Vox-ish build - B+ issue

mallard

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Quick backstory - I'm building AC15ish, based mostly on Mojotone's 3015 with a few minor tweaks to taste. I'm using their chassis but I already had the 18w transformer set from Hammond in stock so I'm using that (and a choke also to be installed).

I've begun wiring in the power supply, starting with the PT (290-0-290) and recto (no filter caps etc yet).

I fired up the PT, checked the HT secondaries (312VAC from each to CT, unloaded).

Installed EZ81 and probed the rectified B+ coming out at 281VDC. The schematic would have me at about 372VDC loaded, so I would expect something a bit higher here.

Thinking maybe something was up with the EZ81, I rigged up a full wave recto with 2 4007s and a 250R sag resistor, and measured out the exact same raw B+ at 281VDC.

So it's been a while since ive done a build - is what im reading correct, or did i miss something somewhere?
 

mallard

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So Ive got the amp built out, checked voltages unloaded and with the preamp tubes only - looks fairly close (a little high unloaded as expected). The final filter cap is reading a hair low but that can be adjusted via the dropping resistor later.

When I install the power tubes, my voltages are dropping about half across the board. Not sure why - any thoughts? I've tried 2 sets of power tubes but I cant vouch for their condition as i havent tested them yet in a working amp.

No signal getting thru....but if i probe the OT primary wires i get the scratchy sound out of the speaker....so maybe im close...
 

Scooby9261

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Wish i could help, but i am very interested to see what the solution ends up being. One of my next builds i want to do a AC15 type build. How did you like their kit ? When i first build an amp i like to get a kit and then the next time ill stray a little and do my own thing with it. But curious what some of you might recommend for an AC15 ish kit out there.

Good luck on figuring out the issue ;-)
 

D'tar

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How bout some photos? Wheres your HT CT? Power and filter pics may help. Or just a full photo dump cause we like amps!😜
 

mallard

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OK i'll work on some photos. Can I attach them here or need to upload to a 3rd party site and link?

@Scooby9261 I didnt buy the kit, just their chassis. I had most of the parts in stock already.

Mods:

I did my own layout on turret board loosely based on theirs.

Ran a small filter cap and voltage divider to supply a panel LED (instead of 6.3v pilot)

Preamp filters are a 16+16 cap can instead of separate electrolytics

Single input jack feeds a dpdt switch to change channels, instead of a mess of input jacks (also grounds the not in use channel)
 

Kevin Wolfe

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You can snap pics with your phone. Then at the bottom of you post you’ll see a box marked attach files. Click on that and it’ll give an option to link your “photo library”.
 

mallard

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Less dress ain’t my bag - don’t judge harshly…




C180E083-B354-4C17-9011-426A4CC272AC.jpeg

11180B60-125B-4842-AEB9-A88FA616AE8A.jpeg


2D2FAF81-8755-4A96-8F3F-04BFE445CC58.jpeg

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D'tar

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This looks dangerous😳... How do you connect the uninsulated clips to the circuit?

1659100490094.png



Is 100v sufficient here?

1659103931746.png


Not that I see anything wrong but I would like to see a clear view of the EZ81 rectifier socket wiring and if you could identify pin 1, and explain the pin connections that would be great!

1659104457964.png
 

mallard

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1) that is cap bleeder resistors for safety. Connected before startup so the caps drain quickly.

2) the 100v cap feeds the panel LED from a voltage divider. It’s connected directly to the raw rectified DC. Perhaps this circuit is putting a load on, but voltages look fine until power tubes go in.

3) it’s a standard ez81 arrangement, except I have backup diodes jumping from 2 unused pins to the active pins. If that doesn’t make sense I’ll get a better pic or sketch it.

Like this but Noval not octal:

1659117886853.png
 

BigDaddy23

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Are you missing a wire from pin 8 on V1? Looks like u have a spare turret in the vicinity there for the bias resistor and cap to be installed.

Also looks like a shield may be close to shorting on that signal switch??
 




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