Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bangy on January 27, 2017, 06:41:06 pm
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I have recently acquired a (5C4) Fender Super Chassis with all original caps, OT, and most CC resistors. I have yet to plug it in. The Astron Filter caps, all caps are original Astrons except one filter caps is a Sprague and looks to have been changed iOns ago. The circuit is original except an NFB pot in the place of the 2nd speaker jack -- This was done decades ago as well. I am wondering if this amp has ever been in a chasis as there is no eveidence of screw head marks on the chasiss mounting holes. Tube line up is 5U4G, (2) 6L6G, (3) 6SC7. The AC power cord is original lamp shade type. The OT is a chasiss mount.
I am not the type that would rip everything out and replace with new parts to save time, in fact Im on the other side of the spectrum, I want to keep whatever I can from the original circuit. I fully understand the Electrolytics will all need to be changed. But beyond that I want to test for DC leakage before removing an old Astron. I have no idea of the history of the amplifier, for example when it was last fired up. So Id love to hear from my fellow amp restorators on their experiences, and method for resuscitation.
QUestion is where to start? Should I change the electrolytics before I begin?
My naive brain says:
1). Test PT and OT, Can someone layout how to do so.
Ive seen a 360-0-360 PT High voltage is normal for the original PT, however I am unsure of the PT type. I do know Triad 6516 was OEM. However, no markings on the bottom ( common in the era) or anywhere for that matter. The wire is all cloth covered and the solder looks original however there are two extra holes drilled for a larger footprint PT, which leads to at least recognize the possibility of a replacement PT. The center taps are neatly soldered directly to the chasiss with no evidence of a 're'-solder. Sorry no pics as I everytime I try to post a pic I get shut down because my pic is too big and I have them reduced well below 1024K. Not sure whats up there.
Anyway, I am pilot and can follow a checklist. So if anyone has one for resuscitating mothball amps back from the dead, I would be oh so grateful to utilize your work as a template????
Thanks ya'll
I also want to post my three first amps but havent figured that out yet- Anyone have a photo storage site they reccomend. Then I can just post the link.
Bangy
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Hi Bangy! I use Photobucket....full of adds these days but it get the job done for posting images.
There are people here that will give you some great advice. Give Uncle Doug's you tube channel a look. He uses an ESR meter to check his caps for leakage. He restores a bunch of amps.
Good luck.
Bart
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This is from AmpGarage, there is no Malware There to Beware but here it is enc'd. And also a web link, maybe there is something too on the Hoffman wiki pages, you can make one in 5 mintes cutting up an extension cord. Pull the rectifier and try the DBL, this should OK your PT.
https://antiqueradio.org/dimbulb.htm
You need to replace the filters, maybe hollow them out and stick new smaller caps inside for aesthetic mojo, you can do that later. There's info on the web for reforming caps but since this is not an intact collectable combo I wouldn't bother at all. Myself I would just shotgun all the e-caps first thing. Everything else one thing at a time.
Don't just shotgun the old molded caps, they might be ok, it's very hit or miss. Do the e-caps, then try it with your new DBL, if pass, then check for sound, check the voltages, check the tubes, tune it up right, then check for scratchy pots and listen to the amp, if it's bright, alive, punchy and dynamic good chance the coupling caps are still good, or good enough. You can then lift each one at the far end and see if it's leaking with your meter. Google 'grid leak bias' if you don't know what it is regarding the input tubes, it has its limitations and coolness factor that you should be aware of before before judging the amp. You could set up one channel with normal cathode bias too, seeing it's not a museum piece. Cool find, have fun.
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I have recently acquired a (5C4) Fender Super Chassis with all original caps, OT, and most CC resistors. I have yet to plug it in. ...
If it were me, I'd plug the amp into a lamp limiter to verify no short-circuit. If good, then plug it in a wall outlet. Check voltage on each tube grid, verify you have 0v where there should be 0v, and some kind of negative voltage for the input stage grid-leaks. Significant positive voltage on any tube grids could indicate a leaking coupling cap.
If that is all good, I'd play it & see how it sounds. It would be nice to have an ESR meter to check the condition of the filter caps, then you can use your judgment on whether to replace them.
Other than that, I'd just enjoy it as-is.
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HBP- Do you see anything wrong with bringing it up on my variance and watching the current draw as a show of issues? I havent made the light bulb litter yet but I have a very mice variac with amp draw. . .
1). With no tubes and check the High voltage, and filament voltages at 110VAC, assuming i saw no high current draw.
2). Then bring in the rectifier, and check it again
3).. Then the power tubes.
4). Then the preamp tubes.
I have buddies who swear by a slow slow powerip initially to allow the caps to form. One guy very successfully restores old radios with a three day power up. . . Im not as patient.
Opinions?
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> I havent made the light bulb litter yet
Why not?
I'm glad you have a current meter. But the lamp will limit faster than the human eye/hand.
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bringing it up on my variance
did you mean Variac?
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Yes variac, with an amp meter. I HATE aPPLE SPELL CHECK!
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Here's an other link: https://www.nmr.mgh.harvard.edu/~reese/electrolytics/ (https://www.nmr.mgh.harvard.edu/~reese/electrolytics/)
If it was a museum piece I would try to reform the caps, you can do it with a DBL with different wattage bulbs too for a few hours each, or your variac, I recall from pre internet days that Gerald Weber’s book recommended something like 40V for 12hrs then 10V/hr till you got to 120V. I’ve done that w/ old amps and it worked fine, but the amps weren’t barn finds either. Besides the annoyance that you need to be home all day or get out of bed continuously overnight, my worry was always that the thing would start smoking at some point when I was in the bathroom, or had fallen asleep, or ran out for a slice of pizza. Plus, your only real value is the original iron here, even if the caps resuscitate, then what? You have 60y/o caps ready fail at any moment - though that relaxed old worn filter cap sound is nice and appealing in a vintage amp. You could also put a temporary or permanent B+ fast blow fuse after the rectifier will doing this, you can use an inline car fuse holder for testing, use a fuse half than normal. So you gonna get a nice hide glue repo cab and speakers for it? Make it stand proud again? Post picts.
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If it was a museum piece I would try to reform the caps, you can do it with a DBL with different wattage bulbs too for a few hours each, or your variac, ......
I think this works much better from reforming caps than with a light bulb limiter. The key is the limiting resistor so the cap can only draw so much current if it's leaking. I built one and used a variac with it. Worked well and I did not have to worry very much about the cap over heating.
http://www.electrojumble.org/reforming.htm (http://www.electrojumble.org/reforming.htm)
I recall from pre internet days that Gerald Weber’s book recommended something like 40V for 12hrs then 10V/hr till you got to 120V.
Weber was recommending that more for new caps after he got a batch of new caps that were leaking.
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Im really not much for today's tweed, and I have a dwindling stash of 60 year old pine that I use for my cabs, and I painstakingly finish them naturally. And I do use vintage speakers as well. I experiemented till i was blue in the face, over the 3 5F2A builds, caps, (sozo's, Mallorys, blue molded), cabs (mine, Leo's, Sears), Speakers and cab made the most deifference, then OT, then caps. I did not experiement with resistors I used CC except for the voltage drop resistors between Filter caps. I have made two 2x10's with low powered Celestine from a Vox PA, and a 1x12 cab with a '55 P12R. Now my evening ritual--I set my 2x10's up spaced 10 feet apart running stereo effects, and the 1x12 dead center and the B a of a AB box gtr-amp, nothing to crush the dynamics. Its a great way to Not watch TV.
Thanks for the advice on the 5c4 resuscitation project. I ran into an interim project as I found a tweed GA-5 in need, (original as well), and it has a 20/20/10 cap, frayed speaker wire that I think I'm just going to replace the filter cap with three separate new ones rather than trying to reform and then check the amp further. I tried bringing this one up on a variac and it had a buzz a 60 volts that ws not volume dependent.
Here are 3 50k pics of my trilogy.
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Nice looking cabinet work!
Bart