New Amp Hiss

Discussion in 'Amp Tech Center' started by Gringo13, Mar 19, 2015.

  1. Gringo13

    Gringo13 Tele-Holic

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    I picked up a new '68 Custom Vibrolux Reverb yesterday. This amp rocks and is quite versatile. I really love the tone! However, there is a white noise hiss that is audible even without anything plugged into the input once the volume is up to 5 or so. It's not noticeable when playing, but it's definitely there when the guitar is quiet.

    Here is what I've observed so far...

    The hiss on the Custom "Bassman" channel is quite a bit louder than on the Vintage Channel, but it's there on both channels. The Custom channel is hotter, so I guess this makes sense.

    The reverb/trem seems to have nothing to do with the hiss because it doesn't change when 'verb/trem are turned on or off.

    The noise is worse the higher the volume is.

    I checked each tube one at a time. When I removed V1 on the Custom channel, the noise was completely gone. Same with V3. In fact, I couldn't even tell the amp was on when I pulled either of these tubes.

    Questions:

    Is it possible the amp came with two bad tubes and that replacing them would improve the sound? I have an extra 12ax7 that I'm gonna try tonight.

    Does this sound more like a lead dress issue?

    Besides the tubes, what are some simple things for me to try? I really don't want to ship the amp back without knowing I'll get one that doesn't hiss... especially if it's a simple fix. Also, I can have an authorized amp service look it over, but the closest one is two hours away and they'll need the amp for multiple days. Before I do that, I wanna make sure I cover my bases.

    Thanks.
     
  2. Tele-phone man

    Tele-phone man Tele-Afflicted

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    It's not a lead dress issue. That can result in hum or in squealing, but not in hiss. Hiss is the result of gain. It's not really "bad" tubes, although the best quality NOS tubes could reduce it.

    You can dramatically reduce this hiss by changing some of the preamp tubes to lower gain versions such as the 5751 or 12AY7, and/or changing the phase inverter to a 12AT7. Such changes WILL change the gain structure and the amp's response, which may or may NOT be to your liking. However, the real bottom line is that there is nothing wrong with your amp. That's how these amps behave due to the high levels of gain in the preamp stages. It's designed in.

    I recently went through this with a Custom Vibrolux Reverb that I bought in January. On the floor of the GC, with its high levels of ambient noise, I really didn't hear how hissy the amp really was. I sure noticed it at home. The hiss level was so high I would NEVER be able to record with this amp. I went under the hood and made changes to it (added a negative feedback loop, which took several attempts to get it where I wanted it) and I changed the phase inverter to a 12AT7, and three of the other tubes to 5751s. I also changed all of the plate load resistors to metal film versions. I had one that was definitely defective, causing random crackling and sizzling.

    The whole thing kinda pisses me off, though. First, I'm mad at myself for not being more discriminating before I purchased. I should have been more careful, but it was an impulse purchase. My fault. Fortunately, I have the experience under the hood to make the changes I need.
    But I'm also a little miffed at Fender for putting these things out there like this. Most buyers won't have the resources I have to customize the amp to my tastes, and will thus find frustration dealing with an issue that isn't really a defect. That hiss is part and parcel of the amp's design and the tubes they selected. You and I shouldn't HAVE to "fix" a new amp.
     
  3. Wally

    Wally Telefied Ad Free Member

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    Telephone man, is this amp that you have a CVR or the newer '68 Custom Vibrolux REverb???
     
  4. Gringo13

    Gringo13 Tele-Holic

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    Thanks for the input. I agree that one shouldn't have to "fix" a new amp. I've never played the CVR, but I've heard that the hiss is worse on it that it is with the '68 CVR. My amp already has two 12at7 tubes in V3, but I may try a 5751 in V1 of the Custom channel.

    A few developments...

    I called Fender customer service yesterday. The rep was very nice and willing to help any way he could. I mentioned the hiss and he said he has taken a couple calls on it, but that most of the units are very quiet. This makes me think it's not necessarily something inherent in the circuit.

    I compared mine with my brother's '69 Twin and they were comparable in noise. The 68 CVR's vintage channel was about the same as the Twin and the Custom/Bassman channel had just a tad more white noise on 10 than the Twin. However, there's a 50 watt difference in the two amps, so that may not be great.

    I also had my band's bass player and other guitarist check it out and they thought the hiss was minimal and I was making too much out of it. Perhaps this is a case of message boards influencing what I heard before I actually heard it. Hmmm.

    I'd still like to minimize hiss and white noise, so any more info is welcomed!
     
  5. Wally

    Wally Telefied Ad Free Member

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    I just listened to a '68 Custom TR that I have just done repairs on...bad connection..one of which was the bias circuit on V8...which would cause it to redplate!!
    Yes, there is a bit of hiss....comes with gain. Not objectionable, imho. My basic problem with the amp???? I don't like those Chinese GT 12AX7's. I put a JJ in V1--, and that modded channel came alive...much more musical, less harsh on the high end, better range of response. The V2 tube is also a tone generation stage---- for the Vintage channel. replace it with a better tube.
    V3>??? ARe you sure you pulled V3? Ime, V3 would be the reverb driver tube and will not prevent either channel from functioning...albeit there will be no reverb. What does the tube chart show for V3?? A 12AT7? IF so, that is the reverb driver and should not 'kill' either channel'. IF you pull V4, that will stop all signal processing. IF this is not the case, then Fender has totally rearranged a scheme that they have used for over a half a century..just for this one amp. I just checked this '68 Custom TR, and V3 is the reverb driver...both channels will function without V3.
     
  6. Gringo13

    Gringo13 Tele-Holic

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    Wally... thanks for the response! Maybe my terminology is off. If looking at the back of the amp, the biggest differences were made when I pulled the 1st, 4th and 6th tubes from the right. When I pulled any of them out, the white noise completely stopped. I understand the 1st and 4th tubes, but not the 6th. I thought the third tube on each side was the reverb, but I don't really know much.
     
  7. Gringo13

    Gringo13 Tele-Holic

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    Again, I don't think the hiss is as bad as I originally thought, but it's just something I'd like to address if possible. I really think the amp will sound killer when the speakers break in and I get some better tubes in there.
     
  8. Ted Witton

    Ted Witton Tele-Meister

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    Mark Moyer (sadly deceased) offered these mods on this site some time ago in order to address the hiss issue with the CVR which I personally found intolerable as did many others. Possibly relevant to the new '68 version. Check out:
    http://www.fenderforum.com/forum.html?db=&topic_number=719412

    The point is that the CVR was designed to offer more touch sensitivity, and for stage use at relatively high volume where the hiss would probably be subsumed - but still there - and therefore the amp useless in the studio. The change of valves and the mods certainly improved the situation but anyway I later sold mine and have not regretted that decision. In my view, not one of Fender's greatest achievements but many would disagree..
     
  9. Wally

    Wally Telefied Ad Free Member

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    Ted Whitton, the OP's amp is NOT a Custom Vibrolux Reverb. It is a '68 SF Custom Vibrolux reverb...thanks to Fender for confusing the issue, right?

    Gringo13, here is the layout for the tubes...from right to left while looking at the back...
    V1--Custom channel preamp.
    V2--vintage channel preamp
    V3--reverb driver
    V4-- one triode is a gain stage for both channels, one triode is the reverb return.
    V5---trem fucntion tube
    V6--Phase inverter
    V7 & 8--power tubes.
    V9---tube rectifier if there is one.

    So, when you pulled V1, you killed the Custom Channel. The 'hiss' from that channel stopped. The noise from the Vintage channel would still be produced....albeit at a lower level IF you have the controls in the two channels set at the same points because the Vintage channel is a slightly lower gain channel.
    When you pulled V4, you killed all of the preamp and all of the noise that either channel might produce.
    When you pulled V6, you disconnected the preamp from the power section...and killed all production.
     
  10. Gringo13

    Gringo13 Tele-Holic

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    Wally.... Thanks for the tube clarification. I was thinking each channel had V1-3 as opposed to V1-V9. Oops!

    So you're saying that the reason pulling those tubes stopped the hiss was because it just killed the power and not because of shoddy tubes? Makes sense. I'm still going to replace the GT's in V1 and V2 with something better.

    Could the hiss be due to the reduced negative feedback? Unlike the CVR, this amp actually has a NFL, but I'm not really sure if that is the culprit. I do like the responsive sensitivity of the amp.

    Since you have a '68 Custom there, have you compared its circuit and parts to those of the CVR? Besides the cheap tubes, any common "problem" spots that may need to be addressed?

    Ted... thanks for the suggestion. I'm hoping Wally will have some comparisons between the two amps so maybe we'll see if they share some common issues.
     
  11. Ted Witton

    Ted Witton Tele-Meister

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    Hi Wally, I have read the very interesting thread that you participated in around last Oct. As you say, thanks Fender for the confusion! I especially liked your pics of the innards of the Pro Sonic v CVR. Seems like the "suits" are beginning to wave their financial wands again?
     
  12. Wally

    Wally Telefied Ad Free Member

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    Gringo, when you pulled those tubes, you interrupted the signal chain. Review that tube layout above. IF you pull only V1, the Custom channel will not work because you will have removed the input preamp stage for that channel...but the Vintage channel will still work. If you pull only V2; the Custom channel will work, but the Vintage channel will be dead. IF you pull only V3, the reverb will not work...but everything else will work. IF you pull only V4, nothing works...because that tube is common to both channels in this amp. If you pull only V5, the trem/vibrato will not work...but the rest of the circuit functions.
    IF you pull only V6, the amp will not process signal.

    Yes, the resistance in that negative feedback loop allows this amp to be 'hotter' than say a stock vintage Fender two channel reverb amp and also hotter than the BF reissues.....'hotter' meaning it has not had as much of the signal in the circuit cancelled with a large negative feedback signal. IT is not as 'hot' as teh CVR, which has no NFB loop at all. Big noise there. One can install an adjustment pot in these circuits to allow one to go from a large feedback signal to a less powerful negative feedback signal....which will allow one to go from a cleaner amp to a hotter amp. I like versatility.

    Re: this '68 Custom SF VR amp and the earlier CVR. I can't find a schematic for this amp today. IIRC, the two amps are somewhat similar in that they both have the bias vary trem circuit...as in the 6G16 Vibroverb. IN this respect, the CVR and the '68CSFVR are different from the vintage Vibrolux Reverb, which had the opto-isolator trem function as found in all two-channel BF/SF reverb amps. I may be incorrect on this bias vary thing...but through the fog of a really big allergy headache, this is how I remember things with the '68CSFVR.
    The '68CSFVR differs from the CVR in that that one channel has been modified to be more tweed like.
    This '68CSFTwinReverb here is different from the '68CSFVR in that trem function..IIRC that bias vary thing in the CSFVR correctly...because the CSFTR has the opto-isolator...as in the vintage TR and also in the TRRI's.

    I have modded the CVR's to have an adjustable NFB loop, a bias adjustment circuit and a really hot tweed channel...more so than this '68CSFVR amps does. The '68CSFVR can be modded to yield a more versatile amp regarding the NFB loop, as I noted. ONE can get back to the cleaner soudn of those BF/Sf reverb amps...or make it hotter than the '68CSFVR is right now.
    I wholeheartedly suggest getting rid of the Chinese tubes and then rebiasing the amp. Whoops...there goes the warranty!! (;^)


    It seems I have figured out how to easily keep these two amps separate....CVR versus '68CSFVR....it lays under the fingers on the keyboard really well....'68CSFVR.
     
  13. Wally

    Wally Telefied Ad Free Member

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    Ted, as I noted back then in that other thread....the Pro
    Sonic is the best built Fender since 1985 except for the handwired Custom Shop amps. Suits always have the last word, ime. OR...is it the consumer???
    No one wants to pay more for an amp that LOOKs just like another amp sitting beside it.....because the general consumer is not aware of the build quality thing. IT is all external to the vast majority of consumers of these things....and the sonics usually don't enter into it either because the experience necessary to decide between bad, good, and better sonics takes a long time to accumulate, ime.
    That said, the build quality on these BF Ri's and the '68 SF Custom Reissues is good---much better than the Blues/HRD series....just not as good as that Pro Sonic or the handwired amps.
     
  14. Gringo13

    Gringo13 Tele-Holic

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    Wally... You're awesome. Thanks!

    I'm definitely interested in having adjustable negative feedback. I really like the sound of this amp, so making it a little more versatile would only be a good thing, imo.

    I'd like to get some high quality NOS 6L6 for it, but until then, what modern production 6L6 would you recommend? JJ?
     
  15. Tele-phone man

    Tele-phone man Tele-Afflicted

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    Wally, I have the older CVR. It seems to me that these amps have a similar circuit, so I'm not surprised that they have similar hiss issues.
     
  16. Wally

    Wally Telefied Ad Free Member

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    Telephone man, similar but not identical. Your CVR had no negative feedback loop in stock form. The establishment of a 'stock' NFB loop from the 6G16 Vibroverb, which is what your amp is based on, will deal with the hiss in your CVR. The '68 SFCVR has a NFB loop, IIRC....but it has a larger resistance than does the 6G16 circuit and therefor has more floor noise than that 6G16....or any of the stock vintage Fenders from the 6G/BF/SF era. Gain equals 'noise'....some don't like it. IF a player wants gain, a player will have this type of 'noise'. IF a player wants clean, then they can have an amp that doesn't have this type of 'noise'.
    So...use a pedal for overdrive and distortion, right? hey, hit that button on a pedal gain/OD device....and listen to the quiescent noise increase that has to come with that added gain.
     
  17. Tele-phone man

    Tele-phone man Tele-Afflicted

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    I had to add much more NFB than the 6G16 schematic to reach a noise floor I could tolerate, plus replacing some of the tubes with lower gain versions. I'm probably going to go back in one more time and put the PI and output stages to AB763 specs.

    You confirmed my point, which was that gain makes hiss, and these amps have a lot of gain. Personally, I want that clean tone and low noise at recording volumes.
     
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