6V6 versus 6L6 schematics

[jazzbass]

It all started with a request of mine posted on a "New 183 build" thread:

Hi everyone,
Sorry for the trivial question but to use 4 x 6V6 as power tubes instead of 4 x 6L6 as well as the different output transformer which other components will I have to change to exploit the tubes in the best way?
a Hug, Franco

Dorrisant's answer was:The power transformer, power tubes input grid resistors and dropping string resistors

I obviously have a limited knowledge of electronics, even if not very low, and I replied:

Thanks Dorisant,
The Power transformer obviously suitable for powering the 6V6s, the output transformer suitable for the Raa impedance of the two or four tubes, but for the input grid resistors and dropping string resistors what are the values suitable for these tubes.
In which discussion on TAG can I find a comprehensive answer on the subject? I built a Dumble 2nd generation clone with 4x 6V6 with a VOX AC30 output transformer that sounds really good (thanks to erwin_ve and rootz) but I wish I could say it's not just luck but a good build !
Thanks, Franco

[jazzbass]

Martin Manning, as always very generously replied:
A 100W PT could obviously handle the current requirement for 4x6V6 (~40W), but the output voltage will be high for 6V6, and higher still due to the reduced current draw wrt 6L6. Heater voltage will be high too, since 6V6 use less than half the heater current of 6L6.You could easily bring the plate voltage of the preamp tubes back in line by increasing the first dropping resistor, but the value will depend on where the plate and screen node voltages land with the reduced load.A power transformer suited to 2x6L6, with ~400V B+ would be a better solution.Grid resistor values are not critical, but they do have an effect on bias excursion and the onset of blocking distortion.1k5 should be fine.
Best to start a new thread on this topic...

[jazzbass]

And here I am with a new tread,

any suggestions for tthe ransformers for 4x 6V6? maybe the Hammond 290DEX or some other transformer?
and for the output transformer the hammond 1750JA?
Martin Manning writes that: Grid resistor values are not critical, but do have an effect on bias excursion and onset of blocking distortion. 1k5 should be fine: any other thoughts on this value?

Thanks for every contribution.

A hug, Franco

[martin manning]

Blackface Bassman transformers would be a good bet for this. 290EEX PT and 1760L OT.

[jazzbass]

Thank you Martin Manning,

as usual you are a great coach and a generous person.

A hug Franco

[dorrisant]

You can use a "MOSFET B+ Dropper" or other means to crop your B+ down if needed. I have used this to use 6V6s with a PT designed for 6L6s.

Another thing that can be done is to use JJ 6V6S for your power tubes. They can be run at a higher B+ than other 6V6s in current production. I have used them at 460-470v for quite some time now with no I'll effects. Beware of any other brand... They can't take it. Look for posts about Jim Kelly amps. They are 4 x 6V6. But read all about them, even the bad things.

As far as values around the different power tubes, I suggest looking for amps that have the power tubes you want to use... Use Google to search for "4 x 6V6 amp schematic"... The Google search at the top of the page will show results from this site...very handy. You will start to see and notice differences... The input grid resistors are usually higher with 6V6s. Be sure to read and find why. It will be there. Once you find a range of target values, try them out. Do this same thing with the screen grid resistors. Fender schematics are great to study this kinda stuff. If you see it done in one of those amps, it probably worked well for a long time. If you see something that stands out as odd, research that.

I don't mean to sound rude at all about any of this. Just assuring you that you are in the right place to find all of this out.

[FUCHSAUDIO]

You can use a MOSFET B+ Dropper or other means to crop your B+ down if needed. I have use this to use 6V6s with a PT designed for 6L6s.

Another thing that can be done is to use JJ 6V6S for your power tubes. They can be run at a higher B+ than other 6V6s in current production. I have used them at 460-470v for quite some time now with no I'll effects. Beware of any other brand... They can't take it. Look for posts about Jim Kelly amps. They are 4 x 6V6. But read all about them, even the bad things.

As far as values around the different power tubes, I suggest looking for amps that have the power tubes you want to use... Use Google to search for "4 x 6V6 amp schematic"... The Google search at the top of the page will show results from this site...very handy. You will start to see and notice differences... The input grid resistors are usually higher with 6V6s. Be sure to read and find why. It will be there. Once you find a range of target values, try them out. Do this same thing with the screen grid resistors. Fender schematics are great to study this kinda stuff. If you see it done in one of those amps, it probably worked well for a long time. If you see something that stands out as odd, research that.

I don't mean to sound rude at all about any of this. Just assuring you that you are in the right place to find all of this out.

FWIW: I use the Mosfet "active zener" on the Mosfet Follies pages as a screen dropper. I pull the screens down 50 volts in most of my models, and have reduced power tube failures substantially and it reduces the drive requirement (less bias voltage required on control grid to set bias and to drive the tube), and lets you run higher plates safely. An IRF series FET a zener and a couple of resistors...

[jazzbass]

Thank you dorrisant and FUCHSAUDIO,

dorrisant wrote:"I don't mean to sound rude at all about any of this. Just assuring you that you are in the right place to find all of this out."

dorissant, I know yours are great advice, surely reading and researching like I have done so far will help me learn a lot.

FUCHSAUDIO to date I have never lowered the voltage with FET or zenner, the amplifier I built with 4x 6V6 (2nd generation hybrid) works great with the values Dumble used for the 6L6s, but what you write concerns me a bit for their duration. Where can I study the circuits (An IRF series FET a zener and a couple of resistors) for voltage reduction?

This site is really full of good and generous people.

A hug,

[dorrisant]

https://ampgarage.com/forum/viewtopic.php?t=27035

[jazzbass]

Hi dorrisant, hi everyone,

Thanks, I found a lot of pages to study, I'll start right away.

A hug, Franco

[martin manning]

If you get plate voltage around 420, and use the 1760L OT (4k2 primary) I think the screen voltage could be left as-is. If It's higher than that, the 14W JJ 6V6S is a good idea, and a screen voltage reduction might be the best bet. I believe what Andy F is suggesting is placing the "B+ Reducer" from the MOSFET Follies between the screen capacitor positive and the screen grid resistors (B+2 to drain terminal, source terminal to screens). The screen voltage reduction will be approximately equal to the Zener voltage.

[mhuss]

Andy has a good point. A lot more stressed output tubes fail via screen grid vs. any other mode. It just cost money to reduce the screen voltage significantly (particularly before the advent of HV FETs) - this is the main reason why traditional designs don't do this.

[jazzbass]

If you get plate voltage around 420, and use the 1760L OT (4k2 primary) I think the screen voltage could be left as-is. If It's higher than that, the 14W JJ 6V6S is a good idea, and a screen voltage reduction might be the best bet. I believe what Andy F is suggesting is placing the "B+ Reducer" from the MOSFET Follies between the screen capacitor positive and the screen grid resistors (B+2 to drain terminal, source terminal to screens). The screen voltage reduction will be approximately equal to the Zener voltage.

Thanks Manning,

I'm studying the articles you recommended .......

I'm studying the articles you recommended.......would you be kind enough to send a diagram of the connection you recommended? .

Thank you
A hug Franco

[martin manning]

Here is a layout for the B+ reducer/screen voltage dropper. Chassis space is limited, but you may find room for it between the power supply board and the power tubes nearest the preamp.

[jazzbass]

Hi Martin,

thank you very much. I will put your advice and support to good use.
I'll be home on March 15th and immediately start testing it on the 2nd Gen Hybrid with 4x 6V.

A hug, Franco