Help! I tried to recap my amp!

[Phil_S]

One thing that puzzled me was that there was high DC voltage going out to the output transformer. I thought that the speaker only wanted AC? [\quote]
Good question. You have it backwards. The first node on the B+ rail, where you have 300VDC...this supplies power to the EL 84 through the OT. The OT drops 30V and what comes out the other side provides plate voltage of 270. What you are not seeing is the signal output from the plate to the transformer, yes that is AC. Don't be concerned about this. The voltage is appropriate for an EL84.

Strangely this "rats nest" is beginning to make sense to me I can now easilly spot the different sections on the schematic and inside the spagetti of components.

Yes, things become clear with time and effort. It is not surprising. Soon, you will sort this out.

In general, the voltages you posted are reasonable. This is a good sign. As MBD115 says, we'd also like cathode voltages. On 12AX7's expect a between 0.5V and maybe 5V at the upper end, or maybe a bit more.

Once we have a complete voltage chart, I think suggestions for trouble shooting will be offered.

While you are in there getting the cathode voltages, probe each grid pin on the 12AX7's (pins 2 and 7). Confirm that you hear a noise (pop, buzz, hum) when you probe each one. This will help us to know if the ciruit is passing a signal to the EL84. DC grid voltage should be zero or very close to zero.

[docz]

Ok, here is a complete chart. I can also confirm that touching pin 2 and 7 makes it buzz. Also quickly touching pin 1 on V2 and releasing it just after a loud pop removes the hum temporarily, touching pin 6 on V1 in the same fashion removes the static temporarily - only it comes back much faster than the hum.

DocZ

[MBD115]

Can you post a sound clip? It don't need to be anything fancy, just a sample of what this amp sounds like.

I would like to hear you playing it normally.
I would like to hear the static and the hum when you're not playing.
I would like to hear how the volume changes the noise level.
I would like to hear how the tone control works.
Hearing the verb and trem on/off/full range would also be good.

A clip would be most helpful I think.

[docz]

Ok, I will post a sound clip later.

[docz]

Ok, now I just completed recording.

I used a cigar microphone as close to the amp as possible, and I did some different takes. It is played on my '56 GoldTop Reissue with Fralin P-90s.

I did notice something, the neck pickup gave me less hum than the bridge, and in hum canceling mode (center position) it didn't affect the hum, this is strange..

The distortion you hear is not from the amp, it is all the junk on my desktop rattling so just ignore that..

[docz]

Seems like I only could upload some clips in one post so here are the rest.

[ampgeek]

Hey Docz,
Forgive me for not following this all in great detail and this has been covered previously.

But....I couldn't help but notice how much the "Noise...with volume..." clip sounded EXACTLY like an old Silvertone amp that I repaired recently.

Turned out being the V1 socket which looked like the same material of construction as yours.

One recommendation first: If you have another amp and speaker of some type that you know works, check out the Amp Stethoscope at the end of the pager here:

http://www.el34world.com/Hoffman/tools.htm

It really is the most simple and useable diagnostic tool in my box. A picture of my implementation is attached. The volume knob is a must from my perspective.

This tool will allow you to "walk" through your ailing amps circuit path and hear exactly where the noise is being injected. I always start the trek by probing the grids of the tubes. Then work back through each component between a good spot and a bad spot. You will narrow down the problem spot in a jiffy. It is also excellent if you have multiple issues to sort through.

It is well worth the time it takes to build!

Good luck,
Dave O.

[docz]

Hm, I think I can build that.. just need to find another amp though.
Is that cap electrolytic?

Did you replace the socket, or did it just need cleaning?

Doc-Z

[MBD115]

I like the tone of that old amp. It has that old school bluesy finger plucking tone. Very nice.

To me the signal sounds fine. You might have a scratchy volume pot, I can't tell with these speakers here, but the signal sounds good to me.

I think I'd check out all the grounds. IMO its very likely a ground connection, but I don't know. How I do that is hook the black lead of my multimeter to the chassis then touch the red lead to the chassis to make sure I have a good connection then I start going through the amp (using the schematic) checking for ground at each grounding point and also checking the value of each component thats connected there. For instance: Your schematic shows a 38K resistor to ground on the input jack. I would touch my meter to the grounding end of that 38K resistor and get a reading then touch it to the other end. This should give me a reading of good ground on one end and 38K on the other. Sometimes resistors won't read correctly unless you lift one end of them so if I don't get a good reading from a resistor I then honestly consider lifting one end to get the reading. Besides checking the condition of the grounds this also checks many of the components.

The digital meter I have can also check capacitors. I don't know if it will work when a cap is actually installed in the amp but I do know it works on caps that are not installed. I would try it on all of the installed caps and even lift one end of them if necessary to get a good reading.

I would also check the condition of all of those ground lugs. A tiny bit of corrosion there can make a huge difference. All of this with the amp turned off and the caps drained of course.

This sounds like a lot of work but its actually easy to do and isn't all that bad. This will check many of the components. I like to have a printed copy of the schematic and I highlight what I've checked as I go and I like to write down the values I find for each component for future use. When I get to the end I know what I've got.

I'm sure someone will know exactly whats wrong by listening to those clips you posted.

Good luck

[martin manning]

RE amp "stethoscope," check out this simpler version suggested in another thread here:

https://tubeamparchive.com/viewtopic.ph ... 270#105270

MPM

[Cornelius]

Just got a tip that old carbon resistors at the plate often makes that scratchy noise when they're ready for the bin...

Maybe try to swap those plate resistors; at least on V1?

[ampgeek]

Hey Doc,
Nope. That is a paper in oil cap. It was the only 0.1uf/600V cap that I had on hand at the time. Any style of capacitor with that rating will work just fine for this application.

I ended up having to replace the socket. I tried cleaning, re-tensioning the pins and re-soldering it to no avail. Not sure what technically was going on with the socket but replacing it eliminated the "frying bacon" sound.

Cornelius is absolutely correct on the carbon comp load resistor. I commonly see just that in old amp re-habs. But...it was the character of the sound of your sizzle that really caught my attention. In my experience, failing plate load resistor sizzle sounds a little different to my ear.

Of course, it is a whole easier to replace a resistor than a socket and hearing it live vs. a recording might put a skew on things so slapping in a new resistor should be strongly considered.

I should also mention that the test amp will need to be connected to a dummy load (as opposed to a speaker) to prevent OT meltdown when you are probing the signal path.

Good luck!
Dave O.

[docz]

I don't have any .1uF I have 10uF .022uF and 33uF any of those work?

Can't it be connected to the speaker?

DocZ

[Cornelius]

A speaker would present a too low impedance load for tubes; that's why a Output Tranny are necessary...

I would guess that your .022 would work. Might filter out hum frequencies though?

If you're short of amplifiers, you probably have a line-in on your pc?

[Structo]

From GEOFIX:
Hiss and noise can be caused by:

noisy tubes (try different preamp tubes)
Noisy plate resistors. The carbon composition resistors used on the plates of preamp tubes often go very noisy, especially in older Fenders. If swapping tubes does not fix the trouble, locate the circuit that seems noisiest by tube swapping, and then replace the plate resistors in that circuit with equal-value metal film or metal oxide resistors. You can use carbon comp if you can find them, but the problem is likely to recur.
Noisy resistors in the B+ decoupling string, often around 10K in value
Unusual: An ultrasonic oscillation can cause an intense insect-like hissing that sounds very much unlike normal hiss.

Rare: leaky coupling capacitors or faulty controls
Rare: a slight ongoing arc on the output tube socket(s)
Rare: a bad solder joint somewhere in the signal chain
Rare: internal arcing or noise in almost any part in the preamp section.

A good way to divide and conquer is to turn the volume control(s). If the hiss changes levels as you do this, then the source of the hiss is something that affects the stages of the amp before the volume control. A faulty, hissing preamp tube will be turned up this way very quickly.

Conversely, if the volume control does not affect the hiss, the cause is somewhere after the volume control. In general, the volume level of the hiss is an indicator of where the hiss is occurring - the louder the hiss, the more likely its source is near the input of the amp where the gain applied to it will be the greatest

The procedure of locating by removing one tube at at time working from the phase inverter/driver back towards the input until removing a tube no longer stops the hiss should then localize the problem to one tube's worth of circuitry.