Fender UL 135W x4 - Twin reverb (1) - Bassman (2) - PA135 (1) low output

[spunko]

Hello there friends.
 
A client recently drove his forklift  and dropped four Fender ultra linear amps from late 70's for service. I have only tested the Twin reverb 135 and the PA135. For the bassmans I'll make another thread.

The Twin 135 and PA135, both work, power up and sound super clean, but the PA135 lacks of power and sounds like a 1 watt amp versus the twin which sounds super SUPER LOUD. Tested both with a guitar and my shop cab (1x12 / 8 ohm / 100w).

- After recaping both amps, the PA135 stayed the same, still sounds like a 1 watt amp but super clean. The twin 135 is perfect 
- Both amps have the same DC voltages on the dog house and power tubes.
- Swaped all tubes between amps, same result. The twin sounds super loud with the PA tubes, and the PA135 still lacks of power with the twin tubes, so no tube related.
- Using my bias probes, the twin show 5mA and the PA135 10mA (is it too low?).

What else should I check? Could this be a PI related issue? Voltages on PI tubes are not the same between amps, although it seems the same circuit. This could be related to the tube itself, but swaping all the tubes didn't make a difference in sound. I didn't check the readings on the PIs when swapped tubes, but I can do that later.



Phase Inverter readings

[Willabe]

- Using my bias probes, the twin show 5mA and the PA135 10mA (is it too low?).

If those readings are correct, those power tubes are barely on.

Use an on line tube bias calculator and bias up the tubes properly. Should be at least 35mA per tubes, probably  a little more. Bias them up for ~ 60% dissipation. You don't need to go to 70%.

With the calculator link below, you want class AB fixed bias, at 60%.

https://robrobinette.com/Tube_Bias_Calculator.htm

[Latole]

Low power :
If voltages was good before, it is was not a filter caps issue.

5 and 10 ma / 488 volts =  2 ,54 to 5 watts / tube    Someting wrong with your bias probe or reading
At this low power amp will not work
You may read 50 ma and 100 ma = 24 and 48 watts , not 5 and 10

You must use a scope to see where the signal drop .
Check volume pots value
Or the OT is bad ?

[spunko]

I took new readings using the shunt method. Weird thing with my probes, I tested them with a hot rod and they read ok. These probes basically brake the connection on PIN3 and place the metter in between (in mili Amps, as Gerald Weber said  ). Could this be a thing with UL amps?

Well, using the shunt method I got higher readings, and they are basically the same in both amps:


I tested the amps before doing any work, the twin sounded good and didn't show any problems at all, just scratchy pots when tested. The PA135 same thing, but with low output as described.
All the caps in the amps were original, and they all were bulgy so they had to be replaced.

I can swap OTs to test, but I really don't want to lol (so much work  Is it necessary?). No blown fuses by the way.
I don't have a scope, it is my whish list. Hopefully soon.

[tubeswell]

Another possibility is a leaky coupling cap  putting DC on the grid of a following stage, affecting bias and gain

[Latole]

Bias reading are better.
I don't know what is wrong with you probe ( or you meter or how you use it ) . It is nothing to do with UL amps.


If you repair amps for customers , I don't know how you can do this with no scope ; it is a must with signal/ audio generator.

[pdf64]

... These probes basically brake the connection on PIN3 ...

So what resistance do you measure between the probe plugs?

... Phase Inverter readings

Those readings for the PA are weird, but it's supposedly the same circuit (apart from the strangely low 220R value of series negative feedback resistor).
So why are they different, eg has a resistor gone bad, is a cap (or the board!) leaking DC?

[Latole]

... These probes basically brake the connection on PIN3 ...

So what resistance do you measure between the probe plugs?

... Phase Inverter readings

Those readings for the PA are weird, but it's supposedly the same circuit (apart from the strangely low 220R value of series negative feedback resistor).
So why are they different, eg has a resistor gone bad, is a cap (or the board!) leaking DC?

Right

[Latole]

Spunko
Wich late '70 Twin ?
Show model number or best , schematic

[pdf64]

Spunko
Wich late '70 Twin ?
Show model number or best , schematic

In the first post he identifies it as the 135W version and provides a schematic. It's not the Fender original schematic but it seems ok.

[Latole]

Spunko
Wich late '70 Twin ?
Show model number or best , schematic

In the first post he identifies it as the 135W version and provides a schematic. It's not the Fender original schematic but it seems ok.

  

[Latole]

the Fender one ;

From ; https://www.thetubestore.com/later-fender-guitar-amp-schematics

[Willabe]

That Fender schematic shows 500vdc right after the stand by switch, in the play position.

He's not even getting 1/2 that vdc in 1 of the amps, and only ~1/2 the vdc in the other. There both way off.

What kind/brand meter do you have? And change the batteries, that can through a meter way off.

I worked on 1 of those 4 x 6L6GC Twins and the vdc on the B+ was lower than the schematic but no where near as low as your getting.

Edit; My mistake.

[pdf64]

That Fender schematic shows 500vdc right after the stand by switch, in the play position.

He's not even getting 1/2 that vdc in 1 of the amps, and only ~1/2 the vdc in the other. There both way off. ...

The first post reports 488V on the 6L6 anodes and screen grids of the problem PA amp.

[Willabe]

That Fender schematic shows 500vdc right after the stand by switch, in the play position.

He's not even getting 1/2 that vdc in 1 of the amps, and only ~1/2 the vdc in the other. There both way off. ...

The first post reports 488V on the 6L6 anodes and screen grids of the problem PA amp.

Yes, but then in post #3, he's down to ~200vdc or so.

Somethings not right.

Try a different meter with new batteries. Or try the meter your using on a known good working amp and see what readings you get.

Best to do both.

[Willabe]

If you have a 2nd meter, change the batteries in both meters and take reading with both meters in the same known good working amp. 

That way you'll know if the meters good or going bad.

If you have a different set of meter leads try those also doing the same test with them on both meters.

[Latole]

If you have a 2nd meter, change the batteries in both meters and take reading with both meters in the same known good working amp. 

That way you'll know if the meters good or going bad.

If you have a different set of meter leads try those also doing the same test with them on both meters.

Or the meter's probes are intermittent, broken ?

[Willabe]

If you have a different set of meter leads try those also doing the same test with them on both meters.

Or the meter's probes are intermittent, broken ?

I said that.

[Willabe]

Another moderator and I had a little mishap happen and 2 of my post replies were accidentally deleted.

When I get a chance I'll try and re-post them.

But, watch out with those amps, some of the 135 UL series have switches on the speaker jacks that switch the OT secondary taps. You have to plug the right speaker ohms into the correct jacks. 

That 135 UL PA amp you have, you have to plug into both speaker jacks or just the lower jack in the schematic. If you don't, the other jack will ground out both speaker jacks, even though you have a speaker plugged into the other jack. Or you could use a 1/4" dummy plug in either speaker jack and use 1 speaker;

https://el34world.com/charts/Schematics/files/Fender/Fender_pa135.pdf

[pdf64]

That Fender schematic shows 500vdc right after the stand by switch, in the play position.

He's not even getting 1/2 that vdc in 1 of the amps, and only ~1/2 the vdc in the other. There both way off. ...

The first post reports 488V on the 6L6 anodes and screen grids of the problem PA amp.

Yes, but then in post #3, he's down to ~200vdc or so.
 ...

Aren't those anode current mA readings?

[spunko]

Thank you all for the replies. I really apreciate it.

English is not my language so is hard for me to express correctly. I will try to give some feedback.

- I have 3 meters, a fluke 117 and 2 cheap ones from amazon (used with the bias probes).
- All bateries are relatively new, I also tested them arround 9.4v each.
- All volatges on 6L6s seems normal, and are the same between amps (488 VDC). I haven't posted lower voltages, only in the PI but those are kinda ok, at least in the twin 135.
- I will check some resistor values against the schematic, and also will post some voltage readings I got from both amps phase inverter tubes, and also without the tubes.
- My leads are Pomona brand, they are good quality. I got them like 4 months ago. No problem with them.

I have two of these basically (pic taken from euro probes).


I have tested two Hot Rods and a deluxe reverb in this right moment. I get good readings on the three amps. When I come back to the PA135 and the twin 135, I get those low readings with the three meters (even the fluke), that is why I asked if its something with the UL amps. The probes and meters work ok on the amps mentioned above.
I also used the three meters to do the shunt method, and they all read the same.

But, watch out with those amps, some of the 135 UL series have switches on the speaker jacks that switch the OT secondary taps. You have to plug the right speaker ohms into the correct jacks. 

That 135 UL PA amp you have, you have to plug into both speaker jacks or just the lower jack in the schematic. If you don't, the other jack will ground out both speaker jacks, even though you have a speaker plugged into the other jack. Or you could use a 1/4" dummy plug in either speaker jack and use 1 speaker;

https://el34world.com/charts/Schematics/files/Fender/Fender_pa135.pdf

Thanks for mentioning about the speakers jacks. The PA135 have two outputs jacks (switches) rated at 8 ohms each. The twin 135 have a normal jack for the internal speaker and the switched jack for the ext speaker. both rated at 4 ohm.

- The twin 135 sounds super loud even with my shop speaker at 8 ohm (using int speaker jack output as normal)
- The PA135 sounds with the same low output in both speaker jacks. I also plugged a dummy plug into the fisrt jack and the shop speaker into the second jack, and viceversa with the same result.

PA135


TWIN 135

[Willabe]

That Fender schematic shows 500vdc right after the stand by switch, in the play position.

He's not even getting 1/2 that vdc in 1 of the amps, and only ~1/2 the vdc in the other. There both way off. ...

The first post reports 488V on the 6L6 anodes and screen grids of the problem PA amp.

Yes, but then in post #3, he's down to ~200vdc or so.
 ...

Aren't those anode current mA readings?

My mistake, I was looking at the PI #'s.       

[Latole]

What is your language and you country ? German , Poolish ?
It helps to know so I can better answer you by not putting myself in your shoe

I speak Canada french and little bit english.

Battery on digital meter ; if you can read digit; battery is good.
 A pale/ dim screen means a low battery, but the reading will be just right.

[pdf64]

...
Battery on digital meter ; if you can read digit; battery is good.
 A pale/ dim screen means a low battery, but the reading will be just right.

My experience differs. A failing battery can cause the meter to display erroneous readings. Yes, that applies to Flukes too.

[Latole]

If I've been following forums for a long time, the vast majority of reading and testing problems are due to a lack of knowledge on the part of the user.