HoSo56 ECL84

[tubenit]

OK, here is the deal ............... I've got a 65ma 190-190 PT.  (Yes, I know I can change the PT & will if I need to.)

What power tubes in a push/pull design would work with a 65ma max PT?

I'm trying to explore something that has a good tone. I sort of want a version of a 6BM8 or a 6K6 that would draw less current and sound as good as those tubes which I like.

Something other than 12AU7 or 6Au6?

I've got a pair of 6K6 tubes but I am not skilled enough to determine the ma draw at 270 volts on the plates?  I am thinking p/p would exceed 65ma?  I think the data suggests it will draw about 55ma in a push/pull?

http://www.scottbecker.net/tube/sheets/127/6/6K6GT.pdf

IF that is the case, with a Hammond PT of 65ma ........... think that would be cutting it too close?

With respect, Tubenit

[RicharD]

I'd agree that a pair of 6K6's would be totally pushing the 65mA PT.  I just threw together a little bedroom amp using a 6CL6 in a single ended configuration driving ye olde Fender reverb transformer as the OT.  I am totally happy with the results.  Here's the cut sheet. 

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6CL6.pdf

These tubes are not selling for $2.00 to $5.00 each on fleaBay.  Be cheap...... like me.   

[RicharD]

I'm guessing you already have an output transformer, but 65mA seems more like SE territory. 

[tubenit]

OK, this is where my thinking is at this juncture ...........

I already used the 190-190 65ma PT to build a very nice SE amp.  Having said that, I am wanting to experiment. This is in a very small cab head. I'd like to see if I can turn it into a p/p amp & have it fit inside this small chassis and cab?

Using the mosfet CF has worked out really nice for me and I'm thinking I can actually build a HoSo56 with 4 tubes by using a concertina phase invertor. I think that would be a cool feat especially if it sounds good.  And I don't have  a HoSo56 currently. So this would give me a different sound than the Dumblish inspired amps.

I have a 6K6 in the SE amp now.  I am thinking that IF I measure the SE ma's and it does not exceed 27ma, that it's worth the experiment to see if this will work out?  I am presuming (perhaps incorrectly) that the p/p would simply be around double the SE ma's?

Some time back,  DaGeezer was kind enough to give me a p/p OT that would be perfect for this. The Hammond trannies are supposedly overbuilt, so maybe this would be reasonable?  Worse case scenario, I try to find a PT small enough to fit the chassis and cab head that would work.  It is a very very tight fit.

EDIT:[/This schematic design using 6K6GT will NOT work with 65ma PT.

With respect, Tubenit

[PRR]

> 65ma 190-190 PT

Say 5mA off for preampery, 60mA to power stage.

266V nominal DC but more like 240V with filtering.

Right away we know you have 240V*0.060A= 14 Watts available. At this size you normall work near Class A, so you want 14 Watts of plate dissipation. If two tubes, at least 7W each.

That's bigger than 6AU6, smaller than 6V6/6AQ5.... you probably end up running a big tube below ratings (long life!).

The ratio 240V/0.060A is 4K ohms. In push-pull your nominal plate-to-plate load impedance should be twice this: 8K.

You want a tube which will pull well into 4K each side. In power triodes you want a tube with plate resistance 2K or less. In pentodes with G2 near plate supply voltage, it works out that if the triode-wired plate resistance is <2K, the pentode performance will be good, more power output than triodes.

I keep coming back to 6V6-class tubes. EL84, 6AQ5, 6K6, 6F6.... These are "too big", but not expensive.

> 6K6 tubes but ...the ma draw at 270 volts on the plates?

From 285V to 270.... hardly no change at all.

But I think you will be (or can be) closer to 240V. 240/285= 0.84. Total demand at 285V is 74mA, 0.84*74mA= 62mA, which is fine. The problem is that the 6K6 data shows 12Kpp load, an odd value.

[sluckey]

Here's something from ax84.com that you may like. Or it may give you some other ideas. It's a fixed bias 2 watter P/P 6SN7 with LTP PI and NFB with presence. There may be some more info and sound clips on their website.

http://www.ax84.com/static/corepoweramps/2W_PP/AX84_2W_PP_Poweramp_Schematic.pdf

[tubenit]

PRR and Sluckey,  

It does not look like a push/pull 6K6GT amp will work with a 65ma PT.  Just single edged 6K6GT is drawing 27.1ma at idle and cranked pushs 30.5.  There would be almost no margin for error.

Thanks for the responses!  I will try to get some more information and hopefully soundclips of the 6SN7GT idea.

With respect, 10thtx

[PRR]

It will... but if you want to go another path, that's good.

[DummyLoad]

gotcha covered!  

 6AM5/EL91 brimar published data for A1 push-pull @ 250V B+.

http://tubedata.milbert.com/sheets/155/6/6AM5.pdf

plenty of them on fleabay reasonably priced.

another one...  6AR5 - use the 250V conditions for single tube and just double everything - R load, bias current, etc.

http://tubedata.milbert.com/sheets/127/6/6AR5.pdf

either tube should keep you well within your heater windings' budget. you never did mention what that is... i'm guessing 2A or less.

--DL

[JBP]

Musical Power Supplies PT190 -  120ma

[TIMBO]

I'm only thinking out of the square with this, but would using a second tranny help 

[darryl]

The 6DX8 might fit the specification:



In push-pull duty, the plate load impedance is quite high at 17.5KΩ, but is generally achievable with off-the-shelf output transformers by juggling the secondary impedances. These are a triode-pentode, so there are also the bonus triode sections which can be used for a phase inverter.

6DX8's can be purchased for as little as $1 each from online valve ( err... tube ) vendors.

[tubenit]

DummyLoad and Darryl,

You guys have come up with some VERY interesting tubes!  I'm impressed. Thanks!

IF I am understanding the EL91/6AM5 correctly, then it looks like it would produce about 5.8w, use around 30-40ma in a push/pull. Cathode resistor in a p/p  would be 410R shared or 820R (each).    

ECL84/6DX8 looks to use about 36ma in a push/pull and will produce about 4 watts. Cathode resistor looks to be a 2M?

Questions:

Q1)  How would I hook up the OT secondary to an 8 ohm speaker for the EL91?  Can I use the 8ohm secondary?

Q2)  How would I hook up the OT secondary to an 8 ohm speaker for the ECL84?  Can I use the 8ohm secondary?

Q3)  The cathode resistor for the EL91 is 820R (each) so shared would be around 410R (probably would use 430R)?

Q4)  The cathode resistor for the ECL84 is 2M for p/p?  Is that right? (doesn't "seem" right?)

Q5)  Is the triode section of the ECL84 similar to a 12AX7?  If so, that would give me more options for sure.

Q6)  Either of you have any experience with either of these tubes in a p/p amp?  How do they sound?

THANK YOU for the help!!

With respect, Tubenit

[tubenit]

Darryl,

Is this your amp? 

Got a schematic that you can share?

How did it sound?

My understanding is the triode is NOT useable like a triode of a 12A_7?  Is that correct or incorrect?

What cathode resistor value did you use?

What OT secondary hookup did you use?

I am strongly leaning towards this ECL84 tube because of it's low cost.  Looks to be like a baby 6BM8 (sort of).

[darryl]

Is this your amp?

Guilty as charged.        I've now built about 150 of this particular model.

The original construction details are here: http://www.guitargear.net.au/discussion/index.php?topic=14767.0

There was an updated version described here, including some sound clips:  http://www.guitargear.net.au/discussion/index.php?topic=19215.0

And a long and meandering thread which includes end-user comment here: http://www.guitargear.net.au/discussion/index.php?topic=18287.0

In answer to some of your questions:

The triode section of the 6DX8 has about 60% of a 12AX7's gain, but it can be used the same as any other medium mu triode.

The 6DX8 data sheet calls for a 117Ω common cathode resistor. I found 150Ω to be necessary, to keep the output valves' quiescent dissipation to about 3 watts each - 75% of the 4W maximum.

The amplifier pictured above has a nominal output of 2 watts into 8 ohms. Its actual measured output is 3 watts @ 8Ω and 2 watts @ 4 or 16Ω.

The output transformer is a public address line transformer - a Jaycar MM1900    The use of PA line transformers as output transformers is discussed here: http://www.ozvalveamps.org/optrans.htm

The 6DX8 data sheet calls for a 17.5KΩ plate-to-plate impedance, which is higher than most standard output transformers provide. If you already have an output transformer that is around 8KΩ plate-to-plate, and which also has both 4Ω and 8Ω secondary taps, then you can use an 8Ω speaker on the 4Ω tap. The primary plate-to-plate impedance then becomes 16KΩ - close enough to the 17.5KΩ required.

Please don't make the 6DX8/ECL84 too popular, or the online merchants will put their prices up.   

[tubenit]

Darryl,

THANK YOU !!!!!!!  I am very appreciative of your help & info with this. And I am excited to give this a go.

I kept wanting a p/p that was decent under 65ma spec & this should fit the bill quite nicely.

With respect, Tubenit

[JB]

I can vouch for EL91's.  I have a SE amp and a PP amp both using them.  SE amp is pretty much finished, PP version still needs some tweaking.

They are plentiful and cheap - particularly the military coded versions, some of which are supposed to be a bt more rugged, as I think they were used in Eddystone military radios.

They sound great, a proper miniature power pentode.  A pair is more than enough for home use, and just about copes for not too loud rehearsal with our band.  I use a VVR to quieten them down a bit.

I tried ECL82's, which are good, but I think the little EL91's sound better.  Like the look of the ECL84's in that amp.  Nice job.

[kagliostro]

Very interesting thread

have never considered el95 ?

http://crude.mine.nu/sel95.html

Operating characteristics two tubes in class B push-pull

Va    200        250       V
Vg2   200       250       V
Vg1   -10       -13       V
Raa     10         10       kohm
Vi   0   7   0   9   V
Ia   2x7   2x19   2x8   2x24   mA
Ig2   2x1.2   2x5.0   2x1.2   2x7.2   mA
Po   0   4.0   0   6.5   W
dtot       3.5       3.5   

Kagliostro

[JB]

I have tried an EL95 in my SE amp in place of the EL91.  Pin-out is slightly different so I have a switch to take either - re-directs the wiring and changes the cathode resistor.  Also have to change the impedance.

EL95 is basically a 6V6 in a 7 pin package.  Slightly lower max ratings I think, but otherwise it is a 6V6.  Sounds good, as well as being louder than the little EL91's I think it has more bottom end.  Probably just the ability to deliver power better.

[DummyLoad]

EL95 is similar to 6005/6669/6AQ5 & has same pinout. EL90 is direct replacement for 6AQ5. 6AQ5 is also a down-rated 6V6.

--DL

[JB]

Sorry, yes you're right.  I'm muddling my 90's and 95's.  It's the EL90/mini 6V6 that I've been swapping with EL91 for more output.

[darryl]

Hi Tubenit, I notice on your schematic that you are planning to use 5.6K grid stopper resistors on the ECL84 pentodes. I did find it necessary to use larger values - typically 15K to 22k, otherwise the pentodes oscillated.

You may also need grid stoppers on the triode sections, but I'm not sure about this in your circuit, because I used a cathodyne phase inverter, rather than a long tail pair.

[jojokeo]

EL95 is similar to 6005/6669/6AQ5 & has same pinout. EL90 is direct replacement for 6AQ5. 6AQ5 is also a down-rated 6V6.

EL95 is = to 6DL5 and different from a 6AQ5. A 6AQ5 is = to El90 and is similar to the 6V6.
Another tube to consider for 'Nit is a 6AN5 or a EL91/6AM5.

Personally, if I were him and w/ my prior experiences I'd simply modify the Firefly schem and use either a 12BH7 or the newer JJ ECC99 and run those with a self-split PI. You'll get the power and what you're looking for and use less tubes too making something to be said for simplicity.

[jojokeo]

The 6AN5 and 6AM5/EL91 are mighty little "power" tubes run on lower voltages but you must be careful w/ screen dissipation limits or they will redplate easily.

In subbing a 6DL5 (great sounding little tube by the way) w/ a 6AR5 and a 6AQ5, the 6AR5 isn't really worth it as it's very close to the 6AQ5, is expensive and not easily gotten from ebay like 6AQ5s are. Going between 6DL5 and 6AQ5 you will hear a difference btwn the two. But, for your money - just go w/ the 6AQ5 w/ lower voltage if that's that power range you're looking for?

[DummyLoad]

OK, this is where my thinking is at this juncture ...........

I already used the 190-190 65ma PT to build a very nice SE amp.  Having said that, I am wanting to experiment. This is in a very small cab head. I'd like to see if I can turn it into a p/p amp & have it fit inside this small chassis and cab?

Using the mosfet CF has worked out really nice for me and I'm thinking I can actually build a HoSo56 with 4 tubes by using a concertina phase invertor. I think that would be a cool feat especially if it sounds good.  And I don't have  a HoSo56 currently. So this would give me a different sound than the Dumblish inspired amps.

I have a 6K6 in the SE amp now.  I am thinking that IF I measure the SE ma's and it does not exceed 27ma, that it's worth the experiment to see if this will work out?  I am presuming (perhaps incorrectly) that the p/p would simply be around double the SE ma's?

Some time back,  DaGeezer was kind enough to give me a p/p OT that would be perfect for this. The Hammond trannies are supposedly overbuilt, so maybe this would be reasonable?  Worse case scenario, I try to find a PT small enough to fit the chassis and cab head that would work.  It is a very very tight fit.

EDIT:[/This schematic design using 6K6GT will NOT work with 65ma PT.

With respect, Tubenit

need a cap between trim wiper and bootstrapped SLPI.

--DL

EDIT: need another cap between source follower and tone stack - otherwise may have some unhappy effects units - effect unit input caps may not take the DC level at the source.

[tubenit]

I'm going with the ECL84 so I can build another HoSo56 type amp. This will allow me a p/p with an LTPI and only 4 tubes still with a 5879 pentode in the preamp.  The layout board is now built.

These are the updated schematics.

EDIT: I plan to use a dual gang 500k pot (instead of 250k) & I will use the 2.2M resistors with it. 

With respect, Tubenit

[DummyLoad]

i guess i wasted my time... just read your last post. oh well!   

--DL

[tubenit]

DL,

Hey man, THANKS for the drawing!  Very kind of you to do it. I read all of Darryl's thread on the Aussie forum and liked what he was saying about the ECL84. And I was familiar with the 6BM8  triode/pentode thing with an LTPI.  


I think what I'll do is create a ARCHIVES thread for small watt push/pull amps and I'll include this drawing. I'd love to see someone build your amp design. I think it would be a good one!

 

With respect, Tubenit

[DummyLoad]

THANKS for the drawing...

happy to help. :-)

another tube to consider at a later time...

http://tubedata.milbert.com/sheets/009/e/E80L.pdf

they are kind of pricey - esp. on fleabay and they are harder to find here in the states - a US industrial variant is the 6227.

maybe an amp using 6267 & 6227 - full house! 6's and 2's. lol!

best to ya,

--DL

[moonbird]

DL --

Cool schem using the 6AR5 -- will have to put it on the list. NJ7P says they are 7-pin versions of the 6K6. I really like how 6k6s sound in my 5e3 clone. Thanks much!! 7-pin tubes are not appreciated and as a result are still cheap.

In this thread:

              http://www.el34world.com/Forum/index.php?topic=7492.0

Geezer mentions 3 7-pin small sig pentodes for the Hoso56 : 6CB6, 5654, 6au6. Think I might add a parallel 7-pin input (ala HoSo56 II) and then two more 7-pin sockets for the two halves of the 12AX7. I could start with 6AV6s - because they are the same as 1/2 of a 12ax7 -- and go from there. I am going to have some time off soon - maybe I can hide from my fam long enough to really do something <grin>. There are just too many GREAT designs on this forum sometimes (LOL).

Oh and BTW -- I have about 5 E80Ls (2 have gold pins) -- so I will be watching for your E80L design too <grin>

thanks again!!

[DummyLoad]

Cool schem using the 6AR5 -- will have to put it on the list.

thanks!  -)

moonbird, i hope 7 pin tubes stay unappreciated for a while longer... until i can round more of them.

regards,

--DL

[tubenit]

In keeping with generating more options and ideas to experiment with...................

I drew this EL91/6AM5  push pull amp. It's based on the Carolina Blues Special that originally used 6V6's.


With respect, Tubenit

[Jennings]

Really interesting thread...just thought I'd mention that the Torres Tiny Tone is a 1 to 2 watt little beauty that uses an EL90 / 6AQ5 output tube.  Uses two ECC83 / 12AX7 in the preamp.  Sorry, I don't have a circuit diagram to hand for it...but figured I'd add it to the melting pot of ideas!

[tubenit]

6AQ5 is a great tube! Nice tone. I eventually built a HoSo56 w/6AQ5 tubes.

Right now, I am trying to collect ideas strictly for a push/pull pentode low watt amps.  There is a bunch of SE designs and push/pull triode stuff already out there.

Once the info is all collected it will go into a new thread complete with tube data, schematics ................... we'll make comments, then I'll edit it all and put it into ARCHIVES.

With respect, Tubenit

[jojokeo]

In keeping with generating more options and ideas to experiment with...................

I drew this EL91/6AM5  push pull amp. It's based on the Carolina Blues Special that originally used 6V6's.

I like this idea using these tubes and from personally using one in a SE design, they are very loud for their ratings - almost as much so as the 6DL5's were! Two of them would maybe be louder than a Champ!?

[JB]

One EL91 SE is plenty loud for practicing at home.  Too loud if you want power amp distortion and have other people in the house, or neighbours.  A push pull pair of them sounds very good too, but I run mine with VVR to tame it down.  I run an LTPI into them but you have to watch the gain as they don't take much signal to drive to full output.

I think a pair is about the same as a champ for loudness.

[moonbird]

Couple years ago -- I did a bunch of tube research for the heck of it. I made up a series of analysis sheets for various tube pin groups - Here are my 7-pin output pentode sheets. Please note the highlighted ones on the summary sheet. They all have the same pinout and roughly the same limits. At the time, I wondered if they would sound different enough to detect. All are *ideally* in the 1-2W output range. The other doc contains NJ7P (tube site) screen captures for a few of them.

[tubenit]

Thanks Moonbird!

Ironically, I did something very similar this very morning.  DummyLoad is helping me with it.

With respect, Tubenit

[Tone Junkie]

How goes the quest my friend i dont post much but Im always reading for some reason building smaller and smaller amps to preserve my hearing is very appealing to me. Ive started buying some cheap starter amps in the pawn shop to do some conversions on. so this post and the other small tube low wattage posts are very interesting to me right know.
Thanks Bill
PS. I have a bad habit of spending money on things I might do someday <LOL> and my wreck still sits in the amp stand unfinished. 

[tubenit]

The HoSo56 ECL84 is finished!  And it sounds super sweet!

Very touch sensitive like the original HoSo56.  Less harmonics than the original but smoother tone. The original was chimey-er and this is creamier.

This is a high gain amp ........ BUT it is very very transparent in it's tone. It has a very cranked pushed tone.

Good news is that it sounds incredible and has excellent sustain. It's one of those "oh wow, don't want to quite playing" type amps.  If the TOS and TBM are 10's for sustain, then this amp would be around an 8.5.

I built this into a very small  12" x 6.5" chassis ........... so it is remarkably cramped in there. That has to be something of a factor here.  Incidentally, it is super quiet at idle. No hum at all and the gain hiss is hardly noticeable. It's so quiet at idle that when you sturm a chord, it takes you by surpise at the volume.

It is a very loud 3-4 watts.  Louder than the VibroChamp I used to own.  But I can dial the PPIMV down to a pretty low volume and still get excellent tone.  Definitely a good bedroom practice amp.

EDIT:  Initially, I had some oscillation issues if ALL the vol, master vol and PPIMV were cranked to "10".  Turns out that it was a very microphonic 5879 preamp tube.  With a good 5879 and a Sovtek 12AX7 ......... I can dime everything with no oscillations.  

Ironically, the most toneful combination that I've found so far is a 5879 and a NOS 12AT7 into the ECL84's. There is no loss in sustain and the tone is a little more transparent.  The Sovtek 12AX7 sounds good also in V2.

I'll try to post soundbits when I can.

With respect, Tubenit

[DummyLoad]

tubenit; dolled up version. please check for errors. what make/model OT did you use and what pins did you strap?

peace.

--DL

[PRR]

> all have the same pinout and roughly the same limits

These tubes are all the "same chassis", with different grid spacing, and marketed for different supply voltages.

6FX5/12FX5/19FX5/60FX5 is in this class, and has higher power sensitivity.



The load impedance seem to vary 2.5K-4.5K, but would be all-the-same IF adjusted to same plate voltage and current, limited by dissipation.

6AS5 is rated 5.5W and 150V. 5.5W/150V= 0.036= 36mA max. 150V/36mA= 4.2K load (4.5K on sheet). Round up for lowerTHD numbers.

6FX5 is rated 5.5W and 150V, but there was demand for extra-cheap sets with low voltage and current. 5.5W/110V= 50mA max, but 35mA supply costs less. 110V/35mA= 3.1K, but 2.5K sucks more output power from small signal voltages. (60FX5 could play records without a preamp.)

The low-S tubes (6AS5) have lower THD. The high-S 6FX5 can be pleasantly raucous.

You can't directly tube-roll these types. Some need quite different cathode resistors even when conditions are adjusted for similar voltage and current. Sensitivity varies almost 3:1.

[DummyLoad]

thank you PRR, that took some time to compile.

again, thanks and hope you're staying warm up there.

peace.

--DL

[tubenit]

what make/model OT did you use and what pins did you strap?

Geezer gave me the OT a few yrs ago. Nice OT.  It has RCA on it, so I am presuming it's maybe from a HiFi, integrated amp or PA?

I have the 4 ohm tap hooked up to an 8 ohm speaker.  It's about the size of a Princeton Reverb OT.

I decided I am going to call this amp the "So-low watt". 


PRR, THANKS for the list of other tubes we can use to do this with!

With respect, Tubenit

[DummyLoad]

yet another spin w/ another pentode output - 6DS5

should make about 7 watts.

--DL

[Tone Junkie]

Cool you guys this sounds really good thanks Tubnit for always coming up with something new and thanks to the other guys for always adding new Ideas and cool new tubes.
Bill
PS. do you know what rating that ot was

[Geezer]

PS. do you know what rating that ot was

Typical OT used for 2x6v6 or 2xEL84 operation, 12w to 18w, so pretty stout for this application. It came out of an old late 50's or early 60's monoblock/PA-type amp (think Bogen/Knight/BellSound) running 2x6v6's PP.

G

[Tone Junkie]

So its probably 8k I guess that was what I was asking.
 It was pretty late Im working swing shift know Im pretty brain dead when I get home.
Thanks Bill 

[moonbird]

Been away awhile -- life just gets "in the way" at time <grin>

PRR - Thanks much for analysis on the 7-pin pentodes. Very helpful.  I am always on the lookout for an amp with good touch sensitivity and mild grit type overdrive tone (mostly clean) with some healthy warm distortion. Many of my guitars have fairly chimey single coil pickups - so a warm amp balances them abit. Which one would YOU try first??

Thanks again!!

[tubenit]

Here is a quick 1 minute demo of the amp. No song or backing track just chording and few single notes.

The brief demo does a good job of showing the sustain. It will sustain almost endlessly into feedback when cranked.  You can hear the amp feedback at the end of the chording and rhythm part and you can hear it somewhat on the brief lead run. 

I have a hard time getting good recordings of an overdriven nature, so I acknowledge the soundclip is just "ok" but you can get the idea of the kind of sustain it has.

http://soundclick.com/share.cfm?id=11184770

I don't think the recording does justice to it, but this amp has an incredible amount of bass for a very low watt amp of maybe 3-4 watts?

With respect, Tubenit