hiss in my new build - ideas??

[markmalin]

Here's a link to my original post.  THere are pictures of the layout and such.  The only things I've changed from these pictures are:

- I went with Sprague PS caps
- added a 10K pot on the back next to the impedance switch in series with the 820 ohm NFB resistor
- replaced a couple of the ceramics with Mica caps
- replaced 2 of the ceramic's with Sprague PS orange drops

The amp has always been somewhat noisy, but it really has a background hiss.  I'd like to use it in the studio but would like to get it to quite down first.  Please offer any suggestions you can.

I've poked around with lead dress somewhat - and in pushing on wires/components with a chopstick find that the reverb tends to hum when I get near the reverb output gain tube, for what it's worth.  On the bench (wide open) the reverb does hum, even with the tank disconnected, but I find if I put a metal cover over the chassis it's quiet.  Still have the hiss, though. 

Thanks, guys!  Here's that link to the original post with pictures:

http://www.el34world.com/Forum/index.php?topic=8788.0

Mark.

[FYL]

The two main culprits:
- Hissy tubes, mainly in V1. Swap tubes.
- Carbon comp resistors as plate loads. Use oversized metal film R's (typically 1W or 2W models).

A couple lesser evils:
- Std resistors at the input. Use some low noise MF resistors such as Dale RN/CMF55.
- B+ dropping resistors. Use oversized MF or MOX resistors (3W and more).

As you've added an OD section, you should also consider tailoring it's response - you want disto, not noise. Add a low pass filter/attenuator, play with local feedback...

[markmalin]

Thanks so much for the input.  I have some 1W metal films I can use as the plate resistors, and can swap some tubes around (as an aside, I discovered the tube sockets I bought will destroy Electro-Harmonix tubes...they are too fat and though they go in fine with a bit of gentle force, after a few hours of playing they crack!).  I may also have some metal films for the B+ dropping string... but that's also good to know. 

Again, thanks a million!  I especially appreciate the suggestions on the OD circuit.  It works, but is obviously not designed for this circuit. It's way too gainy and therefore does more to increase volume than add OD and distortion.

One question - will the plate load resistors being MF vs Carb. comp. effect the tone at all? I was trying to stay as close tone-wise to this 67 Vibrolux Rev. I designed it after. 

Many thanks,
Mark.

The two main culprits:
- Hissy tubes, mainly in V1. Swap tubes.
- Carbon comp resistors as plate loads. Use oversized metal film R's (typically 1W or 2W models).

A couple lesser evils:
- Std resistors at the input. Use some low noise MF resistors such as Dale RN/CMF55.
- B+ dropping resistors. Use oversized MF or MOX resistors (3W and more).

As you've added an OD section, you should also consider tailoring it's response - you want disto, not noise. Add a low pass filter/attenuator, play with local feedback...

[HotBluePlates]

Are you able to isolate where the hiss is coming from? Could you make a listening amp by making the probe and plugging it into another amp?

Once, a long time ago, another forum member had a hiss problem, and none of our suggestions solved it. He put together the listening (stethoscope) amp setup, and found the hiss occurred at a cathode resistor! Swapped the resistor and the hiss was gone.

It was an atypical place to find it, but somewhat believable.

[markmalin]

Are you able to isolate where the hiss is coming from? Could you make a listening amp by making the probe and plugging it into another amp?

Once, a long time ago, another forum member had a hiss problem, and none of our suggestions solved it. He put together the listening (stethoscope) amp setup, and found the hiss occurred at a cathode resistor! Swapped the resistor and the hiss was gone.

It was an atypical place to find it, but somewhat believable.

That's a great idea.  I'm going to try swapping out the plate resistors first, I guess.  Kind of shotgun, I know, but it would be quick and easy to do.  But this idea of a listening amp, that really sounds useful.  Can you tell me how to make one?  (if not too complex).  Often I seem to have things like this happen and no good resources for tracking them down.

Mark.

[tubeswell]

... make a listening amp by making the probe and plugging it into another amp

This idea of a listening amp, that really sounds useful.  Can you tell me how to make one?  (if not too complex). 
Mark.

fixed 

Any bit of wire connected to the 'listening amp's' input signal wire is what's needed for the probe. You could modify a geetar cable (because its shielded), but use an insulated Probe handle (like a meter probe), and put a 600V .1uF cap in series with the signal wire - so it will block any DC, which will mean that you can (carefully) probe the plates etc without interference.

[Ritchie200]

Mark,
Take a look here: http://www.el34world.com/Hoffman/tools.htm

There is also a current discussion going on on the "Tools" section.  It is easy to build and use.  Really comes in handy when you are scratching your behind wondering why you took on this build/repair/favor/etc.....

Good luck!
Jim

[markmalin]

Mark,
Take a look here: http://www.el34world.com/Hoffman/tools.htm

There is also a current discussion going on on the "Tools" section.  It is easy to build and use.  Really comes in handy when you are scratching your behind wondering why you took on this build/repair/favor/etc.....

Good luck!
Jim

Thanks, Jim.  I'll check it out ;)

Mark.

[FYL]

One question - will the plate load resistors being MF vs Carb. comp. effect the tone at all? I was trying to stay as close tone-wise to this 67 Vibrolux Rev. I designed it after.

MF resistors are more linear and less noisy. You may use CC's as plate loads if you wish, but get 2W models (bulky and expensive).

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#CarbonComps

[markmalin]

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#CarbonComps

This is really an excellent write-up.  Thanks for the link.  I appreciate the guy's perspective, understanding both the "mojo" thing and real numbers.  I've got to print this out....

Mark.

[tubeswell]

FWIW if you use MF plate and grid leak resistors in the V1 (earlier) stage(s), you will cut out quite a bit of hiss/noise floor in the critical 1st gain stage(s). This is a fundamental principle for eliminating noise in recordings (high signal-to-noise ratio, or low noise-to-signal ratio, whichever way around you prefer).

Then you can use your CC in the later (PI) stage where the noise floor isn't so critical because the signal is comparatively bigger in relation to the noise floor (than it is in earlier stages) anyway, and the voltage gradient mojo will kick in more because of the bigger signal swing. 2CW

[markmalin]

Last night I swapped out the plate resistors with Visay/Dale RN65's, which I believe are metal film.  Still have the hiss.  I also pulled V1 and the hiss mostly goes away, so I'm guessing it's in that input circuit.  I tried carbon film resistors as well, but no difference.

Are Dale RN65's a descent choice for low noise?  If so, I'm wondering if one of the caps might be the issue?  Or, maybe the next thing to do is get some metal films for the input jack.  What do you guys think?

BTW - I checked out the listening amp idea.  That's a sweet idea...unfortunaly I don't have the time or parts to put something together.  Then again, if I can't get rid of this hiss, maybe I should take the time(?).

Mark.

[sluckey]

I checked out the listening amp idea.  That's a sweet idea...unfortunaly I don't have the time or parts to put something together.

You don't have another amp, guitar cable, cap, and two gator clip leads?

[markmalin]

Well, they're talking about a "clean" amp.  I have another guitar amp I built, but nothing like an audio amplifier.  Should I just use another guitar amp?

(Sorry if I sound daft ;)  The engineering half of my brain is fighting with the "just get this &%*$*$# project done" half ...not pretty <LOL>  To be sure, the same battle is raging about the sport suspension conversion project on my old BMW as we speak!)

[sluckey]

Should I just use another guitar amp?

I'd just use whatever I had, even if it was a raunchy sounding 18watter. Clean is ideal, but I'm not gonna use my Denon 3802!   

[tommytornado]

Did you change the V1A & B cathode resistors? It looks like they're cc as well.  And you may have to try shielded wire from your input jacks to V1.

[markmalin]

Should I just use another guitar amp?

I'd just use whatever I had, even if it was a raunchy sounding 18watter. Clean is ideal, but I'm not gonna use my Denon 3802!   

That's encouraging ;)  I'll try one of my other amps, then.  It sure would be nice to just get in there and pinpoint exactly where it's coming from

[markmalin]

Did you change the V1A & B cathode resistors? It looks like they're cc as well.  And you may have to try shielded wire from your input jacks to V1.

I didn't swap out the cathode resistors.  I'll need to buy some more MF's... I only had a few 100k's on-hand. 

Regarding shielded wire, I wanted to stay away from that because of lots of discussion on ampgarage.com.  It seems people feel it cuts down on the gain compared to non shielded wire, but I do have plenty on hand and can try that.  That might get rid of some of the 60Hz I can hear in the background.

[tommytornado]

I understand about the shielded wire, I try to get by without having to use it myself.  You might also try routing the wires from the jack(s) to the V1 grid(s) around the right side of the board instead of under (twist them if loosely if there's more than one).  They could be picking up something from the components directly above them.  And.. if I haven't said it, I'm basing these suggestions on your saying the hiss went away when you pulled V1. 

[FYL]

I also pulled V1 and the hiss mostly goes away, so I'm guessing it's in that input circuit.

Have you swapped V1 with a known good low noise tube ?

Are Dale RN65's a descent choice for low noise?  If so, I'm wondering if one of the caps might be the issue?  Or, maybe the next thing to do is get some metal films for the input jack.  What do you guys think?

RN65's are excellent. They are the mil version of the CMF65 (same resistor, different designation and specs).

The input section has two 68K in // and 1M to ground. Noisy, noisy, noisy if you don't use high quality metal film resistors - I routinely use CMF55's there - which other affordable MF is spec'ed for noise at at "0.10 microvolts/Volt over a decade of frequency, with low and intermediate resistance
values typically below 0.5 microvolts/V"?

Check the caps and resistors in your B+ string. Try to kludge a decoupling node for V1 - say 10K + a compact radial 22µF/400V. Check your PT ratings: KT66's suck a lot of current.

[FYL]

Regarding shielded wire, I wanted to stay away from that because of lots of discussion on ampgarage.com.  It seems people feel it cuts down on the gain compared to non shielded wire, but I do have plenty on hand and can try that.  That might get rid of some of the 60Hz I can hear in the background.

A number of people at Ampgarage are card-carrying members of the KF Kult: if the late Ken Fisher used red PVC-sleeved 22 ga wire from the Outer Hebrides, they have to use red PVC-sleeved 22 ga wire from the Outer Hebrides, or their clone won't have "the KF Tone".

Ken didn't use shielded wire or twisted heater wires or proper grid stoppers or... You don't have to: use some nice shielded wire between the input jacks and V1, with the shield only connected on the jack side. Mount the grid stoppers as close as possible to V1, or alternatively on the Hi/Lo jack assy. 

[simonallaway]

One thing I've learned from my short time as an amp addict is that there is no magic here. Yes we all want tone, but you don't have to wave dead chickens over your soldering iron, or sacrifice your first-born to get it.

So I've never quite understood the mystique surround Trainwreck amps. The schematics/parts are available just like any other, right?

[FYL]

So I've never quite understood the mystique surround Trainwreck amps. The schematics/parts are available just like any other, right?

Trainwreck amps are outstanding players' amps, voiced quite forward in order to cut thru the mix - an Express delivering 10 watts to a 4x12 will subjectively sound much louder than any equivalent Marshall delivering the same power - and with an outstanding touch sensitivity - from clean to dirty at your fingers.

If schemos and layos are well documented, wreckish amps really shine when properly built, always staying on the verge of instability. Just like the finest jet fighters are really agile because of their controlled instability...

That said, there is no magick inside these amps. And there shouldn't be any mystique surrounding them.

[HotBluePlates]

We're veering off topic, but Trainwrecks are like Dumbles. The have mystique because they're expensive, no one you know has one, and you've heard a lot of people write about them. So the amp gets to be as fabulous as your imagination will let it be.

More to the point, they were built by individuals who gave their customers exactly what they wanted. Customers raved, others with money bought them, but the lone builders couldn't make enough the way they wanted to make them to generate a large supply. So the resell price skyrocketed, regardless of however much they cost initially.

I once saw a Trainwreck advertised for sale in the Charlotte area. A lawyer owned it, and was selling for somewhere around $10k-15k. I'm sure he sold it, cause the ad did not run long.

If it weren't expensive, if it were low-quantity, if it were readily available, then it would have no mystique. It would be like all the other good-sounding vintage amps available now. After you get it and the initial rush wears off, you think to yourself it sounds good, but not as wonderful as I imagined it would.

I've "been there, done that" with a LOT of vintage amps. I don't own any vintage amps anymore, just a couple copies I could make just as well for a lot less.

[markmalin]

Hey, regarding the "listening amp", couldn't a guy do the opposite?  Like ground out the signal starting from the input jack and see if eventually the hiss went away?  Then I'd know which component it is?  Because I removed V1 and the hiss almost completely went away, I'm thinking I've at least isolted it to the first 2 gain stages.

Humbly,
Mark.

[tubeswell]

Hey, regarding the "listening amp", couldn't a guy do the opposite?  Like ground out the signal starting from the input jack and see if eventually the hiss went away?  Then I'd know which component it is?  Because I removed V1 and the hiss almost completely went away, I'm thinking I've at least isolted it to the first 2 gain stages.

Humbly,
Mark.

Well then, maybe just put MF in the plate resistors for the V1 tube

[markmalin]

Hey, regarding the "listening amp", couldn't a guy do the opposite?  Like ground out the signal starting from the input jack and see if eventually the hiss went away?  Then I'd know which component it is?  Because I removed V1 and the hiss almost completely went away, I'm thinking I've at least isolted it to the first 2 gain stages.

Humbly,
Mark.

Well then, maybe just put MF in the plate resistors for the V1 tube

I actually did  put MF's (Dale RN65's) in the plate risistors for V1, but still have hiss.  I am planning to try the other resistors in that section, but I need to order some MF's....

[tubeswell]

Do MF plate and grid leak resistors for the first couple of stages.  The grid leak resistors are also a potential conduit for an AC noise floor (which in a jumbly sort of way, is what hiss is), because they are in parallel with the plate resistors as far as AC is concerned.