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Thread: Help! valve bias problem

  1. #1
    Join Date: Oct 2017

    Location: Coventry

    Posts: 130
    I'm Mick.

    Default Help! valve bias problem

    Hi All.
    I treated myself to a second hand ARC D115 just before Christmas, and apart from one noisy pre valve all seemed well, I went through the motions of setting the bias and the DC balance with no problems.
    Im new to the world of Valve power amps so was pleased.

    I ordered 3 ECC88 tubes, when they arrived installed them. setting the DC balance again.

    When collected the amp I was told 17 of the valves had been replaced but I doubt this to be correct, The power valves 6550s do look new but who can tell.

    I have been enjoying the sound from this beast daily for the past 3 weeks.

    Today I decided to check the bias again and now instead of reading 65ma as it was set, its closer to 10 ma on all the valves.
    (Some reading 12 and some 8).
    If i wind the pots to full i can got some of the valves to read correctly but not all of them, (I run out of adjustment)
    Ive tried the old tubes back where they came from but it didn't effect the readings.

    I was hoping that someone out in AOS land had an idea what could have happened or be causing my problem.

    Mick

  2. #2
    Join Date: Oct 2009

    Location: Birmingham, UK

    Posts: 394
    I'm Phil.

    Default

    it sounds like you need to get the valves checked out , although you have the bias adjustment its best to have valves closely matched , generally if they start to drift a lot it usually means they are wearing out , I have an avo mk2 valve tester and can check them for you , I think you need to start
    there first and at least rule them out cheers phil.
    Phil.

  3. #3
    Join Date: Dec 2017

    Location: Ontario, Canada

    Posts: 104
    I'm Dan.

    Default

    Did you replace all the 6550's including the one that is all by itself? If that one has failed, your amp won't bias.
    One other thing. At the risk of insulting you, make sure your bias procedure is correct, and make sure your meter is working properly. I once had a customer go around in circles biasing an amp, only to discover his meter wasn't working correctly.

  4. #4
    Join Date: Oct 2017

    Location: Coventry

    Posts: 130
    I'm Mick.

    Default

    Hi Phil thanks for the offer of the test, I may have someone local that can test them for me next week. Not that i know much but seem strange that they would all go out of spec in 3 weeks.
    I think the cause will be common to all the valves Poss Supply related.
    I will PM you if i need to test them on your AVO.

    Mick

  5. #5
    Join Date: Oct 2017

    Location: Coventry

    Posts: 130
    I'm Mick.

    Default

    Hi Dan, I haven't changed the 6550s it was the 3 ECC88 signal valves that are new, You do raise a good point though, i could swap the single 6500 (PSU) for one of the octet and see if that effects the reading.
    That valve looks to be older than the power valves.
    I don't think its a meter problem but did consider it, another avenue to explore.
    Thanks for your suggestions.

    Mick

  6. #6
    Join Date: Dec 2017

    Location: Ontario, Canada

    Posts: 104
    I'm Dan.

    Default

    That single 6550 is part of a voltage regulator, and sets the voltage on the screens of the output tubes. If it is low, bias will be low.
    I misread, and thought all the outputs were new. You may just be in need of new tubes, but for all of them to be low would be unusual.

  7. #7
    Join Date: Oct 2017

    Location: Coventry

    Posts: 130
    I'm Mick.

    Default

    Further update,
    I swapped the regulator valve, no change.
    Did some further investigation, Phoned a friend who suggested testing the voltage supplied to the Bias pots, It should be 40volts but was reading 56volts (Transformer output voltage).
    So it seems I have a voltage regulation problem, and need to replace some components.
    One of them is D41k2 transistor, and the only ones for sale are in Canada....

    So if anyone has one of these spare Pls let me know.

    Or if any one has any comments regarding using an equivalent component, I would value your thoughts.

  8. #8
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

    Default

    It's a 50V 2A PNP Darlington transistor, I know I've got NPN darlingtons, I'll have a look later in my stock.


  9. #9
    Join Date: Dec 2017

    Location: Ontario, Canada

    Posts: 104
    I'm Dan.

    Default

    Before you go crazy looking for a replacement, do some testing of the three transistors that are there. The TIP42 may be the only on you need to change, and they are quite common.
    While not the best thing to use, an NTE269 will work and may be easier to get.

  10. #10
    Join Date: Oct 2017

    Location: Coventry

    Posts: 130
    I'm Mick.

    Default

    I haven't yet confirmed which component (s) have failed. And due to their location testing is difficult, I have a friend due to visit tomorrow who repairs PA amps for a living.
    He is bringing his meter with him and hopefully can identify the problem.
    We have both looked at the circuit and cant quite grasp exactly why ARC designed the circuit as they did. He said " Ive never seen such an over engineered way of controlling the Bias voltage"
    He also suspects there is current limiting involved.

    Alan thanks for looking

    Dan, thanks for the recommendations.

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