Quote from: Roly on July 05, 2014, 04:59:39 AM

Quote from: nashvillebill(merely switch the meter to AC)

Nope - we've been around this one before.  Most common VOM's and DMM's will see DC as AC and give wildly inaccurate readings.  Apparently some of the more expensive DMM's have a true AC input but most don't, they just peak rectify and assume no DC.  To measure ripple with one of these a cap, say around 0.1-1uF, of suitable voltage rating has to be used in series with the probe to block the DC.  On older moving coil multimeters this was sometimes inbuilt as a socket marked "Output" or just "Out". {the one in the multimeter to hand is 0.047uF at only 400V working}

Sorry, I forgot about the averaging meters that can measure a DC battery as AC; I'm using a true-RMS Fluke. 

My old Simpson 260 analog meter did indeed have the Output connection (cap coupled) to measure AC on DC....

Yes, the tube (valve) has lost its integrity physically.  A tiny crack in the glass-- at the base, or around a pin perhaps.

If this is the second one to experience the exact same failure, then I'd look very carefully at the entire mechanical area around the tube.  How is it being inserted, how is it being clamped, is something hitting it or pressing on it.

I certainly don't claim to be an expert, but I've got a few questions:

1) When you checked the amp's output power, were you still on the DBT?
2) The DBT isn't indicating any shorts, so why would we still want to have it on the DBT?  Seems to me that we no longer need any current limiting on the amp's power supply primaries if we're trying to get the amp to full power!
3) The amp apparently still has the original filter caps.  Question a): Is the amp still holding close to its designed 470 and 460 volts when it's pushed to full volume--what do these DC readings fall to?  Question b): How much AC ripple do we see on the 470 volt supply, both quiescent (idling) and at full power? (merely switch the meter to AC)
4) On a similar note with the bias supply, what conditions are the two caps?  Similarly, what are the DC and AC readings on the bias voltage both at idle and at full power?

I'm not sure we can draw a lot of conclusions from the numbers from either amp.  We really would need to see a plot of the frequency response...those numbers you gave are too "simplistic" IMO.  Merely saying one EQ is "100Hz" doesn't tell us a whole lot.

Not only that, you are comparing an active transistor-driven preamp to an all-tube amp!  Can we say comparing apples to oranges?

In theory, yes of course the EQ on the Ashdown could be modified, but given that it's active, it's going to take a lot more than just experimenting with different resistors and caps. Running a SPICE simulation for what you've got, then tweaking it, while maintaining correct operation of the circuit.

I agree with phatt, I doubt the usefulness of messing with the tone stack on the Ashdown.  Maybe try some different speakers, then if that doesn't work, go buy the Ampeg.   No offense to the Ashdown owners but that Ashdown isn't in the same league as a SVT.

There are several electrolytics being used as coupling caps, have you replaced all the electrolytics?

If any caps look suspect--even film caps-- I'd replace them.  And getting that voltage back up a few more volts might help too.

Quote from: joecool85 on May 22, 2014, 06:58:54 AM
You should really go through the thread and test the things folks have asked you to. Once you provide answers to those questions, we will be able to help.

We cannot overemphasize this enough.  If your car doesn't start one morning, do you automatically replace the engine?  Of course not...you would (hopefully) go through some diagnostic steps--does the starter motor turn over, is there gas in the tank, etc.

Same way with amps.  Sometimes we open up the amp and the problem is obvious, but often it isn't.  If the problem isn't obvious, we typically start going through various troubleshooting steps--we don't just start replacing parts.  We measure voltages, trace the signal through the amp, sometimes we observe the signal on a scope.  The goal is to determine where the problem lies, then replace only the defective part(s).  It's the most efficient, most cost-effective way.

Merely replacing parts can sometimes create even more problems.  I've got a friend who sells used stereo equipment and from time to time he brings stuff over to me to repair.  Often, he's tried to fix it himself...but his repair skills consist merely of unsoldering parts and replacing them, he has little technical knowledge and no troubleshooting skills.  Usually, I've found that he has replaced perfectly good parts.  He's also inadvertently soldered transistors in wrong, lifted PCB traces, and had cold solder joints.  In other words, by attempting to fix an amp with blind-luck parts change-outs, he's caused more problems than he's solved.

So let me give you an example.  Suppose you have a 12 volt battery.  Now you hook up two resistors in series.  One resistor, which I will call the load, will be 200 ohms.  The other resistor, which I will call the dropping resistor, will be 100 ohms.  Okay, these are in series.  So the total circuit resistance will equal 300 ohms (it's in series, so 100+200).  Since this is connected to the battery, the circuit drops 12 volts across the two series resistors.  The current can now be calculated: V=IR, or 12=I(300).  Divide the 12 by 300 and that gives a current flow of 0.040 amps.  Each resistor, since this is series, has 0.040 amps going through it.  The 200 ohm load resistor will have a voltage drop across it of: V=IR or V= .040 (200) or 8 volts.  The 100 ohm dropping resistor will have a voltage drop across it of V=IR or V= .040 (100) or 4 volts.  (Sanity check, 4 volts for the 100 ohm resistor plus 8 volts for the 200 ohm resistor should equal our 12 volt power supply.)

Now let's keep the same 12 volt power supply, and the same 200 ohm load resistor.  But let's change the dropping resistor to 400 ohms.  Same series circuit, but now the total circuit resistance will be 600 ohms.  The current flow through the circuit will be 0.020 amps.  The voltage drop across the 400 ohm dropping resistor will be 0.020 times 400, or 8 volts.  The voltage drop across the 200 ohm resistor will now be 4 volts.

Go the other way around, and keep a given dropping resistor--say the original 100 ohm resistor--but change the load resistor to 1100 ohms.  The circuit now has 1200 ohms total resistance, so only 0.010 amps will flow through it.  1 volt will drop across the dropping resistor, and 11 volts across the load.

AS you can se, dropping resistors are not always an accurate way to drop voltage. 

If you are using a resistor to do the voltage drop, the current flow through the resistor determines the voltage drop.  You've got to know the rest of the circuit to determine the current that will be drawn through your dropping resistor.

That's why the formula is V=IR.  The voltage drop V equals the current flow I times the resistance R.

I wouldn't modify it yet, but do some preventive maintenance--replace the electrolytic capacitors...they are 30 years old and living on borrowed time.  This may tighten the bass up and make the amp sound smoother.

Then, play it  a while and decide what you like, and don't like about it.  Then make your decision to modify based on your actual preferences, rather than just some guy's opinion on the Internet.  Maybe a speaker swap would match your sound better...just give the new speaker a chance to break in.

...I see on the last picture the preamp tube sockets are indeed mounted on rubber isolation washers, these are almost certainly dried/cracked/split and not functional.

I will also point out that I doubt I will recoup all of my refurbishment cost, should I decide to sell the amp.  Old Gibson amps don't quite have the appeal for collectors as the old Fenders.

Don't you hate it when great old amps (like this) appear to have been stored in a barn??  I mean, who decides "hey let's take this perfectly decent looking amp and carry it out to the chicken coop to store it"? 

My Epiphone EA-15RV (also made by Gibson, roughly same time frame) looked almost in the same shape as this amp.  I went the complete refurbish route: stripped and replaced the Tolex, stripped the chassis down and sandblasted it to bare metal, had it rechromed, replaced all tube sockets, even made a new eyelet board and repopulated it with everything new.  By the end of it all, the only things original were the wood box, the steel chassis (with its new chrome plating), the power transformer, and the reverb tank and switch.  I tried silkscreening new lettering on the chassis but couldn't get that process down so I stuck on labels instead, that's the only corner I cut.  Not counting the silkscreening effort, it cost a little over $100 for the chrome plating, maybe $75 for the Tolex and grill cloth, and maybe another $175 for the remaining stuff (OT, Garolite, pots, caps, resistors, knobs, sockets, etc).  I replaced the OT because the one in there was nowhere close to original so I used a Deluxe model from Classictone, it bolted right up to the existing holes.  I even replaced all wiring.

It sounds great now, no crackles or noises or hum, so the total refurb was worth it in my opinion.

Oh, mine had a unique way of shock-mounting the preamp tubes, they were riveted in place but through rubber isolation washers.  If yours is the same (I can't tell from the pictures), the rubber washers are available from McMaster-Carr, they offer a silicone hi-temp version.  Instead of rivets, I used #4-40 screws and nuts, then soldered the ground wires just above the nuts, the solder will keep the nuts from vibrating loose. 

Quote from: joshdfrazier on March 27, 2014, 12:13:16 AM
...source to gate  - .733v (no reading with leads reversed)
gate to drain   - .733v (no reading with leads reversed)
....

Gotta quote Enzo here from some of his earlier threads... "what do you mean by "no reading"?  There's no such thing as "no reading".  Either it's open (infinite resistance) or shorted (zero resistance) but regardless, that's a reading.  Which is it?"

I know it sounds like I'm nitpicking but over the Internet some things aren't clear.  (Not trying to bash on you, again, so don't take it personally) 

Listen to Enzo, he is the Peavey guru.

Without a footswitch, the dirty channel is stuck ON all the time, which means of course the Saturation control--which controls the amount of distortion--is active all of the time.

I strongly advise you to download the manual for this amp and read it:  http://assets.peavey.com/literature/manuals/80343008.pdf

Note that some circuit boards in industrial devices will be covered with a "conformal coating" which looks like heavy varnish, it's intended to protect the circuitry from moisture and corrosive fumes.  I have not seen any guitar amps with conformal coating, though I have vague recollections of a pedal which had it...wait a minute, all of my recollections are vague though.

In the case of the OP, though, it's almost certainly the solder flux as already mentioned.  Conformal coating is much more uniform and covers everything. I just wanted to touch all the bases here, but again, in the OP's amp, it's solder flux he's seeing and not conformal coating.

You must have the footswitch to engage the Normal channel, by the way. (for testing, you can ground Pin 5 on the DIN socket).

The manual is available for download on the Peavey website, under Classic VTX.