I'm pretty familiar with these particular HK amps as I have both restored and "played around" with both the A250 and A260 extensively. I love them in fact. (WAY more than ANY of the "Award" series HK integrated amps like the A300/500/700 IMO they cut a lot of corners on those amps in order to offer them as DIY kits)
The A250/260 are pretty similar obviously but I prefer the A260 aesthetics. I prefer the faceplate design and champagne finish with separate power button (as opposed to the treble switch) but to each his own. Actually the A250 is VERY similar to the HK250 power amp (which I've also restored) and they both have a nice copper chassis but you cant really see it on the A250.
The A250/HK250 is 25wpc vs 30 wpc for the A260.(A250/HK250 designed for 6L6GB and the A260 set up for more power for 6L6GC) The A260 uses a pair of 7247 in the AF amp and phase inverter (according to Hegman to reduce both phase shift and distortion levels) as opposed to 12AU7 used in the A250 and 6GC7 in the HK250. If memory serves me right the A250/HK250 are both cathode biased, but the A260 is fixed bias with a circuit very similar to the Citation V. The fixed bias circuit is sourced from the power supply to provide the grid with about -30 Volts (negative) DC to permit it to operate in Class AB1.
The HK250, A250 and A260 (ver1) ALL use the exact same "FT2382666B" output transformers (7000 ohm Pri-Z). But when researching these amps I learned there are actually 2 different versions of the A260 with a different output and power transformer and filter cap arrangement. Both versions have a 150VAC winding and a doubler with solid-state rectification to provide ~400VDC to the OPTs and roughly 390-395 VDC to the plates. But the earlier version (I will call ver1) can easily be identified by only 3 top side electrolytic caps while ver2 has 4 (2 paper 2 metal). The addition being a metal can added in a position on the right hand side just forward of the RCA input panel (as shown in the A260 SAMs I have which only supports ver2)
Version 1 A260 uses the exact same "FT2382665E" power transformer as used the A250 and HK250. That PTX has a 17VAC winding that on the A250 is used for the DC heating of V1 V2 V3 via a full wave diode rectifier. (those leads are blunt cut on the he HK250 as it is a power amp so it doesn't require DC heating for preamp tubes) But on the ver1 A260 in order for the 17VAC to be able to provide DC heating AND create the -30VDC bias voltage it adds a twin capacitor doubler combined with single diode rectification.
But the improved ver2 power transformer (labelled FT2963803) is equipped with a convenient 36VAC winding, eliminating the need for the doubler and also uses single diode to provide DC for heating for the preamp tubes and leaving around -30VDC for the negative bias. The OPTs on the ver2 are labelled FT2963804A and have more ideal loading at 4400 ohm Pri-Z necessary to push 30+ watts of output power in push-pull.
IMHO the ver2 A260 is the nicest to work with and easily sounded the best after a basic recap/restore. In fact, for many, a properly restored A260 ver2 will likely represent the pinnacle of HK integrated amps IMO. I listened to both versions extensively for a couple years in 2 different spaces and really became very fond of the ver2 A260.
After a while I decided to attempt to modify the ver1 amp to see if I could improve it somewhat to match the ver2. I sent the schematic to Jim McShane for advice, and with his help we first beefed up the power supply and doubler. I also completely redid the negative bias generator and added a fully adjustable -DC bias very similar to the Citation II (Thanks Jim!). I then added a triode-pentode switch just to give it try (even though 6L6GC are really not known to be configured in triode). While I was at it I swapped out many of the orange drops I had originally used and went with a comination of Kimbers, Hovlands and Mundorf in the preamp stages and 0.22uf Russian PIO coupling caps.
It turned out very well! Un fact, I liked it so much when I sold by Citation 2 (to pay for parts for my Fisher 200 monoblocks, another story) the modded ver1 A260 became my main amp in my basement listening room. I really like the way it sounds -smooth and powerful. So much so I am now considering redoing the ver2 in the exact same fashion as it's been sitting on a shelf for 4 years.
Anyways that's my take.
If anyone is working on these A260 amps feel free to PM me and I'll share what I have on them. Here's an old thread from 2008 on this amp.
http://audiokarma.org/forums/index.php?threads/h-k-chorale.191936/
I will find a couple photos tomorrow
WOT
A230 will sell after I'd settle between A250 or A260
