About a year ago (Feb 2022) I bought an A10 after following Stephs' excellent upgrade video series on this site. I applied all the mods and am very satisfied with the results. However, in going through the process I inevitably added a few of my I own, the main one being to add a "line" O/P using the unused 1/2 of the pre-amp valve as a unity follower.
I've finally got around to photographing and summarising what I've done, so I thought I'd share them here.
In summary, this is what I've done:
- all the Skunkie recommendations, I settled on a 620K FB resistor.
- re-wired the mains wiring to the on/off switch using screened cable, earthed to the chassis. This greatly reduced a noticeable mains hum in the Left channel (ie. the channel adjacent to the mains wiring cable run to the On/Off switch).
- I converted the A10 pre-amp valves from 6N2 to 12AX7 (ECC83) and only used 1/2 of the triode as a pre-amp (ie. not both sides in parallel).
- I used the "unused" 1/2 of the 12AX7 as a unity gain cathode follower wired to a 3.5mm stereo jack fitted to the rear of the chassis. This gives a line O/P of whatever the amplifier is playing, useful to drive a tape deck, sub woofer or, in my case it drives slave a amp/speaker system in other rooms of the house.
- the I/P transformer does get quite warm with the Skunkie mods, so I added x2 40x40mm CPU heatsinks to the front and back of the transformer to aid heat dissipation, these are readily available online and are cheap enough that it's worth a go, just adhere them with the heat sink paste/glue you use to stick them to a CPU. They are small, black and unobtrusive.
- I'm lucky enough to have a selection of EL34's available, so I did some tube rolling and can report the best are Mullard EL34's, followed by TEONIX, then Groove-Tube, then PS-Vane. Don't get too hung up on this, they all sound pretty good, so I wouldn't spend hundreds on Mullards, but if you have them, use them.
- I also rolled a few rectifier valves, the stock one it comes with, the Sovtek recommended by Steph and a 5Z4G (a whopping great big bottle valve from a 1930's radio). Again, not much in it, but settled on the Sovtek as it gave slightly high HT (B+).
Schematics and pictures:
I've been running this amp now for nearly a year now with no problems. The Line O/P was originally used to drive a slave solid state amp and some ceiling speakers in another room, connected via 30 plus foot of cable. Although, technically an ECC83 (12AX7 is not the greatest choice for this (an ECC82 would be better, as a line driver, but has lower gain), in practice it works fine. In fact I now use this O/P to drive my slave speakers and an active Sub-Woofer, no problem.
Drilling the hole in the rear panel is a real pain, as Steph says in her notes, the chassis is made of "undrillium". I was originally going to fit x2 phono's but after drilling the first hole I settled for a single stereo jack!
Finally, these are just my tinkerings given here for interest and advice only,
FYI I have many years of working with HT and valve equipment, if you are unsure of anything please ask on this forum for advice before proceeding and also please follow all the usual HT safety precautions listed by Steph elsewhere on this site.
Hope you find this useful, happy tinkering.