6X4 with EL95 PP

[spamer]

Hi,

My current project is scavenging the iron from a console stereo that used 2 ELL80's in the amplifier. My online reading has revealed that 2 EL95's
are a direct replacement into that circuit. I would like to copy the circuit and build the EL95 version but I would like to replace the SS rectifier with tube rectification.
The power transformer only has 6V heater supply so I would like to
go with that variety. I assume I have enough power since it powered
6 other tubes in the radio and preamp circuit.
My question is will 1 6X4 work for 4 EL95's ? If I am reading the data sheets correctly it looks like each EL95 needs 20 ma and the 6X4 will output approx 70 ma which would not be enough, correct? If it cant handle both sides, are there other inexpensive choices or is it ok to run one 6X4 per side?
Apologies for any ignorant questions but I will use a quote I read on this forum such that I am a solder by number guy and dont pretend to understand more than I do.
Thanks for any help.

Steve

[DeathRex]

If you use only one 6X4, the tube will have reduced life and the B+ will be alot lower.

You can run one per side, it will make for a more complicated power supply, but you will have better left-right seperation.

Other small rectifiers: 6V4 6.3V @ 0.6A 90ma $10.00; 1 will be fine, but $10
6BW4 6.3V @0.9A 100ma $3.00; 1 will be fine but more filament power required.

The transformer would have to come from a German radio, and with diodes in a bridge configuration, so you'll have to hook up the tube, and 2 diodes to make the bridge. You'll still have the slow tube rectifier ramp up, but with slightly lower B+ voltage.

Probable filament current of your transformer: 2 X ELL80 1.1A, EM84 0.2A, EABC80 0.45A, ECC83 0.3A, EF89 0.2A, ECH81 0.3A, ECC85 0.435 = 2.975, stay under this.

[kheper]

By converting to tube rectification, your B+ will drop 20-25V. Some possible subs for the 6X4:

6BX4 = 90ma

6AV4 = 90ma

6CA4 / EZ81 = 150ma

The above have a current draw of 1A, not .6A as with the 6X4. There are new production 6CA4 / EZ81 tubes. You could probably find the other 2 on ebay for cheap.

[soundbrigade]

Have you seen Frank Nachbaurs paraphase-amp? Not 6X4 but 6X8.



http://www.dogstar.dantimax.dk/tubestuf/paraph.htm

[SVinzant]

It looks like the link to Fred's pages doesn't work at the moment.
If they don't come back up later this afternoon, I will put a a mirror link of his stuff and post the URL here.
Scott

[SVinzant]

Look's like I jumped in here too soon, it appears to be back up now....

[EWBrown]

There's always the octal based 6AX5, it is heftier than the 6X4 and 6X5 rectifiers.

Those 1960s era German radios generally used a full wave SS bridge, so a tube rectifier would also need two SS rectifier diodes in the negative side to make it a hybrid full wave bridge., with the tube rectifier serving as the positive half of the FWB.

6AX5 is the same rectifier as used in the GetSetGo, and it works very well with two 6B4Gs, at about 56 mA each, plus a couple of mA for the 6SL7 plates, so it should be able to handle teh EL95s and the small signal tubes' plate current requirements.

Those ELL80s are hard to find, and expensive to buy, I have one NOS/NIB which I found at a flea market in a large box of mixed NOS and pulled tubes. I also have a radio which uses this tube, so it was a fortuitous find for me

Another related, and even more exotic, tube is the ECLL800, which has an ELL80 and a low mu phase-inverter triode all inside the same 9 pin envelope,

/ed B

[spamer]

Thanks for the responses everyone. I have been reading as much as I can.
In the process you guys helped me learn what a Hybrid Graetz Bridge is.

To try and estimate transformers heater capacity, I dug out the old tubes from the radio and it had
3 x ECC83, 2 X ELL80, ECC85, EBF89, ECH81 and a tuning eye (EM84?)
I think this amounts to
(3 x 300)+(2 x 550)+435+300+300+210 = 3.25A.

The EL95 version would be 4 x EL95, 2 x ECC83, 1 x (EZ81 or 6AX5)
(4 x 200)+(2 x 300) + 1.2 = 2.6

So I think I am ok in the heater department.

The thing I am worrying about is the comment regarding the voltage drop.
I am not sure that it can handle that. Any comments in that area?
That sounds like a stopper to me.

Before I dismantled it I measured a B+ of 200V.

The power supply was a SS diode bridge + C+R1+C+R2+C where C is 50uF and R1=100, and R2 is 1K. The B+ for the output xfmr was taken off of R1.

Maybe I'll build it with SS to begin with and if it works I'll experiment with the tube rectifier.

This is essentially a low budget project for me where I can try my hand at hand wiring and hopefully learn something.

Thanks again for all of the responses.

Steve

[DeathRex]

If it's the radio's original bridge rectifier, it's probably a selenium rectifier. Selenium recifiers themselves will drop your B+. Usually if you replace a selenium recifier with silicone you have to add a 50 to 100 ohm resistor to keep your B+ low enough. 200VDC B+ seems really low. Disconnect your high voltage secondary on the power transformer and see how much AC you have there. German radios from the 50s - 60s usually have a 240VAC secondary, that made more like 270-300VDC. You might have had leaky capacitors around the ELL80s, leaky electrolytics, or the selenium rectifier was going bad.

Selenium rectifiers are either a long tube, black, about 1 to 2 cm in diameter and 5-7 cm long. Or a flat pack, about 5 X 4 cm with leads coming out of the ends. You can open them up and have pieces of senium fly everywhere.

You can also replace R1 with a choke to get more and better filtered B+. Maybe a 0.5 to 1 H will do.

[DeathRex]

One schematic I found that matches your tubes is a Grundig Majestic MS50U/S. It says it has 290VDC at 125ma.

[spamer]

Yes on the flat pack rectifier.

The transformer has multiple settings for the input voltage.
Disconnected,
and input set to 110/117 I get 250VAC and 7.25VAC on the heater line.

When the input is set to 127 I get 229VAC and 6.64.

[DeathRex]

I set all of mine to 125 or 127, 110 makes the filament too high. So you should be able to get +260-270 out of it. With a tube and 2 diodes you'll have to keep the first cap equal to or lower than the tube's max.
6V4 max 50uf
EZ81 - 50uf
6X4 - 40uf
6BW4 - 40uf
Couldn't find anything on the 6BX4 or 6AV4.

[spamer]

Thank you for all of your help. Now all I have to do is build it!

Steve

[spamer]

Finally getting closer to finishing this project.

The original capacitor used in this system was a 3 x 50uf multi section can.
I cant seem to find a replacement with that same configuration.
I dont have much chassis real estate so I need/want to stay with a single
item.
Is it ok to use a 40uf capacitor? CE Manufactoring makes a 4 x 40uf
that would fit but I want to make sure that it will work.

Thanks,
Steve

[spamer]

I was able to build half of the amp and was happy that it was actually making music but it has a pretty loud hum to it.

I am looking for some advice on how to chase this down.
I tried to use a common ground and twist the heater wiring.

Here are some links to the amp:
http://www.flickr.com/photos/31146530@N06/8168198380/
http://www.flickr.com/photos/31146530@N06/8168198850/

and the schematic as I copied it from the original radio
http://www.flickr.com/photos/31146530@N06/5039993895/

Thanks for any help

Steve