Dart amp

[catstrat]

I hope i've finally got using this forum down. Has anyone ever worked on a Dart guitar amp? This came to my shop and it looks like an old Kustom amp with the rolled and tucked covering. My guess is that it was made probably in the early 70s. It was made in Korea and has Samwha caps in it. It's an all tube amp but i can't locate a schematic for it. The problem is the tremolo works but the o/p tube grid bias is at -61 volts. Does anyone have any suggestions?:

[catstrat]

Dart amp. No takers huh.

Ist question i ask myself is why do i even bother with this oddball crap when i can't even find any info on the thing at all. Of course, no schematic. The latest problem i've got on the piece of s--- is the pre amp tube plates have about 480 volts. The o/p tubes are about 480 volts as well. Now, the pc board has the voltages printed in clear view and it shows 260 volts at the preamp tube plates which is about what it should be. I've checked all the way back to the power supply and i have about 500 volts from the power supply. I've got 115 volts ac for the Power trans primary. I know the filter caps need replacing since it is about 30 years old would be my guess. Is it possible that the filter caps are somehow acting as a voltage doubler? I've already removed almost all the resistors in the circuit and checked them with an ohmmeter and replaced the ones out of tolerance as well as most of the other caps. It's funny that i was in Austin Texas over the weekend and i was talking to one of the techs at Austin vintage guitar who does some of Eric Johnsons work and i explained to him what i was dealing with and it baffled him as well. Am i the only one who runs into problems like this? If anyone can shed some light on this please stand up.

[Enzo]

No one has heard of the Dart apparently, at least with any information to provide, but a tube amp is a tube amp, and most repairs can be done without a schematic. Or they would have responded. I'd bet the basic circuit, at least on the power supply and power amp, is pretty much like a Bassman or most anything. Push pull pentode stages are all pretty much alike, and so are B+ and bias supplies.

Sit down with a pad and pencil and draw your own schematic from the amp. I have done that countless times, it is not that hard. Especially if you start with the basic circuit configuration of something like a Bassman in your head.

-61v, instead of what , maybe -45 to -55? What is the amp doing wrong, what is the symptom?

Ist question i ask myself is why do i even bother with this oddball crap when i can't even find any info on the thing at all. Of course, no schematic.

Think about the amp generically. They are all similar. What would you do if this were a Fender Twin?

The latest problem i've got on the piece of s--- is the pre amp tube plates have about 480 volts. The o/p tubes are about 480 volts as well. Now, the pc board has the voltages printed in clear view and it shows 260 volts at the preamp tube plates

OK, the preamp plates are all up to the B+ supply voltage instead of 260 or whatever. I just bet you that the cathodes of those same tubes are at zero volts instead of 1 or 2 or 3 volts or whatever. The only reason the plate voltage of a preamp tube is lower than the B+ supply is that the current through the tube is also flowing through the plate load resistor causing that voltage drop. No current - no voltage drop.

Now if one tube does this, I think bad tube. But when all of them do this, I have to think the only thing that would turn ALL the tubes off is a loss of heater current. Are the heaters glowing on the tubes with the high voltage?

Oh, and I am assuming something here - tubes. Are you taking readings with the tubes removed? Are you actually getting 480v at the plate of the tube? Or just at the plate pin on the socket while the tube is removed? If the tubes are out, then of course the voltages will all read high. With no tube, there is certainly not going to be any current through the plate resistors. Other than to verify voltage is getting places, voltage readings taken with tubes removed are essentially meaningless.

Is it possible that the filter caps are somehow acting as a voltage doubler

No, not remotely.

get all that straight. Voltage on the power supply will rise if unloaded. SO if the tubes are out it is quite normal for the B+ to rise considerably. Depending on the amp, the transformer sag can also affect the bias voltage. Your -61 could wind up lower with all the tubes running.

Look at the bias supply on most any Fender - or most anything. Usually an AC winding feeds a diode half-wave into a filter cap or two, then a couple resistors or a resistor and a pot set the bias voltage which then goes to the powr tube grids via some large vaslue resistors - 220k for example. If we wind up still having to adjust the bias supply, it would be a simple matter to modify that supply if necessary or set the adjustment if it has one. Are there any trim pots on th thing?

[Sweetfinger]

Think about the amp generically. They are all similar. What would you do if this were a Fender Twin?

It IS pretty much a Twin Reverb.

I actually owned one of those. Dart by Hak Myong or something. If yours is the same as mine, its basically a Plush copy "built like a TV set" to quote one tech I showed it to. IIRC it was essentially a twin reverb/Showman circuit.

This was many, many years ago. Got it at a pawn shop. It was still wired for foreign voltage and someone had shoved a car fuse in it. The reason I remember this one is that it had operated long enough to burn away the plate of one of the 6L6's- just like a little pixie had taken a cutting torch and chopped about a third of the plate away! Cool, huh! Then the side of the glass softened enough that it sucked in and lost vacuum. It also had a small fire near the output sockets- don't remember what was missing, and what likely saved the amp was that the power switch fried to a short.

Sold it a short while later and have never seen another.