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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Thu Dec 01, 2011 6:12 pm
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Well, no change. I replaced the improper 1500 ohm with a 1200 and replaced the bad 820 ohm resistor. Same problem. I've gone back and tried the master volume knob in response to a previous question - the volume aspect works (from 0 to 10) but I can't perceive any difference in tone with the knob pushed in or out. Back to the drawing board. :(


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Tue Dec 06, 2011 6:31 pm
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What's the next step? Let's get this thing running!! (Not like I can help or anything, but I offer morale support)


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 7:32 am
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I'm trying to get my hands on an oscilloscope next - I can measure the AC signal strength coming out of the phase inverter circuit as well as the DC voltage applied to bias the output tubes, but I can't tell if the 2 are correctly out of phase with a multi meter. Aside from that, trying to figure out where else the problem may be coming from. Since it happens in both channels (Normal and Vibrato) there should be a limited number of places it could be coming from. Frustrated.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 8:06 am
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What you need is a procedure, step-by-step, to methodically verify that each section of the amps circuit. Randomly replacing components, sometimes they aren't even bad components, can introduce more problems, which can lead to frustration because things are worse, and now you're chasing two problems.

Personally, at this point, I would start over again with the power supply, and follow each supply to it's end. I have no confidence in the results that you've posted, and question several things that you've "verified" already. Have you tried new output tubes, or tried replacing all other tubes? The bias is still way too cold, still not sure exactly how you checked the current, you dismissed that suggestion, why not try it.

One notable point, you said that the output tubes were "running hot" and you changed a capacitor that "fixed" that. The output tubes are supposed to be hot, and what is your measure of hot, and why do you feel that it was wrong? I would double check every part of the amp that you replaced components in, as a wiring mistake or wrong value component might be all that's wrong.

If you are going to use a scope for testing, it will be practically worthless without a tone generator to put a steady AC signal through the amp. A guitar into the input will not give a steady signal for troubleshooting, the scope screen will look like a Spirograph, not of any use. And checking for AC at any point in the signal path without a steady signal into the amp is pointless. A scope will not help your troubleshooting ability. I would bet that you've overlooked something simple, like tubes, bias, bad connection. Over-thinking it.

In fact, maybe even something simple like a coupling cap is bad. Not even sure if or how you verified those. :idea:

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 9:11 am
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shimmilou wrote:
Personally, at this point, I would start over again with the power supply, and follow each supply to it's end.


+1!

The power supply is like the foundation of a house -- if it's wonky, so's the rest of the structure.

shimmilou wrote:
If you are going to use a scope for testing, it will be practically worthless without a tone generator to put a steady AC signal through the amp.


Also a +1!

You need a constant-state tone injected at the amp's input in order to provide the oscilloscope with something to compare to. A 400 Hz signal of roughly 300 mV peak-to-peak from an H-P or Eico signal generator is ideal.

HTH

Arjay

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 10:14 am
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In a pinch, I got one of these when my BK took a tumble. Since my first use with it, I find that I really don't need anything else, very surprisingly good little generator. It does sine wave and square wave, independent variable voltage output for both types of wave, phone jacks for the outputs and the VCO connection, and not much bigger than a pack of smokes. Great if on a budget, and frankly all you will need for amp testing. Just use the scope to set the signal that you want, and use an instrument cable directly from the phone plug of the generator into the amp, no adapters needed.

http://www.mcmelectronics.com/product/72-490

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 12:02 pm
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shimmilou wrote:
In a pinch, I got one of these when my BK took a tumble. Since my first use with it, I find that I really don't need anything else, very surprisingly good little generator. It does sine wave and square wave, independent variable voltage output for both types of wave, phone jacks for the outputs and the VCO connection, and not much bigger than a pack of smokes. Great if on a budget, and frankly all you will need for amp testing. Just use the scope to set the signal that you want, and use an instrument cable directly from the phone plug of the generator into the amp, no adapters needed.

http://www.mcmelectronics.com/product/72-490


+1000!

And the price is right.

Good find, shimmilou!

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 12:43 pm
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Thanks for the continuing guidance, I'll try and go through this a step at a time with what I've done, what I checked and how. Lets start with the filter caps. I replaced them all with new electrolytics from Mojo - they're the black ones they sell as Mojo no longer promotes or carries Spragues. I've used them on other amps but I guess one of the might be bad. If any were installed with the polarity reversed it would have let me know the instant I powered it up...

The voltages I have on the filter caps are as follows:
the 220uFd 350 volt caps - the first has 497 V on the positive end, 248 on Neg
the second has 249 V on the positive end, 0 on Neg

The next are the three 22uFd caps, all with the same polarity orientation.
Starting with the one closest to the 220 uFd caps:
the first has 404 V on the positive end, 0 on Neg
the middle has 390 V on the positive end, 0 on Neg
the last has 354 V on the positive end, 0 on Neg

The last 2 are the 80's on the end.
the first has 0 V on the positive end, -61 on Neg
the middle has 0 V on the positive end, -58 on Neg

The values are not exactly what show on the schematic but the're not radically off. Based on this, I assumed that the filter cap job is good. Sound assumption or am I missing something?

As for the tubes running "hot" - they were cooking - I didn't leave the amp on very long like this. I have a number of tube amps, I know they get hot, but these were screaming hot very quickly. Way beyond what I'd expect. They were radiating heat in a big way when you got anywhere near the amp.

I will go back to the bias suggestion later tonight and reduce the value of the 33K resistor, I agree that the bias seems cold with the tubes running in the mid 20's mA range. I tested these using a bias probe, tube in the probe, probe in the socket, amp powered up, read milliamps on the meter after allowing to warm up until stable readings (3 to 5 minutes each tube).

I have replaced all the tubes with brand new. Suspecting that one or more of the "new" tubes could be bad, I did a full swap with a functioning 100 Watt Twin Reverb amp - these tubes are also essentially new - only a bout 5 hours of time on them. With either set of new tubes, same result.
Gotta run out for a couple hours now, I'll go through the PI and output section in the next email with all of the voltages I have as well as where I've changed any components.
Than ks again for reading and helping, Dan.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 1:16 pm
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danoHSS wrote:
...As for the tubes running "hot" - they were cooking - I didn't leave the amp on very long like this....


Ok, sounds like the DC supplies are all good, including negative supplies, and you also ruled out the tubes, excellent.

The above still puzzles me, were the Plates glowing red? I know that you didn't let this go on for long, but any idea what the Cathode current was at that point? I would closely look at that capacitor that you changed that "eliminated" the problem with the hot tubes. In other words, the tubes at that point may have been drawing excessive current, but not necessarily because of that PI circuit cap being bad, you may have opened/shorted the circuit somewhere at the same time as the cap change. :idea:

Just to be sure, you do have the speakers plugged into the correct jack? The main jack has to be used first, as it is a shorting jack. If you only use the external speaker jack, you will get no sound out, or a very low and distorted sound out. :idea:

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 3:43 pm
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Now that's a good point - the OP tubes may indeed have been drawing too much current because of something else. The cap I changed is the .01 mFd that sits between the yellow/green wire of the output tranny and the MV control ground 100 ohm resistor. It has the 820 ohm resistor in parallel with it. It was visibly damaged, when I changed it the output tubes then ran at a less than nuclear reactor core temperature. I did not check bias on them when they were hot though, would have been an interesting and valuable data point.

I have removed all tubes, cleaned the sockets, inspected them with a magnifying glass and bright LED flashlight looking for evidence of arc traces, I've also done a wiggle test on all tubes to try and detect/eliminate a loose socket pin connection as the culprit. All seem good although a carbon arc trace on the inside of one of the sockets would be impossible to see...

I'm always using the speakers connected to the output jack every time power is turned on ( not the external speaker or line/recording jack). I have measured the speakers and ensured a 4 ohm load is there.

If I had an odd ground/short somewhere in those speaker jacks that might do it as well, I've considered removing (unsoldering) the leads for the external and line jacks to see if that makes a difference. When I check for continuity with the power off, pretty much both sides off all 3 jacks show a ground connection. According to the schematic I'd expect this since the black wire shows a ground connect and it's part of the windings that terminate in the green and green/yellow wires.

I'll post the voltages of all the pre-amp tubes in a bit. I have put a DMM on the output of the PI (plates) and seen that an AC voltage does flow from both plates when the guitar is played, but I can't tell if the signal is clean and in correct phase, or as strong as it should be. I get a max of about 19 VAC on the plates of the PI. They both show good DC voltage as well (according to the schematic).

The hunt continues...


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 4:15 pm
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Ah, sounds like the cap is the one in the feedback circuit. If so, that cap couldn't possibly caused the output tubes to run hot, unless the output transformer is shorted primary to secondary (not likely, but possible). If the transformer was shorted like this, you would have high DC voltage on the feedback resistor/cap, and since that isn't the case, I would bet money that the cap is not what caused the tubes to run hot. But, when you get to it, double check this part of the circuit, that cap in the feedback circuit should also be connected to the 22K ohm, one-watt resistor and another .1 cap, both in the PI circuit.

I like the procedure that you are using so far, carry on. :)

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 5:19 pm
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Not that it amounts to a small hill of beans, but amongst other things, I've been flying helicopters since 1973. My point? It is a business that demands attention to detail and a lot of trouble shooting, and even though this amp repair is not my area of expertise, I love observing the trouble shooting process. It's logical and orderly steps. I guess it's like a good mystery you finally get to solve, one way or the other. Keep it up guys. This is a good read.


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 9:26 pm
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A new twist and FINALLY a glimmer of hope! So I have another 100 Watt Twin Reverb(in great working order, my last project) sitting in the dining room, have been using its tubes and speakers for troubleshooting. I decided to try something I had not done before - I hooked up both the normal speaker jack as well as an external speaker. I used the working twin cabinet as the main speaker, and the ones from the Twin I'm working on as the externals. Sound! Glorious loud clean Twin Reverb sound! Now I need to figure out if it's a jack issue with the speaker jack, or, since it appears that the windings for the output transformer have 3 leads (a center - green and yellow and two ends - one black and one green) I now need to figure out if by some chance there's an issue with the green to black windings or, if it's just something flaky with the jack. Finally something solid to go on...


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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Wed Dec 07, 2011 9:56 pm
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Awesome! The main spkr jack (lower jack on schematic) is a shorting jack, shorts the OT when unplugged. The ext spkr jack is a switching jack and selects the other tap on the transformer when plugged in, changing the output impedance requirement. If the amp works when both jacks are used, the transformer is likely fine, and seems a jack issue. Are you sure you were using the main jack? What if you just use the other jack by itself, what sound then?

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Post subject: Re: Twin Reverb 135W low distrted volume
Posted: Thu Dec 08, 2011 10:16 am
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The plot thickens - using the external speaker jack by itself produces full volume. I've looked in detail at the wiring and switching of the External jack and the regular jack, all seems to be functioning normally. I'm not out of the woods yet - here's my latest observation:
When I trace the wiring, when using the external speaker jack the signal is coming from the solid green wire coming from the output transformer. That wire goes directly from the transformer to the external speaker jack. When the regular speaker jack is used, the signal comes from the yellow and green wire which routes from the transformer, to the circuit board, and a white jumper connects from that contact point to the external speaker jack and then jumpers to the regular jack. The regular jack circuit is driven by the Yellow and Green transformer output. The spot on the circuit board where those 2 wires (the yellow/green and white jumper) connect is the same spot where the .01uFd and 820 Ohm in series items that I replaced terminate, the other end of those components goes to ground and the dreaded master volume pot. The master volume control does increase and decrease volume on both speaker jacks, but I don't hear a difference in tone with the push/pull now that I have a way to run the amp at normal volume levels. I'm trying to understand that side of the master volume circuit, what it looks like is that it send some amount of feedback from the output side of the amp (right at the speakers essentially) back through the master volume pot to the pre-amp section, or to be more precise, adds it to the signal as it exits the pre-amp and enters the phase inverter.
I could try one of a couple paths - replace the MV pot - I did order 2 from a company in Watford England, the only place I found that seemed to have the exact drop-in replacement. Other option is to bypass the MV pot at least temporarily just to prove if it may be the culprit.
Any chance this latest clue leads to an "Ah-ha" moment? By the way, this is still fun! Regards, Dan.


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