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View Full Version : Confusing problem here...

Arthur Dent
03-31-2008, 10:51 PM
I built a modified version of a 5E1, made even simpler than the originals. No controls, just a power switch, no negative feedback. The power transformer is just an isolation transformer, with the filament tranny separate. It's half-wave, solid-state rectified, about a 160v B+. Filter caps are both 40uf. Other than that I think it's standard 5E1.

Now, the interesting part: All the voltages seem to be about right, but there's no output. Every now and then, a buzz very gradually comes and goes. If I touch the jack, it *sometimes* buzzes loudly like it would if you did that on a working amp. If I touch anything attached to the grid of the second gain stage, it buzzes quieter, as one would expect from a working amp, and touching the power amp grid produces a very quiet buzz. However, no instrument input at any stage causes anything at all to happen.

I've searched for a long time and cannot find anything unusual except the strange selective output. Any ideas?
Arthur Dent
04-01-2008, 01:09 AM
Oh, I forgot to mention that this problem occurred on the first power-up, and I couldn't figure it out. Since it's such a simple circuit, I took the whole thing apart and rebuilt it. The only parts that were common to both attempts were the filter caps, OT, a few resistors, and maybe a signal cap or two. The PT, rectifier, tubes, sockets, etc. were replaced.

I started the rebuilt one up, and it did exactly the same thing.
Enzo
04-01-2008, 01:54 AM
Post your schematic. I'd be thinking wiring error first.
Don Moose
04-01-2008, 02:02 AM
A B+ of 160V sounds awfully low.

Your no-knobs approach - you did put a 1M resistor in place of the volume pot, and attached the grid of the second triode to the 'top' of that resistor, right?

-- like enzo said, your schematic would really help.
loudthud
04-01-2008, 03:26 AM
What type input jack did you use? Did you connect to the correct terminals? The widely available import jacks have terminals in a different order than the old Switchcraft jacks.
Regis
04-01-2008, 01:32 PM
What type input jack did you use? Did you connect to the correct terminals? The widely available import jacks have terminals in a different order than the old Switchcraft jacks.

Yes, that happened to me recently. Brand X shorting jacks have the hot and tab reversed from the Switchcraft 12a.
Arthur Dent
04-02-2008, 02:07 PM
A B+ of 160V sounds awfully low.

Your no-knobs approach - you did put a 1M resistor in place of the volume pot, and attached the grid of the second triode to the 'top' of that resistor, right?

-- like enzo said, your schematic would really help.

True, 160v is low, but I've got two other amps that run on 170, and one of those has a 6v6 and sounds great.

I'm pretty sure I put in that resistor. It's possible I could have missed it, I'll check.

I haven't got time to post the schematic now, I'll try to put it up later today.

The jack is the same brand I always use.