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In Reply to: RE: By Jove i might have forgotten to install a grid leak resistor! nt posted by DAK on January 25, 2015 at 20:49:19
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/
Follow Ups:
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
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/
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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