5F11 - Need some guidance

[Crawfish]

Hi all -

I'm having an issue with a It's a 5F11 Vibrolux I'm building and am looking for a little guidance.

Built in an exact copy chassis, Mercury transformers, eyelet board, etc. Have built a few amps before, so I'm not a total novice, but I'm not a guru either!

I brought it up slowly over the weekend on a variac. Cranked up I get nothing at all, no hiss, no hum, nada. Measured voltages and they are a bit higher than the schematic but nothing looks too far off to be worried about. Wiring has been checked during and after assembly numerous times. I was fairly confident all is correct...but clearly I missed something.

Symptoms:

- At idle, the amp is not drawing as much current as I'd expect - about 0.4 A.

- Plates and screens of 6V6s are running at the same voltage (about 372 at 120 volts on the line). Strange since the screen supply is definitely wired after the 470 ohm resistor. As an experiment, I tried an additional 1K resistor in the screen supply too - no change.

- With an input (guitar or just finger on the hot side of input jack), there is a faint, very buzzy tone when cranked all the way up.

- With an input, the B+ drops drastically, about 50 volts, and current draw goes up to almost an amp. So putting a signal on it creates a huge load.

- With an input, there is a arc between the plate and screen socket connections on leftmost tube, and the other tube arced internally (!).

Any help or thoughts would be appreciated.

-Kevin

[cbarrow7625]

Are the power tube cathodes grounded? Just a WAG.

[Crawfish]

Are the power tube cathodes grounded? Just a WAG.

They are, I believe, but I will double check.

-Kevin

[cbarrow7625]

What would really help is if you could go through the amp with an AC voltmeter and signal generator to find out where the signal is stopping.

Does it make it all the way through the preamp? Does it make it to the grids of the Power tubes but no further? Do you see signal on the power tube plates but just don't hear it in the speakers?

At least then we would know what part of the amp to start pondering. And you won't have to put up with more guessing from me.

[Paul P]

It seems to me that to get arcing you need quite a high voltage. The only
thing that I can see that could generate that would be the output transformer.
Are you sure your OT and speaker are connected properly ? I wouldn't worry
about the rest of the amp until I got rid of the arcing.

Paul P

[Crawfish]

It seems to me that to get arcing you need quite a high voltage. The only
thing that I can see that could generate that would be the output transformer.
Are you sure your OT and speaker are connected properly ? I wouldn't worry
about the rest of the amp until I got rid of the arcing.

Paul P

My guess is you are right, Paul. I'm using a Mercury OT. The primary CT is orange - going to B+. The other primary leads (green and brown I believe) are going to plates of the 6V6s.

The secondary may be where I have misconnected. The OT has one white lead and then a 'self-lead' - a copper wire soldered to the OT frame. I have the white lead going to the tip of the speaker jack. I have no connection from the self lead to the ground side of the speaker jack, since I assumed it was ground, but maybe I need one?

Otherwise the jack and speaker did work when hooked up directly to a little Walkman I use as a signal generator.

-Kevin

[tubeswell]

I guess the wire soldered to the OT frame would simply be the ground, but maybe it would ensure a proper connection if it was wired to the speaker socket ground lug like you say?

You've obviously got a B+ voltage. Work back through the power supply rail and check all plate voltages are there. Maybe see if the screen resistor is kaputt? How have you got your bias tapped off the PT? - is it off a 50V tap or off one of the secondaries? You might have a problem with the bias voltage maybe?

[d95err]

If its a push-pull OT (like it should be) it should have three wires from the secondary winding

(I'm sure you mean primary)

[tubeswell]

(I'm sure you mean primary)

Yeah sorry its early in the morning here, and I realised my mistake (too late dang!)

[tubeswell]

The other thing I was going to suggest is see if your OT primaries (thankyou d95err) are wired the right way. Sometimes the wires ain't coloured right. The Plate wires should measure the same resistance either side of the CT. If you've got the real CT wired to one of the plates and vice versa, I guess that could give you odd plate voltages and maybe the push-pull wouldn't work right.

You also could have shorted the screen resistor somehow - is there a resistance measured across it with the filter cap grounds disconnected?

[Crawfish]

It's working!

The amp is working!

Thanks for all the advice, it helped to be pointed in the right direction.

The problem was in the output transformer secondary. The hot lead wire has some sort of clear paint or coating over it, which I didn't realize - it's magnet wire, which makes sense. It looks like it's actually the wire that is wound on the transformer, it just exits and becomes the lead for the speaker. It does have plastic insulation on it too - I had just cut it to length as usual, stripped it and soldered to the speaker jack. With the regular insulation, I assumed it was a regular wire...you know what they say about assuming.

When I was double checking the connections to the speaker, I noticed the joint at the jack looked a little strange - like the wire wasn't covered with solder at all. I tested the wire at two points for continuity and had none! I thought I was losing my mind. Then I began to wonder, and sure enough, scraping the wire took the coating off...connected it and voila! we have a working amp!

Chalk this one up to a learning experience.

The amp sounds amazing. I have a Weber 10" alnico speaker in it and it sounds huge. The tremolo is the bias-modulating type and sounds good too.

-Kevin

[tubeswell]

That's great Crawfish. Are you able to post some pics?