Recent content by AmadeusMozart

  1. AmadeusMozart

    R.I.P.

    R.I.P.
  2. AmadeusMozart

    bias readings in amp do not remain stable

    You do not mention the topology of your amplifier, are the output tubes running in pentode or in UL? It can be as simple as the transformers warming up with the result that their resistance increases somewhat which can effect a voltage change on the screens and that influences the anode...
  3. AmadeusMozart

    Latest build - 6GK6 SEP amp

    The 5670 / EL84 is now obsolete after I read an article on SID and TIM by Walt Jung (The Audio Amateur issue 1/1977) and now prefer my ECC81 / 7868 amplifier with driver in SRPP and 7868 in cathode feedback with no global feedback. It needed expensive output transformers but I'm extreemly...
  4. AmadeusMozart

    Latest build - 6GK6 SEP amp

    A couple of years ago I build something similar with the 5670 and the EL84 using all Transcendar iron. Used a thermal delay tube to slow down the B+ ramp up.
  5. AmadeusMozart

    Ultra Linear vs. Regulated Screen

    Not always will there be space for a choke. In that case an electronic smoothing regulator will work fine and if used for only the screens then it will not need a large heat sink.
  6. AmadeusMozart

    JJ = Just Junk?

    NOS = Next Online Scam In days past tubes were often sold in "a sleeve" of 5 tubes (both driver and output) and the tubes in the sleeve would be matched. Fixed bias is no longer a viable option with the current crop of power tubes, sooner or later a tube failure will destroy something, likely...
  7. AmadeusMozart

    Potted OP Transformer Orientation ?

    If the shielding is made of the proper material, mu metal, then it does not matter. But with today's look alike imitation the shielding does not amount to much. Just use any choke, hook up a mV meter and feed 6V AC through the primary winding and you'll find out how good the shielding is and...
  8. AmadeusMozart

    transformer ratio testing, loaded v unloaded

    The "nominal" impedance but you'll find that this can vary a lot depending on the operating point of the tube. But there are some who will use the lowest speaker load to be matched. I am running the tubes at 90% Plate dissipation and not at 100% (class A) which means that some variance in load...
  9. AmadeusMozart

    transformer ratio testing, loaded v unloaded

    You are right Jack - with PP the DC cancels out. The digital multimeters often do not measure accurately at higher audio frequencies, most start dropping above the 1 Khz.
  10. AmadeusMozart

    transformer ratio testing, loaded v unloaded

    I was not clear in the above message: my current amplifier was designed with 6.5, 10 and 14 ohm output for the use of those three BBC based LS3/5A variants. The crossover I've sofar designed will be used on the 10 ohm tap. In due time testing will tell if the crossover works as it has been...
  11. AmadeusMozart

    transformer ratio testing, loaded v unloaded

    No DC applied and no DC resistance (and losses) taken into account. Plus at what frequency were you testing. Ratio varies with frequency: Above approx 2 Khz the iron core has no impact. FWIW During WW II tests were done of ALL available output transformers as they wanted to use these for...
  12. AmadeusMozart

    Push Pull Clone Amp Builds Using Original or Aftermarket Iron

    The Dynaco 35 OPT are very good value for the money belonging to Dynaco's best. Instead of using the EL84 I would use them with the 7591 but it would need a different chassis due to octal based. The bigger the OPT the lower bass will go but the more HF (and impulse) will suffer. I'll never go...
  13. AmadeusMozart

    Do rectifier tubes affect sound much?

    ^^^ This. Normally PP are sensitive to the amount of resistance in the rectifier tube since the output tubes have a varying plate current when output power varies. Single ended amplifiers normally run in class A where the current through the output tube normally is constant. Don't assume that...
  14. AmadeusMozart

    Ideas about dealing with discolored varnish?

    If the bell covers do not need repainting then with a fine (artist) brush you can repaint the laminations with some satin black Humbrol paint while leaving the transformer in place. Just remove the tubes to get some space to do the job. FWIW I just did the same with a choke and a mains transformer.
  15. AmadeusMozart

    EL34 recommendations

    20 years ago I had a very bad run with Chinese tubes - they did not last long (eight KT 88). Once bitten, twice shy. Different mica's can have a microphonics impact, primarily noticed in driver tubes where often triple mica tubes demand a higher price. Price differences are sometimes...
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