Noisy Squeal from SE amp

sonata149 · 24th March 2008, 09:16 PM

I built the attached SE amp circuit using the EF36 (a 6J7 variant) as driver and a 6L6G as output valve. The circuit is my adaptation of JC Morrison's Fi-Primer 6L6 SE-UL. I used the EF36 instead of the EF86 / 6267 driver and as my OPT does not have a UL point I'm feeding the screen from a second CLC section on the PS.

Today I fired up the amp and as soon as the valves warmed up I got a loud noisy squeal (oscillation?) from the loudspeakers. I changed / swapped the speaker connections on OPT and removed the NF connection. This did not solve the problem.

I then replaced the 0.1uF coupling capacitors in case the original ones were leaky. Same result. I haven't taken any voltage readings because I'm using the speakers as the OPT load so I have to eliminate this horrible noise from the speakers first.

Can some members provide any clues or guidance as to what is happening and how to solve the problem.

Thanks in advance for any input / suggestions.

Joe A

Psychobiker · 24th March 2008, 09:40 PM

The 6L6 is going to 'SG?'?! What is this? Connect the screen grid to the plate with 100R or thereabouts. This straps the 6L6 into a triode

L

hotbottle · 24th March 2008, 09:52 PM

Isn't it more normal to return 4.7uF the screen bias cap to ground? (Don't know if that is causing the problem though)

I have an SE amp with these old pentodes in it. I love the odd arrangement of the input grid being on the top cap! My output tubes are EL34s though.

hotbottle · 24th March 2008, 09:53 PM

Quote:

'SG?'?! What is this?

Look down lower for the second connector with SG in it. It is a bias supply voltage.

Psychobiker · 24th March 2008, 09:58 PM

/me is blind.

I wonder, if that's causing some disparity with the plate, even though it shouldn't. Squealing sounds like a cap problem tho

sonata149 · 24th March 2008, 10:28 PM

Thanks Psychobiker and Hotbottle for your input.

Since you are pointing fingers at capacitors I might as well add that I used a 1000uF across the EF36 cathode bias potential divider (upper resistor) instead of the 100uF specified in the original circuit. I don't know how such a change would affect the performance.

I will attend to this area at the first opportunity.

Thanks and regards.

Psychobiker · 24th March 2008, 10:41 PM

Try strappign the screen to plate with 100R

korey · 1st April 2008, 06:40 AM

Try switching the plate connection (pin #3) and the B+ wires from the OPT around.

SHiFTY · 1st April 2008, 08:02 AM

Try switching B and P around on the output tranny.

sonata149 · 1st April 2008, 07:05 PM

Thanks korey and SHiFTY but I have already strapped the screen to the anode (of the 6L6) and the sound improved dramatically. The only noise I'm getting now seems to be coming from the topcap connection, especially the one which is close to the AC mains switch.

Do you think that a shielded cable (for the topcap) will help? If yes, how and where should I connect the braid (to ground or loose)?

Thanks and regards.
Joe A

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Noisy Amp	vento	Solid State	13	16th January 2009 08:45 PM
PPI a404 amp creating noise (squeal) on AM Band	SRITT3	Car Audio	4	31st May 2008 02:55 AM
noisy noisy noisy noisy noisy tube	dsavitsk	Tubes / Valves	50	20th February 2006 04:07 PM
squeal in amplifier	n0plb	Tubes / Valves	5	5th July 2005 11:16 AM
How can i use a noisy PSU ?	Garbz	Solid State	5	3rd January 2004 12:15 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

24th March 2008, 09:40 PM	#2
Psychobiker diyAudio Member Join Date: Jul 2005 Location: Montréal QC	The 6L6 is going to 'SG?'?! What is this? Connect the screen grid to the plate with 100R or thereabouts. This straps the 6L6 into a triode L

24th March 2008, 09:52 PM	#3
hotbottle diyAudio Member Join Date: Mar 2008	Isn't it more normal to return 4.7uF the screen bias cap to ground? (Don't know if that is causing the problem though) I have an SE amp with these old pentodes in it. I love the odd arrangement of the input grid being on the top cap! My output tubes are EL34s though.

24th March 2008, 09:58 PM	#5
Psychobiker diyAudio Member Join Date: Jul 2005 Location: Montréal QC	/me is blind. I wonder, if that's causing some disparity with the plate, even though it shouldn't. Squealing sounds like a cap problem tho

24th March 2008, 10:28 PM	#6
sonata149 diyAudio Member Join Date: Sep 2006 Location: Malta, Europe	Thanks Psychobiker and Hotbottle for your input. Since you are pointing fingers at capacitors I might as well add that I used a 1000uF across the EF36 cathode bias potential divider (upper resistor) instead of the 100uF specified in the original circuit. I don't know how such a change would affect the performance. I will attend to this area at the first opportunity. Thanks and regards.

24th March 2008, 10:41 PM	#7
Psychobiker diyAudio Member Join Date: Jul 2005 Location: Montréal QC	Try strappign the screen to plate with 100R

1st April 2008, 06:40 AM	#8
korey diyAudio Member Join Date: Mar 2008	Try switching the plate connection (pin #3) and the B+ wires from the OPT around.

1st April 2008, 08:02 AM	#9
SHiFTY diyAudio Member Join Date: Jul 2002 Location: New Zealand	Try switching B and P around on the output tranny.

1st April 2008, 07:05 PM	#10
sonata149 diyAudio Member Join Date: Sep 2006 Location: Malta, Europe	Thanks korey and SHiFTY but I have already strapped the screen to the anode (of the 6L6) and the sound improved dramatically. The only noise I'm getting now seems to be coming from the topcap connection, especially the one which is close to the AC mains switch. Do you think that a shielded cable (for the topcap) will help? If yes, how and where should I connect the braid (to ground or loose)? Thanks and regards. Joe A

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)