The first 6A5G get*set*go is up!

[hembrook]

I think I am the first to use this tube (please disabuse me of this if I am wrong). I pulled by 6B4Gs this morning and stuck in my 1943 vintage Sylvania 6A5Gs and am listening to Thomas Dolby...

I am pleased.

Thanks to Shannon and all the DIY Tubers!

[Shannon Parks]

Cool, Robert. Did the hum change with the different tubes? If not, we might have to play with the 3rd filament ground reference.

[hembrook]

I did a test, both 6B4Gs, then one of each, then 2 6A5Gs. The 6A5Gs are humming noticeably less, virtually inaudible unless you have your ear right on the speaker (95dB efficient, according to Sammi). I LOVE the 6A5G!

[hembrook]

I think some basics like rewiring the extra heater loop with twisted wires to cut down on parallel magnetic fields might be a good start

The hum comes from the right speaker. My voltage divider coming off the heater network ties to the left input, which is noticeably quieter. Any suggestions?

Would a variable pot instead of the 51 and 47 ohm resistors make a difference? Or am I thinking the wrong direction here?

The indirectly heated 6A5Gs work splendidly. I would like to use the $10 a tube JAN 6B4Gs I have. I could get a pile of them cheap and not have to worry about burning them down using them 8 hours a night as bedroom amps. Of course, I can build a Piccolo (Ecolo?) using 6BM8s and those are stupid cheap at 4 Euros a pop from the Czech republic. 2 watts in triode singleended and 4 in pentode SE make for easy listening at night.

[Shannon Parks]

The hum comes from the right speaker. My voltage divider coming off the heater network ties to the left input, which is noticeably quieter. Any suggestions?

Would a variable pot instead of the 51 and 47 ohm resistors make a difference? Or am I thinking the wrong direction here?

Hmmm. Let me think about this. In a standard build, the 51 & 47 ohm ohm resistors are tweaked, but here I don't know.

[EWBrown]

I had found in my first pass at the GSG, that the C354 choke was inducing a tiny bit of hum (actually, more correctly, ripple at 2X the AC powerline frequency) into one of the James OPTs (the one closest to the choke). You have the open-frame OPTs which are very susceptible to this, so it may be the reason that one channel has a bit more hum.

The cure with the James OPT was to rotate it 90 degrees. Nice and simple... I used a "front panel express" aluminum plate, courtesy of Tom McNally, so there was no magnetic shielding as there would be with a steel panel.

Is that diamond plate steel or aluminum? (steel would effectively shield it, aluminum does not).

A quick test for this effect would be to dismount the choke from the chassis plate and let it hang down and moved away from the OPT(s).

It looks like the power trannie is a toriod, those are very good at self-shielding and should not couple AC fields into anything.


In my second GSG, I built it onto a 12 X 10 X 2 inch Hammond steel chassis, and I used two chokes, the C354 in the normal connection, and a Hammond 158Q (5H, 150 mA, 105 ohms DCR) choke in place of the two 180 ohm resistors which are connected in parallel. This lowered the B+ by around 1.5VDC, no big deal... Yellow_Light_Colorz_PDT_02

I mounted both chokes under the chassis, at the two front "corners", so that they would be as far as possible from the OPTs, and this one has no hum at all. I used MQ RH-60 OPTs (5K impedance, 30 H, 185 ohms DCR) and they are open frame, so I wanted to keep the chokes as distant as possible in order to not have any electromagnetically-induced hum problems, and it works very well.

/ed B in NH

[hembrook]

I remembered your experience with the choke, so that was one of the first thing I looked at.

Looking under the hood, you can see my choke is mounted under the chassis on the left side (as you look at it from the front. The left channel is the quiet one.

The top diamond plate is indeed aluminum. Not so good for electromagnetics, I am afraid.

The input jacks are on the right side and run thru shielded 4-conductor microphone cable to the front. Once they pass the volume control, they hit the J1, where the shield wire is tied to ground, and then loop under (still with shielding foil) to J2 for the left channel.

Other than the straight runs for the heater wiring, I can't see what is inducing noise in just the right channel.

[Shannon Parks]

The hum comes from the right speaker. My voltage divider coming off the heater network ties to the left input, which is noticeably quieter. Any suggestions?

OK. Robert, I think you need to connect the divider to one of the cathode bias networks of the power tubes instead of ground. For example, try connecting the divider to TP1 or TP2.

I bet a liter stein of monastery beer it'll fix it. :)

[hembrook]

I have been meaning to ask, what are TP1 and TP2 used for on the GetSetGo?

Need anything from Japan?

[Shannon Parks]

TP1 and TP2 are measurement pads for the bias voltage, or a connector for the bias resistors if a different tube like a 2A3 was used.

What I need from Japan is a visit there myself. I'll get there one day for sure along with my vintage Pentax Takumar M42 lenses strapped on my Canon digital SLR.

[hembrook]

Can I tie a 1 ohm resistor from TP1 and TP2 to ground, then measure the voltage drop across this to calculate tube plate (cathode) current?

[Shannon Parks]

Can I tie a 1 ohm resistor from TP1 and TP2 to ground, then measure the voltage drop across this to calculate tube plate (cathode) current?

No, since the 1 ohm would be in parallel with the bias resistors. Just divide the voltage at TP1 and TP2 by 900.

[hembrook]

Can I tie a 1 ohm resistor from TP1 and TP2 to ground, then measure the voltage drop across this to calculate tube plate (cathode) current?

No, since the 1 ohm would be in parallel with the bias resistors. Just divide the voltage at TP1 and TP2 by 900.

OK. Where can I tie into for my silly meter then?

As far as hum goes, I tried tieing the voltage divider network from the two top tubes (6AX5GT, 6SL7) to TP2. I removed the wire from J2/2 and connected to TP2. Now I get a loud mechanical hum from the amplifier itself, as well as noise in the speakers with both the 6A5G and 6B4G tubes I have. This is a step backwards as the chassis had been quiet before, as had the 6A5Gs.

I dodn't really match the resistors for the voltage divider, nor did I twist the strands. Should I look there instead?

[Shannon Parks]

Dang. Sounds like I owe you a beer now.

First, double check you didn't flip-flop R11 and R13. Just eye ball that R12 and R11 look the same.

Second, take a 1k resistor, bend the leads and solder it across R11. Any better? Worse? Then de-solder it and try it on R13. This kind of simulates us tweaking a hum pot.

[hembrook]

What happens if I don't reference the heater to signal ground? I could try removing the voltage divider network entirely.

I am a little surprised by the mechanical noise I get now. I think it is the choke oscillating. w/o an oscilloscope (on my shopping list now) any good way to track that down?

Thanks for the help. Beer's on me if you get to Germany