Low power problem..100w

[6ringing]

Hi guys,
I'm looking for ideas on where to start looking for this problem. I've just finished a 100w superbass build. It sounds great, but I thought it waan't as loud as I was expecting. Checked the power and sure enough it's only putting out about 50w clean and 75 watts cranked up to 10. All the voltages are good (yes I have all 4 power tubes in!) and like I said it does sound good...but aren't these baby's supposed to do 100w clean?

Any ideas of what I should be looking for?

Thanks a million!

[ThunderOne]

That's exactly the problem I'm facing with my just completed Superbass. All readings check out but the amp pumps out not even half of it's claimed output. Heck, I think my JTM45 cranks out more! I'm using "test tubes" for it's break-in but I must say that it sounds absolutely great even with it's lower output. In any case, there must be a component to tweek and/or replace to make it normal again! I'll research and see what I can find!

[neikeel]

Show a pic of your main board - have you put a 470k (yellow/violet/yellow) where a 470R (yellow/violet/brown) should live. Typical symptoms

[ThunderOne]

Yellow-Violet-Brown here!

[ThunderOne]

I found this in the 50 watters section:

"I just finished a 50 watter following the metro build guide. This amp is a mutt with a weber power transfomer and chassis, mercury magnets output transfomer, and a vox style generic circuit board. I have the el34s running at 454v with 30ma at idle. Everything works but the amp is no louder than a champ. My jtm45 is way louder than this would ever get. I noticed that it doesnt seem to have that thresh hold break over like my jtm45 where the volume comes up significantly after ~2. The volume goes up more in a linear fashion on this unit. Im not sure where to start since the amp works. Any help would be appreciated.

_________________
Finished amps --> Metro JTM45, Champ, Deluxe, Bandmaster, 1974x

Still Debugging --> Metro 50, Trainwreck Express

Last edited by guilds100 on Sat Jan 23, 2010 11:12 pm, edited 1 time in total.




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Flames1950 Post subject: Re: 50 watter works but isnt very loudPosted: Sat Jan 09, 2010 4:31 pm

Moderator


Joined: Sun Feb 08, 2004 2:04 am
Posts: 9073
Location: Waukee, Iowa Sure you didn't accidentally put a 470K in the phase inverter instead of a 470 ohm? (Between the two 1meg resistors?)

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guilds100 Post subject: Re: 50 watter works but isnt very loudPosted: Mon Jan 11, 2010 2:17 pm

Marshall Maven

Joined: Sun May 24, 2009 10:52 pm
Posts: 26 Flames1950 wrote:
Sure you didn't accidentally put a 470K in the phase inverter instead of a 470 ohm? (Between the two 1meg resistors?)



Thanks for the reply. I double checked the value and it appears correct. I think I'm going to reheat a few connections as I'm starting to think I have a weak one somewhere.

_________________
Finished amps --> Metro JTM45, Champ, Deluxe, Bandmaster, 1974x

Still Debugging --> Metro 50, Trainwreck Express




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guilds100 Post subject: Re: 50 watter works but isnt very loudPosted: Sat Jan 23, 2010 12:12 pm

Marshall Maven

Joined: Sun May 24, 2009 10:52 pm
Posts: 26 The problem is solved and I wanted to post this in case someone else runs into the same problem. I resoilder every connection with no progress. I got lucky and heard a little static out of the brite channel pot when turning it. These were the weber pots came with the chassis. I replaced both volumes with CGEs and the amp came right to life. There is a little extra hiss on the brite side I need to calm down, but other than that the amp works great. So if your getting low output volume but the amp seems to work otherwise, check your volume pots.

_________________
Finished amps --> Metro JTM45, Champ, Deluxe, Bandmaster, 1974x

Still Debugging --> Metro 50, Trainwreck Express

Last edited by guilds100 on Tue Feb 09, 2010 5:33 pm, edited 1 time in total.




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PCollen Post subject: Re: 50 watter works but isnt very loudPosted: Sun Jan 24, 2010 10:52 am

George's Right Hand Man

Joined: Wed Jul 12, 2006 1:50 pm
Posts: 1014 guilds100 wrote:
The problem is solved and I wanted to post this in case someone else runs into the same problem. I resoilder every connection with no progress. I got lucky and heard a little static out of the brite channel pot when turning it. These were the weber pots came with the chassis. This was my first amp that I built but it never worked right so I gutted it and started from scratch. I replaced both volumes with CGEs and the amp came right to life. There is a little extra hiss on the brite side I need to calm down, but other than that the amp works great. So if your getting low output volume but the amp seems to work otherwise, check your volume pots.


I hate to read a story without a conclusive ending. Did you go back and determine EXACTLY what was the matter with that 'bad' pot ? Was the resistance across the pots outer lugs incorrect (not 1M) ? Was the wiper contact not picking off the increasing/decreasing resistance as it was being rotated ?





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guilds100 Post subject: Re: 50 watter works but isnt very loudPosted: Sun Jan 24, 2010 11:59 am

Marshall Maven

Joined: Sun May 24, 2009 10:52 pm
Posts: 26 PCollen wrote:
I hate to read a story without a conclusive ending. Did you go back and determine EXACTLY what was the matter with that 'bad' pot ? Was the resistance across the pots outer lugs incorrect (not 1M) ? Was the wiper contact not picking off the increasing/decreasing resistance as it was being rotated ?



I was so happy it worked I almost forgot about the pot in question. Upon futher inspection the pot yielded an intermitemt doubling of resitance from outer lug to outer lug while rotating the shaft. Also there is a slight wiggle to the shaft. The wiggling shaft makes me think I had a bad pot from the get go."

I remember pushing my CTS volume pot shafts a bit hard upon installing the knobs and the back of the volume one pot actually became loose. I recrimped it and it seemed ok but reading this now, makes me think that it could have been damaged. I'll look out for more possibilities and keep this one in mind.

[neikeel]

ThunderOne I take it that was not your problem?

[ThunderOne]

ThunderOne I take it that was not your problem?

Hello Neil! The 470ohm resistor was not my problem indeed. Now, unless I can find something more obvious to try out, I'm in the process of starting to think about changing a pot. You can see I'm quite slow about these things. I also found out that the pot I busted and recrimped was the volume 2 not volume 1. I wonder if this alone could put my Superbass' wattage in the same league as a Princeton's. I left the amp cooking for a few hours on standby to wear in the new power tubes (and just for the joy of looking at the pilot lamp ) and noticed that the PT got fairly hot, not unbearably so but more than just warm. Is this normal for 100 watters?

[neikeel]

Just saw the broken bias feed wire, was that it or is there more?

[ThunderOne]

Well, yes, I also have the same low output problem as 6ringing. I'm checking out possible causes and I ran into this thread. Odd that it should happen at about the same time and for the same circuit... I didn't pay too much attention when my amp finally came alive as I didn't crank it because it was plugged into 1 x Vintage 30 speaker.

[6ringing]

I've got my 470 ohm in the right spot. I actually have an o'scope and signal generator...just not sure how to use them to trouble shoot this. I guess I could inject my signal at the input and scope it through to the output and see if there's a gain drop.

As this superbass is actually going to be used for bass...I need all the power I can get!

I'm glad there's someone else in the same boat. If I figure it out, I'll let ya'll know.

[ThunderOne]

There is a solution, that's certain! I can already tell you that my CTS volume pots weren't defective after all. I changed both for Alphas and I still can't get the output to its normal level. I'll keep looking!
Cheers!

[6ringing]

Well, I checked mine on the scope input to output and the signal only gets bigger and bigger as it goes through. So mines not dumping any anywhere.

Arg. I hate trouble shooting, i'm so awful at it. Anyone know of any good books for fixing amps? I've been building for 10 years, but am an idiot with fixing.
Drives me nuts!

[neikeel]

I did have a 71 100watter on the bench - was mainly used for recording- never had much oomph, then I noticed someone had put a 50k presence pot in.

Swapped it out and the beast was released!

I run my presence pots separately grounded btw (not off pot bus rail) with the dedicated earth wire to the PI cap ground lug

[6ringing]

Hmm, that's interesting. I'm using a 10k presence...but doubt that could be it.

I've consulted with a couple of guys who said that a bad output transformer can do this, I think that'll be my next thing to check out.

[6ringing]

Well, I found my problem. I bought a bad output transformer! I got it cheap (like $50) on ebay...I guess you get what you pay for. I won't mention who made it.
Simple test, I didn't even have to unsolder anything. I checked the resistance from CT to leads and got a very low resistance. Then checked all leads to ground...almost no resistance! Oops.

Good luck with your amp!