Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TerryD on October 31, 2011, 01:51:31 pm
-
I put the Sprague atom caps on my 68 bassman head about 15 years ago. They all look like new. I checked my tube sockets and cleaned and tightened things up and still get sputtering. The sputtering comes and goes, but will stay consisitent for a while.
Could it be the Caps??? What else could it be? Would I have to repalce all the caps? I never checked caps before. My multimeter says it checks capacitance in an F section...I get nothing, but may be doing something wrong.
I just use the amp in the house about 5-7 hours a week at 1/2 to 1 hour intervals.
Have you noticed caps arn't getting any cheaper?
-
are the plate resistors old carbon comps?
-
I think I change those but I'll look.
-
Not unheard of. You can soon tell by tacking (a) fresh filter cap(s) in parallel, as a temporary measure, and seeing if that improves things.
-
If the amp wasn't used at all, then yah the caps could have gone bad. The Sprague Atoms are way overpriced and use old technology as compared to new caps. I have some on the shelf that are from around 15 years ago that were unused...I reformed them on my Sprague Tel-Ohmike and they check out ok now, but I will not use them on a customer's amp. Each one took about a day and a half to reform. I would guess they will not have as long of life as a new cap would. After I used these in something, I doubt if I will ever buy Atoms again unless a customer requests them. My 2 cents....
greg
-
There is no way I can overstate how strongly I will insist or recommend that you replace those 100K plate resistors along with those caps. You are, I take it, replacing those caps, *not* because they are instantly blowing fuses or instantly flaming those 1W resistors between subsequent B+ nodes, but because you suspect they are "iffy" and may be contributing to intermittent amp noise.
I mean, I had a Pro Reverb with its original 1965 electrolytics that was working uhhh, errr, fine (while making tons of noises) and I replaced the caps and it made almost no difference. You want that hissing, popping noise out, do yourself a big favor and replace 4 or 6 or all of those 100K resistors (the sum of which cost less than ONE electrolytic) with extreme prejudice. Just rip them all out and replace 'em. If your experience is anything like mine, the difference is dramatic, and, by the way, improves quite noticeably after the amp has cooked those new resistors for half an hour or so. why that is the case, I have no idea, but it did the same exact thing on 3 separate Fender amps I recently finished recapping. A Princeton Reverb (about 1969) and a Deluxe Reverb (1968, AB763) and this Pro Reverb (BF, 1965) ALL of them made really crappy hiss and pop noises, ALL of them lost 90% of that noise upon replacing 4-6 of those 100K resistors. Dougs' 3-watt metal oxide ones work great and provide plenty of headroom.
-
Actually I shoved and moved tube #1 around and got a lot of crackling. I always have that channel off...but I heard it as I moved it and have gotten "some" respit. If the noise continues I will certainly try what you say.
Are you saying all the plate resisters plus the resisters between the filter caps. I'm sure I replaced this stuff when I did the caps umpteen years ago.
-
Just the 100K plate resistors, generally, they are 1/2 watt in the older amps and 1 watt in the silver-faced ones. Swap 'em all out, I say. The intra-node B+ resistors I generally find no need to replace...it's not the worst sin you can commit to replace them just so you don't have to take the amp apart again, but the ones in my 30+ year old Fenders were not discolored due to heat and were within tolerance as they sat. The 100K plate resistors--those are the villains when it comes to Fender amp noise. Replacing them made a giant difference, not subtle at all.
-
Thanks