Brownface Super Bias circuit diode

[brwnfce]

Hi Martin,
Thanks for the reply, difficult to get a clearer shot than this, Ive checked the astrons, 8uf 160v they have esr of 0.84 ohm & 1.08ohm. it's all original bar the orange drops? and 100uf 25v cap subbed for the spec 4uf 25v. The filter caps 20uf 500v are measuring between 0.44 - 3.00 ohm, the 25uf 25v 36.2ohm/40.00ohm, the 50uf 25v 3.0ohm/2.6ohm, I not have measured the electrolytics out of circuit for capacitance as yet, was hoping to keep as much as possible original , but I'm guessing that is unrealistic with 60yr old components.

Regards.

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[Phil_S]

The post about ESR got me to thinking...I wasn't really sure what was OK, so I found this table. When you consider the internet as a random source, I can't assert that it is valid or accurate, but it seems reasonable enough. It was referenced in what appeared to be a well written article on ESR.

In any case, electrolytic caps that are as old as those are well past intended service life and, in general, even if never used, are simply too old to be relied upon. I suggest they all need to be changed. It's better than cleaning up after one spews it's contents inside the chassis. I think I see one newer yellow barrel (maybe a Sprague?) e-cap that you might keep, but I can't see a reason to do that.

The OD caps are obvious replacements. Look at how they were installed -- by someone who was afraid to desolder the eyelets. If they are OK, you might consider keeping them. Up to you. I don't think I'd be in a hurry to change the coupling caps unless they are obviously bad. See if any are passing DC voltage. They shouldn't and if significant, they are due for replacement.

[brwnfce]

You are right , pretty as it looks with the original parts, replace is the way to go, got all but the diode, will be leaving the coupling caps unless very leaky, got some replacements for the OD's, thanks for the chart and thoughts, helps get to the right decision, end of the day it's for playing and listening to not looking at, not the interior at least.

Cheers.

[brwnfce]

Hi,

Diodes got here today, grid in the photos is 10mm, I am also thinking of replacing the resistors in the bias circuit, filter cap & power tube spots with 2w CC resistors values as per the schematic, or should I leave them alone if they measure in spec? Also the outside foil on the OD's one on left faces up, one on right faces down as you look at them with the transformers on the right, is this correct?

[pdf64]

How far to go with replacing apparently good but life expired parts is subject to a conversation with, and instruction from, its owner. A safe, functional vintage amp can’t be museum standard time capsule.

...the outside foil on the OD's one on left faces up, one on right faces down as you look at them with the transformers on the right, is this correct?

Dunno, it’s hard to tell. Certainly for any whose application is decoupling, ie that are connected to 0V common, that 0V leg should be the outside foil.
For other caps anyway, any benefit is subject to debate.

[brwnfce]

Fair enough, guess that is an unanswerable question without the benefit of clairvoyance, maybe that OD on the left is marked incorrectly from the factory, earth being towards the control panel, the diode/s are correct though yes? I'm just exercising caution and garnering the superior knowledge of others here.

[pdf64]

The diodes should be fine.
Don’t omit to use your light bulb limiter when first powering up! But then don’t forget to take it out of circuit once all proves good.
Both ODs can’t be correct. It looks like their control panel side legs connect to 0V common, so their outer foils should be ‘up’.
The same considerations for the bias supply must also apply to the coupling caps to the output valve control grids. Any measurable degree of leakage there can’t be tolerated, because it’s only going to get worse, and any gradual increase will cause the slow bake scenario.
The cautious approach would just be to default replace those, especially as old Astrons are reportedly somewhat notorious for DC leakage.

[martin manning]

At least with current production OD's, the markings are randomly applied. Those in the photo are older, but I don't know when or if they were ever applied correctly. You would have to remove and test them to find out for sure.

[sluckey]

I would not be concerned about the orientation of those two caps. They are in the vibrato circuit, not the signal path.

[brwnfce]

The coupling caps to the output valve control grids pin 5 would be the two 0.1uf 400v nearest the PT, is that correct?

[sluckey]

correct

[brwnfce]

Thank you all Gentlemen for the clarifications and insights, it's a privilidge to be able to access your collective wisdom.

Cheers.

[brwnfce]

privilege*

[Colossal]

Wow, that is a beautiful amp! Looks mint.

[brwnfce]

Thanks, waited a long time to get one of these, still in the process of selling what other amps I have to pay for it but that is fine, going with the less is more philosophy. Got the original diode out, slightly clearer shot here, swapped out the astrons for sprague 8uf @ 450v, almost exactly the same size as the astrons, the resistors measured bang on spec and don't looked cooked at all so I left them there. Put 22uf 500v F&T filter caps, again all the resistors measured pretty much exactly to spec and looked fresh so left those too. Will be thinking about some new speakers to save the P10R's when this side of things is complete, but plan on using them for a while.