Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: CraigB on August 23, 2011, 07:25:33 am
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Hi all, haven't visited the forum in awhile! One of my customers (he has two 18watt combos I built for him) brings his amp to me in shambles. The speaker is hanging by one bolt and knocking up against the output tranny, tubes are rattling around in the bottom of the cabinet, the safety ground was broken off the AC cord, etc., you get the picture. It has obviously been badly abused, and most likely dropped. I told him I will work on it, ONLY if he promises to treat his amps more like his guitars in the future. We'll see... :icon_biggrin:
So I reinstalled the speaker, new AC cord, plugged in the tubes and it works, but as the volume gets pushed up, there is a horrible out-of-tune overtone/ghost note that rides along with the some notes, not all. I switched to a known set of good tubes. Same thing. I checked the tension of the socket contacts, that's all good. The power tubes are held in by a spring retainer that I make for my amps that holds those suckers in there firmly. I tested the speaker for damage by pumping one of my amps through it. Sounds fine. Probed with my voltmeter and voltages all look good. Checked resistors for correct values, and they're close enough.
I have heard that an output transformer can be ruined from the shock of being dropped, but I'd think that would be more like dropping the iron directly onto the ground or something like that, yes? I plan on replacing it with another used one that I have in the shop, just so I can rule that out (or hopefully rule it in and it would be fixed!)
Any thoughts on what else I should be looking at? Thanks for any help you guys can give!
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Doubt that you have a xformer problem...Try checking the output jack(if it has one). I've got a 18 Watt IIB sitting here now if you want to compare any readings.
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Thanks for the reply labb!
Our voltage readings would be a bit different - this was originally a late-model PCB Ampeg with two EL84s that he had me rebuild to 18wII circuit using the power and output trannys. It's only about 265v on pwr tube plates.
Checked output jack, input jacks. No damage. They're all switchcraft, so good quality.
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What filter caps did you use?The ghost notes on some notes indicate an under-filtered condition.
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Thanks psychonoodler! Thing is, it didn't have this symptom before. They are Xicons, I believe, rather large value because the PT requires a SS bridge rect. I believe I used the same value as were on the Ampeg PCB, 47uF for plate supply and 47uF for screens.
But this is a good lead for me to take, and I really appreciate your help. I did not check to see if the power supply caps were holding charge and bleeding off as normal. They are siliconed to the board, but with a good jar I'm wondering if one of the leads might have been damaged and maybe is intermittent especially when presented with heavy vibration from the speaker, as it becomes more pronounced the louder the volume is turned...I will check it out.
THanks again.
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OK, so the original schematic for the Ampeg indicates that I should be getting about 320VDC on the plates. I'm getting considerably less, around 265. That is close to the same reading I get straight off the rectifier with the OT center tap disconnected. Then my reading at the HT winding disconnected from the whole circuit is 270VAC. With a SS bridge, I can't remember what I should expect (for instance 270-0-270 with FW rect is something like 270*1.4) as voltage right off the rectifier, but seems like the power tranny is fine. I checked the diodes in the bridge circuit with my meter, and they measure "~the same" in relation to each other, but shouldn't I be getting a voltage much higher than 270VDC off the bridge without the rest of the circuit hooked up?
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but shouldn't I be getting a voltage much higher than 270VDC off the bridge without the rest of the circuit hooked up?
The voltage will be low if the filter caps are not connected.
I'm assuming you're only using half the bridge with that 270-0-270 PT? IOW, the negative bridge terminal has nothing connected and your PT centertap is connected to ground.
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Thanks Sluckey - sorry about the confusing piece I put in about a 270-0-270 transformer. This is not. To clarify what I'm looking at, here's the link to the Ampeg schematic, and what I built is merely that same power supply, but the rest of the amp is an 18wLite. (Sorry, I would have sliced out what we're interested in looking at, but Acrobat select tool won't let me cut out just the power supply and paste to jpeg)
http://www.el34world.com/charts/Schematics/files/ampeg/ampeg_j12t_jet_ri.pdf (http://www.el34world.com/charts/Schematics/files/ampeg/ampeg_j12t_jet_ri.pdf)
So again, I'm getting 265VDC at first filter cap, then the same thing straight off the rectifier with NO filter caps connected, and then about 270VAC when I measure just the HT winding disconnected from the bridge rect.
THe schematic says I should get about 320VDC at the first filter.
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So again, I'm getting 265VDC at first filter cap, then the same thing straight off the rectifier with NO filter caps connected
Sounds like that first filter cap isn't doing anything. It may be open. Use gator clip leads or just tack solder another cap across that first cap and see if the voltage comes up. Even a 20uf should pump up the B+ with the power tubes out. If your B+ increases, then replace that cap.
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That was it! I went ahead and replaced all of the filter caps and the diodes for the bridge and she fired right up. Thanks!