Speaker hum/hiss with Eico HF-81

breadhead

Active Member
Hello again,

As I stated in my last thread I am in the process of rebuilding an Eico HF-81 amplifier. I got rid of the very loud hum I reported in the first thread by discovering that in the process of replacing the power supply filter caps I had bent a resistor lead and it was touching one of the tube pins!!! The amp sounds real good now but I have a very mild hum in one speaker and a slight hissing type noise in the other speaker. Any suggestions as to the source of this noise would be greatly appreciated.

Thank you all for your help,
Jim
 
tubes have filiments that use about 6vac. Maybe a tube in that ch is "leaking"
filiment hum. Old caps may be going out not filtering, in filiment supply or main supply.
Or anyplace else in the chain. Flaky resistors can cause hiss. Sounds like you need to poke around in there and locate the hum/hiss sources. Check the pwr suply/s for hum.
Hope you have at least a schematic.
Beware, my advice has been questioned here on AK, so don't do anything that sounds unorthodox.
 
Depending upon the level, the hum and the hiss may be typical and normal.

There is a hum balancing pot on the HF-81... did you adjust it?

My HF-81 is hummier than it should be, and caps are all fine. Folks at the EICO users group at Yahoo suggest tying (signal) grounds together to eliminate any ground loops, and also resoldering ground connections on the I/O jacks (and chassis grounds, too). Since many/most of these were home-built, there's no guarantee that the solder joints are all good!

The EICO manual also recommends shorting any unused high-gain inputs (i.e., the phono preamp or tape head input, if you're not using either or both) to maximize S/N.

Now, about hiss...

Hiss, while it can be pathological, is intrinsic, especially in high-gain circuits With tube amps, it's typically thermionic noise, although transistor amps do it too (some sort of shot noise I guess).

EDIT: One nice thing about the HF-81 is that you can isolate the preamp and the power amp, so you should be able to determine where the hum and/or hiss are originating.
 
I know this post is extremely old but I can’t seem to find any help. I have the EICO HF-81 connected to Heresy II’s speakers and I’m getting a constant flutter noise from both and I’m not sure how to go about this. I’ll take any advice at this point! Thanks in advance!
 
I know this post is extremely old but I can’t seem to find any help. I have the EICO HF-81 connected to Heresy II’s speakers and I’m getting a constant flutter noise from both and I’m not sure how to go about this. I’ll take any advice at this point! Thanks in advance!
Welcome to AK!

As you can see from this, what?, 17 year old thread, which only attracted two responses, questions about tube gear don't get much attention on this forum. I would suggest that you start a new thread over on the Tube forum.

Best to also refer to your problem (flutter) in the title along with the brand and model of the amp. If would also help if you include something about the general condition of the amp. Has it been restored in any way and, if so, what has been done.

What you're hearing is likely oscillation, sometimes referred to as "motorboating".
 
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