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Hi,
I'm restoring a Fisher amp and the cathodes of the output tubes (7189's) are connected to three 12AX7's in series + a 22 ohm resistor (and a 2.2 kohm resistor and a 200 uF,50 volt cap are in parallel with that). Two of these tubes are in the phono stage and the other is the tone circuit. Anyway, each of these 12AX7's only has 9 VDC dropped across the filament (pin 4 to pin 5). This seems awfully low to me. I'm asking because one of the tubes does not seem to be conducting as much current as it should be, there is barely any voltage drop across one of the plate resistors. Should I try upping the voltage a bit across these filaments?
Follow Ups:
The lower voltage is by design (though it should be 10-11V DC) - the 12AX7's transconductance will be lower, but noise is reduced too.
A matched set of EL84Ms may fix things up with no further action required.
I have that situation on my Scott 272 but I'm not getting the DC voltage for the filaments off the output tube cathodes.The voltage was a series string giving me 7.2vdc on pins 4 and 5.
In your case,do what Jim and Eli said with a little torroid.You you can even put the preamp tube filaments on the DC string depending on the size of your filament transformer you get.
Keep in mind that it isn't unusual to find one tube's filament brighter than another even of the same tube brand.Measure the filament resistance of each 12AX7 and get them as close as you can.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
This is typical of the "cheap Charlie" circuit used by Fisher and others to provide DC power to the heaters of specific tubes. It's quite common to see only 9 or 10 volts across them. You could change the value of the 22 Ohm resistor (or the 2.2K) but that also changes the bias on the 7189s. Even f you were able to set it up right - if you change power tubes the voltages will change too. Does this circuit work? Yes, but with a number of compromises built into it.The only real solution is to build a dedicated DC (preferably regulated) to power the 12AX7s - and then to either properly cathode bias the 7189s or build a fixed bias supply for them. The amp as built uses the 12AX7 heaters as part of the cathode bias circuit; so if you remove them from the existing circuit you'll need to have some method to properly bias the 7189s. That's you choice.
Also - as long as you have only one bias resistance for all four 7189 tubes you'll need a cathode current matched set of tubes for the output. You could rebuild the circuit with two cathode bias resistors and bypass caps and then you could use two matched pairs.
IMHO, this cost-cutting by Fisher was a very poor compromise. And fixing it will require you to spend the money that Fisher didn't spend when they built the unit.
Yes, you can make a "band-aid" to get higher heater voltages - but a proper fix (again IMHO) requires some redesign work.
Edited to fix typo
Edits: 12/03/16
Hi Jim,
Been out of town...thanks for the response. I did try bypassing the 22 ohm resistor with a jumper wire and now I'm getting about 11 volts dropping across the filaments and all of the 12AX7's appear to be conducting properly. The conduction problem on one of the tubes may actually be due to oxidized pins on the tube socket; sometimes it works, sometimes it doesn't. I'll hit it with some deoxit. If I were going to hang onto this amp, I'd definitely consider doing the mods you've suggested (maybe I should do them anyway). My plan is to pass this onto my cousin; she expressed an interest in owning a vintage integrated tube amp. I liked this one because it doesn't require any bias adjustment, just plug and play. I've changed out all of the electrolytics and coupling caps and inserted a couple of thermisters to get the voltages down to spec. Also retubed it with brand new tubes (I purchased them from you a while back, thank you very much!). So hopefully this thing will behave itself and give her years of trouble-free service. Thanks again for your help.
I agree with Jim. The "Cheap Charlie" method of DC heaters stinks on dry ice. :> ((Install a proper RC bias network in each channel and construct a regulated 12 VDC supply for the 3X 12AX7s. A $10 AnTek AN-0107 toroid is the heart of the new supply. Phase up and series connect the 2 secondaries. FWCT rectify the composite with a MBR20100CT common cathode twin Schottky diode. Filter the "raw" DC with a 25 WVDC/6800 μF. 'lytic. Regulate the rail with a 7812 3 terminal IC.
BTW, Fisher phono sections are notorious for poor bass extension. After getting the unit squared away, should better bass be on your list of things to do, we'll talk about reworking the phono preamp to use tweaked RCA topology.
Eli D.
Edits: 12/03/16
Thanks for the tips Eli. I would definitely be interested in your mod for the phono section. This particular amp uses those PEC modules for the equalization circuit. Would I have to take these out and replace them with discrete components? Please provide a link if you can. Thanks!
Yes, those blasted PECs go. Replace the source selector switch and hard wire 1 set of RCA I/Ps to the phono preamp. Tape head is passe. Get switches out of mV. level signal lines!
The tweaked RCA setup needs stable 250 V. B+. A TO-92 case LR8 regulator in each channel gets the job done, while improving channel separation.
Experience shows that a protective 12 V. Zener diode should be installed between the tube side of R3 and the ZVN0545A's source electrode.
Eli D.
nt
I had a "Senior Moment". I provided the wrong toroid. The AnTek part needed is AN-0112. Unfortunately, that item is out of stock. The "simplest" work around is to use the larger $11 AN-0212.
Eli D.
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