Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Blind Lemon on January 16, 2017, 07:26:29 pm
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And also when I probe the phase inverter pin 2, that really sets it off. Change tubes around and shortened up some wires. Has a pretty good hum which I am assuming is a result of whatever the problem is. Seems to be 120hz.
Here are some board shots and voltages.
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If this is a new build, try reversing the OT primaries.
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dON'T REVERSE THE ot PRIMARY !
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...this is not your problem. I think that a bad cap is responsable for that: check that every stage has it's own filtering cap, that the cap is good, and that the resistors between the filtering caps are good ( not shorted ).
If this doesn't solve your problem, check the grounds. Make sure power tubes cathode and PT center tap are bolted to the same place in the chassis.
Colas LeGrippa
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when I probe the phase inverter pin 2, that really sets it off
By probing, or hard picking
If you can reproduce probing, try a re-tension, re-flow, and re-verify component values.
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New build, swapped OPT primary's first, won't do that again......... what? I SAID I WONT DO THAT AGAIN. Re flowed all solder joints. Did a cursory look at the components and they look good.
BL
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Hah! I had them swapped when I fired up my Bassman after I did the blackface mod.. yeah that's the worst sound ever.
So just a thought, I know a couple of guys were saying that the Silver Mica (the little black M shaped ones) had like a 1/3 failure rate on them. Maybe if you got a couple of spares try those out real fast. Just an Idea.
-Brett
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Nice call. I have had bad ones in the past when I did a Dumble type built. I lifted the leg of one on the brite channel but not the one in the tone stack after the cathode follower.
And just from my experience I have had nowhere near 1/3 failure rate.
BL
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Also check the cap that is across the Plate wires on the PI. This is common if one is not installed, well I say common but have seen many amps without them.
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Replaced the Mica Caps different kind and color so as not to come from same batch. No love!
Doug's layout (50w) does not have that cap so I put one on. No love.
I can now probe pin 2 of the PI but not 6 the other plate. Tap it and it sends it into an oscillation, a very loud one. I even changed speaker cabs.
Seems to be very sensitive around the PI. :dontknow:
BL
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Replaced pre-amp tubes and no love.
Do the PI voltages look right?
I know this is going to be something I over looked. Double check component values and they look right.
When I play through it lightly its not bad, ie: sounds OK. Pluck hard or with the volume up, either channel, it has a very nasty distortion, it wont always go into oscillation but sometimes it does.
Chassis is setting on the bench isolated from the speaker.
BL
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(https://sp.yimg.com/ib/th?id=OIP.Mf29a03bfd523ce9c9116185fa599c7c9o1&pid=15.1&rs=1&c=1&qlt=95&w=108&h=116)
Guess you all are as stuck as I am.
Thanks for the help.
BL
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When I play through it lightly its not bad
either channel, it has a very nasty distortion
sounds like you have a gain issue, to much. Got a scope?
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How are you grounding your input shielded wires? Sorry, could not see in the pics.
Jim
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What is the voltage measurement for V3 pin 2?
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I assume you have a EFX loop in that circuit, try injecting a signal to and from the amp to isolate which part of the circuit is problematic.
Eg. If you have another amp with a EFX loop take its OUT and plug into this amps IN to prove the power section, and the opposite way round to to prove the preamp section. - this should give some direction.
Some things to try disconnect NFB and possibly OT primaries polarity... they may be incorrectly color coded.
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R200 - At the input jacks,
Shooter - yea but is been a while
Steve - have it written down at home, if I remember right it's about 23v
Attila - Nope - No EFX Loop
BL
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V3 - Pin2 - 24.7V
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Nothing revealing with V3-2 voltage. Now check resistance of pins 2, 3, 7, and 8 with respect to chassis. Pins 3 and 8 should both read 15K. Pins 2 and 7 should both read 1.015M.
Also disconnect the NFB from the OT secondary. I know you've already verified the correct phasing but I'd just like to eliminate the NFB circuit completely. Since the PI tube seems to be a bit touchy I think you should concentrate on that circuit for a while longer.
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3 - 8 = 14.76K
2 - 7 = 1M
NFB will have to wait, I get to pick up my 11 month old granddaughter Eliza from DC. Will check back in a bit.
Thanks Steve.
BL
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The PI input cap... try loosening the part, heat solder and slacken the part just a touch.
See if the problem goes away.
I have had the leads tight on that cap, and when powered...and tapped with a chopstick,
have a sympathetic ring, that turns into an oscillation at the same tone.
Loosen the part a tiny bit...it goes away.
I always tap around the board after first power up with tubes...looking for part/wire contributed noises.
This can also happen at the cap
entry to V2 as well.
Also check lead dress on nearby offending wires too close or not at 90 degrees.
Peace
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Ok, just to get it down. Lifted the NFB and still had the same problem lots of hum and when I probed pin 2 (or was it 7) it went into wild loud oscillations went to SB to stop the run away flipped back on all good.
Damn hummmmm, pulled first tube still there, pulled V2 and gone. Hum gone now lets see if I can still make it oscillate. Easier to get to the turret that pin 7 is connected to - same same. Run away oscillation hit power switch.
Next did what the Dude suggested, but lets just replace the PI input cap, same type Mallory 150, different color - different batch.
Went to turn the amp on and nothing, check the mains fuse and its blown, stick another in it blows - pull the B+ fuse and put in a new mains it blows again. Well shit!!!! Out comes the light bulb and it looks like the PT has gone south.
:BangHead: :BangHead: :BangHead: :BangHead:
I think while I'm waiting on a new PT I'll just pull the board and look at it.
Guys, I have built a lot of amps, Plexis, Rockets, all the big and little Tweeds, BF Fenders, Dumble types, slucky's Revibe (sweet). This thing has kicked my butt and it's not a complex build. :dontknow:
Bet you a dollar to a donut its something simple that I'm missing. Cost me a PT.
Suggestions always appreciated.
BL
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Just for grins I checked my power tubes for shorts on my little Sencore Mighty Might, one of them showed "Bad" for grid leakage. Everything seemed to have biased up ok, I had checked the temps on all the power tubes early on and they were all close in temps. ???????
BL
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Your filter caps don't appear to be wired like a typical 100W Marshall. You should have a total of six dual 50 cans. Five of those cans should have the 2 caps strapped together.
The plate node should consist of two dual cans wired parallel/series for a total of 50µF. The PT center tap connects to the junction of these two cans. That looks OK, but you don't need balancing resistors across these two cans. The CT connection does the balancing.
The screen node also consists of two dual cans wired parallel/series for a total of 50µF. These cans have two balancing resistors across the cans.That doesn't look right.
A fifth can should be located underneath the IEC power connector, but you have a cover plate over the mounting hole??? This can has the caps wired parallel for a total of 100µF.
A sixth cap can should be mounted under the board near the preamp end. (Some people use individual caps rather than the sixth can.
It looks like you mostly have a Hoffman board with some modifications for the bridge rectifier. But it's not clear which layout you followed for the cap cans. I'm attaching Hoffman's schematic for the Marshall 100W and also providing a link to a nice layout that may/or not be useful.
http://sluckeyamps.com/2203.pdf (http://sluckeyamps.com/2203.pdf)
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As I'm pulling the board I notice I have 56K grid resistors not 5.6K.
BL
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Filters re done.
The pot is the bias circuit. I've used this setup on quite a few amps, never had a problem.
BL
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Its been a bit, I've been out of town and putting the amp back together.
I have the Marshall layout now for all the PS smoothing caps. Found a joint that was not soldered along the PS dropping circuit. Replaced the one bad (?) power tube and put in a pre-PI master. Also changed to more of a Marshall layout for the bias circuit. Changed a little bit of the way this amp was grounded and it is dead quiet. I changed no values of the components, I did change a couple of the chick-let style caps to Mallory 150s.
This amp sounds good at normal playing volume (MV @ ~1/2 and normal/brite volume 1/3), but, with ear plugs in, I crank the MV up and strum the open strings it still goes into a nasty osculation/distortion that is louder than hell and the only way to stop it is to hit the SB switch.
Load is 1 8ohm speaker with the impedance selector on 8, if I go to 16ohm it seems to delay the nastiness. Also seems to delay the onset of nastiness if I roll the Presence all the way to the left.
Thoughts?
BL
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Youtube Vid.
https://www.youtube.com/watch?v=2yzGo69SLjs&edit=vd (https://www.youtube.com/watch?v=2yzGo69SLjs&edit=vd)
Notice ear plugs.
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Man tough crowd. Not even a "your video sucks"
BL
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Your video sucks!
Wish I had an idea but I don't. Could be almost anything. I'd make it start that sound then run a scope forward from the input to try to get an idea where the noise first shows up. Look at the B+ nodes with the scope also. May be tough though. It could be that once it begins, it shows up everywhere in the amp.
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1.Checked components
2.Re-flowed solder joints
3.Pulled board completely out and checked it
4.Re-installed PS to mirror Marshalls
5.Have swapped out most caps on the board.
:BangHead: :BangHead: :BangHead: :BangHead: :BangHead: :BangHead: :BangHead:
Do you think it could be the OPT?
My o-scope is crap.
I got it oscillating and I left it that way so I could move some wires around and it started to smoke the screen grid resistors. So I put some true 5W cement block resistors in it and put my DVMs on the screen side of each one and got the thing screaming and it started dropping voltage and pulling big current again. I don't think I could troubleshoot it screaming like that without melting the tubes down.
Don't know how to troubleshoot this thing at this point without just shotgunning parts at it.
This sucks ............................... :dontknow: :help:
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Pull the preamp tubes and rig a jack so you can connect the guitar directly to the PI input cap. Can you make it oscillate?
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I can play it all day long at low volume.
At high volume,,,,, MV almost all the way up and volume at 10 o'clock is when it starts. Light strumming-good, Medium strumming you might hear some distortion (got ear plugs in) bare down on it and it runs away.
I say that to say I can try your PI hookup but I don't think the signal will be large enough to make it happen.
What do you think? :w2:
All this and NASTEP going on too.
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As requested, not enough drive. Could not make it start screaming even with hot humbuckers.
Didn't take long, used the aux. speaker jack as input. Ugly but worked.
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Got a New Fender Twin OPT, won't be able to for a couple of days, but I think I am going to slave it in.
If that doesn't work would you replace the power tubes first (friggin quad of EL34) or the power supply caps first?
At least that's were I think I am heading.
BL
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I'd replace the tubes 1st before replacing filter caps or OT. (Because their already new caps and new OT.)
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Mighta missed it;
what is the bias without tubes, any fluctuation?
with tubes?
what happens to the bias when it starts making bad noises?
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shooter - ur talkin bias voltage? Steady without and with, of coarse current goes up when strumming and way up when screaming.
willabe - I got the caps and transformer now, got to order new power tubes, these power tubes were new also.
Thx guys, someone's is going to hit on something.
BL
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Have you pulled a power tube off of each side yet to test them in pairs?
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Good call SG, I am going with power tubes. There's only so much that can go wrong and through process of elimination iron, caps, and resistors don't work then not work then work again on their own like this.
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Exactly. Mix and match to see if you can ID the possible offending tube. Also early on you mentioned you relplaced the preamp tubes. Did this include the PI?
Jim
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At this point I would suggest taking another amp and cross wiring it. In other words, the other amps preamp goes to the amp under question, power amp, the preamp from the amp under question goes to the power amp of the known good amp. That will indicate a preamp or power amp problem in the amp under question or it will indicate a systemic problem in the amp under question if the problem fails to appear when the two sections are isolated.
I'm going to guess there is a short at the higher power. Check the rectifier and the power caps. Can't remember from all the posts: Have the filter caps been swapped out? That video sounds like an arc.
silverfox.
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Nice thought on testing in pairs. Will try that next. - silver gun
Got a new quad of EL34s coming.
I have a Rocket that I may be able to slave in to it but I'm betting a dollar to a donut it's a PI and out problem.
All preamp tubes have been swapped multiple times. PI included.
Already swapped out the reciter diodes, have not swapped out the JJ 50/50 caps.
Thank you Gentlemen.
BL
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I just had another thought. what are the value of your screen resistors? sluckey mentioned something about the screen supply and now I'm curious if there isn't some sort of interaction between the screens and G1 at higher power levels- and that could be why the screen resistors smoked. sure they were underated? But why didn't they smoke in the beginning.
silverfox.
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Just played the amp with 2 power tubes in PP and it was fine, with all 4 being played (in pairs) to make sure it wasn't a tube. It's not a tube, they all perform perfect at high volume.
Read silverfoxe's post about screen resistors and wattage. Per the schematic they should have been good,,,,,,,,,except for one thing,,,,,,, I'm only using one screen resistor per pair. Used a 50w layout. :think1: Look at my board in the first pics of this thread. Should be using a screen resistor for each OPT.
Give me a few minutes to do a little rewire and test.
BL
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:icon_biggrin:
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Did you replace those 56K grid stoppers too?
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Yep, I'm cleaning this mess up. Seems that took care of it.
I'll post more a little later.
BL
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Well guys the mystery has been solved.
Thanks to Silverfox's questions, it got me thinking about the wattage of the screen grid resistors and how I had used those exact resistors in that exact place on the power tubes before. But, there was twice the load on those resistors because I was only using 1 resistor for a pair of screens. Then it hit me, the amp would behave itself with just 2 power tubes because those screen weren't tied together. (Thanks Silvergun) I needed 4 screen grid resistors for 4 tubes, one for each tube.
Bingo!!!! I had used the Hoffamn layout for the 50w, it works but you need to add a few more parts for a 100w.
Maybe someone smarter than me can confirm or explain this but i think that the problem was an interaction between the tube pairs on the same screen grid resistor. The 2 screens were tied directly together after the SG resistor. I'm thinking that with each tube having its own SG resistor the they are isolated from one another, no problem.
What have I learned? If your going to deviate from a proven design, think it through and be ready to solve problems. I have built enough amps that I expect to have to trouble shoot a new completion, if I don't I am pleasantly surprised.
Also when I'm stumped and looking for help, to listen and think about every question posed. There were quite a few people trying to help and all with good suggestions.
This is a great community of knowledgeable amp techs, I use to hang out here quite a bit in the early days, but life happens and you drift. Hopefully i can hang around more and maybe help someone.
I want to thank you for the helped, this amp was kicking my butt. But as with so may other things, with hind sight it was really an easy problem to see. Right????
BL
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This time the problem wasn't right in front of you! :icon_biggrin:
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(Thanks Silvergun) I needed 4 screen grid resistors for 4 tubes, one for each tube.
The 2 screens were tied directly together after the SG resistor. I'm thinking that with each tube having its own SG resistor
So know we know... SG = silvergun Screen Grid :w2: :icon_biggrin:
To be honest I had no idea about how many "silvergun" resistors you were using or their values & wattages? :laugh:
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... I was only using 1 resistor for a pair of screens. ... the amp would behave itself with just 2 power tubes ... I needed 4 screen grid resistors for 4 tubes, one for each tube.
... i think that the problem was an interaction between the tube pairs on the same screen grid resistor. The 2 screens were tied directly together after the SG resistor. ...
Were the new resistors mounted on the board, or right at the sockets?
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No room on the board, mounted them on the tube sockets.
BL
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(Thanks Silvergun) I needed 4 screen grid resistors for 4 tubes, one for each tube.
The 2 screens were tied directly together after the SG resistor. I'm thinking that with each tube having its own SG resistor
So know we know... SG = silvergun Screen Grid :w2: :icon_biggrin:
To be honest I had no idea about how many "silvergun" resistors you were using or their values & wattages? :laugh:
I don't know about you, but I feel a resistance every time SG posts to bash my boy.... :cussing:
Jim :angel
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Seriously this is all great stuff to put in our back pockets for the next project or problem that crops up on the forum or at home. Unfortunately BL had to suffer through it to find the fix.
Now we need sound clips with this baby dimed and rockin!!!! :guitar1
Jim
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No room on the board, mounted them on the tube sockets.
There's a good chance then the resistors acted as "screen grid stopper" resistors.
Supposedly, "high slope" (high transconductance) tubes are more prone to oscillation due to long screen grid wires. EL34's certainly fall in the category. And the screen grid acts as a low-mu input terminal. Could be simple bad luck with long screen grid wires too close to plate/OT wiring, and just the right inductance/capacitance/phase relationships to oscillate.
It may be the best plan to always install screen resistors right at the tube socket (even if there is board space for them).
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It may be the best plan to always install screen resistors right at the tube socket (even if there is board space for them).
Doesn't everybody?! :angel
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It may be the best plan to always install screen resistors right at the tube socket (even if there is board space for them).
Doesn't everybody?! :angel
Not everybody every time. Would you like to know when I don't and why, or are you good? How about a discussion? I would bet you already know.
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It may be the best plan to always install screen resistors right at the tube socket (even if there is board space for them).
Doesn't everybody?! :angel
Not everybody every time. Would you like to know when I don't and why, or are you good? How about a discussion? I would bet you already know.
Yes please Ed. I was sort of kidding but do practice what we preach here.
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I mount screen resistors on octal sockets. But I also have built a lot of EL84 amps. These tubes don't have free pins to use as tie points so I put screen resistors on the boards for those.
Hoffman put screen resistors on the board for all his amps except the recent Plexi 6V6.
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I mount screen resistors on octal sockets. But I also have built a lot of EL84 amps. These tubes don't have free pins to use as tie points so I put screen resistors on the boards for those.
Understood, however it can be done using a single resistor such as on this organ donor build. The layout and proximity made it easy and a no brainer.
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It's really a personal choice. Placement of screen resistors is not all that critical. The whole reason we are even discussing screen resistors is not because of improper placement of these resistors but because two of the resistors were omitted.
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... The whole reason we are even discussing screen resistors is not because of improper placement of these resistors but because two of the resistors were omitted.
True.
But coming from that angle, the worst you'd expect is that double-screen-current went through a single resistor, causing a little more compression. But there was oscillation...
Once you're looking at a 100w build, the area taken up by the output tubes is bigger, and the wiring to those sockets gets longer. Long "grid wires" in general would be a risk of oscillation (granted, usually for G1 where the tube is much more sensitive).
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a general curiosity from an intermediate amp builder, would the lack of the 2 screen resistors directly result in the catastrophic PT failure mentioned earlier?
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... would the lack of the 2 screen resistors directly result in the catastrophic PT failure mentioned earlier?
If the screen resistors contributed to failure of any transformer, it's only because an oscillation at full amp-power burned something up. In other words, indirectly if at all.
There are many tube amps with no screen resistors at all, and nothing burns up. Screen protection for guitar amps is a special case.
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Ok been out for a few days and am kind of surprised this didn't die. More traffic now than when I was having the problem.
Any way good discussion.
Now there is something I omitted, just forgot to post. The PT did not open, in my moment of anger and haste I miss diagnosist the problem. I had forgot to post that information, it made me happy for about two seconds and I went on.
HBP - my reasoning that the power tube pairs were interacting causing the oscillation was because of the lack of oscillatiion with only 1 pair installed, in either the inside or outside positions. No other changes were made so the wire length was the same. But, that being said I couldn't say one way or the other for sure.
I wish I would have taken a pic of the amp but I was building it for someone else. I had talked him out of it 3 times over the last 5 years, but he got his wish on the fourth time he argued with me. He is suppose to bring it back in a week or two so I can go over it again with fresh eyes and a better attitude. :laugh: I'll get a pic then of how I did the SG resistors.
BL