Octal Deluxe

[Noval_novice]

I just finished up a scratch build, mostly from spare parts. It is a 5e3 with octal preamp tubes (6SL7). I like the sound pretty well. It has a great warm clean sound as well that raunchy Neil Young sorta thing. I included one of Dana's VVR3 units and a 3 way boost switch that switches in a 25 uF or 1 uF cap or none at all on the input stage cathode.

My major issue has to do with hum. It is 120Hz mainly. There with or without anything plugged into the input. Gets worse with higher volume or higher B+ when manipulating the VVR. The noise is never louder than the guitar and it is barely noticable when playing at bedroom levels.

Some observations:

1. Switching preamp tubes doesn't matter. Tried several new production Tung-Sol 6SL7 and 6SN7 as well as a old Sylvania 6SL7.

2. Voltages (with VVR at maximum voltage setting) are as follows:
6V6 plates 325, screens 280, cathode 17

V2b -- PI plate 176, cathode 44

V2a -- 2nd gain stage plate 132, cathode 44

V1b -- input plate 138, cathode 1.3

V1a -- input plate 129, cathode 1.3

3. Grounding scheme was a bus wire from filter caps all the way to the input. Grounded near the input jacks. The center tap for the HT was grounded separately near the IEC receptacle.

I realize my voltages may be a little low and I was contemplating playing with the dropping string some.

I also realize that octal preamp tubes can be a bit noisy, but I wasn't expecting 120 Hz noise as much. I know that some of you have made octal preamp based amps with less problems.

Any thoughts?

[Structo]

I would ground the filter caps differently.
The large ones for the plates and screens of the power tubes should be grounded near the power supply center tap.
The phase inverter can ground there as well but the preamp should ground near the input.
It's OK to use the ground buss but just to ground the pots and cathodes of the preamp.

[M Fowler]

Unless your pots are insolated from the chassis you have a ground loop going on.

I would separate the main filter cap and grounded with the center taps. Then listen to amp for noise.

Since the buss bar is grounded to all the pots and the remaining filter cap stages there may be another ground loop there.

Mark

[dynaman]

I've used Tom's recommended grounding scheme many times with good results. Lately, I've been using something like a single ground buss for everything with equally good results, but I've always grounded my jacks with their respective cathodes and made the actual chassis ground connection with a wire connected to the big power supply caps. OT, speaker jacks and power tube cathodes are also grounded at this same spot on the chassis. I made a few 5E3s recently using this scheme and they're stupid quiet.

Mebbe you could try a quick and easy change by moving the buss's ground connection to the power side of your board and screw it to the chassis near the PT. Mebbe even ground the input jacks at the beginning of the buss. I'd also move that HV center tap to the first big cap or at least the spot on the chassis used for the buss ground.

Where did you ground your speaker jacks and other output stuff?

FWIW, I made a 36W cathode biased Marshall that hummed like mad until I separated the EL84s cathode resistor's ground from the rest of the board buss. I terminated everything at the same spot on the chassis, but those cathodes got their own wire.

[dynaman]

Unless your pots are insolated from the chassis you have a ground loop going on.

I would separate the main filter cap and grounded with the center taps. Then listen to amp for noise.

Since the buss bar is grounded to all the pots and the remaining filter cap stages there may be another ground loop there.

Mark

Yes, I prefer to insulate my jacks so I can control their grounds. Me no likey soldering to pots, so I just run a buss through the pot lugs. Recently, I've started localizing my grounds even more by grounding pots with their respective part of the circuit. Works great, but can be a PIA in regards to layout.

[Noval_novice]

Just to clarify, the buss wire is not attached to the pots. It floats on my turret board. The speaker ground is attached to the second filter cap. Sorry for the fuzzy pics. Maybe I've been too influenced by Merlin's grounding artcle, but in my last build (an 18 watt), I used a ground buss with a single ground to chassis at the input and is quiet as a mouse. I will definitely try grounding the filter caps with the HT center tap when I get home. It seems like the consensus is that this is more likely due to grounding issues than anything to do with the octal preamp tubes, though..

[David Root]

Probably, but if it still hums after you fix the ground loops, think about a DC heater supply.

[Noval_novice]

David, if that is needed in the end what would be the best and/or easiest way to implement it given my limited space?

[Structo]

Before you go to DC heaters.
You may want to try elevating the heater voltage by attaching it to the power tube cathode. This raises the voltage above ground for quiet operation.

[Noval_novice]

Yeah, I forgot to mention that my heaters are elevated. Thanks, Tom.

[Cliff Schecht]

My octal Deluxe pretty much needed DC heaters (I went all throughout, YMMV here) and careful input wiring to be tolerable noise-wise. 6SL7 will hum like a madman with these relatively high-gain builds.

[M Fowler]

I never said the buss wire was soldered to the pots but the pots are running ground wires to the buss bar which is connected to main filter cap.

There are many ways to ground an amp I have found that out building all types of different amps.

Some one saying this how you do it period is incorrect. If you have buzz you deal with it at stages.

I dislike Cliff jacks and see no sense in insolating a jack from the chassis.

[Phil_S]

Ground discussions aside (this needs to be done), try using a lower value dropping resistor in the B+ supply ladder to boost your plate voltage on the preamp tubes. I think the reason you can't hear any difference when you swap tubes is because you don't have adequate plate voltage to wake up the tubes. For 6SL7, shoot for 250V. For 6SN7, the sky is the limit, well, actually 450V. I think this is simple and worth doing on the chance that you might find out you like it. I think you can go as low as 470/500 ohms without any real concern and certainly 1K will be OK.

[Noval_novice]

Hot Damn, Mark! You were right. When I got home, I realized I hadn't actually grounded the speaker jack. Did that (to the second filter cap) and the hum was better. Then I disconnected the bus wire between the second and third filter caps and grounded that section of the power supply/power amp to where I had grounded the HT CT. Dramatically better. Virtually no hum at all now. It is actually quieter than some of my other builds.

Here are some clips. I tried to leave some empty space in these for you to listen to the (lack of) hum.

http://www.soundclick.com/bands/default ... ID=1147161

For all clips, I am using a G&L Commanche plugged into the bright input. Speaker cab is 1x12 with Scumback M55 (not the ideal speaker for this amp, I realize).

[David Root]

Glad to hear it was just grounding!