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Why do i have DCV on the 6v6 input of my RH amp?

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Posted on January 25, 2015 at 19:13:13
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 I just built this stereo RH 6V6 with 6SL7 driver. It uses a CLCLC power supply and with a 5AR4 rectifier provides 280vdc B+. Well after a warm up period, I tested various voltages in the amp and measured 15vdc on the 6v6 input grid. Oddly enough, the cathode voltage is slightly more than this.
OK, first thing to check is the coupling cap which was a .22uf x 600 RTX. This cap was from a previous build but tested ok on my cap tester. So, I pulled the 2 .22uf caps and installed NOS PIO .22uf Westcap. I was disappointed after reenergizing the amp to find the same problem still existed.
I installed the coupling caps by soldering it to pin6 of the tube socket. From there the 220 ohm grid stopper resistor goes to pin5 or G1 of the 6V6. Supposedly P6 is unused on that tube and is actually missing on some 6V6s.
Having 15vdc on the grid can't be good for the amp. So, I am looking for help trying to cure this problem. TIA, Dak

 

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RE: Why do i have DCV on the 6v6 input of my RH amp?, posted on January 25, 2015 at 19:16:37
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 What's your grid leak resistor value?

 

RE: Why do i have DCV on the 6v6 input of my RH amp?, posted on January 25, 2015 at 19:41:16
danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002
 Check the voltage on the other side of
the grid leak and the cathode resistors.
Might be a bad ground.

DanL



 

By Jove i might have forgotten to install a grid leak resistor! nt, posted on January 25, 2015 at 20:49:19
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 .

 

RE: By Jove i might have forgotten to install a grid leak resistor! nt, posted on January 25, 2015 at 22:56:26
Alex Kitic
Audiophile

Posts: 182
Location: Serbia
Joined: June 25, 2014
 Obviously ;)
Actually not such an uncommon mistake among DIYers. It could have been rather catastrophic without the CCS (LM317).
But why 6SL7? I thought we had already explained that you cannot use the 6SL7 in RH84 rev2, but you might use 6SL7 in the RH Universal if Rfb is set (by switch) to ECC83 adequate values (200k).
Though, the 6V6 is not adequate for the RH Universal, too low anode dissipation... so, what hybrid RH schematics have you applied to your amp?
******

http://rh-amps.blogspot.com/

 

RE: By Jove i might have forgotten to install a grid leak resistor! nt, posted on January 26, 2015 at 02:05:51
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hi AK, I was trying to do the universal with a 6L6, but my power trans is not up to the task. When I plug in the 6L6 the B+ will drop down to about 230vdc. Since I am all set up with octal sockets, and the 6V6 is pin compatible, I plugged it in and fired up the amp. The B+ went up to 300vdc and settles at 275 after 30 minutes. The amp sounds pretty good too. For the 6SL7 the plate resistor is 100k and the anode to anode resistor is 220k which is a resistor I had on hand. The cathode resistor is 422 ohms. This gives a cathode voltage of .7dcv, which if my math is correct, is 1.6MA. For the 6V6 I am using cathode bias resistors which right now is at 190 ohms. When the amp is stable I will use the LM317 as a CCS. If there is any other concerns that I should address please let me know. Thank you for your help and guidance. Dak

 

RE: Why do i have DCV on the 6v6 input of my RH amp?, posted on January 26, 2015 at 21:06:05
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Putting in a 226k grid leak resistor was a huge difference. I did not know it could make such a change. The soundscape is now in focus with everything in place. Wow, the amp sounds great. I will post pics when I am finished with the final details. Thanx for pin pointing the problem. cheers, Dak

 

RE: Why do i have DCV on the 6v6 input of my RH amp?, posted on January 26, 2015 at 21:16:05
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014
 Good times.
△This message will self destruct in 10 seconds△

 

RE: By Jove i might have forgotten to install a grid leak resistor! nt, posted on January 26, 2015 at 22:27:34
Alex Kitic
Audiophile

Posts: 182
Location: Serbia
Joined: June 25, 2014
 If you opt for a "mini" RH Universal, you are going to need different OPTs with a 5k primary. The CCS LM317 should be set for approximately 48mA (27 ohm current setting resistor) mimicking the RH84 rev2 output. The CCS LM317 is essential both functionally and performance-wise.
I would prefer 390 ohms cathode resistor for the driver tube, but yours is probably close enough.
Such a "mini" RH Universal can be used with 6F6 and old types 6L6 (ST shaped and metal): could be interesting?
******

http://rh-amps.blogspot.com/

 

RE: By Jove i might have forgotten to install a grid leak resistor! nt, posted on January 27, 2015 at 08:01:28
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 My output transformers are some vintage Revox which measure right around 5K primary impedance. With a 5V4 rectifier and vintage Kenrad 6V6GT the B+ is 265vdc which ends up as 255v at the anode. G2 voltage is 254v.
For the 6SL7 the anode voltage is 142v and the cathode measures .71vdc (422 ohm resistor). If I install some 390 ohm resistors for the cathodes will that give me more drive and perhaps more power? Thank you for your assistance and advice, cheers, Dak

 

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