| Buzz, then blown fuses - Click HERE for Original Thread |
| DrStrangelove |
Hi all,
This week I finished building my first guitar amp, and now I'm in the trouble-shooting phase.
It's a modified JCM800 100W head circuit design.
I added a gain stage and an effects loop, and modified the e.q.
The problem is:
When I first turn the amp on, the tubes heat normally and I get noiseless guitar tone. But as the tubes keep heating, a buzz appears. After about a minute of being on, the 750mA fuses on the secondaries to the rectifier blow.
I should mention, the rectifier is solid-state with two .1uF caps for filtering.
Another thing I noticed is that the 300-0-300V tranny I bought from Hammond actually produces 330-0-330V. Could this cause the fuses to pop?
Any thoughts? |
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| fdegrove |
Hi,
| quote: | | Another thing I noticed is that the 300-0-300V tranny I bought from Hammond actually produces 330-0-330V. Could this cause the fuses to pop? |
It surely could.
You could try some series resistance before the rectifier to limit inrush current and lower the HT supply a bit.
Cheers,;) |
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| DrStrangelove |
Thanks,
I'll give it a try.
What do you think about the buzz, and the fact that it takes almost a minute before the fuses pop?
Tim |
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| Duo |
Your tube might be going into runaway and causing great current to flow from the power supply. This can pull the capacitor down enough that you'll hear hum in the output. It can take a while for the fuse to blow. Are you sure the bias is set correctly? It seems like the tubes are warming up until they hit a sort of threshold point and turning on hard.
Put a current meter in the cathode circuit of the output stage and see what happens as the amp warms up. See if the current flow increases sharply when the buzing starts. If it does, this means the tubes are turning on somehow and drawing too much current... Check out your wiring.
I can see the 330-0-330 causing changing numbers and that but not really throwing the tubes into runaway unless the bias is maxed out or something. |
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| dhaen |
Hi,
750mA Are big fuses. Presumably they are that big to cope with a huge inrush current to the smoothing circuit. All the same, for the valves to take this much current (1.5 amps) means a bias problem, as Duo has intimated. Alternatively it could be rectifier breakdown. A link to the schematic would help the cause.
Cheers, |
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| DrStrangelove |
Sure,
I just posted it at
http://www.geocities.com/tduryee2001
I've managed to figure out why the fuses were popping.
The two 10uF caps on the way to the bias pots were in backwards (I had assumed that negative would go to ground).
I'm guessing this explains the runaway current you were talking about.
However, the buzz is still very audible.
Any ideas? |
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| dhaen |
Hi,
There are things on the schamatic that make no sense.
Are you sure it has the 1K's from the grids? I think it is a drawing mistake;)
Check that there is equal current through each output valve. You can do this by measuring the voltage across the 1 ohm resistors.
Cheers, |
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| DrStrangelove |
Whoa,
Good call. Wrong tubes even (my editor doesn't have EL34s).
The 1k resistors are right but the tubes were wrong.
Check it again (it's updated).
http://www.geocities.com/tduryee2001/
Thanks,
Tim |
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| DrStrangelove |
I forgot to mention:
Yes, I've measured the voltage drop across the 1 ohm resistors and balance them
~Tim |
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| dhaen |
Tim,
That makes sense now ;)
OK, so they're balanced.
Is the buzz affected by the volume control?
Can you measure the ripple on the following points:
B Output stage supply
C Screen grid (g2) supply
You can use a scope to measure, or a digital voltmeter set to the highest AC range.
Cheers, |
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| DrStrangelove |
Hi,
The Volume doesn't effect the hum/buzz.
I measure a clear 120 Hz at B and a clean flat-line at C (g2).
Would I be able to run the output on the other side of the choke?
Thanks for helping.
~Tim |
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| DrStrangelove |
Well, I gave it a try and the buzz is less audible at low levels, but now it's strongly affected by the Volume.
That doesn't make much sence to me... |
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| dhaen |
Tim,
You have demonstrated that the buz /hum is being produced in the output stage..
The output stage is Push-Pull, and if properly balanced, should be able to cancel out huge amounts of ripple. It can't be balanced. Please re-check the current through each output valve. Maybe it would be easier to run with just 2 output valves for the tests.
It is also quite possible that 1 or more output valves was damaged by the bias fault you had.
Cheers, |
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| DrStrangelove |
Today I pulled two tubes and balanced the current between the two remaining tubes and the buzz wasn't affected at all.
Are my 22uF Caps an appropriate size to filter 120Hz?
Tim |
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| dhaen |
You're right, 22u is not much, and I was going to address that later.
The point is, that a Push-Pull stage, when working properly should cancel out power supply hum.
In fact I have heard of an open circuit smoothing cap only being noticed when a valve failed!
In a previous post, I did ask you the ripple value. You do need to find this for us to make real progress ;)
I've just gone back and read your first post - it's easy to to forget where we started...
So this is a new build. It has not worked before. There are a couple of other possibilities to consider:
1 There is some mistake in the wiring, or wrong component.
2 Hum could be induced from the mains transformer or choke to the output transformer.
When you have checked your own work yourself, it is easy to miss the same mistake over and over again. It's often best to get another pair of eyes to the problem. Another way is to "fault-find" the mistake. But that takes more fault-finding experience.
Try unbolting the output transformer, and rotating it through 90 degrees to see if that helps.
Cheers, |
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| dhaen |
Oh, I forgot:
What is the bias voltage? |
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| DrStrangelove |
Hi John,
Please excuse my ignorance. I thought I was answering the ripple question in post 11. So, assuming I didn't, what is ripple current and how can I measure it?
Thanks,
Tim |
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| DrStrangelove |
Oh,
Bias voltage is 228mV at idle.
Tim |
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| dhaen |
| quote: | Originally posted by DrStrangelove
Oh,
Bias voltage is 228mV at idle.
Tim |
Tim,
I didn't make myself clear, and that's not exactly what I asked, but:
Is that the voltage measured across one of the 1 ohm resisitors?
Are you sure you've got 1 ohm, and not 10 ohm resistors?
If it is that voltage, and they are 1 ohm, it means there is too much current being taken by that valve. Your measurement says it's 228 mA. The EL34 max is 150mA.
You need to adjust the bias pot for each valve to give a MUCH lower reading.
If this is an instrument amplifier, I guess it should be running in class B. That means that the output valves should be hardly conducting at idle, but the top set conduct for 1 half cycle of the waveform, and the bottom set for the other.
I'll hopefully be corrected by someone more knowledgable of class B, but I would initially try a current of 20mA
Since the valves have been severely overrun, even for a short time, there is a high probability that they will perform oddly, and night need vrying bias settings.
Whatever you do, don't run them for any length of time at this current.
If you don't have the adjustment range, it could be that there is insufficient bias supply voltage at piont "A". You should measure whatever the zener is rated - 70v 100v -something of that order.
Also, is the zener in the right way, and is the 15k ok?
Cheers, |
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| DrStrangelove |
That 228mA was accross the 1 ohm resistor. However, those resistors seem to be poor quality and vary between about 1.5 and 2.3 ohms. Obviously, that means my voltage drop readings are inaccurate. That would explain why the tubes still sound crystal clear.
Reguardless, I dropped the bias back and balanced the tubes at 82mA, using 1.5 ohms as my divisor. I don't immagine the tubes will be perfectly balanced until I get better 1 ohm resistors.
I do know that this amp is supposed to be Class AB. I don't know what that means for the bias adjust, so right now I'm just using a value that makes the power amp sound good.
The zenar is rated 100v and is in correctly.
From ground to supply voltage at point "A", I read 59V.
The 15k resistor is good.
How can I measure ripple?
Tim |
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| dhaen |
Tim,
Replace the 1 ohm resisitors.
82 mA is still on the high side I think.
The voltage at point "A" should be 100V. Either the 15K, a cap or the zener is ********ed.
Class AB is midway between A and B. You'll have to look up the exact current requirements, or maybe it's specified for the design somewhere.
The ripple can be measured with a 'scope or with the AC range of a digital voltmeter.
How's the hum?
Cheers, |
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