Danny -- I developed a modification to accomplish this for drew, but I'll have to find it again. When I get a chance, I'll see if I can re-post it here for you.
Dave
Dave
Improving the Fisher 400
- Thread starter dcgillespie
- Start date
drewbolce
Audio Analyist
Dave,
The tape head input conversion was in a PM. I can post it here if you wish. Sorry I have not had time to do the work on it. I have my 500c on the bench but this is a "devil is in the details" mod and I have not had the time to do the necessary checks on the physical details.
One thing I did note is that the tape head input is intended to be at the mv not volt level. As such it is grouped and shielded with the phono inputs. I think it may be wise to use some small diameter shielded microphone wire and move it over to the other side of the shield with the other volt level inputs. Probably overkill but the leakage problem with tape monitor switch has already been noted and addressed here.
The tape head input conversion was in a PM. I can post it here if you wish. Sorry I have not had time to do the work on it. I have my 500c on the bench but this is a "devil is in the details" mod and I have not had the time to do the necessary checks on the physical details.
One thing I did note is that the tape head input is intended to be at the mv not volt level. As such it is grouped and shielded with the phono inputs. I think it may be wise to use some small diameter shielded microphone wire and move it over to the other side of the shield with the other volt level inputs. Probably overkill but the leakage problem with tape monitor switch has already been noted and addressed here.
Hi Drew -- that would be great if you could. I can put my hands on it, I'm sure, but right now, I'm all caught up in the X-1000 project for Don.
Also, your instructions for starting up your IBAM should certainly ensure that the output tubes receive full bias voltage when they are first installed.
As for removing the lead from the Tape Head jack when it is converted to a line level input and isolating it or grouping it with the other line level jack leads, I would certainly do this if only to maintain consistency of the build format. But it would also be a good idea since in the TH position, the output of the low level preamps won't be shorted like they normally would. Good catch.
Dave
Also, your instructions for starting up your IBAM should certainly ensure that the output tubes receive full bias voltage when they are first installed.
As for removing the lead from the Tape Head jack when it is converted to a line level input and isolating it or grouping it with the other line level jack leads, I would certainly do this if only to maintain consistency of the build format. But it would also be a good idea since in the TH position, the output of the low level preamps won't be shorted like they normally would. Good catch.
Dave
drewbolce
Audio Analyist
I'll try and make some time over the weekend to verify the mod on the 500c, here is your writeup:
If the selector switch on the 400 is the same as that on the 500 (it likely is for all intents and purposes) then it should be very doable to convert the Tape Head Input jack to an Aux jack. It will require some rewiring of course, but the basic jest of the effort would be:
1. Remove existing lead from Tape Head Input jack, and ground it.
2. Disconnect terminals 2 & 3 of the EQ package from the selector switch. Also disconnect the output of the preamp from the terminal in between the terminals where the EQ package connects to the selector switch. Since there is also an 82K resistor tied to this terminal that allows the equalized signal to flow on to the line level portion of the selector switch, this resistor must be disconnected from this terminal as well, and then permanently reconnected to the output side of the preamp output coupling cap as it was before. You now have three completely empty terminals on the selector switch.
3. Hard wire terminal #3 of the EQ package to the output side of the preamp output coupling cap.
4. Connect the Tape Head Input jack to the selector switch terminal where EQ package lead #2 was connected to.
5. Find the two terminals that connect to the OTHER END of the aforementioned 82K resistor, and determine which one is picked up when Tape Head is selected. Break the connection between these two terminals, so that the 82K only connects to the terminal that is picked up when Phono is selected, and the other terminal (Tape Head) is free.
6. Find the terminal next to where the Tape Head Input jack is NOW connected (that was originally connected to the output side of the preamp output coupling cap and 82K resistor), and connect a jumper between this terminal, and the terminal identified and freed up in step #5.
And that should do it. The reason that the new Aux 2 input (old Tape Head Input jack) has to pass through two selector switch portions, is because otherwise, in Phono Mono, the Preamp output would try to back feed the new Aux 2 jack -- or whatever was plugged into the Aux 2 jack would try to short out the output of the preamp. By passing it through both switch sections, this concern is eliminated.
If the selector switch on the 400 is the same as that on the 500 (it likely is for all intents and purposes) then it should be very doable to convert the Tape Head Input jack to an Aux jack. It will require some rewiring of course, but the basic jest of the effort would be:
1. Remove existing lead from Tape Head Input jack, and ground it.
2. Disconnect terminals 2 & 3 of the EQ package from the selector switch. Also disconnect the output of the preamp from the terminal in between the terminals where the EQ package connects to the selector switch. Since there is also an 82K resistor tied to this terminal that allows the equalized signal to flow on to the line level portion of the selector switch, this resistor must be disconnected from this terminal as well, and then permanently reconnected to the output side of the preamp output coupling cap as it was before. You now have three completely empty terminals on the selector switch.
3. Hard wire terminal #3 of the EQ package to the output side of the preamp output coupling cap.
4. Connect the Tape Head Input jack to the selector switch terminal where EQ package lead #2 was connected to.
5. Find the two terminals that connect to the OTHER END of the aforementioned 82K resistor, and determine which one is picked up when Tape Head is selected. Break the connection between these two terminals, so that the 82K only connects to the terminal that is picked up when Phono is selected, and the other terminal (Tape Head) is free.
6. Find the terminal next to where the Tape Head Input jack is NOW connected (that was originally connected to the output side of the preamp output coupling cap and 82K resistor), and connect a jumper between this terminal, and the terminal identified and freed up in step #5.
And that should do it. The reason that the new Aux 2 input (old Tape Head Input jack) has to pass through two selector switch portions, is because otherwise, in Phono Mono, the Preamp output would try to back feed the new Aux 2 jack -- or whatever was plugged into the Aux 2 jack would try to short out the output of the preamp. By passing it through both switch sections, this concern is eliminated.
Thanks drew for posting the instructions. I'll have to look at the unit while reading 'em to make complete sense of everything.
I got the power amp section of my amp completed. Next I have to do the preamp part and then the EFB, IMAB and replace the volume pot. I have to figure out how to extend the shaft of the new pot. I'm going slow to help minimize mistakes which has paid off because I caught a few tonight.
Dan
I got the power amp section of my amp completed. Next I have to do the preamp part and then the EFB, IMAB and replace the volume pot. I have to figure out how to extend the shaft of the new pot. I'm going slow to help minimize mistakes which has paid off because I caught a few tonight.
Dan
drewbolce
Audio Analyist
I used a shaft extension on my Alps pot. Still had to cut and file the extension. Will post some old pics if you wish.
BrokenHill
Well-Known Member
After reading all the activity on the Fisher 400 lately I have rotated mine back in use and recently I was also thinking
about changing the stock volume pot over to a Alps, I have used those pots in other tube units and it is definitely worth
upgrading too.
The 400 will need more work then most amps to get the Alps pot to fit nicely and still retain the stock face plate although
I'm sure it will be worth the effort, I will be looking into this myself to see what I need to fabricate to make it work.
Recently I modified a pair of Realistic Optimus 45 speakers with 10 inch woofers and I really like the way they sound with
my Fisher 400, great synergy.
Interesting topic.:thmbsp:
about changing the stock volume pot over to a Alps, I have used those pots in other tube units and it is definitely worth
upgrading too.
The 400 will need more work then most amps to get the Alps pot to fit nicely and still retain the stock face plate although
I'm sure it will be worth the effort, I will be looking into this myself to see what I need to fabricate to make it work.
Recently I modified a pair of Realistic Optimus 45 speakers with 10 inch woofers and I really like the way they sound with
my Fisher 400, great synergy.
Interesting topic.:thmbsp:
Hi. I have a quick question on the preamp section. After the tape switch it goes to a 470k resistor on both channels. On one channel stock it has a 18pf cap across the resistor and on the other it has a 24pf instead of the 18. On the revised schematic it has 18pf for both channels. Is this correct or an oversight when it was drawn up? Thanks
Dan
Dan
What's the lowest wattage the 10k trimmers can be in the IBAM circuit for this project? Is a .1watt one calling it close? It's all Radioshack has besides a 3/4w multi turn. One of the legs of a 1/2watter snapped off on me
I'm trying to avoid a mouser order etc.
Thanks guys
Dan
I'm trying to avoid a mouser order etc. Thanks guys
Dan
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drewbolce
Audio Analyist
Those are "trimmer" caps. They are there to compensate for the various wire length and other component differences in the channels. I have seen many different values so I suspect the values used may have been the result of some measurement. Sometimes they are the same value, etc.
I am having some difficulty finding the Alps volume control pictures (I did it over 5 years ago). I will post them in a separate thread since it is off topic to this one.
BrokenHill
Well-Known Member
If the Alps volume control improved your 400 then maybe it would be OK to post your pictures under this discussion, if you find them.
I don't think it's that's far off topic.
Has anyone tried to bypass the tone controls in the 400 and if yes did it improve the sound any?
I don't think it's that's far off topic.
Has anyone tried to bypass the tone controls in the 400 and if yes did it improve the sound any?
Broken -- Read back through the original material of this thread, and you will find where I post the schematics of my improvements offered -- one of which involves adding a tone control bypass switch. If accurate flat performance is your goal, it is a very worthwhile improvement to install.
Dave
Dave
BrokenHill
Well-Known Member
Dave, I looked back at the previous post and downloaded the schematic from post number 128, I must say Luis did a wonderful job modifying the schematic
and many thanks to him for all the time he put into that project.
I would like to modify just the amp section of my 400 for now and try out some other preamp combinations, the only thing that I am not sure about is if
those mods will work on the 400 I have, reading back on previous post you discussed the early 400 transformers and not knowing if they would work because
of different specs, I did not see if that has since become known and resolved.
I have a early version 400 serial NO. 21359J output transformer NO. T1020-116-1
I do have another parts unit in storage and it could be about two or three weeks until I can go there and check what output transformers are on that unit.
Thanks for your work and info on the 400.
Robert
and many thanks to him for all the time he put into that project.
I would like to modify just the amp section of my 400 for now and try out some other preamp combinations, the only thing that I am not sure about is if
those mods will work on the 400 I have, reading back on previous post you discussed the early 400 transformers and not knowing if they would work because
of different specs, I did not see if that has since become known and resolved.
I have a early version 400 serial NO. 21359J output transformer NO. T1020-116-1
I do have another parts unit in storage and it could be about two or three weeks until I can go there and check what output transformers are on that unit.
Thanks for your work and info on the 400.
Robert
eduarsan1
Luis Eduardo Sanchez
Hi Robert
Is a pleausure, if you need something don´t hesitate in ask!!
In my opinion you can make the mods (unity gain buffer, the control section) in the preamp stage to mitigate the miller effect, as you saw in the previous posts the EFB is to control the screen voltage, so if is not posible to get the OT with the specific impedance as Dave says, just make the IBAM control in the EFB. In this point you must check the values of the screen resistors (200K) and the coupling caps (0.1uF)
I´m attaching the IBAM design from Drew Bolce
Regards
Luis
Is a pleausure, if you need something don´t hesitate in ask!!
In my opinion you can make the mods (unity gain buffer, the control section) in the preamp stage to mitigate the miller effect, as you saw in the previous posts the EFB is to control the screen voltage, so if is not posible to get the OT with the specific impedance as Dave says, just make the IBAM control in the EFB. In this point you must check the values of the screen resistors (200K) and the coupling caps (0.1uF)
I´m attaching the IBAM design from Drew Bolce
Regards
Luis
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