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Thread: Ampeg SVT-II no pro no go

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    Ampeg SVT-II no pro no go

    after my last adventure into SVT land, I am back with an older re-issue model-- the SVT II (2) non pro model. This one appears to be from the 90's and is in bad condition. There have been a number of PCB repairs and the like. The main problem after having to replace the fan, replace one of the 12AU7s, and so on is that the signal is severely limited and distorted-- it appears to be almost square and only achieves a few volts output.

    One issue is that this amp is wired for 240V and we have 220VAC here. That leaves the heater voltage around 5V, however I can boost the mains voltage up to 240 and it comes up to a respectable 6V so I don't think there's a problem there per se. In fact, all voltages check out ok except it isn't possible to draw enough current through the output tubes. Although I can adjust the bias voltage quite low, the recommended .072V reading at the output tubes cathodes can't be reached. I get at best around .035 and there is still no change in the output waveform which stays square and very weak. I have also tried another set of tubes with no difference.

    The two 12AU7s pass a clean sine to the output tubes so I tested the output transformer for a short but that also tested out ok. My best guess is still that there is a trouble with the output transformer but normally a test of a short will demonstrate this.

    Any other ideas of something to check before this tired old beast becomes a door stopper? As well, if anyone has a wiring diagram for the power transformer leads please pass that on!
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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by shzmm View Post
    ...any ideas of what else to check...
    Yes. Please measure the plate and screen voltages at each of the 6 power tubes and post the results.

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    Thank you for the reply.

    All 6 plates are sitting at around 610 and screen voltage is around 320-- the voltages are a bit down from the norm due to the mains voltage difference.
    Otherwise, I haven't noticed anything unusual in the amp. All the other voltages are in line with the schematic.

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    I think for your testing you should use the 240V you said you can, I assume you have a variac.
    At 5V heaters you could get weird issues.
    So at 240V you still can't get beyond .035 at the bias test points? What is the DC on your power tube grids?

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    The best I get is .045 at the higher mains voltage... Grid voltage is then around -35V

    Thanks for the help

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    Bent Member Chuck H's Avatar
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    There's something hinky with the filament supply. I've never seen an amp that had low filament voltage from the factory (not saying it doesn't happen, but I haven't seen it). Usually the filament voltage is a little high. So if the amp has 6.3V at 240V from the mains that SHOULD math out to 5.77V at 220V. That's still in spec. Further, if you're getting 5V at 220V you should be getting 5.44V at 240V. Both below spec and not up to 6V which you report. The filament voltages don't math out, why?

    Regarding the bias voltage, have you measured it at the supply with 240V mains voltage? It should be 180V as spec'd on the schematic. If it is at least close then the low bias voltage condition at the grids is occurring on the grid side of the 47k grid load resistors.

    If I'm interpreting the schematic correctly there's a ribbon cable for many of those board connections. Including the OT connection to the output jack. I'd be especially critical of those connections and the condition of the board where the pins contact pads.

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    Last edited by Chuck H; 02-09-2018 at 02:35 PM.
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    Quote Originally Posted by Chuck H View Post
    There's something hinky with the filament supply. I've never seen an amp that had low filament voltage from the factory (not saying it doesn't happen, but I haven't seen it). Usually the filament voltage is a little high. So if the amp has 6.3V at 240V from the mains that SHOULD math out to 5.77V at 220V. That's still in spec. Further, if you're getting 5V at 220V you should be getting 5.44V at 240V. Both below spec and not up to 6V which you report. The filament voltages don't math out, why?

    Regarding the bias voltage, have you measured it at the supply with 240V mains voltage? It should be 180V as spec'd on the schematic. If it is at least close then the low bias voltage condition at the grids is occurring on the grid side of the 47k grid load resistors.

    If I'm interpreting the schematic correctly there's a ribbon cable for many of those board connections. Including the OT connection to the output jack. I'd be especially critical of those connections and the condition of the board where the pins contact pads.
    You make a good point about the filament voltage and I don't really have a good answer. With all the power tubes pulled, the filament voltage jumps up to 6VAC at 220 mains. With the tubes in it drops down to 5VAC-- the last measurement at 240V I got up to 5.6VAC. Either way, I have never had an issue like this with low filament voltage, normally a small reduction in output power but nothing serious.

    Bias voltage is at spec of -180 so I don't think that is a problem. I am not sure what you mean however with a "low bias voltage" The bias voltage can be adjusted from around -58 to -35V. I have also checked all the 47k grid resistors and they appear o.k.

    There are no ribbon connections on the amp but it is a good point to make as these amps have a number of connections that need to be pulled and reconnected each time when removing the main PCB. I have cleaned and reseated all of these.

    Thanks for your help, hopefully we can figure something out...

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    Bent Member Chuck H's Avatar
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    With other voltages being on spec and the filament voltage being low even with no tubes in the amp I have to suspect the filament circuit is compromised. Math says 6.54V at 240V into the primary with no tubes.

    Did you pull the preamp tubes as well as the power tubes when checking the no load voltage on the filament winding?

    Have you checked the hum balance circuit for shorts?

    You mention that you have clean sine out of the PI. At what voltage? Was it sufficient to drive the power tubes? Is it balanced?

    Have you tested for continuity between the actual output transformer secondary lead and the output jack to be sure nothing is open or shorted between those two points?

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    I'm very suspicious of your meter. What kind is it?
    Schematic shows around -47V bias at the grids. You have -35V there which should make the tubes run quite hot. Yet you can't get more than .035V at the test points which would mean the bias is too cold.
    It doesn't make sense, something is not right with the measurements.
    Check that you have not shorts at R35/D20 or R36/D21. They should all measure about 1 ohm.

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    Quote Originally Posted by g1 View Post
    i'm very suspicious of your meter. What kind is it?
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    I have additional schematics for this SVT II. I looked at the Universal Primary, and it only has 100V/120V taps on the two primary coils. But, with them in series, yhou should be able to wire it for 220VAC (100V Primary in series with the 120V Primary> I've attached the schematics below.

    The Output stage is much like the SVT-VR, in it's use of a common cathode resistor for all three tubes in each half. Have you tried using one pair of tubes at a time? I'd do that, so you can measure what each tube's plate/screen current is. It could be these tubes are tired, and can't get full current, even with the bias voltage applied set hot. I've seen that happen over the years with all the SVT's I've serviced.

    Now, in the filament end of things, my schematic only shows the High Voltage transformer having the universal primary. It's possible the Filament transformer also has one.. I don't have the color code for that. It might just be a 2-coil/4-wire primary (120V/240V) I believe we have just one SVT II here in our inventory. It's been a few years since I've been inside it, but could pull it out to have a look.

    "m not surprised at the cage in heater voltage betwen On Power Tubes and all six power tubes installed. They pull a good amount of current

    ampeg_svt_ii-1.pdf
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    Quote Originally Posted by g1 View Post
    I'm very suspicious of your meter. What kind is it?
    Schematic shows around -47V bias at the grids. You have -35V there which should make the tubes run quite hot. Yet you can't get more than .035V at the test points which would mean the bias is too cold.
    It doesn't make sense, something is not right with the measurements.
    Check that you have not shorts at R35/D20 or R36/D21. They should all measure about 1 ohm.
    I too am baffled... the meter is a Fluke, one of the bargain ones but I haven't had any issues with it. And certainly this is the only amp that I have with this problem. I have also measured across the output transformer primary and there is no difference. I have checked the 1 Ohm resistors and they are fine, I also shorted them out just to check if the diodes
    were defective, but everything remained the same.

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    Quote Originally Posted by nevetslab View Post
    I have additional schematics for this SVT II. I looked at the Universal Primary, and it only has 100V/120V taps on the two primary coils. But, with them in series, yhou should be able to wire it for 220VAC (100V Primary in series with the 120V Primary> I've attached the schematics below.

    The Output stage is much like the SVT-VR, in it's use of a common cathode resistor for all three tubes in each half. Have you tried using one pair of tubes at a time? I'd do that, so you can measure what each tube's plate/screen current is. It could be these tubes are tired, and can't get full current, even with the bias voltage applied set hot. I've seen that happen over the years with all the SVT's I've serviced.

    Now, in the filament end of things, my schematic only shows the High Voltage transformer having the universal primary. It's possible the Filament transformer also has one.. I don't have the color code for that. It might just be a 2-coil/4-wire primary (120V/240V) I believe we have just one SVT II here in our inventory. It's been a few years since I've been inside it, but could pull it out to have a look.

    "m not surprised at the cage in heater voltage betwen On Power Tubes and all six power tubes installed. They pull a good amount of current

    ampeg_svt_ii-1.pdf
    The tubes are new-- I tried an older set and got the same results. Even if the tubes are tired I would think the output wouldn't be so distorted. It is impossible to make a sine wave output.

    I will also look into the wave form and voltage going to the power tubes tomorrow but I seem to remember this being ok. If it helps, I can post photos of the output waveforms.

    Thanks again for all the replies!

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    Senior Member nevetslab's Avatar
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    While it still sounds like an output transformer issue, one of the changes made some time back on the SVT output circuit is the change to 220 ohm 1-2W Screen resistors, and the elimination of the diodes across them. I'll see if I have that service not handy.

    Technical Information Bulletin (TIB0003, June 2001) -- Power tube PCB modifications.pdf

    You certainly have sufficient plate voltage, screen voltage as well as sufficient bias adj range to turn the power tubes all on hard. The notes on the screen resistor change don't suggest it would have anything to do with what you're dealing with. I also assume the 5 ohm/5W plate resistors are all good.

    I've never built one of the Transformer shorted turns testers....that might be handy in this case. If you have a working output transformer handy, you could try discoonnecting this one in the SVT-II, then drive the primaries of it with the primaries of the spare O/T, and drive that spare thru the 4 or 8 ohm tap, so you'd get high AC voltage to feed the primary of this suspect transformer, then see what you get both loaded and unloaded on the secondary of the SVT O/T. This would at least get you into more meaningful signal current. I've been hoodwinked before by driving a suspect OT with an oscillator, or for that matter, AC mains via Variac on the primary and see what I get on the secondary. Now, you do have 220VAC available. That would actually be the easiest test.....feed it AC mains on the isolated Primary leads (disconnect the CT!!) and see what you get on the O/T secondary. If it pulls a lot of current, that could be it.

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    Quote Originally Posted by Chuck H View Post
    With other voltages being on spec and the filament voltage being low even with no tubes in the amp I have to suspect the filament circuit is compromised. Math says 6.54V at 240V into the primary with no tubes.

    Did you pull the preamp tubes as well as the power tubes when checking the no load voltage on the filament winding?

    Have you checked the hum balance circuit for shorts?

    You mention that you have clean sine out of the PI. At what voltage? Was it sufficient to drive the power tubes? Is it balanced?

    Have you tested for continuity between the actual output transformer secondary lead and the output jack to be sure nothing is open or shorted between those two points?

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    at 240VAC no tubes filament voltage is 6.8

    hum balance is dead center

    setting the bias to .038 allows for a clean output of 4VAC into 4 Ohms, or a bit off the normal 220 Watts or so!

    at this point, the PI is outputting cleanly and delivering 19VAC-- without the output tubes installed they deliver up 63VAC clean and reasonably well balanced (within 5%)

    continuity check on output jacks made early on and now repeated


    the 22 Ohm resistors and diodes were replaced with 220 Ohm early on

    no 5Watt plate resistors on this model, it uses ferrite beads.

    putting 220VAC across the primary of the OT gives 7.7VAC on the 2Ohm tap which works out to around a 1625r load. That seems about right too and with no strange current load.


    Thanks again for all the replies, I am very curious how this one will end up.

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    Quote Originally Posted by shzmm View Post
    at this point, the PI is outputting cleanly and delivering 19VAC-- without the output tubes installed they deliver up 63VAC clean and reasonably well balanced (within 5%)
    Where? PI V1 or drivers V2 & V3 ?
    It won't give more than 19VAC with power tubes in? Then what happens, it clips square?

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    So we know the 4 ohm tap fails to make power and the 2 ohm tap tests fine... Why didn't you check one tap (or all taps actually) for both tests? But I don't think it matters. A short in the secondary that still works fine on the 2 ohm tap shouldn't be causing the problem you have.

    And g1 is asking a good question here. I would think the PI should be able to deliver a minimum of inverse the bias voltage with tubes in in order to drive the amp to full power. But when you plug in tubes you only get 19VAC. That's way too low unless you're not putting enough test signal into the amp. Which I don't suppose is the case because with the power tubes out you get 64VAC. Since you replaced the screen resistors I guess those are off the plate too. So... The OT tests ok and something about installing the power tubes makes the amp misbehave. Please do like g1 asks and identify the PI test points you're using and what happens to the wave form there when power tubes are installed. I think you may have more than one bad power tube.

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    Quote Originally Posted by Chuck H View Post
    So we know the 4 ohm tap fails to make power and the 2 ohm tap tests fine... Why didn't you check one tap (or all taps actually) for both tests? But I don't think it matters. A short in the secondary that still works fine on the 2 ohm tap shouldn't be causing the problem you have.

    And g1 is asking a good question here. I would think the PI should be able to deliver a minimum of inverse the bias voltage with tubes in in order to drive the amp to full power. But when you plug in tubes you only get 19VAC. That's way too low unless you're not putting enough test signal into the amp. Which I don't suppose is the case because with the power tubes out you get 64VAC. Since you replaced the screen resistors I guess those are off the plate too. So... The OT tests ok and something about installing the power tubes makes the amp misbehave. Please do like g1 asks and identify the PI test points you're using and what happens to the wave form there when power tubes are installed. I think you may have more than one bad power tube.

    Thanks for the replies!

    Sorry for the misunderstanding, I was trying to answer a lot in one post. The voltage readings were off pin 8 V2 & 3 or the 12AU7 drivers' cathodes. The 19VAC reading is just the value at which the amplifier output starts to clip-- I believe the driver will stay cleaner longer than that actually but I will check that tomorrow and try to explain things a little better in the future!

    I think the suggestion of trying out a single pair of the 6550s is a good one. I will run that test as well tomorrow and see if it turns up anything. However, since I have tried two different sets of tubes now, I don't find it likely to be an issue with the tubes themselves.

    Out of curiosity, I wonder whether there is a difference in testing a transformer with current draw or not. When the leads are free such as while testing for shorts with a neon bulb, or passing a large voltage through the primary perhaps there is a difference versus when there is a load? Would it be a disaster to connect the secondary to a power resistor load while slowly bringing up a voltage on the primary? I have had power transformers that tested fine without a load but once under a bit of duress the voltage drops significantly making it unusable.

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    Yes you can test transformers with a resistive load, if you use an appropriate value.
    Have you tried other AU7's in the driver spots?
    Have you had a good look at the impedance switch?

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    Regarding testing the Output Transformer, having been first disconnected from the plates & C/T connection to the P/S (having the power tubes removed would be sufficient, I think), and driving the primary with the output from your Variac, sourced from 220VAC.....under no load, there should be just excitation current, probably less than 100mA. Not knowing the turns ratio, what to load the secondary with is a bit of a guessing game. Maybe 8 or 16 ohms, just to see that it will deliver voltage looking like the same waveform as it does without a load. We're just trying a different test than the shorted turn test, though if there is a shorted turn issue, I think it would show up in this procedure. It would, of course, draw more current loaded than unloaded. Granted, I haven't tried this on the O/T of an Ampeg SVT amp, so I don't know first hand what the open current load would be (excitation current).

    We assume you're driving the amp thru the Power Amp input jack on the rear, and not thru the preamp. Though, if your signal source doesn't have enough output level, then you would need to go thru the preamp to get enough level

    The Screen resistors are sourced from a lower voltage supply, and not off the Plate supply.

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    ok I tested the OT with an 8 Ohm load and really didn't see much of a difference there... at 220VAC on the primary, I got 6.5VAC on the 2 Ohm tap which is about a volt less than unloaded.

    I have already replaced both 12AU7s and they read similar values and certainly deliver enough voltage to drive the 6550s so I don't see an issue there.

    I double checked the impedance switch again and that appears to be fine.

    Then, I checked the amp with only a pair of output tubes and got exactly the same results adjusted for fewer output tubes.

    All tests have been performed at the power amp input, although I also just tried the regular instrument input for kicks.


    I appreciate all the input so far, but it seems we are all grasping at straws with no clear direction any longer. The problem lies within some interaction between the OT and the output tubes I would say.
    Unfortunately, I don't have a spare OT handy and probably won't pony up the money to purchase one any time soon. I guess the SVT won this battle!

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    Senior Member nevetslab's Avatar
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    Later this week I'll look for the one Ampeg SVT II we have (assuming it's still there), and have a look at it for doing some side-by-side comparasons.

    Meanwhile, looking at the older SVT II Power Amp schematic, I see the expected AC drive signal at the 12AU7 cathodes is called out as 30VAC for getting 35VAC on the 4 ohm output. On your amp, with no power tubes installed, you stated getting 64VAC and only 19VAC with the power tubes in (regardless of one pair of all six, based on your stating you got the same results). So, we know SOMETHING is loading down the signal drive to teh power tubes.

    Have yhou looked at the DC voltages under drive conditions? You indicated with the power tubes removed, you had a bias adjustment range of -58V to -35V. What is the bias adjust range with the power tubes installed (1 pair or all )? And, on each bias adjust, since there are two. Do the power tubes restrict the bias adjust range? Are the plate and cathode voltages on the voltage gain stage for each 12AU7 similar to what' shown on the schematic? Plate voltage of 160V, cathode voltage of 7V is shown. And, does the grid voltage of the 12AU7 driver stage sit around -77V. It will change as per setting of each bias adjust pot.

    And, under signal drive, does the plate supply voltage and screen supply voltage change. I assume both do drop some between NO power tubes installed and power tubes installed. We're all pondering why you can't get sufficient bias to obtain the 24mA plate current per tube on each half. What does the raw bias supply change to with the power tubes installed? Schematic calls it out at -180V. Full wave rectifier via diodes D12 & D13, followed by C8 100uF/250V.

    Since the Primary of the HT transformer has a universal primary, each coil having 100V taps, you can strap the primary for 220VAC, and in the process, since one of the primaries is 100VAC with the other at 120V, having the heater transformer across the 120V coil, that should raise the heater voltage output..

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    Quote Originally Posted by nevetslab View Post
    Later this week I'll look for the one Ampeg SVT II we have (assuming it's still there), and have a look at it for doing some side-by-side comparasons.

    Meanwhile, looking at the older SVT II Power Amp schematic, I see the expected AC drive signal at the 12AU7 cathodes is called out as 30VAC for getting 35VAC on the 4 ohm output. On your amp, with no power tubes installed, you stated getting 64VAC and only 19VAC with the power tubes in (regardless of one pair of all six, based on your stating you got the same results). So, we know SOMETHING is loading down the signal drive to teh power tubes.

    Have yhou looked at the DC voltages under drive conditions? You indicated with the power tubes removed, you had a bias adjustment range of -58V to -35V. What is the bias adjust range with the power tubes installed (1 pair or all )? And, on each bias adjust, since there are two. Do the power tubes restrict the bias adjust range? Are the plate and cathode voltages on the voltage gain stage for each 12AU7 similar to what' shown on the schematic? Plate voltage of 160V, cathode voltage of 7V is shown. And, does the grid voltage of the 12AU7 driver stage sit around -77V. It will change as per setting of each bias adjust pot.

    And, under signal drive, does the plate supply voltage and screen supply voltage change. I assume both do drop some between NO power tubes installed and power tubes installed. We're all pondering why you can't get sufficient bias to obtain the 24mA plate current per tube on each half. What does the raw bias supply change to with the power tubes installed? Schematic calls it out at -180V. Full wave rectifier via diodes D12 & D13, followed by C8 100uF/250V.

    Since the Primary of the HT transformer has a universal primary, each coil having 100V taps, you can strap the primary for 220VAC, and in the process, since one of the primaries is 100VAC with the other at 120V, having the heater transformer across the 120V coil, that should raise the heater voltage output..
    Thanks for your reply and the ideas. The 19VAC drive to the power tubes wasn't the overall maximum it just was the maximum to the power tubes before the signal on the OT secondary crapped out. I think this section is working fine honestly.

    I will check again, but I don't think the range of bias volage was at all affected by the fact of the power tubes being present or not.

    I also remember the 12AU7s being within a reasonable range from the schematic but I haven't checked the voltages under drive.

    The -180 raw bias voltage is also present.

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  25. #25
    don't forget the joker g1's Avatar
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    Quote Originally Posted by shzmm View Post
    ok I tested the OT with an 8 Ohm load and really didn't see much of a difference there... at 220VAC on the primary, I got 6.5VAC on the 2 Ohm tap which is about a volt less than unloaded.
    Reading between the lines here, did you put 220VAC on the primary with the secondary unloaded?

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    Senior Member nevetslab's Avatar
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    With 220VAC on the isolated primary, and you stated at the 2 ohm tap, unload is was about 1V higher than it loaded with 8 ohms, which would be about 7.5V. So, the turns ratio found here is 220/7.5 = 29.33. Using the AC voltage shown on the schematic of 372V to gnd, or 744VAC plate to plate, we'd see about 25.4VAC unloaded at the 2 ohm tap. Disregarding the loading for the moment, computing the power, with this turns ratio, 25.4V sq'd / 2 ohms = 322.6W. That sounds about right for a 300W amp.

    With the primary wires disconnected, what do you get at the primary when you feed 1V RMS into the secondary. You might try this at a few frequencies, like 50Hz, 400HZ, 1kHz. Try it thru both the 2 ohm and 4 ohm output taps (where you're connecting your signal generator). It will add further confidence to the O/T being ok, which it's beginning to look like it is.

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    Last edited by nevetslab; 02-13-2018 at 04:04 AM.
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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by shzmm View Post
    I guess the SVT won this battle!
    Oh no you don't... Even if you determine there's no money in this repair, this is just the sort of thing I love to get on my bench. I grump around for days sometimes saying crazy $h!t like "There's nothing wrong with this broken amp!!!" I learn more from the hard stuff than the easy. And it's a lot more gratifying when you find your way to the other side. If I did repairs I imagine the easy stuff would be my daily grind and I probably wouldn't enjoy those things much. Throw in some drama and a puzzle though... That and I hate telling a customer that something is above my skill set. I'm more likely to call them and tell them I need more time if they're ok with that. Then I charge for the repair (or finishing task) based on what time it likely would have taken someone with specific experience to handle. Seems fair and I'm never sorry for the experience.

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    Quote Originally Posted by Chuck H View Post
    Oh no you don't... Even if you determine there's no money in this repair, this is just the sort of thing I love to get on my bench. I grump around for days sometimes saying crazy $h!t like "There's nothing wrong with this broken amp!!!" I learn more from the hard stuff than the easy. And it's a lot more gratifying when you find your way to the other side. If I did repairs I imagine the easy stuff would be my daily grind and I probably wouldn't enjoy those things much. Throw in some drama and a puzzle though... That and I hate telling a customer that something is above my skill set. I'm more likely to call them and tell them I need more time if they're ok with that. Then I charge for the repair (or finishing task) based on what time it likely would have taken someone with specific experience to handle. Seems fair and I'm never sorry for the experience.
    You are completely right, I really didn't want to waste anyone's time any more than I have already, but, yes, I will get back to this beast in a short while. I just need a break from the darn thing!

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  29. #29
    Senior Member nevetslab's Avatar
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    I brought over the Ampeg SVT-II amp from our Guitar Dept so I can open it up and have a look and get some measurements to help (I hope) track down what's happening with your amp. The Fender Hot Rod Deluxe popped up, so I've been occupied with details in a mod I had posted some time back, so it will probably be tomorrow before I can did into the SVT II.

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    Quote Originally Posted by nevetslab View Post
    I brought over the Ampeg SVT-II amp from our Guitar Dept so I can open it up and have a look and get some measurements to help (I hope) track down what's happening with your amp. The Fender Hot Rod Deluxe popped up, so I've been occupied with details in a mod I had posted some time back, so it will probably be tomorrow before I can did into the SVT II.
    That's great, I was planning to dig into this again over the weekend so I can publish the results I find and we'll see where we have differences.

    Thanks a lot for your help!

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    Senior Member nevetslab's Avatar
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    Svt-ii photos-orientation time

    Saturday, and I finally got the covers off the working SVT-II from our rental Dept @ CenterStaging, LLC Burbankc,CA. I first thought I'd capture and present a series of photos of the amp, just to put things into perspective when what is easy to see on the schematic, and a nightmare to try and get to in the real world. My apologies for not having a current-higher-resolution camera than this old Sony Mavica that I'm still beating up...low res, stores on 3.5" floppy disc!

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    There's plenty more images but this will get us started. First, I'm curious to see what the heater voltage is. Here in California, I'm running on 120VAC. 5.84VAC. At 110VAC Mains, its' 5.37VAC, and at 127VAC (hi line), it's 6.24VAC. Interesting. This has the same Heater voltage problem (is it?) as you've found. No matter...that's where they set the Heater xfmr windings.

    Just looking to see what the bias voltage is set at...it's probably been 3 years since I've had this in the shop. TP 1 is 62mV DC, TP2 is 59mV, so it's a little low from the nominal spec of 72mV. Voltage at the cathodes of the two 12AU7 drivers are -47.9VDC, right in the ball park.

    I'll post this for now, and resume either tomorrow after church or Monday morning. My eyes are in terrible shape, and driving at nighttime is now very scary...scheduling for Cataract surgery on both eyeballs.

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  32. #32
    Supporting Member Jazz P Bass's Avatar
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    "scheduling for Cataract surgery on both eyeballs"
    They 'do' one eye at a time.
    So you have to wait a week or 2 until you get the second one done.

    I am here to tell you that you will not want to wait!

    I took the patch off of the first one on the way home.
    I could not believe how vibrant colors where.
    Bluer than Blue.
    Greener than Green.
    Redder than Red.


    Good luck.

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  33. #33
    Senior Member nevetslab's Avatar
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    SVT II Output Waveforms, Data

    I had hoped to have the whole ay with this amp, but didnt' get started until nearly 2PM. But, got some waveform meassurements & scope photos.

    First, the amp won't put out 300W into 4 ohms It will do 200W with slipping, and 225W into 4 ohms with severe clipping, as you'll see. Here's some saveforms, with details below going left to right

    Click image for larger version. 

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    All of the output waveforms are from the monitor output of the Amber 3501a, and aren't to any specific scale. The other waveforms ARE to scale, and will be noted.

    The first scope photo is @ 20VAC Out into 4 ohm resistive load (100W). The positive-going half-wave is the cathode voltage across the 1 ohm cathode resistor with it's diode across it The image next to it is the Amber 3501a Audio Analyzer, monitoring the output voltage, showing 20VAC on the meter, and is also sourcing the 400Hz sine wave input to teh power amp stage.

    The next waverforms are @ 28VAC into 4 ohms, with the output now showing some clipping, along with the voltage across the 1 hm cathode voltage (now @ 500mV/Div), definitely showing saveform clipping, while we don't see much of it on the output. The next two waveforms are showing teh same 28VAC output, along with the positive and negative driver tube outputs to the grids of the power tubes. 20V/Div via X10 probe. We're starting so see some assymetrical clipping on them at this drive level

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    These two waveforms are at 30VAC output ito 4 ohms (225W), and definitely clipping. The driver stage waveform is starting to show a bit more distortion, but still only moderate non-linearity

    The next two waveforms are driving higher resistance loads on the 4 ohm output tap, with theAttachment 47231 cathode voltage showns with the output waveform.

    Click image for larger version. 

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    These last two waveforms are at 40VAC output, 1st one into 16 ohms, and 2nd one into 24 ohms.. The half-wave is the waveform across the cathode resistor on the positive half of trhe power tubes. I'll have to add the voltage measurements on the amp @ idle when I get home

    At 120VAC Mains, I find the following:

    Power Tube Plate voltage: 647VDC
    Screen supply voltage 383VDC
    Driver tube cathode (bias) -47.8VDC
    Driver tube grid voltage -70VDC @ V3B, -72VDC @ V2B
    V3A Plate Voltage 181VDC
    V3A Cathode Voltage 7.4VDC
    V2A Plate Voltage 182VDC
    V2A Cathode Voltage 7.4VDC
    V1A Plate Voltage 217VDC
    V1A Cathode Voltage 70.5VDC
    V1B Plate Voltage 186VDC
    V1B Cathode Voltage 1.2VDC

    I forgot to get grid voltages on V1A, V2A & V3A. The Cathode Voltages @ TP1 & TP2 were recorded in the prvious post, as was the heater voltage.

    I haven't yet made measurements under 2 ohm load, and should get that tomorrow.

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    Last edited by nevetslab; 02-20-2018 at 05:18 AM. Reason: adding voltage measurement data
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    Quote Originally Posted by nevetslab View Post
    I had hoped to have the whole ay with this amp, but didnt' get started until nearly 2PM. But, got some waveform meassurements & scope photos.

    First, the amp won't put out 300W into 4 ohms It will do 200W with slipping, and 225W into 4 ohms with severe clipping, as you'll see. Here's some saveforms, with details below going left to right

    Click image for larger version. 

Name:	20VAC Output @ 4 ohms.jpg 
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ID:	47224 Click image for larger version. 

Name:	20VAC Otput @ 4 ohms-VM.jpg 
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ID:	47225 Click image for larger version. 

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ID:	47228

    All of the output waveforms are from the monitor output of the Amber 3501a, and aren't to any specific scale. The other waveforms ARE to scale, and will be noted.

    The first scope photo is @ 20VAC Out into 4 ohm resistive load (100W). The positive-going half-wave is the cathode voltage across the 1 ohm cathode resistor with it's diode across it The image next to it is the Amber 3501a Audio Analyzer, monitoring the output voltage, showing 20VAC on the meter, and is also sourcing the 400Hz sine wave input to teh power amp stage.

    The next waverforms are @ 28VAC into 4 ohms, with the output now showing some clipping, along with the voltage across the 1 hm cathode voltage (now @ 500mV/Div), definitely showing saveform clipping, while we don't see much of it on the output. The next two waveforms are showing teh same 28VAC output, along with the positive and negative driver tube outputs to the grids of the power tubes. 20V/Div via X10 probe. We're starting so see some assymetrical clipping on them at this drive level

    Click image for larger version. 

Name:	30VAC Output @ 4 ohms + Pos Driver.jpg 
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ID:	47229 Click image for larger version. 

Name:	30VAC Output @ 4 ohms + Neg Driver.jpg 
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ID:	47230

    These two waveforms are at 30VAC output ito 4 ohms (225W), and definitely clipping. The driver stage waveform is starting to show a bit more distortion, but still only moderate non-linearity

    The next two waveforms are driving higher resistance loads on the 4 ohm output tap, with theAttachment 47231 cathode voltage showns with the output waveform.

    Click image for larger version. 

Name:	40VAC Output @ 16 ohms.jpg 
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ID:	47235 Click image for larger version. 

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    These last two waveforms are at 40VAC output, 1st one into 16 ohms, and 2nd one into 24 ohms.. The half-wave is the waveform across the cathode resistor on the positive half of trhe power tubes. I'll have to add the voltage measurements on the amp @ idle when I get home

    At 120VAC Mains, I find the following:

    Power Tube Plate voltage: 647VDC
    Screen supply voltage 383VDC
    Driver tube cathode (bias) -47.8VDC
    Driver tube grid voltage -70VDC @ V3B, -72VDC @ V2B
    V3A Plate Voltage 181VDC
    V3A Cathode Voltage 7.4VDC
    V2A Plate Voltage 182VDC
    V2A Cathode Voltage 7.4VDC
    V1A Plate Voltage 217VDC
    V1A Cathode Voltage 70.5VDC
    V1B Plate Voltage 186VDC
    V1B Cathode Voltage 1.2VDC

    I forgot to get grid voltages on V1A, V2A & V3A. The Cathode Voltages @ TP1 & TP2 were recorded in the prvious post, as was the heater voltage.

    I haven't yet made measurements under 2 ohm load, and should get that tomorrow.
    nice work! I will post my shots tomorrow, but your amp is much cleaner than what I have here!

    Looks like the voltages are pretty much in line with what I had already posted however...

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  35. #35
    Senior Member nevetslab's Avatar
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    Some added notes to what i posted last night.

    V1A Grid voltage 25.2VDC
    Test Point 1 68mV
    Test Point 2 63mV

    AC Mains @ idle 120VAC @ 2.01A/204W

    Power @ 20VAC Output/4 ohms (100W) 4.36A/442W @ 120VAC mains
    Power @ 28VAC out/4 ohms (196W) 5.8A/563W @ 120VAC
    Power @ 30VAC out/4 phms (225W) 6.4A/647W @ 120CAC

    I haven't opened up the insides thus far. And, I don't have component layout drawing for the PCB's. Thus far, I haven't identified the bias supply. Nor have I checked the min/max range of the bias control and the corresponding voltages at the Test points TP1 & TP2.

    Some client work came in so I need to address that for now.

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