5f6a still making trouble

[ThermionicScott]

What are you using for coupling caps to the output tubes? Changing from the stock 0.1uF to 0.047uF (or even 0.022uF) may tighten things up nicely for you.

Unrelated, but what value of grid-stoppers did you use?

- Scott

[j-stylez]

I used 5.6k gridstoppers.

I coulndt try it with another value in place of the 6uF cap because I coulndt find one. I have lowered it to .68uF and it sounds very nice now.

I will still try different coupling caps as you mentioned, just to see what the difference would sound like.

[j-stylez]

Something else came up. I think it might be a failing resistor somewhere, however I am not sure.

It is like the sound of gushing waves, not too loud but definately there. Kind of like a swishing noise, like surf of the ocean.

I switched out the grid stopper resistors, as I wanted to lower them anyway to see how low I can go, to 4,3k. But I guess they shouldnt be the problem anyway?

This strange noise is affected by both the presence control and tone controls... I can still hear the instrument, but more faint than it should be. I was wondering how I could pinpoint this? I checked all resistors resistances with the meter, and they seem fine...

By the way, I implemented the whole hoffman mod for the preamp stage, so seperate cathode bias and component values of the hoffman schematic. So I have a switch that allows me to switch in another gainstage, which is really useful. The problem is persistent on both settings, so I guess I could already rule out the extra gain stage as being a source of the problem.

Could it be a faulty coupling cap? If so, how could I be able to trace it? I faintly remember reading that a leaky coupling cap could sound like ocean surf...

[j-stylez]

I went through the noise checklist again but I cant seem to find the troublesome component. Any ideas welcome!

[Jazz P Bass]

A leaking coupling capacitor will have high voltage dc on one side & ideally, depending on the circuit, zero volts on the other side.
You can check whether or not they are "leaking" with a volt meter.

[Chuck H]

Early on I posted here thinking I had leaky coupling caps or a conductive circuit board. NOT realizing that the charge across the tube would build a small voltage at the grid. When checking caps for leakage you need to either lift the uncharged end of the cap or in many cases you can simply pull the preamp tube prior to testing.

It sounds to be like a circuit fault or a bad tube. If you can trace the signal through the amp that may show where the problem is.

[j-stylez]

You know, I just pulled tubes as I was reading this reply. It was indeed a faulty tube in the first preamp section. Problem solved.

[gonziak]

Hi, I had the same problem yesterday when I changed the power tubes from Sovteks 5881 WGC to Tung-sols 5881. Before, I had no grid stoppers with Sovteks and there was no fizz problem. I searched the forum and found the topic. I added 5.6k grid stoppers to tungsols, it sounds great now. Do the grid stoppers affect the sound? How about different values? Why they're not added to any schematic or contemporary kits' layout? Do you have them in your 5f6a?

[tubeswell]

Do the grid stoppers affect the sound? How about different values?

If the grid stoppers are big enough, they will interact with the inter-electrode capacitance of the tube, to cut highs. But if you keep them small enough (say under 4k7), they don't make any audible difference.

[gonziak]

Thanks. What is the lowest safest value? Should I experiment?

[tubeswell]

Thanks. What is the lowest safest value? Should I experiment?

There really is no 'unsafe' value, it depends on whether they do the (dual) job of eliminating stray induced HF noise, and/or reducing blocking distortion. For an output tube, somewhere between 1k and 10k will have some useful effect without cutting too many highs. For a pre-amp tube, the size of a grid stopper can be much higher (22k-1M or so) depending on the application and the type of tube

[gonziak]

I experimented with different values and hoffman layout sounds the best in my case - 1k5 grid stoppers and no 47pf cap.
Thanks a lot.