Why does killing my EL38 tube sound so good?

[isaacc7]

eh??
If you look on EBAY (UK) you will find 4 off for about 1-2 UKP each. 🙄
I never imagined it would be desirable, but hey 50p was a few years ago when people used to shoot them with airguns for fun.

The question of bias is pretty simple.
The higher the gain of the valve the less bias you need.
Think of it as a switch, where you have to start from a reasonable "slightly on" base within about 70-80% of its maximum.

A typical 807 would need a bias of about -30>-32V at 600V, because it's an ancient design of valve with low gain. That would make it settle at say 35ma or around 20W idle.

By comparison a 8417 (one of the highest gain devices made) would only need about -15>-17V at 650V. (about half). The idle current would be roughly the same.

The 8417 has roughly 4x the gain of the 6L6/807, so it needs less drive to turn it on and off.

Being as the EL38 has higher gain than the EL34 it's likely the using the same bias voltage would turn it OFF, rather than making it melt.

I just checked eBay UK and the ones being sold are all 20-40 pounds and tend to be sold as singles or in pairs. I would love it if I could find 10 or so for 2! Also thank you for the info.

 

[HumbleDeer]

I can't quite recall the name by m7self right now but I recall the factory where Prima Luna buys their 'binned' tubes makes new versions of these tubes with new methods - like that guy that revived the nixie. Would be useful to know if that factory also has something for your tastebuds.. or ear hairs?

 

[sarcastic1]

the factory where Prima Luna buys their 'binned' tubes makes new versions of these tubes with new methods

oxymoron, =

making tubes with new methods.

Something more on the lines with "we've forgot how to make stuff, or all the original metallurgists have retired or are dead", so we will invent some BS about "new methods".

Applies to PRC and Russia, which is why Svetlana don't make, or don't want to make consumer grade valves any more.

There's no competition possible either against sub standard PRC made or Russian Reflector Saratov modern production with trademarks that are either fake or fake rebrands.

They will just churn out that stuff ad infinitum as it costs mere kopeks to make and sell a repro or throw it in the reject pile.

Either way you are way better off with NOS RCA, Sylvania, Mullard, GEC, Brimar, Mazda-Thompson, Telefunken, Toshiba etc, of which there are still zillions in old stocks.

Philips-ECG & the later RCAs marked the first downturns in quality, while Mullard, STC, GEC etc kept going a bit longer into the 70s.

 

[silvaup4]

Isaacc7, your discovery is absolutely correct.
EL38 is an awesome tube for audio.
It has 14 mV/A transconductance, while EL34 and KT88 both have 11.5mV/A. The higher this number, the more efficient the frequency response would pass through its voltage gain stage.
11.5 is considered very good for audio amplification purposes, but 14… you know what you are hearing!

I too found EL38 bass is deeper than EL34, not at the amount of KT88, bot amount is good enough.
Treble is as bright as EL34, mid is much sweeter than EL34, more like DHT. Treble
Is of course much clear than KT88. It creates a stage effect that EL338 sound very deep with 3D dimension better than EL34 and KT88 single ended.
Because of the sweet mid and bright treble, along with deep bass, overall it sounds less fatigue and more lush than EL34 in my opinion. If sounds less power than KT88 SE of course, but more nuance you find in EL38.

I build a universal class A2 Pentode SE amp that can use EL38/34/6L6GC/KT66/KT88 with adjustable bias to accommodate different tetrode/pentode.
Repurposed LaoChen chassis with total rebuild amp, I have 5755 gain drives 12BH7 cathode follower drives tetrode/pentode, which G2 is regulated by another tetrode/pentode tube that can be EL34/Kt66/807 etc you name it. This voltage regulator tube also provides power for 5755 preamp and 12BH7 to sound clean and tuby. With KT88 output, I get 17W max @ 10Khz before clipping.

Hands down, EL38 at the power output seats is my favorite.

 

[andyjevans]

If you're in the UK, try the CV9 instead of the EL38. Rare but usually a few available here, all Mullard. You may prefer it. I used and enjoyed it for a while before changing to 4 of 4P1L in PSE triode. I use ECC40 as a driver - a rimlock tube that used to be popular in European amps. A mesh plate 27 is an alternative. All very European apart from the 27 mesh.

 

[silvaup4]

thanks Andyjevans. I saw CV9 datasheet it’s so similar to EL38 but a slightly higher gm, lower cathode bias, and lower heater voltage. I am sure it would sound very similar and may be slightly better than EL38 given the characteristics

 

[smoking-amp]

I see one Ebay listing for the CV9 that lists KT77 as near equivalent.
The JJ KT77 curves: Looking similar to the EL38 curves.

 

[smoking-amp]

Oops, was an EL38 tube that came up in the lower CV9 listings ( with the KT77 equiv. listed ) when I searched for the CV9 originally.
I Haven't seen any curves for the CV9 to compare with. The JJ KT77 has near remarkable constant gm throughout its curve set. (equal spacing between curves) EL506, 10JA5 and 12HL7 are among the few Pentode like tubes with this property.

 

[andyjevans]

Here's some CV9 data

 

[smoking-amp]

It seems that EL38, CV9, AL60, and KT77 are all known for excellent triode curves. I can only find triode curves for the KT77, which do look quite decent. (much better than EL34 )
So why is everyone using the EL38 in pentode SE with high output Z?
https://www.audiomatica.com/tubes/kt77.htm

 

[stocktrader200]

Triode mode may be the key. Several transistors, a 12ax7 and a 6922 per channel can output 50w in push pull config. Sounding great while doing it.

 

[smoking-amp]

Transistors acting as current multipliers for the tubes? N Fdbk to the EL38 screen grids to get triode tube curves?

 

[smoking-amp]

Or a transistor front end with EL38 P-P triode outputs?