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In Reply to: RE: resistors value posted by Tubenstein on March 12, 2017 at 05:42:00
Yes, close enough is close enough for resistor values. Get familiar with Ohms law and use logic on loading resistors. The 1M resistor just sets the input load. 500K would work as well. The cathode resistor in the second section sets bias as part of the diodes on the grid. 1% variance of a 422 ohm resistor is 4.22 ohms which puts 430 ohms almost within 1%. 5% variance is generally tolerable.
I would not monkey with the cathode resistor on the lower 6922 section. That is a particular circuit that is serving as an active cathode load for the upper section. You are not going to get much an improvement at all, if any. The designer is obviously fully aware of the benefits of CCS cathode loading as they used it in the first stage and would have used it in the next if it was needed.
I think part of the reason not putting CCS in the second stage because of the costs of production. The SFL-2 almost a bargain price back in the 90s' for what they offered back in the day.
Personal I like those positions with Vishay naked resistors other than CCS.
Don't you think this is a wise idea to have carbon comp resistors for positions like LR27, LR30, LR37, and LR38?
We actually have tried 2 x 54.8K 2W in parallel for the positions of LR35 and LR36 that required 27.4K 3W with great result in my friend's unit. Actually the unit should gain a certain degree of sonic benefit if having wirewound resistor for the remaining positions.
I've seen the circuit diagram in the past and the designer build the supply for the grid supply (145v) is not quite the same level as the other 2 stages.......Any thoughts??
I totally disagree, i do not think the decision was made due to cost. A 50 cent transistor is far cheaper than a tube stage in production. Fancy parts are fun, but be careful, you can't improve a marginal circuit with fancy parts to sound better than a good circuit with normal parts.
Balanced circuits were all the rage 20 years ago because of perceived "professional quality". You don't see them much today because they don't sound as good as a quality single ended circuit. Pro's use them in large studio and hall environments as the long cable runs pick up noise, despite the problems they create with inherent distortion of spitting the signal and amplifying the positive and negative separately, then combining them with errors.
What problems do you want to solve with the preamp?
1. How much gain do you need?
2. What circuit are you trying to drive?
3. How long is the cable you are driving?
4. Do you really need balanced patch cords?
4. How many signals do you need to switch?
If all you need is a buffered attenuator for your CD player with two sources to switch and an amp 1 foot away? An AVC, or a 01a, 26, 10, 801a DHT will be worlds better than running your signal through 6 tube sections (12 for stereo) for just some attenuation.
I agree with you most part, however, this is necessary to have a fully balanced linestage like the SFL-2. I listened to a system that has Lynn Olson Amity abd the SFL-2 drive it perfectly as this amplifier is pretty insensitive -- around 5v input sensitivity with 300B output tubes.
The SFL-2 has a pretty high gain and well designed balanced circuit. I don't think a preamp with your listed DHT tube is able to do the job. Unless a DHT preamp build in the way like this circuit with PT8/PT9 WOT that has 24db with tremendous amount of drive, but it has 2 stages.
Unfortunately, I have the circuit, but no values of all components indicated in the circuit.
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