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Eico HF-87 Clone Build


Curious_George
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1 hour ago, henry4841 said:

A PP amp is too much for me these days. I have only built two of them. How many ma of current you running in those output tubes along with the plate voltage? Just curious. 

70mA per tube and plate voltage is about 430V.

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4 hours ago, Curious_George said:

 

I may put a drop of super glue on each of the leads where they meet the main body of the cap to seal it. 

Orange Drop Cap_2.jpg

 

 

If you're keeping the amp yourself I see no harm in just leaving them as is.

 

My comment was more for a reference for all builders following the thread.

 

If you end up getting curious (pun intended) about stability with the amount of feedback you have already you don't need anything special. Just shoot some square waves through it and keep increasing capacitance on the load. Start low, like simulating cable capacitance then keep adding more. Use whatever spare values you have on hand.

 

Also if you want to do the bode plot I'll explain my process. It's kinda fun actually. 

 

 

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9 hours ago, Curious_George said:

70mA per tube and plate voltage is about 430V.

The reason I ask is George Anderson and Modjeski likes to run a high voltage and less current in their designs. If my math is correct you are running 30W of dissipation on your tubes. What are your tubes rated for?

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2 hours ago, henry4841 said:

The reason I ask is George Anderson and Modjeski likes to run a high voltage and less current in their designs. If my math is correct you are running 30W of dissipation on your tubes. What are your tubes rated for?

Plate voltage is 430 - 33 bias voltage, so 397V plate voltage 397*70mA = 27.79 watts

 

Typical EL-34 are rated at 25 watts dissipation. I am using what Eico used based on the schematic and voltage reference measurements listed in the build manual. 

 

My goal was to emulate the HF-87, not redesign it (yet...). 

 

I have a quad of Svetlana EL-34's I bought probably 20 years ago I am using. They should last for the next 20 years in circuit.

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My last build consisted of 6 circuits. I just look for active devices when I look at a schematic and consider the components around that active device as a circuit one has to build to make that active part work as it should. Much easier for me to visualize what is happening in an amplifier. This one is more complicated for me to build being as you said some of those tubes are dual triodes or pentodes with separate circuits needed to make them work. I am thinking of using the parts from my Eico ST70 and just make an amplifier using just the amplifier section of the schematic for a build. Still more than I want to tackle but I do need to do something with that Eico of mine. 

 

Again great work George.  

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21 hours ago, Curious_George said:

Plate voltage is 430 - 33 bias voltage, so 397V plate voltage 397*70mA = 27.79 watts

 

Typical EL-34 are rated at 25 watts dissipation. I am using what Eico used based on the schematic and voltage reference measurements listed in the build manual. 

 

Maynard taught me to measure plate voltage, when finding dissipation of a tube, from the cathode to plate. You did it mathematically.  🙂  I am sure Eico used an output tube that has a higher dissipation probably more in the 30 to 35W range. I would just be careful selecting replacement power tubes when and if it becomes necessary unless you lower the wattage being run in the tubes.

 

I would sure like for you to look at the schematic of my Eico ST-70 and see what you think about just using the amplifier stage for a new build not using all the other circuits I really do not care for. That and I do not care about not having an original Eico ST-70, just want a PP EL-34 type amplifier. 

 

 

 

 

Screenshot (95).png

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2 hours ago, henry4841 said:

Maynard taught me to measure plate voltage, when finding dissipation of a tube, from the cathode to plate. You did it mathematically.  🙂  I am sure Eico used an output tube that has a higher dissipation probably more in the 30 to 35W range. I would just be careful selecting replacement power tubes when and if it becomes necessary unless you lower the wattage being run in the tubes.

 

I would sure like for you to look at the schematic of my Eico ST-70 and see what you think about just using the amplifier stage for a new build not using all the other circuits I really do not care for. That and I do not care about not having an original Eico ST-70, just want a PP EL-34 type amplifier. 

 

 

 

 

Screenshot (95).png

Henry - I think the ST-70 stripped down to just the amp section would make a great little PP amp. I can redraw the schematic for you to just include the amp section if you want. I see that it is fixed bias. Do you want to retain that feature or go with cathode bias? Both have thier advantages, but if you are not interested in every last bit of power (which I know you are not), I prefer cathode bias due to it's simplicity. With fixed bias, if you have a failure in the bias circuit and you don't catch it fairly quick, you will destroy your output tubes unless there is a least a fuse in the output tube circuit somewhere. 

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4 minutes ago, Curious_George said:

Henry - I think the ST-70 stripped down to just the amp section would make a great little PP amp. I can redraw the schematic for you to just include the amp section if you want. I see that it is fixed bias. Do you want to retain that feature or go with cathode bias? Both have thier advantages, but if you are not interested in every last bit of power (which I know you are not), I prefer cathode bias due to it's simplicity. With fixed bias, if you have a failure in the bias circuit and you don't catch it fairly quick, you will destroy your output tubes unless there is a least a fuse in the output tube circuit somewhere. 

Not really sure about fixed versus cathode bias. Either one will work for me. If you take the time to redraw the schematic as stripped down I would appreciate it. I would like to see what you come up with for cathode bias as well. The ST-70 amp has many more circuits and options that I do not care about. At the time it was introduced many of those options were standard on an amplifier. Not so much anymore. I only want my PP amplifier to be the amplifier part. I see this as a project in the future for me since I already have the expensive parts of a build. I paid like $300 for this amplifier on Ebay a few years ago. Worth much more than that in tubes and iron alone. All original tubes from what I can tell. All test excellent as well. 

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2 hours ago, henry4841 said:

I am sure Eico used an output tube that has a higher dissipation probably more in the 30 to 35W range. I would just be careful selecting replacement power tubes when and if it becomes necessary unless you lower the wattage being run in the tubes.

The EL-34 (NOS at least) were very rugged tubes. The 25 watt plate and 3 watt screen dissipation rating were fairly conservative (in my opinion). The guitar guys would really runs the EL-34's hot and they seemed to last a long time. The Svetlana tubes (and most Russian tubes) seem very well made. I'm not worried about running them at the current setting. I also like the JJ tubes. Asian tubes are my least favorite, but if you pick and choose, you can find some good ones (meaning reliable and perform correctly) at a decent price.

 

If the price for a given tube is the same from three makers such as Russia, JJ (Slovakia) or China, I would pick the Russian tube first, then JJ, then Chinese. 

 

I'll probably buy 4 JJ EL-34's as back-up since the price of tubes today will be lower than in the future. 

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4 minutes ago, henry4841 said:

You know what you are doing and know you shorten the life of tubes running them more than say 80%. Many do not know this. 

As long as I am running at 80% or less, that is all I care about. But seriously, I know. At this point in my life, tube life is not important to me, the sound is. 

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More time listening to the HF-87 clone... 

 

I hate to use typical audiophool adjectives, but this amp is one of the best sounding amps I have heard, regardless of price (tube or solid state). This is quite a statement coming from me. Why? Because typically, I can only barely perceive differences in amps when comparing them unless they are just junk compared to something good. 

 

It is totally transparent, but powerful. Sound, soundstage, imaging, noise, frequency extension (high & low) are excellent. 

 

If you are looking for a moderately powered push-pull tube amp, this is the one to make. For Heritage speakers, you do not need more than 35 watts per channel which this is rated at. 

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10 hours ago, Curious_George said:

More time listening to the HF-87 clone... 

 

I hate to use typical audiophool adjectives, but this amp is one of the best sounding amps I have heard, regardless of price (tube or solid state). This is quite a statement coming from me. Why? Because typically, I can only barely perceive differences in amps when comparing them unless they are just junk compared to something good. 

 

It is totally transparent, but powerful. Sound, soundstage, imaging, noise, frequency extension (high & low) are excellent. 

 

If you are looking for a moderately powered push-pull tube amp, this is the one to make. For Heritage speakers, you do not need more than 35 watts per channel which this is rated at. 

Enjoy

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On 9/22/2022 at 7:18 AM, Curious_George said:

Plate voltage is 430 - 33 bias voltage, so 397V plate voltage 397*70mA = 27.79 watts

 

Typical EL-34 are rated at 25 watts dissipation. I am using what Eico used based on the schematic and voltage reference measurements listed in the build manual. 

 

My goal was to emulate the HF-87, not redesign it (yet...). 

 

I have a quad of Svetlana EL-34's I bought probably 20 years ago I am using. They should last for the next 20 years in circuit.

The Marshall EL-34 tubes I just bought used on Ebay were supposedly made by Svetlana. On another subject, have you tried one of these. https://www.eurotubes.com/store/pc/viewCategories.asp?idCategory=6

I have been using the $28 for a few years and I like it to find bias on the octal tubes. Using it and finding cathode to plate voltage and using ohms law makes it easy to find how hot you are running the tubes. 

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My Chinese SE EL34 amp came with two Shuguang EL34’s. They seem to work pretty good in the HF87 clone, so I bought two more Shuguang EL34’s on eBay. 
 

I’ll break in the four Shuguang EL34’s good and use them for everyday use in the HF87 clone. Plus, I’m curious how long the Chinese EL34’s will last. George A said he has had good luck with some of the Chinese pentode/tetrode tubes. 

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