Revising a tube pre-amp: can't find parts?

[Stabby]

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:

[Rundmaus]

Originally Posted by Stabby

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?

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

[DF96]

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.

[Stabby]

Thanks! I think 400V should be okay, since the off-load HT voltage is slightly below 400V.

Cap 43 is a problem. I've searched and I can't find any possible replacement for it.

[DF96]

I would omit it, and ground the chassis. If this creates a hum loop then there are safe ways to deal with this.

[Captn Dave]

Originally Posted by Stabby

Thanks! I think 400V should be okay, since the off-load HT voltage is slightly below 400V.

Cap 43 is a problem. I've searched and I can't find any possible replacement for it.

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.

[richwalters]

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

[Stabby]

Thanks for the advice. What values should I use for the R/C circuit?

[richwalters]

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

[r2k]

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