1987 plexi weird treble noise

sdn25

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Ok, I see what you mean by "crashing" sound. I'm hearing like a "splatty" distortion.
At the beginning of the clip it sounds pretty normal. 7 seconds in I start to hear that splattiness, and 14 seconds in it becomes a lot more obvious.

My bandmates and I were testing ways to boost a bassman for a recording, and we plugged in an Ampex preamp and when we gave it some juice it would sound splatty like that.
would that imply that excess gain is causing it?
 

SmokeyDopey

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To me that seems what was happening, but I'm not a tech, so I can't say it with certainty.

When we set the level conservatively on the Ampex, the amp sounded normal, but past a certain point it started to get splatty. Since that preamp has a lot more gain compared to a normal pedal boost it was easy to go too far.
 

sdn25

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Right so to test that theory I increased the cathode bypass resistor on v1 from 820R to 2k7 and guess what? The issue isn't there anymore ! Might sub in different values such as 2k, 1.7k, 3k etc to figure out the sweet spot.
 

Chris-in-LA

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No my issue is that turning the mids DOWN makes it really bright and the treble frequency has this unpleasant distortion. The amp only sounds good when the mids are maxed out, and this is with a 50k mid pot so wouldn't that let more mids through ? when I turn the mids to about 25k on the pot (stock marshall value) the sound that I am describing is very prevalent, it is not present on other Marshalls with 25k mid pots.
I have seen odd problems with the wrong value pots. The treble on my Bassman clone didn’t work at all, had a 250KA instead of a 250KL I think. You might try putting in a 25KL pot and see what happens.
 

myersbw

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I listened to your clips. But, let's jump to some background first. In your amp signature you're listing a Ceriatone Plexi 50 Kit. So, I'll assume this is the Ceriatone.

Couple questions to verify what we're listening to... Did you build the kit or someone else or did you get it from Nick already assembled? Also, specify the complete chain for your recording....guitar used, any pedals? (if so, just go direct...guitar to amp)

Bak to the amp being a Ceriatone. If it's the same model I'm seeing on their site, I'm seeing the Bright channel with a push/pull switch on that volume control. It adds in a 500pF bright cap when pulled and no bright cap when pushed in. Do you have the 500pF in or out? ONLY my opinion...500pF is too large for that amp...I much prefer a 100pF cap on that volume control. It is possible to rewire that switch so that out is 500pF and pushed in is 100pF (vs. none...which i don't care for either).

Also, post up a photo(s) of that build...especially the backside of the pots and board so we can verify wiring.

Another quick check if not already done...swap V1 and V2...did the issue go away or change dramatically? If so, suspect a tube...new or old.

Now...tone. It sounds like some upper & mids for the gain. That is not a master volume style amp, so...are you power soaking the output? Or, to me...it sounds like an OD/distortion pedal at the front end. If you're pushing the output tubes and patching channels...phasing due to taxing the supply a bit doesn't surprise me. For that style of amp I prefer to center the TMB tone stack and slightly tweak to taste.

Looking forward to seeing where you're at with this. Yeah, I just saw you eliminated the issue, but curious given your issue. :)

Brad
 

sdn25

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I listened to your clips. But, let's jump to some background first. In your amp signature you're listing a Ceriatone Plexi 50 Kit. So, I'll assume this is the Ceriatone.

Couple questions to verify what we're listening to... Did you build the kit or someone else or did you get it from Nick already assembled? Also, specify the complete chain for your recording....guitar used, any pedals? (if so, just go direct...guitar to amp)
I built it. Chain is guitar > amp > suhr reactive load > audio interface > DAW > greenback IR.
Bak to the amp being a Ceriatone. If it's the same model I'm seeing on their site, I'm seeing the Bright channel with a push/pull switch on that volume control. It adds in a 500pF bright cap when pulled and no bright cap when pushed in. Do you have the 500pF in or out? ONLY my opinion...500pF is too large for that amp...I much prefer a 100pF cap on that volume control. It is possible to rewire that switch so that out is 500pF and pushed in is 100pF (vs. none...which i don't care for either).
I have a 4n7 bright cap installed but when recording it is bypassed, I didn’t like 500pf as it was passing frequencies a little too close to high mids than I would like, the 4n7 just adds a bit of air to the sound.
Also, post up a photo(s) of that build...especially the backside of the pots and board so we can verify wiring.

Another quick check if not already done...swap V1 and V2...did the issue go away or change dramatically? If so, suspect a tube...new or old.
Funnily enough I had initially got a set of high gain long plate 12ax7s for this amp thinking it won't have enough gain but oh boy was I wrong haha, but when I had those installed that treble noise was very prevalent, so I swapped them out for regular 12ax7s an the problem disappeared, this again would reinforce the idea that excess gain was causing it. Initially I thought it was something to do with the tubes themselves and not the gain.

In terms of wiring, I’m am pretty sure it’s all correct because I have had a few threads on the forum about issues as it was my first amp build, eventually I re did the wiring and triple checked everything, measured every value individually etc. I have include the picture of the backside of the pots just in case.
Looking forward to seeing where you're at with this. Yeah, I just saw you eliminated the issue, but curious given your issue. :)

Brad
Yeah I think it’s solved, would you have any suggestions for the best value for the bypass resistor?
 

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sdn25

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Awesome! I'd like to hear a clip of it when you have it dialed in.
Here you go man, Ive dialed in the amp (All knobs on 10 🤭) , it sounds absolutely killer IMO, Im using a celestion Blackback IR miked with 2 sm57s here with some plate reverb. Please excuse my sloppy playing I've only been playing for 9 months 😅.
 
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SmokeyDopey

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Awesome, thanks for posting. It sounds great!
What? 9 months? haha dude...it sounds like you have years of experience.
 

myersbw

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Yeah I think it’s solved, would you have any suggestions for the best value for the bypass resistor?

Which resistor are you referring to? (Or, did you mean cap?) For that bright (Volume bypass cap...pop a 100pF in there and see what you think. ) You can do the 4n7 and the 100 on that switch. Wire both caps to the end of the pot and the other end of the caps to opposite sides of the pull switch. Then wire the center lug of the switch to the Volume wiper. That's an arrangement that gives you two values to work with. But, if you did mean resistor, let me know which one.

Glad you're having fun and great job on a first build!

Brad
 

sdn25

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Which resistor are you referring to? (Or, did you mean cap?) For that bright (Volume bypass cap...pop a 100pF in there and see what you think. ) You can do the 4n7 and the 100 on that switch. Wire both caps to the end of the pot and the other end of the caps to opposite sides of the pull switch. Then wire the center lug of the switch to the Volume wiper. That's an arrangement that gives you two values to work with. But, if you did mean resistor, let me know which one.

Glad you're having fun and great job on a first build!

Brad
I was referring to the 820 ohm cathode resistor for v1, in its stock form it had too much gain so I raised that to a 2k7 like the 70s era circuit which solved the issue I mentioned here. I was wondering whether you would happen to know what the possible sweet spot could be in terms of the value so as to get as much gain as possible without having the unpleasant distortion.

And thank you for the kind words :) it indeed was quite fun and got me interested in electronics haha.
 

neikeel

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The 820R is a cathode resistor (not bypass). It sets the bias of that triode. There are two triodes on V1 each with a resistor. The V1a is usually an 820R bypassed with a 250uF capacitor which is pretty much full bypass. The V1b resistor was initially 820R as well (1968 to 1970) but was changed to 2k7 to make the bias a little cooler and less fizzy with hot humbuckers and typically only partly bypassed with a 680nF cap. Fender might use a 1.5k resistor bypassed with 25uF cap. Many other manufacturers use a similar value. I’m not familiar with your amp schem.
 

sdn25

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The 820R is a cathode resistor (not bypass). It sets the bias of that triode. There are two triodes on V1 each with a resistor. The V1a is usually an 820R bypassed with a 250uF capacitor which is pretty much full bypass. The V1b resistor was initially 820R as well (1968 to 1970) but was changed to 2k7 to make the bias a little cooler and less fizzy with hot humbuckers and typically only partly bypassed with a 680nF cap. Fender might use a 1.5k resistor bypassed with 25uF cap. Many other manufacturers use a similar value. I’m not familiar with your amp schem.
In stock form it was 820r + 0.68uf bypass cap, I now have a 2k7 like in the 70s circuits.
 

myersbw

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I was referring to the 820 ohm cathode resistor for v1, in its stock form it had too much gain so I raised that to a 2k7 like the 70s era circuit which solved the issue I mentioned here. I was wondering whether you would happen to know what the possible sweet spot could be in terms of the value so as to get as much gain as possible without having the unpleasant distortion.

And thank you for the kind words :) it indeed was quite fun and got me interested in electronics haha.

Neikeel explained the essence of that resistor. I won't elaborate more on that. Now, you made it cooler regarding bias, however, you also may have got a similar result by replacing maybe a hot 12ax7 (lot of gain) with a 5751 (12ax7 characteristics, but lower gain per bias point). But, then you find you're into the maddening world of "tube rolling" until you find that right combo you like. The bottom line is...if what you did produced the results you like...awesome! The preamp-side is fairly forgiving with tube swaps, but most don't dive into the bias points and the differences you setup when you, say, swap a 12ax7 with a 12at7 which is not a similar tube in gain structure.

Read all you can...the fun will just keep on rolling!
 

mrrstrat

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I have built many 1987 clones - boards and amps (and many of the Hoffmanamps boards/amps). The vintage gear responds and sounds differently than a JCM2000 DSL or a Fuchs ODS amp. Both of those amps have lots of componentry internally to compensate signal and its handling. The recordings don't sound bad - except the previous two sounded like there was reverb on it.

What I did learn when I started building amps (over a 17 year period for money) was that for the first time I could hear the difference in preamp tubes. The gritty breakup of some tubes (like a coarse distortion) verses the smooth creamy distortion was possible in the same amp based on the preamp tubes. I like the sound of the coarser distortion like your amp has and not a more compressed and buzzy (creamy) distortion.

You might be hearing some tube specifics. On my 1987 builds I shipped them with Electroharmonix 12AX7EHs and a 12AT7 driver to tone it down and get greater clean tone. The 1987 builds I did were not the master volume version, 4 input. That amp was tough for many people not used to a two channel bridged input amp - as you basically mix the two channels. The master volume version I build too sounded like what you have.

Try some tubes for the gain and tonestack that are completely different. I would try a set of JJ preamp tubes and then a set of something like the 12AX7EHs as those two on the vintage re-release stuff I build would show a difference in gain and tonestack signature/response. Sovtek tubes seemed to be darker and unremarkable on any of the amps (12AX7WA and the series seemed to sound the worst). The Phillips and RCA and other USA stuff seems to vary more in definition and distortion (not to my personal liking).

Tube rolling can be madness but only if you roll tubes that sound and respond nearly the same. The JJs are bright and responsive, but more microphonic as the elements do rattle. The 12AX7EHs have exceptional microphonic control and are brilliant in highs and lows (I love the way the 12AX7EHs naturally overdrive too). I found my tube of choice to be a Mullard 12AX7 type and settled on the Hungarian Tungsram 12AX7 (The best I found for any Marshall variant - and cost $50-$100 USD).
 
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sdn25

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I have built many 1987 clones - boards and amps (and many of the Hoffmanamps boards/amps). The vintage gear responds and sounds differently than a JCM2000 DSL or a Fuchs ODS amp. Both of those amps have lots of componentry internally to compensate signal and its handling. The recordings don't sound bad - except the previous two sounded like there was reverb on it.

What I did learn when I started building amps (over a 17 year period for money) was that for the first time I could hear the difference in preamp tubes. The gritty breakup of some tubes (like a coarse distortion) verses the smooth creamy distortion was possible in the same amp based on the preamp tubes. I like the sound of the coarser distortion like your amp has and not a more compressed and buzzy (creamy) distortion.

You might be hearing some tube specifics. On my 1987 builds I shipped them with Electroharmonix 12AX7EHs and a 12AT7 driver to tone it down and get greater clean tone. The 1987 builds I did were not the master volume version, 4 input. That amp was tough for many people not used to a two channel bridged input amp - as you basically mix the two channels. The master volume version I build too sounded like what you have.

Try some tubes for the gain and tonestack that are completely different. I would try a set of JJ preamp tubes and something like the 12AX7EHs as those two on the vintage re-release stuff I build would show a difference in gain and tonestack signature/response. Sovtek tubes seemed to be darker and unremarkable on any of the amps (12AX7WA and the series seemed to sound the worst).
I have jj 12ax7s for v1 and v2 and I have a sovtek in v3. I was able to fix the issue without a tube swap, reduced the amount of gain in the circuitry and that solved it. If I had to guess, the output section was being slammed with too much gain and caused that noise.
 

mrrstrat

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Awesome - I can say I had an amp or two where a weaker connection cause an issue or I used the wrong value component even with being meticulous and triple checking things. Re-shaping the gain characteristic is certainly an option. As you know, the schematic shows nominals and not absolutes so that might be an opportunity for safe tweaks within 10-20 percent. Reduction of gain is a safer tweak than messing with the tone stack.

I did find that a Sovtek 12AX7WA/B/C or other Sovtek seems to work well in the phase inverter and not cause tone shift. The Sovtek tubes are bullet-proof and strong. The JJs for me are hit and miss but can sound great. I have about 250 preamp tubes in my personal stash and at least 25 are JJs.

I like a cleaner amp sound as I use a pedalboard in the front end of an amp so I tone mine down for usable clean tone when I want it. When I plug straight in I might drop in a 12AX7 into the phase inverter slot and use "better sounding" tubes for straight-in the amp. I love the sound of a hard driven 1987 variant Marshall and a Gibson Les Paul plugged in with no effects - match made in heaven!


I have jj 12ax7s for v1 and v2 and I have a sovtek in v3. I was able to fix the issue without a tube swap, reduced the amount of gain in the circuitry and that solved it. If I had to guess, the output section was being slammed with too much gain and caused that noise.
 


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