Help! I tried to recap my amp!

[Phil_S]

re: boards: Phenolic is OK. Fiberboard can absorb water and become conductive. Same with wood. You need something non conductive and suitable for electronics use. It needs to be able to withstand the heat inside the enclosure. I'm not sure how hot it gets in there, but I've felt an amp chassis that is hot enough to burn you.

[Aurora]

I've done some research on this for a couple builds I'm planning, and in small scale it doesn't get much cheaper than Hoffmanamps as linked above, either ready made or in bits and pieces. Fiberglass or FR4 PCB laminate is certainly the best material for the task. The regular 1.5 mm we usually get around here may be a bit flimsy for the task, - I have some 2.5mm - 1/8" is overkill in my opinion, and I've been around heavy duty electronics for many years.
As to making your own, I regularly make prototype and small series PCBs, and ideally one uses special carbide drills and very high RPM drills, but for one off's, any decent drill press will do, or maybe a Dremel with the special stand. A good dust mask actually fixes the dust problem quite well.
Phil's comment on dulling of drill bits is correct, but for drilling asingle board, who cares? Then there's also eyelets instead of turrets, - somewhat cheaper. There's good instructions of the Hoffman pages on how to do both. He also lists layouts for several different amps.

[docz]

great page, do you know where I might find those boards and tools in Norway?

DocZ

[Aurora]

I haven't found any of this stuff in Norway....
Elfa have some turrets ( norsk:loddetaarn) in their range, but these are quite smaller than the ones at Hoffmans. AS for fiberglass plates, there should be users in your area, it's just about finding them......
- boatyards, mechanical industry, larger electrical companys,- there must be plenty in your area

[malt]

great page, do you know where I might find those boards and tools in Norway?

DocZ

You can find them in the rs catalog, and purchase them trough Kompotek in Haugesund.

Group Panel http://no.rs-online.com/web/search/sear ... &x=12&y=18

Turets http://no.rs-online.com/web/search/sear ... &R=433-573

http://no.rs-online.com/web/search/sear ... &R=433-589

Board http://no.rs-online.com/web/search/sear ... &R=1941126

[docz]

UPDATE:

Thanks to Malt, who sent me a bag of goodies in the mail AWESOME!!!!, I was able to change out the other coupling caps. For each that I changed the noise got lower and lower, it is still there. It first starts as amplified hum - the kind you get when you don't touch your guitar strings, then it starts to crackle like a frying pan. But now it is much lower in volume, and it disappears faster.

I'll try to change the last tube cap to see if that kills the noise.

But what I think is weird is that hum seems to excite the noise. That hum is not present when I turn the amp on, it comes after a while of playing. Is there anything I can do to get this hum lower?

DocZ

[Phil_S]

Doc: here are a few things I'd look for, in no particular order.

For hum, look for (and my comment):
loose connections
loose hardware
bad input jack (to test, insert a dummy plug -- tip shorted to sleeve, if noise stops, replace the jack)
faulty reverb circuit
open resistor (have you measured all of them?)
shorted capacitor (you are working on this)
faulty ground connection

For the frying bacon noise:
open resistors
bad caps
loose connections
loose tube socket
loose ground wires

This noise appears after the amp has had a chance to warm up. I'd suspect heat is causing a problem with a cold solder joint. Go through the amp and see if you can touch up any joint that may be suspect. To make sure the joint "wets" you may need to add a bit of fresh solder. Be careful not to overdo it with the solder. Also, be careful that you don't overhead any resistors or caps.

To check resistors, sometimes it is necessary to lift one leg out of the circuit to get a proper reading. I am guessing this is not likely to be your problem.

I hope this helps. BTW, you're doing a great job with this!

[docz]

I was just wondering, how hot can the terminals get before I start to break stuff? I usually have to hold the iron there a long time before the solder melts, and sometimes the components to get really hot. My iron is not regulated, it is just rated at 25W. I was just thinking that I might be doing more harm than good with my poor soldering skills.

DocZ

[Aurora]

Hm... what kind of iron and tip do you have? Plain copper or nickel plated?
How do you clean the tip? - A clean tip and a tiny blob of fresh solder works wonders for loosening old joints...
A 25W iron is on the low side for tube amp work....

[docz]

Ok, clearly I do not know much about solder irons....

The tip was shiny, so I guess nickel? clean it? I didn't know I had to...
I usually tin it before I do any work, I read that somewhere - I can't remember where.. It is sharp, not flat. I used to have a 30W and a 50W the 50W burnt everything up, so I stopped using it, the 30W burned out because I left it overnight without unplugging it - I know it is stupid... So that leaves with my 25W.

Update: Changed the last caps, now I only have a .05 and that square brown thing left of the old capacitors. Still have the noise problem and that new hum problem. The noise is less dominating then it used to be, also certain frequencies seem to distort, but only the G string, from fret 5 - 10 this is weird to me... The same notes on other strings don't behave like this - I don't know if it is the amp or the guitar..
Went over all solder points in the signal chain, found two that looked a bit crusty, and two that had loose leads, so I desoldered those, clamped the leads and resoldered them on. Still didn't have any effect though. Went over all wires, checked for continuity, cleaned the pots again still no effect. Cleaned the jack sockets, and tested with shorting the cable like Phil said, that made another lower pitched hum appear did not silence the noise or the other hum. The only thing that effects the noise is after it makes it's apperance, it goes away and comes back sporadically, but if I switch on the tremolo the noise is always present. (NB this happens only after the noise makes it's first apperance, before that the tremolo works fine.) The tone wheel acts like almost a gain knob for the hum, the pitch of the hum is about "B" or "Bb" so it is not that low "E" hum I hade earlier, that is gone. Changed the 130Ohm resistor into 200Ohm that I got from Malt, it changed the sound of the amp, it compresses more now than before - actually kind of pleasant - no effect on the problems though.

DocZ

[Phil_S]

Doc, you should be able to make an OK solder joint with a 25W iron. 30W would be better. If you have a V shaped chisel tip, place the small flat face of tip so it makes contact with the two parts -- like the cap and the terminal strip, heat briefly and feed the solder so it gets sucked in.

Between every use, dip the tip into a very wet sponge. Sometimes it needs a rub to clean off. The steam will clean the tip, then tin the iron if needed. A clean tip will heat more effectively. A dirty tip will tend to be cold (don't touch).

That problem on the G string could be a bad power tube. I hope not. I have a pair of Sovtek EL83's that buzz on the low A (A2 - = 110Hz).

The tone pot cranks up the hum? Not good, but good. It's a hint. I'd like to bypass the tone stack.

Before you do that, it is really easy to pull V2 and see how the amp operates without reverb and tremolo. If pulling V2 fixes the hum, don't bother bypassing the tone stack. That tells us the problem is on the lower half of the schematic and we can work through that. If the hum stays the same with V2 pulled, put V2 back in the socket and bypass the tone stack.

To bypass the tone stack. Look on the schematic, upper right corner, just after the 1st half 12AX7 for the input and find the .01 coupling cap that is wired to the plate of the tube. Place the jumper after the .01 cap and connect the other end of the jumper on the non-grounded outer lug of the volume pot. Disconnect the .01 cap from the 100K resistor/500K tone pot. (This is not the 100K plate load resistor.) Leave the connection to the reverb tube (goes to V2 grid) in place.

What happens? If this doesn't work, we can explore further. This is the divide and conquer technique. Remove part of the circuit. When the hum dies and the amp works OK, then you know about where the problem is.

[docz]

I tried pulling V2, didn't help. I'll try bypassing it on Sunday. I'll better go to bed soon, it's 3.15 AM and I'm going away on a gig this weekend. I'll update on Sunday once I get to testing without the tone circuit.

DocZ

[Andy Le Blanc]

I've got to jump in and ask.... Is this still the teisco?

If it is... yank all the tubes except the power tube and the rectifier.

Disconnect the coupling cap the provides signal to the power tube from the
plate of the the preceding tube and connect it ( the cap) to ground with a
jumper clip, does it still hum?

If it is still audible the hum is in the power supply or power tube.

Always start on the power side first. An amp is a speaker connected to a power supply.

One issue with old amps is that the insulation in the power transformer starts
to break down. If it hums like this with the input cap grounded, it'll be the PS,
the filament supply, supply filter caps, the power tube or the power transformer, and that's it.

next, un ground the cap and give it a pop test and then run signal.

If it didn't hum while grounded, but distorts with signal it could be ......
the coupling cap, the power tube and/or cathode circuit, or filter caps.

work backwards from the power side thru the pre, adding one stage at a time.

I know I'm sticking my nose in but I'm surprised the issue hasn't been resolved with a little single ended amp.

[Phil_S]

Doc, Andy is a real amp guy and I'm a self taught amateur. Listen to his advice before mine.

Andy, let's say this amp had multiple problems. For example, he finally found a leaky signal cap. The filament string was not properly ground referenced -- this was a major fix in my view. I think, at this point, he's picked the low fruit. It's good that you stuck your nose in. I happily yield to your expertise, which is considerably more than mine.

Phil

[Andy Le Blanc]

You dont have to yield nothing..... I stay busy with repairs, don't give up the discussion you've got going.