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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Nov 23, 2011 9:49 am
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I know, I'm with you, it's like watching a TV show. My electronic skills are minimal :(
You guys! Point us in a direction to learn the basics, which book?


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Nov 23, 2011 9:51 am
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Hi Breakrite,

You're in for a rare treat here, don't see these amps often. A Fender, ultra-linear, 135-watt, all-tube beast of an amp! I am also anxiously anticipating the outcome, not there yet though (we're still guessing :lol: ).

If successful, the OP owes us pictures! :D

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Nov 23, 2011 12:15 pm
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Thanks again for the help, may not be able to get to the resistor change for a couple days due to the holiday activities starting in the next couple hours, but I'll get to that by Sunday evening at the latest and let you know what the outcome is. Regards, Dan.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Nov 23, 2011 12:20 pm
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danoHSS wrote:
Thanks again for the help, may not be able to get to the resistor change for a couple days due to the holiday activities starting in the next couple hours, but I'll get to that by Sunday evening at the latest and let you know what the outcome is. Regards, Dan.


Dan...it took I think like 4 or 5 months following the "Ceri" rebuilding the "Nikininja" burned up guitar. (another classic cliff hanger btw) This thread is moving a ward speed in comparison....and just as enjoyable. :D Happy Holidays.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Nov 23, 2011 2:56 pm
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atolleter wrote:
...Point us in a direction to learn the basics, which book?


Good website to start, all about tubes:
http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm

As for books, there are dozens to choose from, just a few good ones to peruse.

http://www.amazon.com/Desktop-Reference-Vintage-Guitar-Amps/dp/0964106000

http://search.barnesandnoble.com/The-Guitar-Amp-Handbook/Dave-Hunter/e/9780879308636

http://search.barnesandnoble.com/The-Tube-Amp-Book/Aspen-Pittman/e/9780879307677

http://search.barnesandnoble.com/Tube-Amp-Talk-for-the-Guitarist-and-Tech/Gerald-Weber/e/9780964106017

http://search.barnesandnoble.com/All-about-Vacuum-Tube-Guitar-Amplifiers/Gerald-Weber/e/9780964106031

Much of this info can be found on line, just do a Google search for whichever subject that you want. :idea:

Oh yeah, Happy Holiday everyone! :)

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Fri Nov 25, 2011 10:45 am
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I will indeed post some pics as well, this amp was BEAT - I've done a Tolex job on it and will replace the grillecloth as well. I've gotten faily good at working with Tolex, the biggest hassle was getting the cab clean once the old maeril was removed but it's looking great. At my lady's house for the holiday weekend, I'll be back into the Twin on Sunday evening. Stay tuned...


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Mon Nov 28, 2011 8:56 pm
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I'm back at it and still looking at the phase inverter section as the possible culprit. I'll come back to the bias resistor later - my thought is that a cool bias running in the 25mA range shouldn't cause the volume to be so low and distorted - worth addressing later but probably not my immediate problem. One thing I have noticed - my PI tube is warm but I cannot see any glow in the filaments at all. I've compared this to a couple of my other old Fenders and all the other PI tubes show some glow. I've measured all the resistors coming off the PI tube, they all check out within +/- 10%. The only thing I have not been able to check is the .01 ceramic cap that sits between the volume pot and pin 2. What I'd like to know is what voltage and polarity should I have on the grids - I have 77VDC positive, I've read that these should be negative voltages but the schematic doesn't show any info. I've ordered the Pitman book which may help me some. I have ordered some additional .1 ufd Orange Drops to replace the blue blobs although none show any voltage leaking, some ceramics, and some resistors - I may replace the lot to see if by some strange chance they measure good but one may be bad under load. Still scratching my head...


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Mon Nov 28, 2011 10:17 pm
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The ceramic cap is likely good, they just don't go bad. If a 12AT7 in PI, you may not see the glow as brightly as a 12AX7, but if the tube is warm, it has filament voltage, and of course you checked the filament voltage at the socket? Try a 12AX7 here to see the filament glow. Double check the filament voltage at the PI and tell me what you read.

If you look at the PI tube, it is Cathode-biased and you should have a positive 110 VDC at the Cathode. This voltage is also connected to the grids, through a 470 ohm, and then 330K ohm resistors, one for each grid. So the drops across the resistors means that the voltage at the grid would be lower than the Cathode. This is similar to fixed bias such as the output tubes have, in that the output tube Cathode is at ground, so a negative voltage is required on the grid for bias, keeping the grid voltage lower than the Cathode. In other words, your positive voltage on the grid seems about right.

Just a thought, what about your push-pull knob, any change when using it?

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Tue Nov 29, 2011 6:28 am
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The voltages on the filaments are 3.24 at pin 9 and 3.16 across pins 4/5 - pretty normal. With the huim balance pot I can get them to match on any given tube if I need to, not really necessary.

My cathodes are running at 97 VDC, the grids are at 77 - what I wasn't sure about was whether this voltage differential was enough for proper operation since the schematic doesn't show any values for the grids.

I too have wondered about the push/pull pot since it ties in to the grid side of one half of the PI. One thing to note on this - the .01 blue blob cap that sits between the output of pin 2 and the pot showed visible signs of failure - it looked like it had overheated at some point. I replaced it with an orange drop expecting it to fix the problem (it was the only component showing any visible signs of failure) but to no avail. Since I was working in that area I also replaced the .1 with an orange drop as well. Of interest - before I replaced the .01 the PT's seemed to be running too hot, once I replaced it they came back down to a more normal level.

Thanks again for the time, effort, and help, Dan.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Tue Nov 29, 2011 7:06 am
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CORRECTION - after I posted that last one I thought through it and realized I made a mistake - looking at the schematic and not the chassis. Sorry for the bad info - the .01 blue blob cap I replaced goes between the yellow/green Xfrmr OP and the MV pot, NOT the ceramic .01 between the PI and MV pot.

NEW INFO - I've been focusing so much on the PI I missed this one - the 820 ohm 1 Watt that sits directly under the .01 cap I replaced - it measures WAY low (it's bad). Not sure how it didn't get checked before but that's my fault - I'll blame it on the myopia I had around the phase inverter. I'll see if my local Radio Shack has one (it'd be a miracle but I'll try), if not, it'll be ordered. Since it sits between the yellow/green wire from the transformer and ground, I'll bet that's it - too much power "getting away" to drive the speakers. Stay tuned!


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Tue Nov 29, 2011 8:04 am
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danoHSS wrote:
...I too have wondered about the push/pull pot since it ties in to the grid side of one half of the PI...


So, did you compare the sound difference when the pot is in vs out?

BTW, what do you mean by "way low"? Components have tolerances, check the resistor band for the tolerance, but could be as much as 20%.

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Tue Nov 29, 2011 12:51 pm
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The MV pot seems to work - it will increase/decrease volume and there is a slight change in tone - with the volume so low it's about what I'd expect. The volume and tone pots all seem to be working as well but once again, with the volume low it's not real easy to tell.

The resistor is supposed to be 820 ohms (it's a 1 watt big boy) but it's reading around 90 - that's what I meant by way low. I couldn't find an 820 1W locally so have some on order, I may see what I have that's close to drop in there in the meantime to see if I can get a difference. I figure as long as I don't go much above 820 (or up to about 10% over) I shouldn't risk any damage.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Tue Nov 29, 2011 3:17 pm
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Sigh

That feedback resistor would certainly be out of tolerance if checked while disconnected from the circuit.....did you unsolder one side of the resistor and check again?

You're all over the place...... :?

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Tue Nov 29, 2011 6:11 pm
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The troubleshooting plot thickens I see..... We're hangin in there!


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Nov 30, 2011 11:30 am
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I am indeed all over the place! It's the frustration of not being able to nail it down. My assessment thus far is that my trasformers, phase inverter, and power tubes all show correct voltage levels, I've measured all the resistors coming from the phase inverter to (eventually) ground, I've checked all the 470 and 1500 ohm resistors on the power tubes, I've changed the filter caps and all voltages (both positive and negative) as well as grounds in that area check good. Since the problem is in both channnels I'm assuming I shouldn't be looking "upstream" as there are few components common to both channels on the input side of things. The bias is a little cool but not enough to be causing this problem (I may address by upgrading the amp by replacing that 33K resistor from the tubes matching pot to ground with a potentiometer, allowing for true bias adjust as well as matching between the 2 pairs). The wiring of the 3 output jacks checks out as well. The only components in the power supply and output section that I have found to be bad/wrong are a 1500 ohm resistor where a 1200 is called for in the schematic in the power supply (next to the .5A diode) and the 820 ohm in the speaker/output jacks circuitry. I will replace both resistors as soon as my order from Mojo comes in, hopefully later today! Stay tuned...


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