5F6A power section mods

[electrical]

I'm building a 5F6A and want to make it as spongey, saggy, warm, fat, and round as possible. I don't care about definition or higher wattage output on this amp build, I want violin-like sustain at lower volume. I have a 300-0-300 PT to get the amp singing at lower volumes and a 35W 6.6k OT.

My first thought is to graft the power section of the 5E5 onto the 5F6A. I am attaching a rough schematic of the modified power section between the LTP and the OT.

I decided to make the following tweaks:

-remove NFB
-cathode bias
-5U4 rectifier tube (rectifier tap is rated for 5V 3A)
-Add a treble cut to the power section
-add grid stoppers

My questions:
-If I have the tone cut, do I still need the 100 pF stability cap? The original schematic calls for 47 pF, but I only have 100 pF handy.
-Do I need the grid stoppers? I'd like to omit them if blocking distortion will help the amp sag more.
-Do I have that junction between the 220k grid leaks and the ground of the cathode resistor/bypass cap configured correctly? Is there a smarter way to wire everything connected to that junction?
-Can I omit the power section cathode bypass cap? I want the amp to be darker and saggier.
-What values should I consider an upper limit with for the screen grids? My intuition says increasing them from 470 ohms may give me more compression.
-I am concerned with a directly heated rectifier AND the cathode bypass cap that having the standby switch may cause problems for the bias of the power tubes with inrush current and getting the cathodes correctly elevated to the right voltage relative to the grids. Should I move the standby switch elsewhere (my gut says it may be best to use standby to disconnect the negative terminal of the cathode bypass cap)? Would moving to a GZ34 and/or removing the cathode bypass cap help?

[passaloutre]

I have a cathode biased Bassman-ish amp and I’ve tried some of these things.

Mine doesn’t have the pf cap across the phase inverter, so I can confirm you don’t need that.

You can remove the cathode bypass cap, but I don’t find it changes the sound much. The push pull nature of the power section keeps the cathode pretty stable. You can remove this effect by giving each tube its own cathode resistor (without bypass caps).

The Marshall JTM45 has 1k screen resistors, and I’ve tried 2k with success.

Can’t help with the rectifier. Mine has diodes and a sag resistor.

Switching from a choke to a resistor in your power rail may give you more sag as well. Like the 5E5 and 5E3.

[mresistor]

The 5F6A used 5881 tubes IIRC.  I'd use them however I have also used EL34's in a 5F6A and they sounded pretty sweet.  Consider changing the standby to cathode interruption or just ditch it altogether and turn the vol down when on break.

[pdf64]

Why do you need standby?
GZ34 is the best rectifier. If you need more sag, add a sag resistor.Most 5F6A mains transformers have an inadequate degree of protecting resistance anyway, so additional resistance is good practice.
To get most HT sag, you need to maximise the change between HT current draw at idle and at full output.
Hence cool idle fixed bias.
Low value screen grid resistors to maximise peak anode current.

[electrical]

Switching from a choke to a resistor in your power rail may give you more sag as well. Like the 5E5 and 5E3.

This is curious to me, can you explain why? My guess is that an inductive load would filter better, but concurrently a higher resistance might choke off current flow when the circuit demands more current...

Consider changing the standby to cathode interruption or just ditch it altogether and turn the vol down when on break.

To clarify, do you mean putting the standby switch between the bottom of the cathode resistor and ground? That seems like a logical switch to me. I also don't mind ditching the standby switch.

Why do you need standby?
GZ34 is the best rectifier. If you need more sag, add a sag resistor.Most 5F6A mains transformers have an inadequate degree of protecting resistance anyway, so additional resistance is good practice.
To get most HT sag, you need to maximise the change between HT current draw at idle and at full output.
Hence cool idle fixed bias.
Low value screen grid resistors to maximise peak anode current.

I don't "need" standby. But my chassis has a hole drilled for it, and I have a DPDT switch handy. A mute switch seems like something handy to have between sets if I should decide to gig this amp locally (and less of a hassle for a sound guy than trying to find the right volume setting again).

A GZ34 will work just fine, but I have a 5U4GB on-hand and would rather use it up, and I would appreciate the larger voltage drop it provides. I guess a sag resistor (as you suggest) probably does that better.

I've read a bit on sag resistors, is it just placing a resistor after the diode to limit current draw on B+? I'm guessing that will also drop the voltage a bit, which I'm okay with as once again high wattage is not my goal. Do you have a suggested value, or is there some discussion you can point me to? I'll be searching sag resistors after this post as well, to help cut down on extra effort on your behalf. edit: thinking about this a bit more, does the sag resistor eat up some of the current available from the PT secondary? This would in theory increase the B+ current draw at idle, right?

"To get most HT sag, you need to maximise the change between HT current draw at idle and at full output." I will meditate on this for a while and think about my designs...

[sluckey]

"To get most HT sag, you need to maximise the change between HT current draw at idle and at full output." I will meditate on this for a while and think about my designs...

That statement goes hand in hand with "Hence cool idle fixed bias." Very hard to do with a cathode biased amp because they usually run wide open at idle.

[Willabe]

A mute switch seems like something handy to have between sets if I should decide to gig this amp locally (and less of a hassle for a sound guy than trying to find the right volume setting again).

No, you still turn the volume to 0 before you hit the stand by.

I gigged for a number of years in Chicago blues bars and all the guys used the stand by switch on the Fender amps.   

[Willabe]

Since you don't want the volume why not build a 5F6A with 6V6's?

[mresistor]

Since you don't want the volume why not build a 5F6A with 6V6's?

I was thinking that too.. 


Yes   disconnecting the cathode ground (interrupting the connection) to the power tubes is the idea for a standby which doesn't turn the B+ on/off.   


 

[Bieworm]

My Brutus amp is basically a 5F6A with a 6V6 power amp. I wouldn’t want that one to be louder. It is plenty loud, but I can get good OD at moderate volumes. It is more though.. there’s bias vary tremolo and AB763 reverb. Hear this thing sag even without a sag resistor. It’s diode rectified.

eature=shared

[bmccowan]

Since you don't want the volume why not build a 5F6A with 6V6's?

As I started reading this thread I thought the same thing - or even the Mini-Bassman Little Wing with 6BM8s which is a very cool lower power amp. But I also think 5881s have a certain character to their sound that is missing with 6V6s and 6BM8s, and running them at lower than typical voltage produces a nice glassy sound and lets them live long. No need for costly NOS as the reissue Tungsol 5881 sounds great.
Cool project!

[electrical]

I decided to ditch the standby switch altogether, as I naturally turn my guitar volume down to 0 when I'm not playing, it's subconscious.

6v6 Bassman is intriguing for sure. I have the 6L6s handy and want to try my hand at cathode bias, but for my next build a 5y3 and 6v6 driven by a 5f6a pre sounds like a fun time...

Edit: post-build post mortem... the amp came out great and I'm very happy with it. B+ around 325v. Robrob's bias calculator has my 6L6 pair putting out just north of 30 watts. Cathode bypass cap on the power section made a significant difference in tone imo, whether it sounds preferable is something I had to try for myself.

[jjasilli]

I think 2X  6L6 is a great option.  You could go with low voltage, say 270 VDC, for about 18W output.  Use cathode bias, a tube rectifier & choke.  This will be spongy!