Tube Lab #28 Prototype E80cc Preamp Circuit & Grounding

[@pedrova8058]

The chassis has resistance. What we do by creating separate star ground points is put a litle resistance (the distance across the chassis) between those 2 points. The largest amount of noise (main source, heaters, decoupling capacitors, etc.) should be close to the main ground side as possible. Then, the secondary grounds (audio) should be "chassied" on the opposite side, so they are slightly "higher" in potential to ground respect to the main/safety ground-chassis connection (we are talking about the resistance of the metal of the chassis, probably in order of milliohms) When there are multiple stages and high gain, the first gain stages should be the "furthest away" (more resistance) in the ground path, then the middle stages, and at the end (with less resistance, near the main chassis ground connection from the outlet socket), the exit stage. The ideal is to have distributed ground connections (a bus wire/bar for example), so you don´t put signal (audio) grounds inside a return ground path from a noisy section

[@tubelab194]

Excellent summary of our standard grounding topology and why it works so well. Years ago when I first started building gear for myself I struggled with a good ground topology and then I learned about the hybrid star ground method and I've employed it ever since on our commercial Kit Amps. Even the very high gain stages of the Universal Kit Phono Preamp are very quiet (-56db) which for the circuit type is excellent and well below the surface noise of the stylus in the record groove!

[@EsotericArctos]

With Switch mode power supply, how d oyou deal with the very high frequency switching noise, which is inherent to switch mode power supplies. The brick used to power up a laptop, even in brand names, spews all sorts of radiation into the air and onto the power lines. You can here it very clearly on the Broadcast and Shortwave bands. What was your thinking in deciding switchmode PSU over a linear PSU?

[@tubelab194]

Hey Brendan! First not all smps are made equally, some are very noisy others very quiet, with most quality units falling somewhere in-between. The first thing I do when sourcing a new smps, is put it on the scope and see how it does. Now remember the dc supply is only going to an indirectly heated tube, so there's no electrical connection with the rest of the tube. Yes it's still possible with a very noisy smps to induce noise onto the grid, but in my experience with the units we carry the circuit stays extremely quiet. All of the testing data is published in the store under "Information" "downloads". Note when you compare an AC filament feed to a DC (even a smps) there's no comparison, the AC supply is incredibly noisy compared to the DC. Why go this design route? Convenience and practicality and so far we haven't had a single problem with this approach.

[@LawsonsStudio]

I've just used a pair of Hytron Det20s in a prototype amp I've built. Be careful as the topcaps were the reverse of the datasheet. I'm very lucky not to have blown them when I plugged them up.

[@tubelab194]

Thanks for the heads up! You can actually see the connections, so it's possible to visually confirm. I'll just make sure the top cap wires can reach across if required.

[@tubelab194]

How do you like the Hytron's?

[@LawsonsStudio]

@@tubelab194 That's what confirmed it eventually. Lucky escape.

[@LawsonsStudio]

@@tubelab194 It's hard to say. Coupled with a pair of Sylvania 6V6s they sound warm with smooth mids and detailed highs. But when paired with Philips 6v6s, I don't like them. Strangely the opposite is true when I replace the DETs with a Brimar 6sn7. Synergy I suppose.

[@proffessasvids]

Hey Jim, thankyou for your wonderful videos sir! Random question. I have a hybrid headphone tube amp (sabaj ph3) that takes 6j9 e180f 6688 tubes. They are run in a starved plate setup, simply as a buffer, in series with a pair of op amp chips. I was wondering what the difference between the 6j1 and the 6j9 was as I'm looking at getting an exduoo mf-602 aswell (6j1). Forgive my ignorance but can one be used in place of the other if the pin outs are the same in the case of my amp? They are both b9f fittings. I wondered this as mine aren't creating gain as such... merely following.. or would I have to change resistances and pin positions etc? Please tell me if I'm being an idiot. Cheers xx

[@tubelab194]

I have zero experience with any of those tubes, what I suggest is you see if a Google search will find a user's group, they may be able to help - luck!

[@proffessasvids]

@@tubelab194 thanks Jim I've been scrolling through data sheets trying to grasp a clue lol. Turns out I was mistaken. The 6j1 is a b7g fitting and the 6j9 is a b9a. Lol xx

[@LawsonsStudio]

I thought MWT was Marconi Wireless and Telephones.

[@tubelab194]

The plot thickens! I was happy to pickup someone else's interpretation of MWT, I initially thought they were Marconi tubes, I'll take a good look at them and see if any further information comes to light. I'm pretty sure they're military surplus, hence the GEC labels slapped on them. Thanks for the additional information!

[@LawsonsStudio]

@@tubelab194 My MWTs don't have any etched codes. My rebadged Mullards do. That being said, the MWTs glassware I have is nice stuff. I don't know what tbe difference between Marconi and MWT is. There's even a 'Marconiphobe' to muddy the waters further.

[@tubelab194]

@@LawsonsStudio thanks for your help, I confirmed this in last week's video, but thought I'd post it here as well. I finally found a picture of a tube box with both the logo MWT and "Marconi Wireless and Telephone Co. Ltd." together, so that confirms it.

[@LawsonsStudio]

@@tubelab194 I wonder was it just a logo change or a different arm of Marconi?

[@tubelab194]

@@LawsonsStudio Marconi was a huge international company for the time, based in London but with plants around the world including a four block x four block plant in Montreal Canada. Given these are probably WWII surplus, it's not surprising that the logo is unfamiliar, not a lot of these tubes survived with boxes and Marconi changed over the years, the last name that I'm aware of was Marconi-Osram. To complicate things further Marconi both made tubes for other well known tube manufacturers and possibly rebranded as well, I've got tubes made in Japan and branded Marconi that I'm not sure who made them.

[@gnpd07]

Jim, as a fellow R8 owner i'm interested. If you added sound clips to your channel, I think you would gain a lot more purchases. It is hard to buy and build an amp on mere suggestion. I believe some would even pay to ship a pre built amp, with a CC deposit to demo at home before commitment.

[@tubelab194]

Good ideas, the problem with sound clips, is to have any value they'd need to be recorded at a fairly high level and even then the largest single influence on the sound would be my custom open baffle speakers! If any acoustic jazz fans ever heard my speakers I'd be in the speaker business, not the vintage tube biz haha!

[@tubelab194]

Currently I'm not selling kits for any of my prototype builds, though I've been quietly doing some development work, the biggest problem is time, I'm running a busy online business on my own and spend a day each week producing Tube Lab, which means I work 6-7 days a week. To develop kits I really would need a partner and a larger facility and more capital...

[@gnpd07]

@@tubelab194 Well, you have me interested, Could I obtain a schematic of this amp? What sonic characteristics did it imaprt on the Wilsenton final sound ?

[@tubelab194]

@@gnpd07 the schematic for the prototype is available for free download, link is just below video, or go to my online store www.valvesnmore.com go to information (top right) and you'll find all schematics. Note no single part of the design is unique, taken as a whole, everything works well. The dual mono power supply and dual mono preamp circuitry is a big part of the magic, that and a very simple circuit.

[@mazakaudio]

Very nice design, but how can you make such an ugly casing and screw up a cool design like this, Oh you English. Greetings from Poland.

[@tubelab194]

Thanks for the mostly kind comments! I would point out that in a past life I was a traditional cabinet maker and happen to really like the chassis design AND my mother's parents were both Polish!

[@dextergilford8336]

Your chassis design looks great!