5F6-A Build - opinions wanted

[5E34EVR]

Hello, first post here by an occasional lurker. I finally registered to give thanks to all that have come before for the wealth of info I have found here.

And... I am seeking some input that I have not been able to find answers for, either here or with Uncle Doug, RobRob or Lyle @ Psionic.

I am about to get into my 5th amp build based on the 5F6-A Bassman circuit.

My first amp was a 5E3 kit from Boothill (thanks Dave... good starter kit - I went back over this recently and upgraded the tube sockets, added a master volume and 3 way NFB switch... then gifted it to a friend).

Then I built another 5E3 from scratch using Jupiter Cosmos for filtering and Sozos for coupling... and did several of RobRob's suggested mods. (thanks Rob! - your work and generous sharing have really elevated my understanding of amp circuits)

I also built a Ruby in a pedal enclosure - just to keep busy while I planned my next tube build...

The last one I built was a customized Princeton for a gigging player I met when I sold him an old Peavey combo. I let him play my Modded Deluxe, and he was blown away - convinced me to build him a 20+ watt Princeton... (getting enough clean headroom was a real challenge with that cathodyne phase inverter)

I had built a beautiful highly figured black walnut head cabinet with a flame maple front that I had planned to do a Trainwreck Express build... but now will use that for this Bassman. So... I am looking for some input on the following:

1. Speaker cabinet - Is there a certain magic to having 4 x 10? Would a 1 x 15 lose some of that mojo? What about a 2 x 12? Alnico of ceramic? (the 12" Allesandro 1964 I used in the Princeton is probably the nicest speaker my ears have heard) If 4, what wattage and wiring would be best from a single output jack (1750JA, so I have 4 and 8 Ohm secondaries to chose from)
2. Favourite coupling caps for a bassman? - I have a surplus of CDE Orange Drops on hand... I used Mallory 150's in the Princeton and Sozo's for the Deluxe... all sound good to my ear. But in a Bassman, what is your preference?
3. I like using Hammond iron as they are easy to get and nicely priced here in Canadastan... I have a 1750JA for the OT and a 194A Choke on hand... is a 4H choke enough? which PT would you go with? 290DX or 290D2X... I will run a GZ34 Rectifier, but also going to have a switchable diode rectifier mod, my wall voltage is a perfect 120VAC. I am going to build for 5881 tubes, but want to try 6L6GC's as well with a rebias (yep, putting in an adjustable bias pot too)
4. I did an adjustable slope resistor in the Princeton and was thinking of replacing the 56K slope in the bassman as well... would you do this? Although a different approach than the presence control, the net effect on the sound is similar... would having both be a waste of effort or maybe have a negative effect on each other?
5. Summary of mods... NFB switch, Adjustable Bias, Rectifier Switch, Rob's Lead Channel mod, adjustable slope, and... a master volume...
6. that's a lot to pack into a 14" x 6" chassis... so will need to carefully plan grounding and lead dress. Has anyone ever done a mod to use a single volume control for the 2 channels - a single 1M pot after the 270K mixing resistors?
7. Trying to decide whether to use Carbon Comp throughout or Metal Film with just the V2A 100K Plate load and V2B 100K cathode resistors carbon comp.. dropping resistors will be wire wound... I have a good supply of 3 watt metal film in all values needed.
8. Presence pot... original schematic shows 5K Lin, but I have seen 25K as well with a 4.7K resistor... thoughts on best configuration for the presence? My understanding is that the 25K and resistor will remove the crackle when adjusting... but is there something lost in the tone?

Thanks All!

 

[King Fan]

Those are smart, detailed, sophisticated questions; the practical/factual ones are beyond my experience, cuz I haven't built a 5F6a. To keep it simple, tho, I'd suggest it's entirely your choice on a couple questions that are 'pure opinion' or 'highly subjective.' This includes coupling cap type -- you know enough to form your own opinion -- and CC throughout vs. highly selective CC. The 'science' and logic of CC placement are pretty clear; you've probably seen this article:

http://www.geofex.com/Article_Folders/carbon_comp/carboncomp.htm

So there is no logical advantage to CC throughout, but some folks think it 'looks cool' or is 'more authentic.' Selective carbon comp is quieter and may be 'sweeter' based on logic, but you may get a bit more noise and you'll need to replace the CCs when they get noisier or drift. If you go selective, just make sure you select the right spots. Oh, and folks forget those few CCs should be 1/2W (bigger = quieter, but the 'sweet distortion' is a form of noise).

 

[corliss1]

Oh fine, I am a glutton for punishment, I'll take a shot at some of these.

1 - I personally think so, but remember it was originally a compromise. They initially didn't have single speakers that would handle the 40W output, so they split them up. This was obviously fixed by the early 60s, but the 4x10 format stuck and is a classic for a reason. Lots of people have built bassman(ish) heads and used them with everything you can think of, so I really don't think you can go "wrong"

2 - I don't believe in this one. Electrically, it shouldn't matter. Some people say they can tell, but I've never seen an actual blind test with real science where people have been able to reliably tell cap A from cap B. As long as the values are right, you're good.

3 - The switchable rectifier will just bump your voltages a hair. I'd honestly pick one or the other so you can ensure your supply voltages are where you want them. A 5AR4 is also about as solid state as you can get, with minimal voltage drop, so I don't think this is worth it, personally.

4 - You'll never know til you try . You could always do it on the bench and pick your favorite resistance and then solder that one in. It's not something you're ever likely to adjust later once you set it.

5 - These all seem like sane choices to me, except see above about the rectifier thoughts I had.

6 - Standard Fender chassis size for this is 20'' x 4 1/8'' x 2 5/8'' but there's a lot of dead space over by the rectifier and bias areas. I'm sure it can be done, but yes, plan ahead. Graph paper is your friend.

7 - It's not the dark ages, metal film is fine. BUT THE MOJO!!! I doubt a resistor is what's holding any of us back from being a guitar god.

8 - I'm not sure how a resistor would remove DC, but I'd need to see a layout/schematic of how that resistor is implemented to have an opinion. The original is known to work well, although you'll get the crackle, but I doubt you're going to have anyone adjusting the pot while you're playing.

 

[King Fan]

@corliss1 hits on something I was gonna say. Just in general, too many mods can be unwise, especially if space is limited. Even more (you probably know this) build stock, modify later. You need a baseline to depart from and return to. Because you're a smart experienced builder, you can plan ahead for possible mods -- an extra eyelet, a hole for a tag strip or mini-switch, etc. But also as a skilled builder, make sure you can set the amp back to 'stock' with a switch or 'knob on 0' option (OK, whatever iteration of 'stock' you choose in amps like this where Fender may have evolved the amp to a more classic or iconic version).

 

[2L man]

1. I think there come "phasing/richening" effect when sound come from four cones forward and four cones out of rear opening which all can reflect from surfaces.
3. 1750JA is smaller than Bassman typically has!

GZ34 sag HV less than other tube rectifiers but much more than Silicon Diode rectifier especially when output power is high and distorted.
194A choke spec is only 40mA and Screen RMS current can be higher when power is high. Screen current peaks can easily exceed 100mA but filter capacitor handle some of it. Some Marshalls which base to BM have 200mA chokes.

6. For grounding I have only two options which do not use Chassis for HV current so they are legal and have less possibilitys to cause problems!

If you DIY the Circuit Board then single solid Common Bus for all HV0VDC (return current) is simple and then HV capacitors come closer to actual stages they supply the HV

If HV capacitors go to "doghouse" then I would use "Star". Nowadays when we have many wire colors using black for 0V and different color for each stage B+ make it easy to "follow". Wounding DC wires is not electrically needed but they look neat and stay better where you want them to stay but it is good practice to tie long wire runs. Easily available Red/Black loudspeaker pair cable also work fine but it woes not wound! In theory Star is bit better because external electromagnetic field "move both" the same "amount/direction" where Single 0V bus there come two inches to "Plus".

I have used both and honestly think this difference has no effect for audio amplifier and HV capacitors define which to use!!!

When I make Common Bus I drill holes to CB and install Radial electrolytes legs up. For same price Radials Spec might be 105C/10000h where Axial might be only 85C/1000h. Radials come few height/diameter and larger diameters are shorter and installation look quite good.

I use isolated output jack and wire Reference to 0V (usually shielded cable) so that minor OT HV current leak go straight back to 0V (and not thru input jack). For my first high gain build this did stop "ringing oscillation".

7. CC resistors function better as Grid Block for Radio Frequencys but obviously modern radios do not cause same problem the AM radios did. CC should slightly increase 2nd harmonic distortion but unfortunately also increase hiss to audio. I sometimes use two CC in LTPPI anodes and Cathodyne A and C (or SE amp driver anode) and power tube GB but avoid CC in actual pre amp.

 

[sds1]

I believe in dielectrics not brands. So my answer to #2 is use any reputable polyester film cap you want.

 

[5E34EVR]

Thanks for the insight. There are some good things in here to consider carefully. I appreciate the time you folks have taken.

 

[Mr Ridesglide]

I still think that the "stock" 5F6-A I put together for my friend Jack is one of the most versatile amps I ever made. Granted, I've only made about a dozen, but it's loud or not, and I do love the 4x10 alnico setup. It's not as beamish as my single 15" in in my "Pro". I did use 5881's in both.

 

[Pete Farrington]

8. Presence pot... original schematic shows 5K Lin, but I have seen 25K as well with a 4.7K resistor... thoughts on best configuration for the presence? My understanding is that the 25K and resistor will remove the crackle when adjusting... but is there something lost in the tone?

The action and range of control (AKA sweep) is somewhat different. If interested, sign up to TAG and view roberto's sims of the various options https://ampgarage.com/forum/viewtopic.php?p=445915#p445915

 

[5E34EVR]

I have the chassis template mostly laid out on the drafting table... Space will be tight - my mounting surface is 14" x 6" with 12 1/2" x 1 3/4" front control panel and 14" width out back. 4 input jacks, 6 control pots on the front panel does not leave room for a master volume and I do not want it out back. So, I deleted the master volume from my schematic and added in RobRob's half-power mod with a mini toggle right next to the nfb toggle. Using a single power switch with off/stby/on and have moved the fuse to the back... It is a bit busy up front, but not too crowded. Will need to run shielded for any signal carrying runs and keep them as short as possible. Also scrapped the tube/diode switch idea as per advice above. Scrapped the adjustable slope resistor idea as well as it also requires another pot... The Power Transformer needs to be surface mounted on standoffs instead of through chassis in order to fit the PT, Choke, OT and Filter Caps up top... All looks good, on Paper...

Question - Instead of 4 power filtering caps in a doghouse, I am considering a horizontal mounted cap can in it's place... top mounted - seems economical and less build effort. Any drawbacks? The B+1 is 2x 20uF in parallel, so thought I would use the CE 40/20/20 (although the JJ 40/20/20 is only $20... so I wonder about it's quality compared to CE @ $85) Thoughts? I used the JJ in my Princeton build and now am wondering if I may have been able to get more clean headroom had I used the better quality CE Cap Can...

 

[Supertwang]

I have the chassis template mostly laid out on the drafting table... Space will be tight - my mounting surface is 14" x 6" with 12 1/2" x 1 3/4" front control panel and 14" width out back. 4 input jacks, 6 control pots on the front panel does not leave room for a master volume and I do not want it out back. So, I deleted the master volume from my schematic and added in RobRob's half-power mod with a mini toggle right next to the nfb toggle. Using a single power switch with off/stby/on and have moved the fuse to the back... It is a bit busy up front, but not too crowded. Will need to run shielded for any signal carrying runs and keep them as short as possible. Also scrapped the tube/diode switch idea as per advice above. Scrapped the adjustable slope resistor idea as well as it also requires another pot... The Power Transformer needs to be surface mounted on standoffs instead of through chassis in order to fit the PT, Choke, OT and Filter Caps up top... All looks good, on Paper...

Question - Instead of 4 power filtering caps in a doghouse, I am considering a horizontal mounted cap can in it's place... top mounted - seems economical and less build effort. Any drawbacks? The B+1 is 2x 20uF in parallel, so thought I would use the CE 40/20/20 (although the JJ 40/20/20 is only $20... so I wonder about it's quality compared to CE @ $85) Thoughts? I used the JJ in my Princeton build and now am wondering if I may have been able to get more clean headroom had I used the better quality CE Cap Can...

From my experience I’d say it’s doubtful. I’ve never known a new cap can being cheaper than component caps. My last filter cap jobs have used F&T and MOD caps bought on EBay for $20-30 shipped for filter + bias E caps. The MOD caps are “good & cheap”

 

[corliss1]

Where are you seeing an *$85* CE can?

Yes, in general radials (including multi section) are cheaper than axials as those just don't get used much in modern manufacturing.

 

[2L man]

JJ max temperature in only +70 Celsius but I don't remember reading that is a problem! I have not found operating hours and internal resistance specs!

 

[Supertwang]

From my experience I’d say it’s doubtful. I’ve never known a new cap can being cheaper than component caps. My last filter cap jobs have used F&T and MOD caps bought on EBay for $20-30 shipped for filter + bias E caps. The MOD caps are “good & cheap”

I haven’t used JJ caps so I wouldn’t comment except to say that overall I like what JJ offers,…except their less expensive pre-amp tubes

 

[Supertwang]

I built this for @ $10-15. It’s dropping resistors + filtering + voltage doubling rectifier. There are def cheaper ways to build if you don’t have to have the vintage correct layout

 

[5E34EVR]

Where are you seeing an *$85* CE can?

Yes, in general radials (including multi section) are cheaper than axials as those just don't get used much in modern manufacturing.

I am in Canadastan, so that is 85 cdn pesos... I typically purchase north of the 49th to avoid shipping, duty, brokerage fees, and now tariffs... nextgenguitars dot ca is my go to... part # CE-C-EC40-20-20 the JJ is CA-JJ-40UF-500V and has an extra 20uF unused from thetubestore dot com (another great cdn supplier...)

Several older Marshalls did use a lot of cap cans, as did the old Traynors. The AA1164 Fender circuit and several others use cans... vintage correct layout is unimportant in this build.

 

[2L man]

I am in Canadastan, so that is 85 cdn pesos... I typically purchase north of the 49th to avoid shipping, duty, brokerage fees, and now tariffs... nextgenguitars dot ca is my go to... part # CE-C-EC40-20-20 the JJ is CA-JJ-40UF-500V and has an extra 20uF unused from thetubestore dot com (another great cdn supplier...)

Several older Marshalls did use a lot of cap cans, as did the old Traynors. The AA1164 Fender circuit and several others use cans... vintage correct layout is unimportant in this build.

I mostly use Nichicon and Panasonic Radial Electrolytes because 105 Celsius, 10 000h or more might be cheaper than 85C 1000h Axials. I drill holes to Circuit Board and install Radials legs up. Radials come few diameter height ratios and shorter do not come "ugly high" above the CB even only 10mm posts are used. Sometimes Radials legs do not reach as far as Axials but when DIY the CB turretts/eyelets can be placed closer.

I like single Common 0VDC Bus for all HV return current and then each stage RC filtering can be placed closer the stage curcuit which "use" the current.

 

[5E34EVR]

I mostly use Nichicon and Panasonic Radial Electrolytes because 105 Celsius, 10 000h or more might be cheaper than 85C 1000h Axials. I drill holes to Circuit Board and install Radials legs up. Radials come few diameter height ratios and shorter do not come "ugly high" above the CB even only 10mm posts are used. Sometimes Radials legs do not reach as far as Axials but when DIY the CB turretts/eyelets can be placed closer.

I like single Common 0VDC Bus for all HV return current and then each stage RC filtering can be placed closer the stage curcuit which "use" the current.

It would be very interesting to see a gut shot of some of your builds. Do you have any posted somewhere?

 

[telemnemonics]

Modding and lots of added options is popular in the amp hobby building but more due to curiosity than because say a 5f6a is improved by all those extras.
But if you don’t know what you like, add as many as you wish.
Or add them without drilling holes, like hanging out the chassis or simply swap one for the other after hearing the amp.

Seems well established that orange drops are not special or similar to 1950s spec, but I have not swapped and listened.
Nobody reputable builds vintage spec and uses orange drops.

My big warning comment would be changing from 4x10 to 1x15.
No no no no no no no!
Certain players seem to like 15 tone but it sure ain’t similar and it really limits the results.
Ive run various 15 speakers and always found better from tens or twelves.
The origin four by cab does do something special but I think 2x12 is fine and the circuit is heard well that way.

Tube recs I used to like but grew tired of because their influence is largely a loss to bass clarity, while any perceived or claimed fatter more tactile toanz can be gotten with the SS rec by adjusting the knobs and RH touch.
That said, the GZ rec is not really squishy in that amp which is still punchy strident and not like some tube rectified amps we may assume are squishy due to the rec when it is the whole amp not just the tube rec.

Go to a 5f8a with gz34 and yeah the tube is more heard and felt.
I never found my 5f6a response the least bit squishy, even too loud in a bar where it stayed clean and snappy while I was asked to turn down.
Maybe instead of adding SS on a switch, try 5r4 or 5v4, or I forget what all may get more feel.
Ive also not run proper 5881 which may really bring out the best in that fairly strident amp.
My experience is all SC pickups though and maybe a LP or 335 player will think otherwise.

Note: I sometimes fix and mod amps but do not build them and this is just 45 years playing perspective. Three 5f6a so far.

 

[5E34EVR]

Modding and lots of added options is popular in the amp hobby building but more due to curiosity than because say a 5f6a is improved by all those extras.
But if you don’t know what you like, add as many as you wish.
Or add them without drilling holes, like hanging out the chassis or simply swap one for the other after hearing the amp.

Seems well established that orange drops are not special or similar to 1950s spec, but I have not swapped and listened.
Nobody reputable builds vintage spec and uses orange drops.

My big warning comment would be changing from 4x10 to 1x15.
No no no no no no no!
Certain players seem to like 15 tone but it sure ain’t similar and it really limits the results.
Ive run various 15 speakers and always found better from tens or twelves.
The origin four by cab does do something special but I think 2x12 is fine and the circuit is heard well that way.

Tube recs I used to like but grew tired of because their influence is largely a loss to bass clarity, while any perceived or claimed fatter more tactile toanz can be gotten with the SS rec by adjusting the knobs and RH touch.
That said, the GZ rec is not really squishy in that amp which is still punchy strident and not like some tube rectified amps we may assume are squishy due to the rec when it is the whole amp not just the tube rec.

Go to a 5f8a with gz34 and yeah the tube is more heard and felt.
I never found my 5f6a response the least bit squishy, even too loud in a bar where it stayed clean and snappy while I was asked to turn down.
Maybe instead of adding SS on a switch, try 5r4 or 5v4, or I forget what all may get more feel.
Ive also not run proper 5881 which may really bring out the best in that fairly strident amp.
My experience is all SC pickups though and maybe a LP or 335 player will think otherwise.

Note: I sometimes fix and mod amps but do not build them and this is just 45 years playing perspective. Three 5f6a so far.

I have ordered Mallory 150 Coupling caps.
I shelved the SS rec switch (for now) but may come back to it - I was entertaining a switchable trainwreck express feel... (I will be jumpering my octal sockets should I want to try an EL34 swap)
Leaning toward a 2x12 Cab (I have a 2x10 and a 1x12 to sample before I build) The Cab top and sides will be matching figured black walnut - the dimensions of a 4x10 will be very spendy and heavy...