my firstbuilt hybrid-a style - got some problems

[alex]

first of all hello to everybody!
i know this is my first post here, but i learned a lot by reading all the different topics!

my amp (hybrid-a style) is almost finished.
first powerup with bulp limiter and no tubes installed seemed to be ok!
but at the second powerup with the installed tubes and the bulp limiter plus the connected speaker there is a really loud humm! voltages seem to be ok. but the humm changes a little when i'm turning the master volume.

any suggestions what to do?
thanks for helping a newbie!!
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[groovtubin]

first of all hello to everybody!
i know this is my first post here, but i learned a lot by reading all the different topics!

my amp (hybrid-a style) is almost finished.
first powerup with bulp limiter and no tubes installed seemed to be ok!
but at the second powerup with the installed tubes and the bulp limiter plus the connected speaker there is a really loud humm! voltages seem to be ok. but the humm changes a little when i'm turning the master volume.

any suggestions what to do?
thanks for helping a newbie!!

make sure the BIAS voltage is set to MAX NEGATIVE DC VOLTAGE , also reverse the output tranny secondaries at pin 3 on the power tubes, a good start anyways!

jp

[alex]

i did not try with bais voltage at negative maximum ... only at about -40V
but it makes the same noise without the bulp ... but much louder


and pin 3 from the 6L6 goes to the output transformer

but is it correct to set the negative side of the speakeroutput to ground?
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[Funkalicousgroove]

How are you powering your relays?

Unless it is SCREAMING LIKE A BANSHEE you have your leads correct on pin 3, Blue wire to the tubes closest to the preamp, brown wire to the tubes closest to the PT.

Can you describe the Hum? is it more of a buzz or a hummmmm?

Are you using shielded wire on your v1 & V2 Grid leads? If so, is the shielding grounded at one end only?

Have you checked all of your grounds with your DVM for continuity?

Can you give us some pictures??

[heisthl]

First of all isolate the power amp from the preamp at the poweramp in point. If the noise is power amp only try a clip lead from the 24K to ground in the PI - if that clears up the problem it doesn't like where you grounded the presence circuit. If it's preamp only make sure the cathode resistor/bypass cap pair for each tube is grounded at the same point that the filter cap for the power to that tube is grounded.

[alex]

How are you powering your relays?
i'm using an additional powertransformer @ 15V for mechanical relays

Can you describe the Hum? is it more of a buzz or a hummmmm?
i think its more like a buzzzzz

Are you using shielded wire on your v1 & V2 Grid leads? If so, is the shielding grounded at one end only?
yep

Have you checked all of your grounds with your DVM for continuity?
i dont know what you mean exactly with that? if the voltages @ my powersupply are constant?

Can you give us some pictures??
i'll try to make some ... but i think they might be confusing because i tried someting on my own ... but i'll describe when i get the photos


and i'll try to seperate the preamp from the poweramp to look where is the problem ... and my presence is grounded at the frontbus
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[Funkalicousgroove]

Just because your DC voltage is constant doesn't mean that one of your AC Grounds isn't good, and if you have the AC and DC voltages grounded at the same points or something you could be getting hum from that.

Your preamp (Sans presence), your Buss, and the filter caps from V1,2,&3 should ground near the input.

Your Presence circuit/PI grounds at the FX loop jacks

The Screen supply should ground between v4&v5

The Plate supply & PT ground near the PT

The relay supply including the relays grounds at the same place as your voltage regulator.

Hope this helps, I find that these are usually simple problems.[/quote]

[Bob-I]

Another question. How does it sound otherwise. If you're getting nasty sounds mixed with the hum I'd suspect some miswired signal lines.

[alex]

thx for help ... now i need some days to check these things, because i have a lot of other things to do as well.

and i think i have to redo all my groundings, because i got a kind of groundbus with one point connected to the chassis ... that sems not to be good! ... i gonna check the other things before ... perhaps i need not

by the way ... i grounded my output jack ... is that right??? or might this be my source of humm?

and bob-i ... cant tell you how it sounds ... i can only hear my buzzzzzz...
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[heisthl]

Any amp that uses a presence circuit based on negative feedback from the output transformer has to ground the speaker jack(s).

[Bob-I]

and bob-i ... cant tell you how it sounds ... i can only hear my buzzzzzz...

That's what I was getting at. Something is miswired. I'd start by tracing the circuit from the input through. Simplify the amp by disconnecting the ODS, connect from the CL output cap direct into the PI.

[alex]

thx for all the help ... i'll try this
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[Bob-I]

Also, close up pics posted might help.