deluxe reverb distortion

[pdf64]

I know (well, I'm almost certain) it was the PI clipping because I also instaled a post PI MV by connecting a large cap and a pot between both PI outs (that cancels 180 degree out of phase signal, for those who might not know). So with little signail to the power tubes I could tell that the PI was clipping.

That type of master vol works by forcing the PI to clip at progressively lower levels. I'm not sure that your test proves that the PI is clipping; rather it is proving that your master vol is working, by forcing the PI to clip before the power tubes.
With any properly implemented design, the power tubes must clip before the PI, which, given suitable control settings, must clip before the preamp.
Otherwise the amp could never produce full power.
Pete

[el_Fela]

Hi pdf64, are you sure about that you said? how does a MV force the PI to clip earlier?

AFAIK, phase cancellation doesn't have anything to do with tube clipping. Of course the PI is going to clip earlier than the power tubes, and thats exactly what I wanted, to be able to distinguish between preamp and power clipping.

I tried everything on this thing, and colder bias was the answer. Still don't know what is the cause.

I ended up with 2,2UF tantalum bypass caps for the preamp section, 1,5k cathode res for V4, smaller coupling caps at the PI, etc. Bass is kinda tighter, more clear, but everything is more "woody" sounding, not so pleasant sometimes.

[mmmoser]

In a recent build of mine for this circuit I do notice the modern transformers being listed as the proper AC voltage and using a 5AR4 rectifier will hit those power tubes with some pretty high voltage.
Fender did this in its designs but on the schematic I have seen from Fender the voltages are lower than I was getting by some 75-90 watts. Once I dropped the rectifier down to a 5U4 and did some zener tricks, I got the voltage back to near the Fender spec- I was using a Mojo Deluxe Reverb PT in this amp. I could not keep my NOS 6V6 tubes from red-plating (and what a waste that is to see those little beauties red-plate!). The sound from them was terrible until voltage was tamed and bias brought back into line.
I think your cooler bias is right on and check your DC hitting those power tubes. I suppose we could switch to JJ 6V6 (more like 6L6) power tubes that can take the higher voltage.

[pdf64]

The cross line type of master volume, see link below, works by driving each PI side into each other - matter meets anti-matter!
If you are hoping to assess PI performance, eg output voltage swing, using this method you'll get a distorted impression.
http://music-electronics-forum.com/atta ... volume.jpg

Better to use the dual section pot method, see http://imageshack.us/photo/my-images/115/kfmvrr3.jpg/
as, when turned down, that allows the full output from the LTP, unecumbered by loading of the grid clamping from the output tubes or the cross line type master.
If you just want a master volume then use whichever suits you best.
Pete

edit - sorry about previous image links

[rooster]

jjman, I don't see this resistor in series to the cathode cap that you talk about. What year was this or can you make a drawing of it, or well, just state the orientation and values please? Thanks.

I definitely think this comes from the reverb circuit and typically just turning off the reverb will demonstrate this. The screen shots from jjman were priceless.

Since the reverb tranny receives so much voltage and current, it also seems possible that the transformer itself is responsible, as Mark suggested. I currently have a used Fender part in my SR and I can't help but wonder if the reason I ended up with it is because it displayed this behavior in someone else's repair. It's possible IMO, but I am too lazy to change it I suppose.

Anyway, I thought I was the only one dealing with this and reading this post was a nice surprise. If Fender solved this problem with lead dress then my hat is off to them.

[rooster]

BTW, why is this page fubar? It extends beyond the typical window view. Has TAG reached its posting limits?

[Structo]

It is sideways because of the the links above.

Too long.

[rooster]

Thanks for the explanation, Tom..

Jjman, are you in the house? I am still wanting to pursue this cathode change.

[jjman]

I used the 100 watt Twin Reverb schematic as a reference which shows a 470 ohm cathode resistor and no bypass cap. The DR uses a 2200 ohm + a 25u bypass. Twin shows 6v which means 12.8 ma total (6.4ma each side.) 12at7 can handle this, 12ax7 wouldn’t be happy. DR shows 8.7v so 4ma (2ma each side.)

I think I used a 470 for the mod but it may have been a little higher for paranoia curtailment. Running this way with no bypass cap (as shown in the Twin) in my DR made the reverb signal output weaker than the stock setup. But the idle was hotter and the bottom of the wave no longer reached cutoff. I then put the 25u back in (to bypass the ~470R,) and it was about 50% stronger than stock. I added a resistor in line between the 25u and it’s ground connection to bring the swing back to the stock output at the same input. I can’t find that I took a pic and I don’t remember the value that worked well. I’m pretty sure it was less than 1k and more than 220R. The scope shots are not max volume and clipping can still be reached. But there is less clipping at the same output strength as shown.

Keep in mind this change increased the idle current significantly. The 12at7 should be ok and they are cheap. A vintage reverb tranny is an important component. I was not able to find any spec on line regarding the max idle current rating of the vintage reverb trannies.

[pdf64]

Twin shows 6v which means 12.8 ma total (6.4ma each side.) 12at7 can handle this

?It seems to be above the 12AT7 max ratings?
Based on a plate to cathode voltage of at least 430V (significantly above the 300 max) and 2.7 watts plate dissiaption for each section, above the limit of 2.5W.
My experience of these amps is that the reverb driver V3 was almost always the first pre amp tube to fail, and that was using Made in USA GEs; They may have lasted 10 or 20 years even with these condions, but I doubt modern tubes will cope with such stress anywhere near as long.
Agreed that guitar amps have generally exceeded tube max voltage ratings, but tubes seem to cope with that provided that plate dissipation etc isn't exceeded alongside.
My preferred solution for these SF Fenders is to use the 2k2 bias resistor and partially bypass it with 25uF in series with 470 ohms or 330 ohms.
There's a benefit to not fully bypassing the reverb driver cathode, in that it helps avoid V£ grid 'clamping' the top half of the dry waveform to ~9V, which can cause an unpleasant early distortion with the SF master vol amps. With non MV amps, the power amp overdrives well before this, masking the pre amp distortion, so the user doesn't notice it.
Pete

[rooster]

Jjman, understood. I will try this this morning.

I also ran into that TRII and notice the non cap equipped 680 ohm cathode resistor there, but also a 560 pf cap value across pins 1 and 3. That pf cap is high frequency cut and I may try that, too, but not for stopping the what I am sure is a lower frequency signal overdriving the tube. It may just reduce some hiss and that's not a bad thing. I also notice a 10k 2watt resistor in series with the primary of the reverb tranny.interesting because I added a 4.7k value resistor to my amp and I can't hear that that affected the distortion.

I find it interesting that I have played a ton of BF amps and not noticed this distortion. And yet here it is in my build. Bothersome. You have to wonder if the SF units were just more poorly built, or if changing wire and such - and perhaps layout - caused these issues and the Fender science at the time was just so hip to notice these things and create perfect bandaids. Eh, but maybe a bandaid is never a perfect conclusion. Still, I will now try some bandaids. Thanks jjman, and pdf64, too.

[rooster]

Ok, tried a 680 ohm cathode resistor bridged with a 10uf cap in series with a 470 ohm resistorto ground. It's better but still there. Hm. I guess I wil try just the single 680 ohm cathode resistor. Shitzpa.

[rooster]

Ok, the 680 ohm cathode is the fix for me. I can still shake the tank pretty good and there can be some slight distortion on the low strings if I slam double stops on the 4 and 5 strings - but this is much much better.

I can tell you this, the next time I am in the vicinity of a real deal BFSR I am going to check this out. I just don't see how the circuit can escape this phenomina, ya know? It has to be there, at least in some small way I think.

[rooster]

Alright, this is just a measure of my insanity to be posting again on this subject, but I still went back to this amp and changed again the cathode resistor of 680 ohms to 1500 ohms. And it sounds even better.

I am also sure where this distortion is coming from: the .02/.1 bass control in the tone stack. At least in my amp. For whatever reason, it is very easy to slam my input (OD pedal) and have this bass frequency distort the reverb circuit. If I turn the bass control off it doesn't happen.. Sounds like shite, but it doesn't happen.

So in my case, I saw two options. Either change out the .1 to a .047 or .02, which might help, or leave that alone and change the 500 pf cap feeding the reverb circuit. I decided to change the 500 pf cap, and to a value of 360pf. Too, I realize this circuit was fine tuned by Fender years ago, but, for whatever reason, the frequencies the 500pf cap should roll off are not rolled off when the input is slammed.

Also of note, my 808 doesn't have a lot of bass and keeping the 500 pf cap as it was would have been OK. However I am using a Lovepedal Super 6 pedal after an LTD OD recently and it has huge bottom. It was this S6 pedal, and I love it, that really affected the SR. Not that the I couldn't distort the SR with the 808 in its original form, before changing the V3 cathode cap/resistor.

Anyway thanks for the post, it was time to deal with this!

[el_Fela]

talking about reverb, is it normal that some dry low/mid signal goes trough the reverb pot? i can tell when I turn it up, even with the 500pf cap-to-V3 disconnected. Sounds way too muddy! How do I get rid of it??