6v6 5f6a bias circuit resistor values

[Guy77]

Hi everyone. I am building a 5f6a bassman style amp but running 6v6 instead of the 6L6 tubes.

I am using the bias circuit found in the Ceriatone 5f6a layout.

I was wondering what values resistors should i use for the bias ciruit.
Currently it uses 15K right after the diode and another 22k which connects to the bias pot. It is a 25k bias pot.
The bias voltage on the transformer i am using is -45v. My B+ will be about 340Volts

Any help appreciated.

[scrimpus]

Fenders don't vary too much between models. Look at all the schematics for the 6v6 fixed bias fender amps, I am sure you can find one with voltages close enough to yours to give an idea of what values are needed..

[Chuck H]

With a high Vp (5f6a puts about 430V on the plates) I'm not sure -45V will be enough for 6V6's to get them into a safe idle range considering circuit losses to the bias voltage. You may only have -40V of available bias. May be enough.?. You may need to employ a voltage doubler circuit.

[Guy77]

With a high Vp (5f6a puts about 430V on the plates) I'm not sure -45V will be enough for 6V6's to get them into a safe idle range considering circuit losses to the bias voltage. You may only have -40V of available bias. May be enough.?. You may need to employ a voltage doubler circuit.

Ok I have decided to go with another Power transformer, one that will give me 400v B+ and 50v Bias tap.

What I am wondering is how does changing the values of the 2 resistors in the bias circuit effect the current going to my power tubes.

My bias circuit has one 15 k resistor right after the diode and another 27K resistor going to leg of the 25K bias pot.
Lets say I wanted to be able to run my tubes hotter and my bias circuit would not let me do that how would I change the value of these 2 resistors?

Thanks everyone.

[elipsey]

Hi,

I built a 6V6 push pull with a deluxe reverb power transformer which yeilded about 410V B+, and had a 45V bias tap. What Chuck said perfectly describes the outcome, wrt the bias supply. I was not able to create an adequate bias supply from the bias tap, so instead I derived the bias supply from one leg of the HT supply. A schematic follows. YMMV.

I have also tried the 5F6A bassman circuit in a 6V6 amp, and I had to make some changes to reduce bass output and driving voltage to the output section. If you want the details I'll tell you, but maybe you've already got it under control.

Cheers.



EDIT:
Sorry, on closer inspection, I realize that I didn't answer the question as posed.

Specifically, decreasing the values of the 15K resistor, the 27K resistor, or the bias pot would allow you to bias the power tubes "hotter" by cause the bias voltage adjustment range to include voltages which are less negative. The 15K resistor seems to be part of an RC filter, so maybe leave that alone if you can, or at least don't change it first, since that could introduce noise. However, if you have a problem I expect that it will be in the opposite direction, that is, you won't be able to reach a safe bias point because the bias supply cannot provide a large enough negative voltage, in which case you man have to change the supply. If I recall, I needed about -43V, which my 45V tap could not supply after rectification, etc. Maybe 50V will be enough though.

If the circuit you have can't get there, I would probably start by experimenting with the value of the 27K resistor. Use a bigger resistor to make the supply more negative, and smaller one to make it less negative. You might want to use a different pot once you find a good value, such that the sum of the resistances of the pot and the resistor are the same as the some of the old pot, and the chosen resistor value. This would allow you to prevent adjustments that are dangerous to your tubes; any setting less negative than -30V or so would probably be dangerous and unreasonable.

If my advice sucks, hopefully someone with more experience will say so.

Good luck.

[Guy77]

Hi,

I built a 6V6 push pull with a deluxe reverb power transformer which yeilded about 410V B+, and had a 45V bias tap. What Chuck said perfectly describes the outcome, wrt the bias supply. I was not able to create an adequate bias supply from the bias tap, so instead I derived the bias supply from one leg of the HT supply. A schematic follows. YMMV.

I have also tried the 5F6A bassman circuit in a 6V6 amp, and I had to make some changes to reduce bass output and driving voltage to the output section. If you want the details I'll tell you, but maybe you've already got it under control.

Cheers.

[ATTACH=CONFIG]28656[/ATTACH]

EDIT:
Sorry, on closer inspection, I realize that I didn't answer the question as posed.

Specifically, decreasing the values of the 15K resistor, the 27K resistor, or the bias pot would allow you to bias the power tubes "hotter" by cause the bias voltage adjustment range to include voltages which are less negative. The 15K resistor seems to be part of an RC filter, so maybe leave that alone if you can, or at least don't change it first, since that could introduce noise. However, if you have a problem I expect that it will be in the opposite direction, that is, you won't be able to reach a safe bias point because the bias supply cannot provide a large enough negative voltage, in which case you man have to change the supply. If I recall, I needed about -43V, which my 45V tap could not supply after rectification, etc. Maybe 50V will be enough though.

If the circuit you have can't get there, I would probably start by experimenting with the value of the 27K resistor. Use a bigger resistor to make the supply more negative, and smaller one to make it less negative. You might want to use a different pot once you find a good value, such that the sum of the resistances of the pot and the resistor are the same as the some of the old pot, and the chosen resistor value. This would allow you to prevent adjustments that are dangerous to your tubes; any setting less negative than -30V or so would probably be dangerous and unreasonable.

If my advice sucks, hopefully someone with more experience will say so.

Good luck.

Thank you for explaining the bias circuit changes in the 5f6a ceriatone layout.

I like the bias circuit you included in your post , nice and simple! I also just found this post at el34world
Bias circuits

It describes the bias circuit you posted.
My amp started life as Weber Smokin Joe II kit.

I then changed the el84s to 6v6 and it sounded 1 million times better to my ears LOL!
The amp has a master volume . It has phase inverter much like the 5f6a (long tail).
The Master volume overdrives the amp with the most amazing distortion with the 6v6 tubes.

I will use the changes you mentioned to try it out as fixed bias amp and then add a few switches and change on the fly from cathode biased to fix biased.

If you don't mind I'd like to get the changes you made to your 6v6 5f6a for future use!

Thanks . My soldering iron just got hot ....got to run lol !

[Gingertube]

My experience with 6V6
At a B+ of 345V then 6V6 bias at -27V
B+ vs bias readings are in this post.
Power Scaling - diyAudio

You will need to be able to turn the bias down to that.

Cheers,
Ian

[elipsey]

Thank you for explaining the bias circuit changes in the 5f6a ceriatone layout.

I like the bias circuit you included in your post , nice and simple! I also just found this post at el34world
Bias circuits

It describes the bias circuit you posted.
My amp started life as Weber Smokin Joe II kit.

I then changed the el84s to 6v6 and it sounded 1 million times better to my ears LOL!
The amp has a master volume . It has phase inverter much like the 5f6a (long tail).
The Master volume overdrives the amp with the most amazing distortion with the 6v6 tubes.

I will use the changes you mentioned to try it out as fixed bias amp and then add a few switches and change on the fly from cathode biased to fix biased.

If you don't mind I'd like to get the changes you made to your 6v6 5f6a for future use!

Thanks . My soldering iron just got hot ....got to run lol !

Sounds like a fun project, I'm curious:

What plate voltage do you want?
What PT do you have now?
What PT were you thinking of using instead?
Are you going to use a tube rectifier?
Have you picked out a new chassis?

It looks like the Smokin Joe II kit specifies a Weber 025130. If that's what you have, I don't see any reason not to use it. It's pretty much a multi-tap Deluxe Reverb tranny, and it's quite versatile. I have used one myself, and liked being able to experiment with plate voltages by changing HT taps and rectifiers. I also had one fail after a year or so in amp I built for someone. Some people think Weber's Chineese PT's are unreliabe, but I only have one data point so I'm not really sure. If you thought it sounded good before, maybe you should keep the PT, and aim for the same plate voltage.

When using that transformer's 280-0-280 HT taps, you should have no trouble creating an adequate bias supply from the bias tap. With the 330-0-330 taps it might be close, but using the bias tap worked for me when I used it that way. I think the bias tap may have actually been a bit more than 45VAC. The tranny whoose bias tap I had trouble with was a Deluxe Reverb reissue PT, and I was running it with a pretty stiff rectifier tube, and had a plate voltage close to 420V. You might want to try converting your Smokin Joe to fixed bias before you proceed, and if that works ok, you could use the same power supply and bias circuits in the new bassman.

Smokin Joe II is an interesting amp. I glanced over the schematic and it looks sort of like two thirds of a bassman pre-amp. There's a cathode follower, a low loss tone stack, and a long tail PI, but no first gain stage. Also, the solid state power supply with that 91 ohm sag resistor looks like it approximates a 5Y3GT tube recitfier -- I got the about the same plate voltage from that tranny with the 280v taps and a 5Y3GT tube. I would not be supprised if Weber takes that same 10Watt resistor and 1N4007 diodes, stuffs them in a can, and calls it a 5Y3 copper cap. I bet I couldn't tell the difference between a 5Y3GT and that sag resistor either.

It's interesting that you prefered the 6V6 tubes to EL84 in that amp. I'm curious about how you would characterize the difference in sound between the EL84 and 6V6 in that amp. Did the 6V6 sound more solid, especially in the bass, while the EL84 seemed sort of messy and overwhelmed at high master volume settings? Did the EL84 sound harsh of fizzy? Did you change the bias resistor when you converted? I think EL84's sound better at lower plate voltages around 300V or a bit more, while I prefer 350-400V for 6V6.

Did the overdrive sound of the Smokin Joe sounds kind of Bassman-y, or marshally? If you like the sound of that amp, but want better tone controls, it might be interesting to try installing a Baxandall tone stack with a concentric pot in place of the single tone pot, so that you would have seperate bass and treble controls at the expense of a little bit of gain.

I'm asking about all this stuff partly from curiousity, but also because it kind informs what you might want to change on the 6V6 5F6A. Long story short: Mine had tons of gain, and kind of overwhelmed the 6V6's so it was really farty sounding. I switched to a 12AT7 PI tube, reduced the value of the first stage bypass cap to 4.7uf, used smaller PI grid resistors, cut the PI input and output coupling caps down to about the same values as they use in Smokin Joe. Acutally, I tried lots of other stuff too, but those are the first things that come to mind. There are really lots of ways to go about this and it probably depends on taste, and on how you want to use the amp. Do you want a vintage tweedy type of lightly overdriven sound? Big volume headroom? Lots of pre-amp overdrive with MV? Balls out marshally sounding distortion?

I will post more after I know what you're looking for if you want, or if you need more detail about the changes I already said.

[Guy77]

Sounds like a fun project, I'm curious:

What plate voltage do you want?
What PT do you have now?
What PT were you thinking of using instead?
Are you going to use a tube rectifier?
Have you picked out a new chassis?

It looks like the Smokin Joe II kit specifies a Weber 025130. If that's what you have, I don't see any reason not to use it. It's pretty much a multi-tap Deluxe Reverb tranny, and it's quite versatile. I have used one myself, and liked being able to experiment with plate voltages by changing HT taps and rectifiers. I also had one fail after a year or so in amp I built for someone. Some people think Weber's Chineese PT's are unreliabe, but I only have one data point so I'm not really sure. If you thought it sounded good before, maybe you should keep the PT, and aim for the same plate voltage.

When using that transformer's 280-0-280 HT taps, you should have no trouble creating an adequate bias supply from the bias tap. With the 330-0-330 taps it might be close, but using the bias tap worked for me when I used it that way. I think the bias tap may have actually been a bit more than 45VAC. The tranny whoose bias tap I had trouble with was a Deluxe Reverb reissue PT, and I was running it with a pretty stiff rectifier tube, and had a plate voltage close to 420V. You might want to try converting your Smokin Joe to fixed bias before you proceed, and if that works ok, you could use the same power supply and bias circuits in the new bassman.

Smokin Joe II is an interesting amp. I glanced over the schematic and it looks sort of like two thirds of a bassman pre-amp. There's a cathode follower, a low loss tone stack, and a long tail PI, but no first gain stage. Also, the solid state power supply with that 91 ohm sag resistor looks like it approximates a 5Y3GT tube recitfier -- I got the about the same plate voltage from that tranny with the 280v taps and a 5Y3GT tube. I would not be supprised if Weber takes that same 10Watt resistor and 1N4007 diodes, stuffs them in a can, and calls it a 5Y3 copper cap. I bet I couldn't tell the difference between a 5Y3GT and that sag resistor either.

It's interesting that you prefered the 6V6 tubes to EL84 in that amp. I'm curious about how you would characterize the difference in sound between the EL84 and 6V6 in that amp. Did the 6V6 sound more solid, especially in the bass, while the EL84 seemed sort of messy and overwhelmed at high master volume settings? Did the EL84 sound harsh of fizzy? Did you change the bias resistor when you converted? I think EL84's sound better at lower plate voltages around 300V or a bit more, while I prefer 350-400V for 6V6.

Did the overdrive sound of the Smokin Joe sounds kind of Bassman-y, or marshally? If you like the sound of that amp, but want better tone controls, it might be interesting to try installing a Baxandall tone stack with a concentric pot in place of the single tone pot, so that you would have seperate bass and treble controls at the expense of a little bit of gain.

I'm asking about all this stuff partly from curiousity, but also because it kind informs what you might want to change on the 6V6 5F6A. Long story short: Mine had tons of gain, and kind of overwhelmed the 6V6's so it was really farty sounding. I switched to a 12AT7 PI tube, reduced the value of the first stage bypass cap to 4.7uf, used smaller PI grid resistors, cut the PI input and output coupling caps down to about the same values as they use in Smokin Joe. Acutally, I tried lots of other stuff too, but those are the first things that come to mind. There are really lots of ways to go about this and it probably depends on taste, and on how you want to use the amp. Do you want a vintage tweedy type of lightly overdriven sound? Big volume headroom? Lots of pre-amp overdrive with MV? Balls out marshally sounding distortion?

I will post more after I know what you're looking for if you want, or if you need more detail about the changes I already said.

+++

I have uploaded a youtube video of me playing through this amp here.
The picture of the amp below has the vol control on the far right and master volume on the left. The settings you see in the pic are the settings I used to record the video. ( vol at 2 o'clock and master at 10 o'clock)
https://www.youtube.com/watch?v=p4th4AoFOrE

Right now it lives as a Smokin Joe II running the W025130 power transformer and 40 watt weber output trans(using 4k tap with 8ohm speaker)
I am running it cathode biased with 335v plates in this video. I switch from 3 x 40uf caps to 3x 22uf to get a richer sound like a Tweed Deluxe.
I attached a pic of the chassis here. I have also installed a choke on the amp which i can switch in and out. There is a 3rd preamp tube on the amp that will be used for the future 5f6a project.


In regards to your questions:
1. As far as plate voltage goes i am flexible, anything goes.
2. I have the W025130 power trans
3 I may stick with this trans but can be flexable if required,
4 I am running a GZ34 rectifier in the video but i have a switch to use solid state and i can also switch from 270v or 340v HV taps on the tranny
5 I am using a hammond 16 x 8 chassis but will switch to a longer chassis soon.

In regards to how the 6v6 sound as compared to the EL84, the 6v6 have a more smoother and rich sound to my ears, the EL84 were more messy and edgy sounding when i started to turn down the Master volume to get more distortion ( Its a PPIMV master).
I like the thick solid sound of the 6v6 ( but still sweet and rich sounding much like a Tweed Deluxe which is another amp i made and love using) .

One of the things I am really looking for is to get more "Balls out marshally sounding distortion" as you said but still keep some of sweet sound of the 6v6 tubes. I placed 6L6 tubes in this amp as well and the overdrive was great , more ballsy but it lost the sweet tweedy character that the 6v6 tubes had, Another reason I like the bassman idea is that I would like more tone range that a bass treble control would give me. I was also thinking that the extra gain stages of a bassman might help give me a nicer thicker overdrive,

Thanks in advance! Keep the iron hot !!!

[Guy77]

I finally got the amp to work in fixed biased mode. Thanks everyone for your help!
I used a 4.5K resistor instead of the 22k one in the diagram you uploaded elipsey. I connected the 45v bias line directly to the diode.

6v6 groove tubes sovteks are running at 22ma and 383v on the plates. This was just the sound I was looking for! Much more focused gain and overdrive when the amp is in fixed biased mode! Here is a youtube clip , excuse my amateur playing......... this thing is a little screamer has now!!
In the video I am running through a little 8in 15 watt speaker from my Fender G-Dec.

https://www.youtube.com/watch?v=em6WrXsHhPA

I should also note that I am running my preamp tube in cascode mode as explained here.
AX84.com - The Cooperative Tube Guitar Amp Project

Time to build a nice head cabinet for it now. I think I will do this.
http://www.haarguitars.com/haar-guit...7/P1160008.jpg