Tubelab SE with 2A3 Hum

[Doc Jr 8156]

Hello,
Can someone help me how to solve the hum on my Tubelab SE with 2A3? I followed the wiring schema per George's website and I'm getting 14mv of hum per channel.

The RCA inputs are isolated from the chassis as well as the speaker posts. The common of both speaker posts were connected to IEC ground. RCA ground tabs connected to the IEc ground. The aluminum chassis is then connected by a single wire to the RCA ground tab.

The chassis are 2 aluminum boxes 8X12 bolted together. One side for the power tranny, choke, and oil cap and the other for the board and opts.

I'm using Hammond 370hx, Hammond 193j choke, 50uf oil cap parallel to C5 and Electraprint opts.

The hum is present even if the 5842s are removed.

All voltage checkout are fine. B+ of 300, 5842s at 175 volts, 2A3s bias 55ma per tube. I can hear music when connected to source but there is the hum.

I built 2 Simple SE and never had a hum problem. The wiring schema were the same.
I tried every variation of grounding but to no avail.

I searched the forum but none seem to be using a 2A3 with their Tubelab SE.

The output tubes are JJ 2A3-40, matched.

Thanks for your time. Godspeed.

John Revilla

[tinitus]

I remember something with Aksa tubepres having hum when pot housing not being grounded

[Brit01]

I remember something with Aksa tubepres having hum when pot housing not being grounded

I had significant noise problems with the alps pot housing not being connected to ground also.

I attached a thin wire just under the screw of the pot and then to the ground.
Hum disappeared instantly.

[Doc Jr 8156]

Hi Tinitus and Brit 01. Thanks for replying. My Tubelab SE employ no pot. I'm using a preamp. Godspeed.

John Revilla

[Doc Jr 8156]

George...are you there? I'm getting 15mv of hum even without a case and input tubes. Thanks.

John Revilla

[tubelab.com]

George...are you there?

Yeah, just checked in.

You have hum without a 5842, so the input wiring is not the issue. You are using "hot rodded" 2A3's which were not available when the Tubelab SE was conceived. The issue arises because some of these tubes draw a bit more filament current than a stock 2A3 causing the filament regulator to drop out of regulation momentarilly during the low point of the input voltage cycle. In many cases this can be remedied by adding a large value capacitor in parallel with C1. I have not tried the JJ tubes but I have experimented with a pair of Sovteks that caused problems for another user.

In order to completely kill the hum with the Sovtek tubes I needed to add a 100,000 uF 10 Volt capacitor mounted off board and connected to the PC board with fairly heavy (#18) wire. Adding capacitance to C3 may help too.

[Doc Jr 8156]

Thanks George,
Is there a regulator that has more amps than what was specified in the parts list? What are the maximum capacitance I can use with C1 and C3?

I am hoping to make my amp with 2A3 work because I do not want another 300B amp. I already have a 300B amp.

My hum were originally 14 to 15 mv per channel but when I added a 4700 uf cap to C1 the hum lowered to 10.3 mv and adding another 4700 uf cap once again lowered it to 8.4mv. It seems that adding some more will eventually null them.

Worse comes to worse, I'll revert back to a 300B. I can increase the B+ by increasing the value of C4 right? 60uf max for C4?

Tahnks for your time once again, Sir.

Godspeed.

John Revilla M.D.

[Doc Jr 8156]

George,
Can I use the 7A Sharp voltage regulator PQ5EV7J0000H available at Digikey? The tubes are 2A3. Thanks. Godspeed.

John Revilla

[Doc Jr 8156]

I'm about to run out of patience in this Tubelab SE amp. Last night, I switched to 300B output from 2A3 because George told me that my 2A3 were drawing to much current from the regulator hence the hum.

Above, I was asking if I can use the 7A regulator but have not receive a response. Anyhow, I converted the amp for 300B hoping to have a working Tubelab Se while waiting for George's response on using the 7A regulator for the 2A3s.

Tell you what, swithching to 300Bs went fine at the start but now the amp is shutting down after 4 minutes! No blown fuse. Biased at 64mv, B+ at 300v. Amps starts again after 5 minutes cool-off. The Sharp regulator extra heatsink was not even hot! What is wrong now???

I think a precaution about building a Tubelab SE amp using newer 2A3s should be "highligthed" in Tubelab's website. I bought brand new 2A3 tubes just for this amp and have I known that this will not work, (because the Tubelab SE design was conceived before the newer 2A3 tubes like my JJ 2A3 were available), I'd go for the 300B from the start.

Frustrating, very frustrating.

Godspeed.

John Revilla

[Diomedian]

Not to worry Doc - I'm sure it'll get sorted out. It's Sunday, and you know as well as I do how little free time George has, and how much of that is devoted to helping folks on this site. Also consider how much money you've saved building a Tubelab SE over buying commercial - you likely saved more than the cost of both the 2A3s _and the 300Bs for the sound quality you get, and if for some reason you end up not liking one pair, with so few hours on them they can easily be resold on eBay or Audiogon.

-j