S T A A L H O O F D mod

[Colas LeGrippa]

Hi,

I,d like to know if anyone tried the staalhoofd mod. ( and if you like the tone ).  I am building an amp with the power section of a 2204 but with the staalhoofd preamp. I was just curious to know the typical voltages at different test points of the preamp. ( a, k and B+ ). BTW, I know that those voltages will vary according to the PT and power section, but he relatives voltages will be the same.

Regards

colas legrippaaaaaaaaahhhhhhhhhhh

[tubenit]

I have NOT built that amp nor heard one.

Having said that,  PLEASE notice you have 5 gain stages in that schematic.  Even Trainwrecks and Dumbles and other highgain amps typically have less than that.  I would think the amp will be really noisey at idle.

And note that V3a is showing a 330uf cathode cap before the cathode follower. I can not imagine what that would sound like?  I typically will use a 2.2uf or LESS in that position.  When I've tried a 4.7uf there is was WAAAAYYY too gritty for my taste and lacked smoothness.  Maybe the schematic is drawn incorrectly there?

Also notice that V2 is recommended as a 12AU7 (30% gain factor) or 12AY7 (45% gain factor). My conclusion from that is maybe a 5th gain stage really isn't needed?

Here is an amp design that I offer for consideration. I haven't built it, but I'd really like to. My current favorite amp of a TOS 2CF is headed that direction. This design would give a nice clean or a nice overdriven tone, IMO.

with respect, Tubenit

[tubenit]

If you look here, you'll see a great amp design by OldHouseScott.

He has a layout and relay channel switching diagrams and info there also.

http://www.el34world.com/Forum/index.php?topic=745.0

Here is his sound bit which I think sounds superb!

http://www.soundclick.com/bands/default.cfm?bandID=870873&content=music

Listen all the way thru it because the OD clips are close or at the end.  It starts off spanky clean and goes to OD tone.  It doesn't begin to sound "metal" until the latter clips.

With respect, Tubenit

[Colas LeGrippa]

thanks for your posts, Jeff. I have listened to the sound clip and that's not the tone I am after for the moment, though it sounds good. I took a look at the schem you posted and I noticed your standby switch is a dpst on the AC side. Any particuliar reason for that ? AC is less stress on the switch ?  I once wired the standby your way ( and many people's way in fact ) and the switch shorted in the inside causing the loss of the PT ( I must admit the switch was not of the best quality available..... ).  Since then, I use a SPST ( no risk of a short )  on the DC side, IMO. But since I am a newby compare to you in amp building, I'd like to have your opinion on that particular matter.

Best regards

Colas

[sluckey]

A member named Aro started a thread about that project and with some info from him I put together that schematic plus a layout. Here's the original thread...

http://www.el34world.com/Forum/index.php?topic=8470.msg76869#msg76869

And here's a direct link to the pdf... http://home.comcast.net/~seluckey/Aro_Project.pdf

Maybe there's some info you may find helpful. Also, try contacting Aro. He posted a valid email account.

[HotBluePlates]

If it works, then it works... but 4 cascaded gain stages fed by a single filter cap seems to be asking for motorboating and unwanted feedback. Stage 1 and 3's signals should interact, same with stage 2 and 4's signals.

Or maybe that's the mojo. Just seems poised for trouble.

[PRR]

> PLEASE notice you have 5 gain stages

But many of them are low-gain stages.

2 3 4 are 10K cathode resistor. With 100K load the gain can't be even 10; in fact closer to 5. These are connected with 470K+250K networks, loss of about 3/1. So each stage has gain near 5/3 or 1.66. These three stages give gain near 5, much less than one "normal" stage.

The obvious intent is to have the stages overlod 3 2 1 in quick sequence; or rather (since guitar is pluck/decay) to come out of overload 1 2 3 for a large change of color in a short range of dynamic.

That's ignoring caps. There's 100pfd to trim the highs and 470pFd to boost the highs; not going to try to figure that out. There is one with 0.1uFd cathode cap which will raise gain from 5 at 100Hz to maybe 40 at 800Hz. One with 2u which actually gives full gain of 40-50 across the guitar band. So the 3-stage gain may be more like 1.6*40*40= 2500 above 800Hz and 1.6*1.6*40= 100 at lowest note.

The gain below the guitar band "may" be low enough to evade motoboating. The worst case seems to be gain of 1.6*1.6 or 2.8. A 100K plate resistor into a 33uFd rail cap will have a loss of 2.8 at far below 1Hz.

IMHO the major bass-cut in the 3-stage means the 5th stage may as well have full bass. In the clean range, 20uFd would be ample. However as these stages sequence in/out of FULL CLIPPING there will be a lot of bias-shift superimposed on the signal. It may be useful to have the 5th stage VERY stable against bias shift, hence a solid bypass.

Voltages? Plates 2 3 4 will be quite close to the rail. Stage 1 near 85% of rail. Stage 5 will be more like half-rail. Main thing here: V3B cathode normally should be positive of V3A plate; if not, the CF will clip (which is often done and not "wrong" for highly-shashed guitar).

[Colas LeGrippa]

Also notice that V2 is recommended as a 12AU7 (30% gain factor) or 12AY7 (45% gain factor). My conclusion from that is maybe a 5th gain stage really isn't needed?


I have experienced with 12ax7 and 12au7, and even if a 12AU7 has less gain, it sounds different. The inside capacitance, the heater to cathode max voltage are different so it gives the tube its own tone. That's why I have always preferred a 12au7 in the PI position than a 12ax7. Not for the gain, for the tone.  Anyway, even if I would put a 32uF min. to feed each tube, I will build the STAALHOOFD anyway, as is, ready to mod it according to the level of........motorboating. The power amp is already wired with the PI, and the 4 X 12a????????7 are standing on top, ready to be wired up. I'LL GET BACK TO YOU WITH THE RESULTS hey hey....I think it is a good experimental preamp to build.

Colas

[tubenit]

Sounds like you're clear with what you are wanting and that is a good thing!  You are absolutely correct that the 12AU7 and 12AY7 have both a different gain and a different tone.  IF you have not tried a 12AV7 ........ I can recommend those also.  12AY7= 45  12AV7 = 41  12AU7 = 30


A couple of things to consider.  You can still maintain the design integrity of the amp BUT use internal trim pots instead of the 470k/470k voltage dividers.  That would allow you to adjust the gain to find the "sweet spot".

Trim pots are very easy to install and they have been useful to me in dialing in "my" tone.

I would be inclined to have one of those 3 ........ NOT be a trim pot but be a full sized pot on the front.

Using a 1k dropping resistor and another 33uf or 20uf filter cap may help you reduce the noise that you're probably gonna have.

Have fun with the build and good luck with it!    With respect, Tubenit

[tubenit]

Is this the kind of tone you're wanting ??????  

5 of 5 - SIR #36 AFD mod FINAL RESULT

[HotBluePlates]

Well, PRR's explanation mitigated my fear of instability, and the video clip confirms the mod (if BRBS did it as shown in our schematic, which they probably did) lives up to the sound.

[Colas LeGrippa]

thanks for the clip Tubenit, it's exactly the tone I'm after, for that particular project.

Regards

Colas

[DummyLoad]

RCA touched on the problem in the 50's.

please see attached. two stage pentode: av 9000 

--DL

[Colas LeGrippa]

very interesting............next time we experience motorboating, we know where to head for...........

Colas

[gldtp99]

Here's a simpler way to get that type of tone, the #34 Mod---- just one very important thing, IMO---- DON'T use the "hot shield" on the shielded cable from the input jack to the first gain stage-- they're running the shield to the V1 plate---- if the shield somehow works it's way to conduct with the signal conductor of that shielded cable, you'll get high voltage on the guitar--- an unsafe condition if i've ever heard of one. Some people doing these mods insist that it's important to get "the tone". I've built several versions of #34 and #36 modded amps and i don't agree.
But the other mods are quite simple and will produce the tone from GnR's 2nd album using the standard Marshall 3 gain stages plus the standard cathode follower.
The #36 Mod (1st GnR album) adds another tube stage (4 gain stages + CF)---- i built one of these with 6L6GC x 2 and a guy came over to talk about a possible amp build, played this amp and bought it on the spot--- no faceplate, just Sharpie-maked controls--- beat-up old Marshall head cab--- but he had to have "that amp".
#34 Mod Layout (again don't use a hot shield on the input gain stage):
http://img819.imageshack.us/img819/3811/slash34.jpg

These mods will give added preamp distortion and boosted hi-mids/highs--- they can sound good--- they also add noise and the amps tend to feedback easily----i've gone in another direction with my higher gain amp building---but if that's the tone you crave then build one.
I can't see how five gain stages are needed for this type of tone..........................gldtp99

[gldtp99]

Here's one version of the 4 gain stage #36 Mod--- again DON'T use the hot shield on the first gain stage:
http://img526.imageshack.us/img526/6923/slashmodv4c.jpg

I lost interest in the 300+ page thread at Metro after i got the basics of these Marshall mods and built a few amps using different versions--- i really was disappointed that some of them seemed to advocate building potentially lethal amps---- ground the shield of the cable from the input jack to the first gain stage and don't kill yourself or anyone else.
The layout i linked to is just one of about a bizillion versions of this mod that they came up with---- they all sound about the same.....................gldtp99

[Tone Junkie]

The way around it is to use some shielded cable coiled over by the unput side its just 4 or 5 inches wrap it in a circle,shrink wrap it and viola same capacitance facter hooked to the tube without any lethal voltage .
I tried most of them that worked just as good as the other way.
 I also subbed in caps didnt sound the same tried #39 and # 34 Go look at ampgarage and find the one by darkbluemurder i think its called high gain marshall or something. also ceriatone is showing a layout for one of them now , he has trimmer pots inside to dial it in and gives some readings on the layout.
Tubnits suggestion about the trimpots is very good this is one of those amps that you need to dial it in. I wish I would have known that trick when I built mine.
 Transformers, guitar pickups, speakers all those things make a big differance in dailing the sound in.
 After building a couple renditions and tweeking them I gave up on the thread over at metro, To much in fighting over stupid crap several of the guys there nailed the tone perfect and did it a little differantly.
But good luck after enjoying several of the builds, I went a differant direction a little more high gain .
I actually liked a 12ay7 in several positions in that amp so i had to change a few things to get a little higher gain, but the 12ay7 is such a musical tube to my ears .
I have to think Tubnit and Geezer for turning me on to that tube. If you cant find the other ones Im sure I have half a dozen differant schematics in my computer somewhere I could send them to you.
Thanks Bill 

[Colas LeGrippa]

The purpose of the shield connected to the plate is to inject back to the plate any noise picked up by the guitar, but 180 deg out of phase thus cancelling it out. What I have done in the past is using a twisted pair of leads ( like heaters wires ) , one lead from input jack to the grid, and the other one connected to the plate with the other end wrapped up over the signal lead to approx one inch from the input jack and with the end ''cauterized'' and hung up firmly from any safe manner ( terminal strip eg ). It is a good noise cancelling simple circuit and with no risk if done properly. Look at the plates of all the tubes close to the chassis from 1/8'' in , often, with no problem. Anyway, if the chassis is grounded ( and the wall outlet too I hope ), the B+ touching the chassis would blow the fuse right away ( if you have built the amp with one fuse,  hey hey hey..... )

[jerrydyer]

ive built similar amps with just the 4 gain stages and the preamp NEVER goeas above 1pm so I see overkill.

[jeff]

I think it's about how each stage overdrives the next and how the distortion changes in relation to how hard you pick or as the note decays, not just about the amount of distiorion you can get from one stage. More stages more complex texture at different volumes.

[gldtp99]

I was at my shop pulling parts for a 50 watt Plexi build for someone and i took a break to plug into the 14/7 (pentode/triode) #34 Mod amp i built a couple of yrs ago------ Three gain stage #34 Mod preamp into cathode biased EL84 x 2 (GE 6BQ5's). Ran it thru a Hiwatt 4x12 cab.
What a fun rock amp to play !!! I don't see that more than 3 stages are needed to get similar tone to the vid above--- and this 14 watter doesn't need an attenuator-- --- it also has an adjustable NFB pot to get more/less overall hair into the tone.
I recently got this one back from a local studio that keeps borrowing it for recording purposes:

[Colas LeGrippa]

very neat amp