I have found a seemingly very good vacuum tube amplifier (only the schematic not the phisical thing). The Eico HF-52. It is a push pull amplifier that according to the specs sheet delivers 50W audio power. It has a pair of EL34s in the output stage.
I have already built the Mullard 20W amplifier which sounds great and is also powerfull, but I have been looking into more powerfull amplifiers and this seems to just fit my "wants".
I have not seen any videos on youtube and it was difficult to find the schematic at all but one really helpfull site popped up and they even had the service manual which is really handy.
So I actually want to build this amplifier from the ground up. Maybe leave out some parts of the amplifier (no need for a phono preamp and the entire front end really except the voltage amplifier and all of the rest. I can get new toroidal audio transformers for relatively cheap so building the amplifier would not be a problem.
My question really is it worth building this monster. On paper it looks good and 50W is some real power to have acess to. I dont really want to waste my resources on a amplifier that might not sound good or really eat vacuum tubes after just few hours of operation.
(the manuals are on this site : Eico HF-52 - Manual - Mono Integrated Amplifier - HiFi Engine
unfortunately you have to register to be able to download them, I cant upload them to the forum due to the filesize limitation)
Thanks for any replies.
I have already built the Mullard 20W amplifier which sounds great and is also powerfull, but I have been looking into more powerfull amplifiers and this seems to just fit my "wants".
I have not seen any videos on youtube and it was difficult to find the schematic at all but one really helpfull site popped up and they even had the service manual which is really handy.
So I actually want to build this amplifier from the ground up. Maybe leave out some parts of the amplifier (no need for a phono preamp and the entire front end really except the voltage amplifier and all of the rest. I can get new toroidal audio transformers for relatively cheap so building the amplifier would not be a problem.
My question really is it worth building this monster. On paper it looks good and 50W is some real power to have acess to. I dont really want to waste my resources on a amplifier that might not sound good or really eat vacuum tubes after just few hours of operation.
(the manuals are on this site : Eico HF-52 - Manual - Mono Integrated Amplifier - HiFi Engine
unfortunately you have to register to be able to download them, I cant upload them to the forum due to the filesize limitation)
Thanks for any replies.
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I assume it's an HF-50 with an added preamp section. It looks like the same iron as an HF-50, which is a very good amp. There are a couple of threads here and on other forums indicating that they shared the same output transformer.
The claims for the amp predate current legal requirements for "accurate" specifications. While I have little doubt that the unit is a capable performer, don't believe that 50 W. number. 35 W. is a more realistic expectation from PP EL34s.
If you want upwards of 50 W. from a PP pair, build with KT88s or 6550s.
If you want upwards of 50 W. from a PP pair, build with KT88s or 6550s.
The HF-52 is an excellent integrated. Some folks consider the OPTs used in the HF-52 and HF-50 to be even better than the Acro TO-330 used in the HF-60. If I were you, I would strip it of all parts and then rebuild it as just a power amp using the HF-50 schematic.
I have a pair of HF52s and they sound just marvelous. Since they are mono-integrates the downside is that you have to adjust volume at two controls and get it just right, unless you use a preamp which I recommend. But then you have to deal with excessive gain, which is usually not difficult. If they are in good condition, and working, I would advise against stripping them for transformers, Rebuilding the circuits with premium parts would be the way to go if you plan to keep them - they are worth it. Good luck with your find!
You can believe the 50 W number, IF you use Mullard tubes. The HF52 puts 460V on the plates of the EL34s with fixed bias. (Check the Mullard data sheet for PP EL34 to see what is possible). I agree that those numbers may not be feasible with new production tubes, which may not last long in this application.
You can believe the 50 W number, IF you use Mullard tubes. The HF52 puts 460V on the plates of the EL34s with fixed bias. (Check the Mullard data sheet for PP EL34 to see what is possible). I agree that those numbers may not be feasible with new production tubes, which may not last long in this application.
CaterpilarSK,
Do you have an HF-52? Or, do you want to build an HF-52 from other parts? You want to use Toroidal output transformers, Yes? Be prepared that if you do not have and use the original output transformers, you may need to adjust the negative feedback, and other poles in the amplifier. Otherwise, you may have an oscillator instead of an amplifier. Take a look at a few threads on this forum, and what some are going through to make their amp stable. It is a doable thing, but does not always come together when first turned on.
Do you have an HF-52? Or, do you want to build an HF-52 from other parts? You want to use Toroidal output transformers, Yes? Be prepared that if you do not have and use the original output transformers, you may need to adjust the negative feedback, and other poles in the amplifier. Otherwise, you may have an oscillator instead of an amplifier. Take a look at a few threads on this forum, and what some are going through to make their amp stable. It is a doable thing, but does not always come together when first turned on.
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Guys don't worry I do not have the original unit. If I would it would be a unforgivable thing to strip it.
I want to build the amp from bare parts. I have a toroidal output transformer (6k8PP) and that is what I plan to use. I want to build the amplifier from the ground up.. Possibly on a PCB. All new components and probably some modifications too. I may not even use the voltage amp and phase inverter of the original schematic. (I will probably use the voltage amplifier and the inverter from the mullard 20W amp with some modifications to get the gain correct. Despite the 30dB feedback the amp is stable)
Basically I will build the entire amplifier from its schematic. I have checked the HF50 as sugested from another member. It seems to be practically the same thing but the HF50 really seems to stretch the EL34s really a bit too much with that anode voltage. I saw a post somewhere on the web about modifying the amp to accept 6550s which makes it longer lasting I guess
A power oscilator is not what im looking for so surely thats something I want to avoid 🙂 . I have already read a bit of literature about negative feedback so i know whats coming at me.
JJ offers EL-34 that are made appparently better than what the original tubes. They offer the EL34, EL34L, EL34 II which is the apparently best out of all. But I also have a pair of 6L6GCs on the way to me. Currently I am using the shuguang EL34Ls. Were cheap so why not for 20 dollars a pair? Deal right? Pair and brand new unused. Also JJ is local for me so yeah I may just get KT88s in the future if I will get some more finances (im a student so yeah I only built one tube amp and the transformer comes from that amplifier. Yes I do know for KT88 I need a different impedance transformer.
Yeah Im planning to built two monoblocks in the future for stereo operation. And the volume balancing is the last thing Im worried about.
I want to build the amp from bare parts. I have a toroidal output transformer (6k8PP) and that is what I plan to use. I want to build the amplifier from the ground up.. Possibly on a PCB. All new components and probably some modifications too. I may not even use the voltage amp and phase inverter of the original schematic. (I will probably use the voltage amplifier and the inverter from the mullard 20W amp with some modifications to get the gain correct. Despite the 30dB feedback the amp is stable)
Basically I will build the entire amplifier from its schematic. I have checked the HF50 as sugested from another member. It seems to be practically the same thing but the HF50 really seems to stretch the EL34s really a bit too much with that anode voltage. I saw a post somewhere on the web about modifying the amp to accept 6550s which makes it longer lasting I guess
A power oscilator is not what im looking for so surely thats something I want to avoid 🙂 . I have already read a bit of literature about negative feedback so i know whats coming at me.
JJ offers EL-34 that are made appparently better than what the original tubes. They offer the EL34, EL34L, EL34 II which is the apparently best out of all. But I also have a pair of 6L6GCs on the way to me. Currently I am using the shuguang EL34Ls. Were cheap so why not for 20 dollars a pair? Deal right? Pair and brand new unused. Also JJ is local for me so yeah I may just get KT88s in the future if I will get some more finances (im a student so yeah I only built one tube amp and the transformer comes from that amplifier. Yes I do know for KT88 I need a different impedance transformer.
Yeah Im planning to built two monoblocks in the future for stereo operation. And the volume balancing is the last thing Im worried about.
I dont have the original amplifier. It would be unforgivable to totally rip out the internals and rebuild it that way. I looked at the HF50 and it looks awfully the same to the HF60. Oh wait it is also the HF60 schematic. Im mostly interested in the power stage of the amplifier, I have a driver from a different amplifier which needs just a small amounth of modification to work with the power stage.
The HF-50 and the HF-60 are the same except for the OPT. The 60 uses the Acro TO-330. If you are scratch building using the Eico circuit but not an Eico OPT, you are on your own. The input/driver circuit works well but the OPT is a key ingredient in the final product. I for one would not use a toroid OPT.
I dont know if you heard about the polish manufacturer fo these transformers. They seems to do a proper job on them. They also have cathode feedback transformers. So they sure do have to know what they are up agains.
If you want you can check them out here Transformatory toroidalne - Producent transformatorow Toroidy.pl
@ CaterpilarSK
Toroidy manufactures and sells tube amps since 2015 or 2016.
I think they start to know what they are doing with their torics transformers 🙂
Toroidy manufactures and sells tube amps since 2015 or 2016.
I think they start to know what they are doing with their torics transformers 🙂
I cant really tell if theyre good. Good is a subjective opinion. So I cannot tell the difference between old iron and new production. Perhaps someone who owns one of their transformers and also has old iron stuff could tell.. I think it sounds good.
The sales-bumph has an interesting weasel: "within 1dB of 50W". They actually promise 39.7W, at 1% THD, 20cps-20kc. (The 20Hz performance obviously depends on the iron used.)
I'd suspect it does 50W 1% midrange, falling off insignificantly at the extremes.
This is not BS. The Marshall 2-EL34 "50W" guitar amps have been shown to deliver 70W at 5% with small NFB. This would be 50W-60W at 1%. The Ampeg VT40 did an honest 60W at few-% THD on a pair of 7027 (which isn't EL34 family, but EL34 can be used with slight change; they did call 6550 the "Pro option").
Toroidy transformers
If you want to know how the Toroidy output toroids perform check out this thread on Baby Huey (BH) amp building. Members are using Toroidy transformers mostly and member Bfpca had in the last week posted some measurement of his BH amp using KT88 power tubes and Toroidy 6.6k UL outputs. He is very pleased with the Toroidy outputs.
EL34 Baby Huey Amplifier
If you want to know how the Toroidy output toroids perform check out this thread on Baby Huey (BH) amp building. Members are using Toroidy transformers mostly and member Bfpca had in the last week posted some measurement of his BH amp using KT88 power tubes and Toroidy 6.6k UL outputs. He is very pleased with the Toroidy outputs.
EL34 Baby Huey Amplifier
So if I would build the amplifier from the ground up according to the schematic, it would be a mplifier that would certainly not dissappoint in terms of power output.
The output transformer Im planning to use and already own is a 6k6 50W transformer. Primary inductance is 591H and the bandwidth is 8Hz-54kHz (-3DB). It should perform good.
Toroidy may make nice transformers. I have no idea since I had never heard of them before. However, a toroid OPT will most likely sound different from a vintage Chicago or Acro that were used in the Eicos.
Im not really looking for the ultimate vintage sound. I dont know how to put this but I kind of want the most accurate sound out of the amplifier. My guess is that the toroidal transformer has less leakage inductance and that also results in better coupling and in a wider frequency range.
I already own the EL34PP transformer from toroidy and to say it sounds good is purely subjective. I have not done measurements because I dont really have the gear to measure it with.
The fursthest I got is having a compareable transistor amplifier set to same volumes using a oscilloscope and really I could hear a bit more clarity in the sound. But this is purely a subjective view. I built the mullard 20W amplifier on a PCB and that is really my first tube power amplifier I ever built. And it was meant to be a school project and it did suceed to some degree on the competition (yeah im 17 and I am well aware of the dangers with HV)
I already own the EL34PP transformer from toroidy and to say it sounds good is purely subjective. I have not done measurements because I dont really have the gear to measure it with.
The fursthest I got is having a compareable transistor amplifier set to same volumes using a oscilloscope and really I could hear a bit more clarity in the sound. But this is purely a subjective view. I built the mullard 20W amplifier on a PCB and that is really my first tube power amplifier I ever built. And it was meant to be a school project and it did suceed to some degree on the competition (yeah im 17 and I am well aware of the dangers with HV)
Got it. Well, the HF-50 input/driver is a very nice circuit and it isn’t very critical like a Williamson can be. So it should be a good place to start. Good luck with your project!
The input amplifier sure is nice but I kind of want to use the input amplifier from the mullard twenty watt amplifier with very little modifications to get the drive (heres the link where the schematic can be seen of the entire amplifier, I only inted to use the input driver from it. The 30dB feedback works flawlessly I found. Twenty-Watt Amplifier )
6.6k load with 460V supply, fixed bias, will give around 30 Watts.
To get to the 40W-60W zone you need a higher supply or a lower load impedance.
Of course from 30W to 60W is not a big difference. Or rather: if 30W is "not enough", you really want to be thinking 100W-200W. (Or more efficient speakers.)
"disappointed"? I've been utterly satisfied with 1 Watt, and let-down with 150 Watts, in different situations.
To get to the 40W-60W zone you need a higher supply or a lower load impedance.
Of course from 30W to 60W is not a big difference. Or rather: if 30W is "not enough", you really want to be thinking 100W-200W. (Or more efficient speakers.)
"disappointed"? I've been utterly satisfied with 1 Watt, and let-down with 150 Watts, in different situations.
If I would have used the EICO HF60 schematic that has 500V on the tubes. What power could I really expect from an amplifier like that ? And 500V on the EL34s, kind of really on the edge with the lifespan. with a 6K6 transformer? I dont really want to go 100W on the thing. It would make it overly expensive at that point for my budget
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