6V6 Iron With EL84 Tubes?

[neddyboy]

OK, will do. FYI before I saw your post, I plugged in some tubes a fired it up (on the variac). It passed signal, sounded pretty dang good, but not as loud as I expected. I know it was rash, but it seemed safe enough.

I have 2 extra gain stages with two preamp tubes plus half the PI, but it's not as loud as a normal Deluxe. The 6V6s tubes are an unmatched pair, but that's never bothered any of my amps very much, at least for testing.

I'll check the pins and make sure the tubes aren't damaged from the internal short and report back.

[shooter]

It passed signal, sounded pretty dang good

once you fix the gray R you should be able to go to the wall
and

Measure voltage at all B+ nodes. Measure voltage on every tube pin. 

[neddyboy]

I measured all the voltages this AM. The 12AX7 was higher than normal, but proportionally correct. Voltages as follows:
1= 183v
2=7mv
3=2.2v
4&5=3.34
6=270v
7=20.8v
8=63v
9=1v

The 6V6 voltages were not normal, like 5-10% of expected. I suspected the tubes, and indeed, they're really really bad. They were old and didn't have long to live anyway, and the internal shorts must have killed them. On the last round of testing, the tubes got insanely hot. Could the fried tubes explain the low B+?

To be clear, I was measuring voltage to ground with tubes installed, the preamp plugged in with volume to zero, and a speaker plugged in.

At this point, the only pair I have left are a pair of black plate RCAs, so I'm going to get a new set that I'm willing to test with. I'm sad -- the fried pair were also RCAs, just old and nearly worn out. Still, hard to toss 'em in the bin! Anyway, I'll post when I have some new tubes.

[shooter]

what value cathode R are you using on the 6V6?

[neddyboy]

240ohms. Couldn't find a 250 locally. I got all excited because I thought I could drop in a pair of 6K6s that I have, but they require rewiring. I have an offer in on an affordable pair of Sylvanias -- we'll see.

[sluckey]

The 6V6 voltages were not normal, like 5-10% of expected.

What were the actual measured voltages?

[neddyboy]

I obviously misspoke with my 5-10% comment. They're low, but not 5% low. Except for pin 4 on 6V6 #2 at 167mv! Honestly, it's such a simple circuit that I don't get where that extra voltage is going unless it's because of the failed tubes.

6V6 1
2=3.3v
3=138v
4=152v
5=.2mv
7=.2v
8=1.23v

6V6 2
2=3.3
3=somewhere in the range of the other tube. Voltage wouldn't settle, and the tube starting making a whining noise. Not through the speaker, the tube itself. Anyway, I got nervous and stopped.
4=167mv (!!@%#?)
5=0
7=.17v
8=.8v

[sluckey]

Except for pin 4 on 6V6 #2 at 167mv! Honestly, it's such a simple circuit that I don't get where that extra voltage is going unless it's because of the failed tubes.

Easy to prove. Just pull the tube. What voltage is on the pin 4 now?

[neddyboy]

Tomorrow! My afternoon has been dominated by the chore list.

[sluckey]

Where is your schematic?

[neddyboy]

Good morning! I used the attached Weber 5E3 schematic for the PS and PA. I measured the 6V6 pins without the tubes, and saw the following. The high voltage is still too high (I think!?) even for unloaded, but please correct me if I'm wrong. I just ordered a pair of Sylvanias on eBay, so I'll be able to test with tubes in a week.

6V6 1
2=3.5v
3=523v
4=523v
5=bouncing around in the low MV range
7=3.5v
8=0

6V6 2
2=3.5v
3=516v
4=523v
5=.3v
7=3.5v
8=0

[shooter]

8=0

should be in the 15-25vdc range
suspect an open

[66Strat]

8=0

should be in the 15-25vdc range
suspect an open

No tubes ---> No current drawn ---> No voltage across cathode resistor

[sluckey]

Don't sweat the high voltage with no tubes.

[neddyboy]

I did one last sweep through my storeroom for 6V6s and found a pair of EHs that I was willing to risk. I just put them in and fired it up. Good & loud, and no red plating! The PT stays cool, but it does vibrate more than any I've used. Huh. I'll be measuring voltages later today and will let you know. Thanks!

[neddyboy]

Both Pin 3s are at 372v, both Pin 4s are at 325v, and Pins 8 are at 27v. I think the upper limit on 6V6s is 370v, yes? Do I even need to lower these voltages?

[sluckey]

Fender wasn't afraid to run 6V6s at 415V.

[neddyboy]

Wow! I'd imagine these EHX 6V6s are pretty robust too. Well, the last part is arriving today, the jewels for the pilot lights. Once I get all the boxes pretty, I'll post photos.

[66Strat]

Both Pin 3s are at 372v, both Pin 4s are at 325v, and Pins 8 are at 27v. I think the upper limit on 6V6s is 370v, yes? Do I even need to lower these voltages?

Those 6V6GTs can tolerate higher plate voltages, but excessive current not so much. Based upon the numbers posted, the plates are dissipating ~ 18 watts. Design Max rating for the tube is 14 watts.

[neddyboy]

Thanks 66strat! You're also a JT in real life? Anyway, how would I address that? I see online that folks recommend a zener diode before each plate to lower the voltage.

[sluckey]

Use a real 5Y3. Use a 500Ω cathode resistor. Post your new voltages

[High Voltage]

You know, there's nothing etched in stone that says that you must build using a metal chassis.

Hey sorry to hijack, but Im building a wooden amp (actually a prototyping station) as well and wondering how you did your grounding on this? Are you taking everything back to the B+ side of the rectifier and then patched across to earth ground, or something different?

- Steve

[neddyboy]

Hey Sluckey, wow I'd forgotten I was originally going to use a diode bridge! I actually switched back to a 5Y3 pretty early in the PS planning. It's a vintage US tube, Hytron maybe? You'd think I had a silicon rectifier in there tho, with those voltages. I will try the 500k resistor.

[neddyboy]

Steve, the stuff I just built are in aluminum Hammond project boxes, not wood, but my grounding scheme was pretty straightforward. I'll also point out that I'm likely the wrong person to ask! Anyway, the power amp chassis and the PS chassis are linked via an umbilical. Each chassis has a star ground, and the PA is grounded to the PS via the umbilical. Honestly, I'm not sure what you mean about taking it back to the B+ side of the rectifier. As Sluckey and Shooter will tell you, my technical skills leave a lot to be desired!

[sluckey]

I will try the 500k resistor.

It's not a 500K resistor. It's a 500Ω (That's five hundred ohms). Gonna need to be 5 or 10 watts.

[neddyboy]

Sorry, yes, slip of the index finger. 500 ohms.

[66Strat]

Thanks 66strat! You're also a JT in real life? Anyway, how would I address that? I see online that folks recommend a zener diode before each plate to lower the voltage.

Yes, I am a JT in real life. There are several ways that you could use to address the problem. Sluckey's recommendation of a 500 ohm cathode resistor should put you in the ballpark. Alternatively one could add a zener diode between the PT secondary center tap and ground, add a 250 to 300 ohm resistor between the center tap and ground, or add a 250 to 300 ohm resistor in series between each PT secondary and the rectifier plates.

[neddyboy]

Thanks JT. I'm going to start with the cathode resistor, then try the center tap resistor if that doesn't do it. Is 5 watts enough on the center tap? These are either/or options, yes?

[66Strat]

A 5 watt resistor will suffice. The zener diode and center-tap resistor are either/or options. The center-tap resistor could be used in conjunction with a higher cathode resistor value to lower the dissipation, or could be used in lieu of a higher value cathode resistor. It would be a matter of tweaking the resistor values. The center-tap resistor will lower the B+ voltage. A 250 ohm resistor should put you in the ballpark.

[neddyboy]

I just replaced the 240 ohm with a 470ohm on the cathode, and the voltages have risen. 6V6 1 is now 420v on pin 3, 411v on pin 4, and 31v on pin 8. 6V6 2 is 3=410, 4=406, and 8=31v. The replacement resistor is a new Dale and tests correct. I'm somewhat suspicious of my DMM tho -- it's been cutting in and out and acting weird. The measurements are inconsistent even tho the batteries are new.

I also realized that I likely can't do the center tap resistor as the interior of the PS is filled to the brim. I MAY be able to run a series of 2watt resistors, but there's just no room for a cement block.

[shooter]

math says ~ 12W a tube so you're making heat 

a good meter protects good tubes 

[66Strat]

I just replaced the 240 ohm with a 470ohm on the cathode, and the voltages have risen. 6V6 1 is now 420v on pin 3, 411v on pin 4, and 31v on pin 8. 6V6 2 is 3=410, 4=406, and 8=31v. The replacement resistor is a new Dale and tests correct. I'm somewhat suspicious of my DMM tho -- it's been cutting in and out and acting weird. The measurements are inconsistent even tho the batteries are new.

I also realized that I likely can't do the center tap resistor as the interior of the PS is filled to the brim. I MAY be able to run a series of 2watt resistors, but there's just no room for a cement block.

The 470 ohm puts the plate dissipation ~ 12 watts, good to go IMO. How does it sound?

edit:
What shooter said.

[neddyboy]

It sounds great! I don't have an appropriate cabinet for it (yet), so I'm using my Blues Deluxe with a Warehouse ceramic 12 as a cab. It's not as loud as I expected, but that's most likely my perception as my everyday amp is an Ampeg VT-40. Very nice break up with a Tele. My only mild complaint is that the bass is a bit "bloomy", but I suspect a more appropriate speaker & cabinet will change that a lot. I have one '60s Jensen C10Q and I may look for another one for an open back 2x10.

[66Strat]

It sounds great! I don't have an appropriate cabinet for it (yet), so I'm using my Blues Deluxe with a Warehouse ceramic 12 as a cab. It's not as loud as I expected, but that's most likely my perception as my everyday amp is an Ampeg VT-40. Very nice break up with a Tele. My only mild complaint is that the bass is a bit "bloomy", but I suspect a more appropriate speaker & cabinet will change that a lot. I have one '60s Jensen C10Q and I may look for another one for an open back 2x10.

That's the nature of a 5E3. The interactive volume and tone controls offer the means to lessen the boominess somewhat. If it's still to boomy, swapping out the .1 uf coupling caps for .047 uf in the preamp and .047 or .022 in the power amp.

[jojokeo]

If scaling down in size was important I'm surprised Ned didn't think of using 6AQ5's instead? Might be too late (stepping back into the shadows now)?

[neddyboy]

Thanks jojokeo. Heh, yeah, too late! I chose 6V6s for no better reason than I already had a few 6V6s and sockets left over from an earlier build. Early on, I was thinking about EL84s, but I doubt minis would have saved that much room since most of the size difference is outside the chassis anyway.

[neddyboy]

Well, the project is nearly complete. The three components are finished and running well, and I finally trust the power amp enough to put vintage tubes in it! I've attached some pics of both the chassis (plural) and the 5E3-style cabinet I built for 'em. Yes, that's Marshall grille cloth. The PA and the PS will sit in the bottom of the cab, and the preamp will sit on top.

I also wanted to thank everyone who contributed. I feel like I passed some sort of knowledge threshold on this project, and it's due to the patience of this forum. See you next time!

[shooter]

 
Now it's annoy the neighbors time 

[neddyboy]

Not 'til I can afford a speaker! 

[ALBATROS1234]

hey man i only read half of the posts so maybe this was brought up later. you can get a cheap 6.3 volt transformer that handles 1 to 2.5 a for like $10-$15. i used one to run a 6ax5 in a build, about the size of a doorbell tranny. the last one i bought i paid $10 each and they handle 1.4a @6.3v. you can use the main tranny for either preamp filaments or power filaments or vice versa.

[neddyboy]

Hey, thanks for the suggestion! I actually solved the filament issue by using the correct tubes for the PT. I have a Hammond 6.3v that I keep thinking I might use for something!