Hum is gone in scratch 5f6a; new problem

[lmolter]

When I last left here a month ago, I had a hum problem in my scratch-built Bassman. It was volume control dependent and definitely in the first preamp. Shielded cable and some ground mods did not help, so I decided to take out the eyelet board and the cap board and seriously rethink my grounding.

What transpired was a removal of the brass plate, the addition of a bus wire across the backs of the controls, the eyelet board ground and bus wire and preamp reservoir ground and phase splitter reservoir ground all grounded to the input jacks, and the bias supply ground connected to the main reservoir ground at the PT bolt. The result? No hum. Quiet as a church. And since I replaced the preamp plate carbon resistors with metal film resistors, I perceive the hiss to be a bit less. Whatever hum there is seems to be related to the guitar cable being connected. I can live with that.

So what's the problem, then? It 'thunks' when the standby switch is turned off. It's actually a 'thunk, squeal' kind of sound. It dies down quickly, but the amp didn't do that originally. It almost sounds like the amp wants to oscillate a little when the B+ is suddenly applied. Should I be concerned? I keep expecting the lid to blow off the cap doghouse each time I throw the switch. However, other than the presence pot not working (probably a bad solder job on the back of the pot -- it took a lot of heat to get the solder to stick) and the thunks, I'm pleased again.

In retrospect, I think the hum problem may have been the bias supply ground all along. I had it connected to the preamp ground originally.

Larry

[lmolter]

Hmmm... no responses. Odd. Anyway, I had a revelation that when I rewired the cap doghouse, I somehow switched the leads for the screen supply and the phase inverter supply. I haven't proven this yet because it's too cold in my Tampa garage this week to play with it. All that I've noticed so far is that the screen voltage and the plate voltage are the same, at about 474V. This is higher than it was before I rewired it. And the screen voltage should be a tad lower. I'll let everyone know (as if anyone wants to <g>).

Larry

[tubeswell]

Hi Larry

Filter caps discharging probably

Re: the screen and plate voltages, it should be okay if they are the same, although text books say to keep screens lower. If you're concerned enough to put 3W-5W 470R-1k5 screen grid resistors in, that wouldn't do any harm

[txstrat]

It 'thunks' when the standby switch is turned off. - It almost sounds like the amp wants to oscillate a little when the B+ is suddenly applied.

I'm a little confused.
Does the amp make the thunk-squeal sound when you switch it into PLAY mode or when you switch it into STAND BY mode?
If it does when switched into ST BY I would agree with tubeswell and vote for discharging filter caps.
Is there a difference if there's a cable (instrument) in any input jack?

Matt

[lmolter]

It IS confusing when discussing the state of the stand-by switch. So, to clarify, the 'thunk-squeal' is when going from StandBy to Play mode (standby off). The HV is being applied. And it doesn't matter if an instrument is connected or not. I also forgot to mention that the amp clips badly at high volume.

A little history: When I originally completed this build, it did not thunk. The tone was clean at low volume and it broke up nicely at the higher volumes. However, there was an annoying hum that was volume control dependent. To remedy this, I rewired all the grounds according to a diagram I found on the el34world site. In essence, the power tube, screen and bias supply caps, and all center taps were grounded to the PT bolt. The controls' buss wire, eyelet board, and PI and preamp supplies were grounded to an input jack. Hum gone. But I obviously wired something differently because the amp behaves differently now.

What I noticed, aside from the plate and screen voltages being higher and the same, was that the B+ bleeds off much more quickly than it used to. This is a good thing, I guess, but nonetheless it is a change in behavior. I firmly believe I have mixed up the screen and PI supply wires, with the consequence that the choke may be in parallel with the 10k resistor, or something like that. As soon as it warms up a bit in my garage, I'll venture out and connect the DMM.

[tubeswell]

Okay - 1st thing to do is move everything into your house for the winter.

Then check the re-wiring

[lmolter]

We Floridians can't do a darned thing when it gets below 65F -- blood's too thin. Luckily, the cold spell is temporary. Anyway, I plan to take a closer look tonight (between tv shows).

L

[lmolter]

Here's what I have so far: Took out all tubes except output and rectifier. No thunk. PI back in. No thunk. Cathode follower, no thunk. Preamp. Bingo! Squeal, thunk. Swap with 12AX7. SQUEAL, THUNK.

I didn't mention earlier that I also rewired the input jacks. Because I didn't want to get too far and discover I had hum, I only wired the low-gain inputs (the ones without the 1M resistors). Also replaced 100K plate resistors with metal film. As a side note, these resistors have 4 color bands, brown-black-black-orange. I've never seen 4-band resistors, but they measure 100K, so there's nothing to this.

Back to the jacks - tips connects through 68K via RU-174 shielded cable to grids. And shorting pin is grounded. Hmmm... not sure if I checked the standby switch with inputs grounded. But it sounds like (no pun intended) the problem might really be in the front end. I feel a little better that I didn't screw up the power supply wiring. I'll have more later.

Film at 11.

[lmolter]

All fixed. No hum and a little hiss. I _believe_ I had a cold solder joint somewhere that was causing the HV on the preamp stage to be way above nominal. So while I was moving wires around trying to isolate the bad connection, a lug broke off the normal channel volume control. I ordered all new solderable-back controls (I will never, ever, use the non-solderable ones again), ran a nice bus wire across them, cleaned up the input jack wiring, moved the 68K's back to the eyelet board (tsk, tsk) mainly because it was real tight around the jacks with all the resistors, and crossed my fingers.

Well... I must have fixed the problem because all the voltages are right on (20% high because of the vintage Mercury transformer), signals are clean (via the scope), and I'm satisfied once again. As you know, I went through a low period there where I couldn't find the source of the hum. Well, I'm ok now.

Now, it's time to start saving for speakers and a cabinet.

Thanks to all who helped.

I think I'll try a 5E3 combo next. And maybe guitar lessons. It's kind of embarrassing testing the amp with only a 3 chord progression. I wish I could lay down a Chuck Berry intro or a good blues solo. Oh, well...

[gonziak]

Hi I have a question. Do you have noticeable hum on bright channel with vol max and no guitar jack?
I convert my JTM45/100 to 5f6a (transformers, tubes, pwr supply to gz34, choke 4,7H). I have a noticeable hum on bright channel with vol max. When jumping channels together with a short cable, there is much more hum (good quality jacks and cable). Would you send me your grounding draw? I tried different grounding. I found the best result with brass plate but not perfect. Thanks for any help

[loudthud]

Do you hear the Bright channel hum when no cable is plugged into the Bright channel? That could be a defective input jack.

Perform the resistance checks in post #10 in this thread: http://music-electronics-forum.com/t12667/

[gonziak]

Hi, thanks I found the problem, I change the grounding to Hoffman way. It was a ground loop in input jacks. I linked the channels on board and the hum disappeared.