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In Reply to: Re: need to see a schematic posted by groverg on December 25, 2004 at 18:43:56:
Dang, I thought for sure I had it! No such luck. It went a little longer this time, but the same thing happened. When it goes on the old speaker it makes a sound like air going out of a ballon when you let go of the end after blowing it up. More of a squeak than a screech. If you guys could please take a look at the post where I describe the differences between my amp and Grover's schematic I would appreciate it. I simply must find out what is causing this.
Well, okay, here we go :-). First, reapply the bypass cap to the 6N7 cathode since this seemed to stabilize it more. Next, remove the preamp from the circuit, short the input of the amp with a shorting plug, turn it on and see if it oscillates. If not, then there's either an excess of gain or an RF or ground loop thing with the preamp that's causing a problem. If you still get oscillation, try fixing the bias supply as Jim suggested--use a 6.3 volt tranny, hook the secondary to the 6.3 volt filaments, and use the 120V primary for the bias supply. Take one 120V primary and attach a single reversed diode, which goes to the filter cap, and ground the other primary. This makes a half-wave supply and it may add some noise but it should be stable.
Let me know what happens when you short the input.
Also, describe for me how you're powering the 845 filaments. 10VAC with a center tap? Is the supply grounded properly? How are you measuring the current? Your bias should be around -100 volts. If you're bias voltage is excessive (-170) the 845 may be going into cutoff.
I put the bypass cap back in and I took out the preamp and put in the shorting plugs. It made the noise again. It seems to take about 3-5 minutes for it to get to that point. I will check and see if I have a 6.3 volt tranny in one of my junk boxes. I measured the bias as -130 vdc by putting the positive probe of my DVM on the grid of the 845 and the negative probe on one of the filaments of the 845. Is that the right way to measure it? Is that too high a bias voltage? My B+ is running about 785vdc right now because I am only using one 845. The filaments on the 845 are 10vac with a center tap to ground through a 1k/25w resistor. I am not sure what you mean by is the supply grounded properly. I am using a variac to bring everything up. It is plugged into a multiple outlet which is plugged into a grounded wall outlet. I am using a ground buss on top of my bread board chassis that I hook the ground from the variac to. The other two wires go to the primary terminals on the power tranny. All ground connections from the bread board components go to the ground buss on the bread board chassis. I have used this same set up for bread boarding many other amps and have had no trouble with it, there is always a first time for everything.
Aaaaaahhhh... You're using a 1000 ohm/25W resistor between the filament CT and ground, along with -130 volts on the grid? If so, in effect you're "double-biasing" the 845. You're running the tube way down into cut-off, like Class C region :-). Remove the 1K resistor and ground the 10VAC center tap. Set the grid voltage to about -110. That should put you at between 60-80mA. Good enough for testing purposes. You really want 10 ohms/2 watts from CT to ground, and measure .8 volts to ground for 80mA.
Well, it went about twice as long this time, so I guess we are making progress :) . I took the resistor out and took the CT to ground and I adjusted the bias so that it read -110 between the grid and the filament of the 845. I ran it for about 7 minutes with the preamp out and everything seemed ok so I decided to put the preamp back in. It ran for about 5 minutes with the preamp in before making the noise. Guess I had better go back and let it run 20 minutes or more with the preamp out and see if it goes bonkers then. The thing I don't understand is why it doesn't start making the noise right away. Why does it run awhile first?
I'll report back after running it without the preamp for awhile longer.
Took the 6n7 bypass cap out. Gain was definetly down, but it did the same thing after about 5 or 6 minutes. A bit different this time though; it just made a sort of breaking contact type of noise and quite. I turned it off for about two minutes, cranked it back up and it started playing again. This all a plot to drive me crazy!!!
I think I will go watch television for a while.
I just ran the amp for 20 minutes with the preamp out and the shorting plugs in. It never made a peep. I am going to go back and try taking out the bypass cap on the 6n7 and put the preamp back in and see what happens. Oh, one other thing; it got real quiet in the house for a short while and I noticed that the potted power tranny has a pretty good hum to it when full mains is applied. Is that anything to worry about? I have never heard a potted power tranny hum before.
If the oscillation is subtle, it can take a while for it to ramp itself up and set the amp to singing. Have you tried a grid-stopper on the input pins of the 6N7? If not, solder a 1K carbon resistor right up against the adjacent grid pins and attach your input lead to the other end.
What size output cap are you using in the preamp?
I believe the output cap is 1.0 mfd/400vdc. The specs on the power tranny are 1206vct/0.35A.
I was just reading through your suggestions and realized that I use relay sockets on my breab board. Is there any way I can hook up the grid stoppers on those and have it still work?
I really appreaciate all the help Grover!
Good morning Grover. I have today and Friday off due to the holidays, so I am taking vacation leave for the rest of the week to try and get some remodeling done around the house.
I tried the grid stopper just a short while ago. I put a 1k resistor to pin 4 of the 6n7 and attached the input wire to the other end. I kept the end on the relay short by doubling over the lead and sliping it under the clamp. Pins 4 & 5 are tied together with a short lead. They don't need separate resistors do they? I left the bypass cap for the cathode resistor of the 6n7 out, but the preamp was in.
Well, it went about 12 minutes this time before it went pop and stopped playing. Does this mean we are making progress? What do you think I should try next?
What was your take on the output cap of the preamp and the specs of the PS tranny?
The power tranny and the output cap would appear to be fine. The power tranny may be buzzing because you're powering it off the variac. We'll look into that more later.
Here's what I would do next:
Restore the bypass cap to the 6N7. Make sure the 6N7 is wired properly--you have 4 & 5 tied for the grids, and you should have 3 & 6 tied for the plates, and 8 for the cathode resistor.
Disconnect your preamp from the amp, turn it on and make sure you don't have any measurable DC from the output to ground. A leaky output cap could cause the problem you're having.
Is there any way you can place a volume pot at the input of the amp (anything from 25-200K would suffice) and test the amp with your CD player or source directly inputted? That would tell us a lot.
Just so happens my bread board is semi permanent and has volume pots built in for each channel. I will first check the preamp for a leaky cap and then I will put the bypass cap back in and run the cd player directly into the amp. I just checked the wiring on the 6n7 and it is correct. I will get back to you as soon as I do these things.
Most interesting. It only lasted 5 minutes this time with the CD player going straight into the amp. I did have the volume pot up a bit higher than usual, but not that much. So it seems we only have a problem when we have a signal, yes? Of course I may not have heard it when it went off with no preamp hooked in and the shorting plugs in place because there was no music to stop playing. This is quite a puzzle my friend. Next move?
Vinnie, why don't you call me at 301-699-8227 and I'll call you back on my dime--don't worry it doesn't cost me anything.
To any who have been following this thread to see how it ends, Grover was kind enough to march me through my whole layout step by step on the phone until we found the problem. It was the bloomin' EL 34! Grover says that most likely it shorted out when the B+ voltage hit the plate before the filaments had a chance to warm things up. It would run for awhile then shut down. Once it cooled down you could start it up again and run it for awhile longer. It about drove us nuts until Grover had me put the DVM on the plate of the EL34 and watch it while it was running. When it quit the voltage on the plate spiked up, indicating that it wasn't drawing current anymore. At his suggestion I have now installed a separate 6.3 supply for the filaments that will be turned on first.
Bottom line is this is one great sounding amp! Once I got it running I just kept trying one CD after another and they all sounded great. I can't wait to hear it in stereo. Now will come the longer process of deciding just how to turn this into a real amp instead of a bread board. Lots of work and fun ahead. Many thanks to Grover for both the design and helping me get it operational. You're a good man Grover!
Thanks for the suggestions Grover. I think it's best if I take a break for tonight and start again tomorrow. I'm tuckered. I will try the these tomorrow and get back to you with the results.
Is that the proper way to measure it? I tried that just now and it measures the same as from grid to filament +/- a volt or two.
Yes, now that you have the filaments grounded! Before you had -130 on the grid, plus probably +70 volts on the cathode, effectively given you a grid bias voltage of -200 or more (in relation to the cathode/filaments). Always measure your negative grid voltage from grid to ground, not the cathode.
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