Eico HF-12 Rebuild

[kpaxfaq]

Hello Folks,

I am in the process of completely rebuilding my 2 HF-12's from scratch. I am recapping with 716p orange drops and replacing all resistors with metal film equivilant. I am also going to relace the bass/treble/volume pots with new ones. The only parts I'm not replacing are the small value ceramic caps.

I replaced the ceramic coupling caps with 0.047uf orange drops and replaced all of the electrolyics (including the can caps) underneath with individual caps.

Are there any recommended mods for these units or am I where I need to be?

They sound good right now to me, but like all of us I am looking to bring out the best sound possible! Let me know if there's anything I've done already that you recommend changing significantly.

Thanks! :thmbsp:

-Kory

Here is a link to owner's manual with schematic http://akdatabase.org/AKview/albums/userpics/10004/Eico HF-12 Owners.pdf

 

[Bluelobster]

So far it looks like a good job. Did you think about voicing though when you chose your resistors and caps?
Listen for awhile then get some carbon comp. resistors for those in the signal path. I have had good results with these KOA and you can get them at Mouser Electronics.
http://www.koaspeer.com/products/resistors/leaded-resistors/cf-cfs/

Then after another long listen try mixing in some different caps. Electrolytics are good for power supply filter duty. Illinois Capacitor MPW are good in the signal path and maybe try some Russian K40y as the coupling cap on the output tube.
It all makes a difference.

 

[kpaxfaq]

Well here's the interesting part. I first recapped and replaced electrolytics on both units. Today I replaced all the resistors in one and didn't have time to do the other. I measured all resistors I removed from the first one and not a single one was out of spec. There were 1 or 2 that were pretty close to out of tolerance but they technically were still OK. In the second unit with original resistors, all voltages were very close to unit with all new metal film. I played both for an hour and did my best to compare and could not tell the one with carbon comp from the one with all metal film. I know it's a big can of worms whether resistors affect the sound but honestly based on my short test I see not reason to not just replace all with 1% metal film for stability, reliability, and accuracy.

I am open to listen to opinions...and will plan on giving a longer listen to see if I can tell a difference.

I guess there are people who also believe in retaining ceramic coupling caps but I honestly cannot buy that one!

 

[Dmannnnn]

There might be some information at the eico group on yahoo. I just joined yesterday. One thing I saw there was a list of capacitance and resistance values for the PEC cards in the tone section. I'm thinking about replacing those in my HF81.

Figuring out a new grounding scheme might help as well. That is something I am currently investigating. Here is some very good information. It is written for guitar amps but still applies.

Have you found good pots? I think I spotted some decent ones at a guitar supply site, but they aren't long shaft. I think finding long shaft dual gang pots is going to be difficult. Let me know if you find some. I'll send a link to what I can find. I figure I will just have to make some short shaft extensions and use shaft couplers.

 

[kpaxfaq]

There might be some information at the eico group on yahoo. I just joined yesterday. One thing I saw there was a list of capacitance and resistance values for the PEC cards in the tone section. I'm thinking about replacing those in my HF81.

Figuring out a new grounding scheme might help as well. That is something I am currently investigating. Here is some very good information. It is written for guitar amps but still applies.

Have you found good pots? I think I spotted some decent ones at a guitar supply site, but they aren't long shaft. I think finding long shaft dual gang pots is going to be difficult. Let me know if you find some. I'll send a link to what I can find. I figure I will just have to make some short shaft extensions and use shaft couplers.

Hmmm didn't think about the long shafts, just looked at the values and figured it would be easy. I'll have to look into it further.

 

[Bluelobster]

I guess there are people who also believe in retaining ceramic coupling caps but I honestly cannot buy that one!

In the signal path in any other amp (except an hf81 that I will get to) I would say yes. As a shunt to ground or for noise suppression I would leave them.
I don't know if it's too late yet but if you have not dug into the secound amp try just replacing the electrolytics set the other hf12 aside and have a listen, in mono.
Next listen to your restored amp with all new parts and ask yourself which one you would rather have for the long term.
I can't explain it but the HF12 and HF81 just seem to click with all those parts that are supposed to be second rate. They are not highly detailed, wide soundstage or explosive mids kind of amps but are very commfy, musical amps you can sit with for a long time.

 

[kpaxfaq]

Have you found good pots? I think I spotted some decent ones at a guitar supply site, but they aren't long shaft. I think finding long shaft dual gang pots is going to be difficult. Let me know if you find some. I'll send a link to what I can find. I figure I will just have to make some short shaft extensions and use shaft couplers.

Correct me if I'm wrong but dual ganged pots would be unneccessary as because this is a mono amp.

This switched 500k pot looks like it would replace the treble+on/off switch nicely.


http://www.tubesandmore.com/scripts...clients/cemirror/ELEVATOR.FXP?item=R-VS-500KA


PS - I've been at work since 2pm so I am unable to poke under the hood to compare while looking.

 

[Dmannnnn]

Correct me if I'm wrong but dual ganged pots would be unneccessary as because this is a mono amp.

This switched 500k pot looks like it would replace the treble+on/off switch nicely.


http://www.tubesandmore.com/scripts...clients/cemirror/ELEVATOR.FXP?item=R-VS-500KA


PS - I've been at work since 2pm so I am unable to poke under the hood to compare while looking.

You are correct. We have essentially the same amp but I need to find the dual gang pots.:yes:

 

[Dmannnnn]

This switched 500k pot looks like it would replace the treble+on/off switch nicely.


http://www.tubesandmore.com/scripts...clients/cemirror/ELEVATOR.FXP?item=R-VS-500KA

That should work, but the only problem is the same as the orginal - You need to adjust the tone control every time you turn it on.

I don't really see a problem with that, but you might consider putting a power switch on the back. My amp already has a hole drilled in the faceplate so I will keep the power switch there I guess.

 

[kpaxfaq]

That should work, but the only problem is the same as the orginal - You need to adjust the tone control every time you turn it on.

I don't really see a problem with that, but you might consider putting a power switch on the back. My amp already has a hole drilled in the faceplate so I will keep the power switch there I guess.

Actually I could use that same switched pot for volume control and wire the power over there and replace the treble pot with an un-switched unit.

I guess I look at it like I could smother the old pot with a bunch of DeOxit or put a brand new one in for under $3. Seems like an easy choice to me...

 

[Dmannnnn]

Actually I could use that same switched pot for volume control and wire the power over there and replace the treble pot with an un-switched unit.

I guess I look at it like I could smother the old pot with a bunch of DeOxit or put a brand new one in for under $3. Seems like an easy choice to me...

It makes more sense to me to put a switch on the volume. I guess their thinking was that you could leave the volume at a set level all the time. I'd rather leave the treble and set the volume.

It is a no brainer. Just replace it. I've disassembled a few of those pots and gave them a complete cleaning of the element and contacts. I also tweaked the contacts to give a better connection. The pots are still very noisy. I think something has happened to the carbon elements with age. If I can't find replacements for mine, I will try sanding the carbon a little and baking them to remove any moisture. Hopefully I can just find some new ones.