Hi. I'm new to the forum. I've recently build a guitar amp for myself from a kit that turned out really well. I learned some things that got me started thinking about these old hf12 amps I've had for a while. I was using them a couple years ago with my garrad rc/88 turntable to play my vinyl, and was thinking about firing them up again, but I'm worried they might not be safe.
Looking inside I see quite a few paper caps, some coated with wax. Should I just go ahead and change these? Also, do I need to upgrade to modern wiring with a 3 wire system and ground the chassis? If I can figure out how to post pics, I will put a couple up.
Is this worth the trouble? I'm not expecting much and don't need something high end. I think these will do nicely, rather than put out $400-600 for a nice kit. I don,t have much money invested in these.
It seems the Eico equipment is well respected. Any thoughts or advice on these hf12 amps will be appreciated!
Looking inside I see quite a few paper caps, some coated with wax. Should I just go ahead and change these? Also, do I need to upgrade to modern wiring with a 3 wire system and ground the chassis? If I can figure out how to post pics, I will put a couple up.
Is this worth the trouble? I'm not expecting much and don't need something high end. I think these will do nicely, rather than put out $400-600 for a nice kit. I don,t have much money invested in these.
It seems the Eico equipment is well respected. Any thoughts or advice on these hf12 amps will be appreciated!
If you were using them a couple of years ago they are most likely still ok. The one thing for sure that isn't safe is C16 on the AC mains input. Remove it! And yes, install a 3 wire power cord with a chassis ground for your own safety. Many here are quick to say replace capacitors just because. If the electrolytics were ok a couple of years ago, they probably still are. The only paper caps that might be suspect are the two coupling caps to the output tubes. (C12, C13) You'll know if the output tubes run too hot and get red or you have distortion.
Attachments
You have something to work with. 
Get a copy of the schematic off the WWW and study it.
Yes, installation of 3 wire, safety grounded, power cords is in order. You also eliminate C16, AKA the Death Capacitor.
All electrolytic caps. require replacement, as they have literally dried out over time. While they may "work", they have to go.
Replacing the waxed paper signal caps. is a good idea. A mixture of 716P polypropylene "Orange Drops" and Soviet surplus K40 PIO will do nicely.
Carbon composition resistors should be checked for drift out of tolerance.
As is common in units of that "vintage", the low level mag. preamp is switchable between the RIAA and NAB curves. IMO, switches in the lines carrying mV. level signals is a very bad idea. I routinely advise replacement of the source selector switch and hard wiring a single I/P to the mag. preamp section, which is permanently configured for RIAA. Make all the other I/Ps "line" level. Mouser carries low cost Lorlin parts suited to the job.
Speaking of the low level mag. preamp section, the OEM circuitry employs AC heating. If AC heating is retained, it is imperative that V1 be a Sovtek 12AX7LPS. The 'LPS is a genuine 7025 equivalent that contains a spiral wound, hum bucking, heater. It is possible to obtain satisfactory hum levels with AC heating. However, it's ANYTHING BUT easy.
Get a copy of the schematic off the WWW and study it.Yes, installation of 3 wire, safety grounded, power cords is in order. You also eliminate C16, AKA the Death Capacitor.
All electrolytic caps. require replacement, as they have literally dried out over time. While they may "work", they have to go.
Replacing the waxed paper signal caps. is a good idea. A mixture of 716P polypropylene "Orange Drops" and Soviet surplus K40 PIO will do nicely.
Carbon composition resistors should be checked for drift out of tolerance.
As is common in units of that "vintage", the low level mag. preamp is switchable between the RIAA and NAB curves. IMO, switches in the lines carrying mV. level signals is a very bad idea. I routinely advise replacement of the source selector switch and hard wiring a single I/P to the mag. preamp section, which is permanently configured for RIAA. Make all the other I/Ps "line" level. Mouser carries low cost Lorlin parts suited to the job.
Speaking of the low level mag. preamp section, the OEM circuitry employs AC heating.
If AC heating is retained, it is imperative that V1 be a Sovtek 12AX7LPS. The 'LPS is a genuine 7025 equivalent that contains a spiral wound, hum bucking, heater. It is possible to obtain satisfactory hum levels with AC heating. However, it's ANYTHING BUT easy.- Status
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