Ampeg SVT-II no pro no go

[shzmm]

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!

[Tom Phillips]

...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.

[shzmm]

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.

[g1]

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?

[shzmm]

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

Thanks for the help

[Chuck H]

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.

[shzmm]

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...

[Chuck H]

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?

[g1]

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.

[Chuck H]

i'm very suspicious of your meter. What kind is it?

bing bing bing bing bing ^^^^^^^^^^^^^^^^^^^^^^^^

[nevetslab]

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

[shzmm]

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.

[shzmm]

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

[ATTACH]47039[/ATTACH]

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!

[nevetslab]

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.

[Chuck H]

...

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?