6EM7 into EL34 ?

PeteC · Post by **PeteC** » Sat May 18, 2013 5:18 pm

Hi all,

my main amp for last 2 years has been the 6EM7 into 300B.
http://www.audiodesignguide.com/my/new3 ... 0bxls.html

Now pretty much the last pair of 300B's has packed in this week ( well one of the pair and I'm left with one lone EH300B ) - I've had bad experience of both EH 300B's and JJ's failing early now, and frankly I'd like to try something else.

I have loads of El34's and 6L6's knocking around from guitar amp builds, as well as some Rusky 6C4C's which I quite like. I just agreed to sell a pair of PX25s to Paul , the day before the bloody EH300B died on me ! I will still honour that sale though.

Would it be possible to sub the 300b outputs for triode strapped EL34's for example ??

I want a quick fix to get the amp back up and running for the missus's film watching, and also fancy moving away from the 300Bs to be honest.

Any thoughts ?
Cheers
Pete

Cressy Snr · Post by **Cressy Snr** » Sat May 18, 2013 5:44 pm

If you have 5K output transformers, which I assume you would have if using 300Bs then the EL34 should work well, driven by the 6EM7.

I built a single ended EL34 amp for a mate 5 years ago. He and his wife love it, play it every day, listen to the telly through it and have just replaced a worn out pair of Svets with JJs.
Not a bad lifespan for a pair of output valves.

I think you'd love it if you went with EL34s.
400V HT with 820R - 1K 10W cathode resistors would be a good place to start, which would give around 6WPC in SE.

pre65 · Post by **pre65** » Sat May 18, 2013 6:02 pm

Should be OK Pete.

I use 6DN7 to drive KT66 as a triode, and that sounds good on it's own, and also as a driver for the mighty 833a.

PeteC · Post by **PeteC** » Sat May 18, 2013 6:14 pm

Cheers Steve

that's what I hoped someone would say...

I will give it a go - I have a really nice pair of Svet's sitting on the shelf in front of me waiting to be used.

triode best I presume ?

Rgds
Pete

Cressy Snr · Post by **Cressy Snr** » Sat May 18, 2013 6:32 pm

PeteC wrote:Cheers Steve

triode best I presume ?

Rgds
Pete

In this case, probably yes

Paul Barker · Post by **Paul Barker** » Sat May 18, 2013 7:45 pm

In the interim I could send you a pair of used 300b's which should keep you up and running. Free of course.

Regards your plans.
For single ended you might want the extra power of say kt88/6550.

Paul Barker · Post by **Paul Barker** » Sat May 18, 2013 7:55 pm

In the interim I could send you a pair of used 300b's which should keep you up and running.

Regards your plans.
For single ended you might want the extra power of say kt88.

PeteC · Post by **PeteC** » Sat May 18, 2013 7:57 pm

Hi Paul,

thats a really kind offer.
Will send you a pm later

Cheers
Pete

Paul Barker · Post by **Paul Barker** » Sat May 18, 2013 8:18 pm

Oh dear I found out the playbook browser suffers the dreaded double post bug.

PeteC · Post by **PeteC** » Sun May 19, 2013 10:19 am

Hi chaps,

got up ( very ) early today and rewired the 6EM7/300B into a 6EM7/EL34 trioded amp.
pics later ....

found a really neat trick for moving the 2A 5V DC heater supplies to 6.3V supplies for the EL34's - lifting the ground leg of the 5V regulators and inserting red LEDs to raise the reference voltage - seems to have worked a treat. Found this on Merlin Blencowe's excellent Valve Wizard site.
http://www.valvewizard.co.uk/heater.html
Not yet fired it up - will do tonight with some "sacrificial " old EL34's first before using the new Svets

Thanks for the tips.

Rgds
Pete

Post by **Nick** » Sun May 19, 2013 2:55 pm

found a really neat trick for moving the 2A 5V DC heater supplies to 6.3V supplies for the EL34's - lifting the ground leg of the 5V regulators and inserting red LEDs to raise the reference voltage

Andy Grove does that all the time, but normally with a couple of Si diodes to give the 1.3v Vf

PeteC · Post by **PeteC** » Sun May 19, 2013 7:16 pm

So,
bunged a couple of old brimars in to test things out before trying the new Svets.

have 450v across the el34s. plate to cathode.
-39v across the 880 Ohm cathode resistors - so 44.3ma each side.
44.3ma x 450v = c. 20watts dissipation - so about 80% of recommended - quite hot but not crazy ?

Haven't changed anything in the PSU yet - maybe might try choke input to get the plate voltage down - but so far the valves seem happy.

Sound is , i think , more impactful and deeper than the old 6Em7/300B combination.

I'm very used to the sound of el34 in guitar gear so maybe I am hearing something I am used to ?

Anyway - like the sound of this amp quite a lot.

Rgds
Pete

Cressy Snr · Post by **Cressy Snr** » Sun May 19, 2013 7:37 pm

PeteC wrote:So,
bunged a couple of old brimars in to test things out before trying the new Svets.

have 450v across the el34s. plate to cathode.
-39v across the 880 Ohm cathode resistors - so 44.3ma each side.
44.3ma x 450v = c. 20watts dissipation - so about 80% of recommended - quite hot but not crazy ?

Haven't changed anything in the PSU yet - maybe might try choke input to get the plate voltage down - but so far the valves seem happy.

Sound is , i think , more impactful and deeper than the old 6Em7/300B combination.

I'm very used to the sound of el34 in guitar gear so maybe I am hearing something I am used to ?

Anyway - like the sound of this amp quite a lot.

Rgds
Pete

Hi Pete,
That dissipation figure is about the right compromise between power and valve life, so those EL34s ought to last a decent period of time.

Post by **Nick** » Sun May 19, 2013 8:32 pm

I'm very used to the sound of el34 in guitar gear so maybe I am hearing something I am used to ?

I may do that as well, but to me I like the sound of a el34, far more than the 6550 or kt88. But I dont think its guliar bias, as I prefer a Fender Twin R over a Marshal.

Paul Barker · Post by **Paul Barker** » Sun May 19, 2013 8:40 pm

I also prefer el34 to kt88 variants. However gu50 is so cheap I ordered 4.

audio-talk

6EM7 into EL34 ?

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