Question about PT & OT for el95 PP amp

[kagliostro]

Some time ago I've find this schematic

http://www.bensykora.com/7cylinder/7cylinder_final.gif



I'm wondering about to build a reduced version using el95 tubes instead of 6aq5 without the 6x4 rectifier

and as OT the Hammond 125c ( https://www.tube-town.net/info/hammond/hammond_ot_pp_universal.pdf )

connected for 22.5k or 17.6k (8ohm speaker)

as the el95 tubes plates are rated for 250v max

http://www.nj7p.info/Common/Tube/SQL/Tube_query.php?Type=6DL5

I think I must use a 175v-180v AC transformer

a transformer with such voltage isn't so easy to be find however I've seen this one
( http://www.musikding.de/Passive-parts/Transformers/Toroidal-transfomer/Toroidal-transformer-230V-185V-6-3V::943.html )
185v AC 0.04A - 6.3v AC 1.4A

but I'm not sure if the current disposable for B+ is enough and I'm afraid the PT can warm up

Opinions ?

Thanks

K

[zendragon63]

Hi kagliostro, while you may have other objectives in mind, you can certainly sub in a pair of the EL95s without much change to your current circuit. Data sheets give a Ra-a as 10K for both 6AQ5 and EL95 in that voltage range though you probably want to reduce the shared cathode resistor to 200R or so. EL95's also need less to drive--somewhere around 9 volts vs about 15 volts so you may have to adjust the gain structure.

That said, you won't reduce volume much and they are a little less compressed IMHO; just a little different flavor but a good tube for sure. If you want to scale down volume on a PP, you might even consider the EL91. Regards,

dennis

[kagliostro]

There are two reasons about a possible use of the el95 tubes instead 6aq5 and the omission of the 6x4 tube

the first is I've an old HP Psophometer instrument I would like to use as cabinet




there the space is reduct, so I was wondering to avoid an excessive heat on it and to use the smaller component I can

My intention will be to try to don't use a fan, use the window of the instrument as a ventilation window and to add some ventilation holes on the bottom of the front panel (the front panel however will be totally or partially rebuild)

so the use of the el95 tubes that are underrated if compared with 6aq5 and can use smaller PT & OT

more, I have a bounch of 6aq5 & 6005W but I've also 4 x el95 tubes in my stock

If the original project can be fit in that cab (at the moment I haven't under hand the exact measure, but I can post it) without heat problems I would like to go with the original plan, only maybe I'll omit the 6x4 rectifir (or not dependings on the PT I can find)

May be this is one ( http://www.tube-town.net/ttstore/product_info.php/info/p5391_Toroidal-47VA.html )

250v AC 0.1A - 6.3v AC 1.5A - 12.6v AC 1A

that I can use without the tube rectifier

---

About the cathode resistor I've a pair of schematic that uses 180R for the el95

K

[DummyLoad]

respectfully, PT transformer isn't a good choice.    

data sheets say 180R for Rk under EL95's use a 3W resistor - strap speaker load for 10K Z-pri. output is about 7W.

http://tubedata.milbert.com/sheets/030/e/EL95.pdf     <---pg. 2 class AB1; 250V Va; 250V Vg2

use hammond 369EX PT

http://www.tube-town.net/ttstore/product_info.php/info/p440_Hammond-369EX.html

ciao! :-)

--pete

[kagliostro]

Ciao Pete

nice transformer but BIG for that small case (however may be it fits, I must give an accurate look to measures)

do you mean to use it with the 6x4 rectifier ?

---

What about this one ( http://www.tube-town.net/ttstore/product_info.php/info/p2192_Toroidal-30VA.html )

200v AC 0.1A - 6.3v AC 1.5A

(I can use a mixed bridge - 2 x 1n4007 + 6x4)

---

About el95 OT for PP the impedance with the Hammond 125c that I can consider are 22.5k & 17.6k (8ohm speaker)

are this correct ? or I must connect the OT for near 10k ( 11.6k or 8.2k with 8ohm speaker) ?

https://www.tube-town.net/info/hammond/hammond_ot_pp_universal.pdf

Is the 125c underrated for this project is better a 125d or 125h (the datasheet is the same) ?

Grazie

Franco

[DummyLoad]

k, the only PT that website that gets  close to 250V B+ on simulations is the hammond. the 200V .1A get B+ over 320V on simulation. the 10K load is specified as optimum load in the datasheets. you could go higher with a higher B+ though. the 125C part is sufficient but the 125D part would give a better low end response, but then again you may not want something.

--pete

[kagliostro]

Hi Pete

thanks for your support

I think I've a problem understanding the OT impedance required in SE mode and in PP mode of the same tube

the el95 tube datasheet say

http://www.nj7p.info/Common/Tube/SQL/Tube_query.php?Type=6DL5

Class A amplifier 200v 23mA 2.3W 8K

Class A amplifier 250v 24mA 3W 10K

As a PP architecture isn't specified I assume this as an SE architecture

so, if I go PP, I think I must double the Z out

8 + 8 = 16k for 200v B+

10 + 10 = 20k for 250v B+

also, with my math, the use of a 200v AC PT gives (200v * 1.4) = 280v as B+ not more

Where I'm wrong ?

---

One other thing

assuming to build the stock circuit with 6aq5 tubes, if I simply go with a lower B+ can I tame the output power to a lower level as to use an under rated (and smaller) OT ? (a 15W OT is much more large than an 8W OT)

doing this isn't the best for my purpose as the filament consumption of the 6aq5 is 0.45A and the filament consumption of the el95 is 0.2A, so I've 0.25A x 2 more that will give more heat, but may be it is a chance

Thanks

Franco

[zendragon63]

Kagliostro, there are probably more data sheets that you can shake a stick at but this is the one I have using. Class AB shows Ra-a as 10K and the associated load charts graphs are helpful as well.

 http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/EL95.pdf

And of you drop the operating voltage and hence your power, you should be able to run a smaller rated OT all other things being equal. But they are not because the optimal impedance for the tube is a function of voltage. I could be wrong (and often am...) Good luck with what ever you decide on! Regards

dennis

[kagliostro]

Thanks Zendragon63

I think my eyes were lined with ham

Now I've seen the part were there is clearly wrote Class AB (two tubes)  

I apologize with all for my stupid carelessness

today I opened the instrument as to verify better the disposable space

and I ended that it will be very nice to use a low profile toroidal transformer (those for U1 units) for the PS, if I can find one adequate in voltage

if I use the 0-200v toroidal and a mixed rectifier (2 x 1N4007 diodes + 1 x 6x4 tube) I think the B+ voltage will be in order of 250-260v B+ (don't understand why the simulation kindly done by Pete says 320v ??? may be is due to the disposable current that in this transformer is 100mA against a request of I think 70mA of the circuit with el95 tubes)

more I would like to avoid further heat and so I would prefer not to use the tube rectifier, so a lower AC transformer must be found, I think a 170v will be fine (but difficult to flush out)

K

[DummyLoad]

simulation kindly done by Pete says 320v

for SSR only...sim mixed mode.


--pete 

[kagliostro]

A mixed rectifier (2 diodes + tube) didn't has a drop in voltage as using only a tube rectifier ?

K

[DummyLoad]

A mixed rectifier (2 diodes + tube) didn't has a drop in voltage as using only a tube rectifier ?

K

yes, i believe that is the case. you should see around 240V to plates?

regards,

--pete

[jojokeo]

Kag, in my experience with low powered tubes/amps you have a lot of leeway with OT impedance. Don't get hung up on this. You can't really hear any differences between 10k, 12k, 6k and you can't hear any difference in overall loudness, bass response or anything else. It's LOW POWER so it doesn't push the speakers barely enough to make a whole lot differences either way, it's very subtle and subjective even if you can. On some small output tubes you can get away with a bit of extra or low voltage for much the same reason. But, if on the higher side beware of your screen voltage/current. This can light up your tube to a nice red glow with everything else being spot-on. Don't ask how I know!    Go for it! BTW, I really like those tubes and you can also try 6AR5s if you can find them besides the el91s zdragon suggested. If you go with DL's PT suggestion of a higher amperage tranny you can even sub 5AQ5s too! All three are interchangeable if you bias it right and have the PT capacity.