Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JustMike on March 08, 2018, 07:04:10 pm
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I built this amp about a year ago and it's been working fine. about a month ago I added an Iron Sounds effects loop and it worked fine. I "thought" it affected the sound so I removed it. Now I have no sound at all. I double/triple checked everything. It was a simple mod/addition. It just interrupts the signal between the MV and treble pot- a one wire mod.
I've since checked my voltages and found that I only have about 190VDC B+. I disconnected the feed right off the rectifier diodes and I'm getting 380VDC. I reconnected it and measured voltages on my output tubes.
pin 3 90vDC
pin 4 81vDC
pin 5 -80vDC
pin 6 81vDC
What is causing this?
Thanks Again,
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I think I would pull the output tubes and check the voltages on their sockets again.. if the voltages are good then ,, I would try another set of tubes
are you running 6550's or EL34's ?
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34's
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I think you messed something up when you removed the FX loop.
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standby on ?
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I built this amp about a year ago and it's been working fine. about a month ago I added an Iron Sounds effects loop and it worked fine. I "thought" it affected the sound so I removed it. Now I have no sound at all. I double/triple checked everything. It was a simple mod/addition. It just interrupts the signal between the MV and treble pot- a one wire
It should go between the MV and input to the Phase Inverter. But that doesn't make any sense why voltage would decrease.
Mark
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That's why I think he messed something up when he removed the loop. He had to tap into the B+ string to power the loop. That's one logical point to examine. Also, there could have been unknown unrelated collateral damage. Hi-rez pics may reveal some clues. Shouldn't be too hard to find.
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I'll dig into it tonight...and likely I'll find something stupid as Sluckey says. Although I can follow a layout, I wish I had a better grasp of how the circuit works. Like B+ voltage points, Signal flow, etc.
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Not saying it's something stupid. Just saying something unintentional may have happened. Like a wire breaking or coming unsoldered when you removed the FX loop.
What layout did you follow? And how closely?
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The "stupid" comment wasn't a reflection on you or your commentary Sluckey. You've helped me out in the past and I respect your knowledge and delivery. Humor and sarcasm, even if it's self deprecating doesn't translate well in text...anyhoo, stay tuned!
BTW, when I built this, I followed a Ceriatone 2204 layout pretty closely.
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Here are some pics;
https://www.dropbox.com/sh/0h03onx6cekxm6r/AAD7nBs6RaJqXcrOg7JTvjxza?dl=0 (https://www.dropbox.com/sh/0h03onx6cekxm6r/AAD7nBs6RaJqXcrOg7JTvjxza?dl=0)
also, with the output tubes pulled, I'm still getting 172 VDC at the rectifier.
So I pulled the HT fuse and measured the DC voltage at the HT fuse. It started at about 190v and over the course of a minute, it ramped up to 550ish.
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With the Vdcs all screwy like this, I suggest to go back to the beginning (it's a very good place to start) and check the Vac at the PT primary and secondaries.
And leave the power tubes out until we have a better idea of what's going on.
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How about checking under board B+ connections.
Mark
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With the output tubes pulled and the HT fuse in I'm reading 80VDC B+. When I pull the ht fuse the voltage ramps up (about a minute) to 600VDC. I also measure 90VDC at the speaker output at 16 ohms and 70v at 4 & 8. I think this tells me that the choke & OT are ok(?).
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With the output tubes pulled and the HT fuse in I'm reading 80VDC B+. When I pull the ht fuse the voltage ramps up (about a minute) to 600VDC. I also measure 90VDC at the speaker output at 16 ohms and 70v at 4 & 8. I think this tells me that the choke & OT are ok(?).
Is it possible that your reading of 80VDC and 90VDC and 70V are really 80mV and 90mV and 70mV? I ask because it is impossible to have 90VDC at a 16Ω speaker jack.
I don't see any transformer wires connected to the rectifier board? I suppose they may be tack soldered to the bottom of the eyelets. If so, that's not a very good way to make a secure connection. Usually when connecting a wire to the bottom of an eyelet, the wire would be poked through the eyelet and be visible from the top. Set your meter to read AC volts and measure AC voltage from chassis to each of anode of the rectifier diodes (the eyelet where the transformer should be connected.) What have you?
If your meter is indeed reading mV (millivolts) you could just call that zero volts when talking about B+ voltages. Turn the amp off and measure resistance from chassis to both sides of the fuse. Do you measure zero ohms or very low resistance?
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Yes, the readings at the speaker output are mV.
Update- with no tubes is it normal for the B+ right off the diodes to ramp up slowly? With the HT fuse out it takes 2-3 minutes to reach 600v I then plug in the HT fuse and it drops the B+ to 260ish and takes another 2-3 minutes to ramp back up to 511v.
And yes, very low resistance to the chassis/gnd.
With the fuse (and output tubes)in, it almost immediately drains the B+ to negligible after I disconnect power. With the fuse out, it drains at about a volt a second (thru the meter I assume).
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And yes, very low resistance to the chassis/gnd.
If you are telling me that you have very low resistance between the chassis and either side of the HT fuse, you need to find out why.
I still want to know what the AC voltage to the diodes is.
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Very HIGH resistance. More than my meter can read. Maybe even open. Sorry bout that. AC voltage is 700v+
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AC voltage is 700v+
Surely that is from red lead to red lead?
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yes, red to red.
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OK. Now check the DC voltage at the orange wire on the other side of the diodes. I assume there is a jumper between the cathodes of those two diodes? How much voltage do you measure? Is it stable?
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OK. Now check the DC voltage at the orange wire on the other side of the diodes. I assume there is a jumper between the cathodes of those two diodes? How much voltage do you measure? Is it stable?
This is the voltage I referred to in post #15 above.
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With the HT fuse out, that voltage should reach max positive in only a couple seconds, maybe less. Check all 4 of the diodes. Probably best to remove the red wires to do so. If you can't check them just replace them.
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OK. This brings up another question; The Marshall circuit has what I believe is a full wave rectifier configuration. I have a 4 pin device that I believe is also a full wave rectifier. The pins are labelled 2x AC and + and -. I think I would just leave the DC- output disconnected. Correct? I have to ask because on the layout it doesn't seem obvious.
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The Ceriatone 2204 layout clearly shows four individual diodes mounted on the board exactly as you have. And yes, that is a full wave CONVENTIONAL rectifier. The 4 pin device you have is a full wave BRIDGE rectifier. CAUTION! DO NOT USE IT ON THIS AMP!
Theoretically it will work as a conventional FW rectifier by not connecting the negative terminal. But the diodes inside the block may not be rated for the voltage they will see from your PT.
Besides, you may not even need new diodes. Check them first.
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Diodes check good. .446 forward, open reverse. All 4.
I think k I said this before, but when disconnect the downstream wire from the rectifier, the voltage instantly goes to 450. But with the ht fuse pulled, it still creeps up slowly. The only thing in the ckt at this point is the first 50/50 cap and the 100k resistor across the standby switch. When I measure across the cap to ground there is no short.
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Here's the exact layout I used and the amp worked fine for a year as built. Now when I power it up with no tubes installed, DCV starts around 80-90v and slowly climbs to 450ish at about 2v/sec. With or without the HT fuse in.
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I've attached the voltage chart for this layout.
Mark
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Thanks Mark. I have a similar voltage chart. My ratios are the same, but something weird is causing my B+ to ramp up very slowly and the only clue I have is as I stated above. It defies logic. I just checked with the output tubes in and the B+ only reaches 370 after about 3 minutes of slowly ramping up.
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As we all suspected, it was a wiring issue. But not the one I was chasing. We thought it was something I did when I took out the fx loop ckt. but it was the PT HV center tap that was cold soldered on the bottom side of the ground lug. I've circled it in yellow. What clued me was at one point when I was troubleshooting it I heard some output for a second. When I flipped it over and plugged a cable in, there was no output. This made me think something was loose, but throughout the troubleshooting process, I unsoldered and resoldered every wire to the board at least once so I sterted tugging on all of the chassis connections and voila!
Maybe this can help somebody in their troubleshooting nonsensical B+ voltage problems.