Amp project output problem

bubbab74 · 19th January 2010, 08:02 PM

I'm building a little "Orange TT type" push-pull amp and I'm having an output problem. With the amp on, all voltages I measure are acceptable (maybe not exactly ideal, but workable). Gain, Master and Tone controls all work as they should, but the overall volume level is extremely low. I can only tell the controls are working because I can hear them working when it's all the way up and I'm right next to the speaker. Thought it might be the OT, so I wired in a known working one and got the same result - no change.

Here's the schematic w/ voltage readings:

rotaspec · 19th January 2010, 08:47 PM

Pretty hard to read. Can you get a better pic of the output stage, because it looks like you have 120k as your cathode resistor. 120 ohms would be the correct figure.
If you have 120k, then the output stage is virtually turned off.

rotaspec · 19th January 2010, 08:52 PM

Same again with your screen resistors. Should be 470 ohms, not 470k.
That's if I am reading your diagram correctly

rotaspec · 19th January 2010, 09:04 PM

Compare yours with this readable diagram here:
http://www.mtcom.jp/~schematic/orang...iny-terror.pdf

bubbab74 · 19th January 2010, 09:12 PM

You are correct on the values being "k" ohms. That's interesting, because I was going off the schemo that came with a friend's Holy Terror mod kit and it shows them both as being 120k and 470k. It also showed the plate load resistors on V1 as 100R instead of 100k. I'll swap them out and report back. Thanks guys.

tomchr · 19th January 2010, 09:41 PM

29 volts across 120 kOhm corresponds to a cathode current of about 240 uA. I'd expect the current to be a few orders of magnitude greater than this. And now that we're at it... About 67 V across 470 kOhm is 142 uA. So 100 uA of anode current in the output stage. That would explain the low volume... Obviously your output current would depend on the turns ratio of the OPT, but I'm guessing it's somewhere around 5-ish mA. So a theoretical max output current of 5 mA peak into 8 ohms is 100 uW...

~Tom

bubbab74 · 19th January 2010, 11:05 PM

Ok, so I changed out those values and I got much more signal, but it's still not right. Here's my new voltage readings:

Sorry if it's hard to read, the forum seems to be shrinking things.

Voltages at the power supply points are:
A: 247v
B: 246v
C: 221v
D: 198v

My problem now is that with the gain and master all the way up, it sounds good, but volume is still about 1/3 of what it should be and the harder I play, the more it compresses (like a compressor with the threshold set too low or even a brick wall limiter). Also, the tone control does nothing and there's a bad hum that goes away instantly when I either a) turn the power off or b) touch my multimeter leads (-) to chassis and (+) to pin 2 of V1. The buzz also "swells" when the amp is first turned on. Starts off bad, gets quieter, then comes back again, all in about 3-4 seconds.

wakibaki · 19th January 2010, 11:26 PM

It really would help if we could see the diagram. If the forum is shrinking it, perhaps it's huge? ...it certainly takes forever to open.

w

bubbab74 · 19th January 2010, 11:47 PM

Ok, reuploaded pic. Much better.

rotaspec · 19th January 2010, 11:50 PM

Couple of quick things, but they won't be the cause of the existing problem:
2M2 to ground from the output stage grids should be 220k (make sure that 1k5 resistor is as close to the grid pin as you can get in case the problem is oscillation). You have 2x 0.047uF in series with grid of V2a, should only be one of them.
Bias looks wrong on V2, but the resistor values tally with the other circuit (I would have thought the 47k would have been MUCH lower value).
I'll look further when I can

Gary

19th January 2010, 08:02 PM	#1
bubbab74 diyAudio Member Join Date: Nov 2009 Location: Mesa, AZ	Amp project output problem I'm building a little "Orange TT type" push-pull amp and I'm having an output problem. With the amp on, all voltages I measure are acceptable (maybe not exactly ideal, but workable). Gain, Master and Tone controls all work as they should, but the overall volume level is extremely low. I can only tell the controls are working because I can hear them working when it's all the way up and I'm right next to the speaker. Thought it might be the OT, so I wired in a known working one and got the same result - no change. Here's the schematic w/ voltage readings:

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19th January 2010, 08:47 PM	#2
rotaspec diyAudio Member Join Date: Nov 2007	Pretty hard to read. Can you get a better pic of the output stage, because it looks like you have 120k as your cathode resistor. 120 ohms would be the correct figure. If you have 120k, then the output stage is virtually turned off.

19th January 2010, 08:52 PM	#3
rotaspec diyAudio Member Join Date: Nov 2007	Same again with your screen resistors. Should be 470 ohms, not 470k. That's if I am reading your diagram correctly

19th January 2010, 09:04 PM	#4
rotaspec diyAudio Member Join Date: Nov 2007	Compare yours with this readable diagram here: http://www.mtcom.jp/~schematic/orang...iny-terror.pdf

19th January 2010, 09:12 PM	#5
bubbab74 diyAudio Member Join Date: Nov 2009 Location: Mesa, AZ	You are correct on the values being "k" ohms. That's interesting, because I was going off the schemo that came with a friend's Holy Terror mod kit and it shows them both as being 120k and 470k. It also showed the plate load resistors on V1 as 100R instead of 100k. I'll swap them out and report back. Thanks guys.

19th January 2010, 09:41 PM	#6
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	29 volts across 120 kOhm corresponds to a cathode current of about 240 uA. I'd expect the current to be a few orders of magnitude greater than this. And now that we're at it... About 67 V across 470 kOhm is 142 uA. So 100 uA of anode current in the output stage. That would explain the low volume... Obviously your output current would depend on the turns ratio of the OPT, but I'm guessing it's somewhere around 5-ish mA. So a theoretical max output current of 5 mA peak into 8 ohms is 100 uW... ~Tom

19th January 2010, 11:05 PM	#7
bubbab74 diyAudio Member Join Date: Nov 2009 Location: Mesa, AZ	Ok, so I changed out those values and I got much more signal, but it's still not right. Here's my new voltage readings: Sorry if it's hard to read, the forum seems to be shrinking things. Voltages at the power supply points are: A: 247v B: 246v C: 221v D: 198v My problem now is that with the gain and master all the way up, it sounds good, but volume is still about 1/3 of what it should be and the harder I play, the more it compresses (like a compressor with the threshold set too low or even a brick wall limiter). Also, the tone control does nothing and there's a bad hum that goes away instantly when I either a) turn the power off or b) touch my multimeter leads (-) to chassis and (+) to pin 2 of V1. The buzz also "swells" when the amp is first turned on. Starts off bad, gets quieter, then comes back again, all in about 3-4 seconds.

19th January 2010, 11:26 PM	#8
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	It really would help if we could see the diagram. If the forum is shrinking it, perhaps it's huge? ...it certainly takes forever to open. w

19th January 2010, 11:47 PM	#9
bubbab74 diyAudio Member Join Date: Nov 2009 Location: Mesa, AZ	Ok, reuploaded pic. Much better.

19th January 2010, 11:50 PM	#10
rotaspec diyAudio Member Join Date: Nov 2007	Couple of quick things, but they won't be the cause of the existing problem: 2M2 to ground from the output stage grids should be 220k (make sure that 1k5 resistor is as close to the grid pin as you can get in case the problem is oscillation). You have 2x 0.047uF in series with grid of V2a, should only be one of them. Bias looks wrong on V2, but the resistor values tally with the other circuit (I would have thought the 47k would have been MUCH lower value). I'll look further when I can Gary

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