HoSo56 amp project...design input sought

[pullshocks]

Once I got the amp sounding "pretty good", I remembered that the OT I had used was way oversized (and cheap, a 40-50w Weber China unit).
I subbed in an old 1963 Schumacher 18w unit I had & the amp CAME ALIVE!  :o

Can you elaborate on this, and explain the change made it come alive? 

[Geezer]

The smaller OT (about 1/4 the size) just gave the amp more "feel", which I suspect is simply more output distortion(?) The BIG Weber OT (intended for a Fender Bassman/Bandmaster type amp/2x6L6's) just didn't let the little EL84's breathe, I guess......

G

[prs_matt]

Doug has em for .75 cents a piece.

I missed them all the way down at the bottom. I just looked in the section with all the other small value e-lytics. OOPS!

[Dynaflow]

 Sounds like the littler output transformer is probably like you said distorting (saturating?) compared to the big iron, but hey in the amp world the right kinda distortion is a good thing no matter where it comes from. I like the new 5e3 head I did but even though its only a 20watt transformer (ot) the 1600 series of Hammons are probably way overkill, my other one sounds good with a universal 125e but doesn't have as much clean as the new head, then again I'm running 6l6's but I think thats less of a factor, the 6v6's still sound cleaner in the new head than with the littler Hammond iron. With el84's I've had good luck with a fender deluxe reverb replacement tranny which was cheap and works great for those. Happy accidents are what this amp stuff is about sometimes I think (what little I know of it). :D

Regards,

Dyna

[prs_matt]

PRS-Matt, I am not sure exactly what you mean by the spst and dpdt right next to each other? If you mean literally the answer is no they're not but having them next to each other would be fine. I use the PAB playing quite a bit. I tend to leave the 2.2uf (spst) on most of the time.

I was completely confused by your answer so I went back and looked at the schematic, then compared it to the layout. It seems there is a switch in the schematic that is not in the layout. I have attached examples. Please feel free to call me a moron if I am still missing something.

Thanks,
-Matt

[tubenit]

PRS_Matt,

Yikes an error in the schematic drawing. There is no spst below the dpdt. 

Sorry 'bout that, Tubenit

[prs_matt]

No worries :) Just making sure I was not missing something.

[SoundmasterG]

Ordered an RCA 5879 today....

Cool!
I'm making the faceplate for my amp (former Epi VJr head) right now..... ;D

What transformer setup are you using on this thing Geezer?

Greg

[tubenit]

I don't know what DaGeezer used but I'd think Hoffman's 18w PT and OT that he sells would be a great match for the build!

With respect, Tubenit

[Geezer]

18w OT for sure would be great, Doug's "Mojo" 18w is very similar to old 60's Schumacher unit I'm using.
The PT on mine came out of a 60's Hi-Fi amp, & I think it's actually 250-0-250, because the Little wing uses a 250-0 PT with a SS bridge recto, & I'm getting the exact same B+ rail voltages on both amps.
Tubenit is using a 270-0-270 PT & getting great result, also...so I think your OK with anything from 250v to 275v 2ndaries.

[Geezer]

A note to those building this amp (or any amp w/ 5879 pentode):

In adapting this preamp to the Little Wing, I copied the B+ rail exactly. All the voltages were correct (nodes A, B, C & D) with the exception of node "E", which feeds the 5879. It was approx 163v, just slightly higher than the HoSo voltage. I thought "it won't make much difference"....oh yes, it did!
The amp had much more gain (in a bad way). I could only turn the gain pot up 1/4 way before it was totally distorted.
So, I changed the 5k dropping resistor  between nodes D & E to a 22k. This brought the plate voltage on the 5879 down to HoSo levels (~138-140v).
Totaly changed the character of the amp! Still have a few "tweaks" to do, but the amp is currently VERY close to the HOSO tone.
Moral of story = watch the 5879 plate voltage! Keep it below 150v, preferably in the 130-140v range for the best tone.

Geezer

ps...I will draw up a "Little Wing Five" (pentode) schematic as soon as I get a change (and have it "finalized...yeah, right  ::) ) & post it.

[tubenit]

Interesting info, DaGeezer!  Thanks for sharing it.  I've got 158v on my 5879 plate and have the volume at 7 with it. It doesn't seem too distorted to me.   However, I only have about 300v on the EL84 plates.  I'm guessing all of those interact somewhat.   Maybe it's a difference between the 6BM8 triode/pentode vs. 12AX7 & EL84?

I won't have time this wkend, but I'll experiment with lower voltages on the 5879 and post what I find sometime next wk.  Maybe it will improve an already excellent tone!

With respect, Tubenit

[jojokeo]

Please give me feedback on this: when dealing w/ preamp pentodes, if you increase the Vp, don't you get more "gain" but in a "cleaner" less distorted way? However, you're saying that you "lowered" it and it cleaned up the distortion?

For my only pentode experiences (EF86), when the Rp was 100k I got cleaner sound than when I used a 470k it became more distorted.

When I left the Vp alone and changed the screen grid resistor's values - going down to 470k gave much more distortion and then going higher the tube cleaned up a lot more - I think I experimented up to 5.6M?

Another thing I found that was interesting was that when I lowered it to appx. 1k, the amp got a spongie/sag kind of feel and using the standard 2.2k to 3.3k gave much firmer/normal response.

Is the 5879 tube different than the EF86 in any of these ways? Also, aren't most if not all pentode's basic behaviors similar from one to another?

[tubenit]

I think one of the factors in talking about this issue to consider is that gain and distortion are not the same thing, if I am understanding that correctly?  And DaGeezer was commenting that the higher voltage had more gain and then distorted "too early".

I think I might know what he is referring to? At times when I've experimented ....... I would find the voltages changed the distortion from a harsh to a sweet type of distortion. And I found it wasn't so much the amount of gain that I wanted to dial in but the type of gain meaning did it have the best tone to my ears.

Hot Blue Plates gave me a resistor box where I can hook it up and simply dial a variety of plate resistors in while using a 25k pot to dial in the cathode resistor value. I've done that on a few amps and found I could find what was a tonal sweet spot for me.  I think I'll try that approach on the 56T 5879 tube. However, it sounds fabulous as it is already.

With respect, Tubenit

[Geezer]

Please give me feedback on this: when dealing w/ preamp pentodes, if you increase the Vp, don't you get more "gain" but in a "cleaner" less distorted way? However, you're saying that you "lowered" it and it cleaned up the distortion?

jojo, thanks for you help here.
I'm a real newbie when it comes to pentodes in the preamp, so anything I'm doing is just trial & error. When I adjusted the Vp (which was the only thing that was different than the "good sounding" HoSo amp) it "sweetened" the tone/distortion to a more pleasant type.

For my only pentode experiences (EF86), when the Rp was 100k I got cleaner sound than when I used a 470k it became more distorted.

When I left the Vp alone and changed the screen grid resistor's values - going down to 470k gave much more distortion and then going higher the tube cleaned up a lot more - I think I experimented up to 5.6M?

Yes, I suspect that (screen grid resistor's value) is where I should do some adjustment to find the sweet spot....it's very close right now, just needs a little more adjusting. It still breaks up much earlier on the gain pot dial than the HoSo, and it's not the power tubes (6BM8's), it's definately the preamp distorting.

Another thing I found that was interesting was that when I lowered it to appx. 1k, the amp got a spongie/sag kind of feel and using the standard 2.2k to 3.3k gave much firmer/normal response.

Did you mean to say "1Meg", 2.2 & 3.3M?? I don't see where 1k, 2.2k, or 3.3k fit in here(?).
If that's what was intended (which I suspect, typo) then that makes some sense to me. I'll try raising the Rs value & see what the results are.

Thanks again.......G

[Geezer]

Another thing I found that was interesting was that when I lowered it to appx. 1k, the amp got a spongie/sag kind of feel and using the standard 2.2k to 3.3k gave much firmer/normal response.

I had alot of "drive time" today w/ work & was thinking about this....are you possibly referring to the Rk/bias resistor??

[Satellite^Guy]

While comparing this amp's schematic to the layout, I noticed that one of the caps is different from one file to the other;
in the layout, the cap on lug #3 of the board is a 1uF, whereas its corresponding cap in the schematic is .1uF
Which would be the correct value? or would it matter? Another also, is on lug 14: schematic, .047uF; layout, .0047uF. which is the correct value?

Also, the trim pot; I'm assuming it's a hidden trimpot, and not on the front panel, as the layout would seem to suggest?

One more thing; the bright switch -- DPDT -- the way it's drawn suggests that in one position, neither of the caps are in the circuit, and in the other position, they're both in parallel to ground. Is this the case?

S^G

[Geezer]

While comparing this amp's schematic to the layout, I noticed that one of the caps is different from one file to the other

The last schematic on the schem thread ( http://www.el34world.com/Forum/index.php?topic=6176.0 ) has the correct values for my version. I've not been updating the layout...just sub in the values from the schem.

Also, the trim pot; I'm assuming it's a hidden trimpot, and not on the front panel, as the layout would seem to suggest?

No, it's on the front panel....it's very important in being able to adjust the signal strength/gain into the phase inverter, to control PI distortion.
The "trim" designation was just something I called it....it could be called "volume" (so then you'd have "Gain/Volume/Master Volume"), or "Drive", or anything you like.

One more thing; the bright switch

Mine doesn't have that feature....Tubenit will have to answer that one.

Geez'r

[jojokeo]

Another thing I found that was interesting was that when I lowered it to appx. 1k, the amp got a spongie/sag kind of feel and using the standard 2.2k to 3.3k gave much firmer/normal response.

I had alot of "drive time" today w/ work & was thinking about this....are you possibly referring to the Rk/bias resistor??

Yes the cathode's bias resistor! Sorry if it wasn't clear before. I originally thought that I could lower it for changing the gain like a 12AX7 but pentode's gain doesn't work the same way.

[Satellite^Guy]

One more thing; the bright switch

Mine doesn't have that feature....Tubenit will have to answer that one.

Geez'r

Sorry.. after looking at the schematic again, I realize it's the boost relay, relay 1a, 1b, not a bright switch... I think Tim Horton's slipped me some decaf this morning... yeah, that's what I'll blame it on... ;)

Aimed at both Geez'r and Tubenit, as well as anyone else who has built this amp: --> Do you have any pics detailing the inside of the amp, that might help in supplementing the layout? (as well as show the newcomers "how it's done")

S^G

[Geezer]

Thx jojo.

I have been trying everything (different Rk/Rp/Rs for the 5879) on the 6BM8 version & keep coming back to the original values as the best > Rk=470R/Rp=56k/Rs=470k.

I think I've finally got it right (tone-wise) by going back thru the B+ rail & getting all the voltages correct at each node by adjusting the dropping resistor values to get the correct voltages all down the line. I started by measuring node "C" (as there is not much I can do about nodes "A" & "B"....the plates & screens). The "C" voltage was too high, so I increased the 10k up to 15k. Then I checked nodes "D" & "E"....they were now at correct voltage (per the HoSo voltages I had measured & noted on the schematic). That greatly helped to bring the tone more in-line with the original EL84 amp.
As with most higher gain amps, small variations seem to have large effects on tone. Alot of tone tweaking may be needed with each indivdual build of this amp to get the best sound results.

Another change I've made (for the 6BM8's) is a different primary imp on the OT. I originally had an 8k OT in there, but was getting some harshness, so I tried higher & lower imps. I had an old OT from a Fender Musicmaster Bass amp (1978) that is supposed to be 8k, but I had measured & marked as actually being 11k primary......put that in & it's the best sounding of all the 6 or 7 units I tried.

The 6BM8 version is definately lower output than the EL84's. I did put a MV on it (had the old "Presence" control hole to fill) but find I have it "off" or fully dimed most of the time. Of course, I will probably still want it for use in really small/quieter venues.

I'll post a "6BM8" version of the schematic soon. I'm in the process of tweaking the "boost" circuit values now & am getting "pretty happy" with the overall results, although the original EL84 amp still has the better "tone edge" ..... as of right now......still working  ;)

[Geezer]

Do you have any pics detailing the inside of the amp, that might help in supplementing the layout? (as well as show the newcomers "how it's done")

S^G

I don't (although Tubenit may)...I'm not posting any pics of mine  :-[ .
They wouldn't help you much anyway. It's a "prototype" unit & very messy inside (especially compared to my normal standards...see below  ::) )

Right now, I wouldn't recommend this build to anyone not experienced in full scratch builds....it's still in the evolutionary development stages & so far needs lots of "tweaking" to sound it's best.

I'll see if I can do a layout that acurately depicts what the amp layout "should" look like, although Tubenit's drawings are usually pretty good to go by. If I do it, it will probably be later this week.

Here's what my amps usually look like (& the prototype doesn't!)

[jojokeo]

All I need is a schematic that has the latest correct information on it, maybe also w/ voltages would be helpful. I'm not sure that I know which of your guys latest drawing is being used? However, I do have a brand new 5879 tube on it's way :), not sure if I'll use the 6BM8? Not sure why this and not EL84/6V6 (not that it's a big deal at the moment)? The only issue is that the tube site NJ7P doesn't show much in the way of specs for this tube, especially for class AB1 use...
I'll check back in a couple days (funeral out of town). Thanks guys.

[tubenit]

My final schematic and layout  (and DaGeezer's final schematic of the HoSo 56) are here:
http://www.el34world.com/Forum/index.php?topic=6176.0


This layout board follows the layout I drew with the except of the B+ cap to the 5879 being on the board.

With respect, Tubenit

[prs_matt]

Quick question... Schematic shows 5Y3 rectifier and 32uF first cap. I thought the 5Y3 should only handle a 20uF as a first filter cap.

Am I missing something?

[tubenit]

Fair question about the 32uf and 5Y3. It has worked for me in a couple of amps with no problems so I will continue using it. I've seen amps that were build that exceeded the rating of a particular rectifier tube significantly.  I had a Bogen CHA-20 that was a good example of that.

You can always use a 20uf in that place & it will still sound good.

With respect, Tubenit

[prs_matt]

Thanks Tubenit. I just soldered the 32uF in the chassis yesterday afternoon--just figured I should ask before firing it up.

Another question. I am using a 6AK5 for V1 (cute little tube) and in order to make the most of its own design, I grabbed the data sheet for it. http://tubedata.milbert.com/sheets/137/6/6AK5.pdf One column reads 120V for both plate and screen and the next column reads 180V plate and 120V screen. Which way should I go there? Shouldn't the screens be a fraction of the plates by like a ratio of 5:1 or is that plate/screen current?

[bnwitt]

The Hoffman Stout uses a 40uf filter cap on the first node off the 5Y3 so it must be ok.

[Dynaflow]

 You guys using newer 5y3's or NOS? I just wondered, urban legend was that nos stuff didn't like bigger caps much, true or no? Just wondering... I know like the sovtek can take it (not as much voltage drop and I've had a 50% failure rate, but the 2 good ones worked fine).

Regards,

Dyna

[prs_matt]

You guys using newer 5y3's or NOS?

I am using NOS GE and Sylvania. Guess we'll see how they handle 32uF...  :)

[Fresh_Start]

You guys using newer 5y3's or NOS?

I am using NOS GE and Sylvania. Guess we'll see how they handle 32uF...  :)

Leo's 5F2-A had two 16uf filter caps in parallel for the first node.  He sure wasn't using Sovtek tubes! ;)  Effective capacitance is 32uf, but I don't know whether or not having two caps in parallel reduces stress on the rectifier tube.

Chip

[rzenc]

Tubenit

Hot Blue Plates gave me a resistor box where I can hook it up and simply dial a variety of plate resistors in while using a 25k pot to dial in the cathode resistor value.

While you were dialing the pot to smack the sweetspot did it ever made scratch noises?
I´m considering something along this lines but as a permanent feature. I also considered the possibility of using a rotary switch - make before break - but a pot would be the ticket...

Thanks in advance
With Respect

Best Regards

Rzenc

[tubenit]

I don't remember if it made scratchy noises dialing that pot?  The Soma 84 out of TUT 5 uses an interesting treble boost that uses a pot that is dialed.  I had something similar on the original Carolina Blues Special.

I'd probably use a 25k pot paralleling a resistor if you were going to try that.

With respect, Tubenit

[bluesbear]

I have always without fail used a 40-20-20-20 can with any P-P amp using a 5Y3. That's probably 25 amps. There's never been a problem with the 40mf. The only rectifier tube I've ever had fail was a 5AR4. It started arcing after 4 years of 3 to 4 nights a week of jobs. Sadly, it was a Mullard. I'm willing to bet the 40mf didn't have anything to do with it. It just got moved around and shaken up too many times. Few of those gigs were more than 1 night per venue (that'd be too easy).
Dave

[Lucid_Alice]

Would this preamp design work well with 6V6's? Or is the 18watt PI and power important to the sound?

[Geezer]

Tubenit is using it w/ 5881's & seems like it even better!
http://www.el34world.com/Forum/index.php?topic=6316.0

I have just about got it tweaked into the Little Wing/6BM8 format. I think you could sub in 6V6's with very little changes in circuit values, if any were needed at all.

I'm thinking of a quad of tubes......a pair of 6V6s (or 5881's) & a pair of EL84's, cathode biased in pairs w/ the ability to switch off each pair or combine both pairs.

[tubenit]

Yes, it works beautifully with 5881's  &  I do like them better than the 6V6's which also sound good. The CBR-5879 can change out between those two with no rebiasing.  I've tried them both but keep coming back to the 5881's which have better harmonics.  The 6V6's do break up more quickly.

I love this amp & really feel it's the best 5881 amp I've built to date. Certainly the most touch responsive and expressive.

Having said that, ........ IF I could only build one amp with the 5879 ....... it would definitely be DaGeezer's HoSo 56.
 ;)

With respect, Tubenit

[CraigB]

Anyone contemplating a 4xEL84 version?  I've got a good, working layout that can be adapted to a 2xEL84 version easily that is also a bit different from Jeff's.  It's not drawn, but I would take the time to do one up, if anyone is interested.  (I need to practice making my express drawings look as good as these other guys'.)   Craig

[tubenit]

Sure, why don't you post a layout if you time and are willing to.  Not very many guys seem to use the paralleled terminal approach that I use & your layout may be more to their liking and/or a better layout.

When you're done, just put it in the SCH file for DaGeezer's HoSo 56

With respect, Tubenit

[CraigB]

Tubenit, your layout looks great as usual, perfect for those universal-type terminal boards like you used, or terminal strips, and best for tweaking values until you get it right.

I'll post mine in the .sch area when I get it drawn up. 

Craig

[pullshocks]

My 6BM8s came in.  Any word on the 6BM8 version?  Just paste the hoso preamp onto the Little Wing power supply and power amp?

[macula56]

Please post your layout CraigB. I am building a 4 EL84 version and would like to see what you have come up with. Thanks, Jmac.

[Dynaflow]

My final schematic and layout  (and DaGeezer's final schematic of the HoSo 56) are here:
http://www.el34world.com/Forum/index.php?topic=6176.0


This layout board follows the layout I drew with the except of the B+ cap to the 5879 being on the board.

With respect, Tubenit

 This answers the question I was gonna ask on this, it appears those cheap tagboards can be used quite well for this amp. Think I've seen em for like 6-7 bucks, I still would like to build one of these sometime.

Regards,

Dyna

[tubenit]

Those tag boards are "OK"  if you don't experiment and remove and replace parts very much.  If you do alot of tweaking and experimenting the flimsy tags rip off. One of my boards is pretty beat up looking from all the experimenting & soldering.   

But if you know exactly the layout you're gonna use, they are reasonably easy to work with if all you need are paralleled terminals in your layout.

Something like Hoffman's turret boards would be sturdier and less prone to problems, I think. I think if I were building for someone else who played out frequently, I would NOT use the tagboards.

With respect, Tubenit

[Dynaflow]

 Thanks for commenting on them, I wondered since they were pretty cheap. Theres other options, I know were to get the same using just eyelets with another row down the middle I think.

Regards,

Dyna

[PhatTele]

Quick question on the Root 666 designed posted earlier in the thread.  What power transformer would work for that amp?  Are they using something like a JCM900 100 watt PT to get that 370-0-370 vac?

[Geezer]

My 6BM8s came in.  Any word on the 6BM8 version?  Just paste the hoso preamp onto the Little Wing power supply and power amp?

Sorry for the delay....very busy lately (Thankfully!)

Yes, to start, just take the HoSo pre & paste it onto the LW power section, which would include the 6BM8's triode section as the PI.
In tweaking this amp, it was a bit of a challenge & took a long time to get right. It seems the 6BM8's take less signal to drive them into overdrive, as there was too much distortion at 1st. I tried different PI values & such, but ended up using a 12DW7 (ECC832 JJ) in V2 to get the gain down to acceptable levels. That tube (if you're not familiar) is 1/2 12AU7 & 1/2 12AX7, so that dropped the gain a good bit & got the amp to sounding more like the EL84 version. I also raised the Rk on V2A to 3.3k to further clean it up.
There are probably other ways to go to reduce the drive to the power section, but being pressed for time, & it worked very well, I stuck with the tube substitution instead of stressing over it too much. I realize that most folks aren't going to have a 12DW7 around, so you may want to try other methods of signal reduction(?)

[Geezer]

Theres other options, I know were to get the same using just eyelets with another row down the middle I think.

I usually put another row of Hoffman's eyelets down the middle, but only in the holes where I need them.

[Dynaflow]

 Yeah the one in question has turrets with a row down the middle, its a universal board and a bit pricey, if I were going to do it I'd probably just get some eyelets and board from Doug and do my own. I know some don't like eyelets, but I don't mind them and they're easier to set with the modest tools I have. I find a wide pointed centerpunch flares the backside just fine, I also do the board ghetto style with a hand drill and no drill press, I'm in a APT so it makes it hard to have a drill press around like I did in my Pre-divorce days. Provided you get a clean bead of solder I haven't had problems with them. (If you tweak things all the time, then yeah Turrets are the way to go, I tend to get it working, tweak a little and then button it up so I can play it out)

Regards,

Dyna

[pullshocks]

[/quote]
 I realize that most folks aren't going to have a 12DW7 around, so you may want to try other methods of signal reduction(?)
[/quote]

Thanks DG.  I really appreciate your help.
When I started the amp hobby I hacked up an Eico Stereo 40, and have a couple 12dw7s left over from that.  Finally--something to use them for.  The transformers are doing very well in the world's only dumble clone with a cathode biased 7591 power amp.