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Old 24th July 2014, 11:32 AM   #1
Zekk is offline Zekk  Singapore
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Default Failed Converting UL to Pentode!?

Hi Guys!
Kindly advice how I can simply failed for such straight forward mod?

Converted my PP Ultralinear 6550 to pentode mode by firstly disconnecting UL tap of OPT to pin #4 of 6550.

From B+, soldered a 220K to a series of 5x IN5369B, bypass with 47uf@450v , and then branch out to the two EL34.

Bias voltage was reduced to -44vdc prior to inserting EL34.

It's DEAD! Every point measures right except pin #8 of EL34. No reading at all despite pin #5 reads -44v before the 1K grid stopper.

What should I do with the CT of the OPT? Just tap it off?

Before all this, it was sounding good. Since I've a quad of Amperex brown base 6CA7, I've decided to do a switch.

As someone said: "....6550 sounds industrial, EL34 sounds sweet!" Can't get this off my mind yo.

As usual, sorry for the rambling and thanks for the help forecoming.

Zekk
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Old 24th July 2014, 11:57 AM   #2
DF96 is offline DF96  England
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A circuit diagram might help.

I am unclear what the 220k resistor is for. It is clearly far too large a value to provide a g2 supply for an output pentode. As I don't carry EL34 pinouts around in my head it would help if you tell us which electrode you are checking rather than just the pin number.

The CT of the OPT should be left as it was.
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Old 24th July 2014, 12:56 PM   #3
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On each output socket, remove the UL tap from pin4 and connect pin 4 to HT (centre tap of output transformer) with a 1K 5W ww resistor. Connect pin 1 and pin 8 to ground.
Forget about the diodes and 220k resitor.
This will work with either EL34 or 6L6 (6550) type valves.
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Old 24th July 2014, 01:01 PM   #4
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Depending upon the HT, the neg bias may be reduced to as little as -32.
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Old 24th July 2014, 01:03 PM   #5
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Quote:
Originally Posted by Zekk View Post
Hi Guys!
Kindly advice how I can simply failed for such straight forward mod?

Converted my PP Ultralinear 6550 to pentode mode by firstly disconnecting UL tap of OPT to pin #4 of 6550.

From B+, soldered a 220K to a series of 5x IN5369B, bypass with 47uf@450v , and then branch out to the two EL34.

Bias voltage was reduced to -44vdc prior to inserting EL34.

It's DEAD! Every point measures right except pin #8 of EL34. No reading at all despite pin #5 reads -44v before the 1K grid stopper.

What should I do with the CT of the OPT? Just tap it off?

Before all this, it was sounding good. Since I've a quad of Amperex brown base 6CA7, I've decided to do a switch.

As someone said: "....6550 sounds industrial, EL34 sounds sweet!" Can't get this off my mind yo.

As usual, sorry for the rambling and thanks for the help forecoming.

Zekk
A 6550 is a pentode! http://www.dougstubes.com/6550-tungsol.pdf
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Old 24th July 2014, 10:20 PM   #6
lechuck is offline lechuck  Netherlands
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Quote:
Originally Posted by Zekk View Post
Hi Guys!
Kindly advice how I can simply failed for such straight forward mod?

Converted my PP Ultralinear 6550 to pentode mode by firstly disconnecting UL tap of OPT to pin #4 of 6550.

From B+, soldered a 220K to a series of 5x IN5369B, bypass with 47uf@450v , and then branch out to the two EL34.

Bias voltage was reduced to -44vdc prior to inserting EL34.

It's DEAD! Every point measures right except pin #8 of EL34. No reading at all despite pin #5 reads -44v before the 1K grid stopper.

What should I do with the CT of the OPT? Just tap it off?

Before all this, it was sounding good. Since I've a quad of Amperex brown base 6CA7, I've decided to do a switch.

As someone said: "....6550 sounds industrial, EL34 sounds sweet!" Can't get this off my mind yo.

As usual, sorry for the rambling and thanks for the help forecoming.

Zekk
What exactly did you do with pin 4 after you disconnected its connection to the UL tap of the output transformer?

In case you did not reconnect pin 4 to either B+ or pin 3 then your EL34's will be cut off and do not conduct current. If this is the case please connect pin 4 with a suitable (I use 3 Watt rated resistors here) 1k ohm resistor to either B+ (Pentode-mode) or pin 3 (Triode mode). Repeat this for all output valves and try again

Last edited by lechuck; 24th July 2014 at 10:30 PM.
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Old 25th July 2014, 04:46 AM   #7
Zekk is offline Zekk  Singapore
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Guys.. no luck so far.

Configuration to run EL34 in pentode mode is as followed: Pin#4 or g2 voltage is 255vdc @ 8mA.

B+ is 505vdc. So, to achieve a constant 255vdc at g2, I calculate voltage drops divide by current, and that'll give me the value of dropping resistor.
Eq: 505v-255v = 250v / .016 = 15K6.

So, 505vdc B+ > 15K6 > 47uf@450v > g2 both EL34.

All points measures ok except pins #1/#8 have no reading at all!?

I can reverse back to my comfort zone and have my music. But then, I won't be any wiser!

Got my inspiration from the Grommes 260A.

Thanks.
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Old 25th July 2014, 07:24 AM   #8
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Quote:
Originally Posted by Zekk View Post
All points measures ok except pins #1/#8 have no reading at all!?
Thanks.
Pin 1 & 8 are at ground potential (make sure pin 1 is indeed connected to the cathode/ground and not left floating), so should have no reading, what is the bias voltage set at?
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Old 25th July 2014, 07:34 AM   #9
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Quote:
Originally Posted by Zekk View Post
Got my inspiration from the Grommes 260A.
Your circuit looks like this?

Click the image to open in full size.
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Old 25th July 2014, 11:10 PM   #10
Zekk is offline Zekk  Singapore
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jazbo8... Pin 1 & pin8 are connected together to a 10 ohms resistor to ground. At UL mode, it measures 0.75v and that equals 75mA.

directdriver... only followed the regulated B+ to g2 of EL34. Instead of 6L6, I use zeners.

Thanks!
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