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DIY Tube Preamp causing Power Amp Protect Mode
DIY Tube Preamp causing Power Amp Protect Mode
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Old 21st August 2019, 06:14 PM   #11
joshvito is offline joshvito  United States
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Quote:
Originally Posted by indianajo View Post
You need to check for ultrasonic oscillation. I had 1 mhz @ 1vac in a mixer I sped up the op amps on, and it sounded okay, but the power amp hated it.
If you don't have a scope, an analog AC voltage multimeter 5000 ohms/volt with 2 vac & 20 vac scales will work. Block the negative probe with .047 600v to prevent the AC scale from reading on DC. If you have output with no sound, very suspicious. You can prove any AC voltage out is ultrasonic by changing the blocking cap from .047 uf to 390 pf. Audio won't pass through 390 pf, but ultrasonic oscillation will.
a 5 v protection diode across the output would prevent huge spikes from destroying your power amp. You also need 33 pf disk caps from center to ring on each input connector to prevent broadcast radio, police/fire/cb band, or cell phone RF from getting into your circuit. You do have a safety grounded metal box don't you? If not, start at the very beginning, a very good place to start. If you have exposed tubes outside the metal case, use ground clip tube sockets and put metal caps over them to prevent RF pickup. Hammond organ did on the H100. Not everybody had a radio transmitter (cell phone) in their pocket in 1930 but many did by 1965. alternately you can put a grounded mesh box over the top to let the hot air out and keep the RF out.
Quote:
Originally Posted by indianajo View Post
If you don't have a scope, an analog AC voltage multimeter 5000 ohms/volt with 2 vac & 20 vac scales will work. Block the negative probe with .047 600v to prevent the AC scale from reading on DC. If you have output with no sound, very suspicious. You can prove any AC voltage out is ultrasonic by changing the blocking cap from .047 uf to 390 pf. Audio won't pass through 390 pf, but ultrasonic oscillation will.
I just want to make sure I am understanding this correctly. Use an analog volt meter with the listed ranges, Attach the common lead of my multimeter to a .047uF 600v capacitor, which is connected to the ground of the RCA output. Connect the other lead of the multimeter to the output signal part of the RCA. This should read 0 AC volts. If not, there is a problem. Then replace the cap with 390pf and that will only pass the ultrasonic AC.

This test is performed with the preamp powered on with no input signal or output amp connected.

Quote:
Originally Posted by indianajo View Post
a 5 v protection diode across the output would prevent huge spikes from destroying your power amp.
So, just put a diode in line with the output signal for each channel? e.g. output from 6V6 => diode => RCA?
Or are you saying a diode from center to ring on each output connector ?

Quote:
Originally Posted by indianajo View Post
You also need 33 pf disk caps from center to ring on each input connector to prevent broadcast radio, police/fire/cb band, or cell phone RF from getting into your circuit.
Great idea! What voltage rating do I need? Probably nothing too large? Is there a favorite you have that doesn't alter sound too much?

Quote:
Originally Posted by indianajo View Post
You do have a safety grounded metal box don't you? If not, start at the very beginning, a very good place to start. If you have exposed tubes outside the metal case, use ground clip tube sockets and put metal caps over them to prevent RF pickup. Hammond organ did on the H100. Not everybody had a radio transmitter (cell phone) in their pocket in 1930 but many did by 1965. alternately you can put a grounded mesh box over the top to let the hot air out and keep the RF out.
I have a safety grounded top plate, with exposed tubes. The transformer has grounded covers and an additional transformer cover is attached to the top plate. All internal signal wires are shielded cable. The sides of the chassis are wood. The bottom is acrylic.

Sorry for all the extra questions. I do realize this is a lot, so thank you in advance.
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Old 21st August 2019, 06:25 PM   #12
joshvito is offline joshvito  United States
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Quote:
Originally Posted by mashaffer View Post
No. Keep it here you are getting good responses.

I agree with adding screen stoppers if either of those tubes oscillate you would have problems. It also looked to me like the grid stoppers on the 6V6s might be metal film. I think carbon comp would be a better choice for stoppers.
Thanks for the heads up on the grid stoppers. I am not sure where to add a screen stopper on a 6V6 in triode. Is it just another high value resistor? Where would I wire it?
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Old 21st August 2019, 07:43 PM   #13
mashaffer is offline mashaffer  United States
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On both the 6V6s and the shunt regulator you do just like the grid stoppers. A carbon comp resistor of 100 or 200 ohms soldered right to the screen grid tab on the socket. The othe end then soldered to the plate connetion. The resistor replaces your shorting jumper.
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Old 21st August 2019, 08:24 PM   #14
joshvito is offline joshvito  United States
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So, my grid stopper is wired between grid and input signal and is 300 ohm. Does adding another 100 ohm resistor between grid and screen cause me to change the grid stopper value? How does one choose a value for the screen stopper resistance? Is 1/4 watt resistor okay? Or higher?
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Old 21st August 2019, 09:33 PM   #15
mashaffer is offline mashaffer  United States
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No, the screen stopper goes between the screen and whatever it connects to, in this case the plate. Like this...
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Last edited by mashaffer; 21st August 2019 at 09:40 PM.
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Old 21st August 2019, 09:52 PM   #16
mashaffer is offline mashaffer  United States
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I think the confusion came from my use of the term "screen grid" . what we usually call the grid is more properly called the control grid. There are three grids is a pentode the control grid (aka grid), the screen grid (aka screen), and the suppressor grid (aka suppressor).
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Old 21st August 2019, 10:01 PM   #17
joshvito is offline joshvito  United States
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Thanks for this clarification, it is helpful. I am putting together a replacement parts list now. Really wish I had a local store to stop at, as I end up paying shipping so many times...such is life. Eventually, I will end up with a bunch of extra parts bin.

Now I have to dig into some reading to figure out what @indianajo meant about a 5v protection diode and what type of 33pf caps to use on the inputs?

Last edited by joshvito; 21st August 2019 at 10:21 PM.
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Old 21st August 2019, 10:34 PM   #18
indianajo is offline indianajo  United States
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Here is a bidirection TVS diode that starts clamping at 6.4 v https://www.newark.com/multicomp/1-5...nal/dp/91R0686

It's $.55 there is a 5 v one but it is $3.55 Tubes could produce 250 v bangs if something came loose & reconnected. 33 pf input RF filter caps can be 50 v rated or higher. The ceramic disks I'm using were in a Radio Shack grab bag from the 70's still have a few dozen. Cheap X7R are suitable for this use. Reputable ceramic disk vendors are Vishay, Aerovox, Kemet.

The AC meter oscillation test can be done both with a signal and with the input shorted. Music will show up as .1 to 2 vac through a .047 uf filter cap. I find rock music useful for tracing signals through faulty circuits, you can see the beats of volume in the pointer of the meter. Oscillation will show up as a steady voltage through the 390 pf cap, which won't pass any music 20-20000 hz. Found some bad solder joints by chasing music through an amp with the simpson 266xlpm VOM on 2 vac (in the front part) or 20 vac (in the back part).

If you're not getting RF oscillation or interference, don't worry about covering with metal. I get an AM radio station, also a CB radio that drives by emitting dogs barking "dixie" on my amps without a faraday (grounded metal) cage. Took a 68 pf across the input to get rid of the AM radio station. Sports talk radio, how vile!
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Old 21st August 2019, 10:46 PM   #19
joshvito is offline joshvito  United States
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THank you! this is a great clarification.

PS. So, how do I wire the diode into a circuit?

Last edited by joshvito; 21st August 2019 at 10:47 PM. Reason: added a question
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Old 21st August 2019, 10:53 PM   #20
john20851 is offline john20851  United Kingdom
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Hi,
looking at your diagram, are you really supplying the el84s with 340v?
It's a long time since I used these, but that sounds a little high.
regards john
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