master volume with pentode preamp valves

[black_labb]

hi guys, im in the early stage of conceptualising a amp im thinking of making. im hoping to use the amp as a bass and guitar amp so i am basing the power section and tone stack on a 100 watt hiwatt, but id like to try giving it a bit of grit if nessisary by overdriving the preamp tubes a bit, but im not a big fan of the high gain sound of 12ax7's. i thought id try putting some ef86 pentodes for some of the preamp tubes.

the idea being to up the gain with a couple triodes (how many ill figure out after some resarch) from a 12ax7 (with a vol pot somewhere in there to control distortion levels), then 2 ef86 pentodes followed by the master volume and tone stack then a 12ax7 after that for the LTP, then into the power amp, which would be the same as a 100w hiwatt using 4 el34's.

does this sound like a viable option, how many 12ax7 triodes would you guys suggest i would need to get the signal to a level that would get the ef86 pentodes to get reasonably distorted? (thats ignoring a second channel) would having 2 of the ef86's be enough to get some juicy distortion? im not looking for an incredibly distorted high gain, more the option to get some dirt in an amp that would blow my ears anywhere but at a stadium trying to get it to overdrive without a pedal.

thanks for the help

[DirtyGeorge]

I have never tried such a circuit so this is just a guess. Considering the high gain of an ef86 I think one 12ax7 section would be enough to overdrive it. Two sections would probably put you in high gain (high distortion) territory. IMHO adding another ef86 would just be overkill.

Anyone with actual experience can jump in now.

DG

[Earl]

Just some thoughts - I've never looked at any "high gain" amp circuits, and don't really know what that's all about.

The typical AV of a typical hi-u tube in circuit, is ~60, without any following tone filter(s). Let's say worse case is AV = 10. So if you put .5v in, you'd get 5v out. Run that into a following stage that has a gain of 30, and then a PI with a gain of 9 (with feedback).

You can see that 10 *30 * 9 is a large number....so what are the extra stages for?

[black_labb]

my thoughts are that the ef86 has higher gain, so it would take a higher signal than what the 12ax7's can get to to get them to distort, hence the second ef86. the first one to get the signal to a level to distort the second. ive been looking at datasheets, and is it the amplification factor that is used or the voltage gain? i cant seem to find the amplification factor of the ef86.
thanks for the input

[J Martin]

There is no fixed amplification factor since its a pentode. Gain is determined by the plate, screen and cathode resistors, voltage, etc. You can crank the gain up into the hundreds, but as you go higher, you will loose bandwitdh. Rather than type in a tutorial on pentode gain stages, I will refer you to the old tube manuals. Usually there is an apendix called "Resitance Coupled Amplifiers" or similar where tables are given with different value plate and screen resistors vs gain. You can use any small signal sharp cutoff pentode as an example if there is no listing for the EF86/6267 such as the 6AU6 or 6SJ7 which are more common in the US.

[black_labb]

There is no fixed amplification factor since its a pentode. Gain is determined by the plate, screen and cathode resistors, voltage, etc. You can crank the gain up into the hundreds, but as you go higher, you will loose bandwitdh. Rather than type in a tutorial on pentode gain stages, I will refer you to the old tube manuals. Usually there is an apendix called "Resitance Coupled Amplifiers" or similar where tables are given with different value plate and screen resistors vs gain. You can use any small signal sharp cutoff pentode as an example if there is no listing for the EF86/6267 such as the 6AU6 or 6SJ7 which are more common in the US.

that makes sense, thanks. ill do some more research and figure out the best way to design this. im keeping my eye on a 100w ashton amp which has a blown OT to use that for the chasis as it runs 4 el34's so the chasis and power transformer and the whole case would work well for the amp. just need to get another OT and redo the circuitry. i wonder if anyone would be interested in a circuitboard for a 100watt tube amp, there are also some effects like reverb and such built into it, might figure where they are and build those into pedals or something of the like.

thanks for the help. does anyone know what ef86's distortion character is like?

[soundmasterg]

The EF86 by itself has a lot of gain, and as such it is usually best suited as the input stage in an amp rather than an intermediate stage, but it will work in either place.

I used an EF86 into a cathode follower using 1/2 of a 12AY7, then a tone stack and volume control, then into the other 1/2 12AY7 for a gain stage. After that it goes into a Silvertone 1484 phase inverter and output section, which is a 6CG7/6FQ7 cathodyne phase inverter into a fixed bias 6L6GC pair respectively. I found that I could outclean a blackface Fender and approach Marshall JCM800 gain levels as the volume went up. Subbing a 12AU7 for the 12AY7 made for a really nice clean tone and more vintage style overdrive as the volume is dimed. Subbing a 12AX7 for the 12AY7 gave too much gain and the sound got harder and less harmonically rich, which I didn't like.

The key with the EF86 or any pentode is the screen voltage as compared to the plate voltage. Usually the screen voltage should be 30% to 40% lower than the plate voltage. You can go with a typcial Vox AC15 setup and the screen voltage will be about 15 volts higher than the plate, and it gives a somewhat interesting sound, but overall going the other way will sound better IMHO. Since it is a pentode, there is a greatly reduced Miller Effect as compared to a triode, so there are more highs and more lows. The coupling cap coming out of the EF86 should be smaller than normal. If you use a cathode follower right after the EF86, it will increase the gain considerably due to less loading as compared to not using a CF. If you do use a CF, then the coupling cap coming out of that is the one that should be smaller to get the same frequency response as your typical .022uf as in a Fender for example. I found a .003uf worked in my application. The pentodes also compress more when pushed, which is cool. I really like pentodes on the front end of amps myself, although the EF86 tends to be microphonic, especially in combos. Pentodes do require lots of tweaking to get right so have some patience.

Greg

[black_labb]

The EF86 by itself has a lot of gain, and as such it is usually best suited as the input stage in an amp rather than an intermediate stage, but it will work in either place.

I used an EF86 into a cathode follower using 1/2 of a 12AY7, then a tone stack and volume control, then into the other 1/2 12AY7 for a gain stage. After that it goes into a Silvertone 1484 phase inverter and output section, which is a 6CG7/6FQ7 cathodyne phase inverter into a fixed bias 6L6GC pair respectively. I found that I could outclean a blackface Fender and approach Marshall JCM800 gain levels and the volume went up. Subbing a 12AU7 for the 12AY7 made for a really nice clean tone and more vintage style overdrive as the volume is dimed. Subbing a 12AX7 for the 12AY7 gave too much gain and the sound got harder and less harmonically rich, which I didn't like.

The key with the EF86 or any pentode is the screen voltage as compared to the plate voltage. Usually the screen voltage should be 30% to 40% lower than the plate voltage. You can go with a typcial Vox AC15 setup and the screen voltage will be about 15 volts higher than the plate, and it gives a somewhat interesting sound, but overall going the other way will sound better IMHO. Since it is a pentode, there is a greatly reduced Miller Effect as compared to a Triode, so there are more highs and more lows. The coupling cap coming out of the EF86 should be smaller than normal. If you use a cathode follower right after the EF86, it will increase the gain considerably due to less loading as comapred to not using a CF. If you do use a CF, then the coupling cap coming out of that is the one that should be smaller to get the same frequency response as your typical .022uf as in a Fender for example. I found a .003uf worked in my application. The pentodes also compress more when pushed, which is cool. I really like pentodes on the front end of amps myself, although the EF86 tends to be microphonic, especially in combos. Pentodes do require lots of tweaking to get right so have some patience.

Greg

im thinking i may use 2 ef86's with a volume pot between them. for some reason i thought 2 tubes after eachother would be normal, but then realised that it isnt. will probably work well with bass as it will be able to cope with higher input voltages from active basses or guitars with pedals. and having the second one there i can use it as a master volume to get some distortion from the second tube. didnt realise that they have less attenuation at the high and low frequencies, which is nice for the use of a bass.

it will be a head unit, so the microphonics probably wont be too much of an issue. maybe just avoid putting the hgead ontop of the cab when playing at high volumes.


sorry to keep asking questions, but are there any speakers that would be suitable for guitar aswell as bass in a 4x12 or 4x10 config (would be close backed for the bass) or would having 2 separate cabinates be inevitable. anyone know of any sensitive speakers for bass use? most bass speakers seem to be a fair bit less sensitive than guitar speakers, and being only a 100w amp, sensitive speakers would be a good thing when using it for bass.


thanks for the help.

[rexindigo]

one half 12ax7 is enough

i've built a simple SE amp with 1/2 12ax7 before the ef86 to overdrive it, and its perfect to dial in from clean to fat overdrive

[black_labb]

i've built a simple SE amp with 1/2 12ax7 before the ef86 to overdrive it, and its perfect to dial in from clean to fat overdrive

thats nice to know, i may use a 12ax7 with 2 channels, i may use another ef86, using the ef86 would improve the way the amp handles higher input signals, but it also sucks more current and would mean a single channel unless i wanted to put another one in. could also mean more microphonics.

how does the ef86 sound when overdriven? is it smoother than a 12ax7 as i predicted? anything different you noticed about the character?

thanks for the help

[Steve Conner]

I was recently messing with an old Selmer T'n'B 50. I kept one of the channels stock, and increased the gain of the other channel by bodging an EF86 onto the front of it. The only reason I chose that tube was that I had a bunch of barely used Mullards from some tube record players that I dumpster dived...

I put a volume pot as a gain control between the EF86 and the original 12AX7 first stage. With this control up full it really screams like someone was trying to kill it! Turning it down gives some good semi-distorted tones. I also tried it with a 12AU7 or 12AT7 in the second stage, which tames the gain down a bit, and made it sound thicker.

I'm not sure how the EF86 sounds overdriven. I've tried overdriving the front end of my creation with a stomp box, and it sounded pretty good, though with that much gain I could hardly tell any more! It's probably best used as the first stage, where you can take advantage of its low noise properties too. I heard that the new production ones can be badly microphonic, though the originals shouldn't be.

I've never seen a bass cabinet with 10"s that sounded good on guitar. I've been using the same amp head with a 2x12" for guitar and a 2x10" for bass for years now.

If I wanted one cab that would do both, I would go looking for some good 12"s. People rave about the EVM12L, they are a good speaker for guitar, and Geezer Butler uses them in his bass rig too! But they are expensive and heavy.

[rexindigo]

thats nice to know, i may use a 12ax7 with 2 channels, i may use another ef86, using the ef86 would improve the way the amp handles higher input signals, but it also sucks more current and would mean a single channel unless i wanted to put another one in. could also mean more microphonics.

how does the ef86 sound when overdriven? is it smoother than a 12ax7 as i predicted? anything different you noticed about the character?

thanks for the help

yes it overdrives smoother.
the amp i made this, was a p1 from ax84 with the ef86 instead of the 12ax7.
This was very clean and a bit bassy. i added the 1/2 12ax7 and now it rocks, last time i played it in rehearsal i used an old transitor bass combos 15"speaker and it blew my mind. it was loud enough to compete with drum and bass. I seem to like 10" and 15" better for guitars than 12" i dont know why.....

[soundmasterg]

im thinking i may use 2 ef86's with a volume pot between them. for some reason i thought 2 tubes after eachother would be normal, but then realised that it isnt. will probably work well with bass as it will be able to cope with higher input voltages from active basses or guitars with pedals. and having the second one there i can use it as a master volume to get some distortion from the second tube. didnt realise that they have less attenuation at the high and low frequencies, which is nice for the use of a bass.

The EF86 by itself has enough gain to easily drive a sensitive power tube like an EL84. If you stick two together with a volume control in between, you may find that you have too much gain and you'll have to incorporate a divider to lose some gain. A master volume will help but there are some other approaches that may work better. The EF86 and other pentodes need a light loading on their output or the gain level that they can produce is greatly reduced, which is why a cathode follower in that position is so useful. If you have the EF86 first, then a cathode follower direct coupled to it, then a tone stack and volume control in between the CF and another gain stage, you'll have as much gain as a JCM 800 Marshall and some really good tone to boot. You can use a 12AU7, 12AY7, 12AV7, 12AT7, 12AX7, 12BH7 as the CF and gain stage and they will all work, though as you get above the gain level of a 12AY7 it starts to be too much. Two EF86's together like you describe may be too much gain and you'll end up with oscillations perhaps, but have fun with it if you decide to go that way.

Greg

[black_labb]

The EF86 by itself has enough gain to easily drive a sensitive power tube like an EL84. If you stick two together with a volume control in between, you may find that you have too much gain and you'll have to incorporate a divider to lose some gain. A master volume will help but there are some other approaches that may work better. The EF86 and other pentodes need a light loading on their output or the gain level that they can produce is greatly reduced, which is why a cathode follower in that position is so useful. If you have the EF86 first, then a cathode follower direct coupled to it, then a tone stack and volume control in between the CF and another gain stage, you'll have as much gain as a JCM 800 Marshall and some really good tone to boot. You can use a 12AU7, 12AY7, 12AV7, 12AT7, 12AX7, 12BH7 as the CF and gain stage and they will all work, though as you get above the gain level of a 12AY7 it starts to be too much. Two EF86's together like you describe may be too much gain and you'll end up with oscillations perhaps, but have fun with it if you decide to go that way.

Greg

yeah, im straying away from the idea of using 2 ef86's. maybe ill put a channel that bypasses the 12ax7 (straight into the ef86)for a channel for use with pedals and the such. not that i ever use them or have any desire to use them, but if it ends up being the 100watt design like ive been thinking about it would be a suitable input for bass also.

if the ef86 had too much gain, couldnt i just reduce the value of the vol pot to let more of the signal go to ground (or just keep a normal valued one lower down)

thanks alot guys, i dont know too much about electronics but i love designing and you need to learn somewhere, thanks for being helpful and letting this be a good place to learn

[soundmasterg]

Without a cathode follower after the EF86, you don't want to go much lower than a 1M volume pot on the output or you choke the EF86 down so much it doesn't work very well and it compromizes the tone you can get out of it too. Using a cathode follower buffers the output and allows you to maximize the gain out of the EF86 (or another pentode type) and get the tone you want out of that stage, and presents a more optimum impedance for any following stages too. It also works well with a tone stack after the CF because the output of a cathode follower is very low so there are much less gain losses than if you put a tone stack right after the EF86. You're just thinking of resistance, but the impedance is what matters here.

Let me put it this way....if you had an EF86 going into a CF using half a 12A*7, and then a gain stage using the other half of the 12A*7, you would get more total gain than if you used an EF86 going into 2 12A*7 stages, and the tone would be better and would cause less problems trying to tweak it to work right. If you haven't yet, check out the cathode follower circuit....its one of the most useful ones out there!

greg