Fender Bandmaster (clone) 5E7 problem

[jezzbo]

Hi all,

I'm (re)building a 5E7 clone for a friend.
It's nearly finished but I'm getting difficulties.

This is what i've done.

1. new PT (tubeampdoctor)
2. new OT, Heyboer OT for bassman. secondary = 2Ohm, 4Ohm, 8Ohm. (8Ohm is not connected)
3. Made a new turretboard with (mainly) Allen Bradley 1W resistors and sozo blue caps.
4. Changed the biasing to blackface specs. (following the Ceriatone layout)
5. Installed a switch to select 2Ohm and 3 speakers or 4Ohm 2 speakers.
At this moment I disconnected it and wired only the 4Ohm wire (green one) to the output. (because I thought maybe this could be the problem.

I'm using a bulb current limiter (with 60W bulb, the only one I have at the moment)

The biaspot is turnet to the most negative value. (measured at the 22OK resitors that go to the powertubes)

This is how I tested (with the bulb current limiter) and what goes wrong.

1. Amp on no tubes. -> No short
2. Amp on rectifier installed (tested with 5U4G and GZ34). ->no short
3. Amp on rectifier installed + standby on. -> no short
4. Amp on rectifier installed + standby on + preamptubes installed -> no short.
5. Amp on rectifier installed + standby on + preamptubes installed + powertubes installed.-> bulb goes bright and stays bright (tested for a few seconds) so a short right?

The problem is that I just can't seem to find it.

I am gonna try to upload some pix tomorrow.

Anybody that can help me please?

Thank you.

Jesse

[dcribbs1412]

Hi Jesse
Is it blowing fuses?

Darin

[jezzbo]

Thank you for your reply.

No, it doesn't blow fuses. (there is a 3.15A 250V fuse in it)
(It's 230V in Belgium)
The PT's primary is wired for 230V.

[dcribbs1412]

Thank you for your reply.

No, it doesn't blow fuses. (there is a 3.15A 250V fuse in it)
(It's 230V in Belgium)
The PT's primary is wired for 230V.

Hi Jesse
if I remember correctly, using a 60 watt bulb there was not a lot of difference in brightness when you hit the standby...
how are the voltages on the power tubes?...they will be lower.
Double check all the power tube pin outs for shorts or small pieces of wires.
I started using a flashlight. My vision sucks.
Do you have other tubes you can try?

Darin

[jezzbo]

Hi Jesse
if I remember correctly, using a 60 watt bulb there was not a lot of difference in brightness when you hit the standby...
how are the voltages on the power tubes?...they will be lower.
Double check all the power tube pin outs for shorts or small pieces of wires.
I started using a flashlight. My vision sucks.
Do you have other tubes you can try?

Darin

Hi Darin,

About the 60W bulb, does this only apply when switching the standby switch on?
Because when I switch only the powerswitch on the bulb goes bright and than gets an orange hue. (as it should).

I allready tested with other (new) outputtubes.
same result.

I'm gonna check all the power tube pin outs for shorts or small pieces of wires. (Done that before, but it never hurts to check again)
Good idea about the flashlight.

thank, Jesse

[amplifiednation]

What is the problem that you are trying to hunt down?? Sound?? Can you post some pics of the amp? How do the voltages look? Did you confirm any of them yet??

[Mark]

You seemed to believe the problem is in the output stage and that seems like a fair assumption.

I would encourage you to try your light bulb current limiter with a known good amp. If the bulb is bright on turn on and then goes dull after 5 to 10 second I'd assume normal operation. (Though these are my observations with a 100 watt bulb.)

If the bulb stays bright for at least a minute then you have a short.

The next thing to do it leave the amp on and measure the current going through the output tubes. A 1 ohm resistor on the each cathode of the output tubes is a good thing.

Keep us posted.

[martin manning]

5. Amp on rectifier installed + standby on + preamptubes installed + powertubes installed.-> bulb goes bright and stays bright (tested for a few seconds) so a short right?

If everything looks good until you install the power tubes then I think the only possibility would be an error in the socket wiring. Measure the bias voltage (standby off) just to make sure you have it set to the most negative position.

How bright is the bulb with all the tubes installed? When you turn on the standby switch does it go bright and then get just a bit dimmer (filters charging)? It could be that there is enough current draw at idle with all tubes installed to keep the bulb near full brightness. How about you connect your voltmeter to the OT center tap/choke output (at B+) and try the bulb test again (all tubes in) while watchung the voltage reading. If there is a short to ground in that part of the power supply it will drop to ~zero.

[Structo]

I agree, double check the current draw at the power tubes.

You do have a speaker load connected when power tubes are installed, right?

Since the Standby switch is before the large filter caps, they don't charge until you turn Standby to the play position.

I think with a 60w bulb you are drawing a lot of current through it with the tubes installed.

Double check wiring then I would be tempted to just plug the amp straight into the wall while monitoring the bias and watching the power tubes for signs of red plating.

If that looks good, plug in a guitar and play!

[jezzbo]

All of you, thank you very much for your replies and help!

I'm gonna try the things you guys mentioned.

Today I found a 100W bulb (they are getting scares arround here.)
The light goes different now.
I filmed it. http://youtu.be/dCj09QraE4g

So is tis normal behaviour?

Next I'm gonna: - doublecheck the powertube sockets.
- meassure the currentdraw of the outputtubes.
- meassure plate voltage
- bias the amp.

Thanks again

Jesse

[jezzbo]

Ok, this is what I've done so far.

1. Tested bulb currentdraw limiter (100W) with known good amp.
(bulb flashes on en goes dim immediately.
So it works.

2. B+ = 255V (schematic says 410V ! )

Powertubes are GE 5881 (max disipation = 23W)
Looking for 60% disipation at idle (13,8W)

3. Plate voltage powertubes = 251V
4a. Current draw left powertube = (with bias most positive setting) 44mA
Could not go any higher. (should be 54mA @ Up of 251V)

4b. Current draw right powertube = (with bias most positive setting) 20mA
Could not go any higher. (should be 54mA @ Up of 251V)

5. Switched tubes. now the tube with low currentdraw is in the other socket.

I'm gonna test with a pair of 6L6GC's (and with GZ34 rectifier because I'm not sure if the 5U4G can supply enough current)
--------

Another thing I noticed: On the original layout the grid resistor on th epowertubes (1K5) goes from pin6 to 5.
In the ceriatone 5E7 layout it goes from pin5 to 6 (so reversed)
Is screengrid on a 6L6 (5881) not on pin 5?
So wha's right?

Thanks, Jesse

[jezzbo]

ok meassured with 6L6GC's and GZ34 rectifier.

B+ = 221V
Both tubes : Plate voltage = 220V

Left tube current draw= 34mA
Right tube = 42 mA

Both tubes should draw 42mA (according to markings on the tubebase. tubes are new from tubeampdoctor)

One more thing that I noticed is if I switch on the standby switch it gives a lot click.
Also when switching on standby, the pilotlight goes more dim. (not much)

[Tillydog]

Hi Jesse,

Is this the layout that you are following?

[img876]http://diy.suregork.com/wp-content/uplo ... 311207.jpg[/img]

If so, I think there is an error on pin 5/6. Pin 6 isn't connected to the valve, so it is only being used as a convenient place to mount the grid stopper - the signal from the PI should go to pin 6 IMHO, not pin 5 as shown. I doubt this is your problem, though! (Unless there are also other errors).

In your first post, you say that the 8 ohm tap isn't connected - what have you done with the NFB connection? As a long-shot, it might be worth disconnecting the NFB and seeing if the amp behaves.

The pilot light dims because of the voltage drop across the light bulb limiter when the amp draws current, and isn't a problem itself.

You will only get the full bias current when you have the full B+ - the bias voltage range will have been designed (roughly) to match the expected B+. Try to power the amp up with the bias most negative, if the bulb goes ot, then you know you can take the limiter out of the circuit and adjust the bias.

What are the other voltages in the the amp?

Gut shots??

Andy

[Structo]

Are those voltages with the bulb limiter in circuit?

If so, then take the limiter out of the circuit and measure your B+ again.

Tillydog is correct, the wire from the 220K should attach to pin 6.

You should probably also install screen resistors (470R) to make power tubes last longer.

If after this the voltage at pin 3 is still ~255vdc then you have a problem with your power supply.

What is the secondary voltage of the PT? ( ac voltage that is)

[jezzbo]

Hi everyone,

@Andy (Tillydog),

1. yes that's the layout, but only for the biasing circuit.
I used mostly the original layout. So the signal from PI is connected to pin 6.

2. 8Ohm is not connected (4Ohm only at this moment)
The NFB is connected to 4Ohm jack.
I have not yet disconnected the NFB. (Maybe I should?)

3. Even with bias fully negative, the bulb doesn't go out.
This can be seen in the film. (http://youtu.be/dCj09QraE4g)

@ everyone:
Gutshot -> http://users.telenet.be/jezzbro/5E7%20gutshots/
I know these aren't the best shots. (I'll try to make new ones this weekend)

@ Tom (structo)

1. All voltages meassured with bulb limiter in circuit.
2. I'm a bit afraid to turn the amp on without the bubl limiter.
(I don't wanna blow things up.)

3. Good idea about the screen resistor.

4. PT secondary voltage:
a. meassured between the two poles(? sorry I'm n,ot sure this is english)
= with standby off = 667V.
= with standby ON = 442V

b. meassured one side (and than the other side) to ground:
= with standby off = 334V.
= with standby ON = 220V

Thanks again for your help.