Fender Dlx Rev build- last Q b4 fire-up.

[Sea Chief]

Hi chaps, happy easter.

Just doing final wiring/ checks before using current limiter to start up a new DR build (all boards/ chassis/ rev xfmr all fender RI. Pwr: Hammond 291BEX. OT: mojotone. Tubes: JJ GZ34S, EH 6V6GT, JJ & Tung-Sol others).

[PDF of schematic in reply #2, kindly added by JazzP on this thread http://music-electronics-forum.com/t39198/

Ok so checking wiring I notice just -one- anomally: a grey wire connecting R64 to the OT green wire/ jack socket to spkr. It has been cropped vv short on the M'board.. suggesting purposefully done.

Q. is this the negative feedback section? and probably grey wire removed to 'reduce neg feedback loop'? in which case I dont need to worry and can use as is? (possibly of benefit to my DR sound too, poss not).

Thanks, Chief.

[Chuck H]

Hi chaps, happy easter.

Happy Easter!

Just doing final wiring/ checks before using current limiter to start up a new DR build (all boards/ chassis/ rev xfmr all fender RI. Pwr: Hammond 291BEX. OT: mojotone. Tubes: JJ GZ34S, EH 6V6GT, JJ & Tung-Sol others).

Start with the current limiter AND NO TUBES IN. Make sure you have bias voltage. If all looks good so far, that is, no power supply shorts and bias voltage is present, plug in the tubes. If the limiter is behaving normal (= dim), remove it. Now, with tubes in, measure voltages, bias current, etc. If all looks good you may now play through it

Ok so checking wiring I notice just -one- anomally: a grey wire connecting R64 to the OT green wire/ jack socket to spkr. It has been cropped vv short on the M'board.. suggesting purposefully done.

I would have though YOU would know what's been purposefully done in the amp. But I guess you never actually said that YOU built it.?.

Q. is this the negative feedback section? and probably grey wire removed to 'reduce neg feedback loop'?

Yes, it's the negative feedback series resistor. Even if you don't want to include NFB in your circuit you should probably still hook it up at least temporarily to see that the OT phase is correct. In case you ever decide to use NFB on the amp. Otherwise it could make for a confusing surprise.

[Sea Chief]

Hi Chuck, thx for the reply.

Omg Im getting all excited..

Right, beforehand: "I would have though YOU would know what's been purposefully done in the amp. But I guess you never actually said that YOU built it.?."

..good Q/ I wasnt clear. You see I bought the boards used; it therefore perhaps looks to have been snipped purposefully short, by whoever had it b4 you see.. I assumed neg fdbk loop but if Im not 100% sure- I ask, on here 1st.

When you say 'make sure you have bias voltage' when I use current limiter w'out tubes in.. where abouts do I read this exactly? am I looking for 50v or so? (Pwr xfmr certainly has the blue bias wire rated at 50v).

Thanks, SC.

[Chuck H]

You probably won't get the full bias voltage with the current limiter in use. You're just looking to see that it's there to avoid instant melt down when you put the tubes in. The bias voltage is on pin 5 of the 6V6 sockets. With the tubes in place you'll want to adjust the bias voltage for the right idle current before continuing. The easiest way to measure the idle current is with a bias test socket type thingy that a few tube vendors sell or you can include 1ohm/1watt/1% resistors in the 6v6 cathode circuit. The actual adjustment depends on whether or not you have a potentiometer in the bias supply. If you do it's just twiddling the pot. If you don't you need to change fixed resistors for adjustment.

[Sea Chief]

Hi Chuck.

Ok Ive soldered back on the fdbk loop 'grey wire' link to be safe. All wiring now thoughrally checked- twice. All continuity (or not) as it should be on each pin of each tube socket checked- twice. No spiders.

Right one last Q and Im ready to go with the C.Ltr w'out tubes, reading for V at pin 5 of one 6v6 socket.

The circuit does indeed use a 10K/B bias pot.. all I had to hand was a 10K/B mini-pot. Q: is it ok being a mini-me? and do all pots of similar value and type (such as my mini 10KB here compared to a usual size one) have parity with regard to their 3 pins layout?

If yes/ yes its all systems go. Thx, SC

[Chuck H]

The wee pot is fine.

The feedback loop isn't a safety issue. It's just good to know the amp is in proper phase and the loop will let you know right away because the amp will be unstable.

Do you have a way to check for idle current once the tubes are installed?

[Sea Chief]

The wee pot is fine.

The feedback loop isn't a safety issue. It's just good to know the amp is in proper phase and the loop will let you know right away because the amp will be unstable.

Do you have a way to check for idle current once the tubes are installed?

Erm check for idle current.. I have DMM- is that a stupid answer?

Oh and do I need to check pin 5 of both 6v6 for bias V, or will one suffice?

[Chuck H]

Check both tube sockets for bias voltage.

Just a DMM can work, but it's a pita. Easier if you have a resistor in the power tube cathode circuit.

[Sea Chief]

Check both tube sockets for bias voltage.

Just a DMM can work, but it's a pita. Easier if you have a resistor in the power tube cathode circuit.

Wouldn't I have said resistor, in the circuit being an 'orig DR RI' as it is in essence?

I must admit Im confused about idle current checking.. I know I used a DMM before in a Champ to set mA/ bias the tube in good ballpark figure. (Didnt I?). I thought it was just a case of adjusting the bias pot, whilst measuring mA with a DMM on pin* of V7 then V8..? surely normal DRRI users dont have to have some sort of extra gagetry to bias other than a DMM?

[Chuck H]

Wouldn't I have said resistor, in the circuit being an 'orig DR RI' as it is in essence?

The DR and DR RI don't have these resistors as stock. Most builder include them for convenience but Fender had/has no such compulsion to pony up for two extra resistors

I know I used a DMM before in a Champ to set mA/ bias the tube in good ballpark figure. (Didnt I?). I thought it was just a case of adjusting the bias pot, whilst measuring mA with a DMM on pin* of V7 then V8..?

The Champ circuit uses cathode bias. That means a resistor in the cathode circuit IS the bias. So with that design it's possible to check current from a tube pin. Also, not adjustable. But it almost always works out because cathode bias circuits are somewhat self governing. This is not the case with the fixed bias amps. Unless extra resistors are added there is no way to check current from a tube pin with a DMM. Checking current instead requires unsoldering connections and using the ammeter in your DMM to bridge the open circuit. Much easier to add the resistors to the cathode circuit.

But don't despair. Close is nearly always close enough WRT idle current. Once we have an idea of your plate voltage, and knowing the spec for bias voltage (-37V BTW) we can just guestimate and if the amp sounds good and the tubes behave it's fine. There's some leeway with idle current.

surely normal DRRI users dont have to have some sort of extra gagetry to bias other than a DMM?

Surely they do! But even more surely, they aren't supposed to be doing their own service. I'm sure Fender would never go out of their way to make service by users more plausible. It's only conscientious and self motivated users that would ever do their own service. This is where the plug in devices I spoke of find their largest market demographic. Players and less equipped hobbyists that need to check bias on personal amps and builds.

[Sea Chief]

Comprehensive answers (of which I understand a majority, I think)- most grateful ChuckH.

I'm still not understanding "idle" current fully and had thought I could read, as I had in store by way of a good YTube hotroddeluxe biasing vid, the current mA from a poking around. Maybe the difference is there the HRDlx has a 'test point'? (but why wouldnt a better DRRI have one?).. maybe not. I'll leave this confusion for now if I can skirt around it ok.

Ok Im happy to go guestimate for now. Maybe I can take over to a techy s'one I know to fine-tune the So tmrw I'll fire her up for 1st time and report back with the bias V readings V7,V8.

[Chuck H]

Do it. The current limiter should be dim because there won't be much current. If it glows brightly it could mean a miswire or failure in the power supply causing a short that IS drawing current. Then it's back to the bench for troubleshooting. If the bulb is dim and there is bias voltage present on both sockets, plug in the tubes. The current limiter will get a little brighter, but not full brightness. If it does that remove the current limiter and measure voltages. If voltages look close to spec (especially 6V6 plate voltage and bias voltage) go ahead and give it a test run.

[g1]

Maybe the difference is there the HRDlx has a 'test point'? (but why wouldnt a better DRRI have one?).. maybe not.

The HR has the resistors and uses them as test points. The DRRI does not have them. I suppose they figure the original amp didn't have them so the re-ish shouldn't either.
It's possible to do it with resistance and voltage measurement at the OT, but that can be left for later.

[Sea Chief]

The HR has the resistors and uses them as test points. The DRRI does not have them. I suppose they figure the original amp didn't have them so the re-ish shouldn't either.
It's possible to do it with resistance and voltage measurement at the OT, but that can be left for later.

Hi g1.

Blimey I asked a non-stupid Q/ I was thinking along the right lines! Ok understood.

Thanks chaps.

[Sea Chief]

Ok 1st test done. I read 50.9v at both pin 5's of V7,V8 (6v6's).. ok that's good..

But the 100w light bulb didnt light. WTF? (Light checked, 2nd one too, switches to ON, plug in normally to wall=bright, plug into my C.ltr and nothing whilst checks done/ nothing before either).