New build thread (5F6A Bassman from Tube town)

[Tintinus87]

Hello,
I start a new project (that I wanted to do for a long time ago) and this is it now : a 5F6A- bassman, but in a combo with two or one speaker mixed together (a WGS blackhawk 100W Alnico and a Eminence Swamp thang 150W).
I already make the all cabinet by myself this summer !

I want add some mod and want/not want to do.

First I took at the 5F6A schematic from Tube-Town, which I bought a kit.

1) The 27K NFB resistor is connected with the 16 ohm tap which is not conform with the standard NFB (27K on 2 ohm tap). So I will use a SPST switch and have a choice of two NFB :
-27K with 16 ohm tap for JTM45 style
-75K with 16 ohm tap for real 5F6 style

2) I kept the original schematic tone slope resistor anc capacitor, but i will change the C6 220pF by 250pF (why TT use this value ????) and also the 10K tail resistor.
Why ?
Because I think Fender change it for better use with the 4 Jensen P10R in the combo. As I did not use this speaker I will see if I am happy with the schematic value.

3) I change the schematic 5K presence pot by 25K and the 4K7 resistor.

4) I want to delete the stanbye switch because I never use it once the amp is "hot" and I think it is a unnecessary with an indirect heated rectifier like Gz34, which have 20-30 second to warm up and bring the B1+.

5) I add an elevated heater using a voltage divider with B1+ (220K-39K) and the center tap of the 6.3V tap, using the free pin of the rectifier. Also add 1N4007 backup rectifier diode.

6) I want also dual adjustable bias, using 10K pot and using the hole for 4 ohm tap that i will not use, and add a master volume (in the hole for standby switch), the LAR-MAR 2, so I delete the 220K bias resistor.

7) I hesitate for the 5k6 screen grid resistor.
Why they use 5k6 instead 1K5 ? Is there a difference ?
And Is there owners of 5F6A WHITHOUT grid resistor ? Can I try without ?

8) From the kit I change most of the resistor by carbon comp and also change the capacitor.

9) I change the first capacitor of the bias tap because which is delivered by TT is 10uF63V, and I prefer use 100V capacitor (so I use 22uF 100V)

10) And I add a pentode/triode switch

That's it for now !

 

[2L man]

Very good looking combocab. You must be anxious to finish the amp fast

3) If you mean to prevent current flow thru pot wiper then it is good mod!

7) You mean 5k6 vs 1k5 grid block resistors? I think the difference is small. 12AX7 phase inverter does not have power to drive power tubes to AB2 but if it is "tried" (and I recall right) the PI recover faster when resistor values are higher when coupling capacitors do not store as much energy "trying" to push current thru higher grid block resistors. PI will try anyway if it is driven For the drive signal amplitude the resistor value is small because there does not flow current when drive signal stay AB1.

Tube grid "Miller capacitance" has more effect as long the control grid drive signal stay below the cathode voltage and when capacitor reactance depends of frequency the Miller restrict highs more. Together with grid block resistor Miller make good block to RFI

Grid block resistors do prevent RFI based distortion to enter to signal and there Carbon Composite resistors function better and later stages their "hissing" effect to already amplified signal is very low. Practically I use CC resistors in PI anodes and power tube grids. In pre amp I use only metal film because their hissing is lowest.

6) I think the bias potentiometer should always be placed between grid and 0VDC and then if the wiper fails full bias voltage drive tubes colder! I have used this dual bias circuit on right and it is fast to "balance" the bias connecting voltmeter between both 1 ohm resistor / cathode junction and using balance just nulling the reading. Next time I will use 10 ohm sample resistors and there come 10 times higher voltage and more "decimals" in my multimeter when its mV range is up to 999.9mV. I have not yet used it with Master Volume but it should work.

 

[Tintinus87]

Thanks for your reply !
I think i use your schematic with 10K balance pot et 25K pot range and 10K resistor to have -43 -53V bias range.

 

[2L man]

Thanks for your reply !
I think i use your schematic with 10K balance pot et 25K pot range and 10K resistor to have -43 -53V bias range.

Lower value potentiometer is nice when adjusting comes less coarse but bias current range comes narrower. I often have changed one resistor when I tune the circuit so that range comes better centered.

 

[Lowerleftcoast]

C6 220pF by 250pF (why TT use this value ????) and also the 10K tail resistor.
Why ?

Imo, TT thinks it is easier/cheaper to source and the small change in value doesn't make a big difference. It is close enough.

Why they use 5k6 instead 1K5 ? Is there a difference ?
And Is there owners of 5F6A WHITHOUT grid resistor ? Can I try without ?

Originally it was 470R. Depending on the tube type, different values can be used. As far as sound, a larger resistance will give somewhat more compression. No resistance will sound a little coarser. It is not much of a sound difference. To hear the difference you can jump the resistor with a wire.
With 6L6 type tubes I would use 470R to 1k. EL34, I would use 1.5k on up to 5.6k. OMMV.

The tubes will probably run happier (longer life) with some resistance here.

 

[Tintinus87]

Originally it was 470R. Depending on the tube type, different values can be used. As far as sound, a larger resistance will give somewhat more compression. No resistance will sound a little coarser. It is not much of a sound difference. To hear the difference you can jump the resistor with a wire.
With 6L6 type tubes I would use 470R to 1k. EL34, I would use 1.5k on up to 5.6k. OMMV.

The tubes will probably run happier (longer life) with some resistance here.

I talk about the grid stopper resistor, which is none on the original 5F6A schematic (deleted by Fender after 1K5 on 5F6). Do you talk about screen resistor ? I use 470R 5W for screen resistor.

 

[2L man]

I wrote long post how grid block resistors should lessen blocking distortion, when they prevent the coupling capacitor charging, when PI output signal voltage peaks on power tube control grids go positive against cathode voltage, which is where control grid current begin to flow and coupling capacitor charging happen.

Then I understood it does not have much effect to sound if grid block resistors are 1k5 or 5k6 or there are no grid block resistors. PI anode resistors are where the possible coupling capacitors charge current comes and they are 82k and 100k. Now if there are 1k5 or 5k6 they can't prevent coupling capacitor charging!

So I think 5k6 grid block resistors do not effect sound lessening the blocking distortion but they prevent better possible RFI noise forming a low pass filter together with Miller capacitance.

This link has good blocking distortion article by Randall Aiken

What Is "Blocking" Distortion?

Guitar amplifiers

www.aikenamps.com

 

[Lowerleftcoast]

Do you talk about screen resistor ?

Yes, I was referring to the screen resistors.

I talk about the grid stopper resistor, which is none on the original 5F6A schematic

Too many screens on a pentode. hah.
The 1.5k *control grid stopper* on several of the Fender schematics is there to try to keep ultra-high frequency oscillation from occurring. (The 47pF snubber cap on the PI plates is there for the same reason.) The frequencies involved are beyond what the ear can hear. This resistor is part of an RC filter formed with the Miller capacitance so, any resistor value chosen below, iirc, ~40k will not be heard by our ears. 5.6k will not be noticed. If the circuit does not oscillate, a control grid stopper is not necessary. The problem is we do not hear the ultra-high frequency oscillation but if it is present, it could damage the amp.

I am cautious with oscillation occurring in this part of the circuit. I use a 100pF snubber cap and 1.5k to 10k control grid stoppers. OMMV.

 

[Tintinus87]

Ok thanks for yours answers ! I will keep the 5K6 of TT schematic. Easy to install or uninstall.

 

[Tintinus87]

I mount the rear panel with tube socket on the amp chassis. There is two switch that I added : a pentode/triode switch and and a 3 NFB switch.
Instead of the 4 ohm jack I put here the bias range and balance potentiometer.
The all stuff is very heavy.
On the TT kit there is :
-Hammond 290DEX ( 325-0-325V@230mA / 50V/ 5V@3A/ 6.3V/ 4A)
-Tube Town 5F6 output transformer 50W (4.2K primary 4-8-16 ohm secondary)
-Hammond 194B for choke (4H 90mA)

I install the main board inside and the cap board outside the chassis. Take care of the mounting order because some screws are under the output transformer !

The tube sockets in the kits are belton phenolic. I added socket retainer (size of 5881 and no 6l6GC).
Apparently the Tube town 5F6 is larger than the original 5F6 chassis (12.6 cm instead of 10.5cm), in order to better fit the board and minimize the risk with electric choc.
The capacitor and resistor are not those with the kit (Carbon film with Mallory cap). The resistor for bias and for gridleak resistor are missing because I use adjustable and a master volume (LAR MAR).
The metal film resistor, which have a "air leg" is the added for NFB switch.

 

[Tintinus87]

I just finish soldering all the wire and component. The chassis from Tube Town, which is 2 cm wider than old 5F6 chassis, help a lot.

I delete the standbye switch and I replace the hole by a Master volume.

Here we see the Pentode/triode switch.

We see the the two potentiometer for adjustble bias balance (replacing the 4 ohm output hole) and the On-OFF-ON switch for JTM45-none-5F6 negative feedback.
I use shelded wire for the master volume.

I have checked the voltage of output transformer and all was fine. Now i have to put the vacuum tube, check voltage an adjust bias.

 

[Tintinus87]

Hi,
I measure Voltage and all were fine :
-B1+ : 446V
-B2+ : 442V
-B3+ : 390V
-B4+ : 325V

Bias 33 mA

V1a anode : 174V
V1ab cathode : 2.5V
V1b anode : 168V

V2a anode : 193 V
V2a cathode : 1.2V
V2b cathode : 194 V

V3a anode : 260V
V3abcahtode : 42V
V3b anode : 261V

V4 (5881) anode : 440V
Va screen : 438V

V5 (5881)anode : 441V
V5 screen : 440V


BUT I have oscillation ("who who who who who" sound) when master volume (LAR-MAR type 2) is over 50% turned....
Do you think it is related to shelded cable ?

Do you think there is a ground loop bewteen the ground connection (bus ground for the enter and cathode for the output) ?

Do I need to ground together the 3 wire only ON ht BUS ground ?

Thanks !

PS : At 50% volume pot it still very loud and I find the presence pot not very efficiency.

PPS : When it oscillate, the bias oscillate too (and plate voltage)

The LAR-MAR master volume is there :

 

[Tintinus87]

Ok problem solved !!!
It was positive feedback, and, when I wired the master volume, I mismatch the phase inverter to the wrong power tube !!! So reversing only the wiring on master volume pot resolved this !!
No more "Whowhowhowho" at high volume, and the amp is far less boomy and the presence pot is working now !!!
Great sounding amp !!

 

[Tintinus87]

I install 5881 NOS Marshall USA (on the left in the photo with Tung-sol, similar to Tung-sol) and a 6l6WGB Sylvania (the other Tung sol was too different on spec).

I also change the 25K pot range bias by 50K potentiometer in order to have a better range of bias.

The amp is very clean, and the master volume (LAR-MAR 2) is very transparent. There is not much hiss or hum.

I made all the cabinet, and I can use the two speakers in // (8 ophm) or each with 16 ohm tap.
If I want smaller and rounder-medium sound I use the Warehouse Blackhawk alnico 100W. If I want a more brillant and "in your face" sound the eminence have a lot a power.

That's the End !!