Eico HF-81 Tube Amp to Restore

[NOSValves]

Why thanks very gracious of you.

Lou on those cathode resistor values realize that changing the value from 165 to 200 or even 220 ohms is not going to cool the bias of those output tubes by the percentage change in resistance, cathode biasing does not scale with resistance percentage change. All vintage companies were very wreckless in biasing....this is hard on all the output tubes and all three transformers.. Why burn up either expensive NOS tubes or modern production tubes that can't handle it when no sonic bendfit is achieved?

Cheers Craig

 

[tube-a-lou]

I ordered some 200 ohm resistors thank you for the help.

Tube

 

[Shadowdog]

Maybe the EFB fixed bias mod would be a great one then as you could bring the total dissipation down to really safe levels (allows any EL84 to last) for very low cost and a small add on (which can be easily reversed) as it has done one the Dynaco SCA/ST-35 which have a similar circuit. It can also help sonically also. I can't see why the lack of UL should make any difference. Info found under Dave's lab on the Tronola Electronics site.

http://www.tronola.com/

 

[NOSValves]

IMHO Nope....


EDIT: I guess I should comment a bit further just to save confusion. I think Dave's modification is a good one if your looking for a more Hi Fi specification bragging rights king... if musicality and over all relaxed long term enjoyment of the piece is the name of the game then absolutely not...

 

[Shadowdog]

But the fact that most Scotts and Fishers used actual fixed bias and are well liked sonically , and I think this is an improvement over regular fixed bias sonically so may be worth a try even on the HF-81. Very low cost anyway and one may very well be very happy with it plus very long tube life.

Craig would you agree that the HF-12 (I think output transformers are the same) is much the same sonically as the HF-12? I know the there is a separation with a resistor on the B+ to the output transformer that may be added to the HF-12 for maybe a sonic improvement and possibly a much lower DCR & cooler running power supply transformer is also maybe in order.

Thanks for posting your info and thoughts!

 

[NOSValves]

actually I think you have fixed bias and cathode biased completely confused. Its kind of funny that the bias arrangement that is adjustable is called fix and the one that is not adjustable is not called fixed I know why but its still funny... I call Scott arrangement and my own "fixed adjustable"

but honestly I do not have time to combat what I call ego based specification engineering... because if I did an absolute nightmare that would close this thread would ensue. I just do not have the energy or desire..so I ain't going there.

If I said it once I've said it a gazillion times "technically better is rarely sonically better". but technically horrible is absolutely horrible. Some day folks will realize that these vintage designers/true tube engineers were in fact audio geniuses that simply were held back by the pencil pushers but there is a huge limiting and none upgradable part of the equation that limits all these wild ego engineering technical specification based modifications to questionable across the board benefit. The engineers had EXTENSIVE college and hands on training IN TRUE TUBE THEORY that should be respected.

 

[Shadowdog]

Hi Craig,

No I know the difference, and maybe the EFB mod is the best of both in some applications. I know what you mean about altering what the vintage EE's did, but I'm open to trying simple things that can be reversed. Anyway when I get some $ I will try it on my HF-12 and compare. Thanks!

 

[tube-a-lou]

Have not done much today on the amp But! the rectifer tubes came in the mail today
three GE's

Tube

 

[GordonW]

No I don't want it bright.

Tube

Mine certainly wasn't bright, with the .039 polystyrene coupling caps in the AMP section. In fact, it sounded best, with both the bass and treble controls right in the center.

I stayed right with the original values, in the phono stage. Replaced the .25uf caps in the phono stage, with .22uf GE 40L poly caps. I've used those for years, and they NEVER make anything overly bright. The phono stage in mine was REALLY enjoyable afterward... quick, tight, great bass.

Regards,
Gordon.

 

[tube-a-lou]

Thanks Gordon I might have a few of those, here's some photos of some of the things
I've done, power supply caps and others. I have 180 ohm resistors in there
there now but waiting for the 200 ohm's in the mail. I had to redo the wiring
in the rectifier area the wiring was pretty much shot, replaced it all.

Tube

 

[NOSValves]

Gordan,
Raising the value probably negated the bright factor with those caps good for you.

 

[tube-a-lou]

It's 7:00 PM Eastern time and for the last 1/2 hour I have the Eico on a dim bulb test
and the bulb is dim which is a great sign. I replaced all the lytics including the can
cap with one of my F&T 50+50 cap with a 20 on the bottom. I am using a set of
those heavy duty Russian 6P14P's I hope that's good.

Tube

 

[tube-a-lou]

More cleaning, More pictures of the chassis, still waiting for the caps
to replace the .1uf's. Second picture is the blue eyed monster (F&T cap)
Third is the selector switch and the mode switch, that was pretty dirty,
I used Flitz polish on the silver switches.

Tube

 

[Dandy]

Nice progress Tube. Those rotary switches are tough to clean and service.

I still wonder about all those bigger ceramic caps below the deck, but I've never even heard one of these (!), so I'll just wait till you report on the sound.

 

[tube-a-lou]

The F&T been out for awhile, AES has them and the prices is better than the
Sprague, the only thing I really have to do is replace the Pyramid caps there
original, But I wonder if there good.

Tube

 

[NOSValves]

The are never truly good...50 year old caps are just that....I would have thought you had already learned that fact Replace all caps in the signal path or that are DC voltage bearing at the very least...electrolytic and film....

Do it once do it right.....

 

[tube-a-lou]

Nope just waiting for the mailman, they should be here by Monday.

Tube

 

[kvflyer]

The are never truly good...50 year old caps are just that....I would have thought you had already learned that fact Replace all caps in the signal path or that are DC voltage bearing at the very least...electrolytic and film....

Do it once do it right.....

I know that it is like beating a dead horse (sorry about the cliché) but that's just the way it is. I have found some of my HH Scott amplifiers with the white American Radionic Ceracap capacitors still testing perfect. Guess what? Gone. Yes, they may test OK. But how can one justify a smoked output transformer when one of those paper capacitors decided to "Go South". Not worth it.

Have I been guilty of trying an amplifier with original capacitors? I would be a hypocrite if I said, "Never". But I was fully aware that it was not the right thing to do.

When those coupling capacitors come out, you can breath a sigh of relief for sure. Finding and exact replacement output transformer would almost have to be from a donor. I hate to admit "downgrading" a Scott amplifier to "parts", but unfortunately it has happened. Most likely because the previous owner already did the damage.

Good luck and keep it up!

 

[tube-a-lou]

As I wait for the Mail tomorrow I did a little more to the amp, I cleaned those lovely
inputs on the back of the amp on small panel. I replaced some of them with original
NOS that I got awhile back and gave the front another cleaning which came out
pretty nice.

Tube

 

[mibag6]

Looking good Tube! Are those faceplates lacquered or bare brass?