Hello All,
Some time ago (2009?) we discussed this. I again have an old phono amplifier that just has a 50L6, and a 35Z5 in it but no preamp. There's a dropping resistor in series with the filaments to take up the slack. Now if I add the filament of the 12AT6 in series with the other two, take a 470K ohm resistor and a .01 mfd capacitor and connect it to the output of the 12AT6, and connect the other end of the .01 mfd cap to the input of the 50L6 and the other end of the 470K ohm resistor to power supply negative, then move the volume control wiper to the input (grid) of the 12AT6 and have something like a 4.7 meg ohm resistor, from the input to power supply negative. Then take another 470K ohm resistor and connect it from B+ to the plate of the 12AT6 to power it up. Then I need to connect the cathode to power supply negative too. Should I also connect the two diodes to power supply negative?
Will this work, will that give me a ceramic cartridge input or line level input? I can add capacitors to prevent hot chassis problems.
I think, with our higher AC line voltages today, then when the amp was made, this should work for the filaments.

Thanks,

Dave

I'm not you but here's what I would do. Search for a schematic that does what you want it to do. You can sub different preamp tubes by playing around with the bias and pinouts. The filament current rating has to be the same as the 50L6, 0.15 amps, IIRC.

I found these circuits at 'schematic heaven', a guitar amp site. These simple circuits are in the Bargain Bin folder. Look thru the cheap amps that used surplus radio tubes. The first Alamo amp above used only one preamp tube. The second one has an added tremelo circuit you could drop, but uses tubes closer to what you have.

What you describe might work fine as well. I'd have to see it on paper. But no need to reinvent an old wheel, eh? :>)

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/bargainbin/alamo_fiesta_1962.pdf

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/bargainbin/alamo_capri_2360.pdf

:I'm not you but here's what I would do. Search for a schematic that does what you want it to do. You can sub different preamp tubes by playing around with the bias and pinouts. The filament current rating has to be the same as the 50L6, 0.15 amps, IIRC.
:
:I found these circuits at 'schematic heaven', a guitar amp site. These simple circuits are in the Bargain Bin folder. Look thru the cheap amps that used surplus radio tubes. The first Alamo amp above used only one preamp tube. The second one has an added tremelo circuit you could drop, but uses tubes closer to what you have.
:
:What you describe might work fine as well. I'd have to see it on paper. But no need to reinvent an old wheel, eh? :>)
:
:http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/bargainbin/alamo_fiesta_1962.pdf
:
:http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/bargainbin/alamo_capri_2360.pdf
:
:

Footnote: I see that the Alamo Capri, the second schematic not the first, while having only 3 tubes, has is a clever 90 volt tap on the secondary winding of the isolation transformer, just for the filaments of the 3 tubes. So this is not very helpful. You will need to add a considerable amount of dropping resistance to your circuit, but with the filament voltage down to 90 vac or so, it will work.

I had and sold a couple of months ago, a Symphonic guitar amp that uses the 50L6 and 35Z5 with a 12AX7 preamp, so again, only 3 tubes. It also has a transformer filament supply, but in this case, they used 50 volts. 50 volts to the power tube, 50 volts to the other 2 in series.

This is fine. ...pretty conventional.. You can run the grid leak resistor all the way up to 10 Meg, but 5 is fine. Use a .003 MFD (or around that) input capacitor. This capacitor is necessary for grid-leak bias. Increase for more bass. Add a series resistor of from .5 to 1 Meg on the cartridge side of the volume control to remove annoying mid-range if necessary.

Increase the plate resistor to 1 Meg for more bass if necessary. This can increase mid-range, though. Response will be tighter with the .5 Meg.

Most AC/DC radios with 150mA tubes were designed for 118 or 121 volt nominal operation, depending on the tube string.

:Hello All,
: Some time ago (2009?) we discussed this. I again have an old phono amplifier that just has a 50L6, and a 35Z5 in it but no preamp. There's a dropping resistor in series with the filaments to take up the slack. Now if I add the filament of the 12AT6 in series with the other two, take a 470K ohm resistor and a .01 mfd capacitor and connect it to the output of the 12AT6, and connect the other end of the .01 mfd cap to the input of the 50L6 and the other end of the 470K ohm resistor to power supply negative, then move the volume control wiper to the input (grid) of the 12AT6 and have something like a 4.7 meg ohm resistor, from the input to power supply negative. Then take another 470K ohm resistor and connect it from B+ to the plate of the 12AT6 to power it up. Then I need to connect the cathode to power supply negative too. Should I also connect the two diodes to power supply negative?
: Will this work, will that give me a ceramic cartridge input or line level input? I can add capacitors to prevent hot chassis problems.
: I think, with our higher AC line voltages today, then when the amp was made, this should work for the filaments.
:
:Thanks,
:
:Dave
:

Thomas,
That works great. It was easy to hook-up. Now all I have to do is fit it inside the amplifier.

Thanks,

Dave
:This is fine. ...pretty conventional.. You can run the grid leak resistor all the way up to 10 Meg, but 5 is fine. Use a .003 MFD (or around that) input capacitor. This capacitor is necessary for grid-leak bias. Increase for more bass. Add a series resistor of from .5 to 1 Meg on the cartridge side of the volume control to remove annoying mid-range if necessary.
:
:Increase the plate resistor to 1 Meg for more bass if necessary. This can increase mid-range, though. Response will be tighter with the .5 Meg.
:
:Most AC/DC radios with 150mA tubes were designed for 118 or 121 volt nominal operation, depending on the tube string.
:
::Hello All,
:: Some time ago (2009?) we discussed this. I again have an old phono amplifier that just has a 50L6, and a 35Z5 in it but no preamp. There's a dropping resistor in series with the filaments to take up the slack. Now if I add the filament of the 12AT6 in series with the other two, take a 470K ohm resistor and a .01 mfd capacitor and connect it to the output of the 12AT6, and connect the other end of the .01 mfd cap to the input of the 50L6 and the other end of the 470K ohm resistor to power supply negative, then move the volume control wiper to the input (grid) of the 12AT6 and have something like a 4.7 meg ohm resistor, from the input to power supply negative. Then take another 470K ohm resistor and connect it from B+ to the plate of the 12AT6 to power it up. Then I need to connect the cathode to power supply negative too. Should I also connect the two diodes to power supply negative?
:: Will this work, will that give me a ceramic cartridge input or line level input? I can add capacitors to prevent hot chassis problems.
:: I think, with our higher AC line voltages today, then when the amp was made, this should work for the filaments.
::
::Thanks,
::
::Dave
::
:
:

Nice. If you are playing any high fidelity records and you do use a .5 to 1 Meg series resistor on the cartridge, you can also parallel this with a capacitor in the neighborhood of .001 MFD, +/- to your taste, plus any resistors in series with this to further tailor it to your taste. This will pass the crisp highs while still limiting the harsh mids that are common with some ceramic pick-ups. Not using the treble bypass capacitor, however, can also make for a very nice mellow bassy sound, somewhat like an old jukebox.

Injecting a small amount of silicone grease into the cartridge can also significantly reduce unnatural acoustic resonant peaks that the cartridge may posses if not already damped properly. Damping does increase necessary tracking force somewhat, however, but does eliminate some incredibly unpleasant resonances that many ceramic cartridges posses. I also like to use a very small amount in my Zenith Cobra RF cartridge. Works wonders to make for an unbelievable high fidelity experience, such as with late-30s/early 40s Victors and Bluebirds.

Negative feedback from the plate of the output tube to the plate of the 12AV6 can further make for a great high fidelity experience. Place a 50k resistor in series with the already present plate resistor on the B+ side of the original resistor. Send feedback to the junction of the two resistors via a capacitor and resistor in series from the output plate. .01 MFD makes for a sharper 1940s studio sound, but you can run it all the way to .05 or .1 MFD for good balance on modern recordings. Series resistance should be from 10k to 1 Meg to your taste, to control the amount of feedback. The resistor in series with the capacitor will also tend to hit more of the mids, where-as elimination of the resistor will tend to make for broader negative feedback. .001 MFD from the junction of the previously mentioned 12AV6 plate resistors to B- will cut out negative feedback at the highest frequencies and really clarify the highs for an amazing experience. Again, adjust value to taste.

Playing with the negative feedback can even make 78s sound amazing, which is what I really use it for, since I rarely play anything else, though recently I've been using my little RCA 45 player a lot, and have been acquiring quite a few 1950s 45s, like Les Baxter, Dion and the Belmonts, etc.