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:
Here's the schematic w/ voltage readings:
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.
If you have 120k, then the output stage is virtually turned off.
Same again with your screen resistors. Should be 470 ohms, not 470k.
That's if I am reading your diagram correctly
That's if I am reading your diagram correctly
Compare yours with this readable diagram here:
http://www.mtcom.jp/~schematic/orange-amps/tiny-terror.pdf
http://www.mtcom.jp/~schematic/orange-amps/tiny-terror.pdf
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.
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
~Tom
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.
An externally hosted image should be here but it was not working when we last tested it.
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.
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
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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
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
You missed a .22uF capacitor right on the input. If there is any DC voltage coming down your guitar lead it could queer the input tube into compressing the sound.
Tone control not working could be miswiring (you did take a wire from one side and a wire from the center wiper terminals of the pot?)
Tone control not working could be miswiring (you did take a wire from one side and a wire from the center wiper terminals of the pot?)
Unless there's something I don't know (which is likely as I don't work with guitar amps), that .047uF capacitor from the V2b grid to ground looks wrong. It will kill one half cycle of the drive to the output stage.
In fact, I don't know how V2b will work as it is. I can't see where it gets its grid signal from, but again it agrees with the other schematic.
I would have thought it would have been set up as a phase splitter, not a basic gain stage like V2a.
Someone more experienced may want to chime in here
In fact, I don't know how V2b will work as it is. I can't see where it gets its grid signal from, but again it agrees with the other schematic.
I would have thought it would have been set up as a phase splitter, not a basic gain stage like V2a.
Someone more experienced may want to chime in here
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Ignore my last post. I looked at it some more, and it looks like a grounded-grid stage (something, being a tube newbie, I haven't had much experience with) so it may be correct.
Alright, I got it fixed. There was a wire hooked up where it wasn't supposed to be and I couldn't see it (underneath the board). Now I got full signal and controls working fine, HOWEVER....now I have this horrendous ground hum that's almost as loud as the guitar. I've tried moving around lead dress and using more shielded connections, but nothing has helped. What does help (and maybe this will help diagnose the source) is when I short pin 2 of V1 to ground, about half the hum disappears. When I ground out pin 6 of V1 or anywhere after that, it goes away completely. I swapped the tube and no change. Any thoughts?
Here's a shot of the guts to show the lead dress (I know, it's a mess).
Here's a shot of the guts to show the lead dress (I know, it's a mess).
An externally hosted image should be here but it was not working when we last tested it.
Try shorting to ground the junction of the 68k resistor and .001uF capacitor that come off V1a. If that kills the hum, them your problem is grounding on the input somewhere.
If not, then likely something around your remote switch jack. Try unbolting it from the chassis and see if that helps. It could be a ground loop between main ground and chassis via the jack, in which case, wire the jack without connecting ground to it (ground only one end of the shielded cable), or use an insulated jack.
If not, then likely something around your remote switch jack. Try unbolting it from the chassis and see if that helps. It could be a ground loop between main ground and chassis via the jack, in which case, wire the jack without connecting ground to it (ground only one end of the shielded cable), or use an insulated jack.
I've reworked the schematic a bit, now more like the original TT, but with a bypass switch on the second gain pot for a gain boost. Now I get good volume and gain, but there's pulsing and hum when turned way up. Also, a couple of voltage readings on the phase inverter are really off, but I can't find what's causing it. Here's what I read:
Pin 1: 223v
Pin 2: 2.49v
Pin 3 + 8: 36.2v
Pin 4,5,9: 6.3v
Pin 6: 147v
Pin 7: 13.8v
I've swapped several tubes, same readings. I've changed the socket and measured every component attached to the socket out of circuit and values are all fine. I replaced them anyway and nothing changes. All voltage readings everywhere else in the circuit are good. What's the issue I'm missing here?
Pin 1: 223v
Pin 2: 2.49v
Pin 3 + 8: 36.2v
Pin 4,5,9: 6.3v
Pin 6: 147v
Pin 7: 13.8v
I've swapped several tubes, same readings. I've changed the socket and measured every component attached to the socket out of circuit and values are all fine. I replaced them anyway and nothing changes. All voltage readings everywhere else in the circuit are good. What's the issue I'm missing here?
An externally hosted image should be here but it was not working when we last tested it.
From looking at the photo you posted, I see what looks like a lot of ground points under various bolts. You really need to bring all your grounding wires to one bolt-down point, preferably at or near your AC line ground.
If there are several grounds, you will get a lot of ground-loop hum.
If there are several grounds, you will get a lot of ground-loop hum.
And parts not obviously soldered. May not be your hum, but will be a problem. See the white cement resistor in the lower left of the eyelet, err, card board. Also along the bus wire on the top.
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Disregard the photo at this point. I've torn all of that out and rewired everything point-to-point. My issue now is the bad voltage readings in the following post. Any help with that issue would be appreciated.
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