5F6-A decopling and main filter stage

[Bub]

Hi Gent's,

I'm wondering what the 22uf 450V on pin 6 does and is it worth putting in.
Also on the first filter stage there is a .1uf 630V cap. What does it do and is it worth putting in.

Thanks for any help

Rob
  

[sluckey]

I'm wondering what the 22uf 450V on pin 6 does and is it worth putting in.

It's not on pin 6. That cap is just another filter cap on the B+ rail. You must have this cap.

Also on the first filter stage there is a .1uf 630V cap. What does it do and is it worth putting in.

That .1µF is a high freq filter. It's there because it responds to high frequencies better than the big electrolytic that it parallels. The original amp used a .05µF. It's worth it IMO.

[VMS]

Chapter 2 explains the use of these caps. From page 2-8 to 2-11.

[Bub]

Thanks Gentlemen,

I guess I'm stuck between building the KOC Bassman or the Hoffman 5F6-A. I've read a lot of posts and motor boarding doesn't seem to be an issue with the stock build, but I am building a platform to play with so maybe I'll throw some extra turrets in to build both.

Thanks

Rob

[FYL]

I guess I'm stuck between building the KOC Bassman or the Hoffman 5F6-A. I've read a lot of posts and motor boarding doesn't seem to be an issue with the stock build, but I am building a platform to play with so maybe I'll throw some extra turrets in to build both.

KOC uses distributed filtering nodes positioned as close as possible to the plates. This is far better than the Fender doghouse approach.

Doug's layo can be very easily modified for distributed filtering. You'll find below a quick and dirty partial scan of one of my templates based on Doug's 5F6A design, with distributed preamp and PI filtering nodes.

[phsyconoodler]

Why not just build the 5F6A bassman the way Fender did it?I've done a few and they all work just fine.Tone out the wazzo and quiet with the right lead dress.and the board fits nicely into the LTD chassis too.
  I just happen to be converting an LTD bassman into a hand-wired one right now.I'll try and post some pictures as soon as I can.

[Bub]

Thanks for the template FYL. I did draw something similar to accomodate KOC mods. I like the idea of using a 12AX7 in V1. I have a huge supply of 12AX7s black plates, grey plates and 7025s of every make I would like to try but I would also like to try the 12AY7s which brings me to phsyconoodlers point, just build the original and be done with it. If I built the KOC version but wanted to run a 12AY7 in V1 can I just swap the cathode resistor and cap to a 820 ohm and 220uf cap?.

Thanks

Rob


 

[phsyconoodler]

The tubes bias is pretty similar.Lots of guys use 12AX7's in V1 and the LTD does too.I prefer the 12AY7 myself.
  The original bassman has 4 inputs that lets you jumper the channels for tone of gain and a nice volume interaction.Same as a JTM45 and plexi.Some nice tones are lurking in there when you jumper the channels.
  I'm adding a Lar-Mar master to this one.All that tone at any volume level.

[FYL]

I like the idea of using a 12AX7 in V1.

Then a 5F6A it won't be anymore. Using a 12AY7/6072 for V1 is a must, ditto for any tweedy amp using originally a 12AY7.

If I built the KOC version but wanted to run a 12AY7 in V1 can I just swap the cathode resistor and cap to a 820 ohm and 220uf cap?.

The original cap value is way too high, leading to flabby bass, lots of intermodulation, nasty blocking disto with humbuckers and a not effects-friendly amp.

Using a common RC network was a good idea when Leo did the bean counting, but not today: using separate RCs allow you to voice differently the two channels, say 1K5/22µ for vintage Fender sound and 1K5/680n for something JTM45-related.

[Bub]

I think I'm going to build the KOC board but make it easy to switch back to a stock 5F6-A. I want to be able to use 12ay,12at, 5751 and 12ax7s in V1.

phsyconoodler, is that a VVR master? .
If I install a switch to jump both channels am I better to jump both at jack input? The different ways I've seen to do this there are always two EQs being jumped.

Thanks for all the help

Rob

[drumslinger2000]

I'd go with the 5F6A as well.  It was a fun little project.  I used one of Doug's chassis and it worked out great.

[FYL]

I think I'm going to build the KOC board but make it easy to switch back to a stock 5F6-A.

Doug's version and derivatives are IMO superior when it comes to noise.

KOC specs generic Hammond iron, a non-no in my book when it comes to building a proper 5F6.

The spec'ed 1650K OT is an UL hifi model with a 3K4 pri. You should use a 4K 1760K if you want to stick to Hammond, or get the excellent 018343 from Doug.

Ditto for the PT : KOC specs a 272JX, which has no bias tap and supplies a slightly too low loaded B+ (300 V) and too high heater voltages (6V3 is sized for 8A, you need only 4 A; 5V is spec'ed at 4 A, you only need 2 A with a GZ34/5AR4 or 3 A with a 5U4G if you want slightly more sag). Use a 291DX, Doug's 022798 or any other quality PT putting out around 330 V @ 200 mA, with 6V3 @ 4 A and 5 V @ 3 A.

And use a proper low Z bias subsystem, which is more reliable and easier to set up.

I want to be able to use 12ay,12at, 5751 and 12ax7s in V1.

No need for mods: just plug and play. OK, working points won't be optimal with all tubes, but they'll be fine.

phsyconoodler, is that a VVR master?

The Larmar is a PPIMV derived from KF's type II. It replaces the bias mixing resistors with a dual pot *and* two safety resistors in //. You should pick pot and R values in order to obtain a resulting 220K value (pot and R in //). It's a good option, quite transparent, but plays havoc with the NFB loop and the associated presence control. It's also a royal pain in the neck if you want to add it to a KOC dual bias system...



From the Metroamp web site.

If I install a switch to jump both channels am I better to jump both at jack input?

A short patch cord is all you need, or a switch with suitable attenuation.

[Fresh_Start]

It's easy to modify a Hoffman layout to conform to O'Connor's "galactic ground" system.  In fact, Hoffman's grounding scheme already is darn close to one IMHO.  I've had success adding a board-mounted ground bus to a Hoffman layout, allowing placement of the filter caps along the edge of the board.  This Super Reverb layout worked well:



It's hard to see there, but the orange/gold line just below the board is the ground bus.  Mounted on either end in eyelets.  Green leads are grounds - that'll point you to the bus too.  The 3 big caps on the right-hand end of the board are for the power amp.  Notice that they are on a separate, short ground bus.  That's Hoffman's grounding scheme, and it works.

In another build, I used radial electrolytics for filtering the preamp stages.  That seemed cleaner in layout terms compared to using axial caps.  Also, I hate that messy bias board and have a much better layout for a bias board now.

I got the idea for caps along the edge of the board from this picture in Doug's "Library"



That's a Hoffman Bassman build BTW.

Cheers,

Chip

[Bub]

The 272JX and 1650K I'm able to get for free so I have to check the 1760K and 291DX and see what my luck turns up. If I have to buy them I will.
I'm going to use radial caps for all supply node filtering just for ease of installation. As you said its easy to mount right on the board.
As far as the ground buss goes I will be running a gound buss down that side of the board but I'm not sure if I should keep V1, V2 and EQ grounds separate from PI and power tube grounds, but then it wouldn't be a galactic ground. I've got Doug's 5F6-A board on order with a blank that I'll add a few extra turrets to fit the mods in. I'm building this amp as my learning plattform so I can play with a bunch of mods, so if I don't go to exact 5F6-A specs thats ok. I was able to get a scope for free so I eventually want to be able to hook it up and understand sine wave patterns.  

Thanks for all the info and help

Rob
 

[JayB]

I'm wondering what the 22uf 450V on pin 6 does and is it worth putting in.

It's not on pin 6. That cap is just another filter cap on the B+ rail. You must have this cap.

Also on the first filter stage there is a .1uf 630V cap. What does it do and is it worth putting in.

That .1µF is a high freq filter. It's there because it responds to high frequencies better than the big electrolytic that it parallels. The original amp used a .05µF. It's worth it IMO.

Does paralleling a metal film cap across an electro really make a difference? Honestly I haven't tried it yet. I understand why it would work in theory but never gave it a shot yet.

[FYL]

Does paralleling a metal film cap across an electro really make a difference?

It does: HF impedance is lowered. Does it matter in MI amps? AFAICT, no. Just use good quality 'lytics or go for full film caps - still very bulky but quite affordable: high quality metalized poly caps from SCR cost less nowadays than their Sprague Atom equivalents!