Rebias after Recap?

[Perkinsman]

I know that with power tube replacement the bias requires adjustment but what if I'm just replacing the main filter caps and keeping the same power tubes?

[Jazz P Bass]

Can of worms with that 'bias' word.
What seems to be missing is the fact that all tubes bias differently.
Enter GT with there bias numbers.
If the amp is NOT pulling obscene current at idle, let it roll.

[DrGonz78]

If it’s a case with really nasty old caps that might drag the b+ voltage down a bit then I would at least check it. In all tube amps that I service I check the bias especially one where the caps are old enough to warrant a replacement. Doesn’t mean you have to adjust anything but you won’t know unless you know.

[Jon Snell]

In theory, capacitors do not conduct DC voltage but in a bias circuit if they change value, then in a doubler circuit will have an effect on the resultant voltage produced. The previous owner may have adjusted the bias voltage and when the capacitors are replaced, the bias will need resetting. Portion of bias voltage generator from a TSL100/22

[pdf64]

My view is that the bias supply ecaps are more ‘mission critical’ than those of the HT supply. So there should be good reason to replace the latter without also the former.
To the query, if the amp chassis is already on the bench, why wouldn’t such basic checks as power tube idle plate / cathode current and full output be done?

[Leo_Gnardo]

if the amp chassis is already on the bench, why wouldn't such basic checks as power tube idle plate / cathode current and full output be done?

As Enzo says, there's no good reason NOT to check. I'll expand that to "there's no excuse." Be thorough, takes a couple extra minutes, no big deal.

[g1]

If it’s a case with really nasty old caps that might drag the b+ voltage down a bit then I would at least check it.

Agree. Worst case is that B+ comes up a lot with the fresh filter caps and idle current becomes unacceptably hot. Not worth the risk, check it.

[Leo_Gnardo]

Agree. Worst case is that B+ comes up a lot with the fresh filter caps and idle current becomes unacceptably hot. Not worth the risk, check it.

The case I all too often see is a set of fresh hi voltage filter caps installed by another tech but the ailing failing bias cap neglected entirely. Fresh bias cap = cheap insurance even if the old one seems to be holding up. I'm sure we're all thorough here. I'd like to encourage that sort of thoroughness to any of our unannounced visitors that may read this thread.

[Perkinsman]

Ok, thanks guys, I have no problem doing it, just curious as to whether its standard procedure when replacing only main caps. The old ones were off spec by 30% so I'm curious to see how that impacted the bias. I also check the bias cap.

[The Dude]

I check the bias on every amp before it leaves the shop- regardless of what the repair was. It's a quick and easy and may prevent the amp from coming back.

[pdf64]

...The old ones were off spec by 30% so I'm curious to see how that impacted the bias. I also check the bias cap.

What spec are you referring to? How are you checking ecaps?

[Perkinsman]

What spec are you referring to? How are you checking ecaps?

i'm referring to each caps value per the schematic. I'm measuring out of circuit with my dmm. Unfortunately my Sprague T0-6 is down right now, i'd rather be using it because it brings the cap up to full voltage and measures leakage.

[pdf64]

In the absence of an old school, high voltage cap tester, such as your Sprague (which I don’t have), my view is that the best way to check ecaps is to measure how they perform in circuit, swap in a known good, new one of the same nominal value, measure how that performs and compare the results.
I don’t see much point in making a judgement based on how close their cap value is to the nominal at a very low voltage. Especially if their capacitance ‘tolerance to nominal’ spec is unknown.
Their ESR and leakage resistance at working voltage seem more significant to their in-circuit performance and suitability for leaving in place than how close their cap value is to nominal.

[Enzo]

Value on old electrolytic caps was all over the map. In the old days, filter caps often had the spec -20/+80%. Seriously, no lie.