Hi, What are these 2 components connected to the Cathode / Ground of the 12AT7 tube. Also, what would be their purpose/function? This tube is the 2nd tube in the phono stage. 12AT7 --> 12AT7 --> line stage
Thanks
Thanks
Here is a close up of the top. It appears to be a tantulum cap, correct? If so, is this a bypass cap? Only 6V.
That is what I am thinking, but why would a 6V be used? 6.3V filament. Seems a bit tight. So, this leads to another question. SInce the 12AT7 is indirect triode, the signal does not pass thru the cathode, correct? And therefore, the sonics are not impacted by the brand/quality of the cathode resistor and cap, correct?
Have you measured the voltage across it?
Can't answer that one off the top of my head 🙂 but I'm sure some of the valve experts here will know.
Can't answer that one off the top of my head 🙂 but I'm sure some of the valve experts here will know.
Cathode voltage is only a few volts.
Incorrect.
Incorrect.
Cathode Bypass Cap allows AC frequencies to bypass the Cathode Resistor. This can affect the signal passing from grid to plate in several ways.
It is a small aluminum electrolytic capacitor, 100 uF at 6Vdc.
So, the 12AT7 is a twin indirectly heated triode. What this means is that the filament is separate from the cathode. The cathode is a hollow metal sleeve with a coating to enhance emission. The filament heater wire is slipped inside the hollow cathode tube and the voltage applied simply heats the cathode, but does not conduct any cathode current.
Looking at the photo, pin 9 is not connected, so the heater voltage is applied to pins 4 and 5 which would be 12.6 volts for the pair of filaments in series (look at the data sheet). Also, the pair of 100uF 6V caps are typical for some newer polymer aluminum caps... I use these for several digital circuits. The photo shows the caps connected to the cathode pins (which are 3 and 8) and it appears the other end, which is attached to the center post, is a ground buss wire.
In short, the signal goes thru the bypass cap. The bypass cap will increase voltage gain by effectively shorting out the AC signal current across the cathode bias resistor. Without the bypass cap, the signal will result in a small voltage change proportional to the signal, resulting in feedback and reduced voltage gain. Hope this clears things up.
Looking at the photo, pin 9 is not connected, so the heater voltage is applied to pins 4 and 5 which would be 12.6 volts for the pair of filaments in series (look at the data sheet). Also, the pair of 100uF 6V caps are typical for some newer polymer aluminum caps... I use these for several digital circuits. The photo shows the caps connected to the cathode pins (which are 3 and 8) and it appears the other end, which is attached to the center post, is a ground buss wire.
In short, the signal goes thru the bypass cap. The bypass cap will increase voltage gain by effectively shorting out the AC signal current across the cathode bias resistor. Without the bypass cap, the signal will result in a small voltage change proportional to the signal, resulting in feedback and reduced voltage gain. Hope this clears things up.
Thanks everyone, great input. So, kmaier, since you have used these caps before, do you find replacing / upgrading them noticeably improves sonics? Also, which brand / model do you recommend. In theory, I could replace these with some film caps. Anyone have recommendations on replacing these caps? Or am I chasing my ***?
As noted above, I've used these caps in digital circuits, like microprocessor designs, not analogue audio designs.
Currently, all of my analogue audio designs, which are all vacuum tube based, use film capacitors, NO electrolytics. I've used several brands and types of film caps, including WonderCap, Solen, Axon and several others over the past 4+ decades. Types include polystyrene, polypropylene, metallized versions of same and some with tin or teflon foil as well. There's a wide range of components out there... but you should look at all components used in a design, not just caps... resistors, transformers, tube brands/types, connectors, sockets, wire, solder, etc. all play a part in getting a final build to perform as required.
Not sure this helps, but there's no magic formulae on how to pick parts for what you're designing and building... even the physical layout can make a difference as well. Posting a full schematic and some decent pics of what you're working with could help in suggesting some other components, but again, no magic here.
Currently, all of my analogue audio designs, which are all vacuum tube based, use film capacitors, NO electrolytics. I've used several brands and types of film caps, including WonderCap, Solen, Axon and several others over the past 4+ decades. Types include polystyrene, polypropylene, metallized versions of same and some with tin or teflon foil as well. There's a wide range of components out there... but you should look at all components used in a design, not just caps... resistors, transformers, tube brands/types, connectors, sockets, wire, solder, etc. all play a part in getting a final build to perform as required.
Not sure this helps, but there's no magic formulae on how to pick parts for what you're designing and building... even the physical layout can make a difference as well. Posting a full schematic and some decent pics of what you're working with could help in suggesting some other components, but again, no magic here.
Thanks kmaier. Very helpful. I too have experimented greatly with various film caps. Never used electrolytics. This is an existing commercial pre-amp that has no discernable schematic (at least I couldn't find it online). My favorite caps are the Russians. K40Y in particular. I tried Duelund and was not enamoured (I have a pair of Duelund 1uF silver/copper cast PIO available if anyone wants to make an offer). The K40Y give a texture / tone that is not found elsewhere, subjectively. I like them.
Replacing cathode bypass caps can be worth doing if they are no longer the capacitance of the original design. What is this phono amplifier? Is it old? Do you have a schematic? You could be sharing a lot more and get far better quality answers. 🙂
Electrolytic caps can dry out, but typically not that fast. It usually takes a decade or more... The most important thing when replacing them is to source similar value new components.
Yes, these ones are small and cheap. That doesn't mean they don't work as intended.
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That depends on the circuit. If the anode load is significant enough, and the valve/tube is running enough current, then an un-bypassed cathode resistor is an option. Usually this is not the case, and the original design with the by-passed cap is probably best.
We don't have a schematic, but it is a phono, and uses 12AT7. The cathode on the 2nd 12AT7 has this 100uF by-pass. This seems pretty obvious old-school phono to me. Likely that removing it will change the RIAA correction too. I would predict that removing it will cause frequencies below 100Hz to drop off more rapidly...
Personally, I am not a fan of 12AT7 for phono (plate curves say a lot about different valves/tubes). But some people claim they made a nice phono amplifier with them...
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The darkened area on top of the cap indicates that the lead wire under it is the negative terminal of the capacitor.
Hi everyone, There is no schematic that I can find for this. It's a Leben RS28CX pre-amp (with phono). THe 12AT7 is the phono stage that has the 100uF bypass cap. The pre-amp is relatively new (< 5 yrs), so I am not concerned about the caps drying out. Rather, I am a believer in cap quality makes a difference, assuming the signal goes thru the cap. Since this is an IDHT, I was unsure if the signal went thru the cathode. Definitely true for DHTs, but unsure on IDHTs. The phono stage is modeled after the Citation IV phono stage which uses a CR design. I don't know if it is an exact replica of the Citation IV, but was certainly the foundation for the design.
The signal 'goes through' almost every component in the whole circuit. But some matter more than others.