Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Willabe on September 12, 2018, 04:23:04 pm
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Next on the bench for my friend is an ultralinear SF Fender Bassman 135 that he wants me to look at. (He has 2 of them.)
Here's a link to Doug's library for the schemo; https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_135.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_135.pdf)
It's soooooo loud he runs the MV on 2. And it's soooooo bright he sets the treb. on 2 or 3 and bass on ~8 and mid on 10. And he faces the speaker cab, 15", towards the wall.
Kevin O'Connor's TUT 1 has a mod for this amp. He just runs 1 pair of the 6L6's as triodes. There's no choke in these amps so 1 pair in triode is an easier mod. It knocks down the volume some and warms up the amps tone a lot.
The schemo shows 500dcv at the PT CT. There is a wall ACV selector switch with a 127ACV setting. The screens on this amp are fed from the UL taps but they do have 470ohm R's also. I was thinking that for a little extra protection for the 6L6 screen's, and maybe a little less volume and a little compression, I'd raise the screen R values to 1K5 on the 2 tubes still in UL?
He sets his mids on 10, so I think I'll just add a mid pot tail R to give him a little more mids? Why change the pot out when he never uses less mids anyway? There's 2 channels, so maybe a 4K7 R on 1 channel and a ~25K on the other?
I'm going to change the LTPI values to BF values. That should warm it up some too.
And if he likes the lead channel mod I did on his Deluxe Reverb, I'll do something like that on 1 of the channels.
I know someone else has been in the amp and worked on it, have to see what they did or tried to do.
It might need a full cap job.
I might need to do something with the MV?
And maybe the -FB loop?
Is there an easy way to add a -bias adjust pot to the -bias balance circuit that Fender used in these amps?
Any thoughts? :think1:
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> He sets his mids on 10
That makes B and T kinda useless.
The "slope resistor" against the Mid pot reduces everything by the mid-pot value. The B and T knobs add-back B and T from there.
If M is maxed, there's little overall cut, little to add back.
It's also more gain than was probably intended.
Kill some treble somewhere else. Without looking, I'd find a 100K plate resistor and bypass it with 0.002uFd in series with 50K. That will fall above 900Hz, -10dB near 3KHz, but flat above to preserve fuzz-tones. Much experimentation needed. (Might want to do this on the far side of the coupling capacitor so you can clip-lead or switch-box without being shocked.)
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If the triode mod doesn't warm it up enough I'll try that PRR. I think I'll leave the mid controls alone and see how it sounds 1st after the triode mod and cap job.
He said he likes the MV, said it works fine for him, so I'll leave that alone.
It doesn't look like anything has been done inside. It does need a full cap job.
I'll try and make out a parts list tomorrow so we can order the parts from Doug. One stop shop, fast delivery, I like that! :icon_biggrin:
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This amp has -bias balance adjust.
Can I put a voltage divider, pot and R, pot in series with -bias and tail R to ground, where the 2K2 series R that comes off the -bias diode to make the -bias adjustable?
Doug sells a 10K bias pot?
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Appears that, this is the same as the Super Reverb 135- minus Reverb and Tremolo.
Why are there screen resistors and a UL tap? I suspect to get a further reduction in the screen voltage and that adds to the clean headroom?
silverfox.
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I'd experiment with replacing the 33K that's on the wiper of the balance pot with a 25K pot and 22K resistor connected in series between the wiper and ground. I'd pull the output tubes to do this experiment.
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Thank you. :icon_biggrin:
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I'd sell the six-pac, install a couple KT120's, self biased, done! :icon_biggrin:
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I'd sell the six-pac, install a couple KT120's, self biased, done! :icon_biggrin:
It's only a 4 x 6L6. And it is loud! TOOOOOOO LOUD!!!!!!
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I'd sell the six-pac, install a couple KT120's, self biased, done! :icon_biggrin:
It's only a 4 x 6L6. And it is loud! TOOOOOOO LOUD!!!!!!
yer too old then! :d3:
try this: clip out the 120pF cap on the MV tap. disconnect .01uF cap in NFB loop.
--pete
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yer too old then! :d3:
This things a beast! :laugh:
He plays it with the MV on 2 and the speaker cab turned towards the wall.
try this: clip out the 120pF cap on the MV tap.
There is no 120pF cap on the MV and that pot has no tap. :dontknow:
disconnect .01uF cap in NFB loop.
Ok, I'll try it. :icon_biggrin:
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They are stupid LOUD, too clean, and heavy and :icon_biggrin:
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I have a question on the diode rating for the FWB rectifier this amp uses. (I should know this already.)
What is the dcv each of the 4 diodes see?
Schemo 400acv per secondary leg, 500dcv at the 1st filter cap and the diodes are listed as 800dcv/1A.
That's kind of low on the diodes voltage rating? Why didn't they use 2 diodes in series for each diode there now to get 1.6Kdcv rating for each diode?
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The diodes work in pairs, ie, two diodes will always be forward biased and two diodes will always be reverse biased. When the AC voltage crosses over to the negative half cycle the diode pairs will flip. Since the diodes work in pairs they are effectively in series with each other and the voltage rating of the entire bridge will be twice that of any single diode. So, those diodes are very adequate for that circuit.
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Thank you Sluckey. :icon_biggrin:
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How much can you increase the screen grid R without excess sag?
The schemo shows B+ of 500dcv with 120acv wall acv input. He does use a variac, but that just keeps the B+ like the schemo shows, and that's still a lot to ask from 6L6's. And it's the screen grid that is probably the weakest link in the tube at these high dcv's?
The amp does have 470R screen grids from the factory, but I'd like to increase the screen grid R's value for 3 reasons;
1. Sag. Give the amp a little sag, amp has SS rectifier B+. (I am also installing some 10w R's in the B+.)
2. Screen grid high dcv protection. Even though it's UL, the amp pushes the 6L6's to/beyond their limits.
3. Volume decrease. Any volume decrease would help.
Didn't Tubenit play with screen grid R values and end up going as high as 2K2?
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I recommend you leave 'em be. It's a UL circuit. You can't make a dirty sounding bedroom amp out of this beast. :icon_biggrin:
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I recommend you leave 'em be. It's a UL circuit. You can't make a dirty sounding bedroom amp out of this beast. :icon_biggrin:
Yes, but - anything - would help.
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Is it having problems eating output tubes? If so, maybe throw some 1K in there, the tubes will thank you. Otherwise, if it aint broke... 6L6's deserve to die anyway.... :blob8: Maybe pull two tubes and see how he likes it, but watch your voltages and re-label your speaker jacks. Might make a little dB difference and allow him to open it up a bit. Could sound better. Hey, UL is cool! Don't be talking trash about UL! :icon_biggrin:
Jim
edit: If he has the master on 2 I doubt those power tubes are even breaking a sweat. However, if he does an outdoor gig and has to crank it, they may start complaining. I'm not familiar with this amp. Does it have a history of eating power tubes - especially new manufacture tubes?
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Is it having problems eating output tubes? If so, maybe throw some 1K in there, the tubes will thank you.
If he has the master on 2 I doubt those power tubes are even breaking a sweat. However, if he does an outdoor gig and has to crank it, they may start complaining. I'm not familiar with this amp. Does it have a history of eating power tubes - especially new manufacture tubes?
It's not eating power tubes. He has NOS RCA 6L6GC's in it. But like you say, he's playing it with the MV on 2. :w2: :laugh:
Here's what I've done so far;
1. Full cap job, including the 2 -bias caps, changed those to 100uF/100v, used all F&T caps. (Nice size and good price.)
2. Since I changed the -bias caps, I put in a new ultra fast -bias diode, and took out the -bias series CC dropping R, changed it to a MO 3w R. And Fender had the -bias caps ground to the preamp B+ filter cap ground. And to do that they had a long loop of wire running across the length of the chassis. So I fixed that, those caps are now grounded with the power tube caps, like they should be. :BangHead: :cussing: :cussing:
3. Rebuilt both preamp channels, the bass channel I modded as a lead channel, 220K plate R, 2K7 K R, 2.2 uF bypass cap and a .0047 coupling cap. He said the amp was hissy so, I change all the 100K CC plate R's and the 2 - 100K TS slope R's to new CC's. (I did this on his '67 BF DR's normal channel and he loved it. It really upped the sustain without getting gritty, very smooth.) And while I was in there I put in a silver mica 500pF for the lead channels TS treb cap, and a silver mica 250pF in the normal channel. On the normal channel I put in a 10uF K bypass cap.
4. Rebuilt the PI to BF specs, for a little more drive to the output tubes and more touch sensitivity. And I used a 1000pF (.001) silver mica cap for the PI input. (This amp can use all the chime it can get.)
5. Fixed some other ground loop wiring problems. Wires running back and forth. :BangHead: :cussing:
6. I've taken out the CC screen grid R's, disconnected the power tube K ground and cleaned up the socket pins.
I'm ready to install the new screen R's and put in 1 ohm bias sense R's. And I'll wire 2 of the 4 6L6's as triodes. Found it in a Kevin O'Connor book he says is/was a standard mod he did with Fender UL amps. Supposed to add some even order harmonics and decrease the volume from full to 3/4. I'm going to try it.
I'm going to add a 10ohm/10w R in series in the B+ before the 1st filter caps, hopping for a little sag.
And I'm going to see how disconcerting the -FB loop sounds. That might be very helpful to this amp? We'll see.
Adding all these things together might/should help to warm up the amps overall sound.
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Is biasing the the 6L6's the same with 2x wired as pentodes and 2 wired as triodes?
I have 1ohm K bias sense R's installed.
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datasheet shows 3ode and 5ode both 30W Pdis, tweak 'em to ~70% and watch for red
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Thanks shooter. :icon_biggrin:
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IF the amp were my own, ………………….. I'd make it cathode biased with each pair of 6L6's having their own cathode resistor and cap. That way I could simply remove a pair of 6L6's when I wanted to for reduced power. And being cathode biased, I think you'd have significantly reduce volume from that?
DaGeezer made an incredibly sweet sounding amp out of a Twin Reverb using a pair of cathode biased 6V6's along with a pair of cathode biased 6L6's.
With 500v on the plates on the amp you're working on, you won't be able to use 6V6's but you could still use two pair of 6L6's cathode biased.
Just a thought …………
With respect, Tubenit
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It's the UL taps on the OT that's killing the even order harmonics and makes it thin. That's why I'm going to try a pair in triode.
Jeff how high have of a screen gird value have you used?
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I measure only ~ 80K from 6L6 grid to ground on each 6L6 instead of the normal 220K.
Isn't that throwing away a - lot - of drive signal to the 6L6's?
They wanted these amps clean, so they used an UL output stage, with a -FB loop, but that was not enough for Fender on this amp. :w2:
They also cut the drive on the PI, 47K plate R's and 330K grid returns. And cut the drive further to the 6L6's with them only seeing ~80K total for grid return. :BangHead:
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If you don't like the UL thing, then don't use it. You could connect the screen resistors to the 410V end of the 2.7K 10W resistor and lift one leg of the 30K 20W resistor and see what happens. That should give it some sag/compression.
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If you don't like the UL thing, then don't use it. You could connect the screen resistors to the 410V end of the 2.7K 10W resistor and lift one leg of the 30K 20W resistor and see what happens. That should give it some sag/compression.
I almost did that, but there's not enough B+ filtering there for the screens. Only 20uF after a 2K7 R, instead of a choke.
Fender must have added that B+ node, with nothing on it, as an extra B+ filtering stage before the PI and down stream preamp tube stages because they didn't use a choke.
I even thought of using the screen taps for the plate dcv to the 6L6's to lower the dcv a little if I put in a screen B+ node.
I've installed 3 x 10 ohm/10w R's, in series, to the FWB, 1 in each leg from the PT's B+ wind going to the FWB and 1 right after the FWB. And I'm increasing the screen grid R's. Both of those things together will give some sag.
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I've used as high as 2.2k/3w screen resistors in a handful of amps.
With respect, Tubenit
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That's what I thought.
Did the larger valued screen grid R give too much compression?
And did they drop the output volume any?
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I could not discern any volume drop. I liked the compression "feel" that it gave. It wasn't too much at all. Perhaps added some smoothness to edge of breakup tone.
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Ok, that's good info for this project, thanks Tubenit. :icon_biggrin:
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It was suggested to me to 'change the ratio' of the -FB loop by changing the R values in the loop.
Is saying 'changing the ratio' just another way of saying more/less -FB? Or does it just mean changing the ratio of the -FB's voltage divider? Which would be more/less -FB, same thing. Or is there more to it involved with the OT, power tubes and PI all being in the loop that I'm missing?
I'm going to try it and see how it sounds, but I'd like to understand a little about what is actually happening circuit wise -IF- 'changing the ratio' is more than just more/less -FB. (I'm also going try it with out any -FB too.) I know it also effects speaker dampening along with cleaning up a power amps distortion.
I seem to remember PRR and HBP explaining -FB loops as extra gain that can be sacrificed in the loop and they did the math to show how much gain was lost? And I get the gain loss part.
Stock values are; 820ohm for the series R and 100ohm for tail R to ground.
Suggested values are; 8K2 for the series R and 1K for the tail R to ground.
Is that 10x less -FB injected into the PI? Or is this just changing the -FB ratio? Or are they the same thing?
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The ratio is the same.
820/100 = 8200/1000
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So changing to those values wont do anything to the sound of the amp's output? :think1:
The PI see's the same voltage.
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The NFB voltage will still be the same because the divider ratio is the same. But the tail resistor is also part of the cathode bias circuit so the dc bias voltage will change and that may affect the sound. :dontknow:
To change the amount of NFB you would only change one of the resistors, usually the 820Ω.
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To change the amount of NFB you would only change one of the resistors, usually the 820Ω.
Just to make sure, increasing the value of the 820ohm R will decrease the -FB, right?
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I'm sorry Sluckey, I deleted your post. :BangHead:
Sluckey said correct to my question and suggested using a 10K or 100K pot in place of the 820ohm -FB series R.
Yes, to dial it in, thank you. :icon_biggrin:
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Sluckey said correct to my question and suggested using a 10K or 100K pot in place of the 820ohm -FB series R.
That's not quite what I said. I said put a 10K or 100K pot ***IN SERIES*** with the 820Ω resistor.
That way you can easily dial the amp back to the original NFB value or turn the pot the other way to decrease the NFB.
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Sluckey said correct to my question and suggested using a 10K or 100K pot in place of the 820ohm -FB series R.
That's not quite what I said. I said put a 10K or 100K pot ***IN SERIES*** with the 820Ω resistor.
That way you can easily dial the amp back to the original NFB value or turn the pot the other way to decrease the NFB.
Again, sorry I deleted your post, thank you for correcting what I quoted you as posting.
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It was suggested to me to 'change the ratio' of the -FB loop by changing the R values in the loop.
I found out what I missed/misunderstood on this. ^^^^^^ :BangHead:
It was suggested to me to NOT change the ratio of the -FB R's, but to make them 10x of what they are now.
It has to do with the 10x values interacting with the OT's primary parasitic's in the output stage.
It's supposed to keep the bass end tight, (keep the speaker damping?) but soften up the mid/higher frequencies so the amps not so stiff in playing response.
If I had time I would wire up the 2 different value R pairs on a switch and listen for any difference. But I'm busy getting the house ready to list for sale and packing to move to Rochester N.Y.
I worked on this amp and the BF DR just for the $$ to help with the move costs. It was a real blessing to have these 2 amps fall onto my bench right now. :icon_biggrin:
I did install the 10x values, and the owner should be able to hear/feel any difference as this is his main gigging amp.
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I finished all the wiring and mods and I'm going to fire it up with the lamp limiter, then I'll check the dcv's and -bias dcv.
If all goes well, I'll plug in a speaker and see how it sounds. :icon_biggrin:
I took before pics and will now take after pics and post them after the testing.
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It's soooooo loud he runs the MV on 2. And it's soooooo bright he sets the treb. on 2 or 3 and bass on ~8 and mid on 10. And he faces the speaker cab, 15", towards the wall.
I have to say...this sounds like a good candidate for a simple power resistor attenuator.
That will kill highs and shave serious db'eez
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I have to say...this sounds like a good candidate for a simple power resistor attenuator.
That will kill highs and shave serious db'eez
Simple power resistor attenuator!!!!! :huh: :rolleyes:
Oh yee of little faith! :laugh:
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I'd experiment with replacing the 33K that's on the wiper of the balance pot with a 25K pot and 22K resistor connected in series between the wiper and ground.
Ok, adjustable -bias installed. I used a NOS PEC mil spec locking pot, 25K with 22K R to ground. It's working good.
All measurements with variac set to 120acv;
At max -bias; NOS 6L6GC's are running ~ 60% dissipation.
-bias pot swing; -46.5 ~ -55.6
6L6GC plate; 439dcv.
I'll post more voltages later and post pics, gotta go eat.
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.....locking pot, 25K with 22K R to ground.
All measurements with variac set to 120acv;
At max -bias; NOS 6L6GC's are running ~ 60% dissipation.
-bias pot swing; -46.5 ~ -55.6
You nailed it Sluckey, but, can this be adjusted for a little wider voltage swing?
(Schemo link is in OP.)
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It can but I'm not a confident fixed bias guy, I'd start by increasing the 22k, no tubes, and see which way that goes
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Put a 50K pot in and see what happens. Pull the output tubes while you are experimenting.
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I'll see if I have a 50K pot. :dontknow:
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..... I'd start by increasing the 22k, no tubes, and see which way that goes.
I think I have to go smaller, because I have too much -bias dcv. Lowest setting on pot now is just getting the tubes to 60% dissipation.
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|| a 100k and you're down to 16.7k. NO tubes though just in case
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I'll see if I have a 50K pot. :dontknow:
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?
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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?
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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?
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Does the balance pot still function properly?
Haven't tested it yet, ran out of time last night.
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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. :BangHead:
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 (https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_135_schem.pdf)
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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.
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Here's some pics.
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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! :BangHead:
Thank you Sluckey! :icon_biggrin:
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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.
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Three more more pics.
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What do you think the taps are? Green wire, regular/single speaker 8 ohm and green/yellow, ext. speaker 4 ohm?
yes
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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.
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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.
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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. :icon_biggrin:
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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. :icon_biggrin:
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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? :dontknow: :BangHead:
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. :dontknow:
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. :w2:
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. :BangHead:
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. :icon_biggrin:
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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.
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or turn them off
A "milestone", knowing you got a guitar player to just use the amp :icon_biggrin:
another, watching them realizing they got that sound with only 2/3 knobs, AND no pedals!
Well done!
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Yes, he told me he was depending on his pedals and was hoping to at least dial them way down.
It worked out. :icon_biggrin:
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. :icon_biggrin:
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+.