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In Reply to: RE: PAYDIRT posted by tweaker456 on October 10, 2021 at 15:27:22
SS coupling circuits usually use 10uF coupling capacitors and all of you are recommending tube type low value capacitors.
Follow Ups:
sony, I respectfully beg to differ, tube gear often requires high value coupling capacitors, if you are going to be driving solid state components downstream, which, as you know, tend to have low input impedance.
Lets go with what manufactures are using. Every SS receiver or amplifier used at least 5uF and 10uF was most often used. No exceptions I saw. Tube amplifiers never used more than .3uF I ever seen over 30 years of rebuilding and most were .01 (phono stages) to .1uF.
Do you also beg to differ with all the manufactures that their engineering staffs were wrong? I await your answer.
No need to get huffy about it. To be clear, I am referring to output coupling capacitors, capacitors placed right at the output. In tube phono stages or linestages that I have taken apart, I have never seen less than 1.0uF used in that position. That's just a fact, not an opinion.
I would agree completely that between stages within a circuit, much lower values are typical.
Every schematic of all the common tube amplifiers & tube receivers such as Dynaco, Eico, Scott, Fisher, Pilot, Heathkit, Grommes, Knight, etc never seen anything close to 1uF in a phono stage. .01uF and .02uF is common.
Edits: 10/12/21
Here's a Dynaco PAS3 schematic. The output coupling capacitor value is 1.5uF, so far as I can make out. (It's difficult to read.) So I don't see how your claim holds up at least for this well known and very popular vintage Dynaco preamplifier. I am referencing C111 and C211 in this schematic, which seem to be 1.5uF/160V.
Edits: 10/12/21
is far different on the output cap, as others were. On the wayback machine you'll find preamps connected to amps with input Z of 470k so the cap doesn't have to be large.
Can someone post a schematic or name a preamplifier that actually did use an output coupling capacitor as low in value as .01 or .01uF? I'm curious.
I would argue that in the "modern" era (say 1965 to the present), no phono or linestage used such a low value of coupling capacitor, unless it was designed specifically to cut off low frequencies above some set frequency, or it was designed specifically to be mated to an upstream component with a very high input Z. I am just curious to be proven wrong, if I am wrong. And this is only a discussion; my ego is not at stake.
nt
your points about more modern design is good and since lowering the accepted output impedances (like using cathode drive for the output) to a medium or low value is the key to prevent hum pickup, which can be a real pita. Hope you didn't get me wrong.
I run my preamp to amp connection with good unbalanced cable 20 feet with absolute quiet. My house, AC wiring, and system makes it possible but I understand the frustrations of many when it comes to system integration.
Cheers to ya
your point still holds
I am talking about tube amplifiers. They do have a preamp and most use .05uF to .1uF to the power tube grids from the preamp output on SE amps or on PP amps a phase inverter tube with .05 input and .1uF out. Read any schematic. Audiokarma has a good data base.
Well, you just changed your claim from .01 to .02uF to higher values.
The Z that determines bass cut-off is the parallel sum of the input Z of the driven component and the output Z of the driver. But assuming a net Z of 470K ohms, you would want about 0.2uF, to achieve a cut-off at about 2 Hz. Yes, that's fine. But most "modern" tube amplifiers present more like a 100K ohms input Z and that is further reduced when you take into account the shunt resistance at the output of the driver. But even assuming 100K, then you need 1.0uF, for a 2 Hz cut-off. Anyway, we are comparing actual observations, and you claimed that you commonly have seen .01 to .02uF. All I am saying is THOSE values are way way lower than anything I have ever seen in working on preamplifiers and phono stages, not that my experience is vast. You mentioned Dynaco, and I dredged up the schematic of the most iconic Dyna preamplifier, the PAS3; the schematic shows an output C of 1.5uF. On my own, because you piqued my curiosity, I also looked at the Marantz 7C, another paragon. The coupler is 1.0uF. I don't doubt that there may be some much older designs, where the expectation would be for driving 470K ohms, you might see lower values, and I guess if bass cut-off was not such a big issue in those olden days, then you could use .02uF with 470K ohms, for a -3db point of 20Hz. I have never taken apart an early 1950s design. (The 7C and PAS3 are probably already late 50s/early 60s.) Anyway, I am just chatting here, trying to learn.
Really? As an output coupling capacitor value? Can you post a schematic of any of those you named?
Lew, tube amps use an output impedance matching transformer, not an output coupling capacitor.
Yes, we are talking about stand alone phono stages or full function preamplifiers, certainly not amplifiers, and I do know how tube amplifiers are generally configured. Thanks. I never dreamed we were talking about amplifiers with output coupling capacitors.
I have a First Watt F1J which uses a 1 uF cap on the input, and a SAE from 1976 which uses a 100 uF. You're right, without the actual required values, which would have been nice to have posted, we're all just guessing.
All values are below 0.1uF, and it turned out that at that level even HV capacitors are small enough to fit. My secondary goal was also to save some $$$, but using parts you already own is also a way to save $$$. (See my post below.)
Thanks to everyone who responded.
Some of your old posts have been very helpful to me, even though you may not be aware of it, so thanks!
All values below .1uF? I never seen over 30 years of rebuilding SS amps and receivers using less than 2uF in FM radio coupling sections and never below 5uF every where else.
I must be hallucinating, in that case.If it helps you to calm down, this is the solid state input PCB from a Beveridge Audio direct-drive amplifier, which has a tube-based output stage. I have posted the schematic here. I replaced C101, C103, C104, C107, C108 C109, and C110.
Edits: 10/12/21 10/12/21
C109 and C110 are 10uF @ 40 volts.
Yes, I certainly do know that. I replaced the OEM ones with ELNA Silmic II. I have also used Auricap 10uF/400V in that position with success, but I decided to stick with an electrolytic in this amplifier.
If true in this case I wish I'd known that before looking into it.
Denied facts are still facts.
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