Ampeg SVT-II no pro no go

[shzmm]

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.

[g1]

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?

[Chuck H]

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.

[shzmm]

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.

[g1]

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?

[nevetslab]

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.

[shzmm]

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!

[nevetslab]

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..

[shzmm]

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.

[g1]

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?

[nevetslab]

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.

[Chuck H]

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.

[shzmm]

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!

[nevetslab]

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.

[shzmm]

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!