Low Power Pentode _ but easily available - for very small PP amp - Councils ?

[kagliostro]

A friend is interested to realize a very low power JCM800 amp in the order of around 1W

his preferable choice will be a PP pentode for the power section and I've think (in the wake of the Third Watt by PRR) to the 6AU6 tube

also the 5902 tube is usable for this purpose, but way less easily available

There are other small pentode usable for this project that are easily available ?

Thanks

Franco

[Latole]

I google "1 watts guitar amp" and found link to a very interesting tread by tubenit

https://el34world.com/Forum/index.php?topic=21664.0

[kagliostro]

Many Thanks Latole

Unfortunately the substantial difference in Tubenit's project is that uses a Triode and not a Pentode as Power Tube

Ciao

Franco

[Latole]

It should be easy to mod to pentode.

Google ; 1 watt pentode guitar amp schematic

https://www.diyaudio.com/forums/instruments-and-amps/326999-mini-amp-output-tube-distortion-2.html

https://el34world.com/Forum/index.php?topic=23081.0

[kagliostro]

Latole, Thanks Again

I'm not sure to what you want refer to on that link

but if you refer to the 6K6 tube .... unfortunately it is a 7W max plate dissipation tube

way too big for an 1W JCM800

also an EL91 (max plate dissipation 4W) is too big for this application

(already the 6au6 is a bit big, with its 3W max plate dissipation)


--


To achieve a low power reducing B+, is a way, but this way the tendency is that tube go earlier in distortion, this is the reason for which I do the search for a very low power Pentode

Franco

[kagliostro]

Latole, Thanks Again

I'm not sure to what you want refer to on that link

but if you refer to the 6K6 tube .... unfortunately it is a 7W max plate dissipation tube

way too big for an 1W JCM800

also an EL91 (max plate dissipation 4W) is too big for this application

(already the 6au6 is a bit big, with its 3W max plate dissipation)

--

To achieve a low power reducing B+, is a way, but this way the tendency is that tube go earlier in distortion, this is the reason for which I do the search for a very low power Pentode

Franco

[Latole]

You don't need to use tube at full dissipation. 7 watts tubes is just more expensive than a 1 watt tube.

IMO these schematics work with easy mods for somebody know how and with any tube.

[shooter]

unfortunately it is a 7W

I'm using a pair now at ~12W and it's NOT in the 1W family 

Not sure is the EF86 has been using in a PA sense, hard to beat the 6AU6 for price, availability and such.  I build a lo-watt 3SE with them and was impressed with how loud they were 

[Latole]

unfortunately it is a 7W

I'm using a pair now at ~12W and it's NOT in the 1W family 

 

You are right if you desing the circuit for 12 watts outputs.
Different tubes may have different sound

[kagliostro]

At the moment the better choice seems the 6AU6 tube

at www.ampbooks.com there is a project that uses EF86 tubes

(for a PP with 65mW RMS power) but also the russian version 6J32P (6Ж32П) aren't really cheap

https://www.ampbooks.com/mobile/amp-technology/EF86-power-amp/

To use a 7W or 12W tube at low power is possible lowering B+, but this will alter the response of the tube that will enter earier in distortion, this is the reason for the search of an adequate tube

Franco

[tubenit]

Franco,

I don't have a specific pentode 1w amp schematic that I can share with you.  Having said that, there may be some useful information with Rob Robinette's 1 watt 12AU7 5F6-A Bassman that may help in a design with pentodes?

http://el34world.com/Forum/index.php?topic=18481.0

The other thing that may be worth considering is the ECL84 tube which in push/pull is only 3-4 watts.  You can find some information with that in the So-Low Watt threads.  With a VVR installed, one could easily dial down to 1 watt.  Plus these area a pentode/triode tube. 

http://el34world.com/Forum/index.php?topic=13151.msg122821#msg122821
http://el34world.com/Forum/index.php?topic=13438.msg127145#msg127145

Here is a thread on push/pull 6Au6 amps

https://el34world.com/Forum/index.php?topic=12454.0

With respect, Tubenit

[DummyLoad]

6AK6


--pete

[kagliostro]

Thanks to All

Till now 6AU6 seems to confirm his Top Position on the more suitable tube for this use

followed by the 6AK6 (II place for diffusion of the tube, not other spec)

Franco

[Latole]

This one ?
Or 2 watts here;

http://www.dogstar.dantimax.dk/tubestuf/4x4howit.htm

[DummyLoad]

that lafayette amp is a deathtrap.  i wouldn't recommend it as is - install a isolation transformer.


--pete

[Latole]

that lafayette amp is a deathtrap.  i wouldn't recommend it as is - install a isolation transformer.


--pete

You are right, I put some isolation transformer on few amps like that over years.

These schematics are for their low power amps section, kind a work in progress...

[DummyLoad]

Thanks to All

Till now 6AU6 seems to confirm his Top Position on the more suitable tube for this use

followed by the 6AK6 (II place for diffusion of the tube, not other spec)

Franco

6CA5

[DummyLoad]

6AN5 - 1.3W @ 120V B+ into 2.5K anode load - for P-P we'd get about 2.6W into 5Ka-a so, hang a 10Ka-a OT off the pair - that should get you close where you want to be. kind of a filament current hog though - same as a 6V6 with twice the sensitivity with 1/4 the power output.

10K P-P OT is going to be easier to source than 20K P-P unless you use hammond 125C"universal" or some such that will reflect upto 25Ka-a with multi output taps. be sure 1/2 primary current rating o the OT will handle what you throw at it: lower volt tubs with lower reflected load requirement will need more current. 


--pete

[66Strat]

Another option for reducing power would be to insert resistors in series with the output tube plates to reduce the voltage swing. Vox employed this method with the AC10.

[shooter]

I'm using this 10k:8 was pretty reasonable in USA

EDCOR XPP15-10K-8ohm but it's good to 15W, just in case you want to upgrade 

[kagliostro]

Many Thanks to ALL

6CA5 & 6AN5 seems other interesting candidates

Off Topic

Pete, some time ago you build the Compactronator, unfortunately I don't have documentation about it

I would like to know if it was an SE or a PP amp and which were the Power Tubes

Thanks

Franco

[DummyLoad]

Many Thanks to ALL

6CA5 & 6AN5 seems other interesting candidates

Off Topic

Pete, some time ago you build the Compactronator, unfortunately I don't have documentation about it

I would like to know if it was an SE or a PP amp and which were the Power Tubes

Thanks

Franco

franco, compactronator is a P-P and i used 6JB5 vert. deflection output tubes + 6AY11 preamp tubes. schema attached.

--pete 

[shooter]

you like uVac ripple at the last tap 

[ALBATROS1234]

my first thought is you could go octal with 6sj7 @2.5 dissipation. but really you could make larger dissipation tube only put out 1 watt or 2 by decreasing b+ i built a single ended amp using a 6aq5 tube which at 325volts would be a 5 watter . i just used 2 toroid 120v to 12v and connected secondaries together to make an isolation transformer for b+ the filaments were supplied with a separate small filament tranny. with a bridge rectifier b+ was like 160volts. it was cool because i could crank it and get that cranked tube amp saturation . i calculated it at 1.4 watts . i did this because i wanted a bedroom volume tube amp i could get cranked tones out of. thats before i remembered evh used a variac for volume control. now i get cranked tube tones at bedroom volume out of virtualy any amp 15 watts and below using the variac as volume control. it takes longer for the tubes to warm up but you can crank the knobs and get the volume you need .i find the tone actually gets a little crisper at lower voltages. i average my variac set between 65 to 80volts to get the volumes i want at home then use the same amps to jam with friends full voltage. if you dont believe me google evh and variac theres several articles online. he did this for years to tame his 100watt marshall for club volumes he said if he turned it below 60 v the tubes wouldnt work and thats about what i found. below 60v the tubes just putter out. i assume because of the filament voltage drops to low but if you build something with low b+ and a separate filament transformer you could prob make 6v6s 1 or 2 watts output. might need like 80volts b+? you would have to experiment. you might can find an old solid state amp and rip the power transformer out. i got a few where the secondary is like 63v but they are nice sized transformers that supplied a lot of current to the power transistors for like 100watt solid state. get something like that with a bridge and you should get like 88volts b+ get a filament tranny off ebay for $10 to $15 an you are set. you are making me want to built a pp 6v6 with super low b+for fun.

[kagliostro]

Ciao Pete - Shooter - Albatros

Many thanks Pete

I was missing that documentation and I'm happy you share it 


Albatros

6SJ7 .... 2.5W plate dissipation, OK, maybe it can be used


About lowering B+ on an amp, it is a simple thing to be achieved, lot of documentation around about the use and the effect of a VVR, but as I tried previously to explain, that will not the preferred way to have low power for my friend amp


Franco

[brewdude]

A couple years ago I started drawing load lines (with the help of a generous member on a different forum) for a 6CA5 push pull amp. 


At 125v on the plates with a 6k primary OT, I figured about 3.5 watts output.


I have not tried to build it yet.

[Joel in Texas]

Kagliostro, I assume the goal here is cranked amp tone at low volume.

All of the helpful discussion so far has been in direct response to your actual question, about low power pentode options.  That’s great - lots of good information so far.

But I have two thoughts, not regarding tube type.

First, if the goal is low volume, do not forget about the role that speaker sensitivity and frequency response plays in overall volume. Selection of speaker plays a huge factor in apparent volume, perhaps more than incremental change in amplifier watts.

I believe you can easily make a 3 watt amplifier drop to 1 watt “apparent volume” levels by using a less efficient speaker.  And yes, some less efficient speakers can still sound great.

I experiment with speakers just as much as, if not more than, experimenting with amp circuits. Changing speakers is just about the fastest way to cause major changes in the tone AND output volume of an amp.

For example, consider a few of the speaker drivers I currently have in rotation. The Celestion G10L35 (10 inch driver, not currently made, but I think the current Celestion G10 is similar) has sensitivity of 95db. The Electrovoice EVM12L is a 12 inch speaker at 100db.  The Eminence Swamp Thang is a 12 inch at 102db.

If I play the same amp through these very different different speakers, the difference in apparent volume is huge. Those G10L35’s at only 95db in a little closed-back Marshall 4x10 sound great with a cranked amp - just much quieter than the louder, more efficient EV or Eminence drivers.

Second, you may know this already, but the difference in apparent volume between 1 watt and 3 watts is not that much. 3 watts is only a fraction louder, and is not three times louder than 1 watt.

In fact, I would guess that all other things being equal, a 3 watt amp, cranked through a speaker with 95db sensitivity, would sound nearly equal in volume or possibly quieter than a 1 watt amp cranked through a speaker with 102db sensitivity. Someone smarter than me could probably do the math to interrogate this claim, but based on my own experience I would not worry about 3 watts versus 1 watt.

[PRR]

...the order of around 1W....

That was a very common rating for US table radios. Ever since the first RCA living-room speaker. It ran a '10 at 400V, and was VERY expensive (as much as a Chevy). Tube development led to low-impedance tubes to get 1 Watt from lower voltage, ending at 25L6, and its 35V and 50V brothers, and the 7-pin midget tubes that dominated the 1950s.

Europe must have a similar class of basic home radios?