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Post subject: Twin Reverb 135W low distrted volume
Posted: Fri Nov 18, 2011 8:14 pm
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Latest project is a mid 70's 135W twin. I have low and distorted volume, both channels. New filter caps, resistors all read in spec, PI tube is showing low voltages on pins that the schematic show 200Volts - I have 80. All other PI voltages pretty close. Ground resistors check out, voltages out of transformer(s) read good. One oddity - when I check grounds with my continuity setting on my digital VOM with an audible indicator, I get a warbled tone on some of the grounds, clean audible on others. I get this one side of the diode in the power supply but not the other. The side that's clean is from the filter cap, the warbled side is to ground winding (brown lead) of Xformer. I read 3 volts (positive DC) on that side of the diode. My assumption is somewhere I have something leaking to groind but I'll be darned if I can find it. Warbled sound is there while amp is on standby, same spots give a clean continuity tone when no power applied. New tubes. Speakers measure 4 ohms resistance. I've looked at and tried a host of things, none have isolated the cause.Any and all suggestions would be greatly appreciated.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Fri Nov 18, 2011 8:16 pm
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Additional info - all voltages on power tubes within spec, all voltages on filter caps good as well. Power tubes were running hot, replacing the .1mfd cap in the PI to ground resisitors fixed this.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Sat Nov 19, 2011 3:42 am
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First take a look carefuly to see if you do not make mistake in wiring. This is the first thing to do after a rebuilt project. Check with schematic.
Almost all the time you find something wrong.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Sat Nov 19, 2011 12:26 pm
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Hi danoHSS,

First, do not ever check continuity with the power on, you can fry your meter and/or get hurt. :!: If you only have 80 volts for the PI Plate supply, that means you have a power supply issue, for one thing (or something wired wrong). Trace back to see where the voltage is lost. If the power supply is good as you say, then the problem might be somewhere in between the power supply and PI. :idea:

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Mon Nov 21, 2011 7:11 pm
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Pins 2 and 8 of the PI tube (12AT7 - new, tried 2 different new ones in fact) are the ones with low voltage - they are the output from the tube, not the input from the power supply. The voltages in to the tube are all correct (they closely match the values on the schematic) so that tells me the power supplies are OK. The output from pins 2 and 8 route through a few resistors eventually to ground. The schematic shows they should have 200V, they read 80. All the wiring checks out, the ground connection is solid, and all of the resistors test in spec. I've mesaured fro DC voltage leaking through capacitors, can't find any. I've started replacing components blindly (resistors and capacitors) hoping to get lucky - not the way I like to solve problems but I've exhausted any that my logic could find. The solution continues to elude me...


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Mon Nov 21, 2011 8:09 pm
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One thing I have noticed - my heater voltages are around 3.6 AC. Not sure if this could be causing it, could be either the hum balance pot causing the lower values or the AC winding giving a low value. The last Twin Reverb I did had less than the spec'd 6.3 volts on the heaters (around 4.6) and it works extremely well so not sure if this is one I should be paying attention to.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Mon Nov 21, 2011 8:16 pm
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It seems that you're missing something from the DC supply to the tubes Plates, or checking the wrong pins. :idea:

The "output" or Plate of the tube (AC) is also the Plate supply voltage (DC) and should be close to 200 - 250 VDC and more for all preamp and PI tubes (some have up to 330 VDC). The power supply should have approx 500 volts from the supply to the OT and output tubes Plates. This same 500 VDC voltage is stepped down to lower values through several resistors for each lower voltage supply needed, and applied to the preamp tubes Plates. The Plate is pin 3 for each of the output tubes, and pins 1 and 6 for the preamp and PI tubes (two Plates in each preamp tube).

80 volts on pin 8 of the PI is close to correct (could be up to 110 volts or more). Pin 8 is one of the two Cathodes in the PI tube, the other is pin 3. Pin 2 is one of the signal grids, not sure what you are looking at there or why, but probably irrelevant to your problem. I don't believe that the schematic shows 200 volts for pin 8 or pin 2 of the PI, you should double check that.

Just saw your new post when I previewed my post. How are you checking the filament supply? Reading 3.6 VAC to ground on each side of the filaments sounds about right.

BTW, which schematic are you using?

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Tue Nov 22, 2011 9:06 pm
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First - thank you very much for taking the time to read this and try and help - it is GREATLY appreciated. OK, here's more info - went back and checked voltages, here's what I have:
Phase Inverter tube -
Pin 1 - 262VDC
Pin2 - 77 VDC
Pin3 - 97VDC
Pins 4 and 5 - 3.16 VAC
Pin 6 - 272 VDC
Pin 7 - 77 VDC
Pin 8 - 97 VDC
Pin 9 - 2.24 VAC

Power Tubes-
Pin 1 - -55 VDC
Pin2 - 3.3 VAC
Pin3 - 485VDC
Pins 4 - 485 VDC
Pin 5 - -55 VDC
Pin 6 - 485 VDC
Pin 7 - 3.3 VAC
Pin 8 - Ground

Power Transformer
Green and Green/Yellow - 3.4 VAC
Red - 242 VDC
Red/Blu - 243 VDC
Brn - 56 VAC
Brn/Blu - Ground
Yell/Red - 242 VDC

So that's the current status, still have the low volume and distorted output. The schematic I'm using is the Fender one posted a bunch of places on the web, it's the 135W art number 013694 (the number in the lower right corner). Any additional info would be great -Thanks again, Dan.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Tue Nov 22, 2011 10:07 pm
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OK, I have the same schematic, and your voltages on the tubes look good (if you read each side of the filament to ground). Forget any AC readings except for the filaments, just read the DC supplies. Any AC readings on the tubes will only be useful with a signal input to the amp. Pins 1 and 6 of the output tubes' sockets are just connection points, no connection to tubes. Usually, reference DC readings to ground (chassis) and read the AC directly across both points.

Recheck the DC voltages on the output transformer primary (forget the secondary, it's meaningless) something doesn't look right there, the five wires are: Blu, Blu/yel, Red, Brn/yel, Brn. None of those wires should be at ground potential, and all should read about 485 VDC referenced to ground.

The power transformer is the one that the electrical plug is connected to, and if your DC supplies are good, the power transformer is good. Make sure the AC supply for the filaments is correct from the power transformer, otherwise it appears good. You're only concerned with AC voltages coming from the power transformer, converted to DC with rectifier, and the DC voltage applied to the output transformer.

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Tue Nov 22, 2011 10:47 pm
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Checked all those voltages, they're all at 490 (+/- 5) VDC so Xfrmr looks good. I have discovered that some of the voltages on "valve 5" - 7025 on the schematic, the second tube from the power tubes, it shows a 12 AX7 on the cabinet sticker - seem off. Schematic shows pin 1 at 250VDC, I have 385. Pins 3 and 8 which should have something on them read 0 Volts??? Pins 2 and 7 are at -41VDC, 6 reads 330VDC which matches the schematic. I've tried swapping the tube out, no difference - both tubes I tried were brand new. AC heater voltages are good. Trying to figure out what would cause pins 3 and 8 to be showing 0 as my next clue...


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Nov 23, 2011 12:02 am
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V5 is the vibrato tube, both halves. It is an oscillator circuit, and not sure if it matters and probably not related to your problem, because the tubes just aren't conducting (off), so no voltage drop on Cathodes and you will read no voltage then. You should read the voltage indicated on the schematic when the tubes are conducting (using vibrato channel). The Plate voltage on the half that is high will change also when the tube is conducting. It wouldn't hurt to check the value of the 220K ohm resistor on pin 1 while you are there (power off, caps discharged).

What is your bias current on each output tube?

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Nov 23, 2011 7:38 am
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Now that was a good question - my bias is only running at 27 mA. My calculations show they should be up around 47 ish - 485 Volts on pin 3 of all power tubes, Sovtek 6L6WXT's run at 30 watts dissipation, 70% of that should be 21 Watts, 21 divided by 485 gives 47 mA for 70% of max dissipation. Since this amp has no true bias adjust and just a bias balance, does this mean the side of that balance (output tubes matching) pot that goes to ground is showing too much resistance keeping the bias too cold? Is it possible that tubes running this cold could account for the low volume distorted output? I'd think the amp should still have decent volume even with these tubes running low but don't know for sure - I've never tried to run one WAY cold to see how performance deteriorates...
Are we having fun yet? Actually, yes. Every one of these problems I've solved has taught me more about old Fender circuits.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Nov 23, 2011 7:45 am
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Almost all Fender amps work very well with colder Bias. Myself I never set a fender Amp to 70 % and I doo many tests at hot and cold bias.
I find tone is not better over 60%, many time less than 60 %

And it save tubes


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Nov 23, 2011 8:18 am
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+1 stratele52

In this case I would also go for 55% TO 60% (34 mA to 38 mA), although you can go up to 70% if you want. Usually, if the bias is less than 50% (as is the case here), you can not get good clean sound or full volume. Since there is no bias adjust, you have to change the 2.2K ohm resistor that feeds the "output tubes matching pot". Try a higher value, maybe 5.6K, to change the -55 VDC at pins 5 of the output tubes to a lower value like maybe -45 VDC to -50 VDC (or whatever voltage it takes to get the right current through the output tubes). It may be a little trial and error to find the value of resistor that will give you the proper bias for these output tubes. :idea:

And the mA values I am using are for Plate current. If you are reading Cathode current, the mA numbers should be about 5 mA higher. So, Plate current of 34 mA to 38 mA, is the same as Cathode current of 39 mA to 43 mA. Make sure to check each tube to be sure that they all have close to the same idle current (within 5% or so to each other).

Be sure to immediately check the idle current through the output tubes after changing the bias resistor, and watch the tubes to see that they don't red-Plate from too much current, in case we guessed wrong on the bias resistor value.

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Nov 23, 2011 9:14 am
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I'm not a tech'ie (no disrespect here), but this is a very interesting thread. I am following the logic and the throubleshooting path. Can't wait for the outcome and what the final soulution is. I'm sure there are others like me who "can't" do this, but find it just as interesting listening to those who "can" as they go through the process. Keep it up!


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