Vintage Ampeg SVT = weird voltages

boroman

boroman

Apr 27, 2009
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#1
I've got two SVTs on the workbench, two of same problems - weird voltages in the preamp section.

See preamp voltages I got from somewhere:

preamp-voltages.jpg


Now in mine:
V1 / pin1= 365v, pin2= 230v, pin3= 240v, pin6= 230v, pin8= 2v
V2 / pin1= 240v, pin3= 2v, pin6= 300v, pin8= 2.6v
V3 / pin1= 150v, pin3= 7.3v, pin6= 233v, pin8= 27v
V4 / pin1= 365v, pin2= 160v, pin3= 170v, pin6= 160v, pin8= 1.5v
V5 / pin5= 365v, pin7= 220v

...so these are WAY off with 20- close to 30% more voltage than this shematic calls for. With an exception of V3, where it calls for 200v on pin1, and I have less, around 150v. Is this schematic wrong or I missed something? Power tube section is ideal (690v plates, 350v screens), 12BH7s and DW7 on the inverter are close to 10% (or less) tolerance... Both amps were rewired for 240V because they had universal transformers... Both are recapped.

Question No.2: That big 8.2k/2W resistor in the preamp section (R47) gets very hot after even short amount of time amp is on (like after a minute with no playing through). So hot that my 1W 8.2k I put there (because previous was shot), is smoking - yes - I see smoke... I know it's 1W and schematic calls for 2W, but anyway... smoking? Voltages across - 450V so quite like schematic says. Same resistor in 2nd SVT (original one) also gets SUPER hot.

Question No.3: Are those C24/C25 caps (0.047/600v) on the mainboard across main power strip needed? I think some of them had only one, and some don't (MTI as far as I remember)? what's their purpose (on a grounded amp)?
 
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#4
Also "(...) It is important to point out that in the Skunkworks this is NOT less filtration, it is actually more because R49 was changed to an 8.2K/15W resistor in the Skunkworks to lower the plate voltages for the 12AX7 preamp tubes, so the RC filter to address rectifier noise at 120Hz is actually increased slightly vs a 4.7K/30uF RC filter. (..)"

= does it mean I can try to source 8.2K/15W there and will get rid of those super high voltages? Or leave it as is?
 
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#7
boroman said:
R47, that big one across 40uf/450V cap is 8.2k/2W
See:
Click to expand...

Hold on, what schematic is this? From there, we might be able to figure out if it's a problem or not.

The 8.2K/2W resistor is a series element feeding the cap as part of an RC filter and a voltage divider to the plates of the common cathode and cathode follower stages.

Here's the original schematic clip:
 

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#8
boroman said:
R47, that big one across 40uf/450V cap is 8.2k/2W
See:
Click to expand...


R47 and R48 are the balance/bleeder resistors across C10 and C12A. These should be 220K 2W.

The 8.2K resistor is R49 and it's specified as 15W.

At 690V (node A), total wattage dissipated by both R47 and R48

690^2/440000 = 1.1W. Each resistor is dissipating 1/2 the total so 0.55W
​
R49 is the first big resistor in the voltage ladder between nodes A and B. This is a 15W resistor but the values have varied over the years. The 69 schematic shows 8.2K 15W. Later 6550 schematics show 4.7K 15W. I do believe the Skunks changed this value back to 8.2K.

Schematics show 660V for node A, but in my experience many vintage SVT run closer to 690V. This means the other voltages derived from the plate supply will be on the high side.

The 69 shows a 230V drop across R49. Revision D of the 6550 schematic shows 198V drop across R49.

If you have a 2W resistor in place for R49, it's no wonder it's smoking.

Based on the 69 schematic values___230^2/8200 = 6.45W

Based on Rev D schematic values___198^2/4700 = 8.34W​

Way over 2W!

The value of the capacitor at node B is 30uF 600V. If you are using a 40uF 450V in that position, it way over the voltage rating.

Not to be rude, but you really need to pay closer attention to what you are doing. Otherwise, you are either going to get hurt, blow up the amps, or both.
 
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#10
Patrice B said:
Higher voltages are probably just a side effect of higher wall AC voltages, maybe?
Click to expand...
Maybe, but it's hard to tell without knowing the revision history. Even Bill Hughes couldn't remember the reasons.
 
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#11
boroman said:
Question No.3: Are those C24/C25 caps (0.047/600v) on the mainboard across main power strip needed? I think some of them had only one, and some don't (MTI as far as I remember)? what's their purpose (on a grounded amp)?
Click to expand...

I don't see C25 on the schematics. C24 is on the preamp schematic and the value is 0.047 1000V.
upload_2023-6-22_17-52-19.png

It's the polarity cap. You won't find this cap on the later amps as there was no polarity switch. The recommendation is to either remove the polarity cap or replace it with a safety cap.

The Revision D schematic shows two series 0.47 600V between chassis ground and the neutral side of the power transformer primary windings and neutral terminal of the AC.

These are labeled C13 and C14.
upload_2023-6-22_17-51-6.png


Rev D also shows a 10 ohm resistor between chassis ground and circuit ground.
upload_2023-6-22_17-53-17.png


My guess is the C13 and C14 are intended to serve as an AC shunt between chassis ground and neutral. I believe they may lower the level of hum a bit. Similar to what the polarity switch previously did. But this level of analysis is getting over my head...so I could be wrong.
 
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#12
Patrice B said:
Higher voltages are probably just a side effect of higher wall AC voltages, maybe?
Click to expand...

AFAIK the amp was designed to run on 120V, so IMHO no.

AFAIK the bias supply and all voltages derived from the plate supply tend to run high.

From what I have seen the only voltages that run close to spec consistently from amp to amp are the heater voltage and screen voltage on the 6550 amps. AFAIK the screens ran about 10V over the schematic on the 6146 based amps.
 
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#13
Hey @Wasnex

I guess we all made mess here (starting with me).
In SVT there are TWO R47 resistors, one in the preamp area 8.2K/2W across 40uf/450V cap (I was talking about that) and second R47 220k in the power amp which does not interest me at all.

This one drops voltage across the cap. And I see smoke on fresh 2W one, it seems it draws more than 2W from my calculations based on schematic and voltage drop. So I'd put there 5W probably to sleep safe...

+ also R49 (that one in the power amp near diodes) 8.2K/15W (4.7K in some revisions). Now, amps are 1975 and 1979 production. Schematic found in the original paperwork glued in the envelope inside 1979 amp indicates 8.2K (schematic is from Jan 1974). Amp has 4.7K (looks original). The 1975 one did not come with any paperwork but if schematic is from earl 74, I guess 1975 model will have 8.2K too (but it has 4.7K, looks aftermarket of two ceramic 8.2K joined together). Did some reading and.. well, the used different cap values for the SVT over the years, hence the 4.7k/8.2k battle... Now, am I thinking right, if I put R49 with value of 8.2K in both amps I'd get lower voltage EVERYWHERE or in preamp only? Is it a good move? What about R47 on the preamp (8.2k/2W)? - would 10k/5W bring voltages down there?
 
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#14
Okay, my apologies for my contribution:

I see 8.2K 2W R47 in the preamp. The 69 schematic shows 430V dropping to 300V. So 130V drop.

130^2/8200 = 2.1W.

So yes, even with the 69 schematic values, R47 is over the wattage rating. I would expect this resistor to run hot at the very least.

Per your OP, you're getting 365V on pin 1 of V1. The high voltage could be because the preamp is not pulling enough current. Or it could be because the voltage at node B is too high, resulting in excess current through R47 in the preamp.

Since you are smoking the 2W resistor fast, it suggest the voltage feeding the preamp is too high, rather than the current being too low to produce a sufficient voltage drop across R47.

The value of R49 on the Power PC card will impact the voltage at node B and thus the preamp voltage. Per revision D (R49 = 4.7K), node B is 462 volts rather than the 430 shown on the 69 preamp schematic. Per the 69 schematic (R49 = 8.3K) and node B = 430V, which matches the 69 preamp schematic. So yes increasing the value of R49 is probably a good idea.

Increasing R49 will also effect the voltage at node C, which feeds the phase inverter tube.

Additionally, skunks have a 150K 3W Dale CPF-2 Resistor strapped across C12B. Adding this resistor should also reduce the voltage at node B and C.
 
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#15
As a 66 year old, long time bassist, I must confess, I don't understand any of this....especially coming from Aged Horse and Wasnex, who are obviously highly skilled with analysis, knowledge and formulas. My only, and I say only contribution to this thread, comes from an old, maybe 10 year old SVT Classic review by Jonathon Herrera of Bass Player magazine (yes, I know, that Bass Player - no longer printed), where he said at the end of the review - "a lot of technician/repair guys shudder at the thought of working on SVT heads due to the potential dangerously high voltage." Now I have no idea if that's really true or not, but if so, I say to you boroman, be careful! There, my Einstein like contribution.
 
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#16
Benj-jammin said:
As a 66 year old, long time bassist, I must confess, I don't understand any of this....especially coming from Aged Horse and Wasnex, who are obviously highly skilled with analysis, knowledge and formulas. My only, and I say only contribution to this thread, comes from an old, maybe 10 year old SVT Classic review by Jonathon Herrera of Bass Player magazine (yes, I know, that Bass Player - no longer printed), where he said at the end of the review - "a lot of technician/repair guys shudder at the thought of working on SVT heads due to the potential dangerously high voltage." Now I have no idea if that's really true or not, but if so, I say to you boroman, be careful! There, my Einstein like contribution.
Click to expand...

The amount of knowledge I have in this regard is... let me put it this way, even my merely using the word knowledge here is itself overstating it. I know just enough to suspect that was probably Jonathon Herrera's dramatic way of saying to people like us, "Don't do this yourself at home. Leave it to professional techs."

The oddly ironic part is that I wouldn't trust an amp tech to service my own SVT if they seemed to shudder at the thought of the potentially dangerously high voltage. If they don't know how to fully and safely discharge the capacitors as a matter of routine before commencing all other work inside any amp, let alone my SVT, then they have no more business working inside any amp of mine.
 
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#17
The plate voltage typically runs around 690V. But this is not all that unusual for a high-powered tube amp. I have several big tube bass amps that run about the same. The standout in my collection is the Hiwatt DR201, which run 711V on the plates of six EL34s. That is the highest voltage I have seen in a properly functioning amp.

It can be a bit tricky working on an active circuit as voltage this high can jump, and also when you place a probe it can arc repeatedly, and make the probe tip squirm around.

I use a test lead with a resistor to discharge the capacitors safely. If you just ground a tool across the terminals, it tends to weld to the contact point.

I don't think voltage is actually the main reason people are intimidated. Some of the circuitry in SVTs is a bit more technically complicated (difficult to understand) than the average tube amp. Also some of the circuit boards in vintage SVTs are a huge pain to access.
 
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#18
Wasnex said:
If you just ground a tool across the terminals, it tends to weld to the contact point.
Click to expand...
It can also damage the cap, the discharge currents under this condition is higher than many caps are designed for.
 
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#19
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.
 
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#20
Grinderman said:
The amount of knowledge I have in this regard is... let me put it this way, even my merely using the word knowledge here is itself overstating it. I know just enough to suspect that was probably Jonathon Herrera's dramatic way of saying to people like us, "Don't do this yourself at home. Leave it to professional techs."

The oddly ironic part is that I wouldn't trust an amp tech to service my own SVT if they seemed to shudder at the thought of the potentially dangerously high voltage. If they don't know how to fully and safely discharge the capacitors as a matter of routine before commencing all other work inside any amp, let alone my SVT, then they have no more business working inside any amp of mine.
Click to expand...

Yeah, it's like I said, I have no idea if that's true or not, but I appreciate your insight!
 
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