Express build 1st startup, B+ voltages too high?

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shipwreck
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Joined: Mon Mar 20, 2017 2:13 pm

Re: Express build 1st startup, B+ voltages too high?

Post by shipwreck »

Disconnected the NFB wire from the 8 ohm jack, and for the first time I have sound! Still a fairly high level of hiss, but before I do more troubleshooting or re-connect the presence control, I'm going to play the amp as it is.
I made a soundclip, what's the easiest way of posting it here?

And what DMM setting should I use for measuring bias through my probe jacks at the back (with 1R bias sense resistor connected to pin 8/1 of the power tubes)? Voltage, resistance, or current?
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romberg
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Re: Express build 1st startup, B+ voltages too high?

Post by romberg »

shipwreck wrote: Sun Jul 02, 2017 10:22 am Disconnected the NFB wire from the 8 ohm jack, and for the first time I have sound!
Nice! This means that (probably) the only trouble you have now is that the feedback is positive instead of negative. So, try swapping the OT wires that connect to the power tubes. Or you can swap the output of the PI (wires from C13 and C14). Either way you will change the phase of the feedback. It needs to be negative or the amp will run away. It is also possible the negative feedback wire was running too close to something else and causing feedback where it was not suppose to. But I'd try flipping the phase first. In either case, not too hard to fix.
Still a fairly high level of hiss,
There is a lot of gain in this circuit. So, it is not going to be hiss free. When the volume is turned up there is going to be some hiss. Super excessive amounts are another thing and may be worth tracking down.
but before I do more troubleshooting or re-connect the presence control, I'm going to play the amp as it is.
I made a soundclip, what's the easiest way of posting it here?

And what DMM setting should I use for measuring bias through my probe jacks at the back (with 1R bias sense resistor connected to pin 8/1 of the power tubes)? Voltage, resistance, or current?
If you have 1 ohm resistors between your power tube cathodes and ground (I put them on all my amps) then you can measure the voltage across the resistor. You will be measuring a DC voltage from the cathode of the power tube to ground. Basically both sides of the 1 ohm resistor. The voltage will/should be in millivolts.

The voltage can be used to find the amount of current flowing in the tube via ohms law (V = I * R. In this case the math is trivial since R is one. So, you have the current in milliamps. You calculate the amount of power the tube is dissipating with power = I * V.

So, measure the voltage on the plates and with the current you got from the 1ohm resistor you have the power the tube is dissipating at idle. The tubes should be biased based on a percentage of their maximum plate dissipation. The rule of thumb for push pull power amps is to bias them around 70% of the max plate dissipation. For an el34 this is 25watts. So, 70 percent of that is 17.5 watts.

Mike

EDIT: Forgot to mention that you will be measuring a DC voltage on the 1 ohm resistor.
shipwreck
Posts: 40
Joined: Mon Mar 20, 2017 2:13 pm

Re: Express build 1st startup, B+ voltages too high?

Post by shipwreck »

Thanks again, Romberg, I would like to buy you a beer for all your help! Thanks to everyone else who has chimed in as well.

I played the amp for about an hour yesterday. Was not really able to put it through its paces, because even with the attenuator at max, this amp is LOUD! It does seem to react nicely to changes in pick attack, although I was not able to get "Glen Kuykendall dynamics" where it goes from clean to screaming blues just by a flick of the wrist.

I'll bias the tubes and also reconnect the NFB later, and I also plan to post some glory shots and a soundclip or two. Just need to find a program/platform for sharing soundfiles.
shipwreck
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Joined: Mon Mar 20, 2017 2:13 pm

Re: Express build 1st startup, B+ voltages too high?

Post by shipwreck »

Measured the bias today, the voltage drop across the bias resistors (which I measured to 1.2 ohm) was 55 mV. So the bias current is 55 * 1.2 = 66mA. Probably too hot, right?

I then attempted to measure the plate voltage (pin 8) with one lead of the multimeter touching the pin, and the other clipped to the chassis. But this created first a spark, and then arcing across the tube socket. I turned off the amp after a few seconds, and it seems there was only a little melted isolation. I am attaching a picture.

Questions:

1. What bias current should I aim for?

2. Can I safely use the amp after this incident, or do I need to replace the tube socket?

3. How to measure platevoltage safely?
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romberg
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Re: Express build 1st startup, B+ voltages too high?

Post by romberg »

Yikes! I never try and measure plate voltage straight at the tube. It is way too easy to touch something else. Instead find some point in the amp that is also at the plate voltage and is easier to clip your meter to. And it is always a good idea to power down and unplug the amp, connect the meter and then plug it back in and turn it on when measuring these high voltages. Treat every amp like it has high voltage (which can kill you) until you have proven it to yourself otherwise.

Chances are that everything is still ok. It can be a spooky experience seeing a 400+V spark for the first time (It was for me). But it would be a good idea to start it up again with your light bulb current limiter just in case.

For the bias. You notice that C15 and C16 as well as the standby switch are also at B+1 which is what is the plate voltage of of your power tubes (after going through the OT). The OT does have some resistance. But it is pretty small in the grand scheme of things so the amount the voltage will drop I don't worry about. I'd measure B+1 at one of the previously mentioned spots and call it good enough. It looks like it will be around 400V.

Assuming it is 400V then the math would be:

Bias Power at idle = 25watts * 0.7 = 17.5w
Bias current to get 17.5w at 400V) = 17.5w / 400V = 0.0437A = 44mA

Since you measure mV across a 1 ohm resistor the mV can be directly converted to mA. So, your target is around 44mV on your meter on the 1ohm resistors.

Right now you are running them hot. 55mA * 400V = 22watts. Which is 88% of their max.
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