Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: sstudley on September 06, 2019, 09:50:44 am
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Hello,
Now that the kit is complete, I have to admit, I don't think it sounds very good.
I also discovered last evening that playing at volume and strumming the low F major chord causes the amp to suddenly buzz loudly, much like it did when the bias wasn't right. The hum/buzz will go away when the Standby switch is toggled, holding it open for a few seconds.
I have some rather crappy 6l6's in it right now. I was holding off on installing a very nice matched pair (part of a matched quad!) 6l6's. The matched quad set cost me $200.00+, so I want to be absolutely sure the amp is stable before breaking them out.
Would it sound noticeably better with better tubes? Should I install them and find out?
I have attached some pictures of the final build. Jeff McMurray at Band Mates did my cab.
Kind regards,
Steve
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if your bias is set correctly then you shouldnt worry about damaging the tubes. Id say give it a try. Components can be weird in a kit from China but if they all measured out then you may be helped.
There are ways to change the coupling caps to reduce how much bass comes through the circuit that will reduce the farting out. also, speaker choice could be a consideration.. if the speaker cant handle the bass frequencies youre giving it it will also fart out.
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Thank you for the advice.
Is there a benchmark or smoke test known to amp builders to somehow quantify where the amp is, maybe compared to known response from a real 59 Bassman? I'm wanting something objective, quantifiable. Not too high-tech. I have a typical tech bench, nothing fancy. Then I might be able to plot changes...
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one thing I ALWAYS do, repair or build on push-pull amps (PP) is install 1 ohm R's between cathode and ground on the PA tubes. This gives you a quick, safe way to measure current through each tube.
look at the schematic, I try and do this on all builds/repairs. DC and AC power measurements along with AC signal measurements. That way If it comes back, I have a point of reference. I pass on a copy if it's a repair so the next tech has a point of reference.
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"strumming the low F major chord causes the amp to suddenly buzz loudly" i bet this is a bad connection in the signal path.
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Thank you Shooter, very nice. That is the kind of worksheet I was thinking of, however I was hoping there might be some known target data to shoot for. I can see how knowing where one is at and what changes have made what impact along the way would be very useful.
I think I need to pull the amp from the cab and try to get this buzz/hum problem figured out first. Then I will benchmark it and after that make some changes that should impact tone.
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Not saying this is the source of your problem, but it needs to be eliminated from the candidates. Swap the OT primary plate leads. Any better? If not, swap 'em back.
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You said that it will buzz out when you are playing at volume? Are we talking dimed? Loud enough to rattle the cab/chassis/tubes? Tubes are an easy swap - if you have them. Just for giggles you may want to re-tension your sockets. If I remember correctly in your other thread, you mentioned they got pretty hot? Another easy check. I always say do the easy stuff first to eliminate the simple before going down the rabbit hole of components and redesign.
You also mentioned benchmark comparisons to the original. Shooter has some great technical suggestions. The objective part might be a little more difficult if you have never played or heard the original blasting in your face. Do you have any vintage dealers near you who might allow you to play around a bit? Take that with a grain too because a lot of these old amps for sale do NOT sound like they should.
Jim
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I won't be at my shop today, so I won't be able to make any changes and post results until late tomorrow. I'm gonna start with sluckey's suggestion of switching the OT plate leads first. Simple change and should yield tangible results. Thank you all. I just wanted to post a quick note so you-all would know I wasn't ignoring your advice.
BTW: my "at volume" comment meant a little less than half power. Definitely not cranked fully.
Steve
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might be some known target data to shoot for
some original schematics have amp data but power might have been 115 vs 120 today, so it's relative. Tube data sheets are great for typical/max, again things change.
If it sounds good, meets ballpark values based on other builds, original data, that's about as close as I need for an Analog thing, my math teacher might not agree but hey! :laugh:
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That's 2 for Sluckey! That strange hum/buz hitting low notes and chords seems to be gone! I have to say as well, there seems to be an overall improvement. Much better low response, tighter. Seems to me this needs reverb, just to make it complete. I think I will add spring reverb sooner rather than later. Sluckey, if you have the time and feel like it, can you please explain to me why switching the plate leads would have, in my opinion, a rather significant impact for the better? Thanks again to you and everyone in the forum.
:icon_biggrin:
Steve
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Check ALL the gnd connections and make sure thereis no gnd loop. ( example: both ends of a signal cabke shield connected to different places, only one end has to be gnded).
Before plugging in your new tubes, measure te neg bias vpltage with all power tubes removed.
If minus 60 or so, there is no danger. More poisitive could be a prob.