my 18 watt rocket-ish build

[wsaraceni]

could it be a bad output transformer? i was getting 334 off of the standby switch and 320 off of the other side of the choke.

looking at my old voltages i had 334 going into the power transformer and 322 out the other side.

i had 320 coming out of the choke and 329 on the other side of the resistor. there is no way i can have +9 through a resistor so i'll look into that and triple verify. i guess i should also take the same measurements with only the rectifier installed.

[rock_mumbles]

I have had a homemade Champ-like amp that had B+1 at higher voltage than B+2 ... but because of the resistance of the output transformer the output tube plate is lower voltage than the screen ... I increased the screen resistor to a larger value

[rock_mumbles]

sorry double post ...

[wsaraceni]

Ill have to wire it up and retest. Because I'm not getting a higher b+2. My measurements are higher on the wrong side of the resistor. maybe it was a bad pair of output tubes.

[wsaraceni]

didnt wire it up and retest yet, have a cold and just fall asleep at night instead of going to work on it, but did verify its wired correctly.

B+1 is red wire off of first capacitor.
B+2 is on other side of choke, grey wire going to two resistors.
pin 7 has both transformer wires
pin 9 has the other side of resistors.


[img2200]http://img.photobucket.com/albums/v217/ ... 5da9ee.jpg[/img]

[wsaraceni]

so i think the impedance selector was messed up. i pulled it to install the backplate and decided to check all my wiring. somehow it seems that no matter what way the switch was on, the 8ohm tap was always on. i wonder if that was causing the issue. i replaced it with another brand/style that i can actually see the contacts. should help troubleshooting it. hopefully it didnt fry the output transformer.

[wsaraceni]

so i put everything back together and am at a loss. pin 9 is 333v, pin 7 is 327v. B+1 (going to the transformer and one side of the choke is 338v and the other side of the choke (B+2) is 334v. i just dont get why i am only getting a 1v drop across those resistors. is it possible they are both bad?

[wsaraceni]

looking at the rocket voltage chart, there is a 11v drop in their example from B+1 to B+2 and a 7v drop across the OT from B+1 to the screen. i have a 5v drop across the OT from B+1 but only a 4V drop across the choke. i'm thinking swapping out the choke for a resistor instead may fix what it.

thoughts?

[wsaraceni]

can someone with more brains than me look at this thread and tell me what they think?

http://music-electronics-forum.com/t25161/

[wsaraceni]

So here's a quick and dirty. Crappy playing and all.

http://youtu.be/6aA5BG4w99w Voltage is still wacky

[rock_mumbles]

Your youtube link is broken. you need to remove the period at the end of the link


I think your OT is dropping more voltage than the choke is ... that will happen if the OT is higher resistance than the choke.

If you are using 100 ohm screen resistors on the EL84's put a single 1k resistor between the screen supply and the 100 ohm screen resistors ...
That change should fix the screen voltage issue

Or you could put a resistor in series with your choke to lower the voltages after the choke ...

[wsaraceni]

Your youtube link is broken. you need to remove the period at the end of the link


I think your OT is dropping more voltage than the choke is ... that will happen if the OT is higher resistance than the choke.

If you are using 100 ohm screen resistors on the EL84's put a single 1k resistor between the screen supply and the 100 ohm screen resistors ...
That change should fix the screen voltage issue

Or you could put a resistor in series with your choke to lower the voltages after the choke ...

i think thats the issue. im just looking to figure out how to figure out the correct value. i'm thinking of trying the choke in series with a resistor to lower all the voltages after that point, or swapping the choke for a big resistor and seeing how that changes it.

what issues will there be if i dont change it? it actually sounds, and feels, pretty good as is. the clip sucked but that was fully attenuated, through an iphone, with me playing, through a blues jr cabinet with the chassis ripped out and the horrible stock speaker.

[rock_mumbles]

One thing is you'll never get the same voltage drop across the choke as a Rocket with only two EL84's ... if a Rocket has 11V I'd expect ~7V in your amp.

What are you using for the OT?

--==--

If you put the resistor in series with the choke the preamp voltages will also be lowered ... by probably about 10 volts ...

If the preamp voltages are over 10V high then put the resistor in series with the choke.

If the preamp voltages are good put the resistor in series with the two 100 ohm resistors in the screen supply

The instructions Bruce Collins gave in the thread you linked to was start with about a 500 ohm and work up to about 1k ...

For a reference a low voltage 18 Watt Marshall (275V CT PT) uses a 1.5k resistor between the plate and screen supply ... what's the DC resistance of your choke? Usually they would be between 100 to 250 ohms ...

my guess is you'll need a 1k in either place

--==--

Is the amp really dirty or does it have pretty a decent amount of cleans?

What's the amp sound like with your guitar volume backed off?

[wsaraceni]

I'll take video again with the amp clean and with the volume backed off. Still a little gainy.

I believe its a gds 18 watt OT. Can't remember where I got it from but I think it's there. Probably could use a little less gain with the amp but it's relatively clean until about 11:00 on the dial

[M Fowler]

Sounds like it's not bias correctly.