amp stages...

[9am53]

Hey guys, I was reading up on this article about turning my Bogen into a guitar amp... http://www.geofex.com/Article_Folders/old2new.htm
and they use the term "amp stages" in the mapping out of the amplifier. My Bogen uses a 6EU7 preamp tube (Which from what I understand is almost identical to a 12AX7 for those not familiiar with the 6EU7). This tube has a triode and a pentode inside of it. My questions are whether this 6EU7 is used for 2 separate preamp stages, the triode and the pentode? Which should come first? the other 2 tubes are ECL86's, are these both utput tubes? or am I just really confused? thanks for any help anyone gives me on the subject.

 

[Sch3mat1c]

6EU7 is a slightly lower noise (including new pinout) version of 7025/12AX7A. Dual high-mu small signal triode; no pentode.

Tim

 

[9am53]

ok, so do the two triodes in the tube account for two separate applification stages?

 

[audiousername]

Depends on how they're used and who is doing the classifying. If they are paralleled, in cascode, or in differential - it is sometimes classified as a single stage even though two triodes are in use.

Two triodes in one envelope is exactly that - what you do with them is up to you.

 

[9am53]

so lets say the plate from one leads directly to the cathode of the other, in that case are there 2 "amplification stages", and I assume they are in cascading format to each other...

 

[audiousername]

Cascaded stages is basically where a the output of a stage is fed to the input of another. Mostly it is where the signal from the anode is fed into the grid of the next stage.

When the anode is directly connected to the cathode of another valve, it is most likely a cascode, which is commonly classified as a single stage. Do you have a schematic of what you mean?

 

[planet10]

6EU7 is a slightly lower noise (including new pinout) version of 7025/12AX7A. Dual high-mu small signal triode; no pentode.

Different pin-out as well...

What output tubes does your Bogen have? (or what model) I was exploring my collection and noticed the 6EU7 for the 1st time in one of the Bogens i have.

dave

 

[9am53]

What output tubes does your Bogen have?

dave

The amp has two other tubes, both of which are 6GW8/ECL86's, but as of yet I dont know if they are both output tubes...I will have to check whether they are cascading or cascode(ing), but if they are cascading, is that 2 stages of amplification? I read that the preamp volume control should go, or rather could go, between the first and second amp stages... is this right?

 

[planet10]

The amp has two other tubes, both of which are 6GW8/ECL86's,

Push-pull ECL86 (think of this as 1/2 a 12AX7 + a slightly derated EL84/6BQ8)... i've a bogen with a similar output stage -- i'll have to check to see what it had for input tubes.

dave

 

[9am53]

Thanks alot Planet10. I have a midterm tonight, so I wont be able to look at the amp till later, but now that I understand that there are 2 potential amplification stages in the 6eu7, I have an idea of where to start reworking this thing. From what I understand, a push-pull circuit eliminates some harmonics and distortion...this kinda sux for a guitar amp right? do single ended amps povide better guitar tone?