New High Gain Build

[silverfox]

Still working out the schematic details. Since deciding to build another amp the design has changed due to space constraints, as can be seen. The chassis is from a Magnavox SE console stereo I took home but didn't have room for. That stereo originally was two channels SE. I had another PP Magnavox amp that used the same EL84 output tubes so I'll be using the OT from that and the SE PT for the build. Still not sure where I'm going to put the choke. One thought is to locate it under the OT on the inside of the chassis. Pictures show: Original chassis, stripped chassis, proposed layout.

I'm leaning towards modifying the OD channel of a SLO 100 by voicing it more towards a TrainWreck. The inspiration came from pauledwards.co.uk and the SLO overdrive preamp he built. I'll have questions for sure. In fact I don't know if putting the choke under the OT is a good idea. My thoughts are the chassis should provide sufficient shielding to prevent interference from entering the power supply, but I'm not sure on that.

img to follow shortly:

silverfox.

[silverfox]

Here are the schematics from which I'm working: img to follow, (the only way I can get an image to post is after the fact due to an error. Hence the img to follow quip). Trinity amp it the power amp section I'll probably use with a cut control and perhaps NFB added.

silverfox.

[PRR]

My gut feeling is that this chassis is too small for a HIGH-gain amplifier chain such as four 12AX7 stages plus a power stage.

You seem to have not a lot over 6 inches from the huge signal at the end of the preamp and the super small signal at the gitar jack.

[silverfox]

PRR: "My gut feeling is that this chassis is too small for a HIGH-gain amplifier chain such as four 12AX7 stages plus a power stage"

Thank for your feedback, (pun?), I was contemplating this today, probably lurking in my mind yesterday and what I'm thinking: A research project. I want to try to cram this circuit into a very small space, keep it neat; make it look like satellite technology from the 60's. Why? If all the components of a 40 channel CB can be crammed into a hand held transceiver, in the 80's, why not an audio preamp circuit?. Consider a laptop running at 3 Ghz clock speed with other misc bus circuits running 500 Mhz and greater. And yet the design engineers have managed to tame all the crosstalk you could possibly image existing between 7-10 layers of PCB packed into an 8x10x3/8 enclosure. And guess what? It has an audio interface that produces HiFi quality sound. But wait, there's more! Intel processor technology in 2013 was striving towards > 1 billion transistors in 133 mm square. https://en.wikipedia.org/wiki/Ivy_Bridge_%28microarchitecture%29. I believe there is enough expertise in the forum here to figure this out if the interest level is there. Well, I guess I need a project too.

silverfox.

[tubenit]

My gut feeling is that this chassis is too small for a HIGH-gain amplifier chain such as four 12AX7 stages plus a power stage.

I am with PRR and I think the same thing on this.  I love experimenting and innovative ideas, but I don't think I could personally be successful and happy with the noise attempting this in a small chassis.


With respect, Tubenit

[kagliostro]

OK you would like to build something looking like this



First thing to say .... no AC heaters, DC heaters will be less problematic

but .... have you considered they were build after long studies done by professional engineers with a large experience ?

and more, have you paid attention to the richness in shields of those constructions ?





All that isn't so simple to achieve, maybe you'll be able to build such a thing after testing two or three different chassis with different shields

but since it is not about military equipment, worth to try ?

Franco

[PRR]

Digital signals are not corrupted with part-Volt leakages; raw analog is.

The cellphone does not run tone control wires hither and yon, like we usually do when we space knobs along the length of a chassis.

Any good guitar-size speaker will be bigger, and how much do you gain by making an amp smaller than the speaker it needs?

Mods and maintenance nightmare.

I think it could be done.

[silverfox]

Been out all day. For one, spent 4 hours in the Urgent Care, for not urgent care. But it was a matter of- Better take daughter to see doc for preliminary ear infection. Don't want to Christmas no doc around problem solve. Anyway, Thanks for the posts. Very much appreciated. I'll explain more tomorrow. Benefit of 3 hours waiting room time: 1989 Radio Handbook 23rd edition, chapter on project design and construction. Of no value to this thread. Figured that out after reading section and realizing- I can't filter out what I need in the 1st place and the filter designs are way out of band. It did get me wondering if there is such a thing as grounded grid in audio? and realizing to a greater extent, most of the early amp designers were influenced by, most likely, HF radio from there military service time in WWII.

To explain why I'm interested in doing this: Just an interesting project. A challenge and the parts to give it a try should be less than $15.00. I'll explain more later. Missed my guitar lesson too. That was the sad part but daughter is fine.

silverfox.

[eleventeen]

Hey, give it a shot. I will just say that a single gain stage, 1/2 of a 12AX7, inserted into a normal signal chain of a normal amp, can furnish enough (over)drive to drive the amp completely out of control. 

[John]

To explain why I'm interested in doing this: Just an interesting project. A challenge and the parts to give it a try should be less than $15.00

Best reason of all! And glad your daughter is okay.

[silverfox]

So last night I realized the first thing to do is build the amp on my test chassis since I'll be modifying it anyway and as PRR stated- it would be a hassle to mod. I've also got to better understand parasitic oscillations, since that will likely be the biggest problem. So in this order, at least for the moment, I'm going to build the SLO style preamp as drawn, couple it to a power amp for test purposes, and then mod the existing circuit to get rid of some of the low end purr- I believe I want more of a Marshally Growl, raw sound rather than the smooth distortion of the SLO. Hence the TW direction.

Perhaps separating the two input cathodes and changing the cathode caps to values separated by an order of 10. ex- .47uf for cathode a. and 4.7uf for cathode b. to obtain a voicing. Got the idea from Ginger-tube's "Junk Box TW".

Progress pictures to follow. Suggestion box in break room.

silverfox.

[silverfox]

Choke Placement.

I'm trying to decide where to place the choke in the scheme of things. How susceptible to inductive noise, from the PT, is the choke? I want to place it on the underside of the chassis in the area of the power transformer- With the PT Iron located on the topside and the other half of the windings protruding below. I should also add, the PT is fully encased in the Bell housings. The choke iron, orientation, would be on a different plane then the PT iron. However, there will also be AC wiring in the immediate area of the choke. OTOH, I've seen the choke placed between the PT and OT with apparently no inductive coupling problem from either transformer.

The objective is to compact the power supply components into as small an area as feasible.

Regards and thanks,

silverfox.

[sluckey]

Choke Placement.

Not critical. Look at these examples...

http://sluckeyamps.com/hammond_2/after_2.jpg

http://sluckeyamps.com/sunn/sceptre11.jpg

[silverfox]

I can't seem to decide on the output tube types due to the fact I don't know enough about the sonic characteristics. I'm trying to decide between 6V6 and EL84. The final circuit design will likely closely resemble an original SLO 100 preamp circuit- If that helps at all. The tone I'm after is in the same neighborhood as the, Marshalls and Soldano families. Lots of low end growl and no high end fizz.

Some progress pics. As mentioned previously I'm going to build an 18 watt version laid out with plenty of space prior to trying to concentrate the build into a smaller area. I recycled a research chassis and as can be seen, had to patch a transformer cutout.

silverfox.

[shooter]

6V6 and EL84

fwiw, the 84's drive easier but with a hi-gain pre you should have enough drive for either.  I get more comments about "Marshall sound" from my 84 builds.  I've found using bigger Iron by X2 helps a lot, so an 18w, I would use a 30W OT.  If you hit either with way to much drive, the 6V6 seem to handle it better than the 84's

[BetterOffShred]

Looks really cool!  I love the SLO-100, I remember that picture of Eddie when he played them with a few of the letters removed from the front so it said "SO DA" haha..   Are you designing the preamp yourself,  or are you just using the layout from Sloclone, or are you copying that Junkbox thing there?  Either way it looks like a super fun project. 
-Brett

[silverfox]

BetterOffShred: "Are you designing the preamp yourself,  or are you just using the layout from Sloclone, or are you copying that Junkbox thing there?"

I've gone back and forth a bit in first deciding to build just a preamp, then someone else's design of the preamp and add a power amp section but jam the whole amp into an exceedingly small space; until deciding to build an 18 watt version on a larger chassis and get that working before any "FrankenAmp" modifications. "Meanwhile", Little River Band, meanwhile, another builder submitted his SLO Clone build and I settled on that schematic but stuck with the decision to build an 18 watt version of the output section. Which means a power supply redesign... The design I finally settled on was submitted to the forum a little while back, but I can't find the "Topic" to refer back to it. Something to do with giving back for all the contributions made to the builders journey. Fortunately, being the information scavenger I am, when I saw the build and schematic I immediately scarfed it up. I really like what I see in that design so I'm building the preamp off this design and the power amp will be either 6V6 or EL84.

Thanks for everyones input so far. Much needed. Here's the schematic:

silverfox.

[silverfox]

Some progress and a problem has come up. The as built power supply puts out 375 volts at the 1st cap, unloaded. I should also mention this PT came out of a Maggy EL84 based, SE power amp. Looking at the various EL84 and 6V6 output sections I don't think I can run the plates that high. The chassis is wired for either the Noval or Octal so I could run 6L6s' perhaps, if the filament current is there. I've currently got both filament taps parallel wired and there were quite a few tubes in the Maggy between tuners, preamps, and stereo SE outputs. I'd hate to have to chop the PT out after all the work. So the question is: Unloaded voltage would drop how much after all the tubes are plugged in- If anyone has an estimate... Rough guess?

I've also attached the Maggy power amp schematic for informational purposes. Thanks for any assistance and suggestions,

silverfox.

[silverfox]

More progress. I just finished wiring up the output section. I finally took the time out to use the input vs. output voltage determination to obtain the impedance ratios for several OT I have here. I decided on the 8.2 K winding of the 16R tap. This OT came out of a Univox amp I had. I used the EL84 output design from the 18 Watt Marshal TMB, attached. I'll have to add a FB circuit at some point but for now I'm just focusing on getting the PA up and running.

The layout is a little cluttered but I consider this a research build for now.

Regards,

silverfox.

[silverfox]

Who wants to go for a "Boat Ride"?

It's Motor Boating on power up. Lower frequency like perhaps 30-50 Hz. I'm going to swap the OT leads, check the wiring and grounds as well as replace the 2nd filter cap with a larger size. Any other suggestions welcome. I've grounded all the PA ground points to the same place and not through the chassis. There is also mid level, (high enough to cause the amp to be rejected from a QC point of view), significant background noise but that may be due to the input just hanging in mid air... for now.

Regards,

silverfox.

[TheKT88KilledJFK]

Who wants to go for a "Boat Ride"?

It's Motor Boating on power up. Lower frequency like perhaps 30-50 Hz. I'm going to swap the OT leads, check the wiring and grounds as well as replace the 2nd filter cap with a larger size. Any other suggestions welcome. I've grounded all the PA ground points to the same place and not through the chassis. There is also mid level, (high enough to cause the amp to be rejected from a QC point of view), significant background noise but that may be due to the input just hanging in mid air... for now.

Regards,

silverfox.

Sorry to bump this thread, but did you ever get this worked out?  I've never had an amp give off low frequency oscillation, that I've detected anyways.  I've built much higher distortion amps that the SLO in 1u or smaller chassis, namely 5150 variants that have 3 more gain stages.   Crosstalk is impossible to eliminate in these cases, but normally a very small amplitude still.  I've never had an issue with it!  The small spaces cater to short leads and star grounding, there's simply nowhere else to run the wires! 

[silverfox]

I missed the posting of the question for several days. Okay, As I recall, I never built the HiGain preamp section. I did build an EL84 power amp section and was pleased with the Marshally sound. However, that was constructed on a full size chassis as an experiment. At the time I was using an amp emulator to get the preamp sounds.

This project came after a Dynaco based power amp design, in stereo, my current rig with a different emulator.

silverfox.

[TheKT88KilledJFK]

I missed the posting of the question for several days. Okay, As I recall, I never built the HiGain preamp section. I did build an EL84 power amp section and was pleased with the Marshally sound. However, that was constructed on a full size chassis as an experiment. At the time I was using an amp emulator to get the preamp sounds.

This project came after a Dynaco based power amp design, in stereo, my current rig with a different emulator.

silverfox.

Our experiences are eerily similar! 

I first started modding tube amps with a sunn sceptre, which is basically the Dynaco MkIII. 

I built an el84 amp this summer in push pull with a modified slo preamp, then switched to engl setup, then switched to a modified 5150 gain staging which is a combination of both styles.  This was done in a 12" combo space with no issues with noise or oscillation.  If you ever find yourself contemplating this again, I can say I've had good results with using many different high gain circuits. 

[92Volts]

The chassis you considered using looks even smaller than the "small" high-gain build I posted here recently: http://el34world.com/Forum/index.php?topic=22528.0 but I expect I encountered similar problems to what you might if you do go ahead with putting it in the smaller chassis.

First off 6V6s will take that voltage fine, EL84s probably will too but I'm not as familiar with them.

I got more results from improving my power supply filtering than I expected. Duncan's power supply calculator was useful, and helped me understand the value of even a small CRC filter, especially for a single-ended output stage: http://www.duncanamps.com/psud2/

Shielding my input from the jack to the first grid helped-- of course.

Regarding oscillations I got more improvement by shielding high-signal wires to/from the tonestack and to the power tube, than I did shielding lower-and-medium level signals earlier in the amp. In other words you want to consider sources of noise, not only parts susceptible to noise.

In a small layout, shielded wire just isn't practical everywhere!

[silverfox]

My expectations are: This project will be in Stasis indefinitely as I've gone over to the other side since starting this build. Using an emulator now with the tube power amp. The tube power amp isn't really even necessary. But then, that's an on going debate that I don't intend to start here.

silverfox.

[TheKT88KilledJFK]

In a small layout, shielded wire just isn't practical everywhere!

Using shielded wire gives more capacitance on the wire, then that factor gets amplified by the same gain factor of the section, which IMO isn't necessarily a bad thing but is worth a consideration.  It can always be compensated for by lowering the grid resistors.  I haven't needed any shielded wire yet as these "High gain" designs distort a stage then set the signal output to ~1/11th by the next stage with a 1M/100k voltage divider in the later stages so things aren't really as out of hand as they seem. 

I love modeling too but I see it as an additional sound rather than a replacement.  I spend a lot of time crudely capturing impulse responses for reverbs hahaha. 

[dennyg]

if you haven't completely committed to digital modeling preamp i'd strongly recommend either Merlin's Ultra High Gain pre-amp or the Uber at AX84.
Both only have 4 stages and you've got enough room in that chassis to give a good 4" between tubes to reduce oscillations. 
i've built both with great success.  The other guitarist in my band actually bought my Uber and sold his Mesa dual-rect.  My main gigging amp is the SEL from AX84 so I guess my band is "powered by AX84" now.