PLEASE HELP!! Ampeg SVT R47 Preamp Resistor headache!!!

[NardisAmps]

Hello,

I'm surely overlooking something simple and its about to drive me INSANE! I'm hoping to get a few words of wisdom and maybe some reminders on something I have overlooked......

I have a '73-'75 Ampeg SVT. The first power resistor in the preamp section, R47 (8.2K / 2w) was smoked when I first opened the amp.

I first checked the C23 (40uf / 450V) filter cap accross it, and replaced it with a newly formed sprague atom to rule out any doubt. I then pulled all the preamp tubes to see if it still was getting hot. No problems at that point.

Still no problems with V5 (6C4) installed, the resistor remained cool. After putting in V4 (12DW7) the resistor again SLOWLY heats up to the point of smoldering. It maybe takes it 2-3 minutes or more of the amp sitting idle in the play mode. In that one tube socket I tried the other 12DW7's in case of a bad tube. They all did the same thing. I read somewhere else on this forum to double the wattage of that resistor. The next size up I had was a 10W so I used it. It still got so hot you could not even touch it.

I then went through and checked all the cathode and plate resistors in the preamp, all of them were dead on the money. Couldn't find a problem there.

I also took some measurements. With all preamp tubes installed I measured the total current coming into the preamp in the play mode at 20ma. (measured between the 8.2K resistor and the power in) Voltage coming in is 513v, other side of 8.2K is 348. So if my math is correct, the 165V drop x .020A = 3.3 Watts. This is almost twice what the 2 watt resistor in the schematic can handle.

Do these numbers tell you other, more knowledgeable techs anything?? Where do I go from here!!?? I have tried NEW preamps tubes, that did not change anything. How can i find this excessive current draw?

Note: With the 10w resistor it DOES work and sounds great. I'm just concerned about the R47 resistor drawing a lot more current (3.5W more or less) that the 2 watt part in the schematic. Could this be normal? The 10W resistor gets quite hot, you can hardly touch it for more than a second, but it seems to stay at that temperature and is not turning colors or anything. Could this also be normal. I know resistors are designed to dissipate heat. I'm Confused.

How much current SHOULD R47 be drawing??

Any help is GREATLY APPRECIATED.

Best Regards,
Nardis
270-576-7030

[Amp Kat]

That 8.2k is also the same connection coming off of the other 8.2k 15 watter from the B+ node. That node has about 750 vdc that gets dropped across that 8.2k 15 watter but some of those schematics out there are wrong or at least dont' show the correct voltage at that node which should be 485 volts. You may have to play with that 8.2 K 15 watter and get the voltage to drop to that level as new caps will raise it some. My guess is that the 12DW7 is bad causing the resistor to smoke. See if you can swap it with another one to see it it will work right and check what the voltage is at that node which is also #5 pin on the square connector and if I remember right the wire is orange.
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[NardisAmps]

Hi KB,

Thanks for the input! The voltage from that node you are speaking of is 513V. Not too for off from 485. The 12DW7's I'm currently trying are BRAND NEW and they are doing the same thing as the previous ones. Any more suggestions?? Thanks again!

Nardis

[Steve Conner]

Are they NOS 12DW7s or some Russian equivalent? I seem to remember that a 12DW7 is one half of a 12AX7 and one half of a 12AU7, and some that are sold as equivalents may have the halves switched. In that case, the part of the circuit that was expecting a 12AX7 would get a 12AU7, and draw much more current.

[Enzo]

Are you running this amp without output tubes? That would certainly make all the B+ voltages rise substantially.

Just a note, the resistor is not drawing any current. SOmething beyond it - the tubes - draws the current through the resistor. All the resistor does is create a voltage drop due to Ohm's Law. And that of course means power will be dissipated.

I have several revisions of this preamp, and power amp, on file. The one dated 6/69 shows the preamp B+ rail as 300v with 430 coming on from the PA. (Never mind that on the PA drawings that voltage node is 465-480) Noted too that mains is specified at 120VAC.

That 130v drop alone dissipates a hair over 2 watts right there. and yet the resistor is a 2 watt. It should be a 5 watt if it will dissipate 2. And that is about 16ma, so with your higher voltages, your 20ma sounds about right. SO clearly the part specified in the schematics is wrong.

Now your voltages ARE substantially higher. You start with 513, instead of 480, 465, or 430. So more wattage is dissipated in the resistor - the 3.3 watts. For 3.3 watts i would use at least a 7 watter, and preferably the 10 watts that you selected.

[Amp Kat]

Yeah I agree Enzo that the higher voltages are a problem and could also be due to the higher voltages of the mains today then back then when they were 115 to 120. I had to tweek that 15 watter 8.2k up to about 12K to get that preamp voltage down and the 12DW7 was fried most likely because of the 600 volts there. If you had to you could disconnect that 8.2k 2 watter and do a current measurement(in-line) just to verify exactly how much current you have with the power tubes in and multiply it by the voltage to get the power dissipation at that point.