I hope you can resolve the hum. The two times I have had hum with my CSP3 in the system the problem was not the CSP3. One time it was the fact that I had my cable tv DVR connected to the system. The hum was coming in on the coaxial cable itself. I put a filter in line and the hum went away. (Later when Time Warner was sold to Spectrum and Spectrum changed over the cable tech frowned on it and told me to take it out I did so. . . the hum had gone away). The second time it was a cable placement issue. I am not sure which power cable was next to an interconnect that caused the problem but I think that was the issue. I ended up dismantling my whole cable loom and re-routing things to more effectively isolate power and signal cables and the hum went away.

Though expensive, the 25th Anniversary mod to the CSP3 is transformative. It's good pre-mods, it's really great after.

I play around with the gain between my DAC, ZTPRE, ZROCK2 and CSP3 and input gain on my Monoblocks. I have the Monoblocks at about 3/4 volume, the ZROCK2 pretty pumped up, the CSP3 outputs about 60 percent, and the ZTPRE about 70 percent. Usually my DAC is at 100 but I sometimes knock it back 5 or 10 percent. I control master volume via the ZTPRE. I can get incredible sound playing about with these gain values.

Great Geno. Lots to be discovered with the CSP3 and its gain and the gain of other components.

There are lots of alternatives on eBay, here's one:

https://www.ebay.com/itm/Amptata-5Z3-274A-80-To-5U4G-GZ34-274B-5Y3-5AR4-Tube-Ada...

Another:
https://www.ebay.com/itm/1pc-Gold-WE274A-5Z3-80-TO-WE274B-5U4G-GZ34-5AR4-Tube-co...

Almost all I have seen are from China or Hong Kong, and you have a 14 to 20 day wait or so. I used to have an American source but I think they have dried up.

As for the input tubes . . . I hestitate to make specific recommendations because I have found that even the ROOM makes a difference--tubes I didn't prefer in my living room I prefer upstairs with the same system, and vice versa. You can use so many different types! I really like the Russian NOS tubes from Steve--if you are going to use 6N1P or 6N6P get them from Steve--it's a crapshoot otherwise out on the internet. Steve tests and matches his meticulously. You can use 6922, 7DJ8, so many more. This page I find to be a useful guide:

http://www.audiotubes.com/6dj8.htm

I've gone beyond the pale perhaps. I use 6N5P, 6N6P, 6SN7 with converters, 6085 with converters, tend to rotate these as the fancy strikes me. Lately the 6SN7 and 6085 have stuck.

Hi Lon, great read.  Sweet monoblocks for sure.  

May I ask, what input jacks are mounted on the chasis, and how many?

I'm curious if I can get 25th anniv monoblocks, each with an XLR input.  I prefer to avoid a ZBit or XLR-RCA adapter cables as described in "bridging into mono" in the owner's manual.

I have really been enjoying these amps over the years. Can't believe how much time has gone by.

I think I'm now ready to try out fuses. I've looked at the manual but still a bit unsure which replacement sizes I should get. 5x20mm 3 amp. 250v, 500v? slo blo, fast blo? I'm thinking I'll start with the purple quantum. Any advice on which size to order or other fuses I should consider?

Thank you, Lon! Just the information I was looking for.

It is astonishing really the great, full sound I get from these Monoblocks. The HR-1 are not the most efficient, but relatively efficient speakers, and the density that the preamps and the ZROCK2 give me are also an important factor.

I have occasionally thought about other amps, but I haven't the dollars to experiment, and I really just am so happy with the sound I have and love settling back into the music and the musical presentation.

Keep in mind Synergistic Research has buy 2 get one free offers for fuses through their dealer network a couple times each year. The Cable Company and VH Audio are both solid sellers.

HK