Hi there. I've salvaged an amp from a Bell and Howell projector and want to re-build the amp for general audio use.It uses 4 EL84 Tubes on the main output stage and 1 ECC83 in the pre-amp stage.
My question is:
I am unable to find new capacitors with a value of 6pF and 350V. (The existing one works, but it gets a bit hot and it sort of looks like it is swollen on the top and could potentially explode!) How can I find a suitable capacitor that will not damage the tube? The data sheet specs a value of 6pF and I can find other values (example 68uF 400V)?
I am a newbie in valve amps and can do with all the help I can get. My aim is to learn this way with an already working amp before I take on a project from scratch.
Many Thanks!
PS, I will post pictures if you need to look at it.
Jaco
My question is:
I am unable to find new capacitors with a value of 6pF and 350V. (The existing one works, but it gets a bit hot and it sort of looks like it is swollen on the top and could potentially explode!) How can I find a suitable capacitor that will not damage the tube? The data sheet specs a value of 6pF and I can find other values (example 68uF 400V)?
I am a newbie in valve amps and can do with all the help I can get. My aim is to learn this way with an already working amp before I take on a project from scratch.
Many Thanks!
PS, I will post pictures if you need to look at it.
Jaco
Hi Alastair, on the data sheet it says 6pF, and no, the cap is about 3 inches in length. Could it be a typo on the datasheet? The actual cap values is a bit faded out, so I can't really see what they are, so my only reliable source is the data sheet.(Which I downloaded from UL84 pdf, UL84 description, UL84 datasheets, UL84 view ::: ALLDATASHEET ::: )
Not sure if this is the best place to look for it?
Thank You
Not sure if this is the best place to look for it?
Thank You
The only value on the datasheet which might look like 6pF and 350V would be an internal capacitance which is part of the valve, not an external component. Component values, perhaps given in an example circuit, would be much bigger: 6nF or 6uF for example.
the symbol for microfarad, can look like a "p".
The u has a tail on the left side. Then looking at an old document... easy to make that assumption of a "u".
The u has a tail on the left side. Then looking at an old document... easy to make that assumption of a "u".
Wow, I've learn't a lot from you guys so far! I will post pictures later today. (My camera is still charging...)
Looks like I am reading the datasheet all wrong - Thanks for pointing that out DF96 🙂. So how do I determine the correct value for the capacitance? I don't want to damage the valves with the wrong values. (I am very cautious and don't like bang sounds!!)
The amp switch on and works, but with capacitors about 40 to 50 years old, I dont want to keep it on for to long.
Looks like I am reading the datasheet all wrong - Thanks for pointing that out DF96 🙂. So how do I determine the correct value for the capacitance? I don't want to damage the valves with the wrong values. (I am very cautious and don't like bang sounds!!)
The amp switch on and works, but with capacitors about 40 to 50 years old, I dont want to keep it on for to long.
Considering the value and the (assumed) Push-Pull topology and the voltage, they are 6 μF (Note the use of Greek font).
Also considering that electrolytic caps were bulky and expensi9ve back then, It woud be very possible to swap them with something like 22μF/ 350V and get smoother Voltage.
Also considering that electrolytic caps were bulky and expensi9ve back then, It woud be very possible to swap them with something like 22μF/ 350V and get smoother Voltage.
Ok, now I am getting excited. I assume it is safe (don't worry, I wont blame anyone if it goes up in flames!) to replace the existing capacitors with something more modern, like a 22uF/350V cap, which brings me to my next question:
The current capacitor has 4 terminals.(Negative, 350,350,350). The new capacitors I can find with the 22uF/350V values only has Negative and Positive (2 terminals). Will it makes any difference if I just join the positives to the one terminal point on the new cap? (Pics will come up later still...)
Thanks guys (and girls?)
The current capacitor has 4 terminals.(Negative, 350,350,350). The new capacitors I can find with the 22uF/350V values only has Negative and Positive (2 terminals). Will it makes any difference if I just join the positives to the one terminal point on the new cap? (Pics will come up later still...)
Thanks guys (and girls?)
Sounds like you have several caps in one can (a few pictures will go a long way). If so, there should be several uf values on the side of the can. 4 terminals usually indicate 3 caps and one common terminal.
Here are some modern versions:
Can Capacitors
Here are some modern versions:
Can Capacitors
I am going to be a spoilsport and gently suggest that the OP does a lot more reading and investigation before attempting anything with this amplifier. Recognising components and knowing their circuit function and likely values is a prerequisite to successful and safe reburbishment.
YOur'e getting mixed up between the scales pF, nF ,uF (pico-12)(nano-9)(uF-6)
There used to be a confusion with the old capacitor markings of mF which really implied uicrofarad and not millifarad.
A 6p2 or 6p8 350V silvered Mica cap for example is readily available from Farnell and other vendors. Even for the first stage zobel, this value is very small for an audio amp.
richy
There used to be a confusion with the old capacitor markings of mF which really implied uicrofarad and not millifarad.
A 6p2 or 6p8 350V silvered Mica cap for example is readily available from Farnell and other vendors. Even for the first stage zobel, this value is very small for an audio amp.
richy
I know I should do some more reading (no harm taken DF96 - I respect your view) I just stumble across the Capacitor issue which was a bit puzzling and seek help in this forum (and of which I got a lot of help to resume my project). Is'nt that what the forum is for?
Anyhow pictures will follow in the next hour or so.Should shed light on a lot it seems. 🙂 Interesting that there can be more than one capacitor in a single cap.(Again, I am fairly new in the tube business, mostly play around with Transistors which uses caps with only 2 terminals).
Thanks again for all your help.
Anyhow pictures will follow in the next hour or so.Should shed light on a lot it seems. 🙂 Interesting that there can be more than one capacitor in a single cap.(Again, I am fairly new in the tube business, mostly play around with Transistors which uses caps with only 2 terminals).
Thanks again for all your help.
+ 1
Incidently, the EL84 DS indicates an internal capacitance of 6pf between plate and other electrodes, it's a design information.
This doesn't mean that such a cap should be added 😱
WessieNC: Please post a schematic.
Yves.
Yes, photos and a schematic will go a very long way. With those you should get all the help you need.
Just one error I need to correct. It is UL84 tubes and not EL84 as per the heading. I also manage to carefully cleaned the caps and reveal the capacitance.It is 60uF 300V(2 of them) and 40uF 350V (one of them) Finally pics. Unfortunately I do not have any schematics of the amp, but it looks like a very straight forward circuit.
Let me know if you need more angles.
Thanks
Let me know if you need more angles.
Thanks
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tripping.......
Wow, those are some crazy photos.......reminds me of the 60's......check "preserve aspect ratio" when resizing your pics...
The cap in question sounds like it is a power supply cap, and based on the age of the amp, it's probably not a bad idea to replace all of the electrolytic caps. You can add capacitance in the PS to reduce ripple and provide stiffness, but if you have a tube rectifier you'll need to be careful not to bust it's specs for the first cap value. Back in the day large value high voltage caps were rarer then hen's teeth. It may be worthwhile (and a good education) to draw a schematic of the PS.
I just use the "manage attachments" button to attach pics to a post. They need to be resized to less than 200K before attaching.
Wow, those are some crazy photos.......reminds me of the 60's......check "preserve aspect ratio" when resizing your pics...
The cap in question sounds like it is a power supply cap, and based on the age of the amp, it's probably not a bad idea to replace all of the electrolytic caps. You can add capacitance in the PS to reduce ripple and provide stiffness, but if you have a tube rectifier you'll need to be careful not to bust it's specs for the first cap value. Back in the day large value high voltage caps were rarer then hen's teeth. It may be worthwhile (and a good education) to draw a schematic of the PS.
I just use the "manage attachments" button to attach pics to a post. They need to be resized to less than 200K before attaching.
If is UL84 then it is almost certain to be an AC/DC live chassis circuit. These are dangerous, and cannot be used safely except for their original purpose in their original case. Even then they will not meet modern safety standards. I strongly suggest you proceed with caution. You cannot use such a circuit as a general purpose amplifier.
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Sorry about the size.This is my first photo post on this forum.
You guys inspired me now to draw a schematic from this amp and re-build it with new parts, except for the tubes of course.I've got basic knowledge of Eagle layout editor and will be using this as a tool to reconstruct the schematic.It sounds like I have to us a different alternative to re-construct the amp as the current design is a bit on the dangerous side.(safety first!) My general feeling based on my reading of various manuals and posts is to use a wiring layout as opposed to a pc board to get good sound quality and a safer design.
Any ideas as to which way to re-construct the amp?(Wooden chassis or metal chassis?)
As for now I am going to strip down the 50 year old amp and start working on the schematic.I see this as a very usefull way to learn how tube amps work!
Thanks again for all your help. I will keep you posted on the progress (and ask for some help, as I definitely would need some)
You guys inspired me now to draw a schematic from this amp and re-build it with new parts, except for the tubes of course.I've got basic knowledge of Eagle layout editor and will be using this as a tool to reconstruct the schematic.It sounds like I have to us a different alternative to re-construct the amp as the current design is a bit on the dangerous side.(safety first!) My general feeling based on my reading of various manuals and posts is to use a wiring layout as opposed to a pc board to get good sound quality and a safer design.
Any ideas as to which way to re-construct the amp?(Wooden chassis or metal chassis?)
As for now I am going to strip down the 50 year old amp and start working on the schematic.I see this as a very usefull way to learn how tube amps work!
Thanks again for all your help. I will keep you posted on the progress (and ask for some help, as I definitely would need some)
You will need a different power supply, including a mains transformer, to make it safe. At the very least an isolation transformer to drive the existing PSU and valve heater chain. You will find the heaters are currently wired in series. You could change that for your redesign.
UL84 tend to be much rarer than EL84, but also less demanded so the price is not so high. Actually, I can't remember whether the UL84 is like the EL84 or the PL84/EL86.
UL84 tend to be much rarer than EL84, but also less demanded so the price is not so high. Actually, I can't remember whether the UL84 is like the EL84 or the PL84/EL86.
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