I purchased a Dynakit Mark IV a about a month and half ago,put it together,the sound was marvelous,about two or three weeks ago my nephew was listening to some of his music,most of it being tracks with heavy bass material,I told him not to play it very loud,but since He was using His iPod to play from,He on accident turn the volume all the way up so He says,and the amp after that point was distorting a huge amount. I powered down,tested the output tubes,they tested OK,put them back in powered up,and all I was getting was LF oscillation,even with the iPod unplugged,I'm stumped. Any help appreciated.
Time to make a DC voltage map. Get out the schematic, *carefully* measure the voltages with respect to ground at the plates, grids, and cathodes of the input/driver tube. If the tube used in contemporary Mk IV is a pentode/triode (like the 7199 in the original), get the screen voltage as well.
By "put it together", do you mean that this was a kit of new components you just built -- or is this an original Mark IV you purchased already built? Is it stock -- or used modified components? Restored -- or otherwise still has original components installed? This information is all very helpful in conveying condition, when trying to diagnose by post. So with nothing more than a speaker connected (no input) and the unit turned on, it will make the noise, correct? If the amplifier will go silent without the driver tube installed, then the problem is most surely associated with the driver board. Certainly try another driver tube if possible. Then take careful voltage readings on that board as Sy instructed. Problems that happen all at once like this can be as much component related as connection related. You might go over all of those on or associated with the board as well to eliminate that possibility.
Dave
Dave
Measure the DCR of the primary windings to the center tap and repeat on the good amp and compare. You need to verify that the OPT does not have a shorted winding.
Is there any chance your nephew disconnected the speaker while the amp was on loud? This could damage the OPT, although why it would then provoke motorboating is a mystery.
OK, wild guess coming up: the damaged OPT is no longer able to provide proper NFB so the circuit now has more internal gain, so more sensitive to feedback via the supply rail. The big snag with this wild guess is that if true, it would predict motorboating whenever the NFB is disconnected - which is unlikely.
OK, wild guess coming up: the damaged OPT is no longer able to provide proper NFB so the circuit now has more internal gain, so more sensitive to feedback via the supply rail. The big snag with this wild guess is that if true, it would predict motorboating whenever the NFB is disconnected - which is unlikely.
Again, it would be helpful to know what circuit we are dealing with here -- whether it is a bone stock Mark IV, or modified with a different driver board, bias scheme, power supply, etc. In any event, since it seems that the damage was related to a blast of uncontrolled high power, I would carefully check any cathode resistors used for current flow measurements in the output stage to make sure one of them (if individual resistors are used -- which is not consistent with the original design) has not opened up, or risen very high in value. Another component to check (not consistent with the original design either) would be any screen stability resistors added to the output stage. If any of these opened up, it would surely produce very distorted sound. A good indicator that the output stage is not drawing its proper current will be B+ voltage readings that are too high. Like DF96, I'm not sure if any of these resistors opening up would cause motor boating too, but I do know that stranger things have happened!
Dave
Dave
You might have hit on the problem. An output stage cathode resistor going high would turn P-P into SE. This means that output currents no longer cancel at the OPT centre tap so the supply caps have to work much harder, even though they have been sized for P-P. More LF will get through on the supply rail. A fault in cathode decouplers could have a similar, but smaller, effect.
I measured all four taps,on one side it measured 34ohms and 140ohms,the other taps measured 24ohms,and 110ohms approximately.
Need to measure the known good transformer, Dyna transformers and close copies are noted for having rather different DCR from side to side relative to the center tap, but I no longer remember the acceptable range. It could be this one is OK or not, comparing it to the other one should tell you..
If this is a real MKIV clone then there should be a single resistor of roughly 13.5 ohms shared between the cathodes. No cathode bypass cap, and fixed bias. When biased ~1.56V across the resistor.. The dynakit mk4 is a very close copy of the original including using the original 7199 based design - a socket adapter for the 6GH8A is provided with the kit..
I'm using a board that was designed for a direct use of a 6GH8A tube,I have only built one channel,I will go ahead and purchase another output transformer,and quad cap,as I will need these components anyway to build the other channel,and maybe even replace the driver board with a new one,I have four of them,any one of these component replacements should narrow down where the issue lies. You don't know how much I appreciate all the suggestions I've recieved,I'm having to re-learn everything,as it has been years since I have gotten back into all of this from my college days.
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