It is currently Mon Mar 16, 2020 3:31 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 12 posts ] 
Author Message
Post subject: Princeton Reverb (Blackface) problem
Posted: Thu Mar 04, 2010 5:24 am
Offline
Hobbyist
Hobbyist

Joined: Thu Mar 04, 2010 5:05 am
Posts: 7
Location: Western Hills of Ct.
I am working on this amp for a friend. The 5U4GB had been replaced with an SSR in the past. The amp worked for about an hour and quit. I found the power transformer cooked (literally) and a bad 6V6GT. A relacement tube for the 5U4GB was ordered (a 5u3C was subbed in) along with new finals (NOS) and a replacement transformer. I checked the rest of the amp, especially the quad can and it looked good (refrormed the quad, leakage measured 2ua). My problem is that when I fired it up the B+ never comes up and the line fuse (1 A S/B) blows after about 20-30 seconds. I checked the ac from the transformer, sans tubes, O.K and the fuse does not blow. I isolated the rest of the amp from the power supply(nothing, not even the filter caps powered) and the fuse still pops. At his point I suspect either the rectifier tube or the socket, which I'll check when I get home from work tonight. Any thoughts or suggestion would be greatly appreciated.


Top
Profile
Fender Play Winter Sale 2020
Post subject:
Posted: Thu Mar 04, 2010 4:24 pm
Offline
Hobbyist
Hobbyist

Joined: Thu Mar 04, 2010 5:05 am
Posts: 7
Location: Western Hills of Ct.
Found the problem. Filaments on the new transformer were shorting to ground through the flux band Hammond puts on their upgrade unit. Solved the problem by using heat shrink on the through section of the mounting screws (Hammond only supplies shoulder washers). B+ and negative grid voltages are back up. Thanks to all who tried.


Top
Profile
Post subject:
Posted: Thu Mar 04, 2010 9:21 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Hi there Ice and welcome to the forum. I'm late to the party, but interested in the source of the short. This is the 290AX, or am I mistaken? Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject:
Posted: Fri Mar 05, 2010 7:45 am
Offline
Hobbyist
Hobbyist

Joined: Thu Mar 04, 2010 5:05 am
Posts: 7
Location: Western Hills of Ct.
I was actually to brief in the post, it wasn't an actual short but 8 ohms measured to the flux band from either of the 5 volt filament leads ( the transformer is a 291AX, the upgraded model). After the transformer was isolated from chassis ground the B+ measured 350vdc sans the rest of the circuitry. I reattached the B+ wire to the cans and the negative grid wire then measured 470vdc with 1vac (peak-250 mv rms approx) of 120 HZ ripple. I am going to replace the can even though it reformed well (2ua leakage) to a new unit with a 525vdc working voltage. I have new finals (6V6GT matched NOS's) that I want to check to bias on also and then I plan on running a full audio spectrum distortion test. I was toying with the idea of changing the coupling caps to Orange Drops, as I plan on changing the rest of the axial electrolytic's anyway and I'm already in there. I heard the Orange Drops make a difference in the amps sound, for the better. Any thoughts on this?


Top
Profile
Post subject:
Posted: Fri Mar 05, 2010 9:48 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Ice, that's pretty interesting about the PT. Sounds like you know your way around electronic gear and have the knowledge and equipment to do some great testing/analysis/diagnostic work. I'm just an old barebones kind of amp tech, without formal electronics education. As far as the orange drops go, yes, IMO good quality orange drops work well in Fender amps. But the blue molded tubular Astrons are superior. Unless you find them to be faulty, I would not replace them, especially the ones in the tone stack. Just a suggestion, you might want to R&R the 250pf disc cap with a silver mica in that tone stack. Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject:
Posted: Mon Mar 08, 2010 4:41 am
Offline
Hobbyist
Hobbyist

Joined: Thu Mar 04, 2010 5:05 am
Posts: 7
Location: Western Hills of Ct.
Thanks for the compliment. I've been working in electronics since the late 70's and I enjoy tube gear, all types. I'm actually a retired police officer, I do this as a hobby (I acquired the test equipment in the early 90's when I was working in electronics during the day and as LEO nights-ex wife liked to spend my money). Thanks for the info on the caps, especially the 250pf unit. At this point I'm tracing a nasty hum, sounds like 60/120HZ, on the Princeton that comes and goes depending on volume position. Besides noisy pots, the rest of the amp checks out with surprisingly good volume and low distortion (providing I have the volume set in one of those "quiet zones").

This amp work is fun when you do it as a hobby, especially when you can talk to and bounce idea's off of others (like you). Thanks,and I'm open to any other suggestions you, or others, may have.

Joe.


Top
Profile
Post subject:
Posted: Mon Mar 08, 2010 10:39 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Joe, that volume pot might just be history. If cleaning and even retensioning doesn't get you a smooth response, oh well. Those carbon paths do wear out. But I'm just wondering if you contacted Hammond about the transformer. I have a real problem with the scenario, you described, about the 5 volt taps, flux band, and having to insulate the PT from the chassis. Or do I have this wrong? Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject:
Posted: Tue Mar 09, 2010 4:58 am
Offline
Hobbyist
Hobbyist

Joined: Thu Mar 04, 2010 5:05 am
Posts: 7
Location: Western Hills of Ct.
Art, you're putting into words whats been lurking in the back of my mind in regards to the transformer. I soldered a .015uf @ 600v cap from flux band to ground which didn't help at all (figured I could shunt any ac noise out while still blocking the dc.) I get that little voice in my mind that tells me to go back to the transformer. I'm going to e-mail Hammond and ask about the 8 ohms from the filament tap to ground.
The only pot I cleaned was the intensity pot so I could get a reliable bias reading on the finals, I then ran out of cleaner. Once I get more I will wash out the rest of the pots, which are very noisy, and go from there. I'm thinking that the hum problem goes beyond the pot. With the volume pot set at zero and the input shorted the hum is fairly loud. As I turn the volume pot up to 2, the hum all but goes away. As I keep increasing the pot it comes back strong at about 5, decreases to nil at about 8 and back again strong at 10. As a side note, when I put a nice clean sine wave in at the low hum levels I get a nice clean distortion free output right to the expected clip level (amplitude varied with the generator). I believe I ran into this years ago, but can't remember the details. I have not done enough testing on this amp to make even an uneducated guess so I'll get back to you.
The weather has been so nice I took the Harley out for a 160 mile ride Sunday and didn't get a chance to spend time on the amp. It's great bouncing idea's off of you, thanks for your help. Looking forward to any more idea's or thoughts.
Joe


Top
Profile
Post subject:
Posted: Mon Mar 15, 2010 2:52 pm
Offline
Hobbyist
Hobbyist

Joined: Thu Mar 04, 2010 5:05 am
Posts: 7
Location: Western Hills of Ct.
Finally got back to the amp. I added the 100 ohm hum balance resistors, since the replacement PT didn't have a center tap, and the hum is completly gone. Bias is perfect, finals well balanced and the amp runs through the audio spectrum with perfect clarity. The owner bought all new electrolytic caps, but before I put them in I'm going to have him bring his guitar over and give it a test drive. As of today Hammond hasn't gotten back to me on my questions, but it's probably a moot point now. I'll post later and let you know how it sounds with the guitar. Hope this helps others avoid some of my problems.


Top
Profile
Post subject:
Posted: Mon Mar 15, 2010 7:20 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
That's good news, Joe. The absence of a center tap for the filaments would certainly make a difference! I didn't have the spec's on that PT, should 've tried to look them up. Actually most folks prefer the artifical grounding to the old style center tap, and I've used that approach on mid sixty amps to keep that filament hum at bay. So was that causing all the noise with the volume pot? Hum noise is a pain in the neck, but sometimes I forget that a 120 cycle hum is almost always associated with the filament supply. Oh well, glad you caught that. Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject:
Posted: Thu Mar 18, 2010 6:39 am
Offline
Hobbyist
Hobbyist

Joined: Thu Mar 04, 2010 5:05 am
Posts: 7
Location: Western Hills of Ct.
Art, the missing center tap seemed to be the root cause of my hum problems. I didn't add the hum balance resistors because a) I like to bring the amps back to their stock configuration and go from there and b) I missed fact that the center tap was missing on the replacement PT,even though I knew it was missing (does that make sense?) Still waiting for the owner to try it out with his guitar before I do any more mods, although I finished testing the rest of the unit (reverb, etc.) and am happy with the results. He has a Twin Reverb that needs some work, so I think that will be my next project. As a side note, after reading some forums on two wire vs three wire line cords I converted this unit over to three wire with the fuse and on/off switch wired per our present standard (both on the "hot" side of the line). I left the "death cap" in, wired to the ground switch so that it can be inserted from the load side of the fuse and ground at the owners discretion (if the cap shorts it'll blow the line fuse), although I don't think it really makes any difference.


Top
Profile
Post subject:
Posted: Fri Mar 19, 2010 5:56 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Yep Yep, I understand the deal with the PT. I've done very much the same thing more times than I count!!! Once replaced the OT in our bass player's '67 Band Master. Had to remove a few bogus mods too. So I figured it would be a good idea to restore the wiring lead dress and all the rest to original shape. Since I have a '67 also, it was easy to put both chassis's on the bench and go for it. Seemed to work out great and I was so proud of my work, but when it came time to test that amp it had a horrible hum. Turned out the OT plate leads needed to be reversed, but it took me a trip to one of the old radio repair guys(one of my teachers) to finally get that one straight. Yeah, we learn more from our failures than our successes. Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 12 posts ] 

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: