Looking for suggestions for a low noise (hiss) practice amp

[JWK]

What you described in your opening post is a Rocket with octal power section either 6L6, KT66 or EL34's.

The amp is pedal friendly, no hiss, super quiet operation, plenty of push when needed, can deliver Fender clean or let the power amp push a little. I add a DPDT bright switch for 100pf and 500pf. You could go with a PPIMV if you wish. To keep the amp even cleaner I change out the 220k plate of V1 to 100k or 120k.


Mark

That sounds interesting. I've never looked into building any kind of trainwreck amp since the schematics I've seen and the clips I've heard make it clear there is too much gain for me - but I've never looked into the rocket. I've poked around and can't find a schematic. Any links I can check out? Thanks!

[NickC]

Everything you ever wanted to know about the Rocket:

https://tubeamparchive.com/viewtopic.php?t=5684

[Jana]

No, you pretty much got it right--total snark. I just find it humorous when newbie amp "designers" come on with their first post and have it all figured out. Seriously dude, do you have any idea what a pair of 6l6 or EL34's in ultralinear configuration can do? No master volume and you want to push the amp to breakup levels in a bedroom? You thought of using UL but will stick to pentode mode? Um--you can't use UL in anything BUT pentode mode. But, whatever. Enjoy the burger while you can still hear it.

[JWK]

Everything you ever wanted to know about the Rocket:

https://tubeamparchive.com/viewtopic.php?t=5684

Thanks! I'll be studying that tonight.

[Firestorm]

For low noise (hiss) use quiet tubes and quiet resistors (good metal films) especially in the early stages of the preamp. As much as possible, avoid too much interstage attenuation, since the signal will pick up noise everytime it goes through a voltage divider (look at something like the high-gain side of a Peavey 5150 for what not to do).

Interesting harmonics are a different matter. Push-pull output stages using 6L6s or EL34s do not have much in the way of harmonic distortion; you can maximize it by deliberately mismatching the transconductance of output tubes and by skewing the AC balance of the phase inverter (lots of discussion on that over on the Dumble side of TAG).

But you can generate a lot of harmonics in the preamp by creating assymetry in the waveform (slightly clipping the positive-going peaks while the negative peaks stay "clean.")

A DC-coupled cathode follower can do this for you (unless it is designed not to). It's the effect that made the 5F6A famous. Merlin talks about it here: http://www.freewebs.com/valvewizard1/dccf.htm

[M Fowler]

The Rocket is basically the bright channel of a Vox AC30 and the circuit is close to the Fender 5f6 bassman.

You don't have to use TW transformers especially since the PT is 300mA overkill and OT is rated to handle 50 watts.

You could go with Fender 40w transformer set, Fender 4 hy choke, etc.
Still would be a loud amp enough for gigging but if you go with PPIMV that would help for home practice.

Mark

[JWK]

I'm sure I have far less experience/knowledge than most here, but from my studies, to remove the noise depends much more on layout, spacing, and grounding methods than the actual circuit used.

JB

Those are all factors, but once you get those right it comes down to number of gain stages and resistance in the signal path. The challenge for me is to get an interesting clean sound (no clipping) with as little hiss as possible. I might never get to where I want to go. It might not exist.

[JWK]

I don't know if this is helpful or not, but I had been working on a new amp using a parallel input 12AX7 into a 5693 Pentode with variable screen voltage into a parallel 12AU7 cathodyne PI into a cathode biased pair of 6V6's. I used an unconventional layout to make it fit in the recycled donor chassis.

It works good at many volumes, including bedroom level. It seems to be mostly clean with the ability to hit some good overdriven tones, but it isn't really suited for heavy distortion--at least as is. However, by manipulating the gain, volume, and VVR's you can find spots to emphasize overdrive at different stages to suit different volume levels. There are some triodes that are running parallel now, but could be used to add more gain stages if desired.

I have not had a lot of chances to put it to a true test with a live band. I expect that it could be tweaked some to suit your taste. As of yet, I do not have any demo recordings. I will have to enlist the help of one of my buddies with recording gear to help me make a demo.

That looks very interesting. Where did you source your 5693? No problems with microphonics? How does this tube compare to a 6AU6? I'm studying your schematic. I might not go with 6V6 tubes, but I might use that PI. Thanks!

[JWK]

For low noise (hiss) use quiet tubes and quiet resistors (good metal films) especially in the early stages of the preamp. As much as possible, avoid too much interstage attenuation, since the signal will pick up noise everytime it goes through a voltage divider (look at something like the high-gain side of a Peavey 5150 for what not to do).

Interesting harmonics are a different matter. Push-pull output stages using 6L6s or EL34s do not have much in the way of harmonic distortion; you can maximize it by deliberately mismatching the transconductance of output tubes and by skewing the AC balance of the phase inverter (lots of discussion on that over on the Dumble side of TAG).

But you can generate a lot of harmonics in the preamp by creating assymetry in the waveform (slightly clipping the positive-going peaks while the negative peaks stay "clean.")

A DC-coupled cathode follower can do this for you (unless it is designed not to). It's the effect that made the 5F6A famous. Merlin talks about it here: http://www.freewebs.com/valvewizard1/dccf.htm

Thanks for that info. I've recently stumbled on Merlin's website and know the very article you're talking about. I might use that. I'm also going to wander over to the Dumble section and see what I can scrape up. Thanks!

[katopan]

...it comes down to number of gain stages and resistance in the signal path.

And as has been said already quiet valves. The valves themselves are the largest contribution of noise (hiss) and will swamp anything coming from resistors. Also there is a huge variation in noise levels generated by one valve to another.

[brewdude]

That looks very interesting. Where did you source your 5693? No problems with microphonics? How does this tube compare to a 6AU6? I'm studying your schematic. I might not go with 6V6 tubes, but I might use that PI. Thanks!

I bought two 5693's about 2 years ago from an internet website, but I can't remember which one I bought them from.

I have not had any microphonic issues. However, I am using a relatively small plate load resistor value which would provide reduced gain and presumably less microphonic issues.

I have never used a 6AU6, or any other small signal pentodes. I am relatively new to pentode preamps.

I don't see why you couldn't use 6L6's with the cathodyne PI. However, this is my first experience with a cathodyne PI.

In fact, this is the first schematic that I designed myself--with the help of many people on this forum, as well as others, and Merlin's books. It was a learning exercise, an experiment. In the end it was a blend of ideas gleaned from research and components I either salvaged from other builds or already had on hand. I expect it could be improved upon (I have started a list of things to try), but I am going to play it for a while before I try anything else.

[Gaz]

In my opinion the goal is pretty moot because any well built clean amp will have negligible to no hiss, so I wouldn't worry about that too much. Honestly, I think the Rocket won't have enough headroom as is, but an octal version may be just the ticket. Also, since it hasn't been mentioned an amp with NFB will have less hiss than one without, all else being equal.

[JWK]

katopan - I have never had the resources to try out a lot of different tubes. I was honestly not aware there is that much noise difference between tubes. You're pushing me toward solid state!

brewdude - I have used a 6AU6 once about 10 years ago. I liked it, but the circuit I was using was extremely noisy. I might revisit an input pentode and will look into the 6sj7/5693 tube. The cathodyne could drive 6L6 tubes. It might be just enough for my purpose here with the plate voltage we're talking about.

Gaz - I'm going for as close to no hiss as I can possibly get, just for the hell of it. Your point about NFB is well taken. I was trying to point that out in my original post when I brought up the NFB pot. I plan on trying to use it as a gain/hiss control.

Thanks to everyone who helped me out in this thread for this project. I definately have more ideas now than when I started.

[katopan]

katopan - I have never had the resources to try out a lot of different tubes.

Don't worry, neither do I. But I've got a couple of spare valves and have noticed different levels of hiss generated by each of them even being the same brand. As the others have said, it's pretty easy to build a quiet lower gain amp.