Tube Swapping Question

[Trout]

Hi,

I have a few DIY Fenders Guitar and want to try swapping the output tubes. Basically, I have PT's & OPT's that are well over rated in these amps and have biased them down a bit. PLENTY of filament current also.

A while back A fellow DIYer here mentioned dropping in EL34's in on 1 of these amps, Basically the recommendation was just ground Pin 1 at the socket and re-bias as needed.

Looking at the Pin-outs on the EL34 Vs 6L6, 5881,6L6 ect ,It looks as like pin 1 & 8 are internally connected??.
My Question is:
Is it better to ground pin 1 or better to just jumper it to pin 8?



I am not really looking to increase output levels as much as I am looking to alter tonal character and have found each different tube to offer their own unique qualities,
Gene

[Giaime]

Hello Gene,

It's better to ground pin1 instead of connecting it to pin 8, in case you have a cathode bias amp. In a fixed bias amp, it doesn't matter because you're already connecting pin 8 to ground.

[dhaen]

Pin 8.

[Trout]

This is the Schematic used sort of, I only used the output stage, In Front end is Bandmaster. The Feedback is slightly different but basically the same.
Output info

Cathode Biased for sure.
Gene

[Tom Bavis]

Either will work, though it will affect bias with EL34 (if cathode biased).

Pin 1 on octals was used for the shield on metal tubes (like original 6L6 6F6 and 6V6) and may be connected to the metal base on 6550 - but it's relatively low voltage if you connect it to the cathode on pin 8.

In old Fenders and other amps - WATCH OUT! Fender sometimes used pin 1 as a tie point for a screen resistor. So a metal tube or metal base could have full B+ on it!

[Trout]

Originally posted by Tom Bavis
Either will work, though it will affect bias with EL34 (if cathode biased).

Pin 1 on octals was used for the shield on metal tubes (like original 6L6 6F6 and 6V6) and may be connected to the metal base on 6550 - but it's relatively low voltage if you connect it to the cathode on pin 8.

In old Fenders and other amps - WATCH OUT! Fender sometimes used pin 1 as a tie point for a screen resistor. So a metal tube or metal base could have full B+ on it!

These are DIY Amps based on Fender designs. I NEVER use extra socket pins as terminal strips Ect.

OK,, Sounds like Its not a big deal Either way as far as the connection point.
I will try it later today and report back.
THANKS GUYS!!!!


Gene

[tubelab.com]

I was the guy who said to ground pin 1. I have tried both ways and it didn't make much difference in the sound in MY amp. YMMV. Try both ways.

I had a shocking experience in an old Stromberg Carlson PA amp that I was using for guitar back in high school. I put in metal 6L6's because they were free. I touched one with one hand while holding the guitar with the other hand. After using bad language, I found that one of the four 6L6's used pin 1 as the B+ tie point. I touched that one. From that moment I have always grounded pin 1 on power tubes of this type.

[aletheian]

i usually put my grid stopper and input on pin 6 because I am a lazy crapper!

kindof an aside here. How about tying the supressor grid on an EL34 to the -v in a fixed bias amp? I tried it once and ended up drawing slightly more current. Are there any drawbacks to that (other than not being able to use tubes that have the supressor internally grounded)?

[Trout]

OK, Just a Thought Here,
Since Pins 1 & 8 are internally connected wont it be a problem to just plug N play? I ask because It looks like it will bypass the Cap/ Resistor on pin #8 when I roll back to 6L6's. If I simply jumper the pins together rather than ground it, wouldnt it then be true plug N play?

I know a switch is also an option.

I was the guy who said to ground pin 1. I have tried both ways and it didn't make much difference in the sound in MY amp. YMMV. Try both ways.

Oh I had not forgoten who mentioned it, Your insight on DIY amps is PRICELESS! and Highly regarded. I just generally dont name drop.

I have 1 amp converted to grounding pin 1, But then I had a second thought because In my eyes it appears I would need to remove the wire to ground.

I have the schematic finished now, And have another bug/issue to address as well.

So, anyway, The Sound

No Re-bias drop in of EL34 is a success. The amp now has a much bolder sound, Not as Muddy on the bottom end (Very Potent Bottom End), Much brighter Highs. Overall ouput is about the same, But It seems to extended the clean sound further up the power range which I like.

Gene

[Trout]

OK,

Here is the schematic I used to build these 2 Modified Bandmaster 5E7's

Schematic

The Only real issue I have not figured out is the Bass Control.

It Seems to control the bass level just fine, BUT when reduced it also kills the volume nearly like a Master volume control.

If I can fix this it might be the best sounding amps I have built yet.
Somehow I suspect That 1 leg of the pot should not be grounded, But the originals I see on the web all show grounded.

Both amps do exactly the same thing, So I am fairly sure its not a wiring thing. Everything else performs great.
Gene