Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on October 08, 2018, 04:45:31 pm
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I've got this Ampeg B15s early 70s on the bench - has nasty raspy hum with power on and no power tubes installed. It blows fuses coming off standby, and did this with the power tubes installed too. I'm thinking it is either a filter capacitor issue, rectifier diode issue, or even possibly power tube sockets. If you have ever been in one, they used these big cap cans rated 40-20-20-20uf and they strapped the 40 and two 20's in parallel to make 80uf. One 20uf cap not used. There are some smaller toilet paper tube caps in there that are 2x20uf with both caps strapped parallel to make 40uf. I suppose it would be pretty easy to install modern 80uf and 40uf electrolytics, securing them would be the hard part. I've not ever heard what a rectifier diode in a fwb going bad sounds like, and I haven't checked voltages yet, but will shortly.
The two 7027A power tubes showed major leakage on my calibrated Hickok 800A tester. So I ordered a JJ 7027A matched duet. The 12AU7 was weak too and I have some on hand.
The circuit board looks very nice and I don't suspect the iron is damaged. So, if you've been inside one, and have repaired it maybe you could assist me in fixing this one, lending me some pointers which would be appreciated. I will post some pics here shortly.
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You'll most likely have to rebend the clamps for those brown caps. I recapped 2 different B-25 B's and I think I ended up using wire ties for the second one.
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Thanks David.. I've already been thinking about it those plasitic straps ( wire ties ) seem like they would work. The old cap cans are quite large, and they soldered two clamps together .. did you notice that the screws holding down the circuit board and the clamps on the brown caps have a very unusual head.? I will probably have to use a special tool to loosen them.
I checked the rectifier diodes and they are ok.
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Yeah, mine had those screws. I believe one of the square head bits is what I used.
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schematic https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_b15s_revc.pdf (https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_b15s_revc.pdf)
And I'd like to add this amp needs a 3 wire power cord..
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those are called clutch head screws. You can get drivers to fit those. 3rd pic down looks like that 40-40-40 cap can was pushed too close to those 1/4" jacks. you can see the dimple crease on the side of the can.
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Phunky.. I see that.. good eyes.. will investigate..
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I've been going through all the screw drivers at garage sales, Estate sales and pawn shops grabbing those clutch head screw drivers to work on old Ampegs.
I did a google search awhile back when google actually did good searches and found some bits for sale, wish I had bought them.
Mark
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Since i've read that the OT's have a history of failing in these amps, probably because they are potted and the heat builds up inside, I decided to isolate the OT secondary and take some measurements.
I have the ground and feedback wires disconnected on the OT secondary. I measure 2.4 ohms from the blue lead of the primary winding to any of the secondary windings. The brown lead of the primary to secondary fluctuates like it's seeing some capacitance.
With power tubes out measuring OT primary blue wire to red I get 85 ohms, and brown to red I get 137 ohms, and reading from blue wire to brown wire I get 222 ohms. That's not very balanced.
It is looking like this OT has an internal short from primary to secondary.
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The original Ampeg spec for the OT primary is 300 ohms. Thank you Bruce at fliptops.net
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In a strange turn of events this flip top B15S is mine now. Hope I can find a good OT for it. MM makes one but $260 + the shipping.
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Davidwpack - what capacitors did you use in your recaps.. and did you stick to original values?
Also did you install a grounded power cord in any of your amps?
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I used a 50/50 can cap for the 40/40 can and x2 20uf for the x2 30uf. Sounds/plays great with no problems. Also, I did replace the power cord.
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I made a recording of the sound the amp makes when turning on the power switch. Sounds like a bad capacitor to me. It also sounds like 120hz .
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I'm just getting an error:
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YouTube is down. Just ignore errors; the monkeys will fix it. (They need the ad revenue.)
https://www.news5cleveland.com/homepage-showcase/do-not-panic-but-youtube-is-suffering-an-outage
9:45 p.m. update: YouTube has issued the following statement: "Thanks for your reports about YouTube, YouTube TV and YouTube Music access issues. We're working on resolving this and will let you know once fixed. We apologize for any inconvenience this may cause and will keep you updated."
Previous story: Trying to procrastinate for work or an upcoming exam might be a bit more challenging this evening as the video-sharing site YouTube is suffering an outage.
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PRR what do you think of using a Hammond 1650PA in this bass amp?
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Well PRR never responded - anyway the Hammond 1650PA OT went in along with a full recap of the lytics.. I replaced all the rectifier diodes and bias diode with 1N5408s.
It lives !
The voltage on the plates was around 615vdc for my wall voltage this morning. So I put it on the variac set at 117vac. The voltages at 117vac align with the schematic. The amp sounds very good, very bass-ie on the bass setting and flat on the flat setting and the guitar setting is great for skinny strings. Now I have to trim the OT wires up to finish the OT install and repair a couple of dings on the cabinet. BTW the can for the OT fits perfectly over this Hammond with zero room to spare.