5G9 Tremolux questions! Calling Tweed gurus!

[Herzog]

I'm cracking on with the build, the eyelet board is finished. Chassis to be drilled for the board, capacitor doghouse, OT and choke.

My PT has two 50v bias taps (it's for a low B+ bassman).

Stupid question but I guess I use one and tie the other off?

[sluckey]

My PT has two 50v bias taps (it's for a low B+ bassman).

That's unusual. Are you sure? You may have a totally separate bias winding rather than a 'bias tap' on the HT winding. If so, there would be two wires.

Do you have a data sheet for that PT?

[Herzog]

My mistake, yes, it's a separate bias winding [IMG1365]http://i288.photobucket.com/albums/ll162/jim50hertz/86C940B3-02F4-464A-9D0F-0A28F7C00302_zpsxmoizcqw.jpg[/img]

[ToneMerc]

My mistake, yes, it's a separate bias winding [IMG1365]http://i288.photobucket.com/albums/ll162/jim50hertz/86C940B3-02F4-464A-9D0F-0A28F7C00302_zpsxmoizcqw.jpg[/img]

So, just terminate one of the brn/white bias leads to ground.

TM

[Herzog]

Thanks for that TM

[Herzog]

I'm using isolated jacks in an effort to keep noise to a minimum. My OT has the common connection soldered to it's casing like the originals.

Where do I ground the speaker jack?

Any ideas?

[ToneMerc]

I'm using isolated jacks in an effort to keep noise to a minimum. My OT has the common connection soldered to it's casing like the originals.

Where do I ground the speaker jack?

Any ideas?

The main issue with the "self lead" is that OT ground connection is only good as the integrity of the two chassis mounting bolt locations. Thus, you want lacquer free and clean mounting surfaces(add ext tooth washer between OT & chassis) on both the transformer and chassis. Also, the older bare wires have been known to break. You could just unsolder the self lead and terminate it how you would have normally.

TM

[sluckey]

Just to add to what ToneMerc just said...

That OT secondary wire grounded to the OT case can cause some expensive damage if you don't have a very good electrical ground path to chassis. A bad ground path could be caused by a poor solder joint to the OT case or it could be caused by a poor chassis connection. I would clip that ground wire from the OT frame and ground the lead to the chassis or connect directly to the speaker jack. Then connect another wire from the speaker jack ground lug to chassis.

Just last month on the Hoffman forum, a member built a Harvard 5F10 using a MM OT that had the OT secondary common lead connected to the OT frame. Short story version, the OT secondary was open circuit due to the frame connection or the chassis mounting. Flyback pulses from the open secondary caused an arc between filament and plate pins on one of the tube sockets, killed the 6V6, burned up his 100Ω resistor artificial center tap for the filaments circuit, and damaged the OT.

[Herzog]

Thanks both for chiming in. That's quite the horror story Sluckey. I'll redo this connection then.

Nearly done. Heaters, speaker and trem jacks to finish up then pre flight checks.

All going well first fire up tomorrow.

Thanks for all the help guys.

[Herzog]

Success. The amp passed all the safety checks and fired up just fine.

It passes signal, trem works fine and from what I could tell with my wife and son sleeping it sounds good. And it's super quiet.

I had slight issue in the bias circuit not producing enough negative voltage which was solved by removing the 100k resistor I had in series with the 1N4007.

Now all the voltages are in line with the schematic. I don't mean close, I mean almost to the volt.

Weird thing is the 6V6s are running very hot.

I have cathode current of 46ma and 365v plate to cathode. This gives me 16watts dissipation. Too much huh? I'm kinda new to fixed bias. Do I divide this by the number of tubes?

I'll look again with fresh eyes today.

[pdf64]

I have cathode current of 46ma ... Do I divide this by the number of tubes?

How are you measuring 46mA?

[Herzog]

I have cathode current of 46ma ... Do I divide this by the number of tubes?

How are you measuring 46mA?

I'm using a bias probe. 1%, 1ohm resistor from cathode to ground. Measuring 46mv.

http://www.ebay.co.uk/itm/111507561954

[ToneMerc]

I have cathode current of 46ma ... Do I divide this by the number of tubes?

How are you measuring 46mA?

I'm using a bias probe. 1%, 1ohm resistor from cathode to ground. Measuring 46mv.

http://www.ebay.co.uk/itm/111507561954

Totally removing that 100K was not the answer, use a resistance that works. FYI, you can't go exactly by the values in the schematic for the bias circuit because those selenium rectifers drop far more voltage than modern si diodes. When you had the 100K in place, what exactly was not enough?

TM

[Herzog]

I should clarify, the 100k resistor is not in the original schematic. It was suggested here.

I have replaced the 56k resistor with a 25k trimpot and 39k resistor.

Prior to removing the extra 100k resistor the maximum negative voltage after the 1N4007 was -16.9v. With this the power tubes were starting to red plate.

Now I'm getting -69v.

[ToneMerc]

I should clarify, the 100k resistor is not in the original schematic. It was suggested here.

I have replaced the 56k resistor with a 25k trimpot and 39k resistor.

Prior to removing the extra 100k resistor the maximum negative voltage after the 1N4007 was -16.9v. With this the power tubes were starting to red plate.

Now I'm getting -69v.

My tap is either 45 or 50v and I used a 15K, 10K and a 35 or 55K trimmer ( i cant remember which one) to ground. It has about 370V on the plates and plenty of adjustment in the 18-32 ish range, IIRC

TM