Bassman 135 Mods ?Purpose

[jrc111]

I have a Bassman 135 with a couple of mods by previous owner(s), I’m trying to figure out.  It’s always been kind of “farty” in tone.  I recapped it 10-15 years ago, but never did any circuit verification at that time.  As I recall, I took the Manual’s procedure for hum and power tube balance as gospel, and played on. 

“Book” Schematic:

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

I’m somewhat familiar with the concept of putting both leads coming out of phase inverter onto the wiper of the balance pot, moving the grounded resistor onto one of the limbs of the pot and moving the bias supply lead to the other side.  On the Twin Reverb, the stock linear PI 47K and 68K resistors were changed to 220K, which also in this case it appears to be changed to 220K ohms. So, I think that’s a Twin Reverb adjustable bias mod there.

Additionally, the SPDT ground switch has been swapped out for a DPST with an 820 and 2200 ohm selectable resistance.  The white wire goes to the GRN-YEL center tap off the output XFMR; the grey wire seems to go ahead of the PI to swap out the standard 820 ohm for 2200 ohm.  What does that do?

Here’s some fotos:

[thetragichero]

for the switch, it sounds like more or less (or no, if it's an on/off/on switch) global negative feedback

[jrc111]

It’s a single throw switch, so no in between. 820 or 2200 ohms, only.  So is that 820 ohm in the original schematic the feedback resistor? 

[pdf64]

Yes.
Well, there’s the series feedback 820 resistor and its shunt partner, the 100. They work together.

The increase in the output valves’ grid leak resistor values will tend to increase the tendency to bias shift / blocking distortion when overdriven.
But that requires devastating SPL being generated, surely?

[jrc111]

SPL?  Sorry, don’t know what that means.

So, doubling+ the 820 ohm to 2200 will decrease the feedback, making the amp more aggressive, decreasing headroom (maybe good for guitar, maybe not so much for bass).

It appears the .01 mfd cap is missing parallel to the 820/2200 ohm resistor.  What effect does that have?

[thetragichero]

at 820 ohms .01uf cap would allow frequencies above ~19k to bypass the resistor. there isn't really anything useful for guitar nearly that high up. whether that would help prevent high frequency oscillation i'll let the more learned engineer types to expound on rather than a "shade tree mechanic" hack like myself

[Willabe]

SPL?  Sorry, don’t know what that means.

Sound pressure level, volume, db. 

[Willabe]

I did a lot of work on one of these 135 UL Bassman's a while back, owner loved it afterwards.

Here's the thread on it, only a couple pages, but a lot of info on this amp.

And important info on the internal switching Fender used for the speaker jacks.

https://el34world.com/Forum/index.php?topic=23749.msg255442#msg255442

[tubeswell]

Bassman 135 ... always been kind of “farty” in tone...

Speaker?


Voltages?


Are all the filter caps ground leads grounding out properly?

[jrc111]

I did a lot of work on one of these 135 UL Bassman's

That’s for sure!  I don’t think I can match your expertise, Willabe.

I noticed your cap can is wired differently from mine (your two lesser caps join at the +, mine are parallel); I have to trace them through the schematic.

Speakers: A pair of late 1970 8 ohm CTS Model 126383 in a V.T. Bassman 15 vertical cab.  It looks like one was reconed by Advanced Speaker Technologies out of Toronto in the ‘80s.

Voltages: waiting for parts

Cap Grounds: will check

Farty Sounds:  I never previously biased the amp, as I thought the now bias control was a power tube balance control.  My bad!  My understanding is cold-bias sounds farty.

Re bias:  should I just put 1 ohm resistors on all the 6L6GC’s cathode pin 8 to ground, and measure mV (and leave the resistors there), or just use my Alessandro “Bias and Matching Meter”?

[jrc111]

Cap grounds ok, electrolytic cap wiring correct per schematic.

[tubeswell]

Cap grounds ok, electrolytic cap wiring correct per schematic.

Use your R-meter to check for DC continuity between each filter cap ground lead and the chassis. (Don't trust your eyes). If one of the connections reads 'open' or 'NC', investigate the wiring for that ground return. (This can be a common cause of 'mushyness', due to the filter cap node not decoupling the PS to ground at that point, resulting in inter-stage feedback through the PS rail).

[jrc111]

Good to know!  The only electrolytic that doesn’t have multi-meter-proven continuity to chassis ground is the 220 mfd circled in red below.  It does have lead continuity to the adjacent 220 mfd and the pair of 100K R’s beyond.

[jrc111]

Oops, here’s the red circle:

[pdf64]

That’s fine.

[Willabe]

I don't see the 100K voltage sharing R's across the 220uF/300v stacked caps. Fender used them, but, if I remember correctly, because the PT B+ CT is attached to the junction of the 2@300uF filter caps it forces voltage sharing of the 2 caps, so you don't need them.

If there not there and you put them back, go up to at least 220k. And use metal oxide. (It's in the power supply.) I think I put in 470K@2w MO? 

[jrc111]

The 100K’s are there, but on the under-chassis side.  The salmon colored wire coming off the negative side of large cap #1, comes thru the chassis near the large ceramic C and passes over to the eight lug terminal strip mounting the 100K R’s.

What’s the advantage for upping the R values?

[Willabe]

You lose a little less B+ current to ground, but it's just a small amount.

[jrc111]

You lose a little less B+ current to ground, but it's just a small amount.

Thanks!

[jrc111]

Finished the amp up, went to a pair of 220K R’s as above, replaced all electrolytics and the bent shaft mid pots.  Biased the existing quad JAN 6L6GC’s to 65% PD, added a pair of NOS Sylvania 7025’s and a RCA 12AT7.  Sounds great, no more fartiness.  And I labeled the mod switch/pot for future reference:

[thetragichero]

how did you label it? i have a pa 135 conversion that's been on the sideline while i try to figure out a labeling solution for the back panel (or stop being cheap and purchase a new color cartridge for my printer to do a waterslide decal in yellow)

[jrc111]

Black electricians tape to cover the existing lettering, and a Brother P-Touch labeler using a 6 mm White on Black cartridge.