Improving the Fisher 400

Dan

The POT from Mark Oppat cost $55 plus 4.5 to florida( this is mi mail in the states).

And regarding the improvements the PS upgrade is done, ( check Sony6060 thread), the recap in the FM and MPX sections also, and just finished the EFB board so tomorrow i'll update some pictures about the PCB and try to finish this stage tomorrow.

Regards
 
Yes Mark sells those pots for $55.00 each


I have two Fisher 400 amps, the one I am tinkering with now is a early unit and will not work with some of Dave's mods, it has been modified and was saved from
a basket case trashed unit, pic below.

AS you can see I already have installed a alternative AC switch so I am more open to volume control changes on this one.

I really don't need the loudness circuit and can live without it although I have seen those Chinese Alps pots with the loudness taps on e-bay and I might just
pick one up to try out on this 400 or my other one.

I have a pair of Roberts 770x RR amps that have been modified to standalone mono amps, I used the Mil spec 500k single mono pots that I got from Antique Electronic Supply,
they are very accurate and have that smooth feel like the alps pots and really sound nice with excellent low level volume control, looking at my amp tonight I was
thinking of removing the balance control and then adding extensions to the mil spec pots and using one for each channel, I guess I will be doing some thinking and
experimenting.


I do have a later model 400 that is stock and will be using that one to install Dave's buffer on, that will be a winter project for me.

 
Brokenhill, that's some real nice work. You nailed the intergrated look with a added touch of class.

Luis, I'm looking forward to seeing your work. I'll look for the other threat.

It's good to know about Mark Oppat replacement volume pot.

Dan
 
That 400 was in such bad shape when I got it and I was only going to use it to experiment with, I had a really clean stock 400 and my Dad had a mint looking 400
and this one ended up sounding better then the other two so I hung on to it and the others were sold to happy new owners.

It definitely came out better then I or my Dad thought it ever could be and then I got kinda attached to it.

btw. That's a clever idea about extending the shaft on your Alps pot with JB weld and a straw, that JB weld is strong stuff.
 
Thanks. About the JB weld straw deal. If the straw is too high where the knob attaches to the shaft it won't go on all the way down. It's easy enough to just cut off the straw when everything is done though. The JB weld didn't stick to mine anyway. I mostly wanted it there so that the shafts stayed lined up while it cured. The results seem strong enough to last a life time. knock on wood.

With your fisher, I guess you just never know how something is going to turn out. Do you think part of the reason it sounds better is because of the power supply doesn't have to do deal with the tuner or is it just the luck of the draw?

Btw, I noticed that my unit was starting to sound like someone V'd a EQ's sliders or turned up the bass/treble knobs so I put in new 12ax7's in the line stage/phase inverter sections and it seems to have cured it. I was starting to wonder if I was necrotic. Actually, there isn't much to wonder about.... haha

Dan
 
Do you think part of the reason it sounds better is because of the power supply doesn't have to do deal with the tuner or is it just the luck of the draw?

I do think the higher voltage and current from the power supply without the tuner present is part of it and also the DC bias voltage is -17 volts on each output tube
and the other stock Fisher 400's me and my Dad had were at -19 volts on each output tube.

This one is no doubt a hot potato but it sure does sound nice, I did loose one output tube a few years ago when I had the volume way to high and driving the amp hard,
when I heard the awful noise it made I looked at the amp meter and saw it bouncing around madly and hit the power off immediately, after replacing one toasted output
tube I am a lot easier on it now and haven't had any problems with it since.

Now you know why I am so interested in these mods in this thread, I know those 400's have a lot of potential and what has been achieved here is really a great thing,
(getting the best of both worlds)
longevity out of your tubes and better performance without hot rodding them and pushing them to the point of the output tubes or even a transformer self-destructing.
 
Dave, I have what I hope is a quick question about your Fisher 400 mods. If one wanted to have the option of playing it with the Fisher stock high pitch etc could they use the loudness switch instead to switch in and out the buffer circuit with the rest of your mods or would this be fruitless due to the rest of the changes done?

Dan
 
Hi Dan -- If I understand your question correctly, are you in essence trying to convert the loudness switch so that in one position, the unit provides the sound of the stock sonic performance, while in the other, that produced from the modifications?

If so, then a couple of thoughts come to mind:

1. From the standpoint of response characteristics, there were changes made to the Line Amp stage, Tone Amp Stage, Power Amp AF Amplifier stage, and NFB circuit to flatten the response of all of these stages/sections. Trying to use a simple DPDT switch to switch all of these modifications in and out could only be done with mini-relays, which would become quite complex.

2. The biggest contributors to flattening the response comes from bypassing the tone controls, and installing the buffer stage. The tone controls can already be maintained in the signal path by way of the Tone Control Bypass switch, and the Buffer circuit could in fact be effectively placed in and out of the signal path by repurposing the current loudness switch to provide that function.

3. Parts of the total modification like the installation of EFB and improving the phase inverter stage drive have no effect on circuit response, and you would likely want to leave these elements in place regardless of how you might proceed with any other portions of the modification.

4. After having lived with the modification now for a period of time, have you come to miss the warmer, lush sound of the original design? This is hardly meant to criticize the original performance, as so many people do enjoy it immensely. After all, it is ultimately enjoyment that we seek from this equipment, and if it doesn't provide that, then why have it?

If that is the case, then I would first consider trying to re-achieve that sound with careful use of the tone controls. The problem with so many in today's audio hobby is that they want their equipment to sound exactly like they want it to with all the controls set for flat. Then they have REALLY achieved high fidelity sound! So they go through all of these efforts to modify, modify, modify until they get what they like with the tone controls all set for flat -- except that almost always, the real response produced by such efforts is anything but flat. But, they can at least "show" that it is flat by the control setting positions, and fool themselves and others then because of their unique and highly calibrated golden ears.

The problem with this approach, is that any reference to accurate, flat performance has all but been lost. It is the same as having no point of reference to know how to proceed with a map or GPS system. Either way, you're lost. I mean no disrespect to those who play out this scenario, but it often ends in a futile search because they have no frame of reference from where to start.

On the other hand, I would much rather produce a known standard of performance with regards to accurate response with the controls set for flat performance. I may not like the tonal quality produced at that setting, but at least then I can have a solid frame of reference as to what accurate, flat performance really is. From there, I can adjust to suit, having full knowledge then of just how much my preference for reproduced sound deviates from that which an uncolored response delivers.

Others often rail against such efforts, claiming that there are a million points in the recording/delivery chain where response is/was intentionally/unintentionally altered. True enough, by why clutter it up with one more piece of equipment that further colors the response as well? How much better to know what the original signal source truly represented so you can make informed decisions from there.

In the end, designing for a flat amplification presentation has served me very well indeed over the years, and is the approach used with the modifications offered for the 400. As I mentioned in my response to you in the X-1000 thread, when various units are all designed to produce truly accurate, flat performance from the get go, then their sonic differences -- from a response standpoint -- all tend to largely disappear, so that other characteristics such as power, damping factor, features, etc., then tend to dominate the differences between the units.

This may tend to take some of the "romance" out of the equation, like how some may prefer some specific tubes, designs, or component brands over another. For those, I am hardly trying to burst any bubbles in that regard. However, there is a cold, hard truth to these facts -- that may not be all absolute in defining sound that we perceive, but none the less, play a significant factor in it. The rest is emotion. An important part to be sure, but one that should always be kept in check with the reality of measured performance -- not to make that more important than the emotional pleasure received, but to maintain the all important frame of reference.

Dave
 
Hi Dave, I was wondering more from a curiosity standpoint and maybe to learn something. I barely used the unit stock so I can't really say that I miss how it was or not although I have a fisher x100-3 rebuilt to stock specs that I'm not too into that maybe one day I can ask you for some quick advice on things to try with it such as tape loop mods and where to get the right power requirement for a buffer circuit. I'm assuming it probably has the fisher house sound you detailed in both this and your 1000 threads.

You probably already wrote this but I can't recall. Where does the amp start to roll off during your sweep test?

If you ever have your 400 back on the bench please let me know about where the AC Balance should be. I set it in half due to the 1khz/offset between tube bias points not working out.

Dan
 
Hi Dan -- regarding the overall response of the modified 400, it is set by the power amplifier section, which in modified form is flat +/- 0 db to 20 kHz, with a smooth reduction beginning at 48 kHz, being down only .4 dB at 60 kHz. The modified control section by itself is flat to 56 kHz, down .3 db at 85 kHz (tone controls OFF). You can see therefore, that the overall response characteristic is dominated by that of the power amplifier, which is typically the case in the best designs.

Dave
 
Just a followup. I was inside my amp checking the bias of new 7868 after I had a little time on em so I decided to toss in a OPA2134 that I had around in the buffer's TL072 place and I believe the sound got a bit better. It seems smoother and a bit more balanced in a good way. It very well could be in my head though.

Dan
 
Hi Dan -- Thanks for the followup!

It is very tough to make truly accurate assessments of sound quality between different components, designs, and configurations, without the ability to do so under strict double blind testing conditions. We are so intimately involved with our equipment, and always hoping that our efforts will return a positive outcome, that most often, pure emotion can deliver that end whether it is real or not. Of course, gross improvements or deterioration can always be detected. But more subtle positive (or negative) changes determined without the use of direct double blind testing can only result in a personal preference. That preference may in fact represent a consensus of improved performance if a number of people participate in the test. But without the proper test conditions, emotion can have way too much of a field day with such a subjective topic as perceived sound quality.

That's not to say that personal preferences are not valid -- they truly are. But personal preferences determined under totally non-controlled conditions get into trouble real quick when they are offered to all as an "absolute" improvement. After all, personal preferences are, well, personal.

It is for this reason that I have always felt that you need both lab measurements AND your ears to determine if an improvement or modification is right for you. You may like a resulting sound better, which is fine. But if it measures poorly, then accuracy must be compromised. The perceived improvement of sound may just be the result of an originally accurate piece of equipment now producing an error from a purported "improvement" to compensate for a completely different piece of equipment in the chain that is the real problem!

And of course, a piece of equipment may measure better with modification. But if it sounds poorly when used with other known quality pieces, then who cares how it much better it might measure.

So you need both measured and audible improvement to judge a given change in the ever subjective audible quest as truly "better". I applaud your efforts to experiment, while also recognizing the influence of the test conditions used.

Dave
 
Finally I did it!!!

Dear all:

After a lot of days spent in the amp, i did the dave´s mods with a special feature: a bias meter with a tube bias selector in a external box where the IBAM pots resistors and caps are included; I want to highlight this external box has a quick conector to remove it.

I know that I´m posting a lot of pictures but i want to show in detail every modification done in the amp to help to clarify the improvements engineered by Dave,

Here the first part

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Epilog of Pics!!

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What can I say:

GREAT SOUND!!!

For my part is pendind to do the RED and GREEN line in the drawings to know where is necessary to modify the amp!!

Regards to the AK Comunity!!!
 
Great work Luis, how's she sounding?

Does anyone know if a balance pot can create misbalance between channels? My right channel is weaker than my left by a good deal. I finally took out a SPL meter and measured to be sure. With a new volume pot I have to have the balance a bit past 3 o'clock to even out. Pinned on either side and measured the right is not as loud. I'm almost 100% sure that the unit had the same issue before I did Dave's mods but I figured that it was just the volume pot being shot. Maybe two resistors drifted in different directions over the years. I swapped tubes etc. No dice. As long as it's not a output transformer.

Dan

Just checked the original volume pot and it's only 10k off which is great. Maybe I should toss it back in...
 
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