1x12" Pro 5E5A or Twin 5E8A?

[Chris / CMW amps]

Hi all

Altough I've build, modified and serviced many amps tweeds are very, very rare over here in Europe and I wanna build a compact tweed "bass & treble" amp.

Right know I'm doubting about a Pro 5E5A or a Twin 5E8A (with modified neg. feedback loop). The chassis does haver enough space for 4 noval-sockets and 3 octal sockets and all the controls but I need your experiences to decide which one I'll build. Speaker will be a Celestion H30/75Hz, Heritage 65 Watt or an Alnico Gold.

Many, many tia!!!

[bob p]

maybe i'm missing something, but aren't those two circuits essentially the same amp with different OTs?

i like that output section, but i ahve to admit i'm not the biggest fan of the B&T controls on that circuit. i think that it would be much cooler to use a 5F6-A front end, and the pro's PI and power section. that is one cool circuit.

[AlAmps]

Right know I'm doubting about a Pro 5E5A or a Twin 5E8A (with modified neg. feedback loop). The chassis does haver enough space for 4 noval-sockets and 3 octal sockets and all the controls but I need your experiences to decide which one I'll build. Speaker will be a Celestion H30/75Hz, Heritage 65 Watt or an Alnico Gold.

What do you exactly mean with a modified NFB? (BTW which NFB?) You can name those speakers but what exactly do you want? A H30 and an Alnico Gold are very different (!?)

Al

[Bruce / Mission Amps]

Chris, not withstanding the obvious speaker configuration, most of those amp chassis are really pretty much the same amp.... but with a minor NFB change and which output transformer was used.
All were 5K5 to 6K5 but with either a 4 ohm or 8 ohm secondary.
In a pinch, a dead stock replacement silver face Pro Reverb OT at +4K@4 ohms works very well anyhow.
The 8 ohm OT was used with the single 15" Pro and the other amps used the 4 ohm tap OT as found in a tweed Super or Bandmaster.
4 ohm secondary OT went with the 2.66 ohm 3x10 load or 4 ohm 2x10 load.
In my opinon, the better sounding ones all use the 9pin preamp tubes but I still have some customers that rave about the amps using 6SC7GTs in the preamp.
If you decide to use octal preamp tubes sockets, seriously consider wiring the preamp sockets for 6SL7GTs instead of 6SC7GTs... works really well, NOS 6SL7s are readily available and they sound very good.

[bob p]

If you decide to use octal preamp tubes sockets, seriously consider wiring the preamp sockets for 6SL7GTs instead of 6SC7GTs... works really well, NOS 6SL7s are readily available and they sound very good.

Interesting comments, Bruce! Can I pick your brain for a little more info?

I have a 1950s RCA 30 watt PA amp that I had bought to rebuild as a dedicated harp amp. The circuit path goes from the Mic intput transformers to a 6SC7, 6SJ7, a 6SL7GT phase inverter, and a pair of 6L6G.

To make a long story sort, it got totally thrashed by UPS and the mic inputs and transformers got demolished. It looks like the path of least resistance for what's left of the amp is to rebuild it into a Fender Tweed circuit, so its probably going to end up becoming a 5E5A Pro or a 5F4 Super (maybe with the 5F6 tonestack).

I had been thinking about using the 6SC7 preamp tubes, like the 5C5 Pro. I still have a couple of questions that I'm hoping that you can answer:

When going back and forth from the octal to the noval preamp tubes, is it necessary to rescale any of the gain stages? Looking at the 5C5 schematic, it looks like all of the resistor and cap values were left unchanged when Fender migrated from the 6SC7 to the 12AY7 + 12AX7 preamp tubes. Is that correct? I've never seen one of these in real life, so I just thought I'd ask. (I've considered wiring up the amp for both types of tubes so that I could do a direct comparison.)

The other question that I have is about your comments regarding the 6SC7 vs. the 6SL7. If you could offer any comments about how the tubes compare tonewise, I'd appreciate the help. As it turns out, I have a bunch of 6SC7 on hand, so 6SC7 availability isn't a problem (at least for now).

Thanks for your thoughts!

Bob

[stokes]

Bob,he actually compared the 6SC7 and 6SL7,not the 6SJ7.The 6SJ7 is a single pentode with a lower amp.factor.In my experience with the 6SJ7,I found it to be similar to a half 12AU7,which I didnt like,to me it was flat and dull sounding.On the other hand the 6SC7 and 6SL7 are very similar tonewise,but different pinouts,and much better,to my taste than the 6SJ7.I like the SL7 better,for one thing you can "drop in" a 6SN7 for lower gain.I find them to be similar to the 12AX7 (SL7) and 12AY7 (SN7).Both are a little lower in gain than the AX7 and AY7's but close.I have a bunch of NOS 6SJ7's I dont think I'll ever use,if you want,I can send you one next time I go to the post office,drop me a PM,if you are interested.

[Bruce / Mission Amps]

Well the 6SJ7 is a pentode with a mu of around 1500-1600 so it isn't fair to compare it to any dual triode 6SC7 or 6SL7.
The 6SL7 and 6SC7GT both have similar mu and plate impedances so, with proper tube base wiring, they can just about be swapped out using the same supporting components.
I like using 240K to 270K plate load resistors with the 6SL7GTs.
They sound very very similar to me so I use the 6SL7GT in place of a 6SC7GTs frequently.
Why?
It is getting harder and harder to find 6SC7GTs that are not microphonic, but vert easy to find 6SL7s that are NOS and still sound great.

[bob p]

aw crap. that was a typo. i wanted to ask about triodes vs. triodes -- i didn't mean to type the name of the pentode. that was just dumb fingers on my part.

[bob p]

... the 6SC7 and 6SL7 are very similar tonewise,but different pinouts,and much better,to my taste than the 6SJ7.I like the SL7 better,for one thing you can "drop in" a 6SN7 for lower gain.I find them to be similar to the 12AX7 (SL7) and 12AY7 (SN7).Both are a little lower in gain than the AX7 and AY7's but close.

The 6SL7 and 6SC7GT both have similar mu and plate impedances so, with proper tube base wiring, they can just about be swapped out using the same supporting components.
I like using 240K to 270K plate load resistors with the 6SL7GTs.
They sound very very similar to me so I use the 6SL7GT in place of a 6SC7GTs frequently.
Why?
It is getting harder and harder to find 6SC7GTs that are not microphonic, but vert easy to find 6SL7s that are NOS and still sound great.

thanks for the fast answers, guys. as it turns out, i have maybe a dozen or so 6SC7 on hand, so i thought i'd combine the extra tubes with the demolished amp to make something useful out of a bunch of extra parts that i have lying around. i have no idea if these 6SC7 might be microphonic or not -- so far all that I've had a chance to do with them has been to put them on a tester. i guess that this will just end up being another experiment.

thanks for the suggestions about using the SL7 and SN7 as swappable gain stages. since i already have a bunch of SC7 i think i'll try them first -- if i end up having problems with microphonics with the SC7, then i'll start over and rewire the sockets for the SL7 and SN7.

[stokes]

aw crap. that was a typo. i wanted to ask about triodes vs. triodes -- i didn't mean to type the name of the pentode. that was just dumb fingers on my part.

I thought so,wasnt sure if it was fingers or eyes,at my age both are suspect!But if you want an SJ7 to experiment with,let me know,I got them for nothing and they are just taking up space.