EL34 pentode output in SSE

[Skorpio]

I am currently running my SSE with EL34 in triode connection. This is a very nice amp with approx. 5W output...

I have no UL connection on the OT and reading about Pentode connection of EL34 shown that this would be a step down in performance?

Therefore I was thinking of the possibility to use some feedback (like in Baby Huey) to lower the output impedance of the Pentode...is that a possible way to go?

What will be the penalty, besiden the need for more drive?

 

[Skorpio]

Here are an interesting alternative..."screen drive":

Screen Driven, DC Coupled, Single-ended EL509 Amp

 

[rknize]

Why not use the local, cathode feedback feature also supported by the SSE? That will clean up the pentode operation for you.

 

[Skorpio]

Yes, that seems to be a good idea, do you know what power to expect in pentode mode?

 

[rknize]

Expect around 10W with the EL34. Up to 14W is possible with a bigger tube, like KT88.

 

[w5jag]

Why not use the local, cathode feedback feature also supported by the SSE? That will clean up the pentode operation for you.

I'm using Hammond 125ESE transformers on my SSE and, imo, cathode feedback by itself does little to tame pentode shrillness on the SSE, although it works great for linearizing (sp?) output transformers.

An additional local plate to grid (Schade) negative feedback loop has worked well for me on the SSE in pentode mode, and is easy to implement, but Schade feedback is not without its critics.

There is more of a discussion on the Schade feedback in the SSE in the 6146 SSE thread, three or four pages into it.

Win W5JAG

 

[rknize]

It's a different sound for sure. Not necessarily bad...just depends on your speakers and your tastes.

 

[Skorpio]

Thank you for your inputs!

I will look into the thread...

Do you have more detailed info about the difference in sound? What are the pros and cons....I am thinking on both Pentode/cathode feedback and pentode/Schade feedback versus a clean triode...?

 

[rknize]

How it sounds to you on your speakers is going to be different than how it sounds on mine. I have switches for triode/UL mode, cathode feedback, and even SS vs tube rectification. I also have a rotary switch that allows me to change the cathode resistor to run different tubes at different operating points. I don't normally run EL34s in my SSE. When I do I run them as triodes because to me that is where they seem to excel. As pentodes (UL in my case) EL34s don't sound any better than bigger tubes and lack the bass punch of something like a KT88. So when I have bigger tubes in there, they are usually in UL mode. There are a few exceptions to this, however.

Getting back to your question, I get that "warmer" tube sound with more subtle detail at lower volumes with tubes (especially EL34s) strapped as triode. In UL mode, I get more power and more bass with bigger tubes. This is great for playing rock or other beat-heavy music while doing other tasks. My speakers have horn tweeters and they get "shouty" at high volume. This is especially true on a PP amp. A triode SE amp tames them for those times when I am listening to music up close.

BTW, you will get a bit more power than I stated earlier. This numbers are for UL mode. Pure pentode mode would yield a bit more.

 

[Skorpio]

Thank you for you experiences!

I am at the time being satisfied with the troide connection with EL34 and I am using an active subwoofer below 50Hz, so the bass part is not handled by the tubes...

This is a Great little amp! Big thanks to George for his work!!

 

[Treybal]

Hello

I'm just finish assembling my tubelab simple se:

http://img803.imageshack.us/img803/9162/etapa5.jpg

This is my very first amp proyect, and after reading as a noob, and see the wiring page at the tubelab's site don't know how to wire this in pentode mode.

I think that perhaps connecting R18 and R28 to the B+ would do, but i'm totally unsure, this idea comes from wikipedia's deffinition of pentode mode.

So, how can i make that connection?

Today i'll get all the transformers from a local shop (custom made) and the choke.

Thanks in advance

Roberto.

 

[w5jag]

I don't have a board in front of me, but from memory T3 Pri:2 and T2 Pri:2 should be jumpered to T3 Pri:1 and T2 Pri:1 for triode operation, or T3 Pri:3 and T2 Pri:3 for pentode operation. It is much easier to get the amp running in triode configuration.

I've been running my SSE in pentode mode for more than a year with good results, mostly with low screen voltage sweep / rf tubes, with good results. I have also used the EL34 with good results, albeit at relatively low screen voltages.

Be advised that, imo, pentode mode does require more negative feedback than the onboard cathode feedback can provide, at least in my setup. Without negative feedback, the frequency response will be skewed to favor the high frequencies,

More can be found in the 6146 in the SSE thread, if you are interested.

Win W5JAG

 

[Treybal]

Thanks, i'm gonna start with triode mode to get the amp running. I'm doing the basic configuration with a 375-0-375 power transformer, so i could get a >400 V B+, perhaps thats not provide low screen voltage, what's your low screen voltage and your B+?