Throwing metal film resistors in Fender amps

[Mick Bailey]

Snubber resistors crept in on audio equipment in designs where they previously weren't used. Possibly this was to compensate for tube specs slackening off. Removing them from (for instance) a Twin Reverb has a beneficial effect on tone, so long as the tubes are good quality. Putting snubbers in means that pretty much any quality tube will work and in production line amp builds this matters, but it isn't good for sound quality.

Consideration has to be given to the fact that suppressing frequencies marginally above the hearing range, or upper frequency response of the speaker, has a detrimental effect on frequencies within the hearing range due to sum and difference components. Removing those frequencies early on can result in a very dull-sounding amp.

[Alan0354]

Snubber resistors crept in on audio equipment in designs where they previously weren't used. Possibly this was to compensate for tube specs slackening off. Removing them from (for instance) a Twin Reverb has a beneficial effect on tone, so long as the tubes are good quality. Putting snubbers in means that pretty much any quality tube will work and in production line amp builds this matters, but it isn't good for sound quality.

Consideration has to be given to the fact that suppressing frequencies marginally above the hearing range, or upper frequency response of the speaker, has a detrimental effect on frequencies within the hearing range due to sum and difference components. Removing those frequencies early on can result in a very dull-sounding amp.

Are you referring to the 2200pF caps from the grid of the power tubes to ground in all the newer Silver Face Fender in the 70s? I am just doing this experiment using a switch it in and out and listen. I can definitely hear the difference. You get more chime, not obvious as the presence control, it just give that bit more transparency.

I would avoid putting any caps from signal path to ground or miller caps if all possible. The only time I encounter oscillation is in the cascade gain preamp that you put a lot of gain. I still try to divide the gain down after the over driven stage to avoid adding caps.

[catstrat]

Here comes my two cents worth. Hisses for me are just about as bad as hums to figure out. I've used carbon film for years and never heard complaints about noise afterwards. Finding the source of the hiss is the key. I would probably look at the plate resistors on V1 and V2 first. I'm sure it's been mentioned before but pull your PI tube. Turn the amp on and see if it still hisses. If it does then check your screen resistors on your output tubes.. If not then it may be the plate resistors. I used to blame coupling caps for being noisy but have since found it's most likely a resistor. But tubes can get noisy as well. Make sure the socket contacts are clean and well tensioned. Clean all your pots and make sure everything is well grounded. Hope this helps a little.

[Wil]

Removing V1 and V2 reduces the hiss but adds a sort of "boiling teapot" noise (see famous movies with boiling teapots). Removing V4 makes the amp dead silent.
Would the main hiss be located between the input stage and the reverb and tremolo recovery stage ?
I did not try removing the phase inverter tube (V6).
I have to mention that the vibrato is very very weak, almost dead. I don't use it. The intensity pot is a SPST 50k which allows me to disconnect the vibrato by virtually cutting the wire from R35 to R48 (aka intensity pot). I changed the roach and various component with no positive effect. But I have a great tremolo pedal I made that I use instead so it doesn't matter.

[Alan0354]

If the amount of noise reduced by pulling V1 alone is same as pulling V2 alone, I don't think the problem is V1 and V2 section. You pull the V4, you cut off the Vibrato channel all together. So normal channel (V1) is not your problem for sure.

I suspect reverb circuit is your problem. Put everything back together, unplug the reverb tank and see whether that reduces the noise. If it does not, your problem is V4 circuit. Take the tube from V1 and replace V4 to make sure it's the the tube.

[catstrat]

Alan is right on that. It may just be the Reverb circuit. Good luck on it.

[Wil]

Playing with only V2 V4 V6 is quite hissy.
Most terrible hiss noise is made by the treble pot. But the "background hiss" which is not affected by the controls goes way louder with a 12AX7 in V6 instead of a 12AT7. With a 12AU7 the overall hiss is acceptable but the amp feels less open.
The more hiss there is, the clearer and fuller the amp sounds...

If the amount of noise reduced by pulling V1 alone is same as pulling V2 alone, I don't think the problem is V1 and V2 section. You pull the V4, you cut off the Vibrato channel all together. So normal channel (V1) is not your problem for sure.

I suspect reverb circuit is your problem. Put everything back together, unplug the reverb tank and see whether that reduces the noise. If it does not, your problem is V4 circuit. Take the tube from V1 and replace V4 to make sure it's the the tube.

Tried with the reverb tank unplugged and even without V3, it doesn't reduce the hiss.
For the moment the only thing that affected the hiss was playing with the gain of the PI tube.
But even if a lower gain tube makes the amp more silent, it affects the overall sound of the amp (less clear, less enjoyable).

Playing with only V1 and V6 and I still have hiss. The amp doesn't perform the same between two shots so it's impossible to have relevant and consistent measurements.
I have to admit I give up. This amp is just a pain in the ***. I do think I'm going back from the start, PTB board and tralala.
Thanks for your help ! I might come back soon with some new problems haha !

edit : Well I won't give up haha, I can afford giving up anyway and I need an amp.

[soundguruman]

Playing with only V2 V4 V6 is quite hissy.
Most terrible hiss noise is made by the treble pot. But the "background hiss" which is not affected by the controls goes way louder with a 12AX7 in V6 instead of a 12AT7. With a 12AU7 the overall hiss is acceptable but the amp feels less open.
The more hiss there is, the clearer and fuller the amp sounds...

Tried with the reverb tank unplugged and even without V3, it doesn't reduce the hiss.
For the moment the only thing that affected the hiss was playing with the gain of the PI tube.
But even if a lower gain tube makes the amp more silent, it affects the overall sound of the amp (less clear, less enjoyable).

Playing with only V1 and V6 and I still have hiss. The amp doesn't perform the same between two shots so it's impossible to have relevant and consistent measurements.
I have to admit I give up. This amp is just a pain in the ***. I do think I'm going back from the start, PTB board and tralala.
Thanks for your help ! I might come back soon with some new problems haha !

"Most terrible hiss noise is made by the treble pot. "

Oh, god, stop. you're killing me! hahahahhahahahahahhahhhahahhhahahhahahhahhaaaaaaaaaaaaaa aaaaaaaaaaaaahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh!
that is sooooo funny. I just about crapped my pants.

[Alan0354]

Playing with only V2 V4 V6 is quite hissy.
Most terrible hiss noise is made by the treble pot. But the "background hiss" which is not affected by the controls goes way louder with a 12AX7 in V6 instead of a 12AT7. With a 12AU7 the overall hiss is acceptable but the amp feels less open.
The more hiss there is, the clearer and fuller the amp sounds...



Tried with the reverb tank unplugged and even without V3, it doesn't reduce the hiss.
For the moment the only thing that affected the hiss was playing with the gain of the PI tube.
But even if a lower gain tube makes the amp more silent, it affects the overall sound of the amp (less clear, less enjoyable).

Playing with only V1 and V6 and I still have hiss. The amp doesn't perform the same between two shots so it's impossible to have relevant and consistent measurements.
I have to admit I give up. This amp is just a pain in the ***. I do think I'm going back from the start, PTB board and tralala.
Thanks for your help ! I might come back soon with some new problems haha !

Have you been to GC and test another DRRI?

[Wil]

"Most terrible hiss noise is made by the treble pot. "

Oh, god, stop. you're killing me! hahahahhahahahahahhahhhahahhhahahhahahhahhaaaaaaaaaaaaaa aaaaaaaaaaaaahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh!
that is sooooo funny. I just about crapped my pants.

Well as I mentionnent in the first post, I try to do my best in English. I mean white noise not hiss. Aren't those two similar ?

Have you been to GC and test another DRRI?

Well I'm from Europe and I've tried two DRRI which where really quieter than mine. But they didn't sound any good next to mine !

[Alan0354]

Well as I mentionnent in the first post, I try to do my best in English. I mean white noise not hiss. Aren't those two similar ?



Well I'm from Europe and I've tried two DRRI which where really quieter than mine. But they didn't sound any good next to mine !

I don't think it's your English he is talking about. I don't think anyone is convinced at this point that you have a problem. Hiss is white noise, you are right about that.

So the problem is only when you crank up the reverb? Try changing the V4? If you don't have an extra tube, use V1. Or, don't crank the reverb that high.

[Wil]

I don't think it's your English he is talking about. I don't think anyone is convinced at this point that you have a problem. Hiss is white noise, you are right about that.

So the problem is only when you crank up the reverb? Try changing the V4? If you don't have an extra tube, use V1. Or, don't crank the reverb that high.

I believe a video is worth a thousand words. I remembered I shot a video about that noise.

Since then, I put the middle/bright pot in the input 2 of the normal channel because I found input 2 of the vibrato channel to be quite useful. And the NFB resistor switch is now an on-off one.
V2 is a Tung Sol RI, V3 JJ 12AU7, V4 Tung Sol RI, V5 JJ 12AX7 and V6 NOS Philipps 12AT7.
Power tubes are JJ 6V6S (1 year old) and the rectifier is a brand new JJ GZ34.
(plus, the scratchy pot has been cleaned up since)

[Alan0354]

What is the pot behind the amp? I don't know of any pot at the back of the DRRI.

[frus]

yes, it says "fuse 1A 250V", but there is a knob, I guess it's some kind of a switch

[Wil]

It used to be a SPST pot now replaced by a simple on off switch. The use was to provide negative feedback variable resistance but also to virtually cut the NFB wire which makes the amp sounds way better but way noisier