Fender Bassman 135 (UL)

[shooter]

|| a 100k and you're down to 16.7k.  NO tubes though just in case

[sluckey]

I'll see if I have a 50K pot. 

You can do a simple test to see if a 50K pot will get you the swing you want.

Replace your pot with a 47K resistor. Measure the voltage on pin 5 of your output tubes. Now put a short across the 47K resistor. Measure the voltage on pin 5 of the output tubes. These two voltage readings is the voltage range a 50K pot will give you.

So, what's wrong with the swing you have now? Need more negative voltage? Or less negative voltage?

[Willabe]

You can do a simple test to see if a 50K pot will get you the swing you want.

Replace your pot with a 47K resistor. Measure the voltage on pin 5 of your output tubes. Now put a short across the 47K resistor. Measure the voltage on pin 5 of the output tubes. These two voltage readings is the voltage range a 50K pot will give you.

So, what's wrong with the swing you have now? Need more negative voltage? Or less negative voltage?

It's just short of  -10dcv swing.

I need less -dcv. Pot is max to get ~60% dissipation. 

But that's with these tubes, what about other tubes?

[sluckey]

If you can get to a swing of -40 to -60 you should be able to bias any 6L6 you put in this amp. Use a 50K pot for a wider swing and decrease the 22K to 15K to set the lower limit even lower. Again, pull tubes while experimenting. When you have the bias voltage swing where you want it, put the tubes back in.

Does the balance pot still function properly?

[Willabe]

Does the balance pot still function properly?

Haven't tested it yet, ran out of time last night.

[Willabe]

I ran into something I don't understand with the output jacks and their switching. The amp is gone but I might have to have him bring it back. 

The OT secondary has 2 taps, green, and a green/yellow.

On the schemo, it shows the ext. speaker jack as a 2P/1T (?) switching jack that has both secondary taps hook up to it.

I think the green is used when plugging in the regular speaker jack. And when you plug into the ext. speaker jack the switch goes to the green/yellow tap and disconnects the green tap?

And the -FB loop and the line out are wired up to the green/yellow tap. That green/yellow tap wire goes to an eyelet on the board, with 1 end of the 820 ohm series -FB R and there's a wire in that same eyelet that runs back to the ext. jack and then over to feed the line out 2K2 series R. The line out jack is right next to the ext. speaker jack.

I disconnected those 2 wires for him to hear the amp without -FB, he likes it much better.  Before I did this, I had 2 speaker cabs hooked up, 1 in the regular speaker jack and the 2nd in the ext. speaker jack. We could hear both speakers working.

- BUT - When I disconnected the green/yellow from the board, we could hear  -both- speakers were still working.

How can that be?

Both speaker jacks must be getting feed from the green speaker tap wire.

I soldered the green/yellow back in the eyelet to hold it down, and took out the -FB series R. And I sniped the other wire out that runs back to the ext. speaker jack. He said he never uses the line out.   
 
Here's a clearer schemo of the speaker jacks. Still hard to make sense of it.

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_135_schem.pdf

[sluckey]

I think the green is used when plugging in the regular speaker jack. And when you plug into the ext. speaker jack the switch goes to the green/yellow tap and disconnects the green tap?

That's backwards. Look at the ext jack. That bar that is connected to the tip is non-conductive. It also connects to one contact of the normally closed switch. When you insert a plug, the tip moves up which also moves the non-conductive bar up, which opens the switch contacts.

When you plug into ONLY the main spkr jack, the speaker is connected to the GRN/YEL wire via the normally closed contacts on the EXT jack. This is the only time the GRN/YEL wire is ever connected to a speaker.

When you plug into ONLY the EXT jack The speaker will be connected to the GRN wire.

And when you plug 2 cabs into BOTH jacks, the cabs will be connected in series to the GRN wire.

[Willabe]

Here's some pics.

[Willabe]

I think the green is used when plugging in the regular speaker jack. And when you plug into the ext. speaker jack the switch goes to the green/yellow tap and disconnects the green tap?

That's backwards. Look at the ext jack. That bar that is connected to the tip is non-conductive. It also connects to one contact of the normally closed switch. When you insert a plug, the tip moves up which also moves the non-conductive bar up, which opens the switch contacts.

When you plug into ONLY the main spkr jack, the speaker is connected to the GRN/YEL wire via the normally closed contacts on the EXT jack. This is the only time the GRN/YEL wire is ever connected to a speaker.

When you plug into ONLY the EXT jack The speaker will be connected to the GRN wire.

And when you plug 2 cabs into BOTH jacks, the cabs will be connected in series to the GRN wire.

Ok, that makes sense.

Good thing we didn't plug in only to the regular speaker jack once I disconnected the green/yellow tap.

I'll have to have him bring it back and hook up that wire I sniped out. This head is a pain to pull the chassis of, and the iron set is heavy! 

Thank you Sluckey! 

[Willabe]

1 more question.

On the schemo it says on both speaker jacks; 4 ohm min. load.

What do you think the taps are? Green wire, regular/single speaker 8 ohm and green/yellow, ext. speaker 4 ohm?

What do you think the ohms load on this amp is supposed? 

And when you plug 2 cabs into BOTH jacks, the cabs will be connected in series to the GRN wire.

So with 2 @ 8 ohm speakers plugged in, 1 in each jack, that will be a 16 ohm load the OT secondary sees?

EDIT ; See reply's #56 and #62.



 

[Willabe]

Three more more pics.

[sluckey]

What do you think the taps are? Green wire, regular/single speaker 8 ohm and green/yellow, ext. speaker 4 ohm?

yes

[Willabe]

Now I get the speaker jack switching.

These amps came with a 2 x 15", 4 ohm JBL speaker cab.

Regular speaker jack, 4 ohm tap, with 1 cab, 2 x 8 ohm speakers wired in parallel = 4 ohm load.

Regular and ext. speaker jacks, 8 ohm tap, with 2 cabs, each cab; 2 x 8 ohm speakers wired in parallel = 4 ohm load, so 2 @ 4 ohm cab in series = 8 ohm load.

"1) The Bassman 135 is designed for speaker cabs of 4 ohm minimum
 impedance, for both the main cab and any extension cab.

 2) The Bassman 135 and the similar 135 watt Twin Reverb amps are
 unique among Fender amps, in that the jacks for the main and extension
 speakers are wired so that when both jacks are in use the cabs are
 connected in SERIES rather than parallel, and plugging in an extension
 cab also switches the speakers to a different tap on the output
 transformer (switched from the 4 ohm "center" tap to the "whole" 8 ohm
 secondary winding)."

And you can put a dummy plug in the regular speaker jack and run a single 8 ohm speaker cab in the ext. speaker jack.

"Another unique feature is that you can run either one or two 4-ohm cabs using the speaker and extension speaker jacks in the back, or just one 8-ohm cab from the extension speaker jack alone."

He's been running 2 speaker cabs @ 8 ohms each (single 15" in each) 1 plugged into each speaker jack.

So because of the way the 2 jacks are wired in series, he's been running a 16 ohm load on the 8 ohm tap.

[pdf64]

What do you think the taps are? Green wire, regular/single speaker 8 ohm and green/yellow, ext. speaker 4 ohm?

yes

I think the above may be a typo, ie main speaker jack only grn/yel 4 ohm, ext speaker jack (+ main) grn 8 ohms.

[Willabe]

What do you think the taps are? Green wire, regular/single speaker 8 ohm and green/yellow, ext. speaker 4 ohm?

yes

I think the above may be a typo, ie main speaker jack only grn/yel 4 ohm, ext speaker jack (+ main) grn 8 ohms.

No, read reply #56 - 1st, then read reply #62.

Sluckey got it right. 

[Willabe]

He had an early day, he's self employed, so he came by and I put that 1 wire back in. 

The amp still has no -FB but now both speaker jacks now work as intended.   

[Willabe]

Amp is finished, sounds much better, fatter, fuller, warmer, more/earlier break up, more harmonics, more chime, more sustain, and at least 1/4 less volume.   

He has an outdoor gig this Sat. so he can turn it up some to test it with the band.

We're moving and I'm getting the house ready to list for sale, so I didn't have the time to document this as well as I would have liked to. I lost most of the pics I took, not sure how?     

A few pics are in reply #57 and #60. 

At the end I took voltages with the variac set to 120acv and then straight from the wall outlet, no difference in PT secondary acv's or any dcv's in the amp. 

The heaters were 6.5acv both ways and the B+ secondary didn't budge, nor did the -bias wind.

The amp has a -bias balance and was set by Fender at ~40% dissipation, with these RCA 6L6GC's anyway, but Fender used RCA's too. 

When I added a -bias adjust pot, sealed NOS mil spec PEC 25K and a MF 22K and biased the tubes up to around 60% dissipation, the B+ dcv dropped way down, 465dcv instead of the 500dcv listed on the schemo. Much better IMO.

The 3 x 10 ohm/10w R's in the B+ I installed, 2 before the FWB, 1 in series on each secondary leg and 1 after, only dropped the B+ ~5dcv's total.

I put in new CC 1/2w R's for the preamp tubes plates and TS slope R's, hiss was now gone. And I got rid of at least 3 ground loops from Fenders wiring and once I got the 4 x 6L6GC's balanced, the low frequency hum all but disappeared. But getting them balanced took some time flip/flopping all 4 tubes in all 4 sockets! I don't know why but they each didn't seem to like certain sockets.     
 
I wired the inside pair of 6L6GC's to triode to get them out of UL operation and triodes sound darker than pentodes and to drop a little output volume, now 3/4 instead of 100%. We tried pulling the 2 that were wired as triodes, to get to 1/2 total output, but the amp sounded warmer/fuller with the triode pair in there than with out them. So we put them back in. I didn't have time to try just the triode pair alone. 

When I did a full cap job, F&T's, changed the PI to BF values, wired the inside pair of 6L6GC's as triodes, upped the screen grid R's from 470 ohm to 1K ohm on all 4 x 6L6GC's, disconnected the -FB loop, and biased the 6L6GC's up to ~60%, the amp came alive. 

I also modded the Bass channel to a lead channel by changing the plate R to CC 220K, the K R to MF 2K7, the K bypass cap to 2uF2 and the 2nd stage coupling cap was already a .0047uF. I put a MF 22K R between the mid pot and ground for more mids. The deep bass switch really kicks now. And I changed the K bypass cap on the normal channel to 10uF.

I rebuilt the FWB with ultra fast UL10047 diodes, 1000v/1w and 3 x 10 ohm/10w R's and 3 x .02/630v poly snubber caps, to kill any SS diode switching noise. I rebuilt the -bias completely too, UL10047 diode, MO R's, 2 new F&T 100uF/100v caps.

And I changed the B+ 1st stage filter caps, A node, bleeder/balance and any/all other B+ R's to MO 3w.       
Here's some final voltages;

PT B+ secondary; 1 leg to ground, each ~ 190acv.

Bias tap; in play; 56.3acv, after diode; -63.3dcv. max -bias dcv at bias/balance pot; -60.5dcv.

In play; plate; ~465dcv, screens; ~465dcv.

Final K bias current;

1. 40.2mA
2. 35.9mA
3. 40.7mA
4. 39.2mA

That's as close as I could get them, 4.3mA difference at most, but that's pretty good. 

[Willabe]

Well, he played the amp at an outdoor party this last Sat., he loves it.

He said it's night and day different in sound now. He said it is now very touch sensitive to his picking attack and has great sustain compared to stock.

He couldn't believe how different it sounded and his band mates loved his new tone too.

He was able to turn down, to almost nothing, his tube boost pedals and/or turn them off and the same for his compressor pedals. The amp didn't need the help it needed before. 

And he was able to get the MV up to 6 and the lead channel volume up to 7.

 

[shooter]

or turn them off

A "milestone", knowing you got a guitar player to just use the amp 
another, watching them realizing they got that sound with  only 2/3 knobs, AND no pedals!

Well done!

[Willabe]

Yes, he told me he was depending on his pedals and was hoping to at least dial them way down. 

It worked out. 

That amp was biased at ~40% dissipation, and UL. The PI was Fenders later SF PI setup.

All the things I did, had to do, and there was many, really changed amp. 

These were the most important changes to the amp -tone- wise;   

1. Wired 2 of the 4 - 6L6's as triodes, took them out of UL. This added even order harmonics.
2. Biased the 6L6's up to 60% dis.
3. Disconnected the -FB loop, with 2 - 6L6's still in UL, that still gives the amps power section some FB.
4. Changed the screen grid R's to 1K.
5. Rebuilt the PI to BF values. Most importantly, 1M grid returns and 82K/100k plate R's.
6. Installed 3 x 10ohm sag R's in the B+.