You might just have the best HF-81 in the US! I'd keep it and just go for a speaker upgrade instead of another tube amp. Altec Valencia's or LS3/5A's, eh!
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A Stereo 70 will have more power of course.
But if your Eico is as heavily rebuilt as you say, the sound differences will be mostly about the output transformers and other parts quality. Eicos came stock with ceramic disk caps for coupling caps but it sounds like those have all been changed out; OTOH the HF-81 had pretty good output transformers. The only big difference I see is the Mapletree stuff uses the 6SN7 family which IMO/IME is a better sounding tube than 12AU7s.
So my opinion is that you will be making a bit of a change for more power, not more bandwidth and otherwise slightly different sound (you'll have to play with interconnects between the preamp and amp which will affect things). I don't think it will be a dramatic improvement; with your amp you don't have to deal with the interconnect issue. It might be possible to have your amp set up with 6SN7s - that would be possible if the power transformer can support their filament requirements.
I owned a stock refurbished 81 in the 70's 80's.
The can caps were replaced and something feeding the rectifier tube was changed.
The coupling caps were also replaced with Yellow Illinois brand caps which I was told would sound similar to the originals.
Sources were various Thorens TT's, Nakamichi cassette decks and believe it or not Beta and VCR video decks used to record LP's and to record/mix live music.
The only thing I found lacking was the 81's phono section.
I patched in the phono section of an Advent 300 receiver which easily bested the stock Eico phono section.
So maybe just try an outboard phono preamp for starters.
Though I haven't used it for years I now own a Pilot 240 integrated amp and the same thing goes.
The line inputs sound great, but the phono section is lacking.
During the time I owned/used the 81 I also used Mac 30's and various modified Dynaco Stereo 70's (in mono block and stereo versions).
The 30's were stock/refurbished and the 70 mono blocks had a tiny transformer and other parts added to the power supply (maybe used as a choke).
The above were used with the preamp section of an Advent 300 and sometimes a Dynaco PAS 3X preamp.
The 81 held it's own against the Mac and Dynaco gear.
I sold them all in 1986 and didn't own another decent rig until the late 1990's.
thanks guys for the responses. It has been helpful.
Been a long time since I rebuild it (maybe 2002_2003).
My recollection is I used cardas wire for the signal paths, Sprague 716p to replace the coupling ceramics and Angela oil for the .1 uf caps, Riken carbon resistors in EL84 grid network with new carbon comps for others, replaced the phono section ceramics with upgraded caps and kept the metal film resistors marked West Germany as they were within 1% of value.
Upgraded the negative feedback resistors and caps and el84 bias caps (Blackgate) and resistors (Mills wire).
I found a schematic for the tone control wafers and build new ones with upgraded caps and resistors. Power capacitors were replaced.
Replaced all the rca sockets and upgraded the speaker terminals (8 ohm) to modern banana gold plated types and replaced all the tube sockets.
Oh and replaced the EZ-81 rectifiers with NOS Mullards. Only thing I didn’t touch were the vol, balance, and tone controls.
I’ve got a spare HF-81 and have often thought about using it for a custom unit without all the tape and mic stuff, tone controls, and with a modern input selector and ALPS volume control I have.
Maybe after I retire and am looking for a project. :-)
I had one, but sold it. Just preferred my Scott 222 and 233, as well as several vintage power amps. My HF-81 was a little noisy and the design and finish were a bit underwhelming, YMMV.
I still have a modded ST-70 with VTA board and other tweaks - so a cousin to the Latino 70. Super reliable, nice power, even in triode, but the little Scott is where the magic is for me. These are simply matters of taste and I can see someone else having a very different opinion. The Scotts do have a delightful phono stage, even stock though.
Funny you suggested trying 6SN7 tubes in my amp. I already bought a pair of octal to noval adapters to do just that in the preamp stage. Got a pair of Raytheon 6sn7 gray plates. Was an improvement in soundstage depth, detail, and air but have edgy highs. Bought a pair of what looks to be Sylvania black plates with bottom getter branded Zenith. OMG do they sound fantastic. Big deep soundstage, luscious detailed midrange, vocals to die for, sweet highs, good bass, and well balanced. The NOS 50s Mullard 12au7 are great tubes, but the Zenith branded 6sn7 mop the floor with them.
The HF-81 sounds absolutely superb now. It was excellent before but it’s on another level now.