5F6 Bassman with 6V6 tubes

I want to build a 5F6 Bassman, but I would like to have lower power. Some people suggest using 6V6s instead of the 6L6 tubes.

From what I understand, the main issues would be:
1) the bias - this shouldn't be a problem.
2) dropping B+ voltages, because 6V6 won't survive 5F6 voltages for long.  Increasing dropping resistors or maybe additional RC stages? Some people suggest using 5Y3 rectifier tube, which has bigger voltage drop.
3) issue with the output transformer not having the right impedance ratio's, which would also be not healthy for the tubes. But I think this could be resolved by using different impedance speakers.
Correct me if I'm wrong here: 6L6s needs 4000 ohm on the primary, 6V6 about 8000 for a certain output impedance. Then I use a Hammond 1760K which has 2, 4 and 8 Ohm. So I use 4 ohm winding for a 8 ohm speaker, then the impedance on the primary should be around 8000 ohm also.

Anything else to consider?

Btw - check out the tweed amps I made so far. (champ head, champ 8 inch combo, 12 inch speaker cab with Jensen Blackbird, and a Deluxe head ). Exposition in Netherlands. I also made my own cabs.

I know you are going to get a lot of input on this one.

from what I read of your post it's "how can I adapt a power section designed for PP 6L6 for pp 6V6?".
if that's the question, the answer is "don't!"

simply use a power section from the many examples of Fender's 6V6 amps and proceed from there.

Deluxe reverb power section with a choke and presence for example and the pre and eq from the Bassman.

just my $0.02.

best wishes!

Andy

P.S.  CJ is now winding an 8k-8ohm OT ;-)

I need to drop it in something, if looks tell the story, it is a sweet item  ;D

P.S. 2. those amps look super duper sweet! great job on the cab

andyfromdenver said:

from what I read of your post it's "how can I adapt a power section designed for PP 6L6 for pp 6V6?".
if that's the question, the answer is "don't!"

simply use a power section from the many examples of Fender's 6V6 amps and proceed from there.

Deluxe reverb power section with a choke and presence for example and the pre and eq from the Bassman.

Click to expand...

I'm not sure if it's even doable, but it would be cool to make this change reversible back to the 6L6. So I would prefer modding the original power section, if possible.
Bias can be easily adjusted, if I'm correct about OT, then you could use the same OT for both 6V6 and 6L6 (just use different impedance secondary) - so if no major changes could be made to the power supply, this might work.

I just came across an article by Rob Robinette.
He says you could use a 4k to 4ohm OT as a 8k to 8 ohm ('downside' having more headroom than you would expect from 6V6, because bassman OT is designer for more output).
For voltage drop he also suggests using 5Y3 rectifier.

andyfromdenver said:

P.S. 2. those amps look super duper sweet! great job on the cab

Click to expand...

Thanks - it sure takes a lot of effort to make good cabs. These are all solid pine wood, halfblind dovetail jointed btw. I also age the hardware. I will add some leather handstiched handles made out of old leather belts, but I don't have the time right now.
Now I'm tempted to build even more amps, because I can put them in any tweed cab I can imagine - the materials are not expensive, and I already have all the know how and the tools I need so why the heck not.  ;D

you could use 6V6 tubes in the 5f6 as the B+ is only 430, but wait, that was at yesterdays line voltage, which means you might have 450 on there from a 120 wall socket,

why not build a deluxe reverb? or a tweed deluxe?

in order to double your volume, you need 4 times the power,  so cutting power in half only lowers your volume by i do not know how much, 30 percent?

and the 5f6 uses a 2 ohm  OPT so watch out or that,

here is a cool link on the 5f6>

https://www.ampbooks.com/mobile/classic-circuits/bassman-summary/

you could put a cathode resistor on the 6L6 tubes and get a power reduction, and nice compression, which is cool unless you play the pedal steel,
you could also put a cathode resistor on the 6V6 tubes and run them at 430 since the resistor eats B+.

Leo Fender pushes the 6V6 tubes way past their max ratings in the RCA book, so if you were to  lower the screen voltage, you could run 6V6 tubes all day on 430,

look at the Music Man, 750 volts on EL34's!  but only 350 on the screens,

Anthon said:

3) issue with the output transformer not having the right impedance ratio's, which would also be not healthy for the tubes. But I think this could be resolved by using different impedance speakers.

Click to expand...

The problem in doing this is that the primary inductance for 4000 ohms is half of what it needs to be for 8000. The result is that the LF response will not be as good as expected.

Anthon said:

I'm not sure if it's even doable, but it would be cool to make this change reversible back to the 6L6. So I would prefer modding the original power section, if possible.
Bias can be easily adjusted, if I'm correct about OT, then you could use the same OT for both 6V6 and 6L6 (just use different impedance secondary) - so if no major changes could be made to the power supply, this might work.

I just came across an article by Rob Robinette.
He says you could use a 4k to 4ohm OT as a 8k to 8 ohm ('downside' having more headroom than you would expect from 6V6, because bassman OT is designer for more output).
For voltage drop he also suggests using 5Y3 rectifier.

Thanks - it sure takes a lot of effort to make good cabs. These are all solid pine wood, halfblind dovetail jointed btw. I also age the hardware. I will add some leather handstiched handles made out of old leather belts, but I don't have the time right now.
Now I'm tempted to build even more amps, because I can put them in any tweed cab I can imagine - the materials are not expensive, and I already have all the know how and the tools I need so why the heck not.  ;D

Click to expand...

ok, gotcha.
Well, without going into too many details, I am just not a fan of pwr tube convertible amps. You can make it come true i'm sure . sounds like you have sorted out much of the logistics.

I have just wanted to do a 6v6 Bassman for a while, it's the thing Fender has yet to do. your post excited me .

I concocted a wiring diagram a while back for a way to do a 6L6-6V6 tweed champ, but with NO compromises. it involves some switching to change the cathode bias, a large and small output transformer, and negfb alteration. it was loco, and I lost interest in pursuing it.

I have to believe that beyond the need to vari your bias, the end result of a single OT for 6L6 and 6V6 may not scale in an awesome way. meaning it might sound mighty when cranking in 6L6 mode, but not as mighty and obv. quieter when in 6V6 mode. I hope I am wrong!

best wishes!
Andy

6V6 amps always sound better than 6L6 amps, power comes at a price,

you can get 6AQ5 tubes for cheap, always wanted to wire up 8 of those  to see what happens,

you want an interesting amp, here is a Versatone schemo we traced out a while back,  8"' and 12'' speaks with  some nice 7591 tubes>

abbey road d enfer said:

The problem in doing this is that the primary inductance for 4000 ohms is half of what it needs to be for 8000. The result is that the LF response will not be as good as expected.

Click to expand...

Quick question from a non-tube guy:

Would a quad of 6V6 work OK into that 4K primary, lower inductance considered?

Just academically curious,

Gene

Gene Pink said:

Would a quad of 6V6 work OK into that 4K primary, lower inductance considered?

Click to expand...

Yes, definitely.

here is a thread on the topic of "quad 6v6 "

both tubes have about the same trans-conductance,  the 6V6 has a bit more distortion,

price is mentioned in the thread and something to consider
a quad of 6V6 tubes would probably cost about twice as much as a matched pair of JJ 6L6 tubes.

http://www.ax84.com/bbs/dm.php?thread=412234

we could wind a trick transformer for 2-4 tube combos, some amps cut the cathodes on two tubes but then you have a mismatched OPT, Music Man lowers the B+ for lower power to avoid this but you need a transformer tap.

this would be a bi-fi pri wind to keep the Z the same when switching out two cathodes>

The only justification of using a quad of 6V6 rather than a duet of 6L6 is the difference in sound. The idea of disconnecting one pair in order to halve power is akin to the 8-6-4 system in Cadillac engines. A complete failure until practical implementation of variable displacement was made possible by the convergence of several engineering progress (in computing and hydraulics).
Just the same, practical implementation of variable power in tube amps requires somewhat more than simply pulling tubes. It takes at least adequate impedance switching. The technology is there, but the mindset is not.

A way of connecting multiple active devices is to have multiple voltage rails, connect devices in series, and arrange for the inner ones to draw power from low voltage rails whilst the outer ones draw power from the high voltage rails when needed. We used this in Head-Up Display yoke drivers, where steady current required low voltage and fast slew needed high voltage. This was done with transistors and diodes.  It would be interesting to see a valve amp version, I guess the multiple voltage rails could be replaced by different transformer ratios. Quad did something similar with beefy transistors driving at low frequency and wimpy ones handling high frequencies. Apologies for thread drift. 

mike-wsm said:

A way of connecting multiple active devices is to have multiple voltage rails, connect devices in series, and arrange for the inner ones to draw power from low voltage rails whilst the outer ones draw power from the high voltage rails when needed. We used this in Head-Up Display yoke drivers, where steady current required low voltage and fast slew needed high voltage. This was done with transistors and diodes.  It would be interesting to see a valve amp version, I guess the multiple voltage rails could be replaced by different transformer ratios. Quad did something similar with beefy transistors driving at low frequency and wimpy ones handling high frequencies. Apologies for thread drift. 

Click to expand...

Indeed there are many ways of complicating a simple concept .
Making a switchable-power tube amp is relatively straightforward but requires a little more sophistication than just pulling a pair of tubes. Something like a tapped-primary output transformer, for example. It seems that even that is too difficult to grasp for most guitar amp designers. In fact, 95% of tube guitar amp design today is in repackaging a 5F6.

CJ said:

we could wind a trick transformer for 2-4 tube combos, some amps cut the cathodes on two tubes but then you have a mismatched OPT, Music Man lowers the B+ for lower power to avoid this but you need a transformer tap.

this would be a bi-fi pri wind to keep the Z the same when switching out two cathodes>

Click to expand...

That wouldn't work like this. Two identical windings in parallels have the same impedance as one. You need a different turn ratio.

thinking more like this>

CJ said:

thinking more like this>

Click to expand...

Whatever... being on the same core, with a high coupling coefficient (about 0.99+), two identical windings have the same impedance as one single. There's no escaping taps.