EL34 based amp... Eico HF-87 maybe? - diyAudio
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Old 5th January 2003, 06:23 PM   #1
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Default EL34 based amp... Eico HF-87 maybe?

I found a schematic for an Eico HF-87 and to a tube n00b, it looks fairly simple- quite within my reach of building, I think. I have done a little solid state- headphone amps mostly, so I know how to solder and read schematics for the most part, except for some of the odder marks on tube amp schematics. I am thinking of giving up the loudspeaker design until I can better understand what the heck to do about a crossover. So, I thought I might do tubes to improve the current listening situation with my speakers. The schematic for the HF-87 I found is here: http://www.triodeel.com/eicohf87.gif

I have some questions, though.

1. These capacitors are marked mmfd, if I am reading that right. What do they mean? Is that just another way to say uF? Also on caps, do I use electrolytic? One cap appears to just say mfd- what is this value? Are these caps polarized? I cannot hardly make out the directions of the polarization if they are.

2. On resistors, I can pretty much get everything. Except for one. Right after the 6SN's, between them and the EL34's, where the 420V in is, there are two resistors, one of which says 33K 1W and does the other say 28750 1W? What value is 28750? IS this a normal one? Will several have to be paralleled to achieve this?

3. I only found one other topic when searching the forums about the HF-87. In it, someone suggested adjusting all cap values to larger ones, and smaller ones on the .1uF values. Why exactly?

4. Do you have to build a separate preamp section for each channel, or does one preamp section work for both? In other words, do I need 5 tubes per channel, or two per channel and 3 shared?

5. What kind of power transformer do I need? I have heard a lot about Hammond, but really know nothing about transformers. What specific voltages would I look for? Can anyone recommend a model number?

6. Same for output transformers. What do I use? What do I look for? Model numbers?

7. What about a power supply? Any diagrams floating around? I did a search on Yahoo and only returned 17 topics with the HF-87 in them. Will almost any power supply? Do I need tube?

I think that is all I am very unsure about... Thanks for the help.
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Old 5th January 2003, 07:05 PM   #2
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Default Re: EL34 based amp... Eico HF-87 maybe?

Quote:
Originally posted by trespasser_guy
1. These capacitors are marked mmfd, if I am reading that right. What do they mean? Is that just another way to say uF? Also on caps, do I use electrolytic? One cap appears to just say mfd- what is this value? Are these caps polarized? I cannot hardly make out the directions of the polarization if they are.
mmF = micro micro Farad, ie picofarad. uF is micro-farad.
If you look closely. you can see the cap symbols are mainly a flat line (+) and a curved line (-).
Quote:
2. On resistors, I can pretty much get everything. Except for one. Right after the 6SN's, between them and the EL34's, where the 420V in is, there are two resistors, one of which says 33K 1W and does the other say 28750 1W? What value is 28750? IS this a normal one? Will several have to be paralleled to achieve this?
You'll need to series a couple of resistors to make a composite one of that value and current rating.
Quote:
3. I only found one other topic when searching the forums about the HF-87. In it, someone suggested adjusting all cap values to larger ones, and smaller ones on the .1uF values. Why exactly?
Larger power supply decoupling caps usually won't hurt. The 0.1uF between the 6SN7 and the EL34 could probably be increased, cautiously. I've had some feedback amps do strange things wjen it's been done before.

Big tip: as you don't know exactly what you're doing yet, build it stock and get it working, then play with it. That way it will most likely work straight away, and be easier for people to help you fault find remotely.
Quote:
4. Do you have to build a separate preamp section for each channel, or does one preamp section work for both? In other words, do I need 5 tubes per channel, or two per channel and 3 shared?
You can share the 12AX7 sections between channels if you build it as a stereo chassis. If you build mono blocks then you will need a 12AX7 per chassis.
Quote:
5. What kind of power transformer do I need? I have heard a lot about Hammond, but really know nothing about transformers. What specific voltages would I look for? Can anyone recommend a model number?
In answer to this and your Q7, I'll calculate out a supply later. No time right this minute.
Quote:
6. Same for output transformers. What do I use? What do I look for? Model numbers?
You could buy pre-loved OPTs from this model, (or ST70 Dyna too probably). The Dynas were 4300:8 and the Hammond PT1650N would do, as would the Magnequest reissues (twice the $). The amp will likely put out 35W/ch or so.

There are a couple of Eico enthusiast sites IIRC, so check them out and they may have the PSU already, from the original drawings.

Off the top of my head;
350v - 0 -350V-> FW rectifier 5AR4 or SS -> (?) uF -> 10H -> 100uF+ -> power dropper resistor to 200V for 12AX7 -> 50uF+
5A at 6.3V per channel for heater, plus 5V at 3A for each 5AR4

Cheers
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Old 5th January 2003, 07:10 PM   #3
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Default EICO

Hi,

Quote:
1. These capacitors are marked mmfd, if I am reading that right. What do they mean? Is that just another way to say uF? Also on caps, do I use electrolytic? One cap appears to just say mfd- what is this value? Are these caps polarized? I cannot hardly make out the directions of the polarization if they are.
1.That would be pico Farad (pF) not micro Farad IMO.
Only two electrolytic caps are shown both are the cathode bypass caps.(marked with a +)
I can't seem to find a cap without its' value marked though.

Quote:
2. On resistors, I can pretty much get everything. Except for one. Right after the 6SN's, between them and the EL34's, where the 420V in is, there are two resistors, one of which says 33K 1W and does the other say 28750 1W? What value is 28750? IS this a normal one? Will several have to be paralleled to achieve this?
2.28750 Ohm.You can put a # of Rs in series of 1 W or 1/2W in // to obtain the same value.Not a standard value.

Quote:
3. I only found one other topic when searching the forums about the HF-87. In it, someone suggested adjusting all cap values to larger ones, and smaller ones on the .1uF values. Why exactly?
3.To increase bandwidth at low frequencies for the cathode bypass caps.I'd leave the 0.100 mF caps unless the amp tends to motorboat (unstable).

Quote:
4. Do you have to build a separate preamp section for each channel, or does one preamp section work for both? In other words, do I need 5 tubes per channel, or two per channel and 3 shared?
4.Only the first tube is half used:1/2 12AX7A so the other half can be used for the other channel if you build a stereo amp.
So that makes 7 tubes for a stereo amp.

5.+6. I don't have a Hammond catalogue at hand.

7.A tube is not necessary but I would recommend it + a choke filter (CLC).
You can have a look at some of the PSU designs in this section (Joel for instance).

Cheers,
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Old 5th January 2003, 08:17 PM   #4
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The original Eico schematic is here: http://www.users.interport.net/~fiddler/HF87S.ZIP

The 170 volt transformer may not be easy to find, but Hammond has a 300-0-300 V @ 250mA that would work - you'd use a half-bridge instead of the voltage doubler. You would need a little more voltage with a tube rectifier.
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Old 5th January 2003, 08:47 PM   #5
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>A tube is not necessary but I would recommend it + a choke filter (CLC).<

I agree. I recently converted an old Heathkit amp to CLC power supply. It's worth the extra effort and expence, in my opinion.
Dave
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Old 5th January 2003, 09:44 PM   #6
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Default POWERXFORMER

Hi,

A xformer of at least 500-0-500 is needed + all the heater supplies if you use a tube rectifier.

Cheers,
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Old 6th January 2003, 01:39 AM   #7
SY is offline SY  United States
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I've got one of these lovelies on hand. Classic circuit- my major change was taking an unused winding from the power xfrmer and converting the output stage to fixed bias. With that change (and EL38 output tubes), I got about 45 watts before any noticeable clipping.

Output transformers are 6600 ohm plate to plate with 43% ultralinear taps. These are different than Dynaco iron, so beware any substitutions.
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Old 6th January 2003, 02:31 AM   #8
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Thanks to everyone who responed thus far.

I downloaded the schematics provided- they helped me to see what was going on a little better. I now have a few more questions, though, about specific components I will need.

1. On resistors, about what wattage would be good? On some in the power supply, 10W or 5W are marked, and on some in the amplifier section, 1W are marked. Does this mean that 1W types are not required all around? Would it be a good idea to use at least 1W for all resistors?

2. Those values which are marked mfd are milli farads? Where do I find these? What value would that be uF? Are most of the cap values really that small? They seem as though they are larger when you see the huge caps on the chassis...

3. The power supply provided in the schematics is not tube rectified... and I would really prefer tube rectified. Anyone know where one of these might be floating around? Also, the most power put out by a power transformer sold by Parts Express (the Hammonds) is 450V-0V- -450V. Is this not enough?

4. Output transformers... Thanks for the info on them, SY. How does the amp sound, BTW? I am curious to know, since this won't be a very cheap undertaking. Anyway, I found a Hammond transformer at Parts Express, the 1650P, which should work according to the info by SY. It is 6600 ohm primary, and cantake 60 watts. The only other 6600 could only take 20 watts, so I figured it wouldn't work. Is this the proper transformer?

I think that is all I didn't know at this moment... probably will find more things I don't understand in the schematics. Thanks all.
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Old 6th January 2003, 02:51 AM   #9
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Default NUIT BLANCHE.

Hi,

1.No,you should stick to the wattage as indicated.A bit more won't hurt but certainly nothing less.
Also respect the voltage ratings indicated according to the variuos types of Rs.
I.e.don't use a rating of 100V for a 200V etc.

2.mfd=micro farad.
Yes,this is a tube circuit working at relatively high voltage so small caps are used.

3.No.450Volts from the xformer won't cut it,the rect tube has a voltage drop due to its' highish internal resistance.

4.Sounds O.K to me although there are better ones.

Quote:
think that is all I didn't know at this moment... probably will find more things I don't understand in the schematics.
We all had to start somewhere.

Cheers,
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Old 6th January 2003, 03:16 AM   #10
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Quote:
Originally posted by hurdy_gurdyman
I recently converted an old Heathkit amp to CLC power supply.
tell us about the fireworks again

dave
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