Stout 18 By-pass caps

[shaun]

I've looked on the stout forum and couldn't find what I'm looking for, so sorry if I missed something.

My question concerns the lack of bypass caps on the cathode of V1b. (There is the 0.68 on V1a. I believe I read somewhere that this doesn't need to be electrolytic.)

I am interested in experimenting with other values - 5mfd, for example - on V1a and V1b.
Are there any reasons, biasing or otherwise, that I should not do this?

There is no bypass cap on V2, and I'm also trying to understand why. My assumption is that the lack of bypass tightens up the tone in some way, but I'm on my second build, so I don't know much.

Thanks for your help.

[sluckey]

Where is this stout forum? Link please.

And can you link to the schematic of the amp you are talking about. Hoffman's Stout is a bit different than what you describe.

[shaun]

Sorry, I just meant the other Stout postings on this site - I did a search and found various threads.

Attached is the schematic I found on EL34.com website.

Since my earlier post, I have found more Stout 18w schematics in this site with suggested mods, so they are answering some of my questions. One of those is attached also. I guess my answer is that I should probably build the thing according to the schematic, and then adjust the bypass caps if I am not happy with the sound.

I'm finding a 0.68uF difficult to come by as an electrolytic, so I was hoping I could simply go with a different size. However, in his mod of a 5F6A preamp, Rob Robinette says:

"The "Lead" channel gets a much smaller, Marshall size 0.68uF (50v or higher voltage) bypass capacitor to attenuate lower frequencies which will clean up and tighten the overdrive tone. This size bypass capacitor is rare but it does not need to be an electrolytic."

[d95err]

0.68u rare? It’s used on pretty much every Marshall amp (and Marshall-clones and wannabees)...

You can use any type of cap with a voltage rating of about 50V or more.

The absence of a bypass cap on the second stage is probably mainly to limit gain a bit. Also, the sound of an overdriven unbypassed stage is considered better by some (matter of taste I suppose).

Feel free to experiment with bypass caps on the second stage. Gain will increase a bit. Your ears will decide what’s best there!

[PRR]

> concerns the lack of bypass caps on the cathode of V1b

Rule of thumb: you need two good gain-stages to get from guitar to power tube.

Another for a high-loss tonestack.

Here we have *three* gain stages. The tonestack is negligible loss. The power stage is more sensitive than most.

You "have to" shed some gain or the thing will be a too-sensitive and hissy beast. (Of course some players like that.)

Not using cathode caps (or only small ones to bring up the highs only) is cheap and cheerful.

But go ahead and play with 47uFd to 0.1uFd on V1A V1B. At first it may be "WOW!". But for real use, it may be too much wow.

[shaun]

Thanks All. Much appreciated.

[PRR]

> I believe I read somewhere that this doesn't need to be electrolytic.

A) You can read all kinds of stuff "somewhere on the innernet".

B) It never "HAS" to be electrolytic (in small audio systems(*)). You can always use a film cap. However a 40uFd 450V film-cap is 5X-10X bigger and costlier than an electrolytic the same value. The whole growth of consumer electronics since the 1930s hinged on lower-cost e-caps; only the telephone company and early church organs clung to large film/oil-caps after e-caps got cheap.

There's traditionally a gap where e-caps are too small/cheap to bother with and film-caps too large/costly. Historically if you figured you needed 1.6uFd 3V cathode cap to make 82Hz bass, you would throw in a 5uFd 25V because anything smaller was no cheaper, less likely stocked, and a little extra bass normally no problem.

But for *lead guitar* a part-uFd cap shaves the bass in a way that gives less mud and more soprano power. Marshall's 0.68uFd cathode cap was a big piece of "the Marshall sound" and helped Marshall sell more than a few amps.

While Marshall used a large-size film-cap, the world of electronics has changed. You actually can buy down to 0.1uFd electrolytics now, and a few cents cheaper than a film-cap.

The other factor is life-span. Early e-caps lasted 5-10 years. Good enough for a home radio or a working-gig guitar amp. Older film(paper)-caps last like 50 years. New e-caps are either cheap junk or will last many decades; modern film-caps worked well within rating are likely to live 50+ years.

So you gotta ask: is this something I will play for now, and repair if needed in the 2030s; or do I hope to live a long time and pass it down to the kids with hope of no-trouble working? For those of us not-young and no musical kids, and always a hot iron around, e-caps are the way to go for anything over 1uFd and for many jobs in the 0.1uFd-1uFd range. If you are Building For The Ages, you may plan different.

(*) Weird fact for the day: in some telephone applications they used "wet electrolytic" as caps but also to absorb surge. A big fat costly e-cap can absorb huge surge energy and self-repair any damage. But this is far outside our field. Image

[shaun]

Wow! That's a great history lesson. Thank you, PRR. That's gold for someone like me, who is learning a hellavalot right now.

The "somewhere" turned out to be Rob Robinette, who seems pretty up on things. I admit I've found 0.68 electrolytics much less easy to come by than, say, a 5uF.

I'm amazed, PRR, that you can look at a schematic and know that "The power stage is more sensitive than most." I hope to have that level of knowledge one day. Boy, am I hooked. Thank you!

[dude]

Also, if you can hear the difference between a 1uf and a 5uf in the first stage, you have "super human" hearing, . These first and second stage cathode caps probably are the most done tone mods, typically used to d-fart an amp or lose some unwanted bass flab. I never understood why Fender and most early amp designers used 25ufs, too much flab, IMO. I find that 5uf on first and 10uf on second stage is pleasing to my ears for Fender like circuits. But have since lowered the first stage to .68u or 1uf.


al

[shaun]

Great info - thanks Dude. I'll keep in mind when attempting my own designs....one day I hope.

[shooter]

I'll keep in mind

I took my 1st store bought new jeep light muddin after I signed the papers, smacked in the rear a little, scratched a little paint;
you don't have to wait, you can modify a brand new amp 

[pdf64]

The signal to noise ratio will be compromised if the input stage’s cathode is not fully bypassed, see tube info, eg p2 of https://bms.isjtr.ro/sheets/093/1/12AX7.pdf
If the objective is to trim the low end there, it may be better for the coupling cap value to the next stage to be reviewed.

[shaun]

Got the Stout up and running today, and man, while it sounds great, it's ice-pick time! I'm running it through an old alnico speaker, so that may be enhancing the trebles. I can see how this design is great for over-driven guitar, but now that I've gotten this far, I've decided to warm it up.

After trying the 0.68uF on the V1a cathode, I swapped it out for a 5uF, and I also added a 5uF on V1b. It tamed the piercing trebles somewhat, but it still has plenty o' bite. I may add a bypass on V2.

However, first I need to understand the tone stack better. The tone knob does almost nothing until it's at about 7 or 8, then it actually starts to cut treble frequencies. Yet if I turn the tone all the way down, there is almost no change in tone. I'm hoping that, if I get the tone stack sorted out, I might not need to make further mods.

The speaker came from a dead organ, and I used the paneling from the organ to build the cabinet. I'll post pics when the amp is running right.

Thanks again for all the help and advice.

[Willabe]

I never understood why Fender and most early amp designers used 25ufs, too much flab, IMO.

Leo had single coil PUP's and he thought his amps would be played clean.

Tele's and Strat's on the lead/bridge PUP is pretty thin/bright.

[sluckey]

I may add a bypass on V2.

Can't do that. V2 is the phase inverter. You can put a small cap (47pf to 220pf) between pins 1 and 6 of V2 to tame the highs. Easy to put directly on the socket.

[shaun]

"You can put a small cap (47pf to 220pf) between pins 1 and 6 of V2 to tame the highs. Easy to put directly on the socket."

Nice! Thanks Sluckey. I'll do that.

[dude]

Also, try a 10u on V1A cathode and maybe a 25u on V2B, if too much flab, lower the 25u. You can experimenting by just lifting one leg and tacking difference values, then fix permanent what your ears tell you.

[Tony Bones]

You can experimenting by just lifting one leg and tacking difference values, [...]

A long long time ago I took a selection of caps and resistors of different values and soldered alligator clips to the leads. Makes experimenting really easy. I spend more time looking for the one I want. 

https://www.ebay.com/itm/100Pcs-Metal-35mm-Small-Size-Crimp-Alligator-Clips-Test-Probes/182089552836?hash=item2a656213c4:g:49QAAOSwjMJXC8~2

[DummyLoad]

https://www.robotshop.com/en/elenco-rs-500-resistor-box.html

https://amzn.to/2Zd6PkH   << same as above from amazon prime. 

https://www.elenco.com/product/1-watt-resistor-substitution-box/

https://www.elenco.com/product/large-alligator-lead-set/

https://amzn.to/2K6XkA2  << 20" test leads with alligator clips

may be of help?

--pete

[dude]

That’s pretty cheap $40, to change almost any value.

[PRR]

> https://www.elenco.com/product/1-watt-resistor-substitution-box/

Elenco 1 Watt Resistor Substitution Box is 3 bux cheaper through Amazon:
https://www.amazon.com/Elenco-RS-500-Resistance-Substitution-Box/dp/B0002KX76M

Their 24-step half-Watt box is $22 shipped:
https://www.amazon.com/Elenco-Resistor-Substitution-Box-RS-400/dp/B00R6SOXLG/

[shaun]

Thanks, PRR. I've been meaning to buy  a couple of decade boxes, and these could be the ones.

I appreciate all the advice received here, but I'm still perplexed by the tone system in the Stout. I can't get the pot to alter the tone unless it's cranked all the way, at which point it kills the treble considerably. I guess it may be a bad pot, so I'll trade that out and see what happens, then post some pics.

Thanks again.

[sluckey]

I never cared for that tone control. It's copied from the Marshall 18W amp normal channel. I much prefer the tone control of the tremolo channel. Easy to change. Take a look...

     http://sluckeyamps.com/18w/18w.pdf

or

     http://sluckeyamps.com/dual_lite/dual_lite.pdf

[shaun]

Thanks Sluckey. I'll give the 1974 18W tone a try. I was wondering about oscillation. Nice schematics. I might try to build the 1974 18W some time.

[dude]

On that M/V layout, do you have a schematic for that mod? Or what are the values of the two R on the PPIMV's dual 500KA pot?


2.2Megs ?