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In Reply to: The coupling caps I see are in about the middle of the diagram, just above and below center. posted by jeffreybehr on May 21, 2006 at 20:05:45:
Hi.I think I know which ones you mean. It's C1L and C2L. But they are not in the signal path, so I don't know...
theo
Follow Ups:
...to the output tubes on the right. But maybe not...
Looks to me like Jeff is correct; C1L and C2L couple the drivers to the grids of the output tubes, i.e., they block DC on the plates of the drivers from affecting the bias on the output tube grids, which is derived from a separate circuit. Your choice of those caps will affect the sound quality. If you want "warmth" go PIO. If you want good sounding but "warm" resistors, consider Rikens; I think they are carbon or carbon film types.
Hmm, sure is something here I don't understand her. Isn't it a feedback loop going from the output back from the output through C7L?Theo
This amp definately has coupling caps, although there is a bit of resistive coupling too (though not very much).If I were you I would start with the tube types in the driver- what types are they?
You might also consider Teflon coupling caps. They give you smoothness without coloration or lack of detail.
Hi.Thanks for the advice Ralph and Jeff. I think I'm starting to figure it out.
The tubes are from left to right: 6922-EF184-6C33C-B. I have experimentet a lot with different types of NOS tubes, and found Amperex 6922 and RFT EF184 to be nice sounding.It's maybe difficult to see from the circut picture, but if you can, is the signal flowing this way ?("upper" side):
In-> R2L-> R6L-> grid 6922-> plate 6922-> R12L-> grid Ef184-> plate EF184-> (R15l+C1L)-> grid 6C33C-> cathode and out.
Theo
After stating that I agreed with Jeff and upon further examination of the schematic, I too am a little puzzled by the circuit. Clearly, as you and Ralph suggest, there is a resistor (can't see the label) in parallel with the "coupling" caps. This would mean that some of the DC plate voltage on the EF184 will indeed be seen on the grid of the following 6C33C, depending upon the value of that resistor and the current. On the other hand the schematic also shows that the plate of the EF184 appears to be connected to what I think is the bias supply for the 6C33C, thru that very same resistor and cap, in parallel. Either the schematic has an error, or I don't understand how the amp works, or both. Nevertheless, the caps in question would affect the sound. What is their value (C1L and C2L, I think)?
Hi.I think I have figured it out now.
The signal goes from the input to the grid of 6922, From the plate of the 6922 in travels to the grid of the EF184 which is used as a triode. From the plate of the EF184 goes the signal through a circut where also the bias is regulated, and to the grid of the output tube.
And then there is a feedback from the output back to the 6922 a/EF184 g.The value of the coupling caps is .1.0 uF.
Theo
including the much vaunted 6H30 'super tube' and its brethern, all have the same issues with microphonics and contribute to unnatural brightness.So if you are concerned about getting some warmth out of the amp, it may be bona fide as the input tube is probably robbing you of some of that.
We used to use 6DJ8s and the like 16 years ago and discovered how profound this problem is. When the 6H30 showed up, we found it had similar traits. It will be a good idea to use a tube that was actually designed for audio.
I recommend perhaps a 12AT7, but there are no direct substitutes so you will have to figure out the bias point for the new tube that you go with. My point here is that until this is dealt with changing capacitors will not help- they may compensate the problem in some way but you will introduce other colorations as well. My suggestion is get to the root of the problem first- then deal with the caps.
Hi Ralph.I totally agree with you, the best way is to deal with the reason for the problem. Unfortunately I can't change the tube to another type. It's to complicated for me right now. So I try some "sweet" caps. And don't misunderstand me, this is a great OTL amp, but everything has it's compromises as you know:-)
Theo
Hi.I'm not a very skilled in reading schematics, so...
Anyone else got an opinion?Theo
Yes, I have an opinion. Listen to Ralph and Lew. They both are usually right on the money.
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