Hi Tonydec
What was the outcome of your checks? Were the KT88s damaged by the faulty 12AT7 tube?
Thanks |
Thanks. I had the tubes tested, and the one that was in the number 2 spot tested poorly, which I think corresponds with the number 2 KT88. I did plan on replacing it because I figured it severly shortened it's life. I'm going to have them all tested just for curiosity to see the differences in numbers.
Tony |
Hey Tony, I have the same amp and have had the very same problem you have. The 4 12AT7's provide the bias to each KT88 output tube thereby if one 12AT7 goes bad, then the corresponding KT88 will glow cherry red. The Mac's are built rugged and there is usually no need to investigate further but to replace the bad 12AT7 and get rid of it or at least mark it so it won't be mistakenly reused. I would also replace the KT88 as the Genalex KT88 that went cherry red several months ago on me finally gave up the ghost in a bright display of arcing a few days ago upon power up which blew the mains fuse. Replaced the fuse and the KT88 and all is well.
Cheers |
I agree with you about that line, which is another reason why I haven't bothered with the ST70 yet. No rush, I love the sound of my Aragon 2004, and only want to get a tube amp running well enough to see what all the fuss is about. |
Well you do have to draw the line somewhere. When I do a mod on someones preamp or amp I take a long hard look at what was the intent was of the designer of the circuit. In the case of Dynaco, there were always cost considerations. This was the stereo for people who couldn't afford McIntosh or Marantz, or wanted to build it themselves. Heath kits were also popular. Obviously with the ST-70 or MKIII I would never replace the transformer unless it was damaged. BTW on the MKIII's I had heard that the older gray painted transformers were better then the newer blacks ones but I was unable to prove if that was true or not. There are other sites with other circuit board replacements. I would check them all out. I have absolutely no problem with an all triode tube input and driver section. If it's something like a 12AU7 coupled to a 12BH7 that works just fine. |
Curcio Audio Engineering has too many (well 3) options to go into on this thread. Basically, it's stick with the 7199's if you are restoring, an all triode model fully direct coupled, or all triode CCDA. Non of which makes any sense to me at this juncture in time. I assumed the other versions came about because the 7199's went out of production and became scarce and drove up the price ( I know an adapter is available). Some threads I've read prefer that ol' Dynaco sound, others, well you get the picture. I planned on researching and learning this craft a bit more before I began the project (I'm more of a solid state guy right now).
If it's not to much trouble, could you explain the differences mentioned above or do you know of a site with that info? I've been torn between restoring or rebuilding, but if it's a MKIII rather than the original 70, I'd probably lean more towards a rebuild using state of the art components for a more controlled sound.
Thanks |
That's a good question. I haven't really looked into them. Find out what your options are and let me know. Dynakit's were available for most Dynaco products including the FM3 tuner. If you use more tubes on a new mother board, depending on the tubes used to replace the 7199/6AN8, the filaments of those tubes will draw more heater current. Hopefully it's close to what the 7199 drew. Search the internet for circuit board updates. I believe Joe Curcio makes one. He designed some of the circuits for Sonic Frontiers. |
The Amp is stuffed away in the box so I'm really not sure what version it is, but if I'm reading into your message correctly, the DynaKIT is based on the DynaCO MKIII. Is that correct? I never realized there was a difference until now. The 7199's say Dynaco Made in USA. Also, thanks for the advice regarding overheating the of power transformer, that's the first time I've ever heard that.
What driver board would you recommend? |
Heyraz. May I suggest you replace the board in it and there by replacing the 7199 as well. Both the 7199 and the 6AN8 used in the MKIII are not very reliable. That tube is a medium mu triode/sharp cut off pentode. Although a good idea at the time of the design, it's hardly enough tube for the job. In fact after about 6 months or a years worth of wear on the tube and one half of the output waveform will prematurely clip. You will never get full output and distortion products will go up from the premature clipping. Also, hum is a distinct possibility and the culprit is that 7199/6AN8! Since the amp was designed around the RCA version of that tube, it's the only one that seems to work the best and those are very hard to come by. There are a number of people making upgrade circuit boards and power supplies. I would however keep the GZ-34/5AR4 tube and not replace that with a solid state device. Also, be careful with the circuit board upgrades as you can run the risk of overheating the power transformer attempting to power too many tubes. |
Hifigeek1-
Thanks. My ST-70 was kit built by someone else with no pride in his workmanship. I mean unbelievably messy in there. I plan to pretty much rip it back to the transformers and replace everything neatly. The only decisions I haven't made yet are whether to use the 7199's (they're in great shape) on a new drive board, or to replace the drive board altogether with a mod board. I also haven't decided if I want to use a cap board or replace the can. That project is on the back burner for a little while. Thanks for the advice. |
Heyraz...Check out the coupling caps in the ST-70. They do go bad. Also if it's really old and the bias supply uses the selenium rectifier, replace it. Those rectifiers leak voltage and the bias supply suffers. Caps in all power supplies as well.. |
Check it out as I described. I'm interested to see if that caused your tube to run away. Someone recently gave me a dilapidated Dynaco ST70 that worked, though not well. The glass on the power tubes were separated from their bases but still functioned as the vacuum was maintained. Basically the tubes were held on by their wires. I replaced the power tubes and as soon as I powered the unit up, one tube glowed bright red and it's cathode biasing resistor blew out, actually splitting in half, thankfully protecting the output transformer. I haven't had the time to figure out what happened yet, whether it was a short, an open, a lousy tube socket or whatever. In fact, I plan on simply overhauling the amp. The power tubes checked out ok and are in another vintage amp. Personally, I suspect a bad socket connection for the affected tube. Reading your situation reminded me of mine. Good luck |
Got it, one of those I have. |
DMM stands for Digital Multi Meter. Before the display's on the meters went digital, they were called VOMs (volt ohm meters).
They are relatively inexpensive starting around $12.95 at Sears on sale. They measure volage, current, resistance and continuity.
What I was getting at by suggesting you test yout suspect tube with a DMM would be similar to testing a light bulb, the filament is either connected (some resistance), open (no continuity) or shorted ( no resistance). Other uses include setting an amp's bias and testing batteries. |
Hifigeek, looking at it from the side, back row is 4 KT88's.
Front row is 2 12AX, 2 12AT, 1 12AX, 2 12AT. I remember reading the first 12AX is only needed when run in stereo and not mono blocked. So I am assuming 2nd 12AX and 2 12 AT for first 2 KT's, 3rd 12AX and 3rd and 4th 12AT for last 2 KT's. But that's just a guess. It is the 2nd KT that gets hotter, which makes me think it's one of the first 2 12AT, probably the 2nd if my guess is correct.
Heyrazz, what is a DMM? Eyeballing the tubes, it appears everything looks connected.
Continued thanks,
Tony |
If you have a DMM, you might find a short or open on your suspect tube by comparing it to a known good tube. Look through the tube, see where the wires are going and check those pins for opens or shorts. I learned that trick from a friend last year. It won't tell you the tube's quality, but it will identify an open or short. You can use this method while you're looking for a tube tester. |
Tony what's the last tube in the chain before the KT-88s? |
Version V, newest version.
Heyraz, you are absolutly right. I am considering purchasing a tube tester.
HiFigeek, the weird thing is, only one tube runs away. I don't have the wiring schematic, but I'm guessing the first 2 AT's run the first 2 KT's, and the next 2 AT's run the next 2 KT's, with the 12AX's leading? That is what makes me think I have it narrowed down to 2 of the 4 AT's.
Tony |
Well whatever the problem with that tube is it's causing that KT-88 to run away. Usually, if the DC voltage operating points are upset, that will upset the balance of the amp and over drive the output. Or for some reason the bias on that KT-88 is being affected and your loosing negative bias voltage on that tube. I believe in the vintage 275 the 12BH7's are the driver tubes. Is this a new 275 or a vintage 275? |
I'd pick up a decent tube tester from ebay. The price you'll pay will be about the price of ONE good tube, so it will pay for itself as soon as it catches the first bad tube. Plus, it helps take the guesswork out of the equation. I'm sorry, I don't have any answers for you.
I had a bad tube or socket that lit up a power tube and split a resistor when I rolled tubes. That's when I decided I needed more test equipment to understand what was going on. In my opinion, the only downside to owning a tube tester is how big they are. But I own every other kind of test equipment, so I figured if I was going to mess with tubes, I needed to tool up. The plus side of owning your own tube tester is how many new friends you will make. It's like owning a truck or a station wagon! Good luck. |
The power tubes were not moved. Only the 12AT7's. I swapped them twice, both times the Tele's ran the power tube hot, but just one. I replaced them in the same spots, so I think I have it boiled down to 2 of the 4. I was thinking about swapping them to the other channel, but don't want to press my luck. As far as I can tell, I didn't damage my speaker, and the power tube seems to be running normal with the old 12AT's back in. I will replace the power tube however, as going really hot like that probably shortened it's life compared to the others?
Thanks for the ideas, I will be getting the Tele's tested. Any other ideas? This is the first time I have encountered any issues when rolling.
Tony |
Was the power tube's socket "stretched out" and not making good contact with all of the tube's pins? That's the first thing I would have checked. You should rule out the simple stuff first. |
No I did not. I will be getting them tested. I swapped them out, seems to work fine. Put them back in, same thing, one power tube started getting very hot. It's obviously something in the 12AT's. I think I have it down to a couple of them, as I placed them back in the same order with the same results, but I'll get them all tested.
I'm not very well versed in tubes, but that was my guess, a short. It made music with no weird noises, just ran that one tube very hot. Good thing I was watching closely as they were new to me tubes, and it appears I shut it down before any damage, other than maybe shortening the life on that one tube?
Tony |
Did you test the 12at7's for shorts before inserting them into your system? |
