The Canadian Guitar Forum banner

1 - 20 of 30 Posts

·
Registered
Joined
·
103 Posts
Discussion Starter #1
have a custom built 100 watt marshall major design . 480/ 560 v switchable plate voltage. merren output tranny , hammond 278 cx pwr tranny . brand new built , iska resistors , mustard caps , kt88’s 2 -12ax7 and 1-12au7 . cascaded input , ppimv ... when i strum a note i get some sort of fizziness and the tail end of the played note , also when i dig in on the note it seems to have this haziness to it , sustain seems shorten . this issue was there from the initial build , then after i had the cascading done hoping it would go away . looking for some skilled ideas on how to fix it , or what ares inthe build would cause it ...have used other guitars/ cords /cabs to eliminate these areas . tried a pwr conditioner and other electrical circut outlets ..... regards jeff
 

·
Premium Member
Joined
·
3,209 Posts
If it's that new (is the the one from that recent build thread?) take it back to the guy you had built it. Personally I would consider that not delivered.

Since tubes are knew (and I assume the heaters light up ), I'd be looking at power supply. If you are getting normal range in volume with the PPIMV, I'd be looking at the plate voltage to the preamp tubes. Pull the tubes (all except rect) and check the DC voltage on pins 1 and 6 (each to ground) on the 12A_7s. Since you're in there it doesn't hurt to check the power tubes too. V should be a bit more than you'd expect with the tubes in/under load; so anything too low is suspect - trace it back to the rect; usually there's a burnt component in the filter/voltage dropping section.

Usual safety caveats apply.
 
  • Like
Reactions: sammyr

·
Registered
Joined
·
211 Posts
I agree with Granny on this one. I would take it back to the builder to straighten out. An oscilloscope in the right hands can probably track down the issue without too much grief. I'm inclined to agree with Granny that it's likely an issue in the power supply. Checking voltages on the preamp tube pins can be a good way to start tracing the issue.
 

·
Premium Member
Joined
·
1,218 Posts
You need the tubes in to check the voltages at the tubes. Without the tubes in, all the small tubes will show too high a voltage due to no current draw therefore no drop across the dropping resistors.
I had this same thing happen to a much smaller amp. Turned out to be a off spec grid leak resistor on one of the coupling caps/tube grid. So you ended up with a small amount of DC voltage on the grid of the tube, and it would go into cut off after a short while.
I would take it back to the builder. He's made a mistake somewhere.
 

·
Premium Member
Joined
·
3,209 Posts
You need the tubes in to check the voltages at the tubes. Without the tubes in, all the small tubes will show too high a voltage due to no current draw therefore no drop across the dropping resistors.
Yes, I addressed this in my post, the voltages would be a bit high, but that's fine; we're not checking for exact voltage here, just that it is at least what we expect (as opposed to below that). It is safer to do this with the tubes out (e.g. no worries about having a load/speaker plugged in), and takes them out of the equation as a variable (internal short etc). As a starting point at least.

I would take it back to the builder. He's made a mistake somewhere.
Or something blew/burnt shortly after he tested it.

Anyway, your other suggestion re the grid leak resistor is pretty good, and now that you mention it, sounds a bit more likely given the described bahaviour. To check that you'd need the tubes in; woulda been step 2 if I were doing it.
 

·
Registered
Joined
·
4,986 Posts
Just to clarify, with the preamp tubes out, the voltage won't be a 'bit' high, they will go all the way up to the B+ rail.
Still will show if there is a gross power supply fault though. And can be done without chassis removal.
 

·
Registered
Joined
·
103 Posts
Discussion Starter #7
i find also when i pwr the amp up , wait untill tubes are nice and warm , turn on the idle switch and i get a crackling sound untill i strum it s few times , then it works normally other than the note haze / fizz i get at the end of the note
 

·
Registered
Joined
·
103 Posts
Discussion Starter #8
64C0F9B2-5480-4F33-98DC-8A4017AD2A17.png
also tried putting at7 in v1, at7 in v2 au7 in PI . still has note distortion ( low e string is where you hear the double sound note , almost sounds like a blown spk at very low volume, so i tried it in clean channels and it does the same thing . checked bias 550v measued plate . not sure on screen voltage (waiting on that from mike) biased it at 25/35/45 problem still there . any other ideas . what should the o/t impedence be .chris merren wound it with 4k impedence , with 560 volt b+ in mind to match his p/t
 

Attachments

·
Registered
Joined
·
103 Posts
Discussion Starter #9
81ABB33E-1FE7-47AA-AB72-EC05D850C6AB.jpeg
A6F0E929-1383-4DBD-B40E-96029D40AA7D.jpeg
B8B938CA-E5BA-4326-888E-9553F81CA83A.jpeg
51FA6FDE-0E1F-4B5A-A50A-C9379769A0B8.jpeg
32E12946-52F6-4F14-A813-9554E57C7BB7.jpeg
also tried putting at7 in v1, at7 in v2 au7 in PI . still has note distortion ( low e string is where you hear the double sound note , almost sounds like a blown spk at very low volume, so i tried it in clean channels and it does the same thing . checked bias 550v measued plate . not sure on screen voltage (waiting on that from mike) biased it at 25/35/45 problem still there . any other ideas . what should the o/t impedence be .chris merren wound it with 4k impedence , with 560 volt b+ in mind to match his p/t[/QUOTE]

]
8767C13B-F1FB-4A2B-8E74-ABD18E619BFA.jpeg
AB839AFB-BB96-4876-A2EF-09220D332CC1.jpeg
 

Attachments

·
Registered
Joined
·
103 Posts
Discussion Starter #10
View attachment 168721 View attachment 168729 View attachment 168737 View attachment 168745 View attachment 168753 also tried putting at7 in v1, at7 in v2 au7 in PI . still has note distortion ( low e string is where you hear the double sound note , almost sounds like a blown spk at very low volume, so i tried it in clean channels and it does the same thing . checked bias 550v measued plate . not sure on screen voltage (waiting on that from mike) biased it at 25/35/45 problem still there . any other ideas . what should the o/t impedence be .chris merren wound it with 4k impedence , with 560 volt b+ in mind to match his p/t
] View attachment 168761 View attachment 168769 [/QUOTE]
 

·
Premium Member
Joined
·
190 Posts
Have you determined there's no fizz being introduced by a speaker ? I've been there once and it was not the amp. ( not to detract from your ongoing investigation, just expanding the scope ) Mine was fizz on the decay of a note, shortened sustain and thinner sound overall. Physical damage in a speaker can contribute these things.

Oops , sorry , after re-reading I see you tested other cabs. So now I'm quite interested in seeing where this goes.
Presuming that sustain with no fizz is evident at stage 1, a scope should be able to isolate where this gets introduced.
 

·
Registered
Joined
·
274 Posts
Steve Moratto knows what he is doing, I would take it directly back to him for service. Problems like this can develop easily with NOS components. I read through the build thread with much interest, it's a cool amp idea that not many have cloned as-of-yet, and Steve does nice work.
 

·
Registered
Joined
·
274 Posts
Sidenote: I have had a similar problem in a stereo build with 4 KT88> I find the filament winding of the 278cx to be marginally rated for a quad, but should be no problem for a single pair. When I added a separate 10A filament transformer, the buzziness went away, and the amp made more clean power especially in the low range
 

·
Registered
Joined
·
4,986 Posts
Sidenote: I have had a similar problem in a stereo build with 4 KT88> I find the filament winding of the 278cx to be marginally rated for a quad,
Not even. :confused: The 278cx is 6A heater winding, KT88's are 1.6A each. Plus your preamp tubes on top of that. I'd guess this amp can run a pair of KT88 or a quad of something else.
But it looks like he only has a pair in there.
Still needs to check the screen voltages, and KT88 idle current.
Agree it should go back to the builder. You mentioned Moratto, but collector said he was waiting to hear from 'Mike'. ?
 

·
Registered
Joined
·
103 Posts
Discussion Starter #19
Yes steve moratto built this mini major for me at a very prime labour cost with a # of months working on it to get it right . He has in the past built 3 other marshalls for me in the past with zero issues . and has worked on a few other heads I own as well . I wanted that marshall major ultra linear tone which differs from a plexi tone , and was ok paying well to get that tone without issues . As for “mike “ Mike is musician ( not a teck guy )who is the owner of a real 200watt major who has played around with modding/ tweaking his 68 marshall major and other marshalls for people for years, and has a few videos up on youtube of his work . “ micheal r/t mods/youtube” . so I took my “one off” build amp to mike for a once over . he got most of the big issues repaired , the note fizz is less now but still there . last step now is to scope it to see if the issue is o/t or p/t .
 

·
Registered
Joined
·
103 Posts
Discussion Starter #20
Not even. :confused: The 278cx is 6A heater winding, KT88's are 1.6A each. Plus your preamp tubes on top of that. I'd guess this amp can run a pair of KT88 or a quad of something else.
But it looks like he only has a pair in there.
Still needs to check the screen voltages, and KT88 idle current.
Agree it should go back to the builder. You mentioned Moratto, but collector said he was waiting to
 
1 - 20 of 30 Posts
Top