Questions on Bassman bias circuit and other bassman build questions...

[1blueheron]

In the process of planning a Bassman build.  I noticed that all but the 5B6 versions have what appears to be a 48V biasing circuit. 

I would like to use the iron I have from a Baldwin organ that ran a 5U4GB,  2-6L6 and a bunch of 12AX7/12AU7's

The transformer has the following taps:

372V HT Red pair with CT
13.6V Green pair
6.5V Brown pair
5.7V Yellow pair

I do not have a 48V tap.

What are my best options?

Can I use the 5U4GB for rectifier or do I need to move to SS or 5AR4?

[dude]

Something like this, play with the resistor value 15K to get what you need. 


Added: 15K should be about right, use a 47K for the load R, should get you around -25v to -60v. I have used this with a Hammond organ PT 270-0-270, biased EL43's, 6L6s and 6V6's. Use a 3 watt 220K.

[mresistor]

Yes you can use a 5U4GB just pick off the bias voltage from one leg of the AC input and then adjust your range resistor for the right amount of voltage to power your bias circuit. The circuit would be the same as the above picture but you will be using a tube rectifier instead of two diodes..  I have used a 5U4GB in my Baldwin organ iron powered 5F6A guitar amp.  But after using a the big choke from the organ at the input of the power supply instead of a capacitor as the first filter, the voltage was a little lower than I wanted so I switched to a 5AR4 to bring the voltage up.. 

[sluckey]

Just copy the bias circuit on page 2 of this pdf. You will have to decrease the size of the BIAS RANGE resistor (maybe 100K, 120K, 150K) to provide the proper bias for 6L6s.

     http://sluckeyamps.com/6v6plexi/6v6plexi.pdf

[dude]

Here's a Princeton bias circuit, like Sluckey says you might have to play with that 3 watt 220K but with that PT ac voltage my guess is 220K.


 

[2deaf]

Couple of tables for the above bias circuits.  Really saves a lot of time.

[1blueheron]

Thanks guys, that's a huge help and glad to now I can use what I've got.

I'm going to attempt to draw up a schematic showing the power supply section to make sure I have it right in relation to everything else.

Yes you can use a 5U4GB just pick off the bias voltage from one leg of the AC input and then adjust your range resistor for the right amount of voltage to power your bias circuit. The circuit would be the same as the above picture but you will be using a tube rectifier instead of two diodes..  I have used a 5U4GB in my Baldwin organ iron powered 5F6A guitar amp.  But after using a the big choke from the organ at the input of the power supply instead of a capacitor as the first filter, the voltage was a little lower than I wanted so I switched to a 5AR4 to bring the voltage up.. 

Mresistor,
What were the numbers on your Baldwin choke?  Wondering if mine is the same as yours?  That's the next question is how to tell what the value of the choke is.

[mresistor]

The Orgasonic choke part # is 512-13926B    it is an 8 watt 30 ohm choke.. according to the schematic..   it is larger than the output transformer and if I remember right it is around 7 lbs..


If you don't have specialized equipment it would be hard to determine the henry rating..   it dropped quite a bit of voltage,, like around 250 volts if I remember..  and I think the insulation
on the choke is good enough to handle the hv off the PT which in my case was very high..   it is still working just fine.. 

[1blueheron]

Here is what I've com up with.  After looking at horde's of Bassman schematics, I think the AA165 is the one I like best so this is based off of it.  I have substituted a 12AX7 for the 12AT7 so I would guess I need to make some adjustments and don't know exactly how to do that and I made a best effort at adapting the power supply and bias circuit.

I may have missed a few things, its still a work in progress but I am putting it up to find out if I am on the right path or way out in left field.

I did find a little bit of info on my choke based on part number.  I put it on the schematic and noted.

Besides how to derive proper voltages I have a couple questions.

1.  Why did fender not use the second set of pins first in the V1 and V2 positions?  Most amps I see do this and i read it is because the second set typically is lower noise?  Is this true or wives tale?

2.  Regarding grid stopper.  I should use 34k if it is only one input and 2- 68k if it is split input correct?  Hoffman scheme shows a single 34K.

3. 525V caps are almost non existent or very expensive.  Is 500V in a modern cap sufficient or do I really need a 600V rated cap for supply filter?  Should I use double 350's at twice the uF?

[ac427v]

My Princeton Reverb clone used that bias circuit. Worked great but I found I had to use a 100k resistor (not 220k) to get enough voltage from the power transformer secondary tap.If the amp will be for bass and you want really clean power, the stock Bassman 135 phase inverter with 47k plate resistors and a 12AT7 tube is hard to beat. Most Fender circuits use a 100 ohm (not 100k ohm) for the bottom negative feedback resistor if the speaker load is 4 ohms. 47 ohm if the load is 8 ohms.--Craig

[PRR]

> I have substituted a 12AX7 for the 12AT7 so I would guess I need to make some adjustments

For one: either AT or AX, the shared cathode resistor is 470 Ohms, not 470K.

After that.... why would you deviate from Fender's tube choice?? And what do you think needs changing?

[1blueheron]

> I have substituted a 12AX7 for the 12AT7 so I would guess I need to make some adjustments

For one: either AT or AX, the shared cathode resistor is 470 Ohms, not 470K.

After that.... why would you deviate from Fender's tube choice?? And what do you think needs changing?

I will correct my 470 cathode resistor error pronto. Thanks for pointing that out.

My thought pattern on deviating from Fenders tube choice is Fender deviated from his own tube choices as he progressed and as tube improvements became available, and as tone demands changed. The 6G6 version used all 7025's.  The 5F6 used it as the PI, as I am doing, so it seems like it fits?  Marshall changed them all to AX7's (not equating myself to Jim Marshall or Leo Fender)  with great success.

My primary motivation however, would be, 12AX7 and 12AU7 are what I have available so if I can make them work, why not use them?  I am not trying to replicate every tonal detail of a particular bassman, (if it were even possible) and I am fine with something a little "off" or different as long as it works ok. I am willing to sacrifice a little clean headroom.  Perhaps this is a rookie mistake but I thought why not try.

As to what needs changed, I am not sure but it seems like I must have more issues than just the 470K resistor problem.

[sluckey]

I am not sure but it seems like I must have more issues than just the 470K resistor problem.

Start at that 470K (should be 470Ω). Go west until you come to a dead end. Turn south and go past one block. You'll come to a 100K in the middle of the road. Change that to 100Ω if your speaker load will be 2 or 4 ohms. Change to 47Ω if your speaker load will be 8 ohms.

[Willabe]

My thought pattern on deviating from Fenders tube choice is Fender deviated from his own tube choices as he progressed and as tube improvements became available, and as tone demands changed. The 6G6 version used all 7025's.  The 5F6 used it as the PI, as I am doing, so it seems like it fits?  Marshall changed them all to AX7's (not equating myself to Jim Marshall or Leo Fender)  with great success.

By the time Leo got to the AA165 Bassman, he was using a 12AT7 for the PI. Once the miniature 9 pin tubes came out, he went with the 12AX7 for the PI at 1st, then went to the 12AT7, no deviating. The 7025 is just a low noise version of 12AX7. A 12AT7 will give more current and a little less voltage gain than a 12AX7. And Leo didn't care about 'tone demands changing'. Leo didn't like R&R he loved Country & Western music. He was always trying to get a cleaner sound. Marshall used a 12AX7 for his PI because he copied and stayed with Leo's tweed Bassman circuit.     

All the amps you list all had different plate voltage on the power tubes and the power tubes were loaded differently, OT's primary impedance. 

My primary motivation however, would be, 12AX7 and 12AU7 are what I have available so if I can make them work, why not use them?

You can use use either of those tubes, you might like their sound in that amp and you might like a 12AT7 better.   

[1blueheron]

OK,

Here's what I think is a complete schematic. I have corrected the values mentioned by Sluckey and PRR (thanks guys).  The numbers in red are things I am unsure of due to unverified, unloaded,  rectified B+ at this point and due to the possibility of changing the V4 12AT7 to a 12AX7, 12AU7 or perhaps a 12AY7 or 12AV7. 

Questions I have at this point are the voltage value of the Bias circuit caps, and what voltage I should expect out of the PT after tapping off for the bias and rectification. 

I read the Fender 4H choke drops voltage by approx 6V so I anticipate my 5.6H will be in the 8-10V ballpark?

Using charts/formulas in other threads  I am estimating the B+ will be between 425V-450V

(unloaded) 372V*1.2= 446.4V - Loss for Bias tap?

6L6GC has a design target of 450 plate 400 screen. 500V Max.  Don't see any danger of hitting that. 

Am I pretty close to target of 425V? 

Is 1W on the 6L6 470 ohm screen resistors sufficient or should they be 3W as I have seen in some other schematics?

[Willabe]

Did you decide to build this Bassman because the bass channel has 3 gain stages?

If so and you plan on playing guitar through the bass channel you might want to change the tone stack because that preamp is voiced for bass. I'd get rid of the deep switch. and depending on how much mid dip/scoop or no mid dip/scoop you want, add a mid range pot, 25K, 50K. You can strip out some boomy-ness by lowering the value of the K bypass caps. Nothing larger than 10uF, guys like to use 4.7uF, 2uF, and even smaller, down to 0.68uF. The more you crank up the amp, the more low end you probably will need to strip out so the distortion isn't muddy.

The plate R bypass caps strip out high end chime/sparkle. You need high end for note definition. I would take them all out.

And maybe change some of the gain taming circuitry in between the 2nd and 3rd gain stages.

And all 3 gain stages are set up with the same plate and K R's. I would set up at least 1 gain stage differently to get a different set of harmonics out of it.

You can try using 220K plate R, 2K7 K R, 5uf or 2.2uF K bypass cap, and 0.0047 for the coupling cap coming off that stage.

You might want to read this if you haven't yet. It's Rob Robinet's web site. A tone of great info in there on modding amps and much more.

https://robrobinette.com/How_Amps_Work.htm
 
 

[sluckey]

Using charts/formulas in other threads  I am estimating the B+ will be between 425V-450V

(unloaded) 372V*1.2= 446.4V - Loss for Bias tap?

I'm betting your B+ will be closer to 500V. Unloaded will be 370*1.4=520. Where did you get 1.2? There will be no loss for bias tap.

Speaking of bias and since you will be building an AA165, why not just use the bias circuit for the AA165, except, connect the range resistor to pin 4 or pin 6 of the rectifier tube and change the resistor to something more like 100K to 220K. Determine the exact value experimentally.

[1blueheron]

Using charts/formulas in other threads  I am estimating the B+ will be between 425V-450V

(unloaded) 372V*1.2= 446.4V - Loss for Bias tap?

I'm betting your B+ will be closer to 500V. Unloaded will be 370*1.4=520. Where did you get 1.2? There will be no loss for bias tap.

Speaking of bias and since you will be building an AA165, why not just use the bias circuit for the AA165, except, connect the range resistor to pin 4 or pin 6 of the rectifier tube and change the resistor to something more like 100K to 220K. Determine the exact value experimentally.

1.2 was derived from this table...  I thought 1.4 was the factor for SS rectification?

https://el34world.com/Forum/index.php?action=dlattach;topic=20455.0;attach=58985

About the Bias circuit... If the AA165 bias circuit will work with my trannie, I'm good with using it.  I wasn't sure it was compatible.  I guess you are saying the range resistor makes it compatible

Did you decide to build this Bassman because the bass channel has 3 gain stages?

If so and you plan on playing guitar through the bass channel you might want to change the tone stack because that preamp is voiced for bass. I'd get rid of the deep switch. and depending on how much mid dip/scoop or no mid dip/scoop you want, add a mid range pot, 25K, 50K. You can strip out some boomy-ness by lowering the value of the K bypass caps. Nothing larger than 10uF, guys like to use 4.7uF, 2uF, and even smaller, down to 0.68uF. The more you crank up the amp, the more low end you probably will need to strip out so the distortion isn't muddy.

The plate R bypass caps strip out high end chime/sparkle. You need high end for note definition. I would take them all out.

And maybe change some of the gain taming circuitry in between the 2nd and 3rd gain stages.

And all 3 gain stages are set up with the same plate and K R's. I would set up at least 1 gain stage differently to get a different set of harmonics out of it.

You can try using 220K plate R, 2K7 K R, 5uf or 2.2uF K bypass cap, and 0.0047 for the coupling cap coming off that stage.

You might want to read this if you haven't yet. It's Rob Robinet's web site. A tone of great info in there on modding amps and much more.

https://robrobinette.com/How_Amps_Work.htm
 
 

Willabe,

Thanks for the tips.

I have a couple things in mind with the AA165.  It is being built for the purpose as being the stage amp for a very small church.  It's primary purpose right now will be for electric Bass amplification.  As the band grows, it will also likely become used by guitar on the second input.  The Bass input will stay as bass, however, I do have my eye on the unused second half of V3 as additional gain stage for lead.  Probably won't do it right away but would like to leave room for migration to it in the future.  Probably relay controlled on a foot switch to go between lead and rythym.

I will take a look at the mods in your drawing.  I may add presence control to the schematic and I have seen some MV mods that look interesting.  The Bassman seems to be a great platform for about anything I can imagine from a Tweed to a Marshall and it seems like the iron I have is pretty well matched up with this duty.  I like the wide gene pool it gives me to grow from.

[sluckey]

1.2 was derived from this table...  I thought 1.4 was the factor for SS rectification?

1.4 (really 1.414) is the conversion constant for converting RMS voltage to peak voltage. All rectifiers (ss and tubes) operate on peak voltage and will pass 1.414*V_RMS to the filter cap. If there is no load the cap will charge to 1.414*V_RMS. Now if you connect a load to the cap and current begins to flow, different tubes will have a different voltage drop, but you cannot just say that voltage will be 25V or 60V. The voltage drop for a 5Y3 is not a constant 60V. The voltage drop depends on the amount of current flowing through the 5Y3. The actual voltage drop may only be 25V, or 43V. Different amps will have different load currents and will have different voltage drops across the tube. Same principle applies to all the other tubes in that chart. The only way to determine the voltage drop for a particular rectifier tube is to know the plate voltage and the load current, then look at the load lines for that tube. Of course you can always measure the voltage drop ***AFTER*** the amp is built. The only thing useful about the "voltage drop" column in that table is the relative voltage drop for different tubes plugged into the same amp. IE, in your amp a 5U4 will have a certain voltage drop and a 5AR4 will have a lower voltage drop, and a 5Y3 will have a higher voltage drop.

About the Bias circuit... If the AA165 bias circuit will work with my trannie, I'm good with using it.  I wasn't sure it was compatible.  I guess you are saying the range resistor makes it compatible

Correct. The original circuit used a source of ≈50vac to feed the bias circuit, so a low value range resistor is all you need. But, your PT doesn't have a 50vac source so you use the 370vac source, which will require a much larger value range resistor to give you the same amount of negative bias voltage.

[1blueheron]

Sluckey,

Is this what you were talking about?

[sluckey]

Yes. You may want to make a note on the schematic calling that 100K the "Bias Range" resistor. Also, that filter cap should be 47µF rated for at least 100V.

[1blueheron]

Well, I guess I have gone almost as far as I can go on paper. 

I changed the B+ to the mathematically derived output but I would like to verify it in real life and play with dropping resistors to fine tune it. 

Can I just power up the PT with the 5U4 and start building the B+ rails tweeking as I go, or should I build everything else and do that last with loads?

I need to keep the B+ to the 6L6 under 500V correct to avoid becoming a gaffer.

[sluckey]

Build the whole amp before you start trying to tweak B+ values.

[1blueheron]

Working on the layout.  Here is a general concept I have in mind. Reading the post on layout/tube spacing.

Chassis is a bucket out of an organ.  19-1/2" L X 9" W X 2-1/4"  pretty clean canvas to work on.

Have all the tubes except 12AT7.  Have power cord, power switch, lamp holder, most of the smaller value capacitors, many of the resistors, some pots.

Need to order filter caps, shorting input jacks cord retainer, maybe a few other misc. parts. not much.

[1blueheron]

Proposed layout...