General FX400 conversion

[uki]

Hey guys !!

This was scored some time ago and was in the shelf since then. No luck on searching for a schematic, so I did wrote part of the schematic, it is been proved quite difficult to follow the connections due to the layout and clutter in the circuit.

Would you guys kindly give some guidance converting it to an guitar amp?

So far what I have done/verified is:

- the reel machine is separated from the electronic circuit,
- test with multimeter the tubes heater filament continuity and shorts with the other pins, all check good,
- main cap can blown,
- PT and OT seen to be working. PT works when the rectifier is in place, there is B+ after the recto tube. and B+ at the OT both ends of primary side.
- made schem of power supply and power tube, may contain errors,

Would you guys kindly guide in converting it to a guitar amp please

Thanks in advance it is most appreciated !

[uki]

More pictures

The tube on that picture is a magic eye, 6ME5

[uki]

Even more pictures:

Tubes from left to right: 6AR5 , 6AV6 , 6X4 , 6AV6 , 6AU6

[uki]

Yet even more pictures: 

[sluckey]

I would harvest the PT, OT, and tubes. Then button it all back up and set it on the curb. Use the harvested parts to build a nice toaster oven.

[uki]

Well the chassis is no fun,

You mean just use the PT OT and tubes to build an amp out of it?

[sluckey]

You mean just use the PT OT and tubes to build an amp out of it?

Yes. Build a nice toaster oven... 

Like this...  https://sluckeyamps.com/smoky/smoky.htm

[uki]

Oh that is a cool one !!  Well i dont have a toaster as case , dang that is cool !!

The idea is to keep the case, attach that chassis inside of it, the speaker is working too its in the case.

I think this is just the right schematic the parts just fit:

[uki]

 

Got an idea !! By moving the power supply section of the chassis to the other side of the chassis, it may be usable, contributing to a better amp layout.
Well i did it already and now all the power section is all in the same side of the chassis. Rectifier not near the preamp section anymore.

what do you guys think ?

[sluckey]

Good start.

[uki]

Ok i got foundation for the amp now ! well almost.

The OT have some letters on it, anyone know what they mean?  I'm guessing E is for earth that point was connect to ground.

In which socket/position would be best to put the tremolo circuit? Schem above.

[uki]

Hey guys !

Trying to figure out on what socket to put the tremolo, socket 3, as in the picture?

thanks

[sluckey]

yes

[uki]

Hey guys!

Been digging my bin for parts! I got a few questions.

For the power supply resistors I got 8.2k 10w and 27k (5w?)  is the 27k enough?

For the PA tube cathode  470r 10w  ,  will this work?

What would be the values of the tremolo pot and mica cap in the tone stack ?

Thanks

[shooter]

10W is probably overkill for PA's Rk but it's free.
25K should be plenty, unless you're starting with real high B+
I would avoid mica caps unless you got a real good touch n heat sinks for soldering.  They tend not to like heat, a "close-enough" ceramic should do just fine.

[uki]

10W is probably overkill for PA's Rk but it's free.

Maybe that is a typo? Usually its 1W yes?

I would avoid mica caps unless you got a real good touch n heat sinks for soldering.  They tend not to like heat, a "close-enough" ceramic should do just fine.

What about its value, and the tremolo value?

[sluckey]

Mica should be 470pF or 500pF. Trem pot should be 1M to 3M. I'd start with 3M.

[uki]

Some update on this, moving slow but moving !!

The power supply is in place, now starting the preamp section.
I got pics !! 

I have two questions:

1 -  There is a adjustable resistor in place for the artificial center tap for the heaters, it is connected at V3 preamp socket, where is the best place to put the artificial center tap ?

2 - There is some schematics for this amp on web and some others where the diode part of the 6AV6 is connected to ground, what is that for ?

Thanks

[sluckey]

I have two questions:

1 -  There is a adjustable resistor in place for the artificial center tap for the heaters, it is connected at V3 preamp socket, where is the best place to put the artificial center tap ?

2 - There is some schematics for this amp on web and some others where the diode part of the 6AV6 is connected to ground, what is that for ?

There is no magical place to put the artificial canter tap resistors. Just connect to the filament string in any convenient place.

Whenever you see the diodes in a 6AV6 connected to ground or simply not connected to anything, it simply means they are not being used.

[uki]

Update !  Moving slow but moving !!   

Some of the preamp is in place, its been interesting to do a PP circuit, trying to get it as clean and organized as possible!

I got two questions about Preamp and PA tubes cathode:

1 - I don't have a 4k resistor for the cathode on V1, only 3k and 5k, which can be used ?

2 - There is no bypass cap on V2 and PA tubes, can those be added and what value?

[shooter]

I'd try the 5K 1st, looks like it's used for the trem circuit.


you can bypass either or both, but i'd wait til it's working then "tweak".  V2 will have more "gain" bypassed, which might not be needed to drive the PA
I use 47uF on most all SE PA tubes.  on preamp tubes I use 5 - 15uF

[uki]

Update !  Its moving !!!

Got most of it done, missing input jack and speaker jack.

The potentiometer arrangement is just for the test phase, will move it to a panel to be added later.

One question, the OT secondary read 0.6 ohms, is that right? (another OT have same reading)

Got pics ! 

Thanks

[sluckey]

One question, the OT secondary read 0.6 ohms, is that right? (another OT have same reading)

yes

[uki]

Update!

Before moving to the test phase when checking my tubes bin... well I though there was another 6av6 but it is 12av6,
so it leaves me short of one 6av6...

Tubes that came with the device: 6AR5 , 6AV6 , 6X4 , 6AV6 , 6AU6

Question: If the 6AU6 is used for the tremolo besides changing the pins connection, what else need adjust ?


Thanks

[sluckey]

Your schematic calls for a 6AV6 triode for the tremolo. To use a 6AU6 pentode will require a lot of circuit change.

[uki]

That is what i though, searching for 6au6 tremolo, here what i found:

https://el34world.com/Forum/index.php?topic=17008.0

Another though is to use the 6AU6 on V1 or V2. Possible?

Im willing to change the tremolo circuit.

What about the 12av6 ?

What would be the best option? 

Last option to buy a tube.

Thanks

[sluckey]

Im willing to change the circuit.

In that case just build the well known proven circuit from the Silvertone 1482 schematic.

[uki]

In that case just build the well known proven circuit from the Silvertone 1482 schematic.

  Just the tremolo part?

Will the amp work w/o V3 ?

Thanks

[sluckey]

Just the tremolo part?

yes

Will the amp work w/o V3 ?

I don't know. What is V3? Show me a schematic that shows the tube reference designators.

[uki]

What is V3? Show me a schematic that shows the tube reference designators.

Here is the schematic, what I mean is, V3 not plugged. W/o changing anything.

Thanks

[sluckey]

The amp will work just fine with V3 plugged in.

What's all the fuss? A 6AV6 is a very cheap tube.

[uki]

A 6AV6 is a very cheap tube.

I did check the prices and it is affordable, buying one.


Have done some tests today, and got some readings! 

Filament voltages for rectifier and other tubes, both check 7vac no load(except pilot light).

With rectifier tube only:
Hight voltage: A+ 365vdc , B+ 360vdc , C+ 344vdc.
readings done at the filter caps +

The filter caps are discharging somehow after power down, not too long after.
There is no bleeder resistor. Any thoughts on this?

So far all seen ok.

What else need to check ?

Thanks

[sluckey]

With rectifier tube only:
Hight voltage: A+ 365vac , B+ 360vac , C+ 344vac.

Probably meant to say DC volts?

The filter caps are discharging somehow after power down, not too long after.
There is no bleeder resistor. Any thoughts on this?

Probably need some fresh filter caps. That's not unusual.

What else need to check ?

Plug in. Play it like you stole it!   

[uki]

Probably meant to say DC volts?

Ooopss  Yeah DC   

Probably need some fresh filter caps. That's not unusual.

Those caps were taken from LED bulbs   

Plug in. Play it like you stole it!   

  Alright !! Well I got to wait for the 3rd 6av6 to arrive.

I got some pics !! 

[sluckey]

Alright !! Well I got to wait for the 3rd 6av6 to arrive.

You can play that amp without the tremolo tube. What you really need is some new filter caps.

[uki]

Almost there...

Plugged the tubes, v1, v2, PA

voltages are dropping a lot, probably due to those filter caps, output is rather weak.
no bad noises or hum 

voltages:
A+ 163vdc
B+ 149vdc
C+ 138vdc

is it cos of those filter caps?
Did the readings with the light bulb(60w) limiter plugged, it stayed on all time.

Thanks

[sluckey]

Low voltage is probably due in part because of bad filter caps and also due to the lamp bulb limiter. How bright is your lamp? Faint glow is 1? Full brightness is 10? Somewhere in between (number between 1 and 10)? I would be concerned with anything over 5 (half brightness). Replace filter caps and repeat voltage checks. If no smoke, plug amp directly into the wall outlet and repeat voltage checks.

[uki]

I'll have new filter caps on saturday maybe monday, and the 3rd 6av6 also on the way. 

I got some pics of the bulb limiter

one directly through the wall outlet full bright for reference.

one when the amp is cold and right when turned on, very faint

and the other with the amp working at full volume. i would say something around 3-4 from 1 to 10 ?

Thanks

[sluckey]

Nothing to worry about. Plug the amp straight into the wall. Measure voltages. Evaluate with guitar.

[uki]

Amp is working alright!!!
Played for about 30 minutes
well need some tweaking

Here a sample:

Note the camera is really close to the speaker, sounds louder than real.

Here is the voltages all i could think about without the limiter:

A+ 310 vdc
B+ 267 vdc
C+ 240 vdc

V1 pin 7 - 90 vdc   is this right?
     pin 2 - 9 vdc
     pin 1 - 0

V2 pin 7 - 191 vdc
     pin 2 - 1.6 vdc
     pin 1 - 0 v

V3 ping 7 - 234 vdc  no tube installed


V4 PA  pin 1 - 0 v
            pin 2 - 17.4 vdc
            pin 5 - 297 vdc
            pin 6 - 265 vdc

The amp start to overdrive at about 6-7 , the range between 6-8 give a nice overdrive,
8 and beyond sounds too distorted, i think the speaker doesnt help much,

Got pics of the speakers   ,  the small one is the original from the device it sounds terrible,
gladly i got another/bigger one with same impedance

the tone control to the treble side gets too much high frequencies
the bass side only start to work at about 8 gets really muffled at 10.

Overall amp sounds ok , no noise no hum. Output sounds a bit low still.

Thanks !!

[uki]

Update !!!

New tube arrived, did recheck the tremolo circuit, also the tube pins, all check good.

When powering the amp to test noticed a smell, so turned off right the way,
checked all nothing burned, everything ok,
but power tube and rectifier tube going super hot, burning finger like, no blisters gladly 
So it was probably the label painting that smells. But the temperature doesn't looks normal...  What to do about it ?  A+ too high?
(maybe related to those filter caps) new ones did not arrive yet.

Without tremolo tube i didn't notice anything like that.

Amp is working, tremolo is working, there is a very small hum now with all tubes on,
with volume at zero, but only noticeable when close to the speaker.

Voltage readings with all tubes:

A+ 310 vdc
B+ 266 vdc
C+ 238 vdc

V1 pin 1 - 0 v
     pin 2 - 1.15 vdc
     pin 7 - 90 vdc
     
V2 pin 1 - 0 v
     pin 2 - 1.6 vdc
     pin 2 - 190 vdc

V3 pin 1 - 0 v
     pin 2 - 2.8  vdc   
     pin 7 - 221 vdc

V4 PA  pin 1 - 0 v
            pin 2 - 17.4 vdc
            pin 5 - 298 vdc
            pin 6 - 266 vdc



Thanks

[uki]

Update !! 

Got some pics !! 

....
you can bypass either or both, but i'd wait til it's working then "tweak".  V2 will have more "gain" bypassed, which might not be needed to drive the PA
I use 47uF on most all SE PA tubes.  on preamp tubes I use 5 - 15uF

I could say its the "tweak" part of the amp now,

A 47uF/25v been installed as cathode bypass PA tube, and the tone did improve a good amount.
Also a different speaker with 8ohms is in place and it give a better tone.

The amp still too wild thou, driving with volume at 6 and very mad at 8 so I tried a negative feedback loop like in the fender champ,
from OT secondary to V2 cathode with 22k resistor in the middle, but the only thing it did was add hum.

I got two questions:

1 -  How to do a proper NFB for this amp to tame it ?

2 - Also i notice there is no ground on the OT secondary in the schematic is that right?

Thanks

[sluckey]

1 -  How to do a proper NFB for this amp to tame it ?

2 - Also i notice there is no ground on the OT secondary in the schematic is that right?

What you did will work but you must ground one side of the OT secondary.

[shooter]

The amp still too wild thou

the pa tube is "easy" to drive, so there's not a lot that you can do to "tame" it.
hunt up someone with a 4 X12 cab, you might like the "un-tamed" results 

[uki]

I've been thinking about the voltages and bias on the PA tube

the 6ar5 tube datasheet says 250 max plate voltage and 8.5 watts plate dissipation,
right now it is way above 310 ish plate voltage and 10.x watts plate dissipation...

I found this power supply from Shooter, is it possible to add another node on the amp power supply to adjust the voltages?
If so what the values for the cap and resistor ?

Thanks

[shooter]

is "D" for PA or pre?
IF it's for the PA, You might be better off using B as the PA plate if the V-drop gets you closer.  Use B for both plate and screen, except add a screen limiting R (470ohm) between "B" and the pin for G2

[uki]

D would be for the pre, i would leave A disconnected and start with B.  B for plate. C for screen,
 would it work or better just use B as A and B ? How to drop even more B voltage ?

Thanks

[shooter]

use a couple gator-clips to "adjust" the A-B dropping R til you're happy, so more R = less plate VDC.  Then solder in.
 SE amps are known to be run HOT at idle (no signal).  I typically "set" dissipation somewhere between 90 & 110% of MAX 
so 10W is close-ish.


here's the "quick-fix" test to see if it gets you closer. 

[uki]

Have done the suggestion above, with B node for A and B, ... the B node drops to about 160ish v , and the AB resistor does heat up a good amount, also added the a 470 ohms for screen(PA pin 6),

then went back using A node with AB R of 15k, and keep the screen R,

here are the readings:

A+ 311v at node, 300v at Pin5 PA
B+ 242v at node; after the screen R, 240v at Pin6 PA
C+ 217v

PA tube pin 2 15.54v  the cathode resistor read 464 ohms

node A still too high, how to drop A voltage ? then can readjust B

Thanks

[AlNewman]

I would put a dropping resistor between the rectifier and node A.

So if you are at 300V, and you want to be at 250V@8.5W...

8.5/250= .034 A

+ some for screens and preamp.  So let's say .045 A max.

50V/.045= 1111R

.045 * 50V = 2.25 W

I've found that less resistance is normally required compared to the "math", or at least my version of it.  So I would order a selection of 5W resistors between say, 470-1500R.  You gotta pay shipping anyways, and 5W resistors are like $1.50/piece.

So, $10 plus shipping and you can dial it in where you like it.