NewYorkDave

Am_peg Gemini II... Super loud hum when powered on.
« Reply #20 on: August 01, 2005, 10:00:19 PM »
He already did that, CJ. Pay attention  :razz:


CJ

Am_peg Gemini II... Super loud hum when powered on.
« Reply #21 on: August 01, 2005, 10:07:31 PM »
1) Ignore previous post by burned out cj
2) Disconnect B+ lead feeding output transformer. (Coresponds to Point A on the schematic, the lead of a bundle of three not going to the power tube sockets.
3) Re-fuse
4) Stand back
5) Turn on amp
6) Report condition of On/Off pilot light
7) Let amp sit for 5 minutes before poking your snoot in there, as I see no bleeder resistors. Check caps with dc meter and discharge with you neighbor's pet parakeet if necessary.
8) Vacuum up feathers

Sovtek now makes a killer 7591 so don't worry about having to shell out for "NOS" (new online scam) power tubes.

Or, for half the price, you can buy this pair of NOS GE's for 19.99, if the skollywags  don't have it marked.

evil bay 7591

I am suspecting the solid state diodes at this point, with or without a short circuit somewhere as a partner in crime. But that is just conjecture, your Honor.
Your witnesss.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

T-Dogg

Am_peg Gemini II... Super loud hum when powered on.
« Reply #22 on: August 02, 2005, 12:22:28 AM »
You guys are awesome.  Seriously man, I appreciate this!  Think I might've found it.

Just tested the diodes (From what I get reading in the schematic, it shows two of type "hv-24c" -- in my amp I found two of type KIM4007 in series for each original diode).  In circuit, they seemed bad, passed voltage in both directions, so I ripped em out...  But out of circuit, they each tested fine.  Seemed wierd from how I understand diodes to work, so I started testin continuity, and point A on the schematic provided a sputtery signal for continuity to ground.  I took off the B+ lead like you suggested -- no continuity between point A and ground...  B+ to ground = continuity.

I think I see where you were going CJ -- shorted output tranny?
Tim

CJ

Am_peg Gemini II... Super loud hum when powered on.
« Reply #23 on: August 02, 2005, 01:36:12 AM »
"B+ to ground = continuity." you mean the output transformer B+ wire to ground shows continuity?

Use ohms just for the heck of it.

Also, check that output transformer wire to ground without the pwr tubes installed, as a shorted tube would complete the circuit from the output iron to ground.

I hope it isn't the output x-former, as those can be $$.

If the output iron shorts, it is usually from the pri to the core, which is grounded.

And no need for thanks. There are people around here that, for reasons unknown, actually "like" doing this. Please don't think of us  us sick, just think of us as a bit, well, deranged.  :razz:
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

T-Dogg

Am_peg Gemini II... Super loud hum when powered on.
« Reply #24 on: August 02, 2005, 09:55:44 AM »
Yeah -- with tubes out, I'm getting a completed circuit from B+ to ground.  So I guess the transformer is trashed.  Last night I thought about abandoning this thing, not wanting to spend more money on a transformer and possibly tubes...  And maybe a speaker if I wanted to do the bass guitar thing...  

But, today I'm feeling saucy again.  F'ck it, I'll see it through -- if nothing else I'm picking up chunky little gravy-covered tidbits of info along the way.

Which leads me to another question...  

Output transformer reads OT-213...  A google search comes up with nothing!  But I did find a website that references an OT-215 as a replacement for the Gemini II output transformer.  Cost: $115.00.  Damn...  Does that seem a reasonable price?  It is a pretty beefy piece of iron...  I'll do it eventually if I have to, but I'm wondering if anyone knows of any other cross references that may be cheaper?  The site just refers to the OT-215 as a replacement Ampeg tranny, no clue who actually manufactures it (maybe it is an actual Ampeg part...)
Tim

NewYorkDave

Am_peg Gemini II... Super loud hum when powered on.
« Reply #25 on: August 02, 2005, 10:02:05 AM »
Output transformer shorted to ground is possible but rare, so don't despair yet--it could be something else. This is important: did you disconnect the lead from the power supply to the centertap of the output transformer? Does the power supply read a short to ground only when the transformer is connected? If so, does the centertap of the output transformer show a short to ground? If you didn't take measurements this way, then it's very premature to assume that it's the OT. From your responses, it's not clear that you're doing exactly as CJ suggested.

Gus

Am_peg Gemini II... Super loud hum when powered on.
« Reply #26 on: August 02, 2005, 01:17:04 PM »
Does anyone have a good ampeg page.  I was just looking around and have not found one with replacement transformers.  

I would guess this has a 6.6k primary like some 6l6 fenders used IIRC.  Does anyone know what the ampegs used?

Like NYD posted it might be something else if someone was inside this before you they might have caused a short to some part of the circuit or chassis with a solder drip or blob or wire etc.

Does it have a circuit board maybe something is stuff under it like a nut etc.  You never know what you will find when you work on amps.

T-Dogg

Am_peg Gemini II... Super loud hum when powered on.
« Reply #27 on: August 02, 2005, 01:35:05 PM »
Dave -- Here's what I did, I think its in line with what you're saying about a bad transformer...  With the center tap disconnected from point A on the schematic (directly following the rectifier), and the output tubes removed, I got a complete circuit between the center tap and ground.  Granted, my definition of a "complete circuit" was based on testing continuity between the disconnected center tap and ground.  Without the center tap connected to point A, there were no shorts (or really, there was no continuity) between ground and power supply.

This is the only thing I found online for that ot-215 Ampeg transformer:
http://www.fliptops.net/catalog/product_info.php?products_id=418&osCsid=ca0643d461fa5fb45dc392731de2982a
Gus, if you can come up with a cheaper alternative that'll work, I'll detail and wax your car!  With a Q-tip.  No, no...  With a small piece of firmly starched thread!

I did look under the main board (it sits on a raised strip of wood so you can see the underside of the points where the connections are made), thought maybe I left some of the capacitor legs that I replaced too long, but I didn't see anything that was grounding out.  Looked over it pretty hard, but I didn't go so far as to remove the main point to point board...  Everything around the pots seemed ok, but I'll admit I didn't pull them out either...  Is is still worth checking based on what I wrote above with the tranny?
Tim

Scodiddly

Am_peg Gemini II... Super loud hum when powered on.
« Reply #28 on: August 02, 2005, 02:05:37 PM »
I wonder what would happen if you removed the output transformer and then measured it?  A fun thing might be to then run a ~60Hz tone from another amp (a nice durable solid-state amp) into the speaker connections on the output transformer, and measure what you get at the tube connections.  

I once had a Marshall JCM-900 on my bench with a funky output transformer.  It tested fine, but under load it would short internally and drag the tubes down.

Gus

Am_peg Gemini II... Super loud hum when powered on.
« Reply #29 on: August 03, 2005, 09:41:42 AM »
NYD

Do you know if the PS will overvolt the 500V caps if T-dogg pulls all the tubes?  

I would like to find out if the fuse blows with all the tubes pulled maybe something wrong with a fil in a tube.

T-dogg

lets do some more checks to see if something other than the output transformer

unplugged and caps checked to make sure they are drained.

Is this still a 2 wire plug with the cap on the polarity switch?

Is the power switch a 2 pole?  4 or 6 terminals? switches sometimes "fall apart"  inside the switch body and if it is like the linked schematic it switches both the line side and B+ to the power light

Check both standby and power for shorts from pole to pole.

Check the hum pot to make sure there is not  shorted or broken.  This might be causing a high current draw on the heater supply section of the power transformer

What is the size of the output transformer LxHxW in inches?

edit    a good place for tube amp repair http://www.geofex.com/

look at the left side for


CJ

Am_peg Gemini II... Super loud hum when powered on.
« Reply #30 on: August 03, 2005, 03:01:21 PM »
I just thought of something. If the output iron was toast, why would it still pass 60 cycle hum?

Maybe someone hooked it up wrong. The three wires on the tube side should be brn-blu and CT=red.

At this point, I would completely disconnect the transformer, unbolt it from the chassis and ohm it out. Check from the red wire to the core-core bracket area. You might need to poke thru the varnish/black paint.In five minutes you will have your answer.

You could turn this thing into a Trainwreck Reverb Rocket quite easily, which is a killer amp.

There is no need to spend 115 on new iron. There are a million transformers that could be used. I will have a look around evilbay on my daily pilgramge for the Peerless S-217-D and see what I see. Older iron usually sounds better.

Also, check the polarity on those diodes. This seems to be backwards diode month around here. The bars should be going to the cap side positive.
cj
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

T-Dogg

Am_peg Gemini II... Super loud hum when powered on.
« Reply #31 on: August 28, 2005, 11:04:16 PM »
Welp, got a replacement output transformer -- pn#C-050438...  Supposed to be the original replacement part for a [email protected] HotRod Deluxe amp.  The guy I got it from actually designed some of the Sunn and [email protected] stuff, needless to say I was pretty siked I was able to grab his ear for a few minutes!

I'm gonna try to wire it up within the next day or two, but I got more questions first!  There are 7 wires coming out of the original transformer...  It has the two secondaries (black and yellow) fed out to the speaker...  A red wire going to the power supply, an additional black going to ground, the blue and brown going to the tubes push pull style...  And a green wire terminating at a resistor (in the schematic it looks to be coming from the secondary side of the tranny)-- what is this for?  There doesn't appear to be multiple outs for 4ohm vs 8ohm operation or anything (the speaker is an 8ohm load BTW).

By comparison, my replacement has red, blue, brown on the primaries (which look to mimic the original wiring from what I could find online about [email protected] trannies), a black (which I'll have to run out to ground and back down to the speaker), a green (supposedly 4ohm) and a green w/yellow (supposedly 8ohm)...  

First off, I've found differing reports as to which is the 4 vs. 8ohm connection, anyone know for sure?  Second -- can anyone tell what the green wire going out to that resistor does, and if I can simply leave it out?

Thanks for all the help in this thread,
Tim
Tim

CJ

Am_peg Gemini II... Super loud hum when powered on.
« Reply #32 on: August 29, 2005, 02:58:36 PM »
You can ohm out the secondary, larger ohms equals the 8 ohm winding.

Blue and brwon go to your tube plates. You might have to swap them in order to get the phasing right.

Just connect the 15 k resistor wire to the top of your 8 ohm transformer winding.

If the amp squeals like it is oscilating, then your blue and brown wires need to be swapped in order to keep that 15k resistor feedback phased right. If the plate wires are backwards, you end up with pos feedback instead of neg. , thus the oscilation.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

T-Dogg

Am_peg Gemini II... Super loud hum when powered on.
« Reply #33 on: September 01, 2005, 11:16:45 AM »
SWEET VICTORY IS MINE!!! :grin:

Thanks everyone for the help, she's alive and kickin.  Nice sounding amp too, although I couldn't crank it cause it was pretty late last night...  Seems to have that punchy clean thing giong on, exactly what I'm into as far as guitar amps go...  And has a big enough cab to eek out a nice full bass guitar sound, certainly loud enough for recording if not gigging.  I really wanted to get this set up as a bass amp, but I think I'm diggin' it too much in its native state as a guitar amp....

I really want to start on the A_P_I boards I got from Fabio next...  But seeing as how this worked out in the end, I might have to move onto the Peavey 5150 sitting in my basement...  sparked up and blew out in a puff of smoke during a band practice a couple years ago...

Thanks again guys :!:
Tim


 

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