5F6A build

[Cliff Schecht]

Back about two and half years ago my friends house was broken into. Of my friends stuff the guy stole some DVD's, a DVD player, TV and (strangely enough) all of my buds new groceries. He completely missed the bedroom and my buds large stash of cash (didn't like banks, does now ). Of my stuff a '91 Ibanez RV470 MIJ super-Strat and my non-LTD Bassman RI were stolen. I was thankful yet somewhat offended that the guy walked past my ugly homemade pedalboard with close to $1000 in pedals on it (usually 12-15 pedals because that's all I built at the time ).

This single event is what got me building amps. I wanted to replace that Bassman but I couldn't afford a new one. Can't beat the clean punch this amp delivers at high volumes. Damned thing used to compete and hold its own against 6505+'s and other stupid loud metal amps.

Now I have built a 5F6A in the past, but this was a thrown together piece of junk that never really sounded or acted quite right. I tried to PTP the fucker and it really backfired badly on me. So I'm building a "new" 5F6A out of more scraps and junk amps, except this time I'm doing it "right". As you guys probably have noticed by now, right for me is an amp that sounds close to the original using whatever I have on hand. Since I have a lot on hand, this isn't usually a big deal.

The chassis is coming from my 6AQ5 Rocket which I consider a failed project, although I now believe that the underrated power transformer was the reason for the weird ghosting I couldn't ever eliminate. Pulling two power tubes might have fixed the problem. Hindsight is such a bitch! Anywho I'm recycling this Rocket project and trying to make a compact yet full featured tweed Bassman.

For the power transformer I'm using one of the $25 beater vintage jobs I bought from Dynaman a while back. It's at around 570VCT with a 12V only heater winding (albeit a big one). I'm going to wire the 6L6 heaters in series for now with the intention of checking the voltage drop across each to make sure it's within maybe 10% of each other, this way tubes don't start dying early because one side is taking all of the current and doing all of the work. If this doesn't work then I have room for a 6.3V transformer (which I would have to buy ). With wall voltage here being /reliably/ high I am thinking the winding will be closer to 610VCT which is close enough with a modern 5AR4 rectifier to hit the target B+. If not I'll throw in some SS diodes and a Zener CT lift and call it a day for now.

The output transformer is going to be a Stancor A-3851. 4.2k primary to 4, 8, 16 Ohm secondary is perfect IMO for vintagey Fender stuff. I don't like using wimpy iron, especially when the amp is meant to PUNCH, and old hi-fi iron seems to punch well.

I'll try to post some pics of the more interesting parts of the amp soon. The one thing I'm worried about is my clearance between one of the 6L6's, the 5AR4 and the transformers are terrible. I can't really scoot anything over and so if the rectifier starts overheating, once again I'll throw in some SS diodes and call it a day.

Does an 8H choke sound good? It's a Stancor C1709 8H, 85mA 250 Ohm and probably the most appropriate choke I have. My only other option is a 16H, 50mA 580Ohm job that I think would drop too much voltage under heavy loading (from internal resistance, not losing inductance). What should I be expecting for screen current under heavy operating conditions?

Also has anybody heard of these mods: http://img94.imageshack.us/img94/6716/5 ... ricked.jpg

A guy that owns a few real 5F6A's says that his amps all have this little "tweaks" (minus the 10uF cap and 100 Ohm heater resistors, that is) from the factory. I'm going to incorporate these small changes and see if they make much of a difference.

Damn, this post got long quickly .

[Structo]

Do you ever have problems resulting from no grid stoppers on the power tubes?

When you say these amps are punchy, does that mean they don't sag much or compress the tone much when you dig in?

[martin manning]

Cliff, there was some discussion a while back about the PI tail and tone stack values. I had a number of photos from the web of real '59's that confirmed the values as shown on the '59 reissue, not the ones on the old Fender schematic and layout. Re the presence shunt, I don't remember what the photos of vintage amps showed, but there is an alternate network on the reissue schematic that does something similar using a 25k pot. This must be to kill the scratchy noise, and would seem to be more true to the original than the Weber scheme.

[Cliff Schecht]

Do you ever have problems resulting from no grid stoppers on the power tubes?

When you say these amps are punchy, does that mean they don't sag much or compress the tone much when you dig in?

Not usually. The reissue amps have like 15 Ohm grid stoppers which is nearly pointless, although something is better than nothing. I'll leave them off and check for oscillations on the scope, if I need them I'll add them. It does open up the sound a bit when you leave the grid stoppers off.

When I say they are punchy, I'm talking about the amp as designed. 12AY7's where they belong and the 10" speakers are part of the equation too, throwing 12AX7's in the preamp makes the amps distort too easily (and muddy up IMO). The big choke filtered screens/preamp with 60 uF of capacitance attached to it, big power transformer, low output impedance output transformer (primary and secondary) and cathode follower also help here.

[The New Steve H]

I can't believe my 3/8"-thick aluminum channel chassis hasn't caught on like wildfire.

[Cliff Schecht]

I'm tired of trying to drill fat aluminum and steel from home. It's messy and hard to get clean holes with hand tools. I'll take my Hammond and Bud thin aluminum chassis any day Mr. Steve.

[The New Steve H]

Okay, but if you ever need to use your chassis as a prop to shore up a mine ceiling, you'll be sorry.

[Richie]

i've worked on quite a few old fender tweed amps. And i have seen the 6.8k tail in some of those amps. Those amps also sported the different value tone caps,more like what you'd see in a normal fender amp.
Trying to think back but i believe they were either .047 and .022 or .1 and .022.I'd have to dig up the pics i have of those amps to be sure. But i do know they were different and not both being .022's.

I'd also go with the 5k presence pot over the 25k style.
It has a different effect on the amp too.
As for the sound, the cab,and good 10" speakers mounted on the thinner baffle. This makes up alot of the type tone the amp has. And yes the real ones used the 12AY7 which i believe is also the 6072 industrial version.
A bump up might be to use some good 5751's.

That amp is what soooooo many are based on or started from. Its just hard to beat.
Also if you use a 4x10 combo cab,wire the speaker phase so the speakers are moving inward. Most all fender combos the speakers move inward.
The old ones i have had here i checked them all,and thats the way they wired them.
Even the later twins that used the JBLs were also wired like that. You can wire them any way you want,and it does have an effect on the tone or sound,but the old ones were wired like what i described
Sealed back cabs you want the phase so the speakers move outward.

[renshen1957]

I'll try to post some pics of the more interesting parts of the amp soon. The one thing I'm worried about is my clearance between one of the 6L6's, the 5AR4 and the transformers are terrible. I can't really scoot anything over and so if the rectifier starts overheating, once again I'll throw in some SS diodes and call it a day.


Also has anybody heard of these mods: http://img94.imageshack.us/img94/6716/5 ... ricked.jpg

A guy that owns a few real 5F6A's says that his amps all have this little "tweaks" (minus the 10uF cap and 100 Ohm heater resistors, that is) from the factory. I'm going to incorporate these small changes and see if they make much of a difference.

i've worked on quite a few old fender tweed amps. And i have seen the 6.8k tail in some of those amps. Those amps also sported the different value tone caps,more like what you'd see in a normal fender amp.
Trying to think back but i believe they were either .047 and .022 or .1 and .022.I'd have to dig up the pics i have of those amps to be sure. But i do know they were different and not both being .022's.

Hi Cliff,

I have seen the insides of quite a few vintage Bassman 5F6-A (5E5-A, 5F6) , worked on one, and used to to collect circuit board pictures of vintage Tweed Bassman every time one was on Fleecebay.com.

Fender would somtimes substitute parts (with shortages) and the Bassman line also has had a history of being a testbed amp. Whether this was out of necessity (parts substitution) or intent for R&D, I won't venture to guess.

I have seen a 10K 2 Watt CC resistor (normally found in the power supply) substituted in place of the 1/2watt 10K. Did the stock room or the assembler run out of the smaller watt resistor? I recall at least one 6k8 (by the resistor codes, noticed it was different).

The 6k8 tail resistor became standard on the 6G6 and 6G6-A Bassmans however as this is roughly half of the 10k, the cathode resistor adjacent to it which was normally 470 ohms in the Tweed was bumped up to 820.

As to the link on Weber's Tweed Bassman schematic:

The only reason to have two 100 ohm resistors in my book is to form a false center tap on filament heaters which don't have a center tap on the PT.

The original Bassman had 5K pot on the presence (negative feed back), switched 25k on 6G6 (and Marshall used the 25k value on the Plexi). I prefer the 5K. The addition of the 4k7 resistor, I dont see the point.

I have seen tonestack values, which appear to be original that were the Blackface 0.1 and 0.047 uf in one vintage Bassman, I can't recall if the 56k resistor had its original value or was 100k. With all do respect, I prefer the original Tweed values.

The 5F6 (5E6-A, too) had 1k5 gridstoppers on the power tubes which disappeared on the 5F6-A and reappeared later. I've included these in builds, and no-one seemed complain about the addition. I omitted the 47 pf cap at PI since most individuals play guitars through bassmans and not basses.

I personally use 1k 5W resistors on the screens (yes, I know heresy) in place of the 470 ohm in the original. Then, again the circuit I build has more amps in the 6.3 heaters so one can substitute most any octal tube. Of course, with Marsh, Kendrick, Clark, Victoria, etc. building excellent Bassman copies, not much point of trying to enter the "who has the ultimate copy of a Bassman contest." Besides who wants to compete with the ghost of Leo Fender? I have yet to hear of someone proclaiming a clone superior to a half century original in a shoot-out.

Dave Funk's book, "bomb proofing" your rectifier tube by adding two SS rectifiers prior to the tube. I always post about these as it saved me an OT. In a new build (a Tweed based amp) I included them for the first time to see if there was any audible difference in the tone. After an audio test, I removed the 1UF4007 diodes, and replaced the 5Y3 tube, and the fuse blew and did its job (thankfully). I had forgotten to test the bargain "tested tube" rectifier I purchased off of ebay for $5.00. A quick test on my Eico 666 Tube Tester showed that one side of the tube was shorted. Too late to leave negative feedback, however the amp was undamaged and had worked perferctly fine for an hour without my being the wiser.

The mass market Reissue was the subject of many rants (not mine, yet) over circuit values, over using the wrong rectifier tube, cruddy transformers, the cabinent size was not the same as the original, the speakers weren't close to the original Jensens, etc, an easy target.

The first reissue had a reputation of being too bright and harsh (the presence on 1 was compared to 11 on the Tweeds) which I believe Fender may have eventually improved on over time. One contemporary amp book had instructions on how to mod a reissue to sound like an original. I think this would have been solved by the most part by using 12AY7 in the first preamp tube.

Marshall made a similar mistake with the Bluesbreaker reissue, so I guess both Fender and Marshall don't give a rat's arse about getting it right, other than spelling the Name correctly and putting a product on the market.

Both companies were trying to capitalize on what smaller builders had proven to be a best-sellers.

Have fun with your build (envious). I wish I wasn't in a manufacture phase with my business so I could get back to building amps.

Best Regards,

Steve

[Bob-I]

In the 60s a friend of mine's older brother, then a bassist in a bar band, had a 5F6-A. Their puppy decided Jensen speaker cones were tasty and ate them. Seeing ts as an opportunity to convince his wife he needed up upgrade to q BF bassman, he sold me the 5F6-A for $50 (a fortune to a 13 year old). I replaced the speakers one at a time, $8.00 each.

I used that amp on and off for maybe 15 years. It had a great grind at high volumes and punchy clean when you backed off the volume a tad.

One night after a gig upstairs above a firehouse, a helper decided to carry it down the fire escape, holding it out over the railing. The 20 year old leather handle choose that minute to give up. We picked up the rubble only to avoid being arrested for littering.

I rebuilt the chassis into a head cab and used it with a 2x12 cab from a bassman but with jbl d120s.

One night in at a bar gig a thunder storm hit. We decided to take a break and wait it out. Just as we left the stage lightning hit and blew the PT on my 5F6A, and the bass players SVT also went up in smoke, the Rhodes piano blew a fuse. We were left with just the PA, porta-b and Leslie. We put the bass and guitar into the PA and finished the gig.

I gave up on that amp, just bad vibes coming from that one.

Today I have a clone in a 1x12 combo I built several years ago. This is simply a great circuit.

Thats my 5F6-A story.

[Structo]

Wow that amp lived through a lot Bob.

I played through an old tweed bassman before, not sure of the circuit but it had that grind you hear on recordings.
Really sounded nice.

[rp]

Thats my 5F6-A story.

A well told tale. I enjoyed that story.

[Cliff Schecht]

Almost done.. Kinda cramped in this chassis but looks good so far.

[Colossal]

Nice build Cliff. What flavor are those brown drops? Polyester? Polypropylene?

[David Root]

Look like Difilms to me (mylar polyester/paper dielectric).