Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Greg on December 03, 2017, 07:48:55 am
-
Hi guys. I wish to use an ac coupled cathode follower at the output of a preamp (that I am working on) to buffer its signal in an attempt to making it less susceptible to pick-up hum on its way to the power amp. Since it's a «new old» 6C5 with 150V on its cathode, the h-k potential max of 90V is indeed exceeded. Now, I wish to use a small dedicated DC supply at 6V/0.3A to power up its filament. Could I just tap the DC operated heater on a 150VDC voltage source for a ground reference? I've seen this arrangement on AC filaments with a DC bias. The voltage amplifiers upstream are either a 6J7 or a 6F5 already on DC heaters, but at ground/earth potential. My goal is to make an old 40's style circuit hum free as much as possible. Downstream, I only have an inverter with a gain of 20, so the whole shebang could be unnecessary after all... Anyways, I made a circuit simulation on a free online applet and it didn't explode on the screen ;) here's the URL, thanks!
-
Sorry, you have to cut and paste the whole text in the browser. There's only one section that appears as a link and I can't merge the whole thing into a single direct link for an unknown reason http://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+10.20027730826997+50+5+43%0Av+256+368+256+224+0+0+40+6.3+0+0+0.5%0Ar+336+224+336+368+0+21%0Aw+256+224+336+224+0%0Aw+256+368+336+368+0%0Av+144+368+144+224+0+0+40+150+0+0+0.5%0Aw+144+224+256+224+0%0Ag+144+368+144+384+0%0Ao+1+64+0+4099+10+0.4+0+2+1+3%0Ao+4+64+0+4099+160+0.00009765625+1+2+4+3%0A
-
http://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+10.20027730826997+50+5+43%0Av+256+368+256+224+0+0+40+6.3+0+0+0.5%0Ar+336+224+336+368+0+21%0Aw+256+224+336+224+0%0Aw+256+368+336+368+0%0Av+144+368+144+224+0+0+40+150+0+0+0.5%0Aw+144+224+256+224+0%0Ag+144+368+144+384+0%0Ao+1+64+0+4099+10+0.4+0+2+1+3%0Ao+4+64+0+4099+160+0.00009765625+1+2+4+3%0A (http://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+10.20027730826997+50+5+43%0Av+256+368+256+224+0+0+40+6.3+0+0+0.5%0Ar+336+224+336+368+0+21%0Aw+256+224+336+224+0%0Aw+256+368+336+368+0%0Av+144+368+144+224+0+0+40+150+0+0+0.5%0Aw+144+224+256+224+0%0Ag+144+368+144+384+0%0Ao+1+64+0+4099+10+0.4+0+2+1+3%0Ao+4+64+0+4099+160+0.00009765625+1+2+4+3%0A)
Where is the problem ?
---
Usually DC elevated heaters use 40-60v DC reference from a volage divider + an e-cap for stabilization
(https://i.imgur.com/ICEFPSF.gif)
http://www.valvewizard.co.uk/heater.html (http://www.valvewizard.co.uk/heater.html)
Franco
-
Et voilą! Thank you Franco. I don't know why I could not assemble it.
-
You should be able to do that, but, the 6.3 supply must float as shown in your sim circuit. Do not connect any part of it to the ground used by the 150vdc source.
-
Thanks Sluckey! It will be as is. I don't recall seeing a proven known design with floating DC heaters. However, I often see the AC version with a DC offset that Franco is describing.
-
Thanks Sluckey! It will be as is. I don't recall seeing a proven known design with floating DC heaters. However, I often see the AC version with a DC offset that Franco is describing.
That's partly because elevating heaters is a technique to reduce AC hum transmitted from heaters (and heater wiring) into other parts of the circuit. This "shouldn't" be needed with DC heaters (if they're well filtered, constant DC). Of course, you have a different reason for doing it here, but it basically works the same.
You could consider actually referencing the DC voltage to the cathode then no matter how big the signal is at the output, it will always be at the same voltage (and obviously within 90v) but if the same supply also powers other tubes one set voltage that should be safe for all of them would be best.
-
Re-emphasizing: that mystery "small dedicated DC supply" must FLOAT. Infinite ohms.
Are you really delivering 100V signals? You should be able to blast power-amps or house consoles with 30-50V on the cathode. Re-bias.
Simplifying: with <90V on CF cathode, you could run *one* 6V supply at maybe 50V elevation to run all heaters.
-
Thanks PRR...well the value of 150 volts was a rough approximation based on an article by the valve wizard in which he suggested the use of a hot biased 12AU7 into 18k with a B+ of 280 volts with 140 volts across the tube for «long cable runs». For my application as a guitar pre-amp, there's a pentode upstream a la gibson, so the wide-open gain will be about 100 coming from a single coil pick-up so I'm guessing 50V for 500mV input. It could go up to 100V with 1 volt input. I would like the C-F stage to be transparent and not clip/cut-off before the phase inverter to retain the saturated "over the top" gibson amplifiers as much as possible. I sketched a more conservative 47k load line on the 6C5 sheet and I have a bias point of -7 volts, B+ of 250, 160 across the tube and 90 volts on the cathode. I guess your entire 60V heater lift suggestion will work in this case.
-
I've never tried to elevate a DC heater supply and I don't know exactly what will happen
but in my archive I've this two examples
(https://i.imgur.com/cNeW83M.gif)
(https://i.imgur.com/fru4eaw.jpg)
The first (small) schematic, if I remember correctly, was posted from PRR in one other forum, freestompboxes
(but also may be I remember badly and wasn't a post from PRR and/or in that forum)
Franco
-
Thank you Franco for posting this... Mine was like the small example above, but with a simpler C-R-C filter at 300ma/6V.... I tried, one time, plugging a 6VDC walwart on a small single ended amplifier and the hum completely vanished as soon as I grounded the negative. Probably from residual ripple. I recall it was a loud hum. There was no purpose to let the heater float in that particular amp, but that's how I first hook-it up...Btw, on my post above, I said that «I would like the C-F to be transparent to retain the saturated over the top gibson amplifiers»...well... I must admit, this statement seems a bit weird now since the rest of the amplifier is going to crumble. It might not be necessary after all. It will go from maximum 250k to 1 meg with about 14 inches of patch cord. In short my plan is the possibility to choose between 3 different style preamp into 3 different style power/amp with a common power supply, set-it up, and play. Simple topology with fast inter-connections. Speaker cabinet is separate.
-
> pick-up hum on its way to the power amp.
> I'm guessing 50V for 500mV input. It could go up to 100V with 1 volt input.
:huh: About all power amps I know will slam with 1V-2V input. "Lines" generally run 1V to 10V max. I have run UN-shielded cable 200 feet with 1V-7V max signal. Are you really proposing 50V-100V in the cable?
Do you have an actual problem or just anticipating a problem?
Unbalanced cathode follower does nothing to address the ground-difference problem, which is common when preamp and power amp end up on different power strips.
-
I was really anticipating a problem at first, but now it is very well clear that my problem is that I don't know s***t.
I knew that a regular phase inverter stage only need a couple of volts to bring a PP stage to clipping so I don't know why I did indulge myself with those numbers.
I have some homework to do. Understanding AC signals, interpreting tube data etc. etc.
Thanks!
JP
-
What do you mean "power amp"?
To me, it is a box that takes a Volt+ and makes Watts. The power tubes, but also the driver.
You could have the driver in one box and the power tubes in another. But that's typically two drive wires, and also a NFB wire from output back to the driver. And the driver outputs may not like driving long wires. So driver and power usually go in one box and take a few-Volt signal.
The preamp recovers a precious but weak signal and brings it up to around 1V.
All this is really old telephone custom. Telephony sucked until we had carbon mikes that made 0.1V-1V signal. Then they could go miles of wire. Telephone earpieces did OK with such signals. Telephony expanded on the output by adding power amplifiers and loudspeakers so a crowd could hear a phone signal. It expanded on the input by adding preamps so weak sources like dynamic mikes and guitar pickups could drive a phone line. All this technology went right into electrical recordings and talking movies. It turns out that "line level" (about 1V or 1mW) is a good line in the sand between problems of hiss/hum and problems of power.
-
Thanks for the brief explanation.
Yes, by power amp, I really meant driver & phase inverter, in my case a 6N7 and some 6L6's. This also applies for the single ended power amp, in this case a 6J5 followed by one 6L6.
My preamps are just a voltage amplifier in the form of a triode or a pentode with a volume control that I plan to construct on a small 5X7 chassis. I'm a guitarist so... different circuits with differents sounds to extend my palette. One could have a creamy midrange gibsonesque sound, one might have the honky valco/supro sound with the amplitude modulation, the other one might have an organ style vibrato circuit, another one, a circuit with tone shaping and a recovery stage, etc...
Then, common to all those circuit is the cathode follower at the output after the volume control. Like you said, it is not a real problem, rather an anticipated problem. Nothing is built. I'm still at the plan... I have anticipated some potential loss of treble from the shielded cable in between the preamp and the power amp. It might not be a problem.
-
Btw, I found an older thread by Franco about the use of a C-F for chassis interconnexion. It is from last year.
The frugality of big amp manufacturers being what it is, it must be there for something.
http://el34world.com/Forum/index.php?topic=20759.msg219306#msg219306
-
Looking at the GA-90 cathode follower circuit, I drew a 10k load line assuming a supply of 250 volts and it has about only 15 volts on the cathode. It must be sufficient then.