New Build. Suggestions over the schematic.

[bigsbybender]

Hi guys,  

It's been a long while since I've built anything, but I've decided to take on a new challenge. After a couple hospitalizations and recent throat surgery, I'm at home a lot so I've got to do something constructive. Nobody is buying amps right now and I have more than I can personally use, so I decided to take on a new challenge.
I needed a small stereo for my house so I've decided to build one, (I know   , a "Best Buy" black box would be far cheaper...) I'm making this out of scrounged parts, like an old mil-spec potted PT and pulled tubes and a chassis I got out of a local factory's scrap metal pile. I found this schematic since I was looking specifically for a single-ended cathode bias design to keep parts-count low and the circuit relatively compact.

I'm simplifying this schematic as I'm going to hardwire in ultralinear and forget the switched operating modes and cathode resistors. The schematic calls for a 10H choke.  I know SE is noisier than P-P but is a choke this size even needed?  The chassis is kind of tight and I'd like to know if a (physically smaller) 5H or 7H choke would be noticeably noisier?

All the Single-Ended stuff I've built so far have been "Champy" where power supply noise isn't a big deal.


Any suggestions and constructive comments are welcome.


j.

[jjasilli]

Looks like a good hi-fi design to me.  The physical size of the choke is probably determined more by the mA rating than the H rating.  I'm guessing the large up-front choke is there for voltage regulation especially for use in triode mode.  In SE pentode mode the plate voltage should be pretty stable, so a screen choke might suffice for voltage regulation.  For hi-fi I think reasonable voltage regulation -- like that provided passively by a choke -- is a good thing.

But for hum & noise reduction, I think the only way to know for sure is to build it and see.  Arguably, 2 chokes could be used:  the big one up front and a 2nd one for the screens!  Note that the schematic is calling for oil caps which, too, are huge.  That sounds good to me too. 

[bigsbybender]

I plan on running Ultralinear although I may experiment with pentode operation. The screens will be supplied by the screen taps off of the output transformer. Initially I thought about eliminating the choke since I wasn't going to have a dedicated screen supply, however I thought it may be useful in Hi-Fi. Most of my ultralinear experience is on late-silverfaced Fender amps, where noise would be (1) not a big deal for rock and roll and (2) largely canceled out by operating in push pull. 

Since I've never dealt with single-ended ultralinear I keep second guessing myself on the necessity of the choke in this case.

I know I should probably find some Hi-Fi board to post this on, but I like you guys better. 

Note that the schematic is calling for oil caps which, too, are huge.

    It calls for PIO, but it's getting basic film caps....because that's what I have. I make a poor audiophile.


j.

[jojokeo]

You could switch between pentode & ultralinear but I'd not use triode mode, definately not for hi-fi.
The choke is normally rated appx. 10% of the mA rating of the power tranny. In your case w/ S.E. it needs to supply the output tube's plate, screen, and whatever is before it unless you put it between the plate and the screen simulating a pp amp. Since it's S.E. just add up what you got and use one suitable in that range.
As for noise/humm reduction - elevate your heater's ground/artificial CT to get quietest operation. I do this in most of my amps from the very beginning.
IF connecting between various peices of equipment, plan your grounds and think of having a ground lift that could be handy just in case. If it's only to be used w/ an iPod then fogettaboutit.

[jjasilli]

The screens will be supplied by the screen taps off of the output transformer.  I think, with no choke, in anything but triode mode, the SE plate voltage will not vary, becuse the plate's current draw is already maxed-out and won't vary.  I assume that applies to UL too.  But screen current draw will vary causing voltage swings (sag).  This is not desirable in hi-fi.

Another factor:  bass response.  Once again, for design purposes, you need to work backwards, starting from the speaker.  Do you plan to get down to 40Hz?  Or just use bookshelf speakers?  If you want real deep bass, I think chokes become more mandatory.  Blah, blah. . .   you get the idea.

[bigsbybender]

I appreciate the replies !

Actually huge bass won't be a factor, the output transformers I have are only rated down to 70Hz. The speakers I have are rated down to about 55Hz. My thought is that I could add a separate powered subwoofer from a 5.1-type home theatre system to boost bass frequencies if needed.
 
I'm also adding a phono preamp, and a mic/instrument preamp in front of this amp just for fun.  I'm using a 6 position rotary switch to allow switch between CD/DVD, Radio Tuner, Tape, Phono, and iPod. The mic will be able to be mixed with each of the signals from the audio components.

[DummyLoad]

what are the specs on the PT you have? you'd need to design around that.

scrap the tube recto - if it's hi-fi and SE, you want as much filtering as you can stuff in a chassis. PS buzz during a quiet passage in music is very annoying. i'd use the LC network in the plan.

<I'd not use triode mode, definately not for hi-fi.  most hi-fi purists would vehemently disagree. less output power is definitely a drawback, however, with a reasonably efficient 2-way speaker system, to me, triode mode sounds better. experiment with all three modes. overall, i prefer the compromise of ultra-linear. your hearing should be the final judge.

some other output tube options are 6L6GC, 6550, 300B, 803, 2A3, and 813 designs. i'm kicking around the idea of a design using the 4E27A/HK-257B that PRR posted in another thread - 30W+ in SE w/ a single tube without insane B+ levels is appealing.

after you sort out what your PT actually can provide, keep your options open, look around and consider other designs.

--ISO

 

[jojokeo]

<I'd not use triode mode, definately not for hi-fi.  most hi-fi purists would vehemently disagree. less output power is definitely a drawback, however, with a reasonably efficient 2-way speaker system, to me, triode mode sounds better. experiment with all three modes. overall, i prefer the compromise of ultra-linear. your hearing should be the final judge.

I had a feeling this would get a response and I hesitated before saying that only because a lot of hi fi nuts like triode tubes but triode mode is different obviously and hi fi as you say like lots of power which triode mode doesn't give you, plus the reduction in high end freq response is diminished and not favorable in this mode.

Another thing I forgot earlier as possibly adding would be a negative feedback option in case more linearity and cleaner response is wanted?

[bigsbybender]

what are the specs on the PT you have? you'd need to design around that.

350-0-350  >200mA. It has 5v and 6.3v taps. It's an old potted military unit.

scrap the tube recto - if it's hi-fi and SE, you want as much filtering as you can stuff in a chassis. PS buzz during a quiet passage in music is very annoying. i'd use the LC network in the plan.

I thought someone might mention the recto.  I worry that my PT may be a bit hotter than I want so I wanted to have the option of the 5U4 to drop some B+ voltage. My PT is supposed to be 350-0-350, but even unloaded it read much much higher than usual. It's an old pull from some old military equipment, and by it's size I think it will supply far more than the 200mA needed. In that case the voltage may remain a little high under load.  I have a plug-in solid state rectifier that I made in case I want to go that direction, then I can parallel filter caps to double the filtration fairly easily. Also, I have more 5U4's than I know what to do with, so I might as well use one.

Another thing I forgot earlier as possibly adding would be a negative feedback option in case more linearity and cleaner response is wanted?

That's a good suggestion that I didn't even think of.   Once the basic unit is built I think it will end up being tweaked here and there.
 

Once the basic element is done, I have to add the tube phono preamp.

j.

[bigsbybender]

I built it... It sounds better than I imagined!  I used an old potted US Military PT, Edcor OTs, NOS 50's Tung-Sol 5U4, EH EL34's, NOS 70's Soviet Military-surplus 6N1P. Mostly wired Point to Point except around the 6N1P driver tube.  It's a cold war hybrid! 

OK. so now that I've built the heart of the unit... I need to go and complicate things.

I plan on adding a Phono preamp and a Mic/instrument preamp.  I have plenty of available current, for plate and heater supply from this big PT so that's not an issue.  My issue is adding more filtering, not so much for noise but for channel isolation. 

Attached is the schematic of the basic 12A_7 preamp design I got from an old RCA manual...  I need some guidance on 1. suggested R/C values on the power rail to achieve isolation.. and 2. what I should use to "mix" this mono preamp signal into the stereo hi-fi.... (mix resistor values?)  Also should I add another R/C filter in the power rail to the Stereo Phono preamp section?

Pics of the project to come soon!

Here is my mic/instrument preamp....

[bigsbybender]

I forged ahead and added the mic/instrument preamp. My ethos has been "If I don't know what to do, copy Leo."  SO  I added a 10K Resistor in the power rail and a 22uF to filter. I used a pair of 220k Resistors to split the mono out into stereo where it meets the rest of the amp.

I followed the above schematic faithfully with the exception of the 5uF 250v cap on the output.  Instead I'm running about 1uF 250v for two reasons, #1. I had one on hand #2. I figured a little extra bass roll off would play nicer with consumer electronic grade speakers.

From a design standpoint I have no idea if what I did is even close to ideal, but I do know it works (pretty well in fact).

Now I've got to move on to the phono preamp... space is getting awfully tight so I may have to scrap that plan....  I may opt for a different one with a lower parts-count too... 


j.

[bigsbybender]

It's more or less done! (until I decide to mess with it again).

It sounds excellent.  Essentially the signal runs in where it is switched, sent out to the EQ (due to lack of tone controls in this design) then signal goes to the remote Subwoofer and back to the 6N1P tube. Right now I only have ~20uF on the first filter.. That will soon be doubled on my next Mouser order, (I'm limited to 40uF by the 5U4 so I have to go with some different caps than I usually keep around). The hum is slightly noticeable now, but if I can improve it I will. The Chassis is actually steel commercial door "cased opening" that I got from a factory that I worked at between teaching jobs a few years back. I have enough left for about 3 more amps.

I decided to put it in an old (c. 1950) console radio/phono cabinet. I originally bought the dead radio for the iron and tubes, then I had this cabinet that I didn't know what to do with... After a few years of it sitting around doing nothing. put in a shelf for Stereo gear and then I put in built-in speakers.  Although they look like there might be a microphonics issue, I built the enclosures to be largely isolated from the rest of the cabinet. Since I intended to have a remote subwoofer, I don't have to worry about too much rattle.  The speakers are Rola Alnicos from my Hammond Organ that is now connected to a Leslie, so it didn't need these any longer. I intended them to be temporary, but they sound pretty good so they may stay.  The tweeters are just OEM Peerless dome tweeters, I'm using a JBL crossover with a switchable pad for the tweeters.

The only other problem I have is the mic preamp.  I can't get it to mix with another "live" signal.  If I turn my switch to a dead signal, the mic works fine however it's not mixing...  right now I don't care so much but I'll have to figure that one out.


j.

[tubeswell]

Very cool use of time. (edit) So are those two schematics you posted the total schematic (between them)?  

[jjasilli]

Congrats!  I too was stunned as to how good simple vintage-style tube hi-fi (well maybe mid-fi) sounds.  I've got it powering my home theater system.

[samato]

Nice work!  I'd like to try something like this once I get my first guitar amp build under my belt.  Thanks for the info.

[bigsbybender]

Congrats!  I too was stunned as to how good simple vintage-style tube hi-fi (well maybe mid-fi) sounds.  I've got it powering my home theater system.

Mid-Fi is probably accurate (to be fair to those who spend 5 figures on their gear)... but it sounds far better than anything that I've had previously. I'm hearing subtle things in recordings that I haven't heard before like pick noise and stick noise from the drums. I've played songs that I've heard hundreds of times and now they're kind of new again.

I'd better stop before I turn into one of those Audiophool guys....

[jojokeo]

Great work! But, I'm surprised it doesn't have a guitar input? Maybe if you get that mic err guitar input sorted out...?