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Hello fellow amp-builders. I'm in need of some advice and would appreciate any bones you could toss my way.

I wired up my sIII/v6 amplifier, and although I got it to make sound, all is not well in Valveland.

I checked tube voltages in a couple of different scenarios:

1. with all tubes installed

Most of the voltages are significantly off, to different degrees. Some are way higher than expected, while many are lower:


2. with 6v6s removed (hoping maybe a bad tube was at fault) - same story, slightly different results:



Playing through the amp, I get sound, but the volume is extremely attenuated (gain/volume need to be pegged to hear clearly, and the quality of the overdrive is pretty ragged).

Also, the 250 ohm 5W power resistor that connects the 6V6 anode to ground gets insanely hot after a couple of minutes, which I'm assuming is a bad sign.

I've got the amp connected to a 'known good' 8 ohm Celestion speaker in a cabinet, so it's running under load.

Based on the troubleshooting hints, something is drawing way too much current, leading to the hot resistor and the low voltages. What I don't understand is how some of the voltages are ridiculously high.

I keep thinking that something has shorted, but none of the caps or resistors give bad readings. So I'm left wondering about the output transformer.

Is there a way to easily test the output transformer to see whether it's bad? Can I do a simple resistance test of each of the coils, or do I need to actually pass current through the primary and measure the voltage on the secondaries?

Any hints or suggestions as to where to start? I'm concerned that I'm frying other components every time I power this thing up...

On a side note, I noticed one discrepancy on the schematic - a 1.5k resistor and the 250 ohm 5W power resistor are both labelled 'R33' (these appear one above the other between the 6v6 pair and the EL84 pair).

Many thanks in advance.

J

Here is some more information that might prove useful:

Voltage Measurements on Power Transformer

• Mains voltage measures 120.9 VAC
• B+ with no tubes: 424 VDC
• B+ all tubes installed: 315.3 VDC

These numbers appear a little low (expect 440 VDC w/o tubes and 350 VDC with)


Resistance Measurements On Output Transformer

Primary:

• Brown - Blue: 624 ohms
• Brown - Red: 281.4 ohms
• Blue - Red: 343.4 ohms

I'm surprised that the tap appears to be asymmetrical; and gives me further reason to think that my output transformer is partially shorted.

Secondaries:

• yellow - black: 1.2 ohms
• green - black: 1.3 ohms
• orange - black: 1.6 ohms

Do these readings look correct?

Please, any ideas or hints would be greatly appreciated.

JD

Does anyone know how to test an output transformer to see whether or not it's blown?

Thanks,

JD

The number with tubes installed looks a little low, suggesting the power tubes may be drawing more current than they should. For the figure without the tubes, I wonder if both sets of power tubes were installed with their heaters running. That might take a few more volts down.



Those numbers look quite normal. The two windings of an OT will always be somewhat asymmetrical. As more an more turns are wound onto the core each turn gets progressively longer and therefore the resistance of the outermost side of the primary will have a higher resistance than the more inner one.


Yes, those look normal. The secondary side has very low DC resistance values.

_________________
Great tones come in small glass jars!

Zaphod -

Thanks so much for your reply.

What a relief to know that the OT measurements look OK!

I will go back and begin re-checking all components to see if something has shorted, and re-examine all my wiring to make sure it matches the diagram and schematic.

Willl post here what I find.

Yours in filimentality,

jeddak

OK, problems are resolved.

At Stephen's suggestion, I tried a second pair of EL84's, with the hope that it was a bad tube: no dice.

Then (also at Stephen's suggestion), I tried eliminating the 6v6s, detaching the power feed and removing the tubes - same problem.

At this point, the R33 power resistor was starting to noticeably smell whenever I left the amp on for more than a minute or two. I also noticed C20, the large electrolytic next to it. was pretty hot to the touch after shutting off the amp.

Definitely a current-draw problem. I went back to the schematic and looked all around the EL84s to see where all the extra current was coming from, and it finally dawned on me that I hadn't connected the junction of R18 and R19 to ground(!)

No sooner had I done so than all the problems went away:

* the hum
* the anemic output volume
* the insanely hot power resistors (R33 and R20)
* the related 'hot' smell
* out-of-whack voltages

My amp now sounds like it should - absolutely beautiful, with crisp, clear, sparkling Marshall-esque grit.

Many thanks to Stephen and Zaphod for your help and ideas. I was definitely out of my depth.

Will post sound samples one of these days - and once I build a cab, I'll post pictures of that.

Cheers,

Jonathan

Good news. Grounding is the #1 issue I have found when peoples amps don't work correctly. Either missed joint, missed connection or bad solder.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Another update, and a small object lesson for other newbies that I wanted to share.

After I got my amp working (two years ago, now!), it eventually dawned on me that the volume on my sIII/V6 was not what I expected.

For months and months, this lack of power puzzled me, but since the amp *sounded* so good, I didn't worry so much about the output level. It was never really a problem, except sometimes when rehearsing with my band it would annoy me that I had to dime the controls to be able to be heard above our drummer (who is not an especially loud player).

In the back of my mind, though, was always this nagging question: why is my 18 watt amp quieter than my little 5 watt Epiphone????....

A couple of weeks ago, after both of the channels crapped out, I was forced to open the chassis and re-evaluate all the connections. That's when I found not one, but two cold solder joints! (Of course, the irony is that the real issue why the amp wasn't making a sound is because I had inadvertently loosened the first preamp tube.....duh.....)

After resoldering these connections, my beloved Trinity is now putting out all 18 watts. And boy, is it powerful - punch, grind, sparkle....it's earplug time again!!

So just a lesson for the other newbies on the forums - be very, very careful when soldering. Check and re-check everything. Then go do something else for a couple of days, and come back and re-check everything again. Just because a wire doesn't move does not mean that the joint is a sound electrical connection!

Building this amp has been a great learning experience.

Thanks!

JD