Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bigsbybender on August 11, 2014, 10:06:19 pm

Title: 5E3 with Weak distorted tone.
Post by: bigsbybender on August 11, 2014, 10:06:19 pm: Hi guys,

It's been a long time, but I'm back and in need of some guidance.

On my bench I have a Hoffman board 5E3 with choke. I built it back in 2006 and sold it to a guy who has gigged with it non-stop since then. He recently played a show and a power tube went south about 2 minutes into the first song. He was using old RCAs so I told him to keep a pair of backup 6V6s at all times, which he did. When he put the new tubes in, the amp was weak and distorted. I worked with him on the phone and told him to try another set and swap out the rectifier too while he was at it. None of that worked so I took it in for repair....and I can't find anything wrong with it, except it sounds weak and badly distorted.

So far what I've done is:

1. Swapped all tubes with known good ones
2. Checked all resistors with DMM for drift or open condition
3. Reflowed all solder joints
4. Replaced cathode bypass capacitors (usually this has solved this issue for me in the past if not tube related)
5. Tensioned tube sockets
6. Checked plate and screen voltages on power tubes, and plate on preamps. (checks within spec.)

Any other ideas?

Any help is greatly appreciated!

Thanks!
Title: Re: 5E3 with Weak distorted tone.
Post by: SILVERGUN on August 11, 2014, 10:32:02 pm: Check OT?
Do you have a scope?.... you can look at the output signal coming off of the plates as compared to the secondary of the OT

I've never tried this myself, but it sounds like it makes sense:
http://www.premierguitar.com/articles/the-super-secret-transformer-tester-1 (http://www.premierguitar.com/articles/the-super-secret-transformer-tester-1)

Good luck!
Title: Re: 5E3 with Weak distorted tone.
Post by: bigsbybender on August 11, 2014, 10:34:10 pm: I don't have a scope. So far I've gotten along without one, but in this case I wish that I did have one. I did build a "listening amp" but hate to connect it to really high voltages.
Title: Re: 5E3 with Weak distorted tone.
Post by: bigsbybender on August 11, 2014, 11:57:06 pm: The super secret test solved it... Short in one of the OT windings. Luckily I had an OT for an upcoming build and replaced it and we're good to go I think!
I was really hoping, for this guy's sake, that it would be a cheap fix. Of course it had to be the most expensive part.

Thanks for the tip!

J.
Title: Re: 5E3 with Weak distorted tone.
Post by: SILVERGUN on August 12, 2014, 07:22:21 am: A scope takes all of the fun out of troubleshooting :icon_biggrin:

Glad I was able to help.

Guess it's not super-secret anymore.
Title: Re: 5E3 with Weak distorted tone.
Post by: SILVERGUN on August 12, 2014, 09:36:28 am: I took a minute and made a GIF of the page so we can all store a copy....
Never know when you'll hit that link and it'll be dead.
Title: Re: 5E3 with Weak distorted tone.
Post by: Platefire on August 12, 2014, 09:45:46 am: Good Stuff! The only time I ever had OT's fail, it was plate voltage was dead on one of push pull sides which showed up on voltage test--and of course this plate voltage was in spec. Platefire
Title: Re: 5E3 with Weak distorted tone.
Post by: SILVERGUN on August 12, 2014, 10:12:56 am: It would be a good idea to make sure that the player didn't cause the failure with an impedance mismatch.

The only thing more painful than buying a replacement OT is buying 2.
Title: Re: 5E3 with Weak distorted tone.
Post by: bigsbybender on August 12, 2014, 01:31:45 pm: I think the short was related to the failure of the tube. I'm fairly certain that he only uses the internal speaker but I'll reiterate that to him to keep it 8ohm.

I've never run into a failed OT in an amp like this. Usually it's in Marshall styles with the impedance selector messed up. That's why I didn't check the OT at first... They just don't often fail in my experience.
Title: Re: 5E3 with Weak distorted tone.
Post by: punkykatt on August 14, 2014, 06:56:28 pm: Stopped by the local electronic store to pickup a NE-2 lamp, all they had was a NE-2A with a 22k resistor already attached on one lead. Tried testing several each known good OPT`s and PT`s. Following the instructions for the Super-Secret test using a 6v lantern battery, I found that the bulb had to be attached across the primary leads of the OPT`s and the battery to any of the secondaries to flash. On the PTs, the bulb would flash either way between the primaries and HT secondaries, but on the 6.3v and 5v secondaries the bulb had to be attached across the primaries in order to flash. In the instructions posted above it says to connect the bulb across any winding? Did any one else try this test yet besides bigsbybender? Do i have the wrong bulb?
Thanks Punky
Title: Re: 5E3 with Weak distorted tone.
Post by: sluckey on August 14, 2014, 08:06:02 pm: Quote
Do i have the wrong bulb?
yes. Get rid of the resistor or get a NE-2 bulb.
Title: Re: 5E3 with Weak distorted tone.
Post by: punkykatt on August 14, 2014, 08:57:57 pm: Thanks sluckey, so all i need to do is cut off the 22k resistor and attach a 100k resistor like in the diagram in the post above?
Title: Re: 5E3 with Weak distorted tone.
Post by: PRR on August 14, 2014, 10:01:07 pm: I have doubts about a 6V winding kicking-up enough to blink a 90V Neon.

I think if you get a kick, battry to LV winding and Neon to HV winding, ANY of the HV windings (typically 120V or 330V), then the PT is not internally shorted. If it were, ALL the windings would be low/no kick.