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**Chuck H** · 02-05-2015, 09:24 PM

If it's the classic high pitched hiss it's probably just a gain byproduct. Not having looked at the schem I can't say if there is gain channel hiss analogous to the clean channel. Some amps (a lot of Mesa designs) do this. If it's a slightly lower pitched hiss it could be a noisy preamp tube. I've had several hissy/noisy Sovtek 12ax7wa/wb/wc tubes. I guess the super short plate Sovtek's must be prone to it. That you tried a couple of lower gain tubes and the hiss reduced would bare out that it's most likely gain related. In this case using a lower gain tube to reduce hiss solves nothing since it results in a corresponding gain decrease. If you have any stock of 12ax7's just try a few in the position that seems to affect the his most and use the quietest one.

re: Using a lower gain tube to reduce hiss... I see this proposed a lot and I don't understand how anyone comes to this as a solution. Here's a scenario that I hope illustrates why:

*A gallon of water weighs eight pounds*

I need to carry a gallon of water.
I only have additional capacity to carry six more pounds.
If I only fill the container three quarters of the way it only weighs six pounds and I still have a one gallon sized jug.
Problem solved.

Q: What's wrong with the above assessment

**blindboybenton** · 02-06-2015, 10:04 AM

He had changed the tubes and i did the same. I only have sovteks in stock for the 12ax7 or ecc83. I will try a used tube later but i don't expect it will change it. I think most people change tubes for lower gain one's mostly to tame the beast. Im getting the impression from him that its noiser than it was. I don't think so, But ive never had one of these. and we don't see them in the uk. Didnt find anything really on the net about the hiss. Was hoping that one of you guys over the pond would say that they are normally quite ot they are noisy

he is a bit of an odd ball chap. don't want me to change anything on the amp. Like for Like!!!. Output tubes were unbranded and i had a slight battle to get a good set of tubes to replace them with. wanted excatly what Mr Rivera would of fitted

BBB

**Dave H** · 02-06-2015, 10:56 AM

Originally posted by Chuck H View Post

*A gallon of water weighs eight pounds*

I need to carry a gallon of water.
I only have additional capacity to carry six more pounds.
If I only fill the container three quarters of the way it only weighs six pounds and I still have a one gallon sized jug.
Problem solved.

Q: What's wrong with the above assessment

A gallon of water weighs ten pounds.

BBB,
Does it have any carbon comp. resistors in the first gain stage?

**blindboybenton** · 02-06-2015, 11:28 AM

Not sure dave. I don't have the schematic either. Aint looked for one as yet

**Chuck H** · 02-06-2015, 11:46 AM

Originally posted by Dave H View Post

A gallon of water weighs ten pounds.

BBB,
Does it have any carbon comp. resistors in the first gain stage?

Apologies for any confusion. I see now that I should have adjusted my analogy for geographic accuracy

**J M Fahey** · 02-06-2015, 01:14 PM

If both of you had used Metric this confusion would never had existed to begin with

As of "taming" an original design, what do you mean, butchering?

If customer isn't macho enough to handle what he bought, on his own decision, suggest him selling it and instead buying

As of the Rivera amp, try 3 or 4 different tubes as V1 ; if hiss changes drastically, leave the best there, but if it stays roughly the same (what I suspect), then leave as is, it's the nature of the beast.

**blindboybenton** · 02-06-2015, 02:05 PM

[QUOTE=J M Fahey;375166]If both of you had used Metric this confusion would never had existed to begin with

As of "taming" an original design, what do you mean, butchering?

If customer isn't macho enough to handle what he bought, on his own decision, suggest him selling it and instead buying "

I hardly think changing a few tubes for a sound that suits you better is butchering the amp. Just because an amp is not 100% what you are looking for, should you get rid and buy something else. The customer is very happy with the sound and tone he gets just reduce the hiss. as he thinks its more than it should have I think the hiss is no more than can be expected from this amp.
Is my customer macho enough to handle it.... WTF has that got too do with it.

As we don't see these amps in the uk. was hoping for Tech's from USA could comment

BBB

g1 · 02-06-2015, 06:18 PM

I'll wager Mr. Fahey was being very tongue in cheek.
As to the noise, as Dave H implied, I'm wondering if any reduction could be found with different type of plate resistor for that stage (metal film?), or if perhaps it is being caused by a noisy resistor.

**Chuck H** · 02-06-2015, 08:47 PM

I looked up the schem for the amp:

http://bmamps.com/Schematics/Rivera/...er%2025-45.pdf

This is a newer amp that uses plenty of silicooties. I don't think CC resistors would have been considered for this sort of design, but I don't know.

This amp uses a pair of FET's as the first and second gain stages. The inf6449 is used in several "low noise" amps I saw posted but it may be more about the circuit and how these devices are being used. It's very likely the hiss could be found in this circuit. Hiss is a problem for guitar amps because of their high gain. It's generally shot and Johnson noise in the early stages being amplified that we hear as hiss. Not being a silicon guy I don't really know how to evaluate the circuit, but if the hiss is here, and you're not supposed to change anything then that's it. Nothing to be done.

**J M Fahey** · 02-06-2015, 10:19 PM

The >>Tech<< suggestion is at the end of the posting

:

As of the Rivera amp, try 3 or 4 different tubes as V1 ; if hiss changes drastically, leave the best there, but if it stays roughly the same (what I suspect), then leave as is, it's the nature of the beast.

As of the macho amp comments, they refer to the very frustrating "service" calls which I get all the time, typically:
* the amp is too loud
* earpick highs
* buzzy distortion
* my guitar feedbacks too much
* no bass
* no sustain
* I have no headroom!! I distorts already on 4, I want to be able to set it to 9!!!!
* it hisses too much
* my single coil hums/buzzes too much
etc. etc. etc.

to which he adds: "I read in TGP/Ultimate Guitar/My Les Paul/The Marshall Forum/etc. (not in MEF

) that I should substitute 12AX7 by 12AU7 to "tame" it".

Most/all above complaints disappear when amps are [played LOUD (as in volume set from 7 to 10) and in a live playing band environment ... as they were designed for.

On the contrary, perfectly fine classic amps present all the above "defects" in a bedroom.

To me , replacing the proper tube which the designer chose, biased, etc. by another with 8X less gain, and which to boot will bias horribly, can be accurately called "butchering" since the amp turns into something bland and completely different to what was intended.

Same as the suggestion of clipping off bright caps, clipping out the normal channel 220k mixing resistor "so the reverb channel has more gain", bypassing the 10k cathode resistor in JCM800 and its derivations, increasing the 500pF coupling cap in VOX, Bassman 50 and Trainwrecks "to have more bass" (and MUD) , etc.

Classic amps are what they are for a reason.
If they are way too much to handle, maybe they were not a right choice.

Same as buying this:

afterwards using it only to go to church, shopping mall or carrying kids to school and then complain "it eats too much gas!!!"

There must be some good reason for the late huge popularity of 5/15W amps.

Even Mesa and Marshall are offering them!!!

Marshall even has a 1W type !!!!

And a friend of mine, who is a Grammy/Emmy award winner, uses this to record most heavy guitars:

As of being qualified to answer on a Rivera, Pappo, *our* national Guitar hero and a longtime personal friend, happens to own the largest Rivera ever made,

160W + 160W RMS, 4 x 6550 per channel, and more channels and modes than a Mesa ... which is a lot .

with matching black carpet clad 4 x 4x12" cabinets.

Guess who keeps it running?

And he got it for free, in exchange for endorsing the brand in Argentina (remember, local Guitar God) .

That said, he finds it unusable

FWIW he tours and records with a Stereo Chorus SS Marshall Valvestate 8240 I modded

here's a free sample:

the Riveras are in the backline, together with Marshall DSL and Plexi heads ... but those were set up for Guests, he played all night with a Fahey modded SS Valvestate.

You can clearly see it straight behind him, for example at 11:00 , etc.

Maybe now you'll understand why I have little patience with 100 post long threads nitpicking of "long plate vs. short plate", NOS vs new, cap colour, silver wire, "period correct parts", etc.

And also why I'm one of the very few who dare to defend SS vs Tube.

**Chuck H** · 02-07-2015, 11:11 AM

Originally posted by J M Fahey View Post

And also why I'm one of the very few who dare to defend SS vs Tube.

In support of SS tone:

RAT pedal (mid 80's

) Ok. and all the other popular pedals, processors and even digital modeling amps (I read that Los Lobos uses Line 6's on stage now!)

All the really cool tones CCR recorded with those Kustom amps.

The FET input circuit that even Dumble decided was a good idea!

Dimebag Darrell (not a huge fan, but proof that SS amps can be suitable for shredding)

Tom Scholz

David Gilmour and Roger Fisher (both use tube amps, BUT only as reference amps for pedal and rack effects for all tones including distortion)

Mark Knopfler. Sultans of Swing (and many others) recorded with a SS amp.

Leslie West. Yup! Many sources report that Mississippi Queen was recorded with a pedal into a SS PA head (IIRC)

Ok, enough endorsement for SS. Musicians just make music no matter what. And people develop a preference for what they're use to. If SS is how you build your game then SS will sound best for you and you'll play your best using it. But I have to say that the first time I heard a tube amp I was intrigued. I insisted on trying it out. I couldn't believe how rich and complex the harmonics were compared to all the SS stuff I had used. The feel was so intuitive too. Not just a reference like the SS amps. I could dig in and the amp seemed to get under the notes for me and I knew this was what I had been missing. It's been like that with most of the tube amps I've played since and I'll probably never be without a tube amp. So, Juan,..

History proves that SS is good. Preferable even for some players. It's great that there are guys like you to make the best available, but IMHO the very best tone is still a tube amp. I mean, really.?. The world wouldn't continue to lust for them if it was just trendy lore. There's definitely still a quality to tube amps that SS hasn't been able to capture yet. So how about we meet in the middle and just use our favorite stomp boxes into a tube amp? I'm in

**J M Fahey** · 02-07-2015, 11:37 AM

Back to the amp and its problem.

1) thanks chuck h for posting the schematic

Most "new" amps are just rehashing or tweaking of same old (killer) circuits.
Not here, Mr Rivera clearly IS a Designer and starts with a datasheet in hand.

Standard checkpoints in most other amps do not apply much here.

2) again as chuck said, the first two cascaded gain stages are FET, and the tube in the preamp is used basically for clipping, so it should have to be defective, horribly noisy, to have any influence.
Since it is not (besides amplifying what it's fed) , it seems to be innocent.

3) pamper the Fets ... they are unobtanium

InterFET Corporation - datasheet pdf

4) they provide very high gain.
As shown, from 50X (like a 12AX7) to 300X (much more than any tube) .

That might explain the apparent hiss.
Not that they are specially noisy but high gain brings up all ugly details ... and is probably also the reason of the owner liking it so much.

If one of these Fets dies, big trouble.

I don't know any currently available 300V FET.
Highest at Mouser is 40 or 50V .

Oh well.

**Chuck H** · 02-07-2015, 08:15 PM

Originally posted by J M Fahey View Post

2) again as chuck said, the first two cascaded gain stages are FET, and the tube in the preamp is used basically for clipping, so it should have to be defective, horribly noisy, to have any influence.
Since it is not (besides amplifying what it's fed) , it seems to be innocent.

That was my feeling when I looked at the circuit. I was sort of hoping you (being the most specifically experienced in this area) might suggest how to reduce some resistance values or some such (and so, maybe some hiss). But being that these are such specific devices it would probably require undue design effort and additional troubleshooting. Not to mention that the owner doesn't want the amp changed anyway. So it's a moot point.

Originally posted by J M Fahey View Post

3) pamper the Fets ... they are unobtanium

InterFET Corporation - datasheet pdf

...If one of these Fets dies, big trouble.

...I don't know any currently available 300V FET.

On this observation I might suggest that BBB keep eyes out for this part (Hey Juan, NOS FET's!!!) since he is the primary service tech on this amp. Could come in handy. And on that note...

If they can be found it might be an idea to swap a couple of new ones in there just to see. I know older op amps can go hissy because I've done repairs on such amps. Perhaps these high voltage devices suffer this failure mode prematurely. Even if nothing changes you'd still have the extras in case it comes up in the future.

You can go on fleabay, for example, and plug in a couple of different searches with specific criteria (spacing variants, dashes, quote marks, etc.) and instruct them to email you whenever something that matches comes up. They keep a constant watch for you and just let you know. Neat feature.

**Jazz P Bass** · 02-07-2015, 08:29 PM

It appears that Interfet changed the part number to InterFET Corp. Page Selector - 2N6450 datasheet

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