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Old 4th August 2006, 01:48 AM   #1
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Default Some questions on non common things

Hi,
New to this forum so be gentle with me
I'm still new to tube amps, but learned enough so far to build a basic (guitar) amp. I got few questions that I'm not able to find answers for, hope you can shed some light on them.

1. Bought an old PA amp that had a filter (?) capacitor between the power transformer and selenium rectifier, referenced to ground. why is that?

2. I'm modifying an old tube radio (a 1947 Philips BX765X if anyone is into radios) to be a guitar amp. it has a choke between the plate and screens B+. the choke is not touching the chassis, it's on a non conductive material, but one of the screws that was holding it there, had a cap on it that was connected to the B+ (or maybe the "minus"? sorry, already took everything apart ). in guitar amps the choke is connected to the chassis, any idea what is going on here?

3. The radio uses a two wire AC connection. ermmm... so how the chassis connect to ground? maybe this has to do something with the way the choke is connected? (I have a feeling that this is a basic thing, but bare with me )

And some general questions

4. I'm looking on all sorts of non common tubes to build guitar amps around them. how can I know if a tube is good candidate for guitar preamp use?
Right now I'm looking only on amplification factor and heater/plate volt/current needs, but I'm sure there are other factors that I should consider. maybe capacitances spec?

5. Is there a reason why most preamp tubes are triodes? other then the EF86 which is a pentode and seldomly being used in guitar amps, you keep finding the same old 12A_7 over and over again. could something like a 6K7 with its Mu=1000 be used in a preamp?

6. Is there any other output transformer manufacturer that has multi tap transformers like the Hammond 125E series?

Thanks for reading,
Amit
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Old 4th August 2006, 02:14 AM   #2
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Default echad mi yodaya?

Echad ani yodayah.

That's a means of generating B- for the output stage grid biasing and any incidental uses. Generally, you want to get rid of selenium rectifiers (they don't age well), but replacement is not always straightforward. Modern rectifiers have much lower drop and dynamic impedance, and sometimes that can kill downstream components unless they're uprated. It can also shift operating points if not compensated for.

Number two, I'm less sure, but I'll bet that the choke was a plate load. A google for the schematic would be enlightening.
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Old 4th August 2006, 02:22 AM   #3
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Default Re: Some questions on non common things

Originally posted by Hendrixon

1. Bought an old PA amp that had a filter (?) capacitor between the power transformer and selenium rectifier, referenced to ground. why is that?

Could be part of a voltage multiplier. It might also be part of a high pass filter to attenuate the AC before rectification if it's part of a low voltage circuit.

2. I'm modifying an old tube radio (a 1947 Philips BX765X if anyone is into radios) to be a guitar amp. it has a choke between the plate and screens B+. the choke is not touching the chassis, it's on a non conductive material, but one of the screws that was holding it there, had a cap on it that was connected to the B+ (or maybe the "minus"? sorry, already took everything apart ). in guitar amps the choke is connected to the chassis, any idea what is going on here?

3. The radio uses a two wire AC connection. ermmm... so how the chassis connect to ground? maybe this has to do something with the way the choke is connected? (I have a feeling that this is a basic thing, but bare with me )

Given these two questions, it sounds like this thing used a transformerless power supply. That's not good.

And some general questions

4. I'm looking on all sorts of non common tubes to build guitar amps around them. how can I know if a tube is good candidate for guitar preamp use?
Right now I'm looking only on amplification factor and heater/plate volt/current needs, but I'm sure there are other factors that I should consider. maybe capacitances spec?

Stick with the more common audio tubes for this. About the only other type you might be able to use with a guitar amp that you wouldn't want to (usually, there are always exceptions) for high fidelity is a remote cutoff pentode.

5. Is there a reason why most preamp tubes are triodes? other then the EF86 which is a pentode and seldomly being used in guitar amps, you keep finding the same old 12A_7 over and over again. could something like a 6K7 with its Mu=1000 be used in a preamp?

Has something to do with better distortion under overdrive that guitar players prefer. The 6K7 is one of those aforementioned remote cutoff pentodes. You could probably use it.

6. Is there any other output transformer manufacturer that has multi tap transformers like the Hammond 125E series?

Lundahl?
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Old 4th August 2006, 09:14 AM   #4
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Not familiar with the model number of the Philips radio but its very likely that it uses a dropper resistor to derive ht from the mains. This would mean that there is no isolation from the mains and that the chassis is not designed to be grounded (it floats somewhere between mains neutral and live). On most of these radios the chassis is isolated from the user.

This is not good for a guitar amp unless you want to join the dead rockstars hall of fame
The only way to make it safe would be a complete redesign and rebuild using a transformer power supply to provide mains isolation. For the same amount of effort you could build something from scratch which would probably work better anyway.
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Old 4th August 2006, 11:29 AM   #5
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Actually, an accessory ground wire (from the mains ground to the chassis) and an isolation transformer (vital!) would turn this unit into something less lethal.
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Old 4th August 2006, 11:57 AM   #6
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Hey SY, thanks for the laugh

Now back to tubes.
The PA amp:
I read that selenium rectifiers are not the best of things, so I built a full wave rectifier (CT transformer) and took out the capacitor that was between the PT and selenium. the power section of the amp is a two tube push-pull, got it with two EL34 but have no idea if it was designed for them (the amp looks like someone built it for stage work MANY years ago, not a known branded shelve product).
This change did influence the readings, plate voltage raised, but it's still fine at 435V.
Btw, the amp is cathode biased.
You say that putting a cap between PT and rectifier, where we have AC, is a way to get DC "minus" to the chassis?

Anyway, the amp looks now more like "amps" I understand, re-built the rectification and the whole B+ string. was just intrigued why that cap was there.

Miles, I don't think it was a voltage multiplier since after moving to a diode full wave rectifier I got just about 20V-30V more B+.
High pass filter that attenuates the AC?
You mean I can lower an AC - and there for DC - with just putting a cap to ground there?
This could be helpful info

As for the tube radio, this is the boy:
http://66.49.188.60/philips/bx760x.html
The 760 and 765 are said to be the same everywhere I checked.
And here is the manual, in Dutch, that inside it there is also a full schematic:
http://66.49.188.60/schema/radio/phi...947/bx760x.zip

Miles, there is a transformer, a pretty big one, but yea, just a two wire AC connection. also there were, what seems to me as grounding points all over the chassis, I just don't understand how that connects to ground in the end...

Anyway, to make things simple, I did took everything apart and will start re-building the amp the way I know while using original main parts like PT, OPT, chokes (there are two) output tubes, chassis and so on. it will be a tribute amp to this nice old radio as much as I can, including cosmetics

Martin:
Yea I assumed that *something* in the design here is floating, again, not something I'm used to in common guitar amps. as I said, I am rebuilding an amp from this radio using its parts and 'vibe", so in that case, if I'll use a three wire AC cable and just connect the ground to the chassis, will this do it and keep me a living rockstar?

Thanks for the comments guys, I really appreciate the help
Amit
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Old 4th August 2006, 01:04 PM   #7
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I don't know where the radio was made, but most electrical devices that were made in the US had two wire power cords up until the 1960's. For those of us who were old enough to remember, this was because all of our electrical outlets only had two connections. The ground connection was not there before the early 60's. If this was the case with your radio you can usually just add a three wire cord by connecting the ground wire to the chassis. You can not do this on a transformerless radio. An isolation transformer must be added.
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Old 4th August 2006, 01:20 PM   #8
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From the photos, it appears that there is a power transformer, so no need for a separate isolation tranny- just install a three-wire mains cord.
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Old 4th August 2006, 09:29 PM   #9
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Quote:
Originally posted by Hendrixon

Miles, I don't think it was a voltage multiplier since after moving to a diode full wave rectifier I got just about 20V-30V more B+.
High pass filter that attenuates the AC?
You mean I can lower an AC - and there for DC - with just putting a cap to ground there?
This could be helpful info
Yup, here's the basic idea. C and R form a high pass filter with a roll on frequency set to attenuate the high AC voltage of the main PSU. That could be several hundred volts, whereas you only need a small negative DC supply for fixed bias. Granted, the power handling capability is rather poor, as is regulation. However, for Class A1 tubes, you don't need either since there is almost no current demand.

It's considerably better than making high DC, then cutting it way down with a voltage didvider/VR tube/Zener diode with big, high wattage resistors.

Doesn't work for Class AB2 or Class B, however. There, you need better regulation.
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