| Author | Message | ||
| ammscray (ammscray) Username: ammscray Registered: 09-2012 |
hello there first post here, just want to say I love this forum, been lurking around for awhile... here's my sitch, I have been playing, and servicing tube amps for 30 some-odd years, owned some of the first HD-130's there were, and have played them off and on since then...always fixing fender, marshall, orange, vox etc and I never have had to really do anything to MM's besides tubes, caps and IC's...but I do know how they're supposed to sound when they're working right I have an early HD-130R chassis 2275-130 with the PI tube dated 1974 that I just got...this amp, doesn't have the headroom it should have and is only about 1/3 the volume it should be...right around 2 1/2 to 3 on the preamp volume, it breaks up and distorts...and actually sounds really good when using the master volume or cranked to 10...maybe a bit more preamp gain than it should but really does sound good otherwise its not the EL34's, and they are biased right to factory spec around 47Ma respectively on both sides...its not the PI tube, and voltages there are 275 on pin 1 and 204 on pin 6...and not really sure what those are supposed to be replaced every LM307H and it didn't make any difference except for one being noisy replaced all the filter caps and no difference could anything in the reverb or trem circuit do this? they both seem to work ok by the way it happens on both channels, at the same place on the preamp volume I have checked all resistance values and they seem to be spot on, checked all solder connections and touched up a few, checked for any open components...so I'm at a loss I do have another HD-130, later version with 1458 driver and non-reverb, and I have tested some voltages around the IC's, and they seem to be different but I have no idea what they're supposed to be, and I know they're diff circuits also I don't have a signal tracer or scope, I've gotten by over the years and fix hundreds and hundreds of amps without them! Maybe this is what this amp needs... plate voltages are good in both hi and lo mode, tubes bias up good, one thing I'm concerned about, is that even tho this is not a surefire way to check the OT, one side measures 100 ohms and the other side about 65, mis-matched more than I'd like, but I've seen plenty of OT's mismatched more than that and not even be a problem...could it be a bad OT? my other HD130 reads 100 and 115 ohms on either side...I could switch it out it seems to be the same one but for some reason I just feel its something stupid that I've overlooked thats loading it down anybody who has any ideas please respond its very appreciated! thanks! AM | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
Hi AM, I'm sorta thinking you have a bad OT. A scope would really help you here. BTW, one side will always be a bit higher in the DC resistance reading of the OT. The part of the OT winding that is on the outside will have a few inches more wire to get the same number of turns. Have you tried injecting signal directly to the PI stage to rule out any issue on the preamp board? Cheers, Lars Verholt | ||
| Angelo Marruzzi (ammscray) Username: ammscray Registered: 09-2012 |
hey Lars thanks for chiming in...yes I know, 65 is a bit low on one side for this amp, but I have seen worse in some other fenders or marshalls and they didn't exhibit these symptoms, I'm not opposed to just grabbing one of the mojo OT's I hear they are really good...I just want to exhaust all possibilities first... no, I haven't done what you suggested, I don't have the equipment, like I said, all these years I've worked on hundreds and hundreds of amps and never used a scope or signal generator...guess I've been lucky one thing I will mention, when I was taking voltage readings in the first preamp stage, vs the one in my working 1458 HD-130, all the readings in that one were like + 1.2 volts and +2.4 volts etc, but the first stage in the bad amp was reading mostly all negative voltages like -10 volts here and there...I wish I knew more about what those readings were supposed to be...I swapped out coupling caps and resistors there in that section to no avail...there was also some continuity readings between resistors and coupling caps there that were different than the 1458 amp | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Personally, Im thinking either overactive preamp or MAYBE, I remember some guys talking about the channel controls having the wrong pots. Don't remember. Maybe it was one of the guys here. Anyway, unless you can jamb the from end and rear end separately with a signal and see which one is doing it, you're just throwing parts at it. Have you tried just plugging into the low signal side on the amp and see of it cleans up? Maybe the gain structure on the front end IC's is out of wack. Like I said, it's guessing unless you can look or hear. Sometimes I like to use an old signal tracer instead of a scope so that I can HEAR when stuff starts going wonky. you be surprised. If you have an amp to use as a sound source, you COULD build one of these as a poor mans signal tracer. http://www.tedsforums.com/forum/index.ph p?topic=8797.0 It might let you track the signal through but you still need a stable sound source. There are a lot of signal generator programs that will let you generate a 1K or so sound source out of your computer. Mike. | ||
| Angelo Marruzzi (ammscray) Username: ammscray Registered: 09-2012 |
thanks Mike...I thought of the pots and I need to check the values closely today...as far as an overactive preamp, if this were the case wouldn't the overall volume of the amp be normal? Its not, its lower than it should be by the way, if I plug into the lower gain jack, it cleans up a bit, but not much I will look into building that signal listener, I've seen that page before, thanks I could also just swap the OT from the other HD-130 but that would be a PITA and I'm trying to keep that as a last resort | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I'm also kind of wondering if the SS driver is hot on one side. Really, it would probably have to be looked at with both sides being scoped. One side of the SS driver could be driving the crap out of it. It's STILL kind of a crap shoot unless you can SEE it or hear it. Incidentally, I have a 210-65 that's a little hot on the same way. I usually just use it the way it is and have never really decided to go in and actually FIX it. It just works fine for me! breaks up after 3 on the channel and doesn't really get any louder after 4, just dirtier. The way I use it, it's OK and there's no reason to change it! I always thought the IC gain structure was wrong but really never took the time to change it. Mike. | ||
| Angelo Marruzzi (ammscray) Username: ammscray Registered: 09-2012 |
the problem amp is actually the tube PI HD-130...voltages on both sides seem to be ok...it behaves just as you mentioned your 210-65 does, but its unuseable b/c its just not loud enough either...I want this amp to be really clean and loud like its supposed to be, b/c my other SS driver HD-130 I modded the crap out of and that one already does the aerosmith thing really well maybe I'll build that signal listener...thanks |
Tue, 03/25/2014 - 09:53
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