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  1. Tubologic

    Pending AB board upgrade

    "ZERO catastrophic tube failures", really ? You must be VERY lucky indeed. Allow me to ask you the number of statistic samples that led you to this conclusion. In over 35+ years in professionally servicing (not only "running") tube audio equipment I could fill a whole container with power...
  2. Tubologic

    Pending AB board upgrade

    The auto-bias board is not a protection circuit and will not prevent red plating if something goes seriously wrong. In the event of a tube catastrophic failure ( very common with currently produced tubes) the servo-bias will try to limit the current through the tube by pushing it near cut-off...
  3. Tubologic

    Barn Find Scott LK-150

    Good find ! In the condition as shown and untested I would say 300 to 450$ if you could find someone willing to take the risk. If you can test (and guarantee) the transformers it could bring up to 600 - 700$ in the same condition. In good working condition this is an excellent amp and worth...
  4. Tubologic

    Can small signal tube cause other parts to fail?

    Not only theoretically: I once had a Mc275 where a defective (direct coupled) 12AZ7 driver has damaged the KT88 output tube. But this is a rare occurrence and defective small signal tubes usually don't damage associated parts. Another (destructive) effect I had in a preamp was due to a dead...
  5. Tubologic

    A few feedback questions.

    There is: the general theory of closed loop stabilized (negative feedback) amplifiers clearly and unequivocally specifies that the feedback voltage must be taken directly across the output load, or as close to it as physically possible to get the best corrective effect (the intended purpose of...
  6. Tubologic

    McIntosh mc2300 becomes unstable at 120 volts

    I'm currently working on the complete rebuild of a trashed MC2300 I found in wrecked condition: missing power transistors, both driver boards destroyed, many traces of previous (unsucessful) repair attempts, etc... the boards are finished now (ALL new parts/upgrades fitted, except for the...
  7. Tubologic

    A few feedback questions.

    The Dynaco (and many other amps) circuit is a compromise: though you can take your feedback point at any taps on the transformer it is allways better to use the tap where your load (speaker) is actually connected. Of course, the feedback resistor (as well as the paralleled compensation...
  8. Tubologic

    Is this tube?

    Maybe both tube(s) and SS: during the transitional era (circa 1963-1966) it was very common to find hybrid circuits in these consoles, usually the tuner section used tubes and the power amp was solid-state. I've seen that scheme in various old consoles from RCA, Motorola, etc...
  9. Tubologic

    Best new production 12AX7 for Fisher Receivers?

    Most current production 12AX7's should work well in vintage Fisher amps/receivers but there's one critical point who needs attention: in some of their amps (X-100 / X-100A / X-100B / X-101C / X-101D etc...) Fisher used their famous mixed bias circuit where the total cathode current of the output...
  10. Tubologic

    Marantz 1060B, high dc offset and no bias voltage.

    This is a fully DC coupled amp thus the fault can be allmost anywhere, any defective part will unbalance the circuit. I would first replace the differential input transistors pair Q701/702 and check all the related parts around this circuit, including resistors which can sometimes be found...
  11. Tubologic

    1060 right channel is weak/distorted, looking for repair

    I was in the service business for most of my active life and I would say this is quite unethical :how could they state "not the same problem" without even looking at your set ? I fully agree with you, and would not deal with these people anymore.
  12. Tubologic

    Marantz 2325 Tripping at high volume - Even without connecting Speakers

    According to the 2325 manual and circuit, the speaker protection circuit is triggered either by a DC offset detected at the output(s) of the power amp(s) OR a very low frequency (7 Hz or less) signal reaching the amp. The former can already be excluded since you have recapped and adjusted the...
  13. Tubologic

    1060 right channel is weak/distorted, looking for repair

    Very obviously the first thing to do, failure only one month after it came out from the repair shop is not normal and you have full rights to invoke the guarantee.
  14. Tubologic

    Fisher SA-1000 L.F Instability (motor boating) issue.

    I should also mention another unusual thing I noticed on ALL SA-1000's I've worked on: the bias meter needle is never fully stable but oscillates at a very low frequency around the set mark (even when the amp looks stable), the frequency of this oscillation being very close to the heavy...
  15. Tubologic

    1060 right channel is weak/distorted, looking for repair

    You must first determine if the problem is in the preamp or power amp section: remove the pre-out/main in bridges at the back of the amp and feed a (variable) line level source into the power amp input sockets. You may use any Hi-fi equipment (cassette deck, recorder, CD player...) provided...
  16. Tubologic

    Fisher SA-1000 L.F Instability (motor boating) issue.

    Of course, shifting the poles by changing the coupling caps values may improve stability but i'm still wondering how the Fisher engineers have missed this and designed (and sold) a marginally stable amp. And I'm guessing that when these amps were new they worked properly, or they would have...
  17. Tubologic

    Need help with Marantz 1060 dead amplifier section

    The voltage at J709 is the main (unregulated) power amp supply voltage rail and anything between 70 and 80VDC is correct, thus your measured voltage is within specs. The first thing to check are the voltages at J715 and J716 which must be about the half of the supply rail (35 to 40 VDC) and...
  18. Tubologic

    Fisher SA-1000 L.F Instability (motor boating) issue.

    Dave, thank you for your suggestions, I'll try them soon and report here about the results. Point#1 can already be excluded since I've tried to remove the 1st stage CF tube and the amp was still unstable without it. I don't think it would be safe to operate the amp unloaded since when under...
  19. Tubologic

    Fisher SA-1000 L.F Instability (motor boating) issue.

    This is the 4th Fisher SA-1000 (NOT X-1000) big power amp I'm working on and all had the same issue, which I suppose is structural and inherent to the design: the amp is unstable and has a tendency to going into strong very low frequency oscillation (motor boating) at a very low pulsating...
  20. Tubologic

    Marantz SR 3100 want to repair it

    The first suspect is a defective (shorted) STK080G power amp module. Unsolder pins 6 and 8 (the power supply rails) on the defective channel module (the one with the smoking resistors), or remove the module completely. Then test again with the DBT (75-100W, no more) to check if the overload is gone.
  21. Tubologic

    Heathkit Sp2 heavily modded. Need experienced opinion

    Usually I'm very suspicious of the so-called extended audiophile upgrades and stay away from"Highly Modified" units. I've seen too many butchered valuable sets damaged beyond repair by well-intentioned hobbyists using the highest quality parts but unable to make a good solder joint. Ask for a...
  22. Tubologic

    Hi all. What the HECK do I have here ?

    Believe it or not, but there are audiophile D.I.Y hobbyists who are actually building audio amplifiers using these big multi-kilovolt transmitting tubes (like the 250TH in th 1st picture , which is a "small" 250 watts plate dissipation transmitting triode). Mainly a japanese (but not only)...
  23. Tubologic

    Let's play Fisher mystery ID

    Maybe the WSR designator was for the (450T) Receiver, and he WSC was for the (optional) wooden Case (or Cabinet) for the above receiver ? Not sure, but it kind of make some sense...
  24. Tubologic

    ELL80 subs; 6T10?

    I remember in the mid-70ies (when the ELL80 being discontinued was already scarce and expensive !) the local PHILIPS distributor offered a subsititution made from an adaptor and two EL95's (6DL5). They were taller and ugly but worked very well (actually they had a longer life !), and that's what...
  25. Tubologic

    Hypothetically - the value of a Thorens TD124

    "Better" (sonically) is mostly subjective and highly debatable. But if you're into vintage high quality turntables there are a few sleepers worth investigating: A few (non-exhaustive) exemples: Fairchild 412 series (very underrated) Braun PS-500 (don't let you put down by the name, these are...
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