Does anyone have any experience with the 5879 phono preamp in the "RCA High-Fidelity Amplifier Circuits" booklet? The schematic is as follows.
I have looked pretty extensively and only found a mention or 2 on the web, but nothing significant. I'm looking to build one as I have a bunch of the 5879 tubes and have used them in guitar and audio amps with good success. I have an entry level (at best) turntable with the Audio Technical AT-3600L cartridge, and I'm totally happy with the sound. That either makes me a noob or a philistine, or maybe both. Anyway, I proposed some changes as follows.
1) Grid stoppers on all sections
2) 5879 screen bypass capacitor to cathode instead of ground
3) Instead of a 7025/12AX7, a 6AV6 and a 6C4 cathode follower (it'll be driving a 100k load). If the gain is too high, a 6AT6 can be swapped in.
4) Larger cathode bypass capacitors, because it's much cheaper in 2022
Also, how much of a difference would there be if I used a 22nF and 4.7nf caps for C5 and C6?
Any comments or insights will be greatly appreciated.
I have looked pretty extensively and only found a mention or 2 on the web, but nothing significant. I'm looking to build one as I have a bunch of the 5879 tubes and have used them in guitar and audio amps with good success. I have an entry level (at best) turntable with the Audio Technical AT-3600L cartridge, and I'm totally happy with the sound. That either makes me a noob or a philistine, or maybe both. Anyway, I proposed some changes as follows.
1) Grid stoppers on all sections
2) 5879 screen bypass capacitor to cathode instead of ground
3) Instead of a 7025/12AX7, a 6AV6 and a 6C4 cathode follower (it'll be driving a 100k load). If the gain is too high, a 6AT6 can be swapped in.
4) Larger cathode bypass capacitors, because it's much cheaper in 2022
Also, how much of a difference would there be if I used a 22nF and 4.7nf caps for C5 and C6?
Any comments or insights will be greatly appreciated.
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That AT-3600L cartridge can be made to perform quite well with a stylus upgrade to something with a hyperelliptical or even a Shibata tip.
That said, as long as your turntable and tonearm are up to it, playing with preamps should be fun.
I have not built this circuit, but I am curious about it. The 5879 was specially designed to be a low noise pentode for low level audio stages like mic and phono preamps.
In an attempt to be helpful, here's what the frequency response looks like in LTspice for the original values of C5 (20nF) and C6 (5nF):
Changing those to 22nF and 4.7nF:
And finally, with 22nF and 5.1nF:
Getting a little creative, I took some liberties and came up with C2 = 10nF (down from 100nF), C5 = 15nF, and C6 = 5.1nF:
That said, as long as your turntable and tonearm are up to it, playing with preamps should be fun.
I have not built this circuit, but I am curious about it. The 5879 was specially designed to be a low noise pentode for low level audio stages like mic and phono preamps.
In an attempt to be helpful, here's what the frequency response looks like in LTspice for the original values of C5 (20nF) and C6 (5nF):
Changing those to 22nF and 4.7nF:
And finally, with 22nF and 5.1nF:
Getting a little creative, I took some liberties and came up with C2 = 10nF (down from 100nF), C5 = 15nF, and C6 = 5.1nF:
Cool, thanks. Looks like there will not be any discernable difference (with my system and stone ears) using modern values for C5 and C6. Makes sense as the caps back in the day were at least +/- 10% anyway. I think I'll build the RIAA networks on mini turret boards so the components can be swapped out easier, just to play around.
Might as well post the build. Hammmond 10" x 6" aluminum chassis drilled and painted. Hammertone paint because I'm lazy.
Cool project!
I'm planning to build this circuit also using octal equivalents.
I'll be following your progress.
JE
I'm planning to build this circuit also using octal equivalents.
I'll be following your progress.
JE
Nobody ever mentions the 5879 here. It’s specifically designed for audio. Low noise low microphony etc etc.
Close enough, I'm sure. 6SJ7 has higher gm than 5879.
Would a metal tube be noisier than a glass bottle? Perhaps that's not a major noise contributor, though.
I was looking at 6AC7, with higher gm than either. Just haven't gotten around to using it yet.
Would a metal tube be noisier than a glass bottle? Perhaps that's not a major noise contributor, though.
I was looking at 6AC7, with higher gm than either. Just haven't gotten around to using it yet.
6J7 is a very near equivalent of 5879, much closer that 6SJ7, plus you get the funky top cap. The 6AC7 has a much higher transconductance. The GE datasheet does not recommend its use with audio frequencies unless you use DC for the heaters. Anyway, some progress, heaters and power supply done:
I'm gonna run a 6V4 rectifier. I found an mint Dutch Amperex tube in my stash. The electrolytics are 47uF. The 120uFs in my initial schematic were a bit overkill. Don't worry about the PT nuts coming in contact with the underside of the turret board. There's an adequate gap, and I have thick mastic tape covering the underside of the turrets.
I'm gonna run a 6V4 rectifier. I found an mint Dutch Amperex tube in my stash. The electrolytics are 47uF. The 120uFs in my initial schematic were a bit overkill. Don't worry about the PT nuts coming in contact with the underside of the turret board. There's an adequate gap, and I have thick mastic tape covering the underside of the turrets.
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Isn't it unneccesary, if not dangerous, to put those SS diodes in series with the 6V4 plates and solder it to lugs labelled »I.C.«?
Best regards!
Best regards!
I looked at the tube beforehand. Pin 2 was free floating, and pin 8 was just a support for the lower mica wafer. Since this preamp will never go out in the wild, I won't have to worry about someone swapping in a different rectifier tube without checking the aforementioned connections first. As for the necessity of the diodes, they offer protection in case the rectifier tube shorts out. It's probably not a concern in this case with a nice NOS tube, but I adhere to the teachings of @Tubelab_com.
The diodes also increase the PIV of the tube / diode combination while retaining the forward characteristics of the tube since its dynamic forward voltage drop characteristics dominate over the 0.7 volt drop of the diodes. The extra PIV will often save a tube and maybe a filter cap if a tube arc tries to start at turn on. An arc may start on a forward conduction cycle reducing the tube's PIV to near zero allowing the arc to continue during the normally non-conducting half cycle. The added diode will block this conduction stopping the arc, saving the fuse and possibly a lot more.
Back in 2010 the three major new production tube vendors turned out some poor quality 5AR4's, so the diodes were added to all Tubelab SSE boards. This eliminated the dreaded "sparking 5AR4 on start up" issues.
Back in 2010 the three major new production tube vendors turned out some poor quality 5AR4's, so the diodes were added to all Tubelab SSE boards. This eliminated the dreaded "sparking 5AR4 on start up" issues.
Some JJ 5AR4's still have an issue regarding tube arcing when used within the specified ratings. Two JJ 5AR4's I recently bought.
I have an old Lafayette 5AR4 that is in good shape and use it as a reference. The Lafayette operated fine in circuit, the JJ 5AR4 would arc. I installed 4007 diodes and it solved the PIV problem with the JJ. I have Sovtek 5AR4's and they seem to be OK without the 4007's. It's hard to pay the price for NOS 5AR4's when you can install 4007's for pennies...
I have an old Lafayette 5AR4 that is in good shape and use it as a reference. The Lafayette operated fine in circuit, the JJ 5AR4 would arc. I installed 4007 diodes and it solved the PIV problem with the JJ. I have Sovtek 5AR4's and they seem to be OK without the 4007's. It's hard to pay the price for NOS 5AR4's when you can install 4007's for pennies...
A rectifier tube has to pass the current of all the other tubes in the device. It must also fill up all the empty filter caps on the B+ line at start up and do this as it's waking up from a nap, or a long sleep. If the cathode and plate are concentric, straight, and the cathode is evenly coated, the current surge is evenly distributed across the surfaces and the tube should meet spec. If anything is out of alignment the current flow will be uneven, causing hot spots. If the cathode coating is not uniform, the thinner areas will heat up faster than the thick areas and try to pass all of this current leading to an arc.
I have an old RCA branded Sylvania 5AR4 that I got about 30 years ago new. It has been through at least a dozen amps over the years and still works fine. I got some Sovtek 5AR4 from a reseller that buys batches and screens them during the bad batch crisis and they were all good and the ones I still have are still good. At the time he told me that his reject rate for new Sovtek 5AR4's was about 50%.
There have been issues with 5AR4's from JJ, all New Sensor brands (EH, Sovtek, Mullard, Gold Lion...) and some Shuguang brands. These all seen to be batch related, and it is possible that rejected batches made it out the door by unorthodox means and wound up on Ebay or similar sites. I bought a bunch of "Chinese 6L6GC's" on Ebay many years ago for $2.50 each for experiments that might have ended badly. Some were junk, but some worked well, even well into the "red zone."
I have an old RCA branded Sylvania 5AR4 that I got about 30 years ago new. It has been through at least a dozen amps over the years and still works fine. I got some Sovtek 5AR4 from a reseller that buys batches and screens them during the bad batch crisis and they were all good and the ones I still have are still good. At the time he told me that his reject rate for new Sovtek 5AR4's was about 50%.
There have been issues with 5AR4's from JJ, all New Sensor brands (EH, Sovtek, Mullard, Gold Lion...) and some Shuguang brands. These all seen to be batch related, and it is possible that rejected batches made it out the door by unorthodox means and wound up on Ebay or similar sites. I bought a bunch of "Chinese 6L6GC's" on Ebay many years ago for $2.50 each for experiments that might have ended badly. Some were junk, but some worked well, even well into the "red zone."