Well, there's an awful lot of gain in that first 12AX7 stage, and then the RIAA boosts the bass frequencies. Any hum is going to be readily apparent. Doesn't hurt to try it! Report back!
Dan
Dan
Testing out the repaired PSU. I am a bit confused/concerned with the Zener diode stack.
With 2 5373 (68V) and 2 5374 (75v) I am getting about 298V on the B+ outs. I measured 74V across the 68v zeners.
I then replaced the 5374s in the zener stack with 5373s, so four 5373 and get about the same 298V on the B+.
Any other thoughts on how to bring down the B+? I dont want to be over-volting the zeners and have another failure. Do I use four of the 5374s?
Edit : could it be a bad Cap, because the B+ just keeps climbing slowly past 285 up towards 300. Even when I add more series resistance in the CLC filter (1k up to 3k).
With 2 5373 (68V) and 2 5374 (75v) I am getting about 298V on the B+ outs. I measured 74V across the 68v zeners.
I then replaced the 5374s in the zener stack with 5373s, so four 5373 and get about the same 298V on the B+.
Any other thoughts on how to bring down the B+? I dont want to be over-volting the zeners and have another failure. Do I use four of the 5374s?
Edit : could it be a bad Cap, because the B+ just keeps climbing slowly past 285 up towards 300. Even when I add more series resistance in the CLC filter (1k up to 3k).
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Did you use the caps from the kit? I didn't trust any of the caps or resistors. They all were replaced with quality parts.
I use the stuff from the kit in other projects in non-critical, it won't blow up if it fails positions in the circuit.
Dan
I use the stuff from the kit in other projects in non-critical, it won't blow up if it fails positions in the circuit.
Dan
Yeah, I used the kit parts. Replacing those all now, as I work through the board.
What is purpose here of the circled voltage divider? And why is it connected to the heater supply?
What is purpose here of the circled voltage divider? And why is it connected to the heater supply?
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The circuit provides an elevated heater voltage due to the voltage supplied to the cathode follower and the h/k breakdown voltage specification.
https://www.valvewizard.co.uk/heater.html
Dan
https://www.valvewizard.co.uk/heater.html
Dan
@jayme here's what I did - "Today I replaced all the Zener Diodes and still it was measuring 298Vdc under load. I removed one of the 1N5374B and put in a 1N5370B which is 56V breakdown., it measured 278.7vdc. So I removed a 1N5374 and put in another 1N5373 and now I have 288Vdc. So now I'm done messing with the B+ voltage." It has been rock solid ever since.
My Last "All Out Build" I use all high quality parts from Digikey and Mouser. Some parts like the Bridge Rectifier for the Heater and Transistor in the were higher ratings. I never liked the parts the Clone came with in completed form or the Clone kit with all the parts supplied. When I saw that the resistor leads were hollow that was it for me. I never experienced and failures or smoking. I did notice after my last Clone build my EAR Clone runs cooler than the original and the pre-built EAR Clone.
I did add a NTC because when I look at the turn on current spike with the meter there was a very high current spike The addition of the NTC the spike was reduced. I do have the Antek Transformer for these clone mods and just need to build my new enclosure to mount the Phono PCB with new better rotary switch and AC Inlet. Then I will build a separate chassis for the power transformer. I think the Power transformer the Clones come with are running near their limit.
Btw, was there anything else you did to the EAR Clone?
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Quite a bit now in the PSU. I have replaced the original torroid transformer with an R-Core transformer, and now switched out the two 3k zener biasing resistors with 8.2k resistors which reduces the current draw and better protects the diodes. I upgraded the Transistor to a better one (I forget the item number). I added a 15H 900R choke and a 300uF cap in the PSU B+. And upped the 20uF caps to 45uF. Not yet implemented, but I have an EZ80 tube rectifier which will replace 2 diodes in the bridge. Rock solid at 285V now.
On the audio board. i have only changed the C1 and C3 caps to a Jenzen 0.10uf and an Auricap 1.0uf. Sounds amazing.
I have a whole separate PSU+Audio unsoldered board, bought it so I can implement my final build after tweeking the original. The original PSU board is now kinda ugly from all of the unsoldering/resoldering,etc with a lifted trace or two.