8 watt, 11MSB8 tube amp

[fragman56]

Hello,

Just assembled my first diy amp kit from here http://www.s5electronics.com/tube2.html and need help troubleshooting. It did not smoke, blow up when plugged in. Tubes light up and I can faintly hear music coming out of the tubes! No hum or sound from speakers.

What to do next? Start at output transformers and check voltages? Go backwards from there? Start at input signals?

I did check each resistor before installing. All connections look good but what do I know. Would rather not send back to supplier until I am certain that some component is bad.

Your tips are appreciated!

Rick

[ultranalog]

When you hear music from the tubes themselves, it's likely they're not seeing any load. Check the connections to your output transformer, it sounds like you've wired it push-push or pull-pull. Result on secondary: nil.

[fragman56]

Thank you for your response.

I believe the design is supposed to be push pull.

I have two output transformers both wired exactly alike at the moment.

Should I de-solder and switch the wiring around on both or leave one wired as is? Seems that I have wired them both incorrectly?

I assume any switching of wires to the output transformers is done safely since, assuming the wiring is incorrect now, nothing has burned up.

Your thoughts?

Thanks again from an aspiring diyer!

Rick

[fragman56]

How can you tell what the correct wiring is? I do have a schematic and instructions but shouldn't I be able to check the pinouts from the tubes as well as the leads to the transformers to determine the proper wiring config for push-pull?

Perhaps you have a website reference that I can go to?

Thanks for reading!

Rick

[ultranalog]

Hi Rick,

I believe the design is supposed to be push pull.

Maybe if you post the original schematic we can all take a look.

Should I de-solder and switch the wiring around on both or leave one wired as is? Seems that I have wired them both incorrectly?

Just try one. It'll only take 2 minutes and if it doesn't help, it won't hurt.

How can you tell what the correct wiring is? I do have a schematic and instructions but shouldn't I be able to check the pinouts from the tubes as well as the leads to the transformers to determine the proper wiring config for push-pull?

Since a picture says more than a thousand words, and I don't feel like typing a thousand words:


The dots in the schematic represent the winding direction. Change one of the primary windings (from the B+ supply to one of the pentodes) so that the phase of the induced signal into the core changes w.r.t. the other phase. If the circuit was in the 'bad' state, it'll now be good. If it doesn't help, change it back to the original state.

[fragman56]

Hey thanks for the advice, Ultranalog!

I appreciate you hanging with me.

So, if I understand you correctly, I will switch the B+ (middle) lead on the primary with one (either) of the other leads on the same transformer. I will do this on only one of the output transformers--I have two (one for each channel).

I will scan the schematic and post tomorrow at work. Also, I will try to get digital photo of the amp for your review.

Thanks again,
Rick

[fragman56]

Just changed the B+ with one of the output transformer leads and got no sound other than what is coming from the tubes.

(I did not switch both output transformers leads--just one pair of leads on one transformer.)

Nothing smoked. Output transformers do not heat up if that gives a clue. Tubes and power transformer do get hot, of course.

By the way, the schematic that you provided looks almost identical in config--I will attempt to post my schematic.

Any further ideas? Thanks for your attention.

Aspiring diyer,
rick

[fragman56]

Just a quick update

Signal traced the amp throughout and found the speaker posts (all) had been isolated from PCB pad by mistake with synthetic washers.

The amp sounds good and with my 106 db Altecs 605As is plenty loud! Can be too loud with this 8 watt amp. I will say, however, there is a slight hum to this amp.

Will still attempt to post schematic for those interested.

Aspiring diyer,
Rick

[dice45]

ultranalog,
welcome and .. good job helping/explaining!

Guess we met in Aarhus Triod Audition Festival 2000, can that be?

[ultranalog]

Signal traced the amp throughout and found the speaker posts (all) had been isolated from PCB pad by mistake with synthetic washers.

OK! So it was without load - just in a different way than I suspected. Good for you!

I will say, however, there is a slight hum to this amp.

My first tube amp was something like yours (4*6BM8) and also hummed. Raising the values of the caps, notably those feeding the input stage, helped considerably.

Guess we met in Aarhus Triode Audition Festival 2000, can that be?

I was there, yes. I've decimated my online life during the last year so I've forgotten many names and faces. Yours does ring a bell though.