mike stehr Posted February 14, 2003 Share Posted February 14, 2003 Last year I bought these HF-12's from Jazman. They had some problems of course, and were my first tube amps. I befriended a local retired EE, Who is no fan of tone controls. Of course that was his first advice to dump the tone controls. Well, the pre-amp circuit was ditched as well. The amps sounded OK without the tone/pre circuit, but seemed thin. One reason is I never got the filtering straight. Then the multisectional capacitors got so noisy that the amps were pretty much to the point of being unlistenable. I gave up on them and played with some SE 6BQ5 amps for awhile. But I still wanted to get these Eico's up to snuff. We tried one with the paraphase inverter circuit and that didn't fly. It was worth a shot. So after twelve hours working on them today,(good thing I'm not a tech)I managed to get the one amp back with the original cathodyne inverter circuit, and then replaced the multicap on the other with some quad JJ tesla's that were on sale for ten bucks a pop. Cleaned up the layout a bit as well. 40 mfd on the center tap, 40 mfd on the screens of the output tubes and 40 mfd on the plates of the driver. They both have .033 mfd Cornell Dublier vintage oils for coupling caps. I just fired them up and they are quiet! A slight mild hum when you put your head right next to the speaker and thats it. And they both have a even hum between each amp. (My little magnavox single ender is still more quiet however, but not by much.) One power supply tranny has a mechnical buzz a bit, but the overall hum is even with the other amp. It makes sense, when I pulled the bell covers to paint them the windings on the power tranny do looked rather baked. (So I better start thinking about what I'm gonna do here.) But they don't sound thin anymore! I can listen at low levels, something I could never do before. They do have sort of a single ended sound to them, the Maggie is a bit more open but the Eico's have more of a ballsy sound,(Of course, they are PP's.) I guess it's safe to say that they are HF-14's now. Everybody is crashed at the moment, so I'll have to wait until Tomorrow to stuff my foot in them. I still need to build me a pre-amp nonetheless. Finally!!! Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 14, 2003 Share Posted February 14, 2003 Mike, That is great !! Isn't it a wonderfull feeling when you finally fix one of these after it gives you fits. So what your saying is when bypassing the preamp and tone controls you were missing some the filtering of the Can capacitor or was the factory can bad ? What was the value of the original ? Where did you find a JJ multi section Lyctic of 4 X 40 at ? I have never seen that value available. The only 4 section I've seen was 4 X 20 . Craig Quote Link to comment Share on other sites More sharing options...
Hofy Posted February 14, 2003 Share Posted February 14, 2003 Glad to hear you have those amps up to snuff now. I am still getting ready to do this to my HF12s. (Mine will be in a new chassis as well). Where did you manage to get the 4 x 10 caps? Let us know how they sound when you get a chance to crank 'em up. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted February 14, 2003 Author Share Posted February 14, 2003 Sorry Craig, I should have been more specific. The first 40 mfd is the Sprague Atom power supply cap from the rectifier to the CT of the OPT. Through a 1K resistor to the first 40 mfd section of the JJ quad can. This then feeds the screens of the output tubes. Then a 10K 1 watt resistor from the 40 mfd section to two 20 mfd sections tied together for 40 mfd. This then goes to the plates of the 12AX7 driver. (I had the resistors bass-ackwards, until I paid attention to the schematic.) I originally had 20 mfd on the plates of the driver, as I had posted earlier. The plates only have 20 mfd on them when you bypass the pre-amp and phono sections of the HF-12 using tape out as input. The original cans were 20,40, and 40 mfd, and they were both bad. The JJ quad is 40, 20,20,and 20 mfd @ 500 volts. I still have a 20 mfd section left over. Hofy, I'm still using the original HF-12 chassis's. The JJ quads are from Angela Instruments, and are still on sale for ten bucks apeice. They are not twistlocks, so you have to use clamps for them and drill a couple holes in the chassis. They are short enough that you can still use the the tube cage. (Like I ever will.) I then took the ground tab on the JJ quad and routed a ground wire from the ground tab of the quad cap to the ground point of the power supply cap. I couldn't pull off complete star grounding, but am down to a minimum of ground points. I'm kinda bummed now, I ran out of projects. Except my speaker project, which is almost complete. Thanks for the kudos, guys! Quote Link to comment Share on other sites More sharing options...
Hofy Posted February 14, 2003 Share Posted February 14, 2003 No, Thank YOU, Mike. Thanks for the information and the inspiration on my own hf12/14 rebuild. I am rethinking the the use of the old chassis. It would save money and I am pretty cheap. I could pull the preamp circuits and put on a blank face plate. Also some black paint like Kelly has done on his HF81. Hmmm. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 14, 2003 Share Posted February 14, 2003 Mike I have a project for you add that extra 20 MFD to the center taps of the output transformers you will like the results. Quote Link to comment Share on other sites More sharing options...
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