Modern HV EL34s?

[pdf64]

Here's some real pretty ones in blue.
https://positive-feedback.com/audio-...-vacuum-tubes/
nosaj

All that comical marketing hyperbole yet not mention of 'immediacy'?

[Justin Thomas]

All that comical marketing hyperbole yet not mention of 'immediacy'?

Aw, crap. In David Byrne "No Compassion" voice, "HERE WE GO AGAIN!"

Justin

[Leo_Gnardo]

All that comical marketing hyperbole yet not mention of 'immediacy'?

I noticed but was going to not say anything. You want immediacy? Pay extra for priority shipping.

Two blue glass bulbs please, with absolutely nothing inside, delivered extra pronto! That should run about $600. Hope nobody drops 'em on the way.

[tubeswell]

Is ST Shug Treasure? I've never seen a 6CA7 or EL34 in that bottle style before.

These ones

https://www.thetubestore.com/shuguang-el34-b

A.K.A. ‘Ruby EL34B STR’ (re-branded STs) http://www.dougstubes.com/ruby-el34-bstr.html

[John G]

tubeswell,
I wonder if you could please expand on your belief that O/P tubes should be very closely matched, and what led you to that belief.
i.e was your comment specific to that particular Marshall model or a general comment for all P-P amps.
Thanks
John

[tubeswell]

tubeswell,
I wonder if you could please expand on your belief that O/P tubes should be very closely matched, and what led you to that belief.
i.e was your comment specific to that particular Marshall model or a general comment for all P-P amps.
Thanks
John

My comment was specific to those Marshall 1959 super leads with the quad of EL34s running plates at 510 and screens at 500ish. When you pump a power chord through, the tube that runs more than 5mA hotter than the rest will start to light up.

[Gregg]

Let's not forget that EL34s'screens are spec'd to 450V so it's kind of "expected failure". The fact that NOS tubes can handle the abuse longer doesn't change the specs so it's a good idea to install a zener to the screens if in order to limit the voltage to 440-450V max. You're not gaining much power if your screens go from 450 to 500V anyway.

[Leo_Gnardo]

it's a good idea to install a zener to the screens if in order to limit the voltage to 440-450V max.

I've heard of this being done but curious how exactly to go about it. I would appreciate some guidance, maybe a quick schematic. I'm sure others would as well. Thanks in advance!

FWIW I've also heard of LED's being used in this application. Also not sure 'zackly how.

[Leo_Gnardo]

oops mis post, now deleted.

[cerrem]

Keep in mind that it is the SCREEN DISSIPATION that is really at the heart of the issue...
The screen voltage is only high at idle ...which isn't really drawing more than 3 to 6 mA ....during dynamic operation ..you get a big dip in the screen voltage..
So you have a 1K limiting resistor in series with screen this drops the B+ to around 400V at full "clean" power output in these 100W Marshalls.. when the amp is cranked into full square wave the screen voltages are around 375V..
You would take the AVERAGE power dissipation per cycle ....which is not constant with a sine wave..however with square wave guitar amp the screen dissipation is constant over the cycle..
Also this is a Class AB amp... So the Duty cycle will roughly be half, thus cutting the screen dissipation in half ....this allows tubes to be used at higher instantaneous peak screen currents..

[John G]

Tubeswell,
Thank for your insight.
One more question please, based on your experiences what idle current (and hence dissipation) do you recommend.
Thanks
John.
PS are your customers using speaker attenuators or cranking the Amps full bore ??

[Leo_Gnardo]

Keep in mind that it is the SCREEN DISSIPATION that is really at the heart of the issue...
The screen voltage is only high at idle ...which isn't really drawing more than 3 to 6 mA ....during dynamic operation ..you get a big dip in the screen voltage..
So you have a 1K limiting resistor in series with screen this drops the B+ to around 400V at full "clean" power output in these 100W Marshalls.. when the amp is cranked into full square wave the screen voltages are around 375V..
You would take the AVERAGE power dissipation per cycle ....which is not constant with a sine wave..however with square wave guitar amp the screen dissipation is constant over the cycle..
Also this is a Class AB amp... So the Duty cycle will roughly be half, thus cutting the screen dissipation in half ....this allows tubes to be used at higher instantaneous peak screen currents..

That's all the case ^^^, sounds very soothing, and would lead one to believe there's no problem to anticipate here.

BUT we do have this problem in real life operation of the big amps. 100W Marshalls, the ones that tend to be a bit lower in B+, not so much a problem. But some run well over 500V B+ with screen grid voltage right behind. And there are those pesky old Orange OR120 and OR80 amps that are notorious for eating modern EL34's of every brand. Similar old Hiwatts & Laneys & other brands. What's a mother to do?

OTOH I've seen some MusicMan amps, that run screen grids at half their 700V B+, survive just fine even with those awful old Chinese EL34's from the early 90's, the ones that had steel splints welded to their plates to keep them from bending in the heat. Go figure...

[drewl]

I see alot of Music Man amps with the original tubes still crankin' after many years due to low screen V.

[Chuck H]

One of my observations designing with EL34's is that the HF spikes that occur during hard clipping can find an easier current path through the screens than the plates. I think that managing impedance @ frequency is an important and often overlooked element. The OT primary plate impedance at HF can be quite high due to the typical guitar speaker curves reflected at the primary. This can make for a lower impedance path through the screen circuit. I typically see this when I'm pretty much crushing the grids with signal. My solution has been to mitigate grid signal and increase screen circuit resistance as much as practical until those two approaches stop the problem. I think that some Marshall amps get away with high-ish screen voltage because they often use comparably big strappin chokes with more Henry's (or would it be Henries?) that have a higher HF impedance than smaller chokes or resistors.

My observations as an amateur designer. Submitted for your consideration

[g1]

I see alot of Music Man amps with the original tubes still crankin' after many years due to low screen V.

Also very low plate idle dissipation which also contributes to the longevity.