Vintage Ampeg SVT = weird voltages

[boroman]

The 8.2k/2W resistor is not across the cap, it's in series with the cap.

It's the second half of the RCRC filter and voltage dropping network. The first half is contained on the power amp board and since it has higher current through it (feeding node C and downstream), the Pd is 15W. The second half is part of the preamp, it's the same value but rated at 2 watts feeding node B.

The 8.2k/2W resistor has a drop of about 180V across it (DEPENDING ON THE SCHEMATIC), therefore, the Pd in circuit may be about 4W, BUT the voltages may not be accurate since experience tells us that this part lasts a long time in most amps. What is the actual voltage drop across the resistor on a normally functioning amp steady state? Wasnex's example shows different voltages and thus different Pd. Many older wire wound power resistors have a rating at a higher ambient temperature than they are used at, therefore at a lower Tamb with some derating allowance 2.1W may be ok (in this case).

It's critical to understand how the voltages are specified, especially since they feed cathode followers whose current is very much proportional to AC signal. It's easy for there to be more to the story than what we are seeing in a static environment.

Right.

Acutally, the 79 SVT has original 8.2k / 5W (!) resistor there - checked now, so I think they upgraded it over time. I just ordered 8.2k/15W ,military spec-ones and will put them in the R47/R49 places in both amps. I'd take measurements then and will be back here. Thank you all for the input!

 

[agedhorse]

Why mil spec? Which spec? Mil spec is not necessarily better.

There’s no reason to use a 15 watt part where the 2 watt part is. 5 watt is plenty, and if it fits the space better then it’s a better choice than the 15 watt.

 

[boroman]

The reason was I could not find 8.2k/5W anywhere here...

 

[Wasnex]

The reason was I could not find 8.2k/5W anywhere here...

Take your pick. Link Removed

 

[boroman]

Take your pick. Link Removed

Thanks. Is there anything wrong with the 15W there? if it's a voltage dropper only, that would be a safe choice to go a bit up with wattage?

 

[beans-on-toast]

The 15W can be used if there is sufficient space to mount it, clear of wires and components. A wire wound resistor is commonly used in that position.

 

[agedhorse]

It needs to fit the space well so the vibration does not create an entirely new problem.

 

[beans-on-toast]

it depends n the revision but in the 70's the resistor was mounted on a terminal lug strip.

 

[agedhorse]

That spacing makes the 5 watt part look like a no brainer to me.

 

[Wasnex]

Thanks. Is there anything wrong with the 15W there? if it's a voltage dropper only, that would be a safe choice to go a bit up with wattage?

I don't know if that particular resistor serves as a fuse. If it does, then using a significantly higher power rating could put the circuit board at risk. Assessing this is over my knowledge level.


AFAIK the most obvious concern is size and mass and how these characteristic relates to mounting the resistor so it fits and is secure.

 

[boroman]

it depends n the revision but in the 70's the resistor was mounted on a terminal lug strip.

View attachment 5109046

View attachment 5109039

Looks like it's burned a bit. I think it's the 2W and schematic, as previously stated, has a mistake and it should be 5W. Mine was looking same, maybe more burnt.

 

[beans-on-toast]

Looks like it's burned a bit. I think it's the 2W and schematic, as previously stated, has a mistake and it should be 5W. Mine was looking same, maybe more burnt.

Yes, that resistor shows signs of stress. It was original. Mistakes in those schematics happened. Errors sometimes even get carried over into later revisions.

 

[boroman]

Yes, that resistor shows signs of stress. It was original. Mistakes in those schematics happened. Errors sometimes even get carried over into later revisions.

Exactly. Later revisions had the same mistake (2W vs 5W) but they had 5W installed from factory.

 

[Fabmars13]

Hi,
i have a 69svt BlueLine and some weird voltage on V3 as well (12dw7).

Pin 1 : 130VDC Instead of 200vDC
Pin 3 : 5V instead of 3.2V
Pin 6 : 180VDC
Pin 7 : 12VDC (Normal???)
Pin 8 : 20VDC

I tried and swapped 3 different 12DW7 on V3
All others voltages on V1, V2, V4 and 6C4 are within 5%. Rail preamp voltage : 300VDC.
All caps jobs done, all resistors tested and changed if needed.
Any help is welcome.
Fabien

 

[agedhorse]

Did the amp work properly and the voltages correct before doing the work? Double-check your work.

If not, my suggestion in the future is to correctly identify and properly repair the problem before changing out anything else because now you have no idea if there is more than the original problem.

 

[Fabmars13]

Did the amp work properly and the voltages correct before doing the work? Double-check your work.

If not, my suggestion in the future is to correctly identify and properly repair the problem before changing out anything else because now you have no idea if there is more than the original problem.

Thank you for your reply agedhorse.
Amp works okay except a little hum that i'm chasing
Voltages in v3 were the same before i changed some resistors out of spec in preamp section.It improved % voltages closed to schematic voltage in preamp except in V3 first triode. I double checked cathode resistors/plate resistors and capacitors around v3 and they are okay. I just write this message because i saw that a guy has the same weird voltage in page 1 so i was wondering if it's a schematic issue (i don't think so) or if someone found a fix/solution.
Cheers

 

[beans-on-toast]

Carefully inspect the tube socket pcb solder joint of Pin-1 V3, reflow it if necessary with the tube removed. If there is oxidation on the tube socket contact or tube pin, it can act like a resistor and lower the plate voltage. If necessary, clean the contact with deoxit and a soft brush.

Ensure that the 47K resistor is within spec with the resistor at operating temperature. Measure the voltage relative to ground on the other side of the 47K resistor, it is normally 300V. Pin-1 on V1 should read the same 300V.

The schematic reference voltages are for a 120VAC line voltage.

https://el34world.com/charts/Schematics/files/ampeg/Ampeg_SVT_PreAmp_RevB.pdf

 

[Fabmars13]

Carefully inspect the tube socket pcb solder joint of Pin-1 V3, reflow it if necessary with the tube removed. If there is oxidation on the tube socket contact or tube pin, it can act like a resistor and lower the plate voltage. If necessary, clean the contact with deoxit and a soft brush.

Ensure that the 47K resistor is within spec with the resistor at operating temperature. Measure the voltage relative to ground on the other side of the 47K resistor, it is normally 300V. Pin-1 on V1 should read the same 300V.

The schematic reference voltages are for a 120VAC line voltage.

https://el34world.com/charts/Schematics/files/ampeg/Ampeg_SVT_PreAmp_RevB.pdf

Hi,

i just did what you said: i reflow socket, put deoxit in tube socket, retentioned pin, and i got the same voltage on v3. The 47k is brand new vishay metal film resistor and test good. Before the (R24) 47k,voltage 300VDC. After (R24) 47k (Pin 1 V3) 140VDC. Here is voltage i measure :

V1 : 12DW7
Pin 1 : 300 V
Pin 2 : 180 V
Pin 3 : 190 V
Pin 6 : 180 V
Pin 8 : 1.73 V

V2 : 12AX7
Pin 1 : 185 V
Pin 2 :
Pin 3 : 1.7 V
Pin 6 : 237 V
Pin 8 : 2.23 V

V3 : 12DW7
Pin 1 : 140 V
Pin 2 :
Pin 3 : 5.2 V
Pin 6 : 177 V
Pin 8 : 20 V

V4 : 12DW7
Pin 1 : 300 V
Pin 2 : 123 V
Pin 3 : 132 V
Pin 6 : 123 V
Pin 8 : 1.26 V

V5 : 6C4
Pin 1 : 300 V
Pin 5 : 300 V
Pin 7 : 190 V
From Power Amp before 8.2k 2W resistor : 430VDC, after 8.2k 2W : 300VDC
My SVT is 220V stock version. Wall Voltage is 230VAC.

Fabien

 

[Wasnex]

In my limited experience the actual DC voltages tend to vary significantly from the values on the schematic. I believe the symptoms you identify suggest high current through the plate resistor and low current through the cathode resistor. Perhaps C7 is leaky. Check to see if there is DC on the wiper of the Bass rotary.

I have a 69 and the preamp has some audible hum. I suspect the hum is being induced into the mid control inductor. The hum in my amp varies with how the mid rotary is adjusted. Minimum hum occurs with the control straight up.

The SVT II service manual mentions this problem, and I suspect it applies to original SVT as well:

The preamp in the SVT II is pretty similar to Channel 1 in the original design.

It's been awhile, but I believe sliding the preamp out of the head case had a noticeable impact on the hum in my 69, which AFAIK supports the comment above.

Legacy user Cerrem claimed the hum in the original design is caused by a ground loop in the preamp. He said it can be fixed, by breaking and reordering the grounds. This requires cutting circuit traces.

I have never come across a vintage style SVT that totally lacked hum in the preamp. I believe my 87 Skunk is better than most and my 69 is a bit worse than most.

 

[Fabmars13]

In my limited experience the actual DC voltages tend to vary significantly from the values on the schematic. I believe the symptoms you identify suggest high current through the plate resistor and low current through the cathode resistor. Perhaps C7 is leaky. Check to see if there is DC on the wiper of the Bass rotary.

I have a 69 and the preamp has some audible hum. I suspect the hum is being induced into the mid control inductor. The hum in my amp varies with how the mid rotary is adjusted. Minimum hum occurs with the control straight up.

The SVT II service manual mentions this problem, and I suspect it applies to original SVT as well:
View attachment 7086348
The preamp in the SVT II is pretty similar to Channel 1 in the original design.

It's been awhile, but I believe sliding the preamp out of the head case had a noticeable impact on the hum in my 69, which AFAIK supports the comment above.

Legacy user Cerrem claimed the hum in the original design is caused by a ground loop in the preamp. He said it can be fixed, by breaking and reordering the grounds. This requires cutting circuit traces.

I have never come across a vintage style SVT that totally lacked hum in the preamp. I believe my 87 Skunk is better than most and my 69 is a bit worse than most.

Wow Thanks a lot. I had the same feeling than you. I suspected a leaking capacitor that force v3 to draw some current. I already checked and swapped with brand new capacitor in C7, C2, C6, C3 , C14 with no change. I suspect maybe an issue in the PEC part but i'm not sure and difficult to test. My SVT has the same hum you described when turn the mid pot. Indeed hum vary when i move the position between power amp and preamp. (it came from power transformer/Mid Inductor's position like you describe).
Hum is okay and i can deal with it. I was just wondering about the V3 voltage but maybe i will leave it like this.
Fabien