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Got a breadboard put together (see link) but I am having trouble getting it to work properly. I am getting sound out the other end, and it sounds ok with low B+, but as I increase the b+ toward what it should be it starts breaking up and distorting.
I had to make a few changes due to not having the parts I needed as follows;
1.I used a 6dj8 driver as I do not have a 6n1p and it appeared from the text that either would work. It sounds better than the 12at7 I first used.
2. For my b+ for the driver I just used a clip lead from the b+ connection to the opt to the 4.3k (I used 3.9k) resistor.
3. I looked in the tube manual and it said for 6bq5 in pp the cathode resistor should be 130 ohms, so I used that instead of 400 ohms.
4 . I used 0.1uf for the two capacitors between the two stages instead of 0.46uf, as per the recommendation in the text of the schematic.
A couple of things I thought I should mention in case anyone is interested in trying this amp (it really is a nice little amp with a low parts count).
I had read on a post on another site that the cathode resistor should actually be 200 ohms instead of the 400 ohms shown on the schematic. I went to the tube manual and it says for a 6bq5 in typical PP class AB operation, the cathode resistor should be 130 ohms and the cathode current should be be between 62 and 72 mA. I was getting a cathode current of 33mA with the 400 ohm resistor, so I tried a 130 ohm resistor instead and got about 67 mA which puts me right in the grove. I also changed the coupling caps to 0.1 uf as pointed out by fender lover and as mentioned in the schem. It wasn't causing the distortion, but it did improve the overall sound. Last but not least, I decided to try a 6dj8 after following up on dak's post that it was a closer match to the 6n1p than the 12at7. Again, it was not the cause of the distortion, but I think it sounds much better than the 12at7 in this circuit.
So, if you are looking for a simple, easy to build PP amp with good sound and a beer budget, you might want to give this a whirl. I would have been able to breadboard one channel in about an hour if the stupid volume pot had not tripped me up. Again, thanks for all the help, it was a great learning experience.
look forward to your sonic impressions vinnie! which output transformers are you using?
It appears that sometimes the very best debugging tool is just blind luck. This afternoon, while trying some of the suggestions offered (thanks guys!) I just happened to notice that when I turned the volume down pretty low there was very little distortion. What the hey says I, and just for yucks used clip leds to wire a stand alone volume control I have to bypass the one on the breadboard. How 'bout that %&$# Mable, no more distortion, everything sounds great. Must be that the carbon film on the volume pot has gone bad or some such thing and was causing an erratic signal to be sent on ahead. The strange part is there was no scratchiness or anything like that to give an indication of trouble or I might have checked it sooner. Anyway, I am going to quit while I am ahead for once and wait until tomorrow or the next day to get back to doing an A/B test between the PP and SE amps to see what difference I might hear. I will report back later. Thanks for all the help folks! Your suggestions did turn up a few other things that needed attention and they helped me eliminate some possible causes.
I thought the 6n1p is more like a 6dj8 tube. Could it be in this circuit the 12at7 is causing the distortion?
Nope. See above. If I ever get one though, I would like to try it in place of the 12at7. Sounds like I should maybe try a 6dj8 too, as it is similar to the 6n1p. Thanks for the tip.
You should fist of alln check the plate and cathode voltages of the concertina phase splitter.
If the input bias is wrong the voltage between them will tend to close too much and the sound gets very distorted.
Since I have been known to make mistakes, is it possible that you have the pin layouts to the 12AT7 wrong since they are different from the 6N1P?
Perhaps a closeup photo might help.
Good idea to check them, which I just did against the tube manual, and they seem to be correct. I think I will just rewire it in the morning since it will only take an hour or so and see what happens. Will check them with fresh eyes in the morning though to be sure before I start.
Check the voltages and did not see any that were way off from what is on the schem, but they were bouncing around a bit instead of holding steady. I think I will have to assume I have something hooked up wrong and strip it down and redo the wiring unless someone has some other idea to check out. At least with the breadboard it will take less than an hour to redo the wiring. Love those breadboards!
I've not done breadboarding with tubes, but I did learn when constructing a pin-changing tube adapter that one can easily make tube-pin-numbering mistakes when viewing the tube socket from above rather than below. If you are viewing the tube sockets from above when wiring them on the breadboard, I can imagine the probability of making a mistake is non-trivial.
Been there myself, so I checked them with a continuity meter this morning and everything is as it should be.
Measure in sections: Get the power supply stable, then install and measure the the front-end/driver section, then plug in and measure the output tubes. It will help you nail down the problem.
Did like you suggested. PSU is ok, nice and steady by itself. Then I installed just the driver 12at7 tube and it's voltages are pretty much in the ballpark. I then plugged in the 6bq5's. I was getting around the 13 volts at the cathode resistor with the 400 ohm resistor in place that the schem calls for, for about 33 mA for cathode current if I am doing the calcs right. The tube manual says it should be about 65 mA, but would that be for a shared cathode resistor too? Still distorting a lot. The other voltages are in the ballpark too. I am at a loss as to what to try next. Maybe I should test some 6bq5's and !2at7's and make sure I have a good set of tubes? Maybe hook a test signal to it and follow it through on a scope and see if I can see where it starts distorting?
I think the power supply is ok because it is one from another amp, but I will check it. I will try isolating the other sections in the morning. Thanks for the tip.
It is distorting so bad I did not want to leave it on long enough to check the voltages yet. Figured something must be wrong to make all the noise, but maybe I will turn the volume way down and try to get some measurements before I see any blue smoke.
Are you using 0.47mfd coupling caps between the 12AT7 PI and the EL84 output tubes (like the schematic)? Maybe, a bit too much capacitance, for your setup?
Yes, but would that be enough to make it break up and distort?
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