Eico HF-20 questions?

[Jenkster]

I recently picked up a pair of Eico HF-20's on CL. The serial numbers are quite far apart, 16XX and 10,7XX respectively. Looking at the inside, the brand of caps/resistors and even some of layout appears different to me? Also, the power transformer and maybe the cans were replaced at some point on the later amp. Looking at the pics, does it appear much of any work has been done to either amp, save for the transformer? Would I be better off to do a complete re-cap on both so they will, hopefully, sound close to the same? Thank you in advance for your heilp!:thmbsp: Pics in this post are of the earlier amp.

 

[Jenkster]

Here's some pics of the later amp. Thanks again!

 

[dcgillespie]

Very nice amps! Both appear to have been fully and nicely recapped from the original build at some point, although how long ago is a question. The one unit clearly has a replaced power transformer as well. Other than this work, everything else appears to be true to the original design with no modifications done, other than the work noted. Good luck with them!

Dave

 

[Jenkster]

Very nice amps! Both appear to have been fully and nicely recapped from the original build at some point, although how long ago is a question. The one unit clearly has a replaced power transformer as well. Other than this work, everything else appears to be true to the original design with no modifications done, other than the work noted. Good luck with them!

Dave

Thanks for the info. Dave, I appreciate it! According to the seller, they were used almost daily until, sadly, the owner passed away a couple of months ago. I plan to take them to a friends place this week and check the tubes, bring them up on a Variac etc. Can't wait to hear them!

 

[Dandy]

Nice amps! Will probably sound great with a passive preamp. If the owner used them regularly till a couple of months ago, they should be good to go. But it pays to be careful. Good luck.

 

[Jenkster]

Nice amps! Will probably sound great with a passive preamp. If the owner used them regularly till a couple of months ago, they should be good to go. But it pays to be careful. Good luck.

Thanks Dandy, they are mono integrated amps which is how I plan to use them. May give me a bit of grief getting them dialed in evenly but I look forward to the challenge. I am always careful, especially with 50+ year old gear.:yes:

 

[ronaldw441]

Is the transformer on the later model a Standard Transformer PC-8412? If so it may be what Eico originally supplied. I also have a HF-20 with that transformer.

 

[Jenkster]

Wow, nice amps!

I took them to my buddy yesterday who did the down and dirty once over. He said it looks like they were re-capped around 1968 judging from the date code on a couple of electrolytic caps. Of course it could have been later but it's a starting point. Power tubes, RCA's, dated mid to late 50's. They tested around 70 to 73%. Not bad for being 50+ years old.:thmbsp: Will check the rest of the tubes later but they seem to be fine. He also checked the larger resistors that were OK but may need replacement before too much use. The power transformer in the later model is a replacement. They work and sound great with just a bit of hum. I am really impressed with how well they drive the woofers in the JBL's! I was tracking down rattles in my room I didn't know I had with my other amps including the Marantz 2330b?:scratch2: I think I will just use them sparingly until I decide which direction to go. I am leaning toward a total re-cap but, again, if it aint' broke......

 

[analog addict]

I'm betting that they will be just fine as they are.

 

[analog addict]

Oops. maybe a look at the multi cap cans is in order. Missed the part about the hum. they may have been "vintage" even at the time they were replaced....

 

[Jenkster]

Oops. maybe a look at the multi cap cans is in order. Missed the part about the hum. they may have been "vintage" even at the time they were replaced....

It is a low but audible hum and, yes, those will likely be the first to get an update. The good news is if I can't find close replacement cans, there's a lot of room inside for new caps. I will probably yard out those electrolytics as well.

 

[dcgillespie]

The nature of the hum will point you to its cause -- 120 Hz will originate in the power supply, 60 Hz in the audio stages. Eico was never known for using extensive power supply filtering in their earlier designs, of which the HF-20 is no exception. Since the output stage is fed straight from the rectifier tube, low power supply hum depends on a well matched set of output tubes to balance out the hum in the OPT. The addition of a small Dynaco style choke would do wonders. There is also a hum balance control in the heater circuit to help minimize any 60 Hz AC influence on the earlier stages.

Dave

 

[funwithampex]

It is a low but audible hum and, yes, those will likely be the first to get an update. The good news is if I can't find close replacement cans, there's a lot of room inside for new caps. I will probably yard out those electrolytics as well.

Get your can caps made by http://www.hayseedhamfest.com/ They look great in eico gear, the chrome covers add a lot of style. There not bad priced either, and you'll probably only need to replace them once in your life time :thmbsp:

 

[RWood]

If you decide to go under-board with the filter caps, here's an example of one that I did. Instead of a 40uF I used a pair of 80uF F&Ts in series, with balancing resistors. I mounted them on a small circuit board, on standoffs above the PT.

With the amp pushing close to 450V I wanted to have ample voltage handling capability. It came out nicely, although the amp's owner never could get used to the the separate Level and Loudness controls.

RWood

PS Long live "The Real Mac." The Albatross is alive and well on my turntable!

 

[Jenkster]

The nature of the hum will point you to its cause -- 120 Hz will originate in the power supply, 60 Hz in the audio stages. Eico was never known for using extensive power supply filtering in their earlier designs, of which the HF-20 is no exception. Since the output stage is fed straight from the rectifier tube, low power supply hum depends on a well matched set of output tubes to balance out the hum in the OPT. The addition of a small Dynaco style choke would do wonders. There is also a hum balance control in the heater circuit to help minimize any 60 Hz AC influence on the earlier stages.

Dave

Hello Dave,

I replaced the 6L6's with NOS JAN Sylvania 5881's and, poof, no more hum!:thmbsp: I also replaced the weak GE 12AX7's in V2 with a pair of strong Mullards. Everything is working great though I am going to replace the filter caps and an electrolytic in the bias supply for drill. In the mean time, I am really enjoying the !

 

[Jenkster]

If you decide to go under-board with the filter caps, here's an example of one that I did. Instead of a 40uF I used a pair of 80uF F&Ts in series, with balancing resistors. I mounted them on a small circuit board, on standoffs above the PT.

With the amp pushing close to 450V I wanted to have ample voltage handling capability. It came out nicely, although the amp's owner never could get used to the the separate Level and Loudness controls.

RWood

PS Long live "The Real Mac." The Albatross is alive and well on my turntable!

Nice looking work! Not sure which direction I am going to go with the filter caps yet. Both amps were re-capped at some point with slightly different value cans. I plan to use them with the cages on so looks isn't that important as you really can't see them.:scratch2:

PS Long live "The Real Mac." The Albatross is alive and well on my turntable![/:thmbsp:

 

[dcgillespie]

Glad you found the hum problem! Your old 6L6s may or may not be bad, but in any event, are clearly mis-matched. As stated earlier, due to the minimal filtering used, if the output tubes are not well matched, power supply ripple will not cancel out in the OPT and lead to hum. This was obviously happening in your case. Your NOS 5881s are wonderful tubes. This tube was manufactured to such high standards, that there were more chances than not that any two such tubes at random could be fairly well matched together. Enjoy your amps!

Dave

 

[Jenkster]

Glad you found the hum problem! Your old 6L6s may or may not be bad, but in any event, are clearly mis-matched. As stated earlier, due to the minimal filtering used, if the output tubes are not well matched, power supply ripple will not cancel out in the OPT and lead to hum. This was obviously happening in your case. Your NOS 5881s are wonderful tubes. This tube was manufactured to such high standards, that there were more chances than not that any two such tubes at random could be fairly well matched together. Enjoy your amps!

Dave

I got the 5881's from a friend of mine who tested them on a Hickok tester, the one with the punch cards. One pair is from 76 the other 79. Very closely matched pairs.:thmbsp:

 

[Kegger]

I've still got a single amp here, one of these days I'd like to match it up, or at least
the output transformer and make a nice pair of amps based off of them, I've got an
pair of chokes with the same potting cans, should make for some impressive looking
and hopefully sounding pieces of audio.
(I know that output was used on a few models, mine is the one stamped on bottom)

Congrats..:thmbsp:

 

[GordonW]

Hello Dave,

I replaced the 6L6's with NOS JAN Sylvania 5881's and, poof, no more hum!:thmbsp:

That can happen. All it takes, is unbalanced output tubes (different gain or transconductance), to cause a differential in current draw through the output transformer, which can allow reproduction of hum from the power supply. Normally, with balanced tubes, this sort of thing 'cancels out'... if each tube draws the same, any noise in the PS is essentially balanced/cancelled by the out-of-phase nature of PP output tubes. But, if one is significantly stronger than the other, you can pick up hum that way, due to a net current differential of one tube, over the other.

The exact substitution may be entirely incidental (you could probably have used matched pairs of 6L6s of any type)... but it's great that you found a pair of tubes that work well in each amp!

Regards,
Gordon.