Parafeed with resistive load

[mashaffer]

I haven't found much discussion of this type of output stage using resistive rather than choke load. I realize that B+ needs to be much larger to get the same plate voltage but would the use of a resistive plate load be more linear than a choke load?

Also in some referrences that I have checked mention is made of tuning the coupling capacitor to the OPT inductance. I don't really understand the point in that. Wouldn't you want the reactance of the cap to be as low as possible so that it provided a short at audio frequencies? Why would you want it to resonate in the audio band?

mike

[pedroskova]

Originally posted by mashaffer

Also in some referrences that I have checked mention is made of tuning the coupling capacitor to the OPT inductance. I don't really understand the point in that. Wouldn't you want the reactance of the cap to be as low as possible so that it provided a short at audio frequencies? Why would you want it to resonate in the audio band?

mike

As I understand it, you want to choose the smallest value capacitor that, in conjunction with the OPT primary inductance, has a resonant point below the audible band. Some choose 3Hz, and others choose slightly higher points. Also, in the case of a transformer that is "lacking" in primary inductance, one can choose a capacitor value that will allow the resonant peak to "augment" a weak bass response. A search of the Bottlehead and DIY tube asylum will bring up the different calculations that folks use to help determine cap size.

hope this helps and is not too factually wrong.

[Shoog]

I wouldn't recommend trying this approach as the voltages will be massively large to get any useful output. If you are looking for linear then using one of Gary Pimms Pentode Constant Current Sources is likely to produce much more usable results. Even so a Tube CCS will require something like 700V to get about 15Watts output.

Shoog

[Sch3mat1c]

I think I calculated 1/2W output from a 6CL6 RC coupled stage such as in this thing:
http://webpages.charter.net/dawill/t...d_6146_Amp.gif
Not only is half your *DC* power wasted in the resistor to start with, but at least as much of the AC signal is burned as well!

Works for the attached (my avatar) though.

Tim

[tubelab.com]

I tried this approach several years ago just to see what would happen. I don't remember the exact details, but I used an old 6B4 tube and a large (50 watt) resistor. I don't remember the value. It needed a lot of B+ voltage, something like 600 to 700 volts, most of which is used up making the resistor hot. I believe I got almost a watt of power. The resistor is effectively in parallel (for AC anyway) with the primary of the output transformer, so some of your signal power is also used to make the resistor hot. On the other hand it did sound pretty good.
If you want to make a parafeed amp without a choke the Constant Current Source is the way to go. These can be made with mosfets, bipolar transistors, or vacuum tubes. For small (up to 4 or 5 Watts) amps nothing beats the IXYS 10M45 integrated circuit CCS. I have used them in every SE amp that I have made for the last year or so for loading small signal triodes. They work well as the plate load for a 45 DHT. I couldn't find the exact schematic, however I have included a schematic of a 45 SET amp that I built using CCS's made with bipolar transistors. The same design can be used with the CCS IC's. Look at the SE amp and the 845SE design on my web site for amp schematics that use these chips.

[yagoolar]

Hi,

You may want to look at vt52.com for plate choke formula
vt52.com software tools

or read a really good article by Bill Ramsey on "Mathematical Derivation of the Parafeed Output Stage"

Link to the article by B.Ramsey

[pedroskova]

Originally posted by tubelab.com
For small (up to 4 or 5 Watts) amps nothing beats the IXYS 10M45 integrated circuit CCS. I have used them in every SE amp that I have made for the last year or so for loading small signal triodes. They work well as the plate load for a 45 DHT. I couldn't find the exact schematic, however I have included a schematic of a 45 SET amp that I built using CCS's made with bipolar transistors. The same design can be used with the CCS IC's. Look at the SE amp and the 845SE design on my web site for amp schematics that use these chips.

Hi,

I noticed in your schematics that you have a 1kohm resistor going from CCS(IXYS) ground to tube plate, bypassing the current set resistor. What funtion does this serve? Is this for stability?

[tubelab.com]

Exactly! The data sheet for this chip does not have a resistor here. I added it to solve a problem with high frequency oscillation (50 MHz) that often resulted in blown parts. The 1K resistor works like a gate (grid) stopper resistor.
When these chips do fail they tend to short out. This will put the full power supply across your output tube. Do not use rare or expensive tubes until your circuit is working good.
If you are using one of these (or any solid state current source) to power an output tube, it will need a good heat sink. The tab on the case is connected to B+, be careful.

[pedroskova]

Originally posted by tubelab.com
Exactly! The data sheet for this chip does not have a resistor here. I added it to solve a problem with high frequency oscillation (50 MHz) that often resulted in blown parts. The 1K resistor works like a gate (grid) stopper resistor.
When these chips do fail they tend to short out. This will put the full power supply across your output tube. Do not use rare or expensive tubes until your circuit is working good.
If you are using one of these (or any solid state current source) to power an output tube, it will need a good heat sink. The tab on the case is connected to B+, be careful.

I'll try that. I'm using them to load 5687's in my PP parafeed linestage, as well as to set current for my VR shunt regulaters. One of the VR tube positions "wigs out" on me every once in a while, independant of the particular tube in place, so I've been suspecting the CCS as culprit.

...but they're so damned cheap and easy to use, I can't resist.

Sorry for hijacking the thread.

[rdf]

Originally posted by tubelab.com
Exactly! The data sheet for this chip does not have a resistor here. I added it to solve a problem with high frequency oscillation (50 MHz) that often resulted in blown parts. The 1K resistor works like a gate (grid) stopper resistor.

Finally found this post! I knew you wrote somewhere about oscillating IXYS's. Have you also tried a resistor in series with the CSS and the tube plate? Do you think this location would be as effective as a stopper?

It's something I mean to try, experiment with different voltage drops across the IXYS for a given current. Right off the bat it drops the device power dissipation. Possible additional benefits are isolating the chip (some are adamant it's a simple depletion mode FET, others a box of high NFB parts) from plate reactances at HF and the resistor acts a current limiter should it short. Gary Pimm also meaured a capacitance of ~2.3 pF for the IXYS and under 0.2 pF for a resistor, the series combination has a tiny capacitance between plate and PS, pehaps an additional benefit at very high frequencies. Spare an opinion?