I am trying to get hold of replacement parts (especially capacitors) to revise a tube pre-amp. However, for most parts I can't find the exact values. I've checked Mouser, RS Components, Digikey and Farnell. Either the parts don't exist anymore or I have to order them in big amounts (2000+).
How much freedom do I have in taking parts with other values? Can I use a 400V cap instead to replace the old 500V cap? A 5.6nf cap to replace the old 5nf one? Will these changes make a big difference?
Here is the schematic and part list of the pre-amp I'm revising:
How much freedom do I have in taking parts with other values? Can I use a 400V cap instead to replace the old 500V cap? A 5.6nf cap to replace the old 5nf one? Will these changes make a big difference?
Here is the schematic and part list of the pre-amp I'm revising:
Hi there,
a 400V cap to replace a 500V one is probably a bad idea, the other way round will be ok. A higher voltage capability does no harm - while an insufficiently rated cap will probably blow up with a bang and leave a mess.
Slightly different capacitance values (5.6nF to 5nF) should be ok, as most capacitors are rated +-20% in capacitance anyway...
Greetings,
Andreas
What is the off-load HT voltage? If it is below 400V then 400V caps should be OK. The smoothing caps are specified as 385V, so my guess is that the original 500V caps were specified simply because they were available in some manufacturers range.
The cap marked 43 should be mains rated, or omitted, as it connects one side of the mains to chassis. Failure here is a serious safety hazard.
The cap marked 43 should be mains rated, or omitted, as it connects one side of the mains to chassis. Failure here is a serious safety hazard.
If your B+ caps are rated for just 385 volts, then any coupling caps you need can probably be rated for a little less as the filter caps must be rated for start-up transients.
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Cap43 would be a mains X or Y type.....NOTE 
There is no earth connection shown in the mains connection !!!...; however I'm strongly against the practise of connecting the audio circuitry with hands exposed to audio sockets and the like capacitively connected to one side of the mains, is not adopted now and the leakage currents of whatever value cap used is dangerous. The idea was probably done to avoid mains modulation hum caused by the HV rectifier turnoff oscillating with increased current demand reacting with the mains transformer leakage inductance. These day the good quality approved R/C components across the rectifiers is the proper solution.
richy
There is no earth connection shown in the mains connection !!!...; however I'm strongly against the practise of connecting the audio circuitry with hands exposed to audio sockets and the like capacitively connected to one side of the mains, is not adopted now and the leakage currents of whatever value cap used is dangerous. The idea was probably done to avoid mains modulation hum caused by the HV rectifier turnoff oscillating with increased current demand reacting with the mains transformer leakage inductance. These day the good quality approved R/C components across the rectifiers is the proper solution.richy
Morgan Jones book Valve amps 4th ed page page 374 deals with a simple 10nF cap across each diode but generally I only add these types of component on the last resort, that is if there is a real detectable problem actually caused by rectifiers. Typ is the EMC rating caps which ideal for this app.
Farnell stock numbers 9520376; 1166662 are typicals
richy
Farnell stock numbers 9520376; 1166662 are typicals
richy
never replace higher voltage cap with lower voltage one, if the cap ends up being too low of a nominal you get an explosion, vice versa is ok, you can always replace lower voltage one with higher voltage one. eg from 500V to 600V. for capacitance nominals these are some really wierd numbers there, for a long long time all standard components have followed the E series, go for E12 series, 10 12 15 18 22 27 33 39 47 56 68 82
so replace your 5nF with a 4.7nF one, you'll be fine, the ones you have probably are not that accurate anyway
so replace your 5nF with a 4.7nF one, you'll be fine, the ones you have probably are not that accurate anyway
oh, and if you do get problems start debugging by calculating all these rc circuits there Xc=-1/(2pi*f0*C), Xl=2pi*f0*L
f0 being the frequency of consern, prolly take highest or lowest frequency of consern depending on cap function(supposed to groung high freq? supposed to pass high freq? supposed to stop low freq? etc, use same f0 throuout circuit) once you have replaced capacitances with reactances on the frequency of consern you basically get resistance networks(dont let the negative ohms bother you, they just indicate the reactance is capacitive not inductive) if you identify a problem anywhere you can then tune your resistances or capacitances by eg putting an extra cap parallel to the one you replaced eg 4.7nF+0.33nF = 5.03nF - much closer to original value
f0 being the frequency of consern, prolly take highest or lowest frequency of consern depending on cap function(supposed to groung high freq? supposed to pass high freq? supposed to stop low freq? etc, use same f0 throuout circuit) once you have replaced capacitances with reactances on the frequency of consern you basically get resistance networks(dont let the negative ohms bother you, they just indicate the reactance is capacitive not inductive) if you identify a problem anywhere you can then tune your resistances or capacitances by eg putting an extra cap parallel to the one you replaced eg 4.7nF+0.33nF = 5.03nF - much closer to original value
A capacitor in a circuit only know what voltage is across it, not what voltage rating the cap it replaces originally had. A cap rated at 500V in a circuit which cannot exceed 350V can safely be replaced by a 400V cap. Why did they originally use a 500V cap - probably because the next rating down was, say, 300V and that would be too low.
fair enough the main point is that caps nominal voltage doesnt matter as long as its lower than operating voltage. if operating voltage exceeds nominal voltage you will have smoke and explosion. otherwise the only significant variable of an capacitor is its capacitance(duh). there is also parasitic resistance and inductance but thats not really relevant in low frequency devices(okey parasitic resistance can be relevant in some low freq circuits but not in audio amps)
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Thanks for the advice, guys! In the meantime I have found the problem: the HF inductor is connected with very thin wires and one of them has broken off. Now it's too short to reconnect it, so I'm afraid I'll have to replace the inductor.
How do I replace it though? The current inductor has 5 connections. Any inductor I find online has only 2. The schematic says "nur Körper mit Kern" which means "only body with core"
How do I replace it though? The current inductor has 5 connections. Any inductor I find online has only 2. The schematic says "nur Körper mit Kern" which means "only body with core"
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thats not an inductor, thats an transformer. and dont replace it, better try to solder an extension wire on the one that broke off. also nonsymmetrically tapped transformers are really hard to find, finding the very right one for your application might prove impossible. got a pic of that transformer of yours?
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