Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on March 26, 2013, 04:55:47 pm

Title: powerless sound caused by OT?
Post by: Leevi on March 26, 2013, 04:55:47 pm

I have built an AC30 based amp and after one hour playing the sound became
very powerless and crackling. I limited the problem to the power amp.

I have checked/changed PI tube, power tubes, tried even with two power tubes.
I also changed some capacitors in the power amp.

Now I'm thinking if the OT is broken. I measured it and both sides on the primary
show ~70 Ohm which should to be OK but could it still be root cause for this?

The OT is Hammond 1750V.

/Leevi

Title: Re: powerless sound caused by OT?
Post by: firemedic on March 26, 2013, 05:49:44 pm

I had a 1965 Ampeg Reverberocket 2 that sounded anemic, because of the tiny stock OT. I replaced it with the OT for a Deluxe Reverb and it opened right up.

But my problem undoubtedly developed over 45 years of use & abuse, your problem sounds like a sudden onset. Your voltages are all good?

Title: Re: powerless sound caused by OT?
Post by: PRR on March 26, 2013, 08:29:41 pm

Check speaker connector.

Title: Re: powerless sound caused by OT?
Post by: Leevi on March 27, 2013, 12:43:07 am

Quote

Your voltages are all good?

All the voltages are good

Quote

Check speaker connector.

Yes, I even tried with 16 Ohm output and with different cables and speakers.

I think I worked a short while after I observed the problem first time but
now the problem is all the time on which is of course good from troubleshooting point of view.

I'm using PPIMV master (dual 500K pot + 2 x 470K resistors) in the poweramp.
I use 1.5K grid resitors on the power tubes.

I have built tens of this kind of amps before and never met this kind of problem.

What I thought to do next:
- secure that the problem is in the power amp
- try with totally new set of power tubes
- try with new PI tube
- replace the master with fixed resistors
- try with another OT

/Leevi

Title: Re: powerless sound caused by OT?
Post by: tubeswell on March 27, 2013, 03:22:02 am

You can't really tell if an OT is kaput unless you test the Pr:Sec impedance ratio by hooking up VAC to one side and measuring the VAC on the other. Unhook the plate and B+ ends of the primary and unhook the speaker from the secondary, and hook up your PT's 5V or 6.3V winding to the OT secondary, and measure the VAC across the primary. Measure the VAC on each side of the primary (to the CT).

Title: Re: powerless sound caused by OT?
Post by: Leevi on March 27, 2013, 05:10:07 am

Quote

You can't really tell if an OT is kaput unless you test the Pr:Sec impedance ratio by hooking up VAC

I own a signal generator and thought to use that
/Leevi

Title: Re: powerless sound caused by OT? SOLVED!
Post by: Leevi on March 27, 2013, 02:54:24 pm

Quote

Check speaker connector.

PRR, You were right.

I tried with two different speaker cable and both of them were faulty.
The first one which was broken during I played with the amp caused the problem first time.
The second cable that I had continuously used was unfortunately failed during my troubleshooting.
I finally noticed that when the cable was totally broken and the sound disappeared.

OT is fine, as well all the tubes, no bad components etc.

Thanks for your responses
/Leevi

Title: Re: powerless sound caused by OT?
Post by: firemedic on March 27, 2013, 07:30:44 pm

Wow, that could have turned into a bad OT pretty quick!