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Old 12th May 2010, 12:52 AM   #1
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Default Am I going to blow up my 7189s

I bought a 20Watt Knight amplifier at a garage sale a few years ago. When the capacitors died, I decided to rework it a little bit for musical instrument use. I've attached my final schematic, wondering if I'm going to blow up the 7189s by running them too hard. I'm keeping the original power transformer, but going to silicon diode rectifiers instead of the tube rectifier used before, and doubling my supply capacitance.

The power transformer output is rated at 320-0-320, so with the silicon diodes, I'm expecting 450V at the plates. Is this going to damage them, or will they just break up a little? I'm also counting on a little bit of fuzz from transformer saturation.

I apologize for the exceedingly light lines in the PDF - That's just the way GSchem did it years ago.
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Old 12th May 2010, 01:48 AM   #2
kevinkr is offline kevinkr  United States
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Schematic perfectly readable in linux version of adobe.

Now to the meat of the matter, your B+ is going to be way too high for the 7189 in this application and the only viable replacement - Russian 6P14P-EV is not going to live long at these voltages either. These are still only 12W tubes (pd) and at >400V are really 8W tubes.. I would keep the B+ below 400V in a guitar amp application, and I would suggest a tube rectifier with relatively little capacitance. I don't think you want a really stiff supply for a guitar amplifier, and I think you will also discover quickly that hifi opts don't saturate the way you probably want them to.

Good US made 7189 actually are worth some money as well..
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Old 12th May 2010, 02:13 AM   #3
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The original amp ran B+ at 400, and sounded good running my brother's guitar through it. This will be for an electric bass. Should I run B+ off the bottom of Q1 (~375V) instead? I'm pretty sure it'll handle the current.
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Old 12th May 2010, 03:50 AM   #4
kevinkr is offline kevinkr  United States
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I just realized you have an electronic switch in the power supply that removes the plate voltage without removing the screen voltage - the first time you do this would be the last time for that set of 7189 - so in addition to fixing the B+ issue you need to make sure the screens aren't powered when the plates aren't.

Also that FQN1N60 may blow up the instant it is switched to operate if the gate to source voltage rating is exceeded - if there is an internal zener in this part (I haven't checked) the bias will get pulled down to the internal zener voltage. In fact you don't need this mosfet at all - take another look..

There should be a gate stopper resistor right at each mosfet gate to prevent oscillation.

No selenium rectifier for the bias supply. Use a silicon diode.

Put 1M resistor and 0.01uF/1KV ceramic disk across each 1N5408 to make sure noise is suppressed during transition to reverse bias and PIV ratings are not exceeded.

I would not power the output stage from Q1 as this point is shared with all of the low level circuitry as well.

Regulating the screens probably is not a bad idea as you probably don't want a lot of distortion in the bass amp..

In terms of limiting the voltage to something more reasonable a 5AR4 would be a good choice and would still offer good regulation under load or you could figure out a simple voltage regulator for the plate supply as well. OR you could use a mosfet buffered voltage divider.

Take a look at your bias adjustment pots, there is no current path in the grid circuit, the bias voltage cannot be adjusted as the pots are incorrectly connected as rheostats - I assume the intention is to make the bias adjustable. You need at minimum a couple of additional resistors. Note that you should configure the pots so that if a wiper goes open your output tube(s) do not get destroyed.

A grid stopper resistor on the input tubes is a great idea particularly since this signal is coming in on a long instrument cable which may not be that well shielded.

I also think this amplifier is going to be too small for duty as anything but a practice bass amp, unless you have phenomenally efficient bass speakers. As a learning experience however I think it is a good idea.

Don't get too discouraged, this is a workable design with a little more effort.

Does this bass have active electronics in it?
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Last edited by kevinkr; 12th May 2010 at 04:14 AM.
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Old 12th May 2010, 05:54 AM   #5
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Quote:
Originally Posted by kevinkr View Post
I just realized you have an electronic switch in the power supply that removes the plate voltage without removing the screen voltage - the first time you do this would be the last time for that set of 7189 - so in addition to fixing the B+ issue you need to make sure the screens aren't powered when the plates aren't.
Good to know - Thanks. This is the first thing I've ever done with tubes, apart from repairing old radios by replacing identical parts

Quote:
Originally Posted by kevinkr View Post
Also that FQN1N60 may blow up the instant it is switched to operate if the gate to source voltage rating is exceeded - if there is an internal zener in this part (I haven't checked) the bias will get pulled down to the internal zener voltage. In fact you don't need this mosfet at all - take another look..
The reason for this mosfet is that the switch is only rated to 120vac, so I don't trust it with 450vdc, even if there's practically no current there. Are you afraid of it blowing up during the split second the switch is open? I guess I could put a large resistor on gate-source, and that should fix it.

Quote:
Originally Posted by kevinkr View Post
There should be a gate stopper resistor right at each mosfet gate to prevent oscillation.
Granted, I'm a microelectronics engineer, but I haven't seen this issue before. Can you either elaborate or point me to a source of info on this topic?

Quote:
Originally Posted by kevinkr View Post
No selenium rectifier for the bias supply. Use a silicon diode.
The bias supply is identical to what was in the original amp. I just figured I'd leave it.

Quote:
Originally Posted by kevinkr View Post
Put 1M resistor and 0.01uF/1KV ceramic disk across each 1N5408 to make sure noise is suppressed during transition to reverse bias and PIV ratings are not exceeded.
The 10M resistors are there for PIV equalization. Should they be reduced? I'll get some ceramics for noise suppression.

Quote:
Originally Posted by kevinkr View Post
I would not power the output stage from Q1 as this point is shared with all of the low level circuitry as well.
In the original circuit, the output stage was powered directly from the main cap (40uF, 400V) driven by the tube rectifier. What Q1 is now powering was powered by a resistor straight off the main cap and a 10uF cap of its own. The rest of the supplies were powered by the same resistors I have, only with 10uF caps instead of 22uF.

Quote:
Originally Posted by kevinkr View Post
Regulating the screens probably is not a bad idea as you probably don't want a lot of distortion in the bass amp..

In terms of limiting the voltage to something more reasonable a 5AR4 would be a good choice and would still offer good regulation under load or you could figure out a simple voltage regulator for the plate supply as well. OR you could use a mosfet buffered voltage divider.
I'll probably get a second mosfet to make a small series regulator for the output stage. I'll just add another 24V on top of the 380V reference for the driver-stage for it.

Quote:
Originally Posted by kevinkr View Post
Take a look at your bias adjustment pots, there is no current path in the grid circuit, the bias voltage cannot be adjusted as the pots are incorrectly connected as rheostats - I assume the intention is to make the bias adjustable. You need at minimum a couple of additional resistors. Note that you should configure the pots so that if a wiper goes open your output tube(s) do not get destroyed.
That's actually supposed to be a volume control. As the resistance goes down, the impedance seen by the 6AV6 goes down, reducing the gate drive to the final stage.

Quote:
Originally Posted by kevinkr View Post
A grid stopper resistor on the input tubes is a great idea particularly since this signal is coming in on a long instrument cable which may not be that well shielded.
Um... What? Are you referring to that first tapped potentiometer? The purpose of that is to act as a volume control and input location selector, so you can use the first half of the 12AX7 or not.

Quote:
Originally Posted by kevinkr View Post
I also think this amplifier is going to be too small for duty as anything but a practice bass amp, unless you have phenomenally efficient bass speakers.
See Is a Class D amp appropriate if you have an external feedback loop?. Unfortunately, I got a bit thread-jacked, though the other topic is sort-of on topic with my question.

Quote:
Originally Posted by kevinkr View Post
As a learning experience however I think it is a good idea.

Don't get too discouraged, this is a workable design with a little more effort.
Thank you very much. I can't wait to see what I can do with this.

Quote:
Originally Posted by kevinkr View Post
Does this bass have active electronics in it?
No. The bass I'm building it for is a passive Gibson Grabber with humbuckers (so very low-level drive).

Last edited by Circuitsoft; 12th May 2010 at 05:59 AM.
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Old 12th May 2010, 08:13 AM   #6
Arnulf is offline Arnulf  Europe
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Grid Resistors

Ditto for gate stopper.
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Old 12th May 2010, 11:55 AM   #7
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One very important thing to include now are screen stopper resistors. About 220 ohms for each grid should be fine. When you regulated the screen supply, the impedance of the circuit providing power to the screens was lowered significantly, making the screen susceptible to transient arcing -- particularly now that the amplifier is seeing band use. The screen stoppers protect against this.

Protect the screens, operate them within limits, and keep the plate dissipation safely within limits as well. Then, the actual plate B+ takes on much less importance. 7189 tubes can safely operate with 500 volts on the plate, and with long tube life as well, as long as the dissipation ratings are respected, and the screens are taken care of.

Dave
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Old 12th May 2010, 12:25 PM   #8
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Not saying its wrong, but why the 1K bypassed by 3.3uF?

If already standing atop 68K unbypassed, is this small DC
drop meaningful? Also the 48Hz corner is maybe too close
to your low E or D. If that drop IS meaningful, the phase
shift around -3db corner frequency might matter too?

I think you can ditch both those extra parts to no effect
whatsoever. If you really need a drop to bias your splitter,
an LED is another option without -3db phase shift.

Last edited by kenpeter; 12th May 2010 at 12:30 PM.
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Old 12th May 2010, 02:50 PM   #9
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"Not saying its wrong, but why the 1K bypassed by 3.3uF?"

The voltage drop IS meaningful, as it creates the bias for the tube, since the grid resistor is returned to the bottom of the 1K resistor in question. The output to the bottom output tube coupling cap should also be taken from the bottom of the 1K resistor as well to maintain a balanced drive to the output stage.

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Old 12th May 2010, 04:48 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Circuitsoft View Post
Good to know - Thanks. This is the first thing I've ever done with tubes, apart from repairing old radios by replacing identical parts

The reason for this mosfet is that the switch is only rated to 120vac, so I don't trust it with 450vdc, even if there's practically no current there. Are you afraid of it blowing up during the split second the switch is open? I guess I could put a large resistor on gate-source, and that should fix it.
Actually my concern is with when you apply a negative supply to the source which exceeds the gate to source voltage rating which is usually less than 20V. You may or may not get away with this.

Quote:
Granted, I'm a microelectronics engineer, but I haven't seen this issue before. Can you either elaborate or point me to a source of info on this topic?
These mosfets are operating in the linear region, not as saturated switches so their transconductance in conjunction with external circuit constants like wiring inductance and stray capacitance + gate capacitance can (and usually will) conspire to create an oscillator. (electrostatic coupling and the phase shift introduced by external wiring.) A small resistor greatly reduces the q of the undesirable tuned circuit that results and also introduces an HF pole that reduces the gain above the break point. Google is your friend, I unfortunately don't have the time to explain in greater detail.

Quote:
The bias supply is identical to what was in the original amp. I just figured I'd leave it.
Perhaps a mark of inexperience, but selenium rectifiers have a defined service life which has by now been exceeded. Good practice dictates replacement whether or not it works. A bunch of bad things can happen:
  • The smoke released when they fail is very toxic
  • The output tubes get destroyed through loss of bias which results excessive plate and screen current - you can't count on the fuse blowing in time to save them
  • The power transformer gets fried in some instances

Quote:
The 10M resistors are there for PIV equalization. Should they be reduced? I'll get some ceramics for noise suppression.
Yes they should be reduced - they are there to swamp the reverse leakage current of the diodes thereby equalizing the voltage drop across them. To assure this happens the swamping current should be at least 10X the worst case leakage current of your diodes. 1M resistors should assure this in most cases. These diodes have fairly abrupt cut off when they become reverse biased which generates some emi which can be suppressed by small caps across them - these caps may also provide a small amount of protection against transient induced over-voltage as well. (Distant lightning strikes and the like)

Quote:
In the original circuit, the output stage was powered directly from the main cap (40uF, 400V) driven by the tube rectifier. What Q1 is now powering was powered by a resistor straight off the main cap and a 10uF cap of its own. The rest of the supplies were powered by the same resistors I have, only with 10uF caps instead of 22uF.
The source impedance of your follower is undoubtedly a lot lower than the circuitry it replaced so this is mostly a matter of semantics, but having gone to the trouble of providing such good isolation I would use it. Note that the use of 10uF capacitors will allow more LF coupling between the individual stages which could lead to instability or not..

Quote:
I'll probably get a second mosfet to make a small series regulator for the output stage. I'll just add another 24V on top of the 380V reference for the driver-stage for it.
Sounds like a good solution to me.

Quote:
That's actually supposed to be a volume control. As the resistance goes down, the impedance seen by the 6AV6 goes down, reducing the gate drive to the final stage.
Not such a good idea, this will grossly overload the phase splitter and result in very high distortion - in addition as you change the resistance at this point the LF -3dB will move all over the place. (Halve the resistance and the -3dB point doubles.) Making the bias adjustable for balance and to tailor the sound (running hot or cold) and trade off tube life for distortion performance is a useful feature IMO.

Quote:
Um... What? Are you referring to that first tapped potentiometer? The purpose of that is to act as a volume control and input location selector, so you can use the first half of the 12AX7 or not.
Yes I know exactly what that is having designed MI electronics professionally for a living.. That said you still need to add the grid stoppers at the grids of those tubes - I can't think of a better way to bring EMI directly into the amp than to leave them out.

Quote:
See Is a Class D amp appropriate if you have an external feedback loop?. Unfortunately, I got a bit thread-jacked, though the other topic is sort-of on topic with my question.
Depends on the design of the class D amplifier, but external loops are used in some instances.

Quote:
No. The bass I'm building it for is a passive Gibson Grabber with humbuckers (so very low-level drive).
You may need additional gain, but I am not sure about this as I have not figured out your actual gain distribution.
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Last edited by kevinkr; 12th May 2010 at 04:55 PM.
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