Wreckit 2xEL84 build

[Smokebreak]

Merlin says : "If you're wondering why all those old amps use a standby switch, its because Fender was designing complicated amps on the cheap. In the bigger versions of the Bassman, money was saved by using power supply caps that were rated only for the working voltage, not the peak voltage which occurred before that valves start drawing current. "

I don't doubt this statement, but am having a hard time seeing in which standby/cap arrangement this would occur. I just measured my Express in different scenarios(stanby always on, turning ac on and of, and vice versa), and do not see a "peak voltage" higher than ~400VDC that the plates charge up to(on the reservoir caps before the standby, or any caps after it) There is no spike. In what arrangement would I see this spike?

[JazzGuitarGimp]

This should be apparent in a SS rectified, 50W or greater amp. And the difference won't be that much. For instance, you might see 425V before the output tubes start drawing current (during the 11-second heater warm up time), and 410V after the tubes are warm and are thus drawing current. Electrolytics used to have two voltage ratings specified on them: Surge and Working Voltage, and the working voltage rating was always a bit less than the surge rating.

[rooster]

Smokebreak, well, that's a funny quote on a few levels. The use of the word 'complicated' is relative. Was it complicated to Fender, Western Electric, or Bell Labs? Apparently not.

As to the cappage of the 59 Bassman and where it went from there, you find two 20uf caps @500/525VDC in parallel before the choke and after the standby switch, eventually moving to two (BF era) 70uf caps @ 350VDC in series on the standy switch. So Fender changed their build. And this was probably wise. But this doesn't mean the 59 wasn't working well because it was. (BTW, the next Bassman after the 59, the (Blond era) 6G6, dropped the .05 cap on the standby switch and stored the initial PS voltage on the two parallel 20uf caps.) But eventually the BF era brought a PS design improvement (?) that came with the times. (FWIW, I think to this day Mesa is having 550-600VDC caps made especially for them - because they can - and it also seems a slight guaranty that the caps will survive a little longer. Eh, but they may have changed at this point, I dunno.)

As to surges in a circuit at turn on, the highest voltage is found on the standby switch while the amp is in standby mode. And a Bassman can hold 509VDC there, something I have measured, which is not a problem for the .05 poly cap. But it can be a problem for the caps downline after the choke, depending on the quality of the cap. The caps used in the 59 and pre BF could survive this because they were quality caps - (but then again they were typically only seeing 490VDC on the standby switch using 110AC wall voltage in the day). So ultimately it was a logical move by Fender to consider a better PS filtering system that could deal with the higher voltage that was coming (and maybe the lessor quality caps, as well), so Fender made a change.

Ha, and we haven't adressed what Marshall was doing. I guess if there's a point to this post, you need to look at a lot of builds to actually understand things, to see how the particular amps survived and evolved as you consider what you will incorporate in your build. Well, but you're doing that, right?

[Smokebreak]

Thanks for the explanations guys, and yes, I'm looking at as many different schem as I can. Every day another piece comes together.
Well this brings up an interesting point in the Express, for example. In mine my B+ is high, with ~440 on plates(that's another story). So at the reservoir caps on the standby switch, in standby mode, I just measured and I'm at ~465. That drops to ~445 when powered up. I meant to install 2 500v 40uf in parallel there, but for some reason I've got 1 450v and 1 500v, which obviously doesn't help the 450v cap at all. I need to change that, but nonetheless, even if I was running normal express voltages, I'd still have around 420v on the res caps, which is cutting it a bit close, ain't it? I would think this alone would be a reason to ditch the standby (or use higher rated caps there).

[cbass]

Could a NTC Thermistor be Useful somehow?When running a largeish cap with a tube rectifier.

I guess thats a no.

[Smokebreak]

Could a NTC Thermistor be Useful somehow?When running a largeish cap with a tube rectifier.

I guess thats a no.

I imagine it has a similar effect as Merlin's resistor across the switch, in an effort to limit inrush current.

[Smokebreak]

I'm getting along pretty good with this build and have a couple of questions. My goal is to make this as neat as I can, and as quiet as it can be.
-I'm going through PT wiring scenarios and I'm wondering which is more advisable in the 2 pics below.

-In both I've got all secondaries twisted together, and the primaries twisted together. The purple represents the ground connection from caps on board. I've got another lug at input side for the preamp grounds. I worried about my power ground interacting with the AC. Should I be?

-Does the lug on the PT seem suitable, or would you drill closer to the IEC and lug it there?

-Would you guys run the secondaries around the edge of the chassis, or down the middle? Does it matter? The only thing that area should see besides AC is the ground wire, then of course the rectified DC from GZ34.

- Also, I noticed in the pic of the Zwreck online that the rectified DC line from GZ34 is twisted with the 5V heaters. I know to keep AC away from signal, but have never considered twisting AC and DC lines. What's up with that?

edit:
- Also, is it safe to use pins 8,6 for ties for screen and grid on 6P14P? It looks that way from datasheet but just checking... Which EL84 subs can NOT use those pins for ties? I'm just going to avoid pin 1


Thanks, J
[img2448]http://www.whiteghostshivers.com/images ... 100044.jpg[/img]

[img2448]http://www.whiteghostshivers.com/images ... 100045.jpg[/img]

[Smokebreak]

I swear I have never "bumped" a post in my life, but judging how quick and attentive you guys always are to questions, in addition to the wackiness on this board the last couple days, I fear my question about my amp got lost in the mix.
I'm wondering about PT wire twisting and placement, as well as how it relates to my power ground run and ground point. Pics are in previous post. Thanks, Jeremy

[NickC]

I twist pairs of wires feeding the rectifier (yellow in your picture), the heaters (green), and the the primaries feeding AC to the PT. I twist the pairs individually, and keep the twisted pairs away from each other as much as practicable.

For the heater string, I use two different wire colors to string up the sockets after the two green wires connect to the first tube in the series. That way I make certain to keep the phase correct. I tend to fly the heater wires above the sockets in Fender type builds. I've seen it laying flat against the chassis in Trainwrecks which seems to work just fine. In the JTM45 clone I built, the heater wires were twisted and ran against the seam between the rear wall and chassis bottom. That works too.

I securely bolt a safety ground (for the green wire, of the three-wire AC feed from the wall socket) very near the location where the AC enters the chassis. I never tie anything else to that connection.

[Smokebreak]

I twist pairs of wires feeding the rectifier (yellow in your picture), the heaters (green), and the the primaries feeding AC to the PT. I twist the pairs individually, and keep the twisted pairs away from each other as much as practicable.

Thanks, Nick. I'm curious as to why each twisted pair of AC needs to be separated, as opposed to twisting each pair, then cable tying all of them together, since it's all just AC

[NickC]

I twist pairs of wires feeding the rectifier (yellow in your picture), the heaters (green), and the the primaries feeding AC to the PT. I twist the pairs individually, and keep the twisted pairs away from each other as much as practicable.

Thanks, Nick. I'm curious as to why each twisted pair of AC needs to be separated, as opposed to twisting each pair, then cable tying all of them together, since it's all just AC

I'm not an expert builder by any means. But I keep them separated so they don't interfere with signal lines, and because they're usually going to different connection points in the chassis. For instance, I don't want heater AC leaching into signal lines in close quarters with the main board or the tube pins. I try to keep some distance between those. A bunch of paired cables, nylon-tied together might form an electro-magnetic field strong enough to introduce noise. It's a "wire dress" issue.

[Smokebreak]

Just fired this up and I'm working on the bias for the 2 JJ EL84s. Right now I've got 150R Rk with 333VDC plate-cathode voltage , 12V across Rk, 40mA per tube, dissipating around 13W each. I know you can run El84s hot, but is this too hot?
Plates are 343V, screens 340V w 470R

[Blackburn]

Just fired this up and I'm working on the bias for the 2 JJ EL84s. Right now I've got 150R Rk with 333VDC plate-cathode voltage , 12V across Rk, 40mA per tube, dissipating around 13W each. I know you can run El84s hot, but is this too hot?
Plates are 343V, screens 340V w 470R

Most people get away with running a hot bias in these kinds of amps and JJs do put up with lots of abuse, so I'd say don't worry about it. If you want some real heavy duty 84s, check out the Russian 6P14P. Those are rated for 14w a piece and Ken liked their tone, as do many TAG members. Pretty cheap on Fleabay.

[Smokebreak]

Thanks a bunch. I actually ordered some 6P14P- K last week(i think those are the anti-vibration?)but of course they're coming from Tbilisi so no tellin when they'll be here. I had forgotten how squishy this type of amp is. Good to have one back in the arsenal. The 3 tone controls are waaaay interactive too. Bottom seems a hair flubby though. I'll have to check it with different tubes/speaker.

[Smokebreak]

Here's some build pics. I finally put this amp through some paces with decent tubes and a Celestion G12-75 and it sounds great! Real throaty and midrangy with nice compression. Thanks for the help!

[img::]https://www.dropbox.com/s/ppt1e0rpl1tv39u/P3070272.JPG[/img]