fender 5G9 bias problem

[jezzbo]

Hi Everybody,

I'm about to finish my new amp.
A clone of a Fender 5G9 Tremolux.
This is the schematic:
http://www.prowessamplifiers.com/schema ... matic.html

Transformers I used are Classic tone 40-18029 (PT)
Classic tone 40-18090 (OT)

I used the low voltage secondary winding for B+
Rectifier is NOS GE 5U4GB

Mains voltage = 240V
Plate voltage is 316V
Current draw of the 6V6GT tubes is 69mA.
When I calculate dissipation I get (316V * 0,069A =) 21.8W!
According to the datasheet, the 6V6GT should get a max of 12W.

The bias resitor (fixed) is 82K.

What must I do to lower the biasing?
Is it possible to make a variable bias with this circuit?

Thank you for your reply.

Jesse

[rp]

Are you forgetting to divide by 2 (for 2 tubes)?

BTW where did you get the MC transformers, did you find a retailer in Europe/UK or did you order from the US?

[billyz]

Your plate voltage is way to low. Most of these run 400+ vdc. If your plate current is for one tube that that is your problem. At 400vdc I would like to see 24ma per tube or a little less. If your plate vdc is really that low then 69ma is only a little high if that is both tubes, better would be 62ma for both or 31ma each.
Probably the high plate current is drawing down the voltage.

Adjustable bias is a good idea. Replace the 56k to ground with a pot and resistor. I would use a 20-25k pot and a 47k resistor in series. Other values work too.

[tubeswell]

I built one about 6 years ago - had a B+ of 370 or 380 IIRC.

[billyz]

I built one about 6 years ago - had a B+ of 370 or 380 IIRC.

Yes, 370 vdc is the schematic value. I have never seen that in the wild though. What with modern AC line voltages. 😎

[jezzbo]

Hi, thanks for your reply.

@rp, I ordered in the US.

To meassure the current draw, I use the 1 Ohm resistor from cathode to ground. (Every tube output tube has one)

B+ is low because I rewired the PT.
The PT (http://www.classictone.net/40-18029.pdf) has two different secondary HT wirings.
One @ 710V AC and one at 610V AC both have 200mA current. (both CT)

First I wired the 710V. Bias was as on the schematic. (resistors 86K + 56K to ground).
I got instant severe red plating.
On the datasheet of 6V6GT it says that Max DC voltage on the plate is 315V I assumed that was the problem. (probably not then.)

To make the bias variable I clipped the 86K resistor and replaced it with a 470K pot. Set at 86K and then rotated the pot very slowly while measuring the 1 Ohm resistors.
So I replaced the wrong resistor (should have been the 56K resistor).

Another question: the schemtic doesn't have a 1500R resistor on the grid.
(in blackface amps there is one). Should I put one in?

I'm gonna order some parts and try again. (pot resistors new tubes)
I'll try to poste some pix later today.

Thanks again.

Jesse

[tubeswell]

I built one about 6 years ago - had a B+ of 370 or 380 IIRC.

Yes, 370 vdc is the schematic value. I have never seen that in the wild though. What with modern AC line voltages. 😎

I had a custom wound PT that delivered 300-0-300 on the HT winding when loaded.

[jezzbo]

I had a custom wound PT that delivered 300-0-300 on the HT winding when loaded.

Nice. The technical page of the 40-18029 should give 305V - 0V - 305V (on the low voltage tap) So that should be ok not?

And your transformer gives a 370V at the plates?

[Phil_S]

I usually figure about 1.2x using a 5U4. If you are working with 305-0-305, you should get ~366. The rule of thumb calculation isn't perfect. Based on the schematic, I'd look for ~380. It also suggests something is not right if you are only seeing 316VDC.

It's time to post a full voltage chart, including all B+ nodes.

[Leo_Gnardo]

Another question: the schemtic doesn't have a 1500R resistor on the grid.
(in blackface amps there is one). Should I put one in?

Yes, deafinitely, 1K5 grid stoppers, and if there's no screen grid 470R's, put them in too.

In the 1950's you could get a fresh pair of 6V6 for lunch money. Nobody thought much about getting lots of lifetime out of the tubes. Save build cost by leaving 'em out too.

[tubeswell]

Have you got a decent amount of (negative) bias voltage? (About -27 to -29V in the stock circuit - not that its a good idea to go by voltage, but it'll give us an indication about the problem)

[jezzbo]

Thanks everyone for your help.

For now I'm gonna redo the bias supply. (changing 56K resistor to pot + 47k resistor in series.)

Buy JJ 6V6 tubes (I don't want to fry another set of NOS Sylvania 6V6GT/G tubes )

Rewire the PT again.

Put some 1K5 grid resistors on the grids
Check if the 470R screen resistors are still working decent.
(Carbon comps, I don't know if they survived me kooking those 6V6's)

I wil post a full voltage chart, including all B+ nodes.

Thanks again, I will be back.

[jezzbo]

One more question for now.
The jack to connect the tremolo footswitch, should that be a switched jack? (like on the guitar input jacks)

Thanks

[tubeswell]

One more question for now.
The jack to connect the tremolo footswitch, should that be a switched jack? (like on the guitar input jacks)

Thanks

No - not if you want to be able to use the trem with the foot switch cable unplugged

[jezzbo]

No - not if you want to be able to use the trem with the foot switch cable unplugged

Can the tremolo influence the biasing in this amp when there is no switch connected?