Desperately seeking solution for Mark III power problems

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Desperately seeking solution for Mark III power problems

Postby frankraw » Sat Jan 01, 2011 3:33 pm

Hi, I am new to Dynaco restoration and am seeking a solution or solutions to my problems blowing fuses in both of my two Mark III's. I am sending pics of amp # 2 which held the fuse until I started trying to set the bias,at which time the 3-amp slo-blo let go. I first want to make sure the wiring is correct. I followed Ty Bowers pics for the wiring hookups and am using Ned's version with the 12BH7 and 5751 tubes. All help will be greatly appreciated as I have a considerable amount of time and cash into this project and I'm anxious to hear it sing,like it should. I will post pictures of the other monoblock,which displayed a similar but somewhat different problem in a followup to this post,later. Thanks to all!

Frank

[/img (???) ]http://s1188.photobucket.com/albums/z419/frankraw/
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Postby TomMcNally » Sat Jan 01, 2011 4:31 pm

Hi Frank -

Welcome to diytube ...

Let me ask a few questions ...
do the amps play music ?
what do you hear in the speakers?
what kind of bias reading are you getting
before the fuse blows?
are the tubes glowing red?
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Postby Ty_Bower » Sat Jan 01, 2011 4:45 pm

Does it still blow fuses if you pull out the 5AR4 rectifier tube? If it passes that test, does it blow fuses if you leave the 5AR4 installed but pull all the other tubes? These two question can help determine if you've got a short somewhere in the power supply section.

You should also check the available range of bias voltage by pulling all the tubes, and measuring the voltage between pin 5 (or pin 6) of the KT88 socket and ground. Pins 5 and 6 are the ones that have the 1K ohm resistor between them. Turn the big bias adjustment pot (the one on the chassis) while you take the reading. You should get roughly -70 VDC at one end of the pot, and -40 VDC at the other end.

Do you have a dummy resistor load connected to the output transformer for your power on testing? Or are you using a pair of cheap speakers?
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Reply to Tom and Ty's posts

Postby frankraw » Sat Jan 01, 2011 9:42 pm

Hi Tom and Ty,

I'll try and answer both of your questions in this single post. The first amp I tried was the one with the bulk of images in the photobucket link. Yes, it played music,but with an abnormal amount of crackling and buzzing in the background at all levels. I don't recall what the bias settings were at the time,but changing it did not seem to help the crackling go away. I did not think that the tubes were glowing anymore than they should have,but there was after a time some arc flashing in the rectifier tube and at least one of the output tubes(all brand new,by the way),and I shut it down. That amplifier was assembled by a friend of mine who is no stranger to schematics and soldering irons,but I always questioned how he assembled the bias supply circuit,even though it looked ok according to the drawing in the poseidon manual. However,after I shut that amp down,I decided to wire up the second amp using as much information as I could get from Ty's photobucket images along with the manual. You will see noticeable differences in th bias supply scheme. I had the outputs hooked up to a speaker and the input connected to an external soundcard from my laptop as the source. I hope this info will help you guys help me out of my dilemma. The second amp did not reproduce music and the fuse let go as I was trying to adjust the bias up. Now I do not have the preamp socket on my chassis,as I placed an IEC socket there instead,so on one of the amps I have flying leads coming off of pin 1 of each output tube,until I can mount some bias point test jacks. At any rate I was bringing up the bias on amp 2 slowly and all was going well but the rectifier tube flashed up near the higher end of the bias pot range and the fuse let go. I have not tried either amp since then for fear of ruining something more expensive if I am wired incorrectly. Thanks for your help!

Frank
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Re: Reply to Tom and Ty's posts

Postby Ty_Bower » Sat Jan 01, 2011 10:04 pm

frankraw wrote:I always questioned how he assembled the bias supply circuit,even though it looked ok according to the drawing in the poseidon manual.


I'd imagine the bias circuit looks "funny" to a lot of people. I think it is because the positive end of the capacitors get tied to ground. Usually you would expect to see the negative end towards ground, but since the bias supply is a negative voltage everything is backwards. Also pay particular attention to the direction of the solid state rectifier diode. Again, the banded end goes opposite of the way you would normally expect.

This drawing is a clip from the schematic in the assembly manual. If you have any doubts about the construction of the bias circuit, check it over again. If you don't have a good negative bias supply, you will have big problems of the kind you've described. This drawing has some typical voltages shown on it, as well as an additional 100K ohm "safety" resistor. The safety resistor isn't in the assembly manual, but it's a good idea to add it. It protects the tubes from going into full conduction if the wiper of the pot accidentally lifts.

Image
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby TomMcNally » Sun Jan 02, 2011 12:51 am

Frank -

What type and brand of rectfier tubes were you using?

If they were Chinese or JJ ... flashing and blowing fuses
if par for the course. I've had the best results with new
Sovtek 5AR4's.

... tom
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Postby Ty_Bower » Sun Jan 02, 2011 7:52 am

TomMcNally wrote:What type and brand of rectfier tubes were you using?


JJ does have a poor track record with their rectifier tubes. However, the Mark III doesn't run the rectifier anywhere near as hard as the Stereo 70. Frank also reports arc'ing in the KT88 tubes too. Could that result from a faulty rectifier?

I will agree that "brand new tube" does not always equal "perfectly good tube". There are an unfortunate number of duds fresh out of the box, both old production and new. If Frank truly does have a problem caused by bad tubes, he should be able to find it by swapping tubes around. But first, I'd want to be awfully sure the amps are built correctly.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby dcgillespie » Sun Jan 02, 2011 2:58 pm

If one of the KT88s flashed, it's cathode sampling resistors should be checked to make sure it was not damaged during that event. Also, that particular output tube should be monitored to see if any permanent damage was done to it as a result of the arc.

Invariably, output tube arcing is always a screen related event, with arcing being the worst form of screen instability. From the photos provided, it does not appear that any screen stability resistors have been added to the output stage to provide for their stable operation. If the value of the power supply caps have been increased over that of the original Dynaco design, then their inclusion is manditory to prevent output tube arcing.

Simply install a 100 ohm 1/2 watt resistor in series with each OPT screen tap lead that connects to pin #4 of each output tube. This eliminates the conditions favorable to producing an arc, by ensuring an adequate amount of impedance in the screen supply to prevent it.

Dave
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Postby Ty_Bower » Sun Jan 02, 2011 3:26 pm

dcgillespie wrote:Invariably, output tube arcing is always a screen related event, with arcing being the worst form of screen instability. From the photos provided, it does not appear that any screen stability resistors have been added to the output stage to provide for their stable operation.


Thanks for the reminder, Dave. I did put those screen resistors into my build after reading your article. I've never had any problems out of my Mark III.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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