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Old 24th February 2013, 08:33 PM   #1
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Default Basic questions on configuring Tube AMP

Well, I am close to finish my Simple SE amp.
This is my first amp and was lovely to work on it.
When I jumped finally to the wiring diagram page, I found too much information I am not familiar with (Not blaming anyone, just lack of information from my side)
I was wondering if you can enlight me with you knowledge regarding the basics of this:
Cathode feedback
UL and Triode mode
Supplemental capacitor (What it intends to do in the circuit)
Choke (What it intends to do in the circuit)
SS rectifier vs. tube rectifier

Thanks in advance,
Sagi
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Old 25th February 2013, 01:01 AM   #2
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Quote:
Originally Posted by sagi4422 View Post
I was wondering if you can enlight me with you knowledge regarding the basics of this:
Cathode feedback
UL and Triode mode
Supplemental capacitor (What it intends to do in the circuit)
Choke (What it intends to do in the circuit)
SS rectifier vs. tube rectifier
Sagi: One could write volumes answering your questions........Here is (hopefully) a condensed version:

Cathode feedback: Global (cathode in this case) essentially takes a fraction of the output signal, inverts it, and injects it back into the front of the amp. This is a form of negative feedback, which has the effect of reducing distortion and it also reduces gain. Some folks like amps with feedback and some feel that it sucks the life out of the amp. Amps with feedback will measure better on the bench, but may or may not sound better subjectively. Try it both ways and see for yourself.

People typically run triode mode without feedback, as the gain and distortion are already relatively low.

UL & Triode mode: There are three modes you can run the output tubes that are compatible with the SSE: Pentode, UL (ultralinear), and triode mode. Do a search here or on google for these terms.

Pentode mode has the highest power out and distortion and usually always needs some feedback. Triode is at the other extreme with lowest output power and distortion, and UL is in-between the other two. UL is acheived by wiring the tubes and output transformer halfway (roughly) between how they are wired for pentode or triode. Your output transformers need to have UL taps to run UL mode.

Pentode mode is usually not used for hifi since the distortion is so high, so your choices end up UL or Triode. The switch that you can optionally add allows easy switching between these modes without having to flip the amp over and re-wire.

It's another fun thing to play with, just never switch the triode/UL switch with the amp powered up, as bad things can happen.

Supplemental cap: The supplemental cap is put in parallel with the last cap in the power supply. Typically a motor run cap or paper-in-oil (PIO) cap is selected, since these have very low ESR (equiv series resistance), lower than an electrolytic can cap. That means that these cap can provide the current instantly when the amp needs it like for loud drum whacks or other dynamic passages. This lowers the output impedance of the power supply which is a good thing, so that this added current draw does not pull down the B+ voltage. The other benefit is wiring two caps in parallel adds their capacitance, so it will help reduce ripple voltage on the B+, resulting in less hum, and can provide lots of current when needed. The downside is that motor run or PIO caps get quite large in larger uf sizes, and they can be a little pricey. The bigger the better if you can fit one somewhere and handle the $$. These are available on Ebay or your local air conditioning supply house (assuming you live somewhere where people need AC ). If you add this, you need a motor run cap and not a motor start cap. A big one is 100uf.

Choke: Using a choke instead of a resistor turns your power supply into a CLC (cap-inductor-cap) instead of a CRC (cap-resistor-cap) power supply and will reduce ripple voltage (hum) as well as offer a few other benefits that escape me at this moment. I imagine someone else will be along to further explain this one. Google CRC vs CLC power supply.

SS rectifier vs tube rectifier: The main differences between the two are:

Given the same power transformer, caps, choke and/or resistors, the B+ will be higher with SS rectification. This is because tube rectifiers drop significantly more voltage than SS diodes.

Tubes take a few seconds to warm up, and are a little easier on the amp than SS diodes, since everything is whacked with full B+ voltage the second your hit the switch with SS rectification. Tube rectification increases the power supply output impedance compared to SS which is a negative. Tube rectifiers are also fussy about how big the first PS cap is; SS diodes are not.

The SSE had a few issues with popping SS rectifers (FREDs-Fast recovery Epitaxial diodes), especially the IXYS parts, but I believe the Fairchild Stealth parts and others work just fine.
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Old 25th February 2013, 08:57 AM   #3
zman01 is offline zman01  Bangladesh
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Boywonder,

Thanks for putting together a wealth of good information under one post. This will help any newbie.
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Old 25th February 2013, 01:48 PM   #4
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Quote:
Originally Posted by sagi4422 View Post
Well, I am close to finish my Simple SE amp.
This is my first amp and was lovely to work on it.
When I jumped finally to the wiring diagram page, I found too much information I am not familiar with (Not blaming anyone, just lack of information from my side)
I was wondering if you can enlight me with you knowledge regarding the basics of this:
Cathode feedback
UL and Triode mode
Supplemental capacitor (What it intends to do in the circuit)
Choke (What it intends to do in the circuit)
SS rectifier vs. tube rectifier

Thanks in advance,
Sagi
Boywonder has given very good answers to your questions. May I suggest
that you follow the designer George Anderson's advice and start out with
just the basic configuration, get it working properly then you can add the
other options one at a time.
Here is what George says in his assembly instructions...........


"If you take some time to study these diagrams, you will come to the conclusion that you can wire your amp up initially according to the basic triode connected amplifier shown in the diagram below. Perform all of the initial testing, get it all working, and enjoy it for a while. Then you can add any of the options shown on this page, often with only a screwdriver and wire strippers! This is by design. It is recommended that you start with the simplest configuration first, then add all of the optional stuff. This is the way that I build an amp. Often all of the switches, and supplemental parts are mounted, I just don't connect it all up until the basic amp is working. This makes the wiring a little easier to handle. "
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Old 25th February 2013, 04:08 PM   #5
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Default Wow!

Quote:
Originally Posted by boywonder View Post
Sagi: One could write volumes answering your questions........Here is (hopefully) a condensed version:

Cathode feedback: Global (cathode in this case) essentially takes a fraction of the output signal, inverts it, and injects it back into the front of the amp. This is a form of negative feedback, which has the effect of reducing distortion and it also reduces gain. Some folks like amps with feedback and some feel that it sucks the life out of the amp. Amps with feedback will measure better on the bench, but may or may not sound better subjectively. Try it both ways and see for yourself.

People typically run triode mode without feedback, as the gain and distortion are already relatively low.

UL & Triode mode: There are three modes you can run the output tubes that are compatible with the SSE: Pentode, UL (ultralinear), and triode mode. Do a search here or on google for these terms.

Pentode mode has the highest power out and distortion and usually always needs some feedback. Triode is at the other extreme with lowest output power and distortion, and UL is in-between the other two. UL is acheived by wiring the tubes and output transformer halfway (roughly) between how they are wired for pentode or triode. Your output transformers need to have UL taps to run UL mode.

Pentode mode is usually not used for hifi since the distortion is so high, so your choices end up UL or Triode. The switch that you can optionally add allows easy switching between these modes without having to flip the amp over and re-wire.

It's another fun thing to play with, just never switch the triode/UL switch with the amp powered up, as bad things can happen.

Supplemental cap: The supplemental cap is put in parallel with the last cap in the power supply. Typically a motor run cap or paper-in-oil (PIO) cap is selected, since these have very low ESR (equiv series resistance), lower than an electrolytic can cap. That means that these cap can provide the current instantly when the amp needs it like for loud drum whacks or other dynamic passages. This lowers the output impedance of the power supply which is a good thing, so that this added current draw does not pull down the B+ voltage. The other benefit is wiring two caps in parallel adds their capacitance, so it will help reduce ripple voltage on the B+, resulting in less hum, and can provide lots of current when needed. The downside is that motor run or PIO caps get quite large in larger uf sizes, and they can be a little pricey. The bigger the better if you can fit one somewhere and handle the $$. These are available on Ebay or your local air conditioning supply house (assuming you live somewhere where people need AC ). If you add this, you need a motor run cap and not a motor start cap. A big one is 100uf.

Choke: Using a choke instead of a resistor turns your power supply into a CLC (cap-inductor-cap) instead of a CRC (cap-resistor-cap) power supply and will reduce ripple voltage (hum) as well as offer a few other benefits that escape me at this moment. I imagine someone else will be along to further explain this one. Google CRC vs CLC power supply.

SS rectifier vs tube rectifier: The main differences between the two are:

Given the same power transformer, caps, choke and/or resistors, the B+ will be higher with SS rectification. This is because tube rectifiers drop significantly more voltage than SS diodes.

Tubes take a few seconds to warm up, and are a little easier on the amp than SS diodes, since everything is whacked with full B+ voltage the second your hit the switch with SS rectification. Tube rectification increases the power supply output impedance compared to SS which is a negative. Tube rectifiers are also fussy about how big the first PS cap is; SS diodes are not.

The SSE had a few issues with popping SS rectifers (FREDs-Fast recovery Epitaxial diodes), especially the IXYS parts, but I believe the Fairchild Stealth parts and others work just fine.
These are very good explanations . Just what I need. Thanks a lot !
Surely, I will start with the basic configuration (if it works).

Thanks again. Just cut and paste this info.
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Old 25th February 2013, 04:26 PM   #6
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BTW:
In case I am not burning the fuse or some fire comes up from the board, what are the basic tests I should conduct ? I didnt see any reference to this in George's site.

Thanks.
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Old 25th February 2013, 05:25 PM   #7
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Ideally, for first power up a variac is handy top apply mains voltage slowly. Without that, you can put a lightbulb in series with the mains power cord. This can keep the smoke inside of the components in case you have something wired wrong. Do a search here if you are interested in the lightbulb trick. It should glow dimly if everything is good. If it glows brightly unplug the mains quickly, as that means that the amp is drawing excessive current, ie a short somewhere.....and of course only fit a fuse of the proper size, preferably a slow-blo. Having a few extra fuses around doesn't hurt, either.

First, check that your B+ voltage is close to what you expect using a DC voltmeter to ground. You'll need to hook up some expendable speakers or 8 ohm power resistors (at least 10W rated) to the speaker terminals. Never power up a tube amp without a proper load on the speaker terminals. I wouldn't recommend connecting your fancy speakers for getting the amp running, as these could be damaged if something is not correct.

The next thing to check is the voltage drop across the cathode resistor, using ohms law I=E/R, you can calculate the bias (idle) current through the output tubes. Multiply this by the B+ voltage for the power that the tubes are dissipating as heat. Both tubes should be flowing roughly the same current.

When testing a live amp, either use clip leads on your meter (preferred method-highly recommended) or use one hand and keep the other hand either in your pocket or behind your back. This keeps high voltage from going across your chest/heart in the event you touch something that you are not supposed to.

Here are typical clip leads for a multimeter: These allow completely hands-off testing of the amp.
Attached Images
File Type: jpg clip leads.JPG (664.2 KB, 386 views)

Last edited by boywonder; 25th February 2013 at 05:54 PM.
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Old 25th February 2013, 06:11 PM   #8
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Thanks.
The potentiometer wiring composed of 6 pins in the George's wiring diagram.
I found very common a 3 pin POT. Will it fit ? what is the wiring consideration in such POT ?
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Old 25th February 2013, 08:41 PM   #9
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Default You need 3 pins for each channel

Quote:
Originally Posted by sagi4422 View Post
Thanks.
The potentiometer wiring composed of 6 pins in the George's wiring diagram.
I found very common a 3 pin POT. Will it fit ? what is the wiring consideration in such POT ?
If you are talking about the volume pot, you need a dual gang pot, one gang for each channel for stereo. So you'll have 2 sets of 3 pins, or 6 pins. Make sure you get a log taper pot (not linear). The log taper is for audio and is tailored to human hearing so that when you turn it up, the turn amount corresponds to the increase in volume that you perceive.
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File Type: jpg stereo pot.jpg (11.7 KB, 362 views)
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Old 27th February 2013, 04:27 PM   #10
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Thanks.
On it....
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