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I have this PPEL84 amp chassis. It has a 5u4 tube rectifier. The old circuit is a mess and i am not going to copy it as almost all the caps and resistors were kaput! The original circuit had a 3 tube front end. A triode input to 12ax7 splitter. I am considering the EICO HF81 or the Rogers Cadet? Anyone out there care to share their experience with either or these circuits or something else? Only requirement is I would like to stick with 3 tubes in the front end.Best regards, Dak
Follow Ups:
Eico HF-14 EL84 monoblock; about as simple as it gets.
Heathkit AA-111 EL-84 stereo basic amp. A but under the radar, but excellent overall.
Good Luck.
All I have to add is that you should maybe consider the Eico HF-86. I built one some years ago and it is the best sounding amp I've ever built. The only change I made to the stock circuit was that I use 12AU7/12AX7 instead of the dissimilar 7247 (12DW7) twin triodes. Okay, one other change - I sourced a new Hammond power transformer, so I used 5AR4 rectifier instead of the original 6v rectifiers.
This should be the ticket, at least for using the 3 tube front end. 12ax7 AF amp to 6C4 splitter. The voltages are just right too. I got bunches of 6c4 that i never use. Thanks Steve, for pointing me in the right direction. At the least i have something to shoot at.
Edits: 08/14/17
You have a "Catch 22" to deal with. The 7 pin mini socket used by 6C4s will not directly fit in the chassis holes used by 9 pin mini sockets. You will have to fabricate "hole shrinkers". Cut rhombic shaped pieces of sheet metal whose long axis accommodates the 9 pin socket's mounting centers. The short axis accommodates the 7 pin socket's mounting centers. "Break" the sharp sheet metal corners, to avoid injury.
Eli D.
My memory was that it was a 9 pin base, but that proved wrong. I checked the holes and like you said i would have to fabricate the plates to hold the smaller tube socket. Too bad.
I am thinking of just using 1/2 of an au7 of the tube per channel. I got lots of them so no big woof. I got the power supply sorted so almost half way to the finish line. As appearances go, i have the chassis painted light blue hammertone and the trannys are painted black. thanks for the suggestions. Dak
Wire the 'U7s asymmetrically, so that 1 channel uses system 1 and the 2nd channel uses system 2. When the tubes show signs of wear, flip/flop them.
Eli D.
Great idea. I was just going to wire them so that the plate closest to the el84 would be used. But i can see if that would work. Thanks for the tip!
Hi Steve, my concern is that the OPT are not the level as the eico units. But, maybe i will just try.
In your amp did you draw out a circuit schematic? It would be nice to see how you implemented the 86 circuit. You know, the 86 is about the only EICO amp i did not collect. cheers.
No, sorry, I didn't draw the as built schematic. I used the exact stock Eico schematic with the following changes:
1. Since I was building stereo and 12DW7 was looking scarce at the time, I used a 12AX7 and a 12AU7, such that each tube is used for both channels. I know this might make some purists blanch, but it works just fine.
2. Output transformers were salvaged from a Stromberg-Carlson basket case pentode-connected EL84 amp.
3. Power transformer was a new Hammond unit with 300v-0-300v HV and a 5v rectifier winding. B+ came out a little high. 360v as I remember as opposed to 350v for the factory amp. I have a 100ohm hum balance pot in there.
4. Because I had 5v available, I used a single 5AR4 instead of the twin 6CA4/EZ81 rectifiers.
5. Because my outputs did not have a 32ohm taps, I adjusted the feedback components and tied them to the 16ohm taps.
6. I used metal film resistors and polyprop film-and-foil coupling caps. Nothing fancy.
7. When I built the amp, I was fortunate that Ei was still in production. So I have a full set of wonderful Ei tubes in the signal chain.
Early on, I had a set of Tesla (pre-JJ) EL84. One of them gave me a light show fairly early on, so I replaced the set of EL84s with Ei. B+ is pretty high and the specified cathode resistor has the outputs running pretty hot.
I also had an early-ish Ruby (Chinese) 5AR4 in there at the beginning. It was fine for a number of years, then it gave me its own light show. Been running it with a fairly recent JJ since then. I've also tried an NOS GE and a Japanese 5AR4. I never can hear any sonic difference with rectifiers, so I just run it with the cheap JJ and keep the "mystical" rectifiers for possible trades!
Did you use the EICO output transformers?
Have a smashed HF-81 with all good iron and so wish I had the tools and place to fabricate a chassis as I have everything else I need, dozens of NOS/NIB 12AX7, 12AU7 and EL-84.
Even all of the Octal and Loktal equivalents of the above (6V6 is very close and will work with the same OT without much fuss).
I've started on the process to rebuild some of my old HK amps. Just cut several raw chassis from extruded stock this weekend. I'll be using the original iron, not much else.
--------------------------
Buy Chinese. Bury freedom.
It has a 12au7 AF cap coupled to a 12ax7 splitter. Pilot's have a nice sound. The voltages are about what this amp's power trans will give as well. Any opinions about that circuit.
Examine the SA-232 schematic, carefully. The voltage amplifiers are 'U7 sections. :> ( The 12AU7 is non-linear. The phase splitters are 12AX7s setup in the paraphase configuration. If you want to use UL mode "finals", the 'X7 will be "in over its head". Also, paraphase topology is out of favor, due to certain linearity issues. Give Pilot full marks for squeezing acceptable performance out of mediocre circuitry. My hunch is that the O/P "iron" used had LOTS to do with that.
If your power trafo has the requisite extra heater current, switching from 12AU7 to 6FQ7/6CG7 solves the voltage amplifier linearity issue, without parts value changes. To get good linearity, along with a 300 mA. heater, change from 'U7 to the Loctal 7AF7 .
Eli D.
and arranging them as an input stage LTP will do very well...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
That would be an interesting exercise but i am just building this as to make use of a good chassis and transformers and hopefully make some money to fund some expensive output transformers for a big budget amp.
If it's the 6bq5 sound in pp you are after you might want to read the article at the link below. I have a stereo pair bread boarded and they sound great. I used the 12at7 option for mine.Also read the thread titled "lesson learned-don't try to be too tricky!" in this forum for more info.
Edits: 08/12/17 08/12/17 08/12/17 08/12/17
Low gain hi-GM drivers, and two TT input tubes switchable from balanced to SE.
A BIG PSU - without valve rectifiers - and as little global NFB as possible, yet enough around the OPT stage to get the OPTZ below 1-2 ohms down to about 100-50 Hz,
But still sounds good.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Hi Timbo, and thanks for the suggestion. I would like to stick with a preferably vintage or classic design. the amp is already supplied with a valve rectifier, as well as output transformers that look like they are designed for cathode biased output stage of about 12 watts per channel.
OT? Pentode? Ultralinear?
I purchased a physically damaged Eico HF-81 on Ebay a long time ago. Chassis was bent, no cover but iron all good. Been wanting to build a decent PP 6V6 or EL-84 amp using the iron.
The HF-81 was about the best sounding PP EL-84 amp of it's day so I'm guessing with the right iron you could pretty much copy the HF-81 or the power amp version which used the same iron and whose nomenclature I can no longer recall.
I don't remember. The amp operated in pentode mode, which i will also need to follow. The eico 81 has a 12au7 in front of the 12ax7 driver/splitter tube, and the au7 is for the tape monitor circuit. I wonder if including that tube would be too much gain if i also used it my amp? Does anyone have any experience with that? cheers.
And lots of NFB / gain killing.Noisy.
Esp. into EL84s where lots of gain in the driver is almost pointless. While low GM / 'grunt', is also not good.
IMO&E many classic PP EL84 power amps and integrateds can sound a lot nicer with the changes I've suggested.
It is, of course, your money, but why re-build an amp so that it will sound less good than it could?
I listen to classical music and vocal acoustic music, live, as often as possible.
Live acoustic classical and choral music is dynamically and tonally demanding.
Sensibly progressive gain structure in any system, and I do mean any system, (not just audio) is a good idea.
IMO&E the best way to get really good - excellent in fact - sound from a small PP EL84 (or EL84M) amp is to
i) get the gain structure right. A 12AX7 splitter driver violates that
ii) a nice big and stiff PSU, which requires soft recovery SS diodes.
The PSU could use i) brute force with BIG electrolytic caps and by passing OR ii) use a choke input filter into a largish film cap, then another choke, and so on with caps.
IME the large e-caps approach works just fine.
For me an amplifier has to do its job, which is getting out of the way of the music.
This is not hard to do.
Classic era PP valve amplifiers, even little PP EL84 items, have the capability of very high energy storage in their PSUs.
This always sounds better.
Refusing to go down this track - for 'vintage' sound reasons - is beyond my understanding.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 08/14/17 08/14/17
Many amps from the '50s and '60s used the 12AX7 (or equivalent) as a phase splitter ahead of the output tubes. It might not be the best choice for this use, but it does work.
--------------------------
Buy Chinese. Bury freedom.
My interest in audio has always, at bottom, been in simply recorded (minimal miking) acoustic music.And, its reproduction at home as well as I can afford, on a low budget.
(NB I don't hate pop/rock music, it's just not very important to me, and the classical music just IS. then, Blues and Jazz, and pop.)
I have learned a LOT about what works, and enough of how / why, and have achieved a very high musical ROI in return. I will keep learning.
When I swapped the LEAK St60 I'd been gifted, for the St20s, it was my intention to use them in OEM form.
But I soon found that was going to mean $$$$$$ for decent NOS GZ34s. And $$$$$ for NOS PSU caps.
Then I found a whole bunch of NOS 6CG7s, 6DJ8s and the amazing EL84M. And, then found out about SS SR diodes. And, CCSs for splitters. And how that all might work. WITH a really high storage PSU, too. ALL for far less money than a restoration, and far more dbw. (? real watts.)
One day soon I - might - come up with the funds to convert one of the LEAKS to CLCL... OR LCLC... using PP film caps, and compare it to SR SS diodes and BIG electro caps, just to check. Valve rectifiers will not be in the mix.
But, ...... I do keep putting it off! I wonder why? ;-)
Viz? I _ will- be investing in two line-level SE/bal traffos so as to run each LEAK 'strapped, balanced' - and safely - into the rebuilt QUAD 63s. Giving 40Watts RMS / into 2/4/8 ohms! I'm expecting 4 ohms to be best, given that they and the QUADs won't be doing much in the bass. 1st HP @150 Hz.
Music? I got into performing music at 9 years old in a cathedral choir and soon was able to hear 'us and the place' in simple stereo. A Permanent convert, if you will.
It was revelatory. Systems that go deep and loud really are necessary. Mid 30 Hz minimum? Search here.
Yes, I do use (and have made) such recordings of choirs plus organs et al instruments, to asssess judge what I've done to the gear, and the layout / room. (Surround IS better if it's done to give more 3d).
So, 'audio' isn't my hobby, learning about it became necessary to progress on listening to music at home.
LBNL and for me. SS can help a lot to get tubes doing audio to sound blameless, whenever the recording is blameless.
Still learning, but I am confident I won't go off using SS to help valves do their excellent work as audio amplifiers.
Nor will I stop sticking to established and audibly superior design principles.
I'll leave you with a question?
How many amplifiers have you built that are capable of matching rise and settling times into real-life loads?
It matters, IMO a lot more than a given topology ever can.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 08/14/17 08/14/17 08/14/17 08/14/17
I feel this arrangement helps give the Princeton and Princeton Reverb its unique tone. Compared to the Deluxe Reverb (with its long-tail pair PI).
Other things (like speaker sizes) and front end arrangement also effect the tone. I really like the PR, esp for practice.
8^)
Not an effects amp.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
My experience with cathodyne or Concertina PI guitar amps is they distort slower and earlier in the volume range, than the usual long-tail PI setups.
Makes for a different overdrive tone. Quite unique. I can tell a usual 12AX7 (7025) cathodyne PI amp just by playing the amp at various volume levels and listening to the response. It's that unique.
Cathodyne PI guitar amps were usually the low-end push-pull amps of the manufacturer's line. So other factors also apply. Like cheaper iron, speakers, and parts.
8^)
I have never heard that before.
"The eico 81 has a 12au7 in front of the 12ax7 driver/splitter tube, and the au7 is for the tape monitor circuit."
Buffering the recording O/Ps with cathode followers is a good thing. Loading down the low level mag. preamp section will result in info. loss.
"I wonder if including that tube would be too much gain if i also used it my amp?"
Excessive gain is a serious negative. Attenuating a signal only to boost in again has a very deleterious effect on S/N ratio. :> ((
As I stated previously, I'd like to see a photo of the chassis being worked on.
Eli D.
I suspect tone controls are present. EL84s are easy to drive. Look at Fisher's X-100. A 12AX7 voltage amplifier and a 12AU7 "concertina" phase splitter are all that's needed. The 7247/12DW7 shown in the schematic fragment is 1/2 'X7 & 1/2 'U7.
You might try a Mullard style circuitry using 6H30П (6n30p) as both the voltage amplifier and LTP. Unfortunately, it's most unlikely that the power trafo has the requisite heater current capability.
Most definitely, get the gain structure right.
Eli D.
I have a pair of Fisher output trans that maybe from an x100 and i will use that schematic for it. cheers.
Please post a photo of the chassis. Perhaps something can be concocted to use all the sockets, without overworking the power trafo's filament winding(s). Also, the OEM tube complement will be of interest.
Simply removing unneeded sockets and using the space freed for filter caps. could be the best bet.
Eli D.
The upcoming Fisher project has already undergone 2 iterations. The first was its original use as a bass amp? for a Hammond organ. I removed most of the components and OPT, and installed the 2 Fisher OPT. Since the original Hammond circuit included taps for fixed bias i used a Scott 299 schematic for building the amp. That sounded really good and i was super pleased with the result, but in a "Homer Simpson" moment i mixed up the 6bl8 and el84 lineup and that kinda toasted the PT. So I got a replacement PT, and decided to follow the Audio Note schematic for their PPEL84 UL amp which is now done, but the amp does not sound as good as the Scott schematic and the gain is too much in the front end causing the amp to be noisy.
So, my future plan is to use the Fisher circuit, after i get this other amp out of the way. thanks for the help.
Good look at the chassis real estate and irons you'll be using will definitely be a nice starting point.
Thanks!
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