6L6 SE build

[joshdfrazier]

I really enjoyed my first build. Love the sound, I've been using it at practice. There is really no headroom, though. Which is okay I guess, but I would like the flexibility, and improved bass response of a more powerful amp. So, I'm going to build something as close to my first build as possible, but with a 6L6GC in the power section, ss rectifier, no tremolo circuit and a larger PT/OT.

The schematic to the build is in this thread: https://tubeamparchive.com/viewtopic.php?t=22949

My B+ will be something like 375-400 with the PT I have available for this project. Aside from changing the values of some resistors, what else needs to be done? I have read on other forums that the 6L6 needs a little more negative grid bias, though I'm not sure how to achieve this. I've already ordered the socket and tube, so I hope this isn't too crazy of an idea!

[Noval_novice]

I'm not sure you're going to get all that much more headroom from a change in output tubes. Now that you've made a nice single-ended amp, why not go for a push-pull build? Much more useable in a band situation. Just my two pence.

[10thTx]

I'm not sure you're going to get all that much more headroom from a change in output tubes. Now that you've made a nice single-ended amp, why not go for a push-pull build?

I agree with that. I don't think you'll get much more headroom either.

With respect, 10thtx

[Smokebreak]

I'm not sure you're going to get all that much more headroom from a change in output tubes. Now that you've made a nice single-ended amp, why not go for a push-pull build?

I agree with that. I don't think you'll get much more headroom either.

With respect, 10thtx

I'm not sure you're going to get all that much more headroom from a change in output tubes. Now that you've made a nice single-ended amp, why not go for a push-pull build?

I agree with that. I don't think you'll get much more headroom either.

With respect, 10thtx

That's the first thing I though too, but hey it's your amp.
To answer your question you will need to determine a proper value for your cathode resistor on the 6L6. This insures the tube is operating properly.
For 6L6gc(30w), you can run it pretty hot in cathode bias, but for now let's say you want to dissipate 25w(~85%):
First, I usually try and find a schematic of a similar amp, with similar plate voltage, to find a "starting" cathode resistor value. Err on the high side.
Let's say 350r and using Ohm's Law:
Check the voltage drop across the resistor.
Divide the cathode voltage above by actual resistor value(measure).
This will be your cathode current.
Subtract screen grid current from datasheet (2.5ma?)from cathode current.
This is your plate current.
Multiply your plate current by plate voltage to find dissipation in Watts.
Depending on which side of 25W(assuming this is what yer aiming for), adjust the cathode resistor accordingly--higher value to dissipate less, and lower to dissipate more.
You'll need to recalculate every time you change the resistor.
I'd make it a 5w resistor.
This is of course one of the many ways to do it.
You can always plug in Weber bias calculator too.

[Firestorm]

Or generate a negative bias supply with an adjustment pot and go fixed bias. If your PT can deliver the current and your OT is up to the task (multiple impedance taps and high enough standing DC) you could plug in almost any output tube.

[funkgang49]

Hey Josh,
I just finished putting the final touches on a SE build that I designed to have more clean headroom than your standard Tweed Champ/Princeton type circuit.
The B+ is around 400v and I am able to use 6L6,KT66,KT88, & EL34, KT77.
I've added a VVR on the Power tube to keep the low volume situation useability, while still being capable of decent clean headroom and volume when needed.
I'm running the output tube kinda cool with a 500ohm 10W Cathode resistor and a 1.5K 5W Screen Grid resistor. I've also tied pin#8 (Cathode) to pin#1 for safer EL34 type operation.
The 6L6 and EL34 type tubes definately offer greater clean headroom before breakup with just a touch more volume.
The JJ KT77 sounds great - with a very musical sounding full spectrum tone with clarity and a nice growl when pushed.

[renshen1957]

Hey Josh,
I just finished putting the final touches on a SE build that I designed to have more clean headroom than your standard Tweed Champ/Princeton type
The JJ KT77 sounds great - with a very musical sounding full spectrum tone with clarity and a nice growl when pushed.

Hi,

That's been my experience with the JJ KT77 tube.


Best regards,

Steve

PS Kevin O'Connor describes how to implement a methodology how to dial from PP to SE or in between in The Ultimate Tone volume 2 in the 1990s. Peavey borrowed the circuit, wired the pot backward in the Valve King, and then went on to Patent the circuit. Tut vol 5 has two amps with the O'Connor's body circuit both in schematic and layout; Tut Vol 6 shows a new method or achieving the same ends. (Please don't let on to Peavey.)

Because of copyright issues, I can't post the schematics. Kevin has been very generous with his time when I've talked or emailed him or a number of his novel circuit designs. I've built one of the two amps in Vol 5 and the amp goes from PP to SE and without the SE hum.

[M Fowler]

What voltage are the anodes of the preamp tube? That could make a big difference in headroom going higher voltage.

[joshdfrazier]

maybe i'm missing something, but why would I not obtain more volume by increasing the B+ with a considerably larger PT? I'm not just changing the bottle out. Also, I haven't touched a beam pentode since I started building, so I figure this will be a good way to get better acquainted with it. my next build will definitely be PP!

I will definitely be considering these replies when I finally get my diodes in the mail and start piecing this together. thanks again everyone.

fowler: pretty low. below 150 i believe. This will definitely be going up. i was thinking 250 or something.

[tubeswell]

I did a SE for 1 x 6L6 or 6V6||6V6 which could take a number of tube rectifiers and also had a switchable HT supply voltage. So it had quite a range f B+ voltages from around 320 to 420V. The best-sounding configuration to my ear for the 6L6 had the plates running at abut 320V, screens about 280, and a 220R cathode resistor. The best speaker I found for this configuration was a 10" 30W greenback. YMMV

[funkgang49]

What voltage are the anodes of the preamp tube? That could make a big difference in headroom going higher voltage.

fowler: pretty low. below 150 i believe. This will definitely be going up. i was thinking 250 or something.

Josh,
Keep in mind that with preamp voltages around 250v, while attempting to increase clean headroom, can begin to wander into sterile/harsh sounding territory. As oft quoted from Spinal Tap - "there's a fine line between clever and stupid".

I flirted with those exact voltages and just recently changed out the dropping resistor to bring my voltages down to around 225V.
I still have the clean headroom but a sweeter overall tone and slight increase in touch sensitivity.