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Improving the Fisher 400

I took some voltage measurements with the line at 117 and the heaters at 6.3v

Power tubes
Pins 1&7 300.8v
Pin 9 415v

Phase Inverter 12ax7
Pin1 240.8v
Pins 2&6 92.1v
Pin 3 93.6v
Pin 8 .742v

Power tubes biased at .21v

ac adjustment seems like 50mv off maybe on one side moreso at 12volts etc. I'd have to check again. could a more worn tube cause this even with the bias set right?

Dan
 
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Okay. I hooked up my normal speakers and the volume seems fine. I remembered Dynaco A25's being more efficient. I'll pop in some other power tubes to check if the ac falls in line. I popped in my old set to just get the unit up and one is probably weak/overworn.

drewbolce, I used your IMAB testing procedure with a variac, swapped fuse etc and it worked good. Thanks.

Dan
 
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dcgillespie said:
Tom -- check in a later post, where I added the grid return resistor. I inadvertently left it out on the first schematic I posted -- but not in the actual unit. I caught the error, and corrected it some posts later, providing a revised schematic.

Dave
Click to expand...

dannyr said:
I'm glad the 1meg resistor was brought up. I also didn't have it installed. It may be missing from Luis's schematics.
Click to expand...

Dear All

I want to apologies if the revision 5 and 6 was wrong. By Dave´s suggestions I added the 1 Megohm resistor in his original drawings without IBAM and PS upgrade.

So here the Dave´s Drawings

And here the final version with IBAM and PS upgrade revision 6.

Sorry for update this till today!!, I was too busy in my work:drool:

Regards

Luis
 
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Luis, no reason to apologize. Thank you for taking all the time to do and share the drawings with us. They helped me a great dealing pulling off Dave's mods.

Dan
 
Ok, is good to know this, that´s why I´m helping with this amazing improvement!!

Next step is make a comparison between the original drawings and the modified drawings (Larry´s suggestions)

Maybe takes more time but I will do it!!!

PD: Dan don´t forget to upload the improvement images in your amp.

Luis
 
Hi Dan -- Regarding your concerns for the sensitivity of the unit, it will help if you tell me exactly what modifications you have done, as there were a number of them offered: EFB, phase inverter, driver stage feedback/HF stability design, Preamp Output/Power Amp Input jacks, buffer circuit, AF/tone amplifier revisions, and tone control bypass option. By the voltages you have provided, it would appear that at least the EFB, phase inverter, and driver stage modifications have been performed, and I think I read that you had the buffer stage all done as well. If this is true, did you also do any revisions to the AF/tone amplifier stages as well? They are required to maintain the stock sensitivity level when the power amp changes are made. Ultimately, when the complete modification is performed, a 270-280 mv signal applied to the Aux inputs will drive the unit to full power output when the tone controls are either centered, or bypassed.

Regarding the AC balance control adjustment versus balancing the output stage: May I assume that you are trying to null the voltage produced at the cathode test points when a 1 kHz signal is used to drive the output stage to about 18 watts output? If you still have the stock phase inverter installed, you will likely be able to null this out, but if you have installed mine, you likely won't be able to. That's because of a little known fact about push-pull output transformers: They ain't always perfectly balanced -- and the Fisher 400 "AX" outputs are just such an example of this fact. There are other transformers I can cite that are not perfectly balanced as well. This is not an issue with an individual transformer, but of the way a particular transformer is designed. In fact, when the phase inverter is adjusted for exactly equal outputs, and driving output tubes are exactly matched in every way, the distortion generated will be very much higher than if the drive is off-balanced to accommodate the imbalance in the output transformer. When the phase inverter is in fact adjusted for lowest distortion levels, the drive to the output tubes will not be balanced, with an offset at the output tube TPs noted. In my unit, I found that the setting for minimum distortion of the AC balance control in one channel, was within 100 ohms of the same minimum distortion setting in the other channel. As a result of this, the range of the AC balance control in my phase inverter modification intentionally targets an offset range, such that when nearly centered, the proper offset drive is provided to the output stage to in fact produce minimum distortion.

If you would like, I can measure the ratio of the cathode resistance to plate resistance in my unit, to get you very well in the ball park.

It sounds like you are rounding third for home, on a major undertaking of your receiver. I will be very interested in your opinion of the results. If I can be of any further help, don't hesitate to ask!

Dave
 
Hi Dave, I swapped in my normal speakers and the amp was fine/normal volume wise. I originally grabbed a pair of Dynaco A25's to do the first out loud test and I falsely recalled them being a lot more efficient. The mods that I implemented were everything that you outlined in the four schematics you shared with us here. The EFB, phase inverter changes, buffer plus preamp changes adding IMAB and a electrolytic recap but increasing a few to caps to 50, 47 and 22uf for others with the doubler now two 680uf/250v's.

If you ever have your unit apart and it's not much trouble please do the measurement and let me know. I'm sure it would help others that don't have a scope adjusting the AC balance. I just set the 5k trim to half for the time being. Maybe it's good enough.

Okay, onto the sound of the unit. Honestly, I don't know if it's pride coloring my objectiveness but I think the results sounds fantastic. Exceptionally so. It still has the sweet Fisher mids but is well balanced now. The sound just really blew me away from the first track that I played. Thank you so much for sharing your hard work with us. I greatly appreciate it and I learned a lot from this experience.

Do you happen to know what the phonostage's input impedance is? I think it's 68k but i'm not completely sure. I'd like to play with resistor loading to compensate for the slight roll off that you measured. I may have to raise the the input impedance to do so though depending on what cart is used. Easier than heavily modifying the stage and pretty easy after using some RCA Y adapters and resistors soldered to spare male plugs to pad it down.

Would you mind if sometime that I asked you for some hints/help on implementing your opamp buffer board to be used with a Fisher X-100-3? I think it may really help it out with a unit that I have. I just got to get the voltage to it.

Luis, I'll be sure to upload photos soon. They aren't going to be near as pretty as Dave's work though. I should took some before I placed the amp on my shelf and hooked everything up. One more nit picking thing I found is that the 1meg resistor in the EFB section should maybe be marked 1%. Thank you also for everything.

Dan
 
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Hi Dan -- Congratulations on a job well done! I'm so happy for you that all your hard work provided the return you were looking for. I would agree completely with your assessment that all of the original midrange performance is still present, but that the entire presentation is very well balanced now. I particularly enjoy the presentation of new found detail information now that I found wanting in the stock design. Did you also install the tone control bypass modification? It produces a forced flat response that I find very satisfying -- it just removes any questions about potential coloration the tone networks can add even when set at flat. In my unit, the switch is wired so that the unit is inherently in the bypass mode with the High Filter switch down. Therefore, I am effectively having to turn the tone controls on in my unit (switch up) when I want them in the circuit. You can just as easily wire the switch the opposite way to make it work as a traditional bypass switch -- since use of the word "bypass" implies the tone controls would normally be in circuit if they were included in the signal path with the switch in the down position.

The input impedance of the phono stage is theoretically 52.267K ohms at the Phono Low inputs, and 51.874K ohms at the Phono High inputs, assuming all the resistors associated with the phono input networks are perfect in value. When I can clear my bench of the X-1000 project I am currently working on, I plan to see what options exist to address the phono stage roll off in the Fisher preamp circuit.

The value of the 1 meg resistor in the EFB circuit is in fact important for proper operation, and should ideally be a close tolerance part for that reason. Thank you for highlighting this fact. This resistor acts (in part) to set the range of the EFB bias control. Sight variations in the value of this resistor can normally be compensated for within the available range of this control, although the addition of an IBAM may require a slight alteration to component values to maintain the best usable bias range. If you have any input on that from you experience, it would be appreciated.

Finally, remember too that one of the best things you can enjoy now is that the improved performance you're enjoying also comes with your output tubes idling at only 60% of the quiescent current they are required to in the stock design, and therefore, only produce 60% of the heat dissipation they used to as well. The advantage this has regarding output tube life, and the life of surrounding components in general cannot be emphasized greatly enough.

Congratulations again!

Dave
 
Dave, I did do your tone bypass mod. Thanks for asking. It's a nice feature to have as are tone controls. I find FM really benefits from a bass cut and tiny bit of a high boost to combat the station's compressors/processing and whatever else.

I'm really excited to read about your phono stage experimentations down the road.

IBAM seems to work great with EFB. I have no issues so far using the values in the schematic Luis did. All that seems to matter is if the EFB's bias is able to get up to or lower than the lowest tube then the IBAM takes over from there.

I am extremely grateful that your EFB design extends output tube life. It was actually the main factor that first got me interested in your extensive modifications. Even a quad of Electroharmonix 7868 are more than $100 these days and are most likely going to go up in price over the coming years.

Dan
 
Okay. I finally got around to taking some photos. Dave's helped me a great deal. Maybe these will be of some help to someone also.









 
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Very Nicely done Danny. I especially like the standoff for the upper can caps.

Larry
 
Thank you Larry. I believe I got the idea for the standoffs from a picture of someone else's rebuild here. I can't recall who though. Anyway, it works great for two Panasonic TS-ED panasonic 680uf/250 volt caps. They come in a few sizes so anyone wanting to do the same make sure that they are mouser part 667-EET-ED2E681EA.
 
Danny -- Congrats on a really super job! With how much circuitry there is in these receivers to begin with, they can look mighty intimidating right out of the box. So to install the full complement of these major modifications into that kind of environment, and to do so as neatly as you have done, and have it all work so well to boot, is a credit to your skills and capability. If the sound of the modified receiver is sincerely to your liking, then the return on your effort is priceless!

Thanks for taking this on, and again, congrats on a job well done!

Dave
 
Apple, Luis, Dave, Thanks for the complements.

Dave, I just cloned your work. Without it I'd just have done the basic rebuild. Btw, your modifications sound even better now that I installed a set of strong output tubes.

Luis, when are you going to start doing your 400? :)

Dan
 
dannyr, I see you installed a Alps volume pot, does that control have the forth tap for the loudness circuit? it's hard to tell from the picture.

I have a 500k Alps pot without the forth tap and I tried it last night in my 400, the smoothness and accurate tracking at low level is just fantastic although the
highs are rolled off to much, lacking when compared to the stock pot.:scratch2:

I also got one of the 500k pots that Mark Oppat sells for the Fisher 400 from his web site, tried it last night also, it sounds really good and the highs come through
very nicely even way better the stock pot although at very low level listening one channel drops out completely before the other and it has a tiny amount of backlash
between the left and right channels when turning up then down and that makes it not real good for low level control.

I would try a stepped attenuator but from my experience in other tube gear they are not the best choice for low level control.

For now I will contact Mark Oppat and see if there is a way around that problem with the control I got from him.
 
Yeah, the Alps was one from China with the extra tap. I moved the power switch to the unit's rear and had to extend the shaft with JB Weld and a drinking straw to hold everything in place while it dried. Seems to be holding up really well. The new pot tracks really nice. Hard to ask for much more. I'm not a huge fan of loudness but wanted it functional if it was possible. I never tried the ALPS with my 400 stock so I couldn't tell you if it changed the sonics of my unit. I did install one in a Fisher X100-3 and didn't notice any change in that regard but the tracking of the original was so off that I doubt I'd be able to tell.

How much was the pot from Mark Oppat? Also, have you thought about maybe using the ALPS and installing Dave's buffer in your unit? If it doesn't work out it's easy enough to go back to stock.

I agree about stepped controls. They never seem to have enough steps for me to nail the right listening level later in the night.

Dan
 
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