5f6a still making trouble

[j-stylez]

I recently built a fender 5f6a alike. Its dead quiet, no noise, very nice sound. But one Problem.

I tried to record the problem, its kind of hard to make out. If you listen to the end of the sample, you can hear distortion that really cuts everything else out. Its hard to record because I dont want to break my speakers, and this is really shrill popping loud harsh noise!

crap.mp3

This problem shows up:

- only when i set the volume of the amp high, maybe starting from 1/4 of maximum

- only from certain strenght of input signal on, so I can set the amp really loud and play a chord carefully...no problem. Clean sound. When i hit the strings harder, there is a certain point when suddelny this extremely ugly harsh shrill motor sound distortion comes in, followed by a bit of fizzy noises.

Any ideas? I already shortened the grid wires... Could it be due to something in the preamp stage? Maybe because I dont have any 12ay7 but only 12ax7 to use in the first tube v1?

[MWJB]

A 5F6A should function "normally " with a 12AX7 in V1...most guitar players actually prefer a 12AX7 over a 12AY7.

What size grid stoppers did you use on the power tubes?

Do the treble & presence settings have any effect on the distortion?

You say that you shortende grid wires, good move, did you shield them too? Can you recap on any remedial changes you have made?

Any pics?

[thumbs]

This is just a wild guess. You may have a bad or slightly bent input jack where the switching tab is not making a solid connection and vibration is setting it off. The jacks that come with some of the amp kits are not of the best quality. You can test for this by plugging into each of your inputs. If the problem goes away when plugged into one of them, replace the jack.

[Tom Phillips]

This is just a wild guess. You may have a bad or slightly bent input jack where the switching tab is not making a solid connection and vibration is setting it off. The jacks that come with some of the amp kits are not of the best quality. You can test for this by plugging into each of your inputs. If the problem goes away when plugged into one of them, replace the jack.

Along the same lines it could be a bad connection somewhere else in the wiring. Unplug the guitar and with the amp on and volume turned up thump on the cabinet with your fist. Do you hear noise/static/popping. If so something is loose. Could also be a dirty tube socket pin etc. Poking around with a chopstick (or other non conductive device) is a common way to isolate these types of problems.

[Chuck H]

I'm going with MWJB. It could be a parasitic oscillation due to layout or grounding. Some inside pics and tube pin voltage readings would be a good thing to have now.

[booj]

Wow. That's nasty stuff! It's similar to what I'm getting from a 66 Super Reverb, which I have a recent post about. I'm staying with you on this one. At this point I'm going to record all DC voltages on all pins, and set up a headphone listening hookup. Then listen to 2 or 3 different output tranny's I've got. If I get a clue from that, or not, I'll let you know.

[j-stylez]

Here are some pictures of the inside. If you are worried about safety, there are still some areas that need shrink insualtion. However I dont want to finish this until I found out what the problem is.

Pin Voltages:




It's biased at 28mV

Might it still be the too long grid wires? I already shortened them and used shielded cable, but I cant get them any shorter. I would have to move a tube socket for that or change a whole part of the layout... that would mean so much work.

[Chuck H]

There are some suspect voltages. It'll help more if we have ALL the pin voltages. It's the only way we can know the bias condition of the gain stages. Photo's are fuzzy byt the work and layout look OK. I might push the input leads to the right and add some 220k 1W voltage balancing resistors to the main filter totems.

[tubeswell]

Hi niggez, If you tap the chassis, or brush your fingertip across the chassis edge (with the amp on), does the amp pick up a microphonic distortion? (I am wondering whether the chassis is possibly not rigid enough and may be shaking everything around a bit).

[j-stylez]

Thanks for the interest in my problem.

I will post more voltages tomorrow, I got interrupted before and couldnt finish.

There is no response when i shake or brush the chassis. Chopsticking some of the wires around, for example OT leads, doesnt have any effect. Up to a certain setting on master volume the sound is nice, clean, no problems.

I checked whether presence and treble have an effect on the problem at high input signal and high volume setting, and it does change the tone a bit. However it sounds to me as only the effect from changing the overall tonality of the amp. So I dont think this influences my problem.

[MWJB]

add some grid stoppers feeding pin 5 of the 6L6s, start around 5.6K, see if that helps, if so you might gradually reduce to a size that works at the least value.

Your screen voltages are a little higher than the plates, not much & may not be a cause for concern but, it is unusual in this circuit.

[j-stylez]

It turns out the grid stoppers fixed it!! I didnt know that this was the sound of blocking distortion...wow...such a simple solution for the big problem!!

I can use the whole range of volume now, no problems. Dead quiet, the only buzz coming from my single coil pickups.

Insane headroom, theres hardly any overdrive even at the highest volume level. I guess the higher b+ is responsible for that.

[MWJB]

500vdc B+ isn't outside of "normal" range for a 5F6A.

Glad you got it sorted.

[j-stylez]

yes thanks again for everyones help.

so the next thing i did after fixing it up was to add this mod:

Hot switch

this gives me very nice control over the overdrive at nice volume levels. One question I have is, how is the tonal difference between jumpering pin 8 and 3 on v1 opposed to seperate different values as in the mod? Is there a major tonal difference?

Now if the bottom end is a little too lose for my taste still, where should i make changes? At the tonestack, or add maybe a treble-bleed on the master volume? But I dont actually think it lacks treble, just too lose bottom end even when i turn the bass pot all the way down. This goes for both channels. So mabe change the tonestack to be more marshall-value? Its too muddy as it is.
Or should i better be looking at the first gain stage? I still have the 6uF cap on pin 3 of v1 parallel to the 2k2.

[Chuck H]

One question I have is, how is the tonal difference between jumpering pin 8 and 3 on v1 opposed to seperate different values as in the mod? Is there a major tonal difference?

Yes, jumpering pin 8 and 3 would parallel the cathodes and make cascading the stages as per the Hoffman mod impractical. You don't have a Plexi though. You have a 5F6A. How much of the Hoffman mod did you do? It doesn't work correctly unless you do the whole mod with the seperate cathode circuits and use the component values shown.

Now if the bottom end is a little too lose for my taste still, where should i make changes? At the tonestack, or add maybe a treble-bleed on the master volume? But I dont actually think it lacks treble, just too lose bottom end even when i turn the bass pot all the way down. This goes for both channels. So mabe change the tonestack to be more marshall-value? Its too muddy as it is.

With those high cathode resistors and .68uf cathode bypass caps I would think that amp is bright, not muddy. For what you seem to want I would change the treble cap to 470pf or 500pf. Do you have a .68uf bypass cap on pin 3 of the second preamp tube or no bypass cap as in the 5f6a schematic? are you using 270k or 470k mixing resistors? there are a lot of variables with component values. The Hoffman mod works on a Plexi circuit, not a 5f6a circuit. If your going to use the Hoffman mod you need to do the whole circuit.

Or should i better be looking at the first gain stage? I still have the 6uF cap on pin 3 of v1 parallel to the 2k2.

See, now this is a mistake right here. That's not suppose to be a 6uf it should be a .68uf. This cap value will strongly affect the bass through the circuit. You either need to make the amp a 5f6a or a Plexi or a Hoffman modded Plexi. Trying to hybrid the three won't work. They are different circuits. It's like doing an oil change on your car. You can't just drain the oil, put on a new filter and then not pour the new oil in. That's not two thirds as good as an oil change. You need to poor in the new oil or it doesn't work.