First build, hum issues

[joshdfrazier]

So, I built a clone of an epiphone model 101. I have the schematic and the amp itself, and I've copied the circuit verbatim. All I get when I power up is loud 60hz hum. No line signal, nothing changes when the pots are turned. I've eliminated the possibility of faulty components, and checked my work over and over again with no luck.

I'm attaching pics because maybe there's something really obvious that I missed.

[Diablo1]

My guess is that you have something wired wrong or forgot to solder something. It's always a good idea to twist your heater wires tightly together to minimize hum. I don't see any of your wires twisted. It would help if you measured voltages at each tube pin and posted the numbers. It would also help if you posted the schematic right side up - I tried standing on my head, but I couldn't concentrate for very long.

[Reeltarded]

New amps have to be taught all the songs.

Yeah. It's why they hum, at first..

[M Fowler]

Hmm the image is fine for me?

But is this an amp or preamp? Where is your power transformer and output transformer on the bench or two chassis unit?

[Blindog]

Hmm the image is fine for me?

But is this an amp or preamp? Where is your power transformer and output transformer on the bench or two chassis unit?

The image appears to be posted upside down. Like Mark said, where are your output and power transformer located?

I see several areas that could be problematic. Did you sand or grind the paint off the chassis areas where your grounds are located?


Mark

[Jack Hester]

Most of my experience, since my return to tubes in recent years, has been the repair/restoration of old amps. 90% of what I've found on them, with the 60Hz hum (regardless of pot position) has been the decoupling caps in the power supply.

As yours is a new build, I'd say begin by lifting the signal wiring and see if you still have a hum with nothing going to the power amp. If it is quiet, then begin by reconnecting the next signal wire.

On the old amps, I ground the signal wire at the grid of each tube, starting at the power amp, and working my way back, grid by grid. Sometimes, when the amp is particularily messy (p2p), I will put a cap on the end of the ground wire, so that if I somehow make contact with the B+, I don't create a bigger mess. Ground works much easier, but that depends on your comfort level doing so.

Jack

[Structo]

Where are the Xfrmrs?

I see a couple suspect connections that aren't soldered.

I circled them in yellow.

[eniam rognab]

its a new take on the OTL, the PTL

but surriously, i hope you get it figured out

lotsa wicked smaht folks round here, ayuh

the schematic appears upside down until i click it weird

[joshdfrazier]

Ok, something wasn't grounded correctly. It was a very simple fix!

And yes, transformers are on the bench! The power transformer has been installed, now that the hum issue has been debugged. Output trans I ordered is bigger than I wanted, so it's going to be saved for another project, but will suffice for testing purposes until I get my tiny one ordered. Thanks for all the tips though, I know the guts look like a birds nest, lol.

Now another issue... There is no line signal getting to the power section.

When I turn the tremolo on, i can hear the very faint hum oscillating, so I know the power and trem circuit are functioning well at least!

I figured the hum issue out shortly after posting this, but this one has me really stumped. Any suggestions?

[Phil_S]

For starters, try the pop test. Set your DMM to DC voltage. Black lead to ground. When you probe the plates of V1, do you get a pop at the speaker? If you get it at the second triode and not at the 1st, you have narrowed the location of the problem. Also do the pop test at the grid of the EL84.

[joshdfrazier]

phil: i did the pop test, that was a no go. turns out i put a cap in the wrong place. bear in mind, this is literally the first thing i've ever built. no pedals or anything, so i think that justifies my stupid mistakes, hah.

i suppose the last (hopefully!) issue to address is the ear piercing squeal accompanied by lots of distortion that occurs when the volume is turned up about a quarter of the way.

i have experience this issue before, but it was due to a tube going bad/microphonic. all of the tubes used in this amp are in perfect health, so no go there.

what should i be looking for to eliminate this horrible noise???

[Jack Hester]

In your schematic, one of the inputs has a closed circuit jack that grounds the signal when nothing is connected. The other jack is an open circuit, but the grid is still tied to ground through R1, acting like a grid leak resistor.

Verify that your closed circuit jack is working, to complete this path. You may want to experiment with a 1 meg resistor tied between the grid and signal ground, as well.

I had a Princeton 5C2 (scratch-built) that had two closed circuit inputs, and it squealed like you said. That's when I found that neither of my cheap Radio Shack closed circuit jacks worked, meaning they stayed open. When I completed the path to ground, squealing went away. Connecting your guitar will serve the same purpose, in completing the path.

Anyway, just another place to check, as it's a new build.

Jack

[Structo]

Also bear in mind that single ended amps such as this tend to be a bit more noisy and hum due to the lack of a push pull type circuit that cancels out a lot of hum when properly built.

[joshdfrazier]

ok, so all of the issues were sorted out. squealing was a result of a lead running from the volume pot to the preamp tubes 2nd triode being too close to one of the OPT leads. also had to reverse the primary on the OPT.

alot of the hum was resolved when everything was assembled and grounded to the chassis, too. there were some strange noises when certain pots were turned all the up or down, but all of it was resolved by moving wires around inside the chassis. it's dead quiet now, and sounds great.

my b+ is 250, almost exactly. i tested the AC from the PT to the rectifier and got about 238v AC. does this seem a bit low for a 500v transformer? it is a NOS (never used) Triad from the 50's.

i would like to put something like 320v on the el84. how large of a transformer would i need to achieve this? also, with a larger PT, will i need to change the values of the resistors supplying B+ to the other tubes?

[Structo]

Can you measure the DC voltages and post them?
(chassis ground the black lead and probe with the red lead)

V3 pin 7 =

V2 pin 7 =

V2 pin 9 =

V2 pin 3 =

V1b pin 6=

V1a pin 1 =

V4 pin 1 =