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    wharf-creek

    Posts: 22
    Join date: 2008-12-18
    Age: 62
    Location: Baltimore, Maryland

    Permission to ask a question

    Post by wharf-creek on Thu Dec 18, 2008 12:04 pm

    Greetings to Bob, Kevin, Roy, and other Dynaco Buffs!! First, thanks for creating this forum!! I'm sure I'll be visiting it often with my Dynaco questions, and just for the education. I'd like to ask my first question, and it concerns "bias". In all honesty, I have to confess that I'm attempting to repair an old Heathkit W7-A, and not a Dynaco. I have a pair of these amps and I'm having a problem with a symptom of fluctuating bias on BOTH of them. They're both doing that exact same thing, that being that there is some kind of cycling in the bias current. The amps load up over about a 15 second interval. You can apply a bias probe on either output tube, and starting at the bottom end, the tube will measure about 20 ma. But, over about 15 to 20 seconds, the bias will creep up to nearly 100 ma or more, then there will be a slight 'pop' in the speaker, a visible spike in the tube itself, and then the bias will drop back to 20 again and the process starts over. I should mention that these amps 'sound' fine other than the pop. No major hum and they reproduce a signal from my signal generator that sounds like any other 'normal' amp. I've replaced the caps in the bias circuit, and changed tubes, and the problem persists. I don't have specs for this amp, and the only resource material I was able to find was a copy of the Schematic from the Vintage-Radio web site. I could use some help here, but I also completely understand that I'm on a Dynaco site with a Heathkit issue. So, if you pull this post, I'll understand. But, if you can throw it out there, perhaps the answer can be a source of further education to all. Either way, I've got to figure this out......and I can use some help!! So, with that, I thank you and I wish you all a safe and pleasant Holiday season!! Good luck, and thanks! Tom D.

    Bob Latino
    Admin

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    Tom - You don't need "permission" to ask a question - This is a friendly place...

    Post by Bob Latino on Thu Dec 18, 2008 6:19 pm

    Tom - just to check one thing - when you are checking the bias do you have your speakers connected? SOME (but not all) amps will give a cycling bias voltage as you describe if you have no "load" (the speakers) on the output transformers. Some amps need that load on the output transformers to stabilize operation...

    Bob

    wharf-creek

    Posts: 22
    Join date: 2008-12-18
    Age: 62
    Location: Baltimore, Maryland

    Re: Permission to ask a question

    Post by wharf-creek on Thu Dec 18, 2008 7:19 pm

    Hi Bob, and thanks for the reply. I didn't want to go into too much detail, as I wasn't sure if you'd approve my using your forum for some generic inquiry. But, I did hope you'd see fit to let me enter the query in the interest of advancing some general knowledge. Yes, I did have a speaker load hooked up, and I had my input grounded. I might also mention that the one voltage I could verify from the schematic was the 40 VAC from the Power transformer to the bias Diode. I've got that! There is also a B+ spec but my printer won't give me a good enough resolution to actually see that number. One thing I've been thinking about is the surgistor that is in this circuit. I've actually looked at this amp when tilted on it's side with the lights off.....so I could watch the tubes as this bias was cycling up and down. I noted that the surgistor achieved a red-hot glow on it's filament, but it didn't appear that the actual bi-metal strip was making contact when fully heated. I thought about the fact that with the bottom off and being tilted up on the side, perhaps the flow of air was keeping the surgistor cool....thereby preventing it from operating properly. But, even with this being so, I was still getting the same cycling.....so...probably NOT the causal component. (I was thinking 'spikes' in the power transformer from the surgistor opening and closing.) Anyway, I've ordered some Current Inrush Limiters from Mouser, some CL90s and CL80s, and I'm going to replace the surgistors with those units just for safety sake. I'm going to ponder the amp some more this evening.....just sit and stare at it I guess!! Maybe it will fix itself....lol. If I come up with any more info, I'll post it. Again, thanks for taking the time to respond, and for letting me use this forum to get some help. Take care Bob....be well! Tom

    tubes4hifi
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    Re: Permission to ask a question

    Post by tubes4hifi on Sat Jan 03, 2009 8:15 pm

    Hi Tom,
    sorry for the delayed response, I hope you get this message before you start bumping your head into the tube cage!
    I haven't worked on W7Ms but have worked on several W6Ms which are kind of similar, Heath made some great amps!!
    The surgister is in the primary of the power transformer, so it would affect the entire amp, not just the bias circuit. You are correct that it should be replaced with probably a CL80 or CL90 (the difference is the current), if in doubt replace the rectifier diode (any 1n4000 series should be fine), and most definitely replace C17, 18, 19. I can't remember if that is part of a triple or quad cap, but replace for sure even with individual caps of similar ratings. Your R30 pot sets the overall bias and then R38 sets the balance between the two output tubes. I would also replace C9 and C10 with modern capacitors of similar value, they don't have to be identical with the originals.
    Other than that, check the octal sockets for a solid positive connection, especially on pin 5 where the bias enters the tube.
    Now - you do NOT want to check the bias ON pin 5, the test points are connected to pin 8 (the cathode) of the output tubes, which measure a voltage across R17 and R18, it appears those are 6 ohms, and not sure what the recommended voltage is, but probably around 0.30vdc from each test point to ground, which would be 50ma per tube. Personally I would adjust the bias so that is more like 0.24vdc across 6 volts (40ma). You could also replace those old 6 ohm resistors with new 10 ohm 1 or 2 watt metal oxide resistors, and then set the bias to read 0.40 volts on each tube, then adjust your R38 balance pot to read 0.00vdc with a probe in each test point.
    You probably already knew most of this, but if not this should just about cover everything you need to know about bias for this amp.
    Roy www.tubes4hifi.com

    wharf-creek

    Posts: 22
    Join date: 2008-12-18
    Age: 62
    Location: Baltimore, Maryland

    Re: Permission to ask a question

    Post by wharf-creek on Sun Jan 04, 2009 6:54 pm

    Roy,

    Thank you so much for the reply. I've sent you a note via your web site 'contact' link.....so as to not fill up too much of this space. But I still have a bit of confusion over the Schematic. If you prefer I post here, I'll gladly repeat the question. I found in trying to repair a problem with my old boat that some of these Forum participants are very particular about keeping ALL info on a shared basis. Yet, others prefer to take it to private communications. Either way, I try to abide by the rules. So, let me know if my email is received, and if I should re-post here....or if we can resolve this off the forum. And....thanks again for your help Roy!! Regards, Tom D.

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