EL95 push pull transformer questions.

[markusw]

Hey guys.
A friend of mine found a box of NOS Mullard EL95 and Philips 12ax7 tubes from his father.
Therefore, we are currently planning an amp around these tubes.
It will have two EL95 push pull and therfore roughly 10W power.
Regarding the output transformer we have several options (all Hammond).

A) 1609, 10W, 10k impedance, frequ response 30Hz-30kHz +/-1 dB
B) the 125 series, with frequ response 150Hz-15kHz +/-1 dB; 8W (125C) or 10W (125D) or 15W (125D).

The 125 series is clearly cheaper than the 1609. However, we have concerns regarding the frequency response.

We would like to have 3 jacks for 4, 8 and 16ohm or one jack and an impedance switch. Shouldn't be a problem with the 125 series.
Not sure how it can be done with the 1609.

The 1609 with 10k would be perfect for two EL95.
For the 125 series primary impedances for 4 ohm would be 11.2k or 8.8k, for 8 ohm 11.6k or 8.2 and finally for 15 ohm 10.2k.

MY questions:

* which transformer would you suggest?
* for the 125 series would you wire it for the higher or lower primary impedance?

Thanks a lot for your help in advance!

Peace,

Markus

[Phil_S]

Hmmmm....choices.

I'm not that technical, but your primary load will vary based on plate voltage. You need to draw the load lines...I can't tell you exacty how to do that.

Get the tube data sheet. http://tubedata.itchurch.org/sheets/030/e/EL95.pdf as a place to start.

I don't know if you have enough tubes, but you might consider 4 power tubes in the amp instead of two. These don't require that much voltage or current. You could make a nice 18-20 watt amp. Find a used output transformer from a pair of EL34's or the like...4K to 5K. I'm thinking you can find a 30-40W OT. On the assumption that you can't have too much iron in your amp, it is likely to sound fabulous!

[Structo]

So am I correct in saying that in a push/pull amp these would only output 6-7 watts?

[markusw]

Thanks a lot for your help!

I thought that 10k primary should be fine. At least 10k is indicated in the data sheet.
We are planning to run the EL95 at 300V plate voltage. Still, you're right, it probably won't be 10W, maybe just 8-9W.

We have enought El95 tubes. Nevertheless, we want to stay with two of them only. My friend wants a small and light weight amp.
Any suggestions regarding output transformer?
Peace,

Markus

[flood]

wow, am i glad i came across this thread. i'm building an EL95 blues express - https://tubeamparchive.com/viewtopic.php?t=9423 - sometime, and was planning to use two 12V back to back for getting the HT, and a small EL84 8W transformer for the OT. probably not a good idea anymore - i was under the assumption that EL95s in push pull would give me max. 5W.

[Phil_S]

Thanks a lot for your help!
I thought that 10k primary should be fine. At least 10k is indicated in the data sheet.

I don't know the answer, but, as you increase plate voltage, the primary Z will generally decrease. The data sheet shows 200v and 250v.

Educate yourself instead of shooting in the dim light. Google is your friend and mine.
Here's an example: http://www.diyaudio.com/forums/tubes-va ... -tube.html
Here you'll find a multi part article: http://greygum.net/sbench/sbench101/

It will take a bit of work, but then you will know for sure and it will become a tool in your toolbox.

[markusw]

Hey Phil,

thanks a lot for the links!
You're totally right.
I will follow the links and try to understand loadlines.

Regards,

Markus

[CaseyJones]

A) 1609, 10W, 10k impedance, frequ response 30Hz-30kHz +/-1 dB
B) the 125 series, with frequ response 150Hz-15kHz +/-1 dB; 8W (125C) or 10W (125D) or 15W (125D).

The 125 series is clearly cheaper than the 1609. However, we have concerns regarding the frequency response.

The less hung up you are about frequency response the sooner you'll be on The Path to Enlightenment.

"Flat" frequency response is a hi-fi ideal and may or may not be suited to guitar amps. I suspect the 125 doesn't roll off too badly down to 80hz which is all you need for guitar. "Flat"? WHO CARES?!! A hump or a dip in the right place can be a good thing...

[markusw]

Hey Casey,

thanks for your advice.
Think I will give the 125 a try.

Regarding primary impedance, all EL95 output transformers I found in the net have 10k.
I also found a data sheet for a EL84 that contained a graph displaying power output, distortion, plate and grid current over plate load.
Within a reasonable range (1k up or down) the parameters didn't change that much.
So I probably will postpone drawing loadlines a bit and just go for 10k.

Again thanks a lot for your help to you all!

Markus

[harryk]

I do not know how tight you have with money, but to my mind a better OT is one key to a good sounding tube amp. If the price difference is not too big, I would use 1609. I have used 125 in some of my projects and if I remember right, I got rid of that pretty soon. Despite guitar does not need any hifi frequency, it is nice to have an amp where you can reduce too much bass etc. if needed. If your amp is lacking bottom, nothing will bring it back if you have a a poor OT. Harry

[markusw]

Thanks for your advice Harry!

Initially we planned to use the 1609. My friend wants the "best" components in his amp (within a reasonable range). I also have a tendency for overkill
Since we most likely will have to do some tweaking of the amp anyway why not get both, the 1609 and a 125 and compare them?

Peace,

Markus

[j-po]

Be warned: 300V will be very tough on the screens if you intend to drive the tubes into saturation. I suggest you drop the PA voltage to 200V...250V with a string of 5W zeners (max 20V each) That'll give you 4W+ and is still earache loud when distorted. 125B/125C will offer more impedance options and is built more in the tradition of guitar OT's, looks a bit cheap of course...
I have built a P-P EL91 amp with a interleaved 10W hifi OT, switched over to 125 with no ill effect on sound, even similar bass. Based on my experienceh I do not recommed the 1609 as it is just more expensive and better looking. Then again I'm not the kind of guy who likes to sniff various cap types etc...

Also with PA distortion you may want to limit the PI output to a moderate level. EL95 biases with pretty low voltages and thus also clips early. Options: low supply voltage, large (50k+) tail resistor, split load plate resistors.

[harryk]

Be warned: 300V will be very tough on the screens if you intend to drive the tubes into saturation. I suggest you drop the PA voltage to 200V...250V with a string of 5W zeners (max 20V each) That'll give you 4W+ and is still earache loud when distorted. 125B/125C will offer more impedance options and is built more in the tradition of guitar OT's, looks a bit cheap of course...
I have built a P-P EL91 amp with a interleaved 10W hifi OT, switched over to 125 with no ill effect on sound, even similar bass. Based on my experienceh I do not recommed the 1609 as it is just more expensive and better looking. Then again I'm not the kind of guy who likes to sniff various cap types etc...

Also with PA distortion you may want to limit the PI output to a moderate level. EL95 biases with pretty low voltages and thus also clips early. Options: low supply voltage, large (50k+) tail resistor, split load plate resistors.

Hi J-Po Are you from federal republic of sovjet Finland too? If you are, I am also from Finland and in represent myself in many forums with alias "harryk". It is nice to have new forum friends especially from home country. Harry

[markusw]

Hey j-po,

of course we are planning to drive the EL95 into distortion
Thanks for your advice also regarding the OPT!
We are planning to add a VVR to control EL95 and PI voltage. Would it help to reduce only screen voltage to 250V and to add screen resistors? Why are the 300V especially tough on the screen when the tubes are overdriven? Sorry for the noob questions

I was already checking the options how to limit output from the PI. Will have to check out split load plate resistors.....

Peace,

Markus

[j-po]

Hi Harry, yes from Finland... Respec for your nice site and many builds!
About the bass of my 125/hifi OT comparison: it was a 'wreck so not very bassy anyway. Maybe one would notice a difference in something more heavy. I've been into this stuff for 2.5 years now. Up till now I've built a 'wreck, Bogner, SLO (cannibalized for parts to build a) Engl. Just need to finish my ODS and I'm set for life! (or so I try to tell myself )

Back to your problem Markus,
The higher the voltage the more electrons=current the tube is able to pass. Higher voltage also accelerates the electrons more before some of them hit the screen, thus the screen may overheat. At saturation the screen attract some serious current which is what produces the rounder distortion characteristic of pentodes by the way.

My EL91's are working at 220V plates and 160V screens and the screens are still dissipating over spec. as do most guitar amps, I guess. I did play them at 245V (they are rated for 250V) for a while though without immediate failure, just a nice orange glow. EL95 screens will handle twice the power of EL91 but I still think you should not go all the way to 300V. Anyway, with VVR just put a resistor in series with the scaling pot to limit the max voltage. Too easy.

However, I do not recommend dropping just the screens when using also VVR as that would mess up the impedance requirement of the output transformer big time. At 300V plates and 250V screens (i.e. 83% of the plates) you would need say 12K but when dropped 200V to 100V plates 50V screens (50% of plates) you would more likely need a higher value, 20K+. Drop them both in proportion and you're better off.