New 180W Clone (Converted Fender)...Oscillations, darn, darn

[ic-racer]

I was toying around with putting a Dumble style OD section in my old Fender Super Twin Reverb. I thought about it for a while while looking at the schematic, and then today, decided to go ahead and see what would happen. The amp already has a knob that says 'Distortion' and it has 1 available tube stages for the distortion (though, they are on a 6C10).

I left the tone stack as a standard Fender Twin (I want to keep that great clean tone!)
I removed the existing components of the Fender "Distortion" circuit.
I disabled the graphic EQ to free up a tube stage.
I moved both sections of the Reverb to the 6C8X (they had been split) and eliminated the recovery/mixing stage (to free up the third stage of the 6C10, for eventual conversion to 12AX7). Reverb is disconnected for now.

I put '124' style components on the existing boards and wired up two stages of the 6C10. So far this is all reversible. If I can get it to work, I'll go and swap out the socket for a 12ax7. As it stands, though, I have more NOS 6C10s than 12AX7s

I used 100k/1.5k/4.7uf for each stage. 220k into 100k trimmer for the OD.

No relay, just a DPDT toggle for now.

Got it fired up today and it works. Nothing wired wrong, all components checked ok.

Voltages: (these are two stages of a 6C10)

V2A 212v and 1.5v
V2B 212v and 1.5v

So, here is the problem. I get a spontaneous 6 to 7 kilohertz feedback signal (oscillation) when the OD level is above 7. Below that, it is not spontaneous, but when I play a note I am still getting some oscillations under the guitar note, as the distortion does not sound stellar.

When it is oscillating I have about a 5v AC signal of about 6 KHZ in the lead that comes from V2B plate.

Moving this lead with the wood stick causes it to change pitch like a Thermin . None of the other leads do anything.

Tried swapping tubes, re-routing that lead 4 times (maybe I need to just keep on trying more possibilities)

Snubbers on/off no difference.

No leaky caps.

I need to step back from it for now, maybe tomorrow I will have some more things to try.

[ic-racer]

Here is the starting schematic. I don't have a current schematic yet, I just put things together on the fly.

[ic-racer]

Initial pictures, of the Fender Distortion circuit.

Picture of the components removed.

Last picture is what it looks like after the mods. Components are just from the 'parts bin' so don't be alarmed by all the mismatched brands and types.

The blue wire was the active one, it has since been moved and changed many times but it is still active.

[heisthl]

Decyphering those pics make my head hurt What happens when you ground the grid of OD2? Gone? If so next try grounding the grid of OD1. I would also try clipleading in a .1 to ground on the supply rail to that tube. The problem with troubleshooting oscillations with scopes and meters is false readings and you need to be sure of the origin of the oscillation. Using the "cap on a cliplead to ground" method is faster many times.

[Fischerman]

Don't know how experienced you are so if this is old hat to you please don't be offended.

Generally speaking, grid wires are the 'receivers' and plate wires are the 'transmitters'. Each gain stage inverts the signal (signal on grid is inverted at plate). Positive feedback (like say a plate wire and a grid wire of the same phase in close proximity) makes oscillations, negative feedback (wires out-of-phase with each other in close proximity) generally doesn't.

An easy way to see where it's happening sometimes (in addition to what heisthl suggested) is to put a .01uF cap across the plate load resistor. It will affect the tone...it's just a test. But if desired you can decrease the cap value until it doesn't adversely affect tone much but yet still prevent the oscillation. And you can also trim off some of the high-highs this way if desired (I have one in my amp on OD2's plate R just for tone reasons).

[ic-racer]

Decyphering those pics make my head hurt

My initial idea was just to do a little 're-wiring' of the existing components. Turns out it was more extensive as I wound up only re-using a 68k resistor and .0047 cap and some wires. I used existing holes in the boards.

Thanks for the trouble shooting tips, you guys mentioned things I had not tired. It turns out ,though, there is something fishy with the 4,10,9 section of the 6C10. See below...

BTW: I don't particularly favor the C610 over 12AX7, I just wanted to get this thing up and running before I do a physical mod to the chassis for the 12AX7 socket. So, far all things that have been done are 'non-destructive' to the chassis and boards.

[ic-racer]

Even though I tried multiple C610 tubes, it turns out the tube was causing the problem. I had initially hooked up pins 9,10, and 4 for the second OD stage. So, just on a whim I decided to try the stage I was not using. That would be pins 5,6 and 7. Well, what a difference!! No more oscillations. This is wierd because, as I wrote above, I tried multiple tubes and having the 9,10,4 stage overdriven was bad news in these 6c10s. Perhaps it has something to do with the 'lead dress' of the tiny wires inside the tube.

Anyway, it is up and running and sounds awesome!

Now I have to figure out how to fill the holes on the front panel where the graphic EQ was.

One hole gets a Master Volume (I'm using the original "output" position for the OD master, because it is right next to the "distortion control" which is now the OD Level.)

One hole gets the OD DPDT switch (no relays at this time)

One hole gets the Preamp Boost. For this I just lifted the Mid pot from ground. I need to tweak this a little because the boost is WAY TOO MUCH.

On hole is filled with a DPDT "Reverb Bypass." Now that I have a "one tube" reverb, I am feeding it in between the Master Volume and the PI input. That 100k resistor across which the reverb In/Out is wired does cut some tone, thus I wanted to bypass it totally when not in use.

Bright switch is already included in the preamp as a "Pull" on the Treble Pot.
I tried the "Mid Boost" but, I guess this amp is already wired with full-time "Mid Boost" as when I jumped the 330p cap that is already there, it became quieter (perhaps I could use this as a "Rock/Jazz" switch function?).

So that leaves one hole to fill. The amp already has a footswitch that grounds the tail of the NFB loop for a nice boost, and I will keep this function.

I am going to use the Presence pot in series with the 850R NFB resistor. As it had been wired, the Presence was just another channel of the graphic EQ.

If anyone knows of any cool Fender type mods that that I can control with that last toggle, let me know.

I'm going on vacation tomorrow, and I'll be back working on it in a week.

[ic-racer]

Well, searching for "Fender Twin Mods" got mostly spam and unrelated sites. A search of AB763 was much more helpful. I wound up back at the Dumble section of Schematic Heaven with Steve Ahola's mods to make a Boost switch and a Rock/Jazz switch while keeping most of the original Fender tone stack in place. This is just what I was looking for.

http://www.schematicheaven.com/dumbleamps/mod3.pdf

I actually WANT the Jazz/Rock switch on this amp, as I have used this amp in the past with small Jazz groups where we don't use a PA. The other thing I might try is to get the OD to work with the ES175. I find getting a good overdrive sound with the ES175 very challenging (ie most of the things I try sound crappy).

So with these mods, my 4 toggles will be:

Reverb Loop Bypass
Jazz/Rock
Preamp Boost
Overdrive

[ic-racer]

Another thing I have been looking at is V1B. Since I want this amp to be different from my 'standard' Clone, I was initially going to keep the original values on this tube. It looks like it is a low gain, low distortion setup with a 47k plate, and 850k cathode with NO bypass cap.

I originally thought they choose these values for low distortion, but the more I look at the original schematic, the more I think they were trying to "dumb down" the gain to match the EQ and Reverb, because all three get fed into the same gain stage in the original schematic.

So, it is pretty simple to just switch this V1b over to 100k/1.5k/10uf and see how it sounds. It WILL certainly drive the OD section better. As it stands now, the good OD sound (non-PAB) requires Volume 8 or 9 and Overdrive 8 or 9.

[ic-racer]

I am going through files from Steve Ahola and Schematic Heaven and there are quite a few switchable mods for the preamp of a Fender style amp.

In fact, I am thinking of leaving off the OD and PAB manual switches on the front to free up two more switches for preamp mods.

The original Fender Super Twin Reverb did not have any other way to switch the EQ other than the foot pedal. Since this is a combo amp, the foot pedal should always be available. So I think I'll hook it up so that it will require the foot pedal for PAB and OD, otherwise these two relays will be off when the foot pedal is not hooked up.

I have some parts on order, I'll post more as I go along.

[ic-racer]

So here is the soundclip. This is has STOCK V1 values and #124 values on V2 with NO snubbers. PI and Output section & speakers all stock.

http://soundclick.com/share?songid=6499149

[ic-racer]

Mods done today were:

1) Change the "PAB" so the tone stack is left hanging by a 47K resistor when engaged. This made the change in level more similar to my Dumble clone. (I will probably convert this over to a Dumble style PAB once my 100k mid pot replacement comes.

2) Changed V1b from 47k/850r/0uf to 100k/1.5k/5uf. The main intent was to get more gain into the OD section. There was an increase in gain, but it was not dramatic.

3) Changed the OD entrance. Again to get more gain in to the OD circuit. Went from 220k feeding 100k pot to 64K feeding 100k pot with a 47k tail to ground.

These changes made it so my good OD tone now comes with Volume on 4-6 and Overdrive Level on 6-8, which is much better than before. Before, both controls needed to be at about 9. The overall change in tone is very small.

[ic-racer]

Here is the schematic so far.

[ic-racer]

Mouser order with the relay parts to arrive today. Put through an order yesterday for the parts to swap out the 6C10 to a 12AX7.

In the mean time a Strat style soundclip:

http://soundclick.com/share?songid=6505212

[ic-racer]

I got some fishpaper but it was gray, rather than black. Turns out the original Fender comes with a "Spare" circuit board under each of the existing boards .

I wanted to save all those chokes on the original board, rather than modifying the original board. There were components to the reverb on that board also, so I removed the reverb components and moved them over to the 'spare' board.

Now I am ready to put the eyelets for the relays in place. I will use my new gray fishpaper for the backing board.