Well, if you hear of a fire in LA tomorrow...

Post by **Scumback Speakers** » Wed Nov 28, 2007 4:56 pm

Whoops, brain fade. Sorry! Yeah it's grounded at the output tube ground tab.

So, I think the real problem was finding that one wire on the first
filter cap that wasn't soldered, but I sure had it wrapped on tight
enough to make an almost good connection, so now the amp is working...now the next really fun part...

It won't bias over 19.5 on my meter, no matter how far I turn the
bias pot adjustment.

I'm pretty sure I'm not supposed to ADD the two readings together,
and this multi meter worked fine on my D'Lite to bias it, so now
I'm trying to figure out why it's not biasing up to 35-40 ma.

And then maybe I could try these Amperex EL 34's and Bugle Boys
out to see how they sound.

Where's that emoticon of you smacking yourself in the forehead?
I think I need that.

If anyone has any ideas on the bias adjustment thing...let me know.

Oh, the bass pot still works, but it's either on, or off, and it's making
the presence pot act funny, too, it's scratchy when turned.

(I should have just had George build it....d'oh! Just kidding!)

mwm523 · Post by **mwm523** » Wed Nov 28, 2007 5:23 pm

Jim, it's hard to tell from the pic, but is your bias resistor (connected to the bias pot) a 47K (yellow-violet-orange)? It almost looks like a 4.7K (yellow-violet-red) in the pic...

How about the one above that between the 2 bias caps? What value do you have there?

Post by **Scumback Speakers** » Wed Nov 28, 2007 5:44 pm

mwm523 wrote:Jim, it's hard to tell from the pic, but is your bias resistor (connected to the bias pot) a 47K (yellow-violet-orange)? It almost looks like a 4.7K (yellow-violet-red) in the pic...

How about the one above that between the 2 bias caps? What value do you have there?

Well, it's colored like a 47K and that one is missing from the pile. I measure each resistor before it goes in, and that one matched the layout for 47K. The resistor between the two bias caps is a 15K, again, as specified in the layout.

Here's a pic to verify, though...

http://southbayampworks.com/m50/biaspotpic.jpg

mwm523 · Post by **mwm523** » Wed Nov 28, 2007 5:52 pm

Ok, that pic is much clearer and they look right.

Does the bias adjust at all when you turn the pot? Do you get the same (or close) measurement on either tube?

Tried different tubes?

Post by **Scumback Speakers** » Wed Nov 28, 2007 5:55 pm

mwm523 wrote:Ok, that pic is much clearer and they look right.

Does the bias adjust at all when you turn the pot? Do you get the same (or close) measurement on either tube?

Tried different tubes?

The bias adjusts from 0 to about 20 (max). Those are some Amperex tubes made by Matsushita in Japan (6CA7/EL 34).

Time to throw in some SED's, or break out the real stuff (the Mullards and Telefunkens, Bugle Boys and Siemens)???

Hell with it, I'll try another pair of tubes. WTF, I have about 10 matched pairs here, so...

toner · Post by **toner** » Wed Nov 28, 2007 5:58 pm

It looks like first voltage dropping resistor is 820 ohms (grey-red-brown). It should be 8.2k (grey-red-red). That would at least explain your high preamp voltages.

It may just be the picture. I circled it here:

Post by **Scumback Speakers** » Wed Nov 28, 2007 6:09 pm

Ahh yes...so much for those Japanese made Amperex's...put in some new SED's that were a matched set from "thetubestore" and it biases just fine now to around 37 ma. My meter works fine, my ANOS tubes from eBay didn't. LOL

OK, now I guess I should just let the amp burn in for about 24 hours with the volume down, speaker plugged in and check back on it tomorrow...right?

Thanks Mike, toner, and everyone else who chipped in on helping me out!

thousandshirts · Post by **thousandshirts** » Wed Nov 28, 2007 7:02 pm

"Take two new tubes and post here in the morning!"

mwm523 · Post by **mwm523** » Wed Nov 28, 2007 7:29 pm

toner wrote:It looks like first voltage dropping resistor is 820 ohms (grey-red-brown). It should be 8.2k (grey-red-red). That would at least explain your high preamp voltages.

Jim, did you check the resistor toner mentioned? It doesn't look right to me...

Southbay Ampworks wrote:Thank you sir! Did your speakers arrive?

They sure did! Got here on Saturday...

Post by **Scumback Speakers** » Wed Nov 28, 2007 7:36 pm

mwm523 wrote:
toner wrote:It looks like first voltage dropping resistor is 820 ohms (grey-red-brown). It should be 8.2k (grey-red-red). That would at least explain your high preamp voltages.
Jim, did you check the resistor toner mentioned? It doesn't look right to me...

Sheesh...wrong resistor, must have read the meter too fast. Figures, getting near the end, rushing to finish...d'oh!

mwm523 wrote:
They sure did! Got here on Saturday...

Great! OK, draining caps (again) and I'm going to replace that one resistor. Can't be burning up the Mullards/Teles/Bugle Boys now.

Has anyone taken a whirl at individual bias pots for each tube? I've got some XF2's and 3's here, and (of course) they're a good 10-15 ma apart in the amp.

Wouldn't individual bias pots allow you to bias tubes that weren't matched?

Post by **Scumback Speakers** » Wed Nov 28, 2007 8:22 pm

Ok, changed the dropping resistor (thanks Toner!), and put in some NOS Telefunken EL 34's, Mullard in V1, Telefunken in V2, and Bugle Boy in V3.

Talk about singing, sustaining, ripping tone...wow!

Guess I better go dig my head box up out of storage before they close and get this amp buttoned up.

Hey guys, thanks very much for working with me on figuring out what the hell I did wrong, and taking the time to post some answers, troubleshooting and other stuff so I could get this finished.

I really appreciate it very much. Thanks!

toner · Post by **toner** » Wed Nov 28, 2007 8:37 pm

Southbay Ampworks wrote:...Talk about singing, sustaining, ripping tone...wow!...

Glad it worked out!

Post by **Scumback Speakers** » Wed Nov 28, 2007 8:53 pm

toner wrote: Glad it worked out!

F**king A! I've been playing it for testing through an H55-16HP now for two hours. I realize now why people use those with their Strats and bright amps. This one is bright, but not painfully bright. But that bass cone really allows it to come alive.

I'll go through everything I've got with it, of course, but for a single 12 that H55 was ripping.

908ssp · Post by **908ssp** » Wed Nov 28, 2007 10:33 pm

Wouldn't individual bias pots allow you to bias tubes that weren't matched?

Yes it would. I asked Brian to do it for me on my BB because matching GEC are so hard to find.

Big Mike · Post by **Big Mike** » Wed Nov 28, 2007 11:22 pm

Sounded pretty good over the phone Jim.

When I saw the # on the caller ID I thought SURE it was Mrs. Jim offering me your guitar if I could scrape your ashes off the neck....