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General Discussions => The Lab => Topic started by: T-Dogg on July 21, 2005, 04:17:37 PM

Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg on July 21, 2005, 04:17:37 PM
So I'm trying to diagnose and fix this amp, which was given to me in non-working condition a while back.  I'm also half trying to understand the basics of tube amp circuitry, and have a couple o' general questions on the schematic, available here:
http://users.aol.com/portaflex/schems/g-15.gif

First off, when the amp is powered up and switched out of standby, there is a very loud, constant hum.  Pretty low frequency.  Adjusting the volume or tone controls does nothing to change the output or sound of the hum.  The input jack does not appear to be shorted.  Based on what I've read, this points to a couple of possibilities --

Bad smoothing caps
Bad power tubes
Bad tranny...  power, I guess?  possibly output?

Does this seem a reasonable assessment?  Because the power tubes are tough to find nowadays (7591's I think) I figured I'd test the caps first...  (BTW, one of the few things I do know is to discharge them first:-) )Then I guess the tranny's, although I'd have to read up on how to do this.

I'm gonna start on it tonight, if anyone has any input I'd love to hear it.  Unfortunately, I don't own a scope (nor would I know how to use one at this point!), so my testing is limited to a digital multimeter that can also check capacitance.  BTW, should I take the caps out of circuit to test them?

Also, if anyone got to check out that schematic, I have a random question about it.  I recognized the two smoothing caps as those following the rectifier stage, and think I understand what they do at a really high level(take much of the ripple outta the DC right?)...  But what is the purpose of the cap coming off of the primary going to ground?  Is this some sort of smoothing cap too?
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: PRR on July 22, 2005, 01:17:54 AM
> 7591's I think

Schematic you linked shows 6L6GC. I think you can still get them.

7591 (or a near-enuff clone) is once again in production, if in fact that is what the amp is made for. 6L6 and 7591 are similar but NOT the same.

The cap on the power transformer connects chassis to either side of the wall outlet. One way will give less hum (once you get rid of the BIG hum!!!). The other way will electrocute you.

Your symptoms really suggest that the main 30uFd 600V cap has failed. You can't get a 30uFd 600V cap. Use two 100uFd 400V in series. Go ahead and get a 10-pack of them: I'd suspect most of the 500V caps have also seen better days and will beg for replacement. Replace the 100uFd 100V in the output stage grid-bias supply too.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg on July 22, 2005, 11:49:14 AM
Thanks PRR.  I tested all the caps last night, and it seemed quite a few were bad, or drifting from the expected value -- at least based on what the meter said, though it might be innaccurate cause I left everything in circuit.  But as you predicted, those big paper 600v and 500v ones seemed to be among the worst offenders.  I'll definately replace them, and see how far that gets me.

Also, thanks for the substitution cap values.  That would've probably been my next question in a few days when I realized I couldn't get one!  I searched a bit online and found some "cap series/parallel" calculators...  But what if I wanted to go backwards, without knowing the cap values... ie -- find out how to create the equivalent of a 30uFd 600v cap by using others in series or parallel like you did, what is the equation to do so?  Couldn't seem to find info on this.  I'm hoping it's semi-easy to calculate ... but I'm probably being overly optimistic.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: Gus on July 22, 2005, 12:13:22 PM
How long did you leave the amp on with the hum?   Sometimes an older amps caps will take about 1/2 hour to reform and the hum will decrease.  I would do this outside and watch the amp the whole time don't leave it unattended.
 If the tubes are glowing red at the plates shut it down

IIRC that amp is liked by jazz players.  

IMO the only way to realy test caps is with a meter like a sencore LC102, with the cap disconnected from the circuit.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg on July 22, 2005, 12:55:44 PM
Hey gus.  To be honest, I only left it on for a couple seconds tops...  The hum was so bad (and kinda loud!), I was afraid something would blow up at any minute!  I'll give it a shot though.   When ya think about it, it's pretty damn funny that my amp has a better chance of fixing itself than me getting it working!  

I read its supposed to be really clean...  In the back of my head,  I'm thinkin I'd love to be able to replace the 15" jensen with something a bit beefier, seal the cabinet, and use it as a mid size bass combo.  Not to go to waste, I tried that Jensen in a Peavey delta blues amp I had layin around, and it sounded pretty good, despite its wear.  I think better than the speaker it came with...  

Just looked at that Sencore...  $$$
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: Gus on July 22, 2005, 01:23:07 PM
I bought an old silvertone Se amp kind of like a champ.  It hummed for about a 1/2 hour then it stopped I let it run for a few hours. let it sit a day and then tested the caps they were OK.  I also did this with a older fender princeton and it seemed to work.

   With other amps the amp hummed still after 1/2 hour and they measured bad as well.


Funny thing I have measured older caps in amps and pulled from amps with the 102 and some might have been low in value but the ESR and leakage(at working voltage.
The 102  secore can go to 999V in 1 volt steps) were good.

Some of the older caps seem to last.  I think "blind" recapping of amps like what you read on the web needs to be examined.  reminds me of the 3,000 mile oil change myth.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg on July 22, 2005, 01:45:25 PM
Pretty cool, for as many times I've read about recapping, I never once heard that they can sometimes be revitalized.  

Would it be really, really bad for the amp if I leave the speaker disconnected while doing this?  It's just so loud -- I have a little deck off my apartment, but I could see someone complaining!  Maybe a Scholtz power soak before the speaker?
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: Gus on July 22, 2005, 02:05:42 PM
It might be better to just change the caps if the hum is that loud,  maybe a power soak would be OK.  Was the amp sitting a long time years? before you powered it up.   I bought a b25 ampeg that hummed IIRC the triode pentode output driver tube ampeg used 7199? was bad I changed that and the hum decreased a lot.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: PRR on July 22, 2005, 10:22:46 PM
> those big paper 600v

Those aren't "paper"; though they may be cardboard-wrap.

Paper capacitor is two layers of foil and a layer of paper insulator. Just one uFd of paper-cap is big; 100+uFd of paper-caps would be bigger than that amp.

These are electrolytic. Aluminum foil in a metal can full of liquid. The insulator is a super thin layer of aluminum oxide, "grown" on the foil by running a current through it. With age, the insulating layer un-grows, dissolves, the cap is sick. While you might be able to re-grow the insulation by leaving it powered-up, it does not work well for me and I would not turn my back on a re-formed electrolytic.

Cut open the old caps. I bet they are two capacitors inside that carboard sleeve. There aren't any 600V electrolytics: max is 450, maybe 500V. Paper and film caps are much more expensive per uFd. So the usual "600V" electrolytic cap was two caps in series, packed together, with overall insulation (the second cap's case is at +300V).

> what if I wanted to go backwards, without knowing the cap values...

Two 100uFd, in series, is 50uFd, etc.

Yes, that's bigger than the original 30 or 40 uFd. But bigger is better, and the $/uFd of modern caps is much better than the old days. Ampeg probably would have used 50 or more uFd, but the cost killed their profit. Now that it has matured into a Classic, and you have invested hours of work, you may as well use better (bigger) caps than their accountant let them use.

You would think that two 400V caps in series could be called 800V. That would only be true if they were perfectly matched in value and in leakage. In real life, you have to de-rate. They are probably running 500V-550V on that "600V" cap (which is probably two selected 350V caps); I suspect that two 400V caps from the same lot, in series, will stand 500V-550V for a decade.

It might be good to put 220K 1 Watt 5% resistors across all the 100uFd 400V caps, to equalize the voltage and swamp the leakage.

> replace the 15" jensen

JBL E-130 for loud, E-140 for deep. Can't kill them with 2x6L6 (I didn't kill mine with 4x8417), and you can't get more air per watt than these guys give. Sadly they are out of production. And the E-series is HEAVY.... D-series would be lighter but pre-1980, maybe hard to find a healthy one. (I knew a guitarist who bought a C-130 new and used it all his life.)
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg on July 29, 2005, 12:02:31 PM
Thanks alot for all the help guys...  My shipment of Orange Drops and Sprague Atoms just came in, courtesy of the kind folks at Angela Electronics.  This will be my Saturday project, hopefully I'll have a functioning amp by the end of the day!

BTW, these "orange drops" have such a vibey look to them!  I'm thinkin, if I was to be severely shocked by a cap, I'd like it to be an orange drop.  I may even fashion a pair into some earrings for the ol' lady.  I'm sure she'll just love 'em...

One more question -- these caps are alot bigger than what was in there.  Especially the two 100uF 450v caps that I'll need to put in series to get the 30uF 600v...  Not to mention originally (3) 40uF caps were in a can hanging off the bottom of the chassis, and looks like I'll have to stuff the new ones inside.  It's gonna get tight in there, alot of the bigger one's will likely be right up against each other.  Is it likely heat dissipation be a problem?
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: CJ on July 29, 2005, 02:07:32 PM
Orange drops are cool in guitar amps, not so cool in studio equipment, so you made a good choice. You can still get a metal multi-section cap, available at www.tubesandmore.com. There is a picture of it around here in one of the threads.

Don't disconnect the speaker! !! You can pop that output transformer. It needs a load. If anything, short it out. This will not hurt it during testing. Thats why they have shorting speaker jacks.

I am worried that your loud hum may be more than just the caps, like some circuit problem somwhere, but either way, the new filter caps are a good idea.

Ampex used both the 6L6 and the 7591 in a lot of their amps. And even some 7027A's. Westinghouse came out with the 7591. If you really wind that tube up to it's limit, you can get almost 45 watts out of it in push-pull AB1. Close to a pair of 6L6's, and half the space. Get rid of the 100 ohm pot in the bias circuit if it has one, and use a 140 ohm 10 watt in there. (Cathodes to ground) Westinghouse did not really publish a tube manual, but RCA started making them so there is some data there. Not a lot of companiies made the 7591, as the profit margin was slim. For instance, back in 1969, a 6L6 went for $8.45, and the 7591A for $2.30 !

cj
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: NewYorkDave on July 29, 2005, 02:19:59 PM
Super-loud hum, unaffected by volume and tone controls, usually means that  your output tubes are about to fry. Are they glowing red or orange? Power up the amp and watch the output tubes carefully; do you see a glowing spot appear on the plates of one or both tubes as the hum comes up?

Check your bias supply. Also check the coupling caps to the output tube grids. When these become leaky, the leakage can overcome the grid bias and cause the output tubes to burn up. Large unbalanced currents flowing through the output transformer cause a loud hum.

You might need a new set of output tubes by the time you've fixed the original problem.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg on July 29, 2005, 02:54:31 PM
In the back of my mind I've been wondering if it could be more than the caps.  But all the large value caps, including the one in the grid bias supply, and a few others  in the circuit were showing 0 capacitance on the meter (well, maybe a couple of nF, but nowhere near their respective values.)  I'm pretty sure this amp sat around for 30 years or so with no juice, which I guess explains their lack of...  ummmm... capacitancificationability :razz:    

Then after my original post, I came to realize I had at some point blown a fuse (I think it may have happened when I switched out of standby, but it is possible it occured on power up after a few seconds...)  Haven't picked up any additional fuses, so I never really got to try "reforming" the caps as Gus suggested or checking the plates...  Hopefully, when the new caps are in, I'll at least be beyond the fuse issue (if it isn't working like a charm that is!)  Is there an easy way to test a tube?  I'd assume not without a scope and lots of *voltage running through it, right?

EDIT: *Just remembered, I should say current, right?!?!
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: CJ on July 29, 2005, 03:21:02 PM
Check a tube by putting a voltmeter from ground to the cathode pin. The bias tells most of the whole story. Whatever that means.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg on August 01, 2005, 12:20:32 AM
Welp...  I replaced the caps, had to get a little creative stuffing them in there...  but it didn't get me very far!  The amp powered up in standby, no problem...  So I let it sit for about 20 minutes or so in standby, watching the tubes.  Didn't see anything that looked suspicious to me, nothing appeared overly bright.  But when I attempted to switch out of standby, I got about 1 second of loud hum and then a blown fuse.  Repeated this twice with the same results.

I guess I'm pretty much stuck at this point, not sure what else to try.  I know it was mentioned it could be an issue with the tubes...  I'm kinda reluctant to go out and buy some 7591's, seems like they'll be pretty pricey for a pair, unless thats all that's left to try...  Gotta admit I'm also a little nervous about testing voltages with the thing powered up.

I also noticed there was a schematic on the inside of the amp chassis -- and surprise, it's pretty different than the one I found online and posted earlier.  Maybe just the interpretation of the circuit, but the rectifier section appeared different as did some of the cap values.  To my eyes, the one in the amp was easier to follow...  Looks pretty much like this one, sorry the image is a bit blurry:  http://www.kbapps.com/audio/schematics/tubeamps/ampeg/g15.html

Even if I don't get this thing working, following the circuit from the schematic was a big step for me -- starting to get it now!

And finally, gotta ask my random question: there's a third tap on the output tranny with a 430v potential across it -- why is it necessary/what does it do?  I think I understand the two main taps tied to the output tubes (pushing and pulling the speaker in and out, hence push pull?)
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: NewYorkDave on August 01, 2005, 10:54:01 AM
They WOULDN'T glow red or otherwise do anything out of the ordinary until you switched off standby. I guess I should have been more specific.

Remove the output tubes, then power up the amp. Does it still pop the fuse?

If not: Set your multimeter to DC volts. Connect the negative lead to the chassis and insert the positive lead into pin 6 on either output tube socket. Do you read a negative voltage there? How many volts? Is it present on both sockets? You can take this measurement without even going inside the chassis.

Your ability to troubleshoot this thing is going to be severely hampered if you don't wanna take voltage readings. Yes, probing inside a powered-up chassis can be dangerous; but if you're not careless and you practice proper safety precautions it's no more risky than driving your car.

You do NOT want to stick expensive new 7591s in there until you've rooted out and fixed the problem.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg on August 01, 2005, 02:21:51 PM
Thanks NYD.  I just removed powered tubes, but same results... A quick burst of hum, then the fuse blew.  Can I also remove the 7199 and 6CG7 tubes from the circuit and attempt powering up?  If these were bad, could it cause blown fuses, etc?  

If not, then there's probably some random issue with the circuit?  What sucks is, someone was in there before I got my hands on it, so I can't be sure other things in the circuit weren't rewired for better or worse.  I don't see any obvious wiring issues/breakages -- guess I can try to follow the whole circuit against the schematic to be sure everything is wired right.  My head hurts just thinkin' about it!
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: Gus on August 01, 2005, 08:19:12 PM
the unit hummed with the power tubes removed?  I don't understand

Do you have a hum balence control?  A bad humcontrol can cause hum, check the wiped to outside ends maybe the wiper lifted from the element.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: NewYorkDave on August 01, 2005, 08:23:37 PM
My Spidey Sense tells me that you may have shorted rectifiers in your power supply.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: CJ on August 01, 2005, 09:46:08 PM
1) Pull the 7591 output tubes
2) Re-fuse the amp
3) Hook up speaker
4) Turn amp on
5) Report results here
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: NewYorkDave on August 01, 2005, 10:00:19 PM
He already did that, CJ. Pay attention  :razz:
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: CJ 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 (http://cgi.ebay.com/General-Electric-7591-Pair-NOS-NIB_W0QQitemZ5794996560QQcategoryZ64629QQssPageNameZWDVWQQrdZ1QQcmdZViewItem)

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.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg 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?
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: CJ 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:
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg 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...)
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: NewYorkDave 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.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: Gus 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.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg 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?
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: Scodiddly 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.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: Gus 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
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: CJ 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
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg 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
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: CJ 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.
Title: Am_peg Gemini II... Super loud hum when powered on.
Post by: T-Dogg 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 :!: