EL34 cathode bias?

[firemedic]

Me again, still planning....
I'd like to try EL34s in this Pro Reverb build- preferably an easy switch between 6V6, 6L6 & EL34s. Is it just a matter of adjusting the bias? Or is there a current draw issue other than heater current? I'll get a 6.3v dedicated filament trans. if that's the only issue. And I don't use my unused socket pins as terminals. 
Also, I'd like to try EL34s in the cathode-biased TOS, has anyone done this?

[FYL]

EL34s are high slope, low vacuum pentodes. Very different beasts when compared with 6L6s, with quite fragile screens and vastly different drive/bias requirements.

I'd focus on 6x6s and always have in mind "Jack of all trades, master of none".

[bobmegantz]

I agree with FYL, but I'd like to hear some other opinions.

I've substituted 6L6s for EL34s in a PP 50W amp (just adjusting the bias), and they sounded terrible (flabby, boomy, distortion, etc).

On the other hand, the quiescent current, OT primary impedance, and PI design requirements are similar.  So that leaves gain of the tube and feedback as possible differences.

Perhaps someone who has thought this through can enlighten us...

Bob

[Ritchie200]

My VERY humble opinion.  PRR always seems to come back to this same statement when people are looking to design the next big thing in amps.  "Look at a proven design and go from there."  I think there is a lot of truth to that statement.  There is a reason Marshall settled on their EL34 design and Fender on their 6L6 design - and that was to compliment the tubes being used to get the sound they(we) wanted.  A couple of years ago I played a THD amp (uni-valve?), a SE amp that you could plug a whole ration of 8 pin tubes into.  Guess what...I thought it sounded like crap with every tube I stuck in except the KT77 (probably because it was an OLD Mullard).  I don't think there is a swiss army knife design that can accommodate (within a reasonable one amp budget), the tubes you are talking about.  But hey, maybe you can design it!  That's why we are here!
Good luck,
Jim

[phsyconoodler]

I recently built an Overdrive Rocket amp(what I call my D style amps) with a pair of 6CA7's and cathode biased.According to the owner it's the best amp he's ever owned.I really love the sound of this amp in clean and dirty modes.

[Tone Junkie]

Phsyconoodler did you use the standard Rocket and just sub in the 6ca7 with cathode bias or did you have to change anything to make it work. Sounds very interesting i like some of the new 6ca7 they have come out with which one did you use or did you go old school with the tube.
Thanks Bill

[jojokeo]

A couple of years ago I played a THD amp (uni-valve?), a SE amp that you could plug a whole ration of 8 pin tubes into.  Guess what...I thought it sounded like crap with every tube I stuck in except the KT77 (probably because it was an OLD Mullard).  I don't think there is a swiss army knife design that can accommodate (within a reasonable one amp budget), the tubes you are talking about.  But hey, maybe you can design it!  That's why we are here!
Good luck,
Jim

Hi Jim! Was the Swiss-army knife invented to cut Swiss cheese with? 
I'm partial to my amp ofcourse but I must say the KT77 sounds really good but breaks up earlier than a few various el34s, e34l, 6l6, etc. but they all sound good - just a bit different characteristics btwn them. Below was my design & build process on this one, lol.

 :book1: > :idea1: > :icon_study: > :sign13: > :sunny: > >

[Ritchie200]

What kind of knobs are those?!?!?
Jim

[PRR]

> There is a reason Marshall settled on their EL34 design and Fender on their 6L6 design

Cheap.

In the USA, 6L6 were the cheapest big tube. When Marshall started building, EL34s were selling cheaper than 6L6.

[bluestone]

A few years ago I built a Kendrick New-JoyZee which would handle 6x6's and EL34's. I guess it was the same as his Climax amp. A good sound, not great, but not a huge difference between tubes and tweeking the"Texas Tea" control did not recreate the Tweed and Blackface sounds IMHO. I never say never but....I do agree that it would be very difficult to recreate all the sounds from one amp...Hey wait a minute...How about the Cyber-Twin

[phsyconoodler]

I just used appropriate screen resistors and a Marshall-style PI.It works and sounds awesome.

[FYL]

Hey wait a minute...How about the Cyber-Twin

Hey, 250 different crappy sounds and a Vibratone, as they say in the promo blurb: "Fender's best just got better".
""It's every guitar sound you've ever wanted or will ever need."

 

[phsyconoodler]

If you are partially deaf the cyber twin sounds great.If you have any hearing left at all,they sound like total crap.

Solid state Fender at it's WORST.

[firemedic]

Well if it's that much of an art to simply change out power tubes I'll stick with the 6L6s. The best amp for me to try the EL34s in would be my TOS but it's cathode biased & I don't know how to CB an EL34. I'm my own worst enemy when it comes to fixing what ain't broke.
IOW, I'll let someone do it who really knows what they're doing.
Thanks for all the feedback gents!

[bluestone]

I actually bought one when they first came out....The great thing about it was, it sounded so bad that it is what got me into building amps!!! I have never looked back!!!!!

[sluckey]

The best amp for me to try the EL34s in would be my TOS but it's cathode biased & I don't know how to CB an EL34.

Cathode bias is also known as self bias and is very forgiving. Just tie pin1 to pin 8 for both tubes and connect the 2 tube cathodes together. Put a 130Ω to 150Ω high wattage resistor between the cathodes and ground and bypass the resistor with a big cap.

Or use separate cathode resistor/cap like Matchless...

http://www.el34world.com/charts/Schematics/files/matchless/matchless_clubman.pdf

[Ritchie200]

> There is a reason Marshall settled on their EL34 design and Fender on their 6L6 design

Cheap.

In the USA, 6L6 were the cheapest big tube. When Marshall started building, EL34s were selling cheaper than 6L6.

Sorry, what I meant to say was the circuits were built to compliment the tubes they had to work with - to achieve that sounds they(we) wanted.  Yes we tend to forget that the amp business, like any business, is cost driven - even back in the good 'ol days.  They were spending more time looking at the bottom line, than sitting in some dark back room filled with solder smoke and components - cooking up the latest in amp tone nirvana.  Accidental brilliance, for Marshall especially.  He built to fill a need due to the high cost of Fender imports, tweaked it based on player feedback, and wah-lah!  Almost 50yrs later we are still trying to replicate the mojo of surplus parts and variations on a schematic based on what was laying around the shop at the time.  

Jim

[sluckey]

Almost 50yrs later we are still trying to replicate the mojo of surplus parts and variations on a schematic based on what was laying around the shop at the time.

Ain't that the truth! 

[firemedic]

For some reason I thought EL34 pin 1 had to connect to ground. But, either way, I can't do it (EL34s) due to only 3A heater current on my TOS PT. I could get a 6.3V tranny but that's how I get started fixing what ain't broke......
Plus my fingers are losing their calluses. Time to play a little guitar.

[phsyconoodler]

quote: "For some reason I thought EL34 pin 1 had to connect to ground."

It does.Through the cathode resistor.

[PRR]

> I thought EL34 pin 1 had to connect to ground

It has to go somewhere lower than Plate's lowest swing, at least much lower than +100V.

It can connect to cathode, that works fine, and is how the data is given.

It can connect somewhat lower than cathode. To ground on a cathode-resistor amp, or to -bias on a fix-bias amp. This reduces gain a little.

In a short accident with modern EL34 in 7027/6550 holes, nothing bad happened when I left pin 1 disconnected. In heavy abuse with pin 1 connected to cathode via 470K, I could not measure any G3 (pin 1) current.

At 6K6 p-p load, 400V supply, 250 ohm common cathode resistor, 2K common G2 resistor, I could run metal 6L6, 6L6GC, 7027, 6550, or EL34, in any combination, around 20 Watts clean output, with negligible difference in objective performance. Subjective performance with EL34 was very fine and I left it at that.