Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => EZ Board postings => Topic started by: Geezer on June 23, 2005, 05:22:56 am
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Hoffman Amplifiers
> Tube Amp Building - Tweaks - Repairs
> trailing distortion arrgh!
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leftypups
Junior tube assistant
Posts: 12
(5/21/05 8:26 pm)
Reply trailing distortion arrgh!
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Ok, I finally solved the hum problemon my 5E5A, most of it was due to the outlets I was plugging into. The other part was my noisy strats. I now have a problem that is really stumping me. When I hit a note or chords, I get some kind of trailing distortion as the the note decays, and it seems to get worse as the amp heats up. It almost sounds scratchy.
Here is what I've tried so far:
1) all tubes swapped out with known good ones, and tubes in 5E5A swapped out to other amps, all check out.
2) unplugged speaker and plugged in 2x12 cab same impedence, still no change.
3) happens in normal and bright channels
4) tried several different chords and guitars
5) checked all solder connections at tube pins, and touched them up to make sure there were no cold ones.
6) checked all voltages and grounds. all check out.
This is a brand new weber build.
Can anyone shed some light on this??
gehicks
Senior tube assistant
Posts: 136
(5/21/05 8:32 pm)
Reply Re: trailing distortion arrgh!
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I'd ask on the Weber support board...
phsyconoodler
I will work on all amps
Posts: 384
(5/21/05 11:53 pm)
Reply Re: trailing distortion arrgh!
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Sounds like parasitic ocillation.Did you use the brass plate that weber supplies?If you did,it may be a source of problem grounds.Do you have series grid resistors?What value are they and how close to the socket are they?They need to be right on the tube socket with the resistor body touching the pin.
Hoffman's ground scheme is far superior to the brass plate.
leftypups
Junior tube assistant
Posts: 13
(5/22/05 8:36 am)
Reply Re: trailing distortion arrgh!
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they are 68k, in the usual spot, on the board very close to the imput jacks. The only things grounded to the brass plate are:
1) 8uf signal cap
2) all signal tube cathode caps and resistors
3) imput jacks
4) all potentiometers
Other grounding:
PT centertap, earth ground, filament centertap, and power supply caps are star grounded at top left PT bolt. Power tube cathodes and potentiometer for adjustable bias are at opposite PT bolts.
phsyconoodler
I will work on all amps
Posts: 385
(5/22/05 6:55 pm)
Reply Re: trailing distortion arrgh!
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Take you DVOM and measure the resistance of all your grounds.when any ground gets soldered,it is in question. i.e. the brass plate.Did you use a ground buss for the pots?Don't trust that brass plate.If the rsistance to ground is too high,you may get the ocillation.
leftypups
Junior tube assistant
Posts: 14
(5/22/05 7:13 pm)
Reply Re: trailing distortion arrgh!
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yes, I checked every pot and jack, along with all cathode caps and resistors. They all zeroed out.
leftypups
Junior tube assistant
Posts: 15
(5/22/05 7:32 pm)
Reply Re: trailing distortion arrgh!
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ok, I am going to tear all the grounds off, and follow the Hoffman diagram step by step. One question, using the Hoffman diagram for proper imput jack wiring, do all four jacks have to be grounded to the buss bar? Which lugs specifically are the ones the buss wire is soldered to?
Lucid Alice
I will work on all amps
Posts: 328
(5/22/05 10:31 pm)
Reply Re: trailing distortion arrgh!
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It might also be a loose pin on a socket. Pull the tubes and clean all the sockets. Then make sure all the pins are tight. Doug gives a step by step in his service material in the library. Even if the sockets are new this sometimes has to be done.
phsyconoodler
I will work on all amps
Posts: 386
(5/23/05 12:16 am)
Reply Re: trailing distortion arrgh!
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you probably don't need to tear out all the grounds.Sometimes the lead dress can be the problem.Check all the leads and shorten any that are too long.They should be as short as possible to sockets from the board.The grids of the tubes are especially susceptible to this problem.Make sure all wires are as close as possible to the chassis as well.
leftypups
Junior tube assistant
Posts: 17
(5/25/05 7:13 am)
Reply Re: trailing distortion arrgh!
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well, I grounded as per the hoffman scheme, I checked and "touched up" every solder joint on the pins, I shortened the imput leads as much as possible, I retensioned all the tube socket pins, and I still have that crackling mess when I hit a chord or single notes. The voltages on V1 look normal, but on the cathodes I have about 1.755 volts. Is this ok? I physically checked all the board connections, they are shiny and tight. This is driving me crazy! Why me!!
Voxbox
Senior tube assistant
Posts: 171
(5/25/05 8:19 am)
Reply Re: trailing distortion arrgh!
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Have you tried a different speaker?
You may have a voice coil out of alignment.
leftypups
Junior tube assistant
Posts: 18
(5/25/05 8:26 am)
Reply Re: trailing distortion arrgh!
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yes, I have a 2X12 cab I plugged into and I get the same results.
HotBluePlates
I only work on Fender's
Posts: 1192
(5/25/05 9:00 am)
Reply Re: trailing distortion arrgh!
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I'd try a 47pF or 50pF cap across the outputs of the phase inverter. I don't recall offhand if that amp uses a long-tailed pair or a split-load inverter.
If it's LTP, the cap goes from one plate to the other. If it's split-load, the cap goes from plate to cathode. The cap is before (on the tube side) of the load resistors.
If it's parasitics (which it sounds like), then that should kill the problem. Going larger on the cap value is fine. A larger cap will stop the problem, but may roll off highs.
leftypups
Junior tube assistant
Posts: 19
(5/25/05 9:23 am)
Reply Re: trailing distortion arrgh!
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according to the schematic, it is split load. What could be causing this? I used shielded wire from the presence control to the cathode of v1, and again from the treble to the grid of v1. This worked in a few other 5E5A's that others have built. Can I try a 100pf cap? will it make a big difference? Thats all I have to try for now.
Thank you so much for the help, it is my first build, and I am trying to understand what causes parisitic oscillation, and how to detect and fix it.
GroundhogKen
Moderator
Posts: 2781
(5/25/05 12:00 pm)
Reply Re: trailing distortion arrgh!
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Yes, just tack the 100pF cap across pins 6 and 8 of the 12AX7 closest to the speaker jacks. See if that helps.
Ken
leftypups
Junior tube assistant
Posts: 20
(5/25/05 12:53 pm)
Reply Re: trailing distortion arrgh!
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To Ken and all who have replied, thank you so much. Now for the kicker. I took the amp and my 2x12 cab and set the up in another room. The 2x12 cab is boomy, and it was rattling everything in the work room. I plugged into the amp, and the noise was gone. I played for a while at all different settings, and I couldn't get it to happen. So I got another head, and plugged the speaker sitting in the combo into it. And it happened. The speaker is the problem. I never had this type noise come from a speaker before. It gets worse as i hit the strings harder. If I hit them faintly, it is almost non existent. I thought when a speaker goes bad, it really sounds bad, or you get nothing at all. This speaker sounds good except for the scratchiness that comes through it. Sorry to raise the 5 alarmer. Thanks again so much!!
Voxbox
Senior tube assistant
Posts: 172
(5/26/05 4:57 am)
Reply Re: trailing distortion arrgh!
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Its the voicecoil - it needs a recone. You should be able to feel it if you put equal pressure either side of the cone near the voicecoil and press in gently. You should feel a slight grating sensation as the coil rubs against the magnet.
Gglad you have cracked it.
6G6
I only work on tube amps
Posts: 420
(5/26/05 5:15 am)
Reply Re: trailing distortion arrgh!
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A really good speaker repair guy *may* be able to realign the voice coil.
Otherwise it will mean a recone or different speakers.
6G16
Junior tube assistant
Posts: 9
(5/26/05 9:07 am)
Reply Voice coil
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I had the same problem with my P2P Vibroverb RI ... one of the Weber P10Q's had developed a rub :( According to someone who know's what he's talking about, Weber designes great speakers, but that the assembly sometimes sucks. They also seem to u
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Voice coil
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I had the same problem with my P2P Vibroverb RI ... one of the Weber P10Q's had developed a rub :( According to someone who know's what he's talking about, Weber designes great speakers, but that the assembly sometimes sucks. They also seem to use(d) a type of glue which can pull the voice coil out of alignment after the speaker has been packaged up. He had a speaker which tested ok on arrival, set it in it's box on a shelf (room temperature) for 6 months and it developed a voice coil rub.
leftypups
Junior tube assistant
Posts: 21
(5/26/05 3:50 pm)
Reply Re: Voice coil
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I talked to Ted on the phone, and I sent it out today, no problem. I am just glad I found the problem. I took a lot of time reading and researching this stuff, along with the gracious advice from you guys, and it was driving me bonkers! Thanks again everyone.