Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ambugaton on June 02, 2016, 04:03:31 pm
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Hi everyone. I am having some issues trying to troubleshoot the nasty/harsh sounds I am getting out of my Stout TMB. Seems the more I raise the volume on this thing the worse it sounds. Rather then getting any smooth saturated distortion... I feel the amp loses any musical qualities, becomes thin/brittle, and loses massive amounts of sustain. A lot of in your face fizz as well.
I have tried reflowing each joint, countless tube swaps, in depth eq'ing, cab/speaker swaps, guitar/cable.
The amp does sound great at low volumes with an OCD or something similar in front of it... I don't know very much about blocking or crossover distortion but from the reading and listening that I have done... I really don't think this is the cause.
Any ideas? I can put together a sound clip later on if that would help.
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* Always suspect tubes 1st.
* Check bias
* Isolate source of problem to preamp or to power amp. E.g., use a listening amp to locate source of issue, which must be arising AFTER the vol pot.
Also, see: http://www.aikenamps.com/index.php/what-is-blocking-distortion (http://www.aikenamps.com/index.php/what-is-blocking-distortion)
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Reverse the OT primary plate leads. Does that help or get worse?
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I've reversed the OT leads already and got loud oscillation. I don't have a listening amp but I would assume that is probably the quickest way of narrowing down the problem.
I will try disconnecting the NFB and see if that helps out any
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Consider getting a listening amp; or use a powered computer speaker, e.g. To do the latter search the Forum and use an in-line cap for safe practice.
If you have another amp: send signal from the other preamp in to the power amp of the amp under test; and vice-versa. (If you don't have another amp, then consider fixing that situation right away! :icon_biggrin: )
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I will need to look into running one of my other amps into the power section of this one... I have no idea how to do that, time to find out.
Do I need an amp with an effects loop? I have a blackstar with a loop but still don't grasp how I would bypass the preamp or power section of the Stout since it only has single line in.
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I'm assuming (after a little reading) that I can run Guitar > Stout > Effects Return on Blackstar. That will allow me to bypass the Stout power section and see if that is causing me the problem. Sound right?
With that configuration do I need to do anything to be extra cautious?
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This is more for the folks on here who know what they are talking about (unlike me...) as I am just looking at the schematic and wondering.... What if that big cathode resistor is bad or for some reason has drifted way off? How would that affect the performance of the output section?
Jim
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There are lots of helpful voltage readings on the Hoffman Stout schematic.
Have you taken voltage readings at all those points on your amp?
Are your voltages all within 5% of the Hoffman stated readings?
I'm guessing you might find at least one of those readings is off.
That would give you a good clue of where to look.
If everything is spot on, your amp should sound good.
Unless your tubes suck, but you said you've swapped them all.
On my Plexi build, one of my voltages was off.
That told me right where to look, and I found a leaky cap.
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I will need to look into running one of my other amps into the power section of this one... I have no idea how to do that, time to find out.
Do I need an amp with an effects loop? I have a blackstar with a loop but still don't grasp how I would bypass the preamp or power section of the Stout since it only has single line in.
Read this on listening amps. Essentially, you will be using one of your 2 amps as a listening amp for the other one: http://el34world.com/Hoffman/tools.htm (http://el34world.com/Hoffman/tools.htm)
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Took new readings this morning. So it looks like I am getting pretty low readings for V1-1, V1-6, V2-2, V2-6, V2-7. My readings are in ()
V1-1: 228 (204)
V1-3: 1.8 (2)
V1-6: 225 (198)
V1-8: 1.8 (2)
V2-1: 237 (229)
V2-2: 53 (39)
V2-3: 78 (77)
V2-6: 246 (220)
V2-7: 55 (35.6)
V2-8: 78 (76.6)
V3-3: 11.2 (11.6)
V3-7: 340 (334)
V3-9: 348 (339)
V4-3: 11.2 (11.6)
V4-7: 340 (335)
V4-9: 348 (339)
B+ was all good. I guess its good to know that my problem is within the preamp section. Have some more learning to do. Whats the best way to approach this? Would these readings cause this problem I am having?
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Clip of what the amp sounds like:
http://picosong.com/Z9zY (http://picosong.com/Z9zY)
Volume at about 5, I am getting this crazy phase type of effect and there is a lot of fuzz/fizz that is unpleasant to my ears. Or maybe I am crazy.
LP > Stout > 57 > Logic. No pedals/effects used.
Let me know. Thanks!
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Can you post pictures of your build?
If it gets worse when you turn the volume up, i'm thinking it could be in-phase signal feeding back to earlier part of the amp.
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Clip of what the amp sounds like:
Volume at about 5, I am getting this crazy phase type of effect and there is a lot of fuzz/fizz that is unpleasant to my ears. Or maybe I am crazy.
LP > Stout > 57 > Logic. No pedals/effects used.
The amp sounds fine to me. Much more gain/distortion/fizz than is my taste, but it sounds like it's doing exactly what it was designed to do. Any "phasey" sound may just be "beating" harmonics of the notes played.
It also seems bright-ish to me, except that it sounds appropriate in the recording in that it is bright in the way it should be to sit well in a mix when you have a bass player, drums, etc. You can knock down the gain a bit if you wish; there are many places to do that in this circuit. You can also tame the bright-ish nature of the EL84's.
For the latter goal, try grid stopper resistors of up to 56kΩ in place of the existing 8.2kΩ.
To lower "gain" try adding voltage dividers to reduce how hard each stage is driven. Marshall did this in many of their amps after the initial JTM-type amps. The volume control itself is a voltage divider; if you add a resistor from the wiper of the Treble pot to the input of the Volume pot (in place of the existing wire), you'll add an upper-limit such that there is some signal strength lost at the Volume control, even if it's dimed.
You could also knock down the gain after the output of the 2nd gain stage & before the phase inverter. V1B has a 100kΩ plate load resistor, a 0.022µF coupling cap, a 470kΩ resistor to ground, and a 0.01µF coupling cap going into the phase inverter. If you added a 470kΩ resistor to the board where the 0.022µF cap sits now, and move the 0.022µF cap to replace the jumper between the turrets for the 100kΩ resistor & location of your new 470kΩ resistor, you'd have a voltage divider which knocks the signal strength down by half.
Those 0.022µF and 0.01µF caps also cut out a lot of bass (because they're in series, they look like a composite 0.0069µF cap). If one of those caps was raised to 0.1µF or so, much less bass would be cut out. But high gain and lots of bass usually sounds bad.
If the above steps don't go far enough, you could also remove the cathode bypass cap on the V1A gain stage. Or adjust the resistances in the voltage dividers to cause even more signal loss, which is then less likely to overdrive subsequent gain stages.
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Thanks for the suggestions! And I will post pictures when I get time. I was thinking about increasing the grid stoppers. Is it correct that it is ok up to 68k before it starts taking out too much of the high end?
Maybe I just don't enjoy el84s if you think everything with the voltages is fine. I'm still concerned about some of the readings. I will look into possible reasons for that, increase the grid stoppers and see if that helps. I did try a 12at7 and that helped clean it up but I play a range of music so I would still like to have decent overdriven tones to work with. Thanks for the help.
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I am not sure what you 18 watt Stout schematic is?
However, IF it is the one below ............. you could try some of these mods to see if it smooths the tone out for you. Some of these would be easy to try safely with insulated alligator clipped wires and the caps.
I'd start with the "enchance cap" 220p across the LTPI plate resistor & the smoothing cap on the 2nd gain stage that is 120p. Then I'd try lowering the V1b .022 coupling cap to .01 or .015. Then I'd try lowering the EL84's cathode cap from 47uf to 10uf. Then I'd try the PPIMV.
These may or may not be useful to you?
with respect, Tubenit
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... I was thinking about increasing the grid stoppers. Is it correct that it is up to 68k before it starts talking out too much of the high end? ...
You're probably thinking of the 68kΩ resistor at the input gain stage; I'm talking about raising the grid stoppers on the EL84's themselves.
In each case, there is some amount of capacitance from the tube grid to the other elements, as well as stray wiring (and for the input stage, cable) capacitance. Often, it is suggested "don't go above ____ or it will shave off treble." Except that if you're fine with the quantity of distortion, but don't like the harsh sound, rolling off a bit of treble might help.
So I'm talking about purposely raising those EL84 grid stoppers to roll off some treble.
... I'd start with the "enchance cap" 220p across the LTPI plate resistor & the smoothing cap on the 2nd gain stage that is 120p. ...
And Tubenit's added caps do something similar, but in different places: they shave treble. However, you may find his approach better, as nixing some treble in the particular stage distorting is probably the best approach.
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Would increasing the grid stoppers on the preamp give me a "smoothing" effect? I believe the harshness is coming from the preamp section and wish to shave off some of that treble as well.
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I am thinking that if you try some of the mods I suggested that you would find them helpful.
I would at least try the smoothing cap of 120p on V1b and the enhance cap.
You can add a 100k to 220k between the volume and the grid of V1b (V1-7) and that should cut some treble.
With respect, Tubenit
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Thanks tubenit, I will look into these mods. In the middle of moving into our new house and figure it will be tough justifying the "need" to do tweaking on my amp at this time. I will let you know when I get some time and see if it helps out. Appreciate it.
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Does this look correct for the mods you suggested tubenit?
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Based off the fact that I did not understand how the NFB circuit worked... I think I found most of my problem. I was assuming that a higher NFB resistor would result in more headroom and less hair... If I remember correctly I may have increased this beyond the 120k on the stout schematic. I will double check this when I get home and lower that substantially.
I am running the amp through a 112 cab @16ohm... should I hook the NFB to the 16ohm tap? What would be the difference between the voltage of the 16ohm vs 8ohm tap and how does that affect the tone?
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I am running the amp through a 112 cab @16ohm... should I hook the NFB to the 16ohm tap? What would be the difference between the voltage of the 16ohm vs 8ohm tap and how does that affect the tone?
It will greatly increase the negative feedback. If you go too far increasing negative feedback, the amp will become unstable; I'd advise sticking with what's on the schematic.
Imagine the taps on your output transformer are wall outlets in your house. Even though you're plugged into the 16Ω tap, there is voltage present at the 8Ω tap. The feedback circuit is scaled with the expected voltage from that 8Ω tap.
Unlike your home outlets, there are different voltage levels present at each OT tap for the same output power. Hence why I don't suggest moving the tapping point for the negative feedback circuit.
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I will look at what tap it is soldered to. Unfortunately I haven't had any time to take a look but I'm thinking I actually have it on the 4ohm tap... I'm not quite sure how much of an impact that would have on the grainy/fizzy distortion I'm getting from the amp now, but it is something to look at for sure. Thanks.
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The NFB circuit is designed for a specific tap on the OT.
You can change the tap, but then the resistors in the NFB circuit must be adjusted.
They need to be adjusted to maintain the designs NFB voltage input, if I understand this correctly.
If you just disconnect the NFB does the output get even harsher or stay about the same?
That is what I expect would happen.
If it does, to me that indicates that whatever tap you are using for the NFB is not supplying enough NFB input.
This also relies on the NFB circuit being the real source of the harsh/dry problem in the 1st place. :icon_biggrin:
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Well it seems it was on the correct tap. Not sure which resistor I put into the nfb though. Still hope that I may have increased that too much. Amp begins to break up fairly heavily at around 3.
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It seems to be breaking up rather early.
The NFB would be something that contributes to that.
Usually I try to follow the build design exactly at first to know that things sound as they should.
Then I will apply a mod and be able to know how that mod changed the sound.
If you make a number of mods to the original circuit at first,
you risk not knowing how those mods are actually changing the sound,
or if the mods are interacting and creating problems.
I usually try to build as designed first, then apply and tweak a single mod to taste.
Then apply another mod, one at a time, making sure that any additional mod doesn't create problems. :icon_biggrin:
If you go through that process and apply a number of mods tweaked to your taste that makes it sound best to you.
Then I don't really consider that to be the original design with a bunch of mods applied.
I consider that to be a re-design of the circuit, and your own original design. :icon_biggrin:
Now if you want to build another amp exactly like that,
you are then following your own original design, and know exactly what it should sound like if built correctly.
Just because someone says this or that mod makes it sound better,
I really want to know if I like the sound of the original design, or the mod better.
I might not think the mod improves the sound like they do.
Or I might like the mod for a certain sound, but not how it changes the overall characteristics of the amp.
Most of the original designs we copy have proven to perform well overall.
Even things that don't seem to be much of a change, can negatively effect the amps overall performance. :dontknow:
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Did you see the demo video at the Library page for the Stout TMB (http://el34world.com/schematics.htm#Hoffman_Stout_TMB)?
It still seems the Stout TMB is doing exactly what it should (meaning what it was designed to do), but that you would prefer a lower-gain design. It's not terribly hard to drive EL84's and perhaps this design just has too much preamp gain for what you want to hear.
On page 2 of the layout/schematic document, R12 is a 470kΩ resistor to ground between the 2nd gain stage & phase inverter. If you cut the link between the 0.022µF (C5) and 0.01µF (C6) caps, you could replace the 470kΩ with a 500k-1MΩ pot, with outer lugs spanning the turrets where R12 is now, and the wiper connected the C6 lug which was formerly connected to C5. Now you have a variable voltage divider to knock down signal level going into the phase inverter, which may help you clean up the sound a bit.
You could just tack in that pot to start with to determine the 2 resistor values needed to stay in the amp as a voltage divider, add it as an internal trim pot, or include it in the extra chassis hole.
If you try tacking in that pot but feel you have too much drive from earlier in the amp, there are a number of ways to reduce gain through the preamp.
Assuming the tube socket is rotated properly to allow (or you have an insulated standoff), you could mount a resistor between the plate of the 1st or 2nd gain stage and the present plate load resistor. This splits the plate load into 2 parts, and forms a voltage divider to knock down signal level. You might replace the original plate load resistor with 47kΩ and use 47kΩ for your newly-added resistor (to equal 94kΩ total, and close to the original 100kΩ total) to cut signal by half. Or 27kΩ/82kΩ to knock signal down to ~1/5th.
That's just one option, and there are other ways to lose some signal strength if the existing circuit is just driving the output section harder than you'd like.
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Just want to say thanks for all the help on this. I installed the suggestions that tubenit gave and the voltage divider on the output of v1-b from hotblueplates and it really made the amp more to my liking. Also did a nfb pot using a 500k and found the sweet spot for me. Now will just determine what value r to tack in there. Thanks again.
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I am not sure what you 18 watt Stout schematic is?
However, IF it is the one below ............. you could try some of these mods to see if it smooths the tone out for you. Some of these would be easy to try safely with insulated alligator clipped wires and the caps.
I'd start with the "enchance cap" 220p across the LTPI plate resistor & the smoothing cap on the 2nd gain stage that is 120p. Then I'd try lowering the V1b .022 coupling cap to .01 or .015. Then I'd try lowering the EL84's cathode cap from 47uf to 10uf. Then I'd try the PPIMV.
These may or may not be useful to you?
Well Thank you, this is something that I wanted to look for soon to voice my Stout (Sherman 18)
{EDIT -- untangled quote --PRR}
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I am glad the mods were useful to you and that you are liking the amp more.
Having reviewed this thread, I noticed how high the voltages are on the 12AX7 preamp tube which are on node D.
I have sometimes found a warmer smoother less harsh tone with around 160v on the 12AX7 preamp plates. I have found 140v to 180v range to be the most useful and frequently to my liking. I typically only aim for 180v to 200v range on D-style clean channels only.
I am thinking the 200+ volts you have on the preamp tube plates may contribute to harshness. IF that is the case, you could try increasing the B+ rail resistor from 4.7k to 8.2k - 10k range (or more) and listen to what happens.
With respect, Tubenit
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I just put a better quality 5y3 in not too long ago and I currently have 170v on the preamp plates. That may have helped a bit with all of this as well.
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I played the sound clip from earlier in the thread and the Stout that I recently completed sounds very similar, which is what I had expected.
I installed a NOS 5751 in V1 and that had changed the amps disposition significantly. It was suggested to me to try a 12 AT7 in V2 as well.
This is a well constructed amp that sounds quite nice now, but I think the amp would be a little more attractive with a little bit of a softer disposition, and I think the mods discussed in this thread are perfect for what I felt the amp needed.
I especially am looking forward to trying to cut in the MV, I think that would really open up that amps potential, at least for playability at home.
Great info, thanks, I am inspired to improve on a nice amp.
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try a 12AY7 in V1.
--pete
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Yeah it definitely did. I am still very new to this world, but what helped the most is tampering with the NFB. With the advice from tubenit I could hear an audible drop in hiss when I connected and disconnected each alligator clip when testing them out. With the voltage divider it definitely dropped the gain significantly. It definitely helped "smooth" out the sound. I think my first mistake was building an EL84 Marshall circuit prior to having any actual knowledge of what I liked and what I wanted. But that being said... with a few adjustments I really do enjoy the sounds I am getting now. As I mentioned though... I am so new to this and barely have a grasp on what this amps voice was meant for. I have a 12AT7 in V1, a voltage divider, and NFB that makes the amp as clean as a fender. I know now what my ear wants.
But... the amp is quiet, takes pedals well, is compact, and with an attenuater it is perfect for a wide range.
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Hey that is a great report!
I'll be ordering parts this weekend.
Dang, it's 2:00 and I am just heating up the iron. Killer nap will make you pay later
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I installed a NOS 7551 in V1.....
5751? :dontknow:
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5771, 5751
Close enough for Rock N Roll!