at least balance under real world conditions. Which is why customers were out purchasing output tubes every year or two. As for your quote about "crappy russian tubes", The EH7868's only problem is the novar pins which are just slightly larger than the original noval pins. Otherwise it is a quite capable tube. Running them for going on 10 years I've not had a bad tube, and run them constantly (10-12 hours a day 7 days a week in my 400). I've never had to replace one either due to wearing out.....yet!

Come on over to the FISHER Forum. There are quite a few threads detailing the whys and wherefore's of each and every "mod" that is done with technical explanations behind each one.

Some light reading for you. Start with the 1st one as this deals with tube arcing and why they do, and what to do about it. It is NOT 400 specific, but the solutions will work in the 400. All of these threads are by Dave Gillespie, who is acknowledged as a Master Electronics Engineer, and Tube GURU on AK. His testing methods are on par with the best of the old school testing labs if not better.

Techniques to Maximize Power Tube Life
http://www.tronola.com/html/maximize_tube_life.html

Brief comparison of 7591 vacuum tubes (as the 7868 is in the 7591 family this is included.)
http://www.tronola.com/html/7591a_tubes.html

FISHER 400 Output Transformers
http://www.audiokarma.org/forums/index.php?threads/fisher-400-output-transformers.508954/

Fisher 400 Output Transformers--------Revisited.
http://www.audiokarma.org/forums/index.php?threads/fisher-400-output-transformers-revisited.524133/

Improving the 400
http://www.audiokarma.org/forums/index.php?threads/improving-the-fisher-400.511867/

Modifying the 400 with EFB
http://www.audiokarma.org/forums/index.php?threads/modifying-the-fisher-400-with-efb.509090/

This is a sample of what can be done to the 400, to increase reliability, and stability WITHOUT impinging on Sound Quality.

Larry

 

[Gary Mattis]

I just do not like newer tubes tone compared to vintage tubes, this is a late design with the lower resistance OTs like in the early models, with a full wave rectifier. With modern line voltage everything is right on spec with SKU. With Tubes running At 500 v. Is it realistic to expect long tube life with any tube amplifier ? , I agree the neg bias is pretty low but how can you argue with Fishers tone quality ? Plus some amps run 350 volts with no neg bias. It is the builder's design that was tested and approved. Fisher decided to trade some tube life for great tone, guitar amp makers always do that !

 

[sgmlaw]

I did not know Fisher 400s had any distortion or tone problems in their designs ? or reliability problems with good caps ? There are a lot of them still working without any real problems.This is the first time I have heard anything bad about a Fisher amplifier. A little fine tuning on the scope after the rebuild checking each stage and the voltages is about all the mod I think should be done to such a fine amplifier. Done After a good break in on the tubes and capacitors of about 30 to 50 hours of use.

I'm with you. These are generally well-designed instruments as-is. More importantly, they are well-voiced with a distinctive house timbre.

To hear it, you would think these were bottle rocket launching pads. Yet many march along perfectly five decades out with only light service. They weren't designed by reckless fools.

Too many mods, and it's no longer a vintage Fisher. But I'll never stand in the way of a hobbyist's enthusiasm.

The 400 is a simpler, cleaner circuit as Fisher receivers go. I would leave it as intended with the basic safety updates, and just enjoy it. The modern production outputs are much better than 15 years ago.

 

[Gary Mattis]

I'm with you. These are generally well-designed instruments as-is. More importantly, they are well-voiced with a distinctive house timbre.

To hear it, you would think these were bottle rocket launching pads. Yet many march along perfectly five decades out with only light service. They weren't designed by reckless fools.

Too many mods, and it's no longer a vintage Fisher. But I'll never stand in the way of a hobbyist's enthusiasm.

The 400 is a simpler, cleaner circuit as Fisher receivers go. I would leave it as intended with the basic safety updates, and just enjoy it. The modern production outputs are much better than 15 years ago.

I'm with you. These are generally well-designed instruments as-is. More importantly, they are well-voiced with a distinctive house timbre.

To hear it, you would think these were bottle rocket launching pads. Yet many march along perfectly five decades out with only light service. They weren't designed by reckless fools.

Too many mods, and it's no longer a vintage Fisher. But I'll never stand in the way of a hobbyist's enthusiasm.

The 400 is a simpler, cleaner circuit as Fisher receivers go. I would leave it as intended with the basic safety updates, and just enjoy it. The modern production outputs are much better than 15 years ago.

 

[Gary Mattis]

I agree the design on this amp is good just like it is , time has proven that. My test voltages are right on spec with the old caps, I'm trying to tell the new owner to leave it stock and let me scope tweek it after my rebuild. And to let me replace the short box plate inverter tube with a Ei or Telefunken smooth long plate. Tube life compared to tone should not be a trade off when restoring one of these amps. Why buy a fisher or any tube amp if you think like that and are not willing to buy tubes every few years for great tone ! These amps run 500 volts with min. Neg. Bias., the nuclear heat is what makes them so linear
.

 

[Brice]

Is tube arcing really a problem ? , The amp I am working on has voltages right on spec with the higher line voltages today.

Yes it is.

Dave G. is the master on that subject and there are pots on the arcing subject.

When you increase the capacity of the high voltage power supply stage caps (things we all do to reduce the ripple), you'll most likely face an arcing accident later. Adding them is simple, cheap and will prevent damage, with no sonic impact.

 

[sgmlaw]

I agree the design on this amp is good just like it is , time has proven that. My test voltages are right on spec with the old caps, I'm trying to tell the new owner to leave it stock and let me scope tweek it after my rebuild. And to let me replace the short box plate inverter tube with a Ei or Telefunken smooth long plate. Tube life compared to tone should not be a trade off when restoring one of these amps. Why buy a fisher or any tube amp if you think like that and are not willing to buy tubes every few years for great tone ! These amps run 500 volts with min. Neg. Bias., the nuclear heat is what makes them so linear
.

I still remember when people were literally butchering these for 6L6 mods. Broke my heart.

There were far fewer of us into them then, and we were but a small voice. So many were lost to the Dr. Stereo modders making them 'better'. Constantly on the fritz and sounded awful, too.

Once you start down that path, it's a cascade effect. You literally have to rework the whole thing, patching one thing after another to accommodate the preceding 'upgrade'.

I think I'm still a voice in the wilderness at times.

 

[gadget73]

The 6L6 is a bad sub for a 7591 or 7868. Needs more drive and more bias to make the same power. The stock inverter design can barely drive the stock tubes, its got no hope of getting a 6L6 family tube to swing to full power before it craps out.

Overall these were a pretty good design, there just happen to be a few spots that can be improved on. The screen resistors to keep the tubes from blowing up are one of them. Its pretty non-invasive and has no negative effects, other than for the tube vendor's pocket books. Bias adjustments and monitoring points are also IMO a good idea. I won't tell you it will sound better, but you can tell whats going on and make the tubes run at the current level originally designed for with them.

 

[sgmlaw]

The 6L6 mod was a question of availability at that time.

This was before there were new production 7591s or 7868s.

Then, all we had were old stock and the occasional NOS stuff that would pop up.

And no Ebay or other audio websites for parts or tubes. Hamfests, old TV/Radio shops, and each other.

The audio salons would laugh at these, and try to sell you a CJ or ARC.

 

[larryderouin]

Gary; Check the output transformer #'s. If the suffix is "A" it's got early model output's and should have 5 IF/RF tubes. Suffix "AX" is the 10K transformer and used in later version with 6 IF/RF tubes. Early models have serials below 48001 and Late model are above 48001 but we've seen some anomolies in serials from 47000 to 49000 with either. If the transformers have been swapped I'd ensure that the feed back and output sections are matched to the transformer as they are some differences in component values with the change in transformers.

 

[Gary Mattis]

Gary; Check the output transformer #'s. If the suffix is "A" it's got early model output's and should have 5 IF/RF tubes. Suffix "AX" is the 10K transformer and used in later version with 6 IF/RF tubes. Early models have serials below 48001 and Late model are above 48001 but we've seen some anomolies in serials from 47000 to 49000 with either. If the transformers have been swapped I'd ensure that the feed back and output sections are matched to the transformer as they are some differences in component values with the change in transformers.

This one has 6 tubes for rf/if , early ot's and silicone rectifier. Got the couplings done and Im waiting on the e caps. I'm gonna scope this and a Bogen DB 230 A golden with the 6av5 tubes to see which one sounds better ! , The fisher has much better pre amp tubes Telefunken's , but that bogen sounds dam good to my ears even with cheap ge tubes in it. The bogen has a 5ar4 mullard rectifier tube. Fishers have great ratings and Bogens have basically none by the audio filles ! , That's because Bogen dropped out of tube audio in the early 60s . This bogen has a little more plate voltage and negative bias about 10 more volts on each one due to a PT change and separate neg bias transformer. It Should be a interesting match up on the scope ! . The Fisher has higher plate voltage but the Bogen puts out more power ! , To make it more fair I will use a Bogen tube tuner for the test also. Then use a CD deck on both of them. I look at the insane prices for these old Fishers and I have to wonder if they really merit those kind of high prices ? , Im not saying weather they do or dont , only that The scope and my ears will tell me if they do or not ! . Sweep Outputs against Audio Outputs ! , It will be funny if the Bogen wins !

 

[gadget73]

Just because you don't hear much about a particular piece of gear doesn't mean its not any good. Conversely, just because you hear a lot about something doesn't mean its worth a hill of beans. Bogen gear is solidly engineered but they just didn't seem to get a lot of love. Don't know why, maybe it was the somewhat industrial cosmetics, but I'd argue that some of their designs were a fair bit more advanced than the "big names". That active screen regulator circuit in the Bogen is miles ahead of the simple dropping resistor that Fisher was using for the same purpose.

 

[Gary Mattis]

Your right about that , Bogen was a mile ahead of Fisher but cheap build dropping resistors are the norm today along with SS rectifiers.