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In Reply to: dual Eico HF-86s/no bass posted by chopper87 on August 11, 2002 at 10:20:46:
Chopper,Probably a combination of certain old gain stage tubes and a tired power supply filter circuit. I'd change out the PS caps, first---as if they pop, they make a horrid mess.
I use Sprague ATOMS and try to go to higher voltage rated electrolytics. Don't forget to change the electrolytics & diode in your bias supply circuit, too. Those caps are just are old---and are critical for maintaining a steady idle bias. WATCH THE POLARITY OF ALL CAPS AND DIODES. I'd draw a diagram to be sure.
After the PS caps and bias caps are changed out. Check the B+ C+, & D+ voltages, as well as the NEGATIVE voltage on the fixed idle bias test points. They should be within 10-15% of nominal value.
My bet is a lot of good bass will return after these mods. Then, go after front end tubes, one-by-one.
Follow Ups:
Hi,
One of the sonic signatures of most EL84 based amps is a mids forward ,light bass presentation.Hey Steven,think Brian Adams riffs.This is a low parts count amp,so check all resistor values,output tube condition.
If you don't have any hum,the electrolytics are probably OK.Upping the values in the PS can change the voicing of the amp,and should be considered only after optimizing the speaker match with this amp.You've probably got about 14 watts rms per side ,and a low efficiency monitor may not work in your room.What are you using speaker wise?
Best,
Pat
Iam using two hf-86s strapped for mono. I am using them to drive a pair of B&W Nautalis 805s to more than satisfactory levels. They are remarkably open, clear, and alive but are, as you said quite light in the bass.
Are these mono-blocks in-phase with each other? Both the IC and the speaker connectors?Best way to tell is flip ONE of the channel's IC and give the system a listen. If no difference, put IC's back to original positions. Now, flip ONE of the speaker leads---and listen.
Of the bass suddenly comes back---the system was out-of-phase. Don't put too much creadance in arbitrary markings on the post and terminals. I seen plenty which have little bearing on the actual phase.
Steven,
They are not out of phase so I will make the other changes that you suggest. Are you also talking about replacing the filter capacitor cans with new larger ones, or, other related power supply capacitirs.
Chopper,The electrolytic filter caps, in your power supply. Use the same mfd value, but go to larger VOLTAGE rating. For example, say the OEM caps are 40 mfd/350 volts try to replace them with 40 mfd/450 or 500 volt Sprague Atoms.
One note, the 500 volt rated ATOMS are big. So be sure you have enough room for the caps.
Also, I find that larger voltage rated caps in the bias power supply filter section also gives longer life. I usually change the diode(S) in the bias supply. I feel that a stable bis voltage is critical to not only good tone, but happy, long life power tubes.
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