EL34 tubes with 540v screen voltage

[chocopower]

Hi there.


I get this old amp from 70´s, made in Barcelona (Spain)
Is a Matamp/Orange clone.


Amp came with a Mullard set and after check the usual stuff i measured some voltages.


Plate: 550v
Screen: 540v.
Heaters: 6.47v.


In 70´s we had 220vac in Spain and now 230-240 is usual.


Could be a problem the screen voltage when i install modern EL34 tubes?
If so, there is any easy/cheap solution?


Pics from a identical model:
http://mrmoustache.es/wp-content/uploads/2015/09/Amplif_CMB.png

[kagliostro]

Nice amp !


....


220v vs 230-240v less than 10% difference


nothing that you can't solve with resistors or a pair of  counterphase diodes, may be you are fine also as is


Franco

[chocopower]

nothing that you can't solve with resistors or a pair of  counterphase diodes, may be you are fine also as is
Franco

I´m worry about the screen voltage for the EL34s
I cant find any modern one who handle more than 450v.


As you said, heater voltage is easy, but for screen regulation is needed something more elaborated, Maybe zener diodes or VVR?

[kagliostro]

Now I'm on telephone, later I'll post a pair of schematics

---------

Here I'm

This is a circuit that PRR draw for my 1625 SE where B+ was around 380V and G2 Max permitted value 250V





(I had a PT plan change so B+ is now around 350v)

This is a different approach with the use of a VVR to feed G2 (a resistor set the Max G2 value independently from the pot rotation)



Using a VVR on G2 give you also control on the power of the amp (like a VVR on plates, but working on G2 voltage), not only tame G2 Voltage

the SAG resistor + Switch ....... well that is one other story .......

if you have no interest on power control or you have no space for an additional pot on faceplate, use a trimmer, set it for the right voltage and discard it (inside the amp)

I hope this can be of your interest

Franco


I forgot to say:

That mosfet is rated till 900V, so, no voltage problems

but I think you can use that circuit for power control only if the amp is a cathode bias, NOT if it is a fixed bias

however, as fixed voltage reducer (with a trimmer) for G2 you can use it safely

[chocopower]

Ok. I found this:


https://www.tube-town.net/ttstore/Kits/Power-Supply/Kit-TT-VoCoM-Variable-Voltage-Regulator::5651.html

https://www.tube-town.net/cms/?DIY/Amp-Tools/VoCo_Variable_Voltage_Control

Is cheap and save a lot of time with the layout.
Did you think is ok for my needs?

[chocopower]

Upss. I found you talked about that kit here in the forum some time ago.
Most of you seems to have some concern about putting 600v on the pot.
My idea is upgrade the pot to a pair of fixed resistors once i find the sweetspot (about 450v).

[kagliostro]

Yes, you can use resistors, or a good quality pot

Use a mosfet that can afford a high voltage, the kit TubeTown sell didn't seem to use a High Voltage Mosfet

you can buy the kit and swap to a better mosfet

I've a turret layout for the VVR using 2W metal film resistors (and it is very small)



The blue on turret are the point to wich is to be connected the pot

I've also a schematic that is a bit more complicated but allow the use of a standard pot, the circuit is planned for that purpose



you don't need the part of circuit that is rounded in blue

Franco

[terminalgs]

In 70´s we had 220vac in Spain and now 230-240 is usual.

240V:12V  bucking transformer out front of the mains?  seems cheaper and less invasive.

[Ugly Distortion]

http://el34world.com/Forum/index.php?topic=19816.0

Maybe this?

[Ritchie200]

I'm sure you already checked this but are there any other mains taps, like (hopefully) a 240/245v?  If they shipped this amp international, you would think they would have this capability?

Jim

[chocopower]

In 70´s we had 220vac in Spain and now 230-240 is usual.

240V:12V  bucking transformer out front of the mains?  seems cheaper and less invasive.

That don´t resolve the problem with the screens.

[chocopower]

I'm sure you already checked this but are there any other mains taps, like (hopefully) a 240/245v?  If they shipped this amp international, you would think they would have this capability?

Jim

There is a un-marked wire coming side by side with the used primary. It finish in a strip eyelet with no connection. I assumed is the 110vac tap, as it was the usual those days in Spain (1970s).
But as i said to Terminalgs, it dont fix the high voltage screen problem. Maybe just a few volts.

[chocopower]

http://el34world.com/Forum/index.php?topic=19816.0

Maybe this?

My amp used a voltage doubler as old Oranges.


Is safe use the middle of the voltage doubler as supply for the screens, as is shown in you link?
The only problem is that voltage will be bellow 300vdc....  maybe too low?

[kagliostro]

You can use the 300v coming from the doubler, but with such a low voltage you lost a lot of power

--

from this thread

http://el34world.com/Forum/index.php?topic=21359.msg226528#msg226528



as G2 current consumption is small, you can use a small transformer backward connecting the secondary to the filament winding of your PT

assuming your heater winding is at 6.3v as to have around 550v you need a 220v/6v transformer

connect the 6v winding to your PT 6.3v and you have around 230v on the other winding, 230v rectified and doubled is around 550v

---

one other (simple) way to tame voltage is to use a 220-230v / 10-15v transfromer to book the line voltage and feed the whole amp using this lower voltage





Franco

[chocopower]

The 10-20 volts difererence in the wall supply is there, but fix that with a bucking transformer dont gonna put screens in less than 500vdc


I like the supply for screens using a  backwards transformer solution
But i'll need a 220v/9v transformer. I'm looking for 450vdc in screens.


12.6v into 9v secondary give me 308vac in the 220v primary.
And after rect.
308vac x 1.4141= 435vdc.


Could it work?

[kagliostro]

Never put a higher voltage on a winding, only a very small difference can be allowed and it is bad practice, transformer will heat abnormally easily

so, no 12.6v on a 9v winding, OK for 9v on a 12.6v winding

do you have a 12.6v heater winding ? Use a 15v / 220v transformer, with 12.6v on a 15v / 220v you obtain around 185v AC

double it and you have around 440v DC

Remember that you require a 15v / 220v transformer able to give more current than the current required by G2

quadruple the G2 current request and you obtain (around) the value of AC current that your 15v / 220v transformer must be able to afford

Franco


p.s.: Which are the voltages of your PT ?

[chocopower]

p.s.: Which are the voltages of your PT ?

Secondaries you mean?
my heater is 6.3v  with a trim-pot center tap
I made a mistake in my calculations using the 12.6v value.
So:
using a 220/9v  transformer.


6.3v  into  9v sec.  is  154v in prim.


rectified and doubled is:  154 x 2 x 1.414 = 431v


is that ok?



"assuming your heater winding is at 6.3v as to have around 550v you need a 220v/6v transformerconnect the 6v winding to your PT 6.3v and you have around 230v on the other winding, 230v rectified and doubled is around 550v"

i dont get the maths.  "230v rectified and doubled is around 550v"
my calculations tell me that is  648v¿?¿¿

[kagliostro]

Secondaries you mean?

I was asking about all the voltages the PT has

--

220 : 6 = 36.6666

36.6666 x 6.3 = 230

230 x 2.4 = 552

--

to me

220 : 9 = 24.4444

24.4444 x 6.3 = 154

154 x 2.4 = 370

--

you require a 7.5v / 220v transformer (may be not so easy to be find)

220 : 7.5 = 29.3333

29.3333 x 6.3 = 185

185 x 2.4 = 443

Franco

[chocopower]

ok.

So the 2.4 multipler is for a voltage rectified and doubled.

I was using  2 x 1.414 = 2.828

my bad.




PT have 220v and 110v  primaries. Here i´m just guessing because the "220v" one is showing 6.5v volts on heaters. i have 230vac in my home.
The other prim. is unlabeled, but measuring with a DMM give me 1.1ohm and the other measure 2.7ohm.... so i guess they are 220 and 110...

Sec. are:

.-High voltage with center tap to voltage doubler.
.-Heaters for preamp tubes with 500ohm trimmer for ground reference.
.-Heaters for EL34 with center tap
.-Bias

i´m gonna draw an schematic...

update:  The first B+ node after the rectifier is unused. The OT center tap and screens share the connection after the choke.





As far for the backwards transformer a 220/9v  could work.

220 : 9 = 24.444424.4444 x 6.3 = 154154 x 2.4 = 370
with the real 6.5volts i have in my preamp heaters it will be 379vdc.
And look at this:
http://www.orangefieldguide.com/OFG_SCHEM/AD140TCmainboard.pdf
http://www.orangefieldguide.com/OFG_SCHEM/AD140TCmainboard2.pdf

Right now i have a transformer with 220v prim.  and  9-0-9 sec.  and 0,6A labeled.
Could I use half of the secondary?



or.... maybe this kit.

http://www.londonpower.com/high-voltage-clamp-kit

[pompeiisneaks]

Couldn't you also convert the full wave bridged rectifier which does about 1.4 x input to a full wave rectifier and get a little less starting B+?  Usually about .9 I think.  I.e. x* 1.4 * 2 = 600

solve for x: x= 600/(1.4 * 2) = ~215V. 

Change that to the other rectifier

215 * (.9 * 2) = 385V... or is that now too low?
I hope my math makes sense and is right. 

[kagliostro]

With a 9v - 0 - 9v transformer the problem is you lost half of the current abilities

(and is not possible to parallel windings, to parallel it you must have 2 x 0 - 9v windings, not 1 x 9v - 0 - 9v winding)

is each winding labeled 0.6A ?

assuming each winding is 0.6A (AC) you can think

AC to DC current ratio equal to 600mA x 0.6 = 360mA

a bit of loss on the transformer (we haven't exact data about) so say -60mA

360mA - 60mA = 300mA

50% current loss in the doubler

300mA : 2 = 150mA

150mA x 9v = 1.35VA

1.35VA : 220v (or we must use 180v ??) = 6mA (or 7.5mA)

that is enough for G2 of a near 10W amp, not your amp

--

if you can say which G2 current is required, we can estimate the VA rating of the transformer you need

--

BTW, you still didn't say which are the High Votages of the secondary of your transformer   

---

Seems that the simpler solution is the use of a VVR for G2, don't you ?

Franco


p.s.: I forgot, I think that stabilized voltage on G2 on guitar amps is detrimental, may be good in HiFi (may be ??) but not in guitar amp


I've a schematic of a stabilized VVR that I helped a friend to plan, but I think that it isn't of your interest

[chocopower]

Im gonna measure some voltages and come back.


Thanks a lot, everybody!

[pompeiisneaks]

I double checked, I was right on FWB it's 1.4 into capacitor, but the value of full wave is just .71... so an even bigger drop, maybe not what you'd want...

[chocopower]

I double checked, I was right on FWB it's 1.4 into capacitor, but the value of full wave is just .71... so an even bigger drop, maybe not what you'd want...

i have been thinking about it.
If i disconnect the center tap from capacitor stack it will work as a standard full wave bridge. The problem is the voltage will be bellow 300vdc.....


here are some measurements.


As said, node A is "unused".  OT center tap and screens take voltage from node B.

[kagliostro]

The problem is the voltage will be bellow 300vdc.....

No, to me the voltage will rise

207v + 207v = 414v

414v x 1.4 = 579v

--

Seems you must do something for the capacitors of node C - D - E  -- 450v in that posistion is not very safe

Franco

[chocopower]

The problem is the voltage will be bellow 300vdc.....

No, to me the voltage will rise

207v + 207v = 414v

414v x 1.4 = 579v

mmmm... Them, whats the reason for the voltage doubler?¿


Those voltages are measured with all tubes and center tap connected to the voltage doubler. Is not an unload measurement.

Seems you must do something for the capacitors of node C - D - E 450v in that posistion is not very safe

yep...  you are right!   


is strange to me that the builder used those 450v caps in that position. Everything else is oversized!


Maybe the diodes in the bridge are the reason for all my problems?

[kagliostro]

.......... whats the reason for the voltage doubler?

If there is nothing connected to the CT node, I really don't know ! 

This Fender is similar but there is a reason for that arrangement, a 100W / 25W switch



Franco

[chocopower]

i "think" the correct math is:


207 x 1.4 =  289v.


In fact, this is the voltage in the middle of the stacked e-caps.

[pompeiisneaks]

i "think" the correct math is:


207 x 1.4 =  289v.


In fact, this is the voltage in the middle of the stacked e-caps.

Yes but if you want to use a full wave rectifier instead of a full wave bridged rectifier, you'll see lower voltages.  207 * .71 = 147 then through the voltage doubler that would be 294 about.   That may not be what you're wanting, too low. 

[chocopower]

I have the feeling maths are working agaisnt me

[DummyLoad]

install 2 x kt88/6550 tap the ~300V CT for kt88/6550 g2 B+. tweak bias supply for kt88/6550. done.


--pete

[sluckey]

I think you worry too much.There's nothing wrong with the math. That 207-0-207v secondary applied to a FWB will produce 585vdc unloaded. There's nothing wrong with the circuit. It's a simple full wave bridge, not a doubler. The center tap is connected to the junction of the stacked reservoir caps to force the voltage to split equally across them.

If the amp was designed for EL34s, then use them. Don't worry about what some tube data book says about maximum screen voltage. Guitar amp designers have been abusing those data sheets forever! If you still don't feel good about running the screens above 300v then the easiest thing to do would be power the screens from the junction of those first reservoir caps. You'll have ≈290vdc unloaded. That's pretty close to 300v.

You have been presented with several cheap alternatives to set the screen voltage lower. Someone even suggested a bucking transformer. That will compensate for the increase in line voltage. That's probably all I would do.

[chocopower]

Thanks.
What i learn is that i need dig more in power supply theory.
All this time thinking that i have a voltage doubler in my amp...


I'll post the whole schematic soon.


Thanks everybody, again

[trobbins]

David,

You didn't detail the output stage circuitry of your amp, only the power supply. 

The assumption is that a quad would be fixed bias.  There are often dropper and stopper resistors in the screen circuit, especially for quads.  Also you didn't indicate if the measured voltages were with the output stage operating at nominal idle power dissipation (and what that level was for each EL34), and if the 'screen voltage' was at the power supply or on the actual screen when in idle.  The PP stage output transformer impedance (assuming you are using matched speaker impedance for the transformer) also indicates, when looking at loadline, how much the amp will push in to overdrive and how heavily the screen current may rise (and hence the screen voltage may sag).

[chocopower]

Hi there,


I opened a new topic with a more defined subject:


http://el34world.com/Forum/index.php?topic=21833.0