Amp porn - safe for work

[gui_tarzan]

I got the go-ahead from the shop (who called the owner) to replace whatever is needed. The owner knows what it's worth, but he wants to use it as long as he has the original parts back.

My knowledge of KT66 tubes is limited, I'm leaning toward the Tung-Sol replacements but I'll take advice in this area.

I did finally get more information, he said it was popping and crackling when he tried to play it. Could be jacks, caps, tubes or connections but it'll be a fun journey finding out.

[jelle]

Shuguang KT66 is my current fav.

[gktamps]

I'm using Valve Arts and really like them.

On sale now, too.

http://www.tubesandmore.com/products/T-KT66-VA

[matt h]

(deleted)

[jelle]

Yes, they are. It is what I used in this one: https://tubeamparchive.com/viewtopic.php?t=25536

[martin manning]

I got the go-ahead from the shop (who called the owner) to replace whatever is needed. The owner knows what it's worth, but he wants to use it as long as he has the original parts back. ... I did finally get more information, he said it was popping and crackling when he tried to play it. Could be jacks, caps, tubes or connections but it'll be a fun journey finding out.

Are you still going to try to reform the caps? If so a couple of things to consider if you want to try to reform them in situ:

With a vacuum rectifier you won't get any DC until the rectifier's filament is up to some minimum temperature, and if you are going to use very low line voltage to start the reforming process this is a problem. You can get around it by substituting in a Copper Cap or putting temporary silicon rectifiers on the rectifier socket.

You can use full or only slightly reduced line voltage by placing a resistor (say 100k 3W, or 220k 2W) across the standby switch (and leaving the switch open) to limit the current to a couple of mA. Monitor the voltage across the resistor and on the reservoir over time (several hours) to see how successful you are, and also to make sure that you don't exceed the max voltage rating on any of the filters. A bulb limiter or a variac could be used initially to reduce the line voltage a bit for some safety margin. I have heard that if the caps have been exposed to full voltage before reforming, then the chance of success is much reduced.

[cbass]

I thought reforming only worked if the cap hasn't seen some voltage for quiet some time like years . I have had some success reforming caps before in amps that hadn't been turned on or hadn't worked for a Kong time

[martin manning]

That's what I heard too, but I don't know the science behind it or if there is some way to get around it. Maybe someone here does?

[gui_tarzan]

After I spoke to the shop owner yesterday it sounds like the owner had been using it (though I don't know how often) so it has had power put to it recently. At this point I'm going to do what he wants, get it working. I'll replace the caps and any needed tubes (like the rectifier that I can see is bad) and we'll go from there.

[Firestorm]

That's what I heard too, but I don't know the science behind it or if there is some way to get around it. Maybe someone here does?

Me too. If the amp has been running for a few hours recently, the caps are as reformed as they'll ever be. Some sites claim that reforming restores the oxide layer on the foil, but I don't think that seems possible. The aluminum oxide dielectric is deposited on the foil before construction using very high voltage and proprietary chemical goop. The reforming after assembly is just to ensure that the electrolyte migrates everywhere, especially coating weld points and other unoxidized areas. If the electrolyte has dried up past a certain point, there's nothing that can be done. You can judge the health of a cap (to a point) by putting a current-limiting resistor in series with the anode. Once the cap is charged, there should be very little current through it, hence very little voltage drop across the resistor. If the current stays up, the cap is pretty much done.

[Roe]

to form filter caps remove tubes and add 100k 1-2w over standby switch. change caps if the voltage doesn't drop to 5-10V across the 100k.

TAD and F&T sells new 16+32uf 500v caps. the filter caps influence the tone and response noticeably

[Roe]

Every single resistor checks out within tolerance. How often does THAT happen on an amp this old??? I like it!

not uncommon with old marshalls. marshall generally used more CF than CC resistors in the 60s.

[gui_tarzan]

As you can see my changes were very minimal - filter, bias and cathode caps only. The cans are also new but they're obviously not in this pic.

[martin manning]

Looking good! Did you find a 32/16 can? I see F&T does make them as Roe says, but the only on-line outlet I found with a quick look was in the UK. How about the crackle and other noise?

[gui_tarzan]

I used an F&T 32/32, I couldn't find a US supplier for the 16/32 either. Honestly in that location it shouldn't change anything.

I haven't fired it up yet, there is a ground lead post missing in the power jack (they screw onto a terminal inside the socket) so I'm going to try to find one before I throw the juice at it.