Hofy Posted February 7, 2003 Share Posted February 7, 2003 Yes, I know, stupid question but I want to know. If you had the choice between an HF81 and a pair of HF14s with a pre-amp, which would you choose and why? Quote Link to comment Share on other sites More sharing options...
jazman Posted February 7, 2003 Share Posted February 7, 2003 Much would depend on your economics. The HF14s are not very common to find, but reported to have slightly more bass to offer. Of course they require you to add an additional piece of hardware, the preamp. The HF14 is essentially the same power amp section as an HF12, or and HF81. The HF81 gives you a nice power amp and preamp in one unit. It would be a very nice choice to have. Klipsch out. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted February 8, 2003 Share Posted February 8, 2003 I'd take either one, but the HF-14's are a little more uncommon. Hofy, I purchased a pair of HF-12's from Jazman last year. I ditched the pre-amp circuit to possibly get better sound, and it didn't. I don't get it, it's pretty much configured like a HF-14, right down to the values, minus the 500K level pot. So in a last ditch attempt configured as power amps, I'm trying the 12AX7 driver set-up as a paraphase inverter to see if I can get a little more guts out of them. One amp has the paraphase inverter circuit, the other amp is still set-up like a HF-14. Tomorrow We will compare the two and see what happens. If they still sound thin with no guts with either driver circuit, then I'll put the pre-amp circuits back in. Or buy or make a pre-amp. You would get all in one with a HF-81. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 8, 2003 Share Posted February 8, 2003 Well if it was me I would opt for a HF-14's you get better than what the HF-81 has to offer in the output section and loose the worst part of the HF-81 which is the preamp section. Now please don't every one jump out of there pants !! The fact is EICO used same cheesy parts through out the preamp. The PS is pretty weak which is why the 14 is known to have better bottom end and I bet upper also. So imagine the sound of the HF-81+ output section mated with a top notch preamp !! Bet this would be a setup that would last a guy a good long time ! Of coarse this would all depend on the budget too !! Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 8, 2003 Share Posted February 8, 2003 Mike, How are you pumping your signal into the HF-12s ?? There has to be some reason that there sounding thin. This really shouldn't happen. The HF-14 is known to have a great bottom end. All though every time I have tried to use a HF-81 or HF-12's as a power amp through the tape outs I have found the same thing. Your not using the tape outs as the input are you ?? I think EICO used some freaky off the wall way to make this work. Craig Quote Link to comment Share on other sites More sharing options...
mike stehr Posted February 8, 2003 Share Posted February 8, 2003 My HF-12's are configured like a HF-14. There is no pre-amp circuit, no knobs, just in and out. It's basically a HF-14 on a 12 chassis, minus the 500K level pot. If you look at a HF-12 schematic, the tape out taps in right to grid 7 on the 12AX7. And if you compare it to the schematic for the HF-14, it does the same. The input goes through a 500K pot to ground, then a 10K to grid 2 of 12AX7. Mine just has a 470K to ground at the input. (The HF-12 schematic has the input to grid 7, HF-14 schem has the input to grid 2, either way it doesn't matter.) I'm pretty sure the HF-12 and HF-14 use the same type of power supply transformer and output transformer. I'm not sure about HF-81, I know the OPT's are different. I don't know, I just use the volume control off my CD player. You'd think I would have enough gain with the 2 volt output from the CDP volume control. I probably just need a linestage, more or less. Quote Link to comment Share on other sites More sharing options...
Hofy Posted February 8, 2003 Author Share Posted February 8, 2003 OK, Mike, you sniffed me out. I want to pull the transformers from some HF12s and build some HF14s. The 'THIN' sound has me confused though. The HF12 and HF14 used the same trannies I think. Could the layout make that much of a difference in the sound?? Quote Link to comment Share on other sites More sharing options...
mike stehr Posted February 8, 2003 Share Posted February 8, 2003 I dunno, I'll find out Tomorrow. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 8, 2003 Share Posted February 8, 2003 There really is something strange hear. I'm going to have to down load the schematic and take a look at both of them. I know the OT's are different from a 12 and a HF-81 not sure about the HF-14. I know that when I had a pair of HF-12's hear and compared them to a 81 the 81 sound a good bit fuller with better lower extention then the 12's but the difference I wouldn't describe as thin . All the transformers looked different to me. Craig Quote Link to comment Share on other sites More sharing options...
mike stehr Posted February 8, 2003 Share Posted February 8, 2003 I think most my problems are source related Craig, and having no pre-amp. I'm running a Eico HF-12 with the stock driver circuit now. Seem's to sound fine, plenty of grunt for what it is. Of course I've been listening to SE 6BQ5 amps for the last 3 months, so a few more watts from a PP is a change. Tomorrow I get to listen to a pair of Webster/Allen 90 watt mono 6550 PP's, before they get shipped away. These things are monsters, they do a real 70 watts. I had seen them, but one had a fried tranny. I've yet to hear a Push-Pull with that much power, I'm curious as hell. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 8, 2003 Share Posted February 8, 2003 If you like it loud and full of dynamics those amps should floor you !! Let me know how they sounded. Craig Quote Link to comment Share on other sites More sharing options...
Hofy Posted February 8, 2003 Author Share Posted February 8, 2003 Craig, I have looked over the schematics of both the HF12 and HF14 and they look the same to me. Maybe I missed something? I think think they use the same transformers but I cant be sure as I dont have a HF14 in hand to compare. The HF12 uses an EICO model 30019 power tranny and a 32005 output. If anyone has an HF14 could they please post the tranny numbers? Thanks. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted February 9, 2003 Share Posted February 9, 2003 Those Webster/Allen amps are loud and dynamic, that's for sure. And clean too! I like 'em, great sound! Too bad they are sold. Pretty much unpractical for someone with high efficiency loudspeakers, but man, they would be nice to have. They can push Magnepans. The Eico HF-12 went on the scope with the paraphase inverter driver circuit. (This was a experiment, to basically see what would happen.) Oscillation big time, on the front end and the output. First I wired the phase wrong on the driver tube, oops! Fixed that, and then after about a hour of swappin' resistors, changing cap values in the feedback loop and on the driver, the thing finally put off a decent 5K square wave and seemed more stablized. I think he kind of concluded that there was too much gain in the driver stage and it would oscillate into the output stage and then the output stage would oscillate. But he did get it stable, and we did listen to it. It did not sound bad, but he was hearing something in the amp that wasn't in the source music being played. I noticed that the high-end seemed kinda dull and muted. (Compared to the stock HF-12 driver circuit.) Then we compared the HF-12 with the stock driver circuit.(Cathodyne?) Nice airy mid and top-end like I remembered. We both then agreed the Eico sounds better with the stock driver circuit. We then put the Eico with the stock driver circuit on the scope. It looked pretty good, went out to about 40Khz, and down to about 25 or 20Hz before core saturation. 8 watts, 10 if you wanna really push it. (Pretty much like a HF-14.) He thought the paraphase inverter circuit would work out pretty good. It seemed like a good idea, and I'm sure it works well in some amps. But not the Eico HF-12. It didn't sound bad, just not as good as the stock circuit. Didn't affect gain much either. Well, sometimes you have to try different things, and sometimes they don't work, but the learning experience is still there. We were playing my Eico's on some rather nice Sansui speakers ported to 25 hertz. Both HF-12' amps with both different driver circuits had plenty of bass, and did not seem thin. So my problem must be source related. I think I need a line stage pre-amp. It seems to all boil down to the worthless volume pot I'm using for gain in my carver CDP. How tough can it be to build a line stage?!! Argh! I did like those mono 6550 push-pulls, choke rectified, output tubes screens regulated with gas tubes, and simple layout as hell. Craig, I bet you had to have a HARD time shipping off those Dynaco's of Dales! Hofy, From the pic on the Eico website,(The little ad with specs and stuff)the HF-14 appears to have the same looking power and output transformer. And I'm sure you can turn a HF-12 into a HF-14, I did. But be aware that you'll gyp yourselve out of a pre-amp stage.(Which may not be a bad thing.) Hmmm... HF-12 only has 20 mfd on the plates of 12AX7, HF-14 has two 20 mfd sections tied to 40 mfd going to the plates of 12AX7. And when you use the tape out as input on the HF-12, it bypasses the two 40 mfd sections for phono and pre stages by the looks of the schematic. Leaving you with 20 mfd on the plates, maybe it's the lack of capacitance on the driver plates resulting in thin sound. (I'm just taking stabs in the dark here!) I need to replace the multicap anyway, I should bump the driver plates up to 40 mfd and see. My Norelco 12" FR's may be better off on a open baffle ala Eric Mandaville. But I need something bigger than a shi**er lid. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 9, 2003 Share Posted February 9, 2003 Mike, Great post and sounds like you had some serious fun there. This friend of yours sound real Knoledgable !! As far as Hmmm... HF-12 only has 20 mfd on the plates of 12AX7, HF-14 has two 20 mfd sections tied to 40 mfd going to the plates of 12AX7. And when you use the tape out as input on the HF-12, it bypasses the two 40 mfd sections for phono and pre stages by the looks of the schematic. Leaving you with 20 mfd on the plates, maybe it's the lack of capacitance on the driver plates resulting in thin sound. (I'm just taking stabs in the dark here!) I need to replace the multicap anyway, I should bump the driver plates up to 40 mfd and see. I bet you hit the nail right on the head with this !!! The Mark III are some awesome amps when Modded and rebuilt. I'm not saying there the end all here. But for the music I listen too they are nothing short of amazing. Dynamics to no end with plenty of headroom even in full Triode mode !! Craig Quote Link to comment Share on other sites More sharing options...
Hofy Posted February 9, 2003 Author Share Posted February 9, 2003 ---------------- On 2/9/2003 4:42:50 AM mike stehr wrote: Hofy, From the pic on the Eico website,(The little ad with specs and stuff)the HF-14 appears to have the same looking power and output transformer. And I'm sure you can turn a HF-12 into a HF-14, I did. But be aware that you'll gyp yourselve out of a pre-amp stage.(Which may not be a bad thing.) ---------------- From what I can tell they are the same output transformers. I just want to be sure before I get to far into this project. As far as the preamp, I plan on building my own. I havent decided on what yet though. Quote Link to comment Share on other sites More sharing options...
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