Eico HF-81 Bass Mod?

shacky

shacky

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OK I admit it I'm a bass fanatic. I know the strength of the Eico HF-81 is in the midrange. But is there a way to bump up the bass a bit via modification? I've got the bass tone up to about 3-4 O'Clock. Any more and it's a little too boomy.

Wondering if there are any tricks I could use on the pre or amp section to give bass more impact. Negative feedback :scratch2:

Eico1.jpg


Eico2.jpg
 
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I see HF-85 pre amp has loudness switch. Since the HF-81 is basically an HF-85 + amp, could I put a loudness contour in somehow?

On most amps I listen with loudness on.
 
Perhaps a subwoofer would be in order? I know what you mean about liking good low end.I have read that having good filter cap reserve in the power supply can firm up otherwise flabby bass in a tube amp.It looks like your tube amp is well restored.I am running a Mc240 with Cizek Model 1 speakers.I don't get building shaking lows,but the reproduction is good for my taste.
Dave
 
Maybe someone can figure out how to change the frequency at which the bass control begins to be active... I guess moving it down a bit could allow you to boost the low bass more without making the mid bass as boomy?
 
Other AKers have indicated that output stage cathode bypass caps can be increased by a factor of 10 or more to slightly improve low-end performance. I am personally suspicious that this model does *not* exhibit flat frequency response as designed. There are a number of peculiar-looking circuit tweaks in the schematic. It's high time someone carried out some basic measurements...
 
One more thing: You could try moving the feedback taps from the 16R terminals to the 8R terminals, assuming that you use 8R speakers. If you don't alter the feedback divider resistors, then you end up with less global feedback, but it's slightly more accurate. Easy thing to try, anyway, and very low risk.
 
JonL said:
Maybe someone can figure out how to change the frequency at which the bass control begins to be active... I guess moving it down a bit could allow you to boost the low bass more without making the mid bass as boomy?
Click to expand...

Still blasting away here...

Just add capacitors from the ends of the bass pot to the center, effectively putting them in parallel with caps internal to the PEC at pins 2, 3, and 6. 4.7nF caps should bring the hinge frequency down by half. The external 1.2nF cap is apparently a factory frequency-response tweak that might need alteration as well. It might be there to bring electrical center closer to mechanical center. Ideally, you would use a squarewave generator and scope to sort this out.
 
Thanks Mike for your ideas. I want to love this little Eico especially after all the $'s I've put into it. It sounds very good and someone looking for this sound would love it.

I'm borrowing a friends Dynaco ST-35 and using my Sansui 890DB as preamp. I like the Sui loudness contour and this combo sounds great - better using 8 OHM tap with my Forte's.

With the Eico I've tried both the 4 and 8 OHM taps and seem to get better sound with 8 OHM. I lost 16 OHM tap in last mod but it should still be there somewhere.
 
In stock form those HF-81 amps have great bass, when modifying and
changing the non electrolytic's to different types you can very easily alter
the sound of the amp and sometimes lose a lot of bottom end.
I have experimented with other tube amps and know that the right
combination of caps can really have a big effect on the bass.
My dad has a Eico HF-81 and he has told me that it is sensitive to cap
change.
 
Upping the value of your coupling caps somewhat will increase the bass. :yes:

Cheers

Lar
 
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You usually get better bass by starting with a better power supply. The HF-81's supply can be easily improved.

On my HF-81, right off the EZ-81s, I go into a small and inexpensive Traid C-40X 320 mHY choke, rated 600 mA. at 10 Ohms DCR. Then, follow with about a 47 uF electrolytic. Then, use a small Panasonic "AC line filter" as a L2 choke, (the 850 Series), forgot the value, with another 47 uF as C2.

This kills two birds with one stone !!

(1) You reduce the OVER voltaged B+ by about 16%.

(2) I was able to get real bass "punchiness" by converting the amp from the stock, (nasty sounding) C input filter to a svelte L1/C1/L2/C2 filter. I loved the low end "form" after I did this change.

The power supply is the key to a good low end. The stock Eico's HV winding is 86 Ohms DCR, (not terrible), and you have dual tube rectifiers, (good) so NOW, just make the input filter L1/C1/L2/C2 like better amp designers should do.

Ohh, there is one other "MUST" HF-81 mod I have discovered. On the input tube of the amplifier, cut OUT the nasty sounding stock amp's bell wire connecting Pin 6 to Pin 2, and solder at least two (or even three) paralleled runs of Kimber Kable TCSS across there. This improves transfer function in a HF-81's ultra sensitive area. Write to me if you try it, hear it, and like it.

Jeff Medwin, Warrensburg, MO USA
 
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Thanks for reviving this thread!

You won't believe the coincidence. Craig of NOSVALVES is shipping my HF-81 back to me as I type this.

Craig went throug my HF-81 and made many changes in caps, cap values, and resistors.

Can't wait to hear how it sounds now. Hoping it comes before the weekend. :banana::banana::banana:

EDIT: FedEx shows it's coming tomorrow. And I have to work...
 
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Well my HF-81 was delivered safe to my door today. Craig changed 22 resistors, 12 caps , and 2 cathode bypass caps. Changing values back to original.

NOWI HEAR WHAT THE EICO MAGIC IS ALL ABOUT!!!

Bass is back. Nice and bloomy. It sounds more 3 dimensional too. I'm going to be doing a lot of listening this weekend!

Here's before:
Eico1.jpg


Here's after:
IMGP0319.jpg
 
You said value back to original isn't it?

Mine is singing and has sufficient low end with all stock values. And , oh shame, I even left the ceramics couplings in there :)

The sound is earthshaking through a pair of Electrovoices cabinets (around 98dB/1w/1m).
 
Patrice B said:
You said value back to original isn't it?

Mine is singing and has sufficient low end with all stock values. And , oh shame, I even left the ceramics couplings in there :)

The sound is earthshaking through a pair of Electrovoices cabinets (around 98dB/1w/1m).
Click to expand...

Yes. Previous recap increase coupling caps - 0.1 I think? Ceramics were already gone when I got this or I would have kept them too :yes:

The previous work tightened up the bass too much. And as Craig taught me, the metal oxide resistors, Auricaps, and changed values all leaned towards a bright with tight bass sound.

I didn't sound bad but I wasn't happy with it. Now I'm extatic :banana: :music:
 
Looking good, it's amazing how much better an amp can sound from
using the right combination of resistors and caps.:thmbsp:
 
It looks like all the Auricaps were given the heave ho. What are the replacements?

The large black caps on the preamp, the four .1 caps in the tone/af amp section, and the 4 little orange guys between the phase splitter and the output tubes, what are they all?
 
The .1 caps look like they might be k40y's with black heatshrink, maybe the preamp caps as well. Still curious about the little orange guys.
 
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