Pair of Heathkit EA-1 with different volume output

Discussion in 'Tube Audio' started by Raul, Jan 5, 2018.

  1. Raul

    Raul Active Member

    Messages:
    284
    Location:
    Italy
    I have two mono integrated Heathkit EA-1 (the same of AA-191, with circuit similar to Mullard 3-3) that I modded as per Eli Duttman schematics (attached below), they both work but one of them has definitely less volume (and sound a little less "clear") than the other one, voltages are good, tubes are OK, resistors were replaced if out of spec and the pots are new. Do you have an idea about what can be the reason and if it can be fixed?
    Thanks
    Raul
    [​IMG]
     
  2. knockbill

    knockbill Addicted Member

    Messages:
    6,910
    Location:
    SE PA
    Have you contacted Eli? I've found him very accommodating... You could also check/match voltages between the two chassis,,, and see where they differ... Check the input switches also, if one is dirty or not making good contact, the signal would be affected...
     
  3. mhardy6647

    mhardy6647 AK Subscriber Subscriber

    Messages:
    51,199
    Did you try strategically swapping tubes between the two?
    I mean, that's the easiest troubleshooting tool imaginable.
     
  4. Raul

    Raul Active Member

    Messages:
    284
    Location:
    Italy
    Yes, I contacted Eli and he replied: "Perhaps a poor match in the NFB gain set resistor pair accounts for the observed imbalance" but both resistors were replaced during the mod and they were tested before installing
    yes I swapped the two EF86 (not the EL84) and nothing changed
     
  5. 6DZ7

    6DZ7 Super Member

    Messages:
    1,414
    Seems to be a coupling cap missing off the EF86 output.
     
  6. Raul

    Raul Active Member

    Messages:
    284
    Location:
    Italy
    Can you point me to this cap in the schematcs? Sorry but I am not very knowledgeable about circuits
     
  7. 6DZ7

    6DZ7 Super Member

    Messages:
    1,414
    In the line between pin 6 of the EF86 and pin 2 of EL84. The coupling cap is missing.
     
  8. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    33,949
    Location:
    Southern NJ
    Looks like direct coupling from the EF86 plate to EL84 grid. Don't need a coupling cap if done that way, provided the cathode resistor is sized accordingly and there is enough B+ to make up for the cathode being offset. I'd also expect something tied to the heater circuit to elevate it for heater-cathode voltage limit reasons.

    treble control inside a feedback loop? No, just no.

    Is the feedback tapped from the correct spot on the transformer? If one was off the 16 ohm and one off the 8 ohm, you'd have different levels of feedback and it would make the output level change considerably. Also would be worth checking voltages between the two to make sure they are comparable.
     
    maxhifi likes this.
  9. 6DZ7

    6DZ7 Super Member

    Messages:
    1,414
    They must be running the EF86 on 30V. That 700R K resistor is interesting, wonder what the current is if they expect 3W output? 40ma there would be 28V.

    Agree, probably no cap. Need voltages , but guessing is fun, too.
     
    Last edited: Jan 5, 2018
  10. Raul

    Raul Active Member

    Messages:
    284
    Location:
    Italy
    Tone controls were removed from the circuit, it was part of the mod
     
  11. knockbill

    knockbill Addicted Member

    Messages:
    6,910
    Location:
    SE PA
    Yep,, I thought the mods included removing tone controls... still think best plan is matching the readings chassis to chassis,, one is working correctly, right?
     
  12. Raul

    Raul Active Member

    Messages:
    284
    Location:
    Italy
    yes one is working correctly, and rather loud for a 3W amp, to have about the same output when this one's volume is at 8 o'clock the other one must be up to 1 o'clock
     
  13. 6DZ7

    6DZ7 Super Member

    Messages:
    1,414
    It's possible that the one with less volume is actually the one working correctly. The one that is louder may have a feedback circuit problem and is not getting the feedback necessary to bring the gain down, so it sounds louder.
     
    trainbuftony likes this.
  14. mhardy6647

    mhardy6647 AK Subscriber Subscriber

    Messages:
    51,199
    Good point. I wondered about that myself -- if so, the "tone" of the louder one should be different (based on my experience with similar circumstances).
     
  15. Raul

    Raul Active Member

    Messages:
    284
    Location:
    Italy
    Actually the stronger one is brighter and it's already loud with volume pot a 9 o'clock (starting at 7 until 5), I have them connected to JR149 speakers, so maybe it's too loud for a 3w amplifier, but the other one must be close to the maximum volume to be acceptable, so maybe it's too low, Maybe both have some problem? I am thinking of opening them again to check (these small amps are a bit a pain in the a@@ to work with)
    And in this case how I can verify if there is a feedback problem?
     
  16. knockbill

    knockbill Addicted Member

    Messages:
    6,910
    Location:
    SE PA
    Disconnect the FB from both and see if they balance?
     
  17. Tom Bavis

    Tom Bavis Audiophool Subscriber

    Messages:
    8,950
    Location:
    Macedon NY
    The 10 meg resistor R3 is part of the original design and should stay as-is -- the EF86 is biased with "grid-leak bias" or "contact potential", which requires a very high grid return resistor. The alternative would be to add an additional cathode resistor for bias, requiring a bypass cap as well. But feedback could be a problem as well - check that the feedback resistor is connected (since you removed the bass control).
     
  18. Raul

    Raul Active Member

    Messages:
    284
    Location:
    Italy
    Thank you Tom, so I should put again the original R3 10M resistors instead of the 1M that replaced it, correct? Any other changes should be made according to you? Are the pot changed to 100k and C2 changed to 0.0068 correct? The goal of the mod was to remove the controls. Now I just see that I didn't put the 100 ohm resistor between pin 9 of EF86 and C2 that Eli added. The feedback resistor R6 is connected to the circuit through a jumper wire that, after removing the bass pot, connects the two green dots indicated by red arrows on the upper right corner of the schematics. I'm open to any advice.
     
  19. Tom Bavis

    Tom Bavis Audiophool Subscriber

    Messages:
    8,950
    Location:
    Macedon NY
    Yes, they should be 10 megs. Pot can be 100K, cap value doesn't matter much. The EF86 doesn't have that much transconductance, 100 Ohm grid stopper may not be necessary, though they can help with AM radio interference sometimes...

    If you still have problems, measure voltages at EF86 and EL84 and we may be able to deduce something...
     
  20. Raul

    Raul Active Member

    Messages:
    284
    Location:
    Italy
    changed the R3 resistor back to 10M but nothing changed, I had no time to check voltages today, will do on next week
     

Share This Page