6V6 in triode mode - big hum and blue glow

[Hallingamp]

Hi.

The amp in question is a home made stereo 5W head. It is basically two Champs in one chassis with a shared PSU.
I am constantly told that I'm too loud on stage, so I tried connecting the 6V6's in triode mode for less watts last night.

I first tried connecting pins 3 and 4 together directly, then with a 100 kOhm resistor by mistake, and then with a more sensible 100 Ohm resistor.
I also tried replacing the 6V6's with 6L6's but got the same result. A verry bassy "there's-something-wrong-in-the-power-supply" hum (100Hz?) and tubes are glowing blue.

When connected in pentode mode the amp works perfectly - just too loud.

What am I doing wrong?
Any help very much appreciated.

Jan

[soundguruman]

Seems like the screen power supply is still connected, but you need to disconnect that from the screen resistor(s).

It's normal for the tubes to glow blue, so that's not a problem.

Between pin 4 and pin 6 (on the socket) should be your screen resistor.



Pin 6 connects to the power supply. Disconnect that...you are no longer using the screen grid power supply connection.
Now pin 6 connects to pin 3. (a short wire that you install)

Now there is a resistor between plate (pin 3) and pin 4 (screen). This would be 470 ohm normally, but you are using 100 ohm.
The resistor is soldered between pin 4 and 6. Pin 6 is connected to pin 3 with a new wire.
Will probably work OK w/ 100 ohm.

In your shared power supply, pin 6 on one tube socket connects to pin 6 of the second tube socket.
These are no longer connected together. AND they are both disconnected from the screen grid power supply.

I am not looking at your amp, so posting really clear, well lit, close-up pictures will help !
It's hard to tell without seeing it.

[Hallingamp]

Hi soundguruman, and thanks for your reply.

The screen grids are disconnected from the psu, that was the first thing I did.

The original Champ 5f1 design does not use a screen resistor, the screen is connected directly to a separate node (don't know if that's the propper english word for it...) in the PSU. This point in the psu is now left connected to nothing. I didn't really think that would be a problem, but maybe that could be it?

Many schematics online says to just hook the screen to the plate, others suggest a 100 ohm resistor to help with oscillations.

I will take some pictures and post when I get home from work.

Champ 5f1: http://www.sophtamps.ca/images/champ..._5f1_schem.gif

Jan

[Chuck H]

If you aren't using individual screen resistors you should add them. Use 1k 3W resistors. Connect the screens to the power supply node via these resistors for pentode operation. Connect the screens to the plates via these resistors for triode operation.

Just like SGM's drawing.

Nothing about connecting an amp in triode should induce hum unless the tubes are over dissipating. In this case I would guess that you're over dissipating the screens. If your plate voltage is high you may not be able to safely operate the amp in triode mode.

[nickb]

If you aren't using individual screen resistors you should add them. Use 1k 3W resistors. Connect the screens to the power supply node via these resistors for pentode operation. Connect the screens to the plates via these resistors for triode operation.

Just like SGM's drawing.

Nothing about connecting an amp in triode should induce hum unless the tubes are over dissipating. In this case I would guess that you're over dissipating the screens. If your plate voltage is high you may not be able to safely operate the amp in triode mode.

If one tube is not working as intended e.g. one is pentode and other is triode, then the imbalance will cause hum.

[Hallingamp]

Time flew last night, but I think I found the problem in that I forgot my B+ is a whopping 360V.
That's a tough job for a single ended 6V6 in PENTODE mode, so no wonder it gave up in triode.
I thought a 6L6 would be able to do it, but I guess not.

So - better start a new 1W build!

Thank you for your replies!

Jan

[Enzo]

360v is exactly the B+ on the single 6V6 AA764 Champ.


Watts is not loudness. And I have a hard time getting my head around a 5-10w amp is too loud on stage, unless you are Peter Paul and Mary. 1 watt will probably be too loud as well.

On the other hand I could be wrong.

Before you go watt hunting, set a can of paint in front of the speaker, or your jug of Sunny-D. Something roundish that will break up any beaming of sound. Often piercing is confused with too loud.

Who says you are too loud? The drummer? the singer? folks out front? And when you stand where they are and play, are you too loud? Are you the only amp in the act? Don't aim the speakers at anyone. Tilt them up so they don't hit the audience in the face, and don;t aim them at other bandmates. Don't make the leap from someone says I am too loud to designing a lower power amp. Cutting the power in half only gets you 3db less loud.


And presumably, the amp has a volume control?

[Hallingamp]

Hi Enzo.

Some very good points you make here, and it is of course the loudness that is the problem here.

It is our sound guy that always tells me to turn down, unless we're playing on real big stages of outdoors. This only really became a problem after we started using in-ear monitors and stopped using wedges. Most of the "noise" on stage suddenly disappeared and I'm mostly the only amp in the act. Our bass player more often than not goes direct to desk. (But I really don't want to follow him on that.)

Our sound guy sometimes puts obsticles in front of my amp to tame the stage sound. I'm using a stereo 2x12" cab, and I agree that it is quite loud. Not piercing, if anything it needs a bit more high end. Maybe I should look for some less efficient speakers. Suggestions are most welcome. (But I really like 12")

And presumably, the amp has a volume control?

Yes, that's the only control it has. The sweet spot is at about 1 o'clock, and I thought that by going triode I could still be at the sweet spot where it juuust starts to break up.

Oh, and we're not Peter, Paul and Mary.

https://www.facebook.com/pages/Split...894489?fref=ts

Thanks.

Jan

[Enzo]

Less efficient speakers still only net you a few decibels.


So the sound guy objects, but what does the rest of the band think? How does your level compare to the drummer?


Consider an attenuator between amp and speakers.

And to get hard core, you could put the amp off stage somewhere with a mic in front of it, and go through the PA that way. Now the noise is remote, you can have your breakup tone, and the loudness issue is gone.

[Hallingamp]

All of the bandmembers are on in-ears and are not bothered by it.
I think I am as loud as the drummer, maybe a bit louder.

5 Watts (or whatever a SE 6V6 produces) is a lot louder than I ever expected. How people cope with 100W guitar rigs on stage is beyond me. (Yeah I know, volume control. But then the amp isn't working properly, is it...)

Putting the amp off stage is in fact a very good idea - where it is possible. Or get a stand to point the speakers up in the air.

Anyway I have to find another solution than triode operation. I'll manage somehow.

Thanks for all the input.

Jan

[Enzo]

I have a friend who puts his screaming Marshall 4x12 cab and a microphone out in his van, and runs cables back in to the stage.

[Hallingamp]

OK, I didn't think to put it THAT far away. But that's why I wanted to try triode mode, and even why I built the amp with just 5W in the first place. Scale down so that I can open the amp up and get that nice break up tone without the ear-bleeding volume. If you have to put your guitar amp out in a van to manage the volume then clearly your amp is too loud. I say divide the watts by 10, leave three of the four 12" at home and start over. I am simply trying to practice what I preach.

Jan

[Jot]

360v is exactly the B+ on the single 6V6 AA764 Champ.


Watts is not loudness. And I have a hard time getting my head around a 5-10w amp is too loud on stage, unless you are Peter Paul and Mary. 1 watt will probably be too loud as well.

A friend of mine bought a couple of used Marshall Major stacks at Manny's in New York in 1971. One stack had "JACK BRUCE" stenciled across the back; the other stack said "PETER PAUL AND MARY."

[Enzo]

Which one of them played the Marshall?



If you divide the watts by 10, you get 1/2 the loudness. All else equal, a 100w amp will be twice as loud (SPL) as a 10w amp.

[eschertron]

Which one of them played the Marshall?

I played bass for a charity gig at a local church a few years ago, the drummer played the church's electric kit. The house had him turned down so low that I had to LOOK AT HIS RIGHT FOOT to see where the beat was. I don't think Splitter Pine is that quiet though. Maybe Peter, Paul and Mary.