1970 Ampeg SVT Troubleshooting Questions. Calling on SVT Experts!

[Trey]

I'm new to the svt but I have cleaned up, recapped and retubed a V4B.

1st off I'm aware of the dangerous hi voltages and am very cautious when working on tube amps and bleed the filter caps each time.

I got a non op 1970 svt with 6146's in it. All the tubes would light up but no output. Upon troubleshooting I found that the power tranny primary wasn't wired correctly (when the standby switch is closed it should be in parallel with heater transformer). I connected it correctly. I then got an output to the speaker cab. I let it warm up in standby for 3-4 min, then brought it out of standby and let it sit for approx. 5 min out of standby, plugged my bass in for a quick test. It worked fine very little hum, until I increased the volume, (not way loud) and then 2 of the 10 ohm 5 watt resistors fried, R32 & R44. I immediately unplugged.

Here are the knowns:
1) When I aquired the amp all the boards looked great no visual signs of burn marks or component damage.
2) I installed all new tubes.(supposely matched)
3) Once the Power transformer was connected properly, 512V @ coming out to the bridge rectifier diodes. (didn't measure plate voltages etc.)
4) I slightly turned the bias pots and measured just to verify a voltage change and I did get a change and set them at .072V
5) 6.23v to the heaters.

Here are the stupid things I did:
1) Used an 8 ohm cab for a quick test ( I'm waiting for my 4 ohm dummy load and my 4 ohm cab wasn't at my house)
2) Adjusted the bias with an 8 ohm load.

I'm waiting for parts. I will be replacing all the 10 ohm 5 watts as well as the 22 ohms and 47 k's and will not power up until I have the proper non inductive 4 ohm load.

Questions:
1) Is 512V out of the power tranny to the rectifier diodes ok?
2) Did the brief test with the 8 ohm cab kill it?
3) Will the associated tubes always fry if the 10 ohms opened up?
4) Even though it was brief test did messing with bias cause the fault?
5) Could the 10 ohms be old or compromised from previous bad tubes and finely failed when pushed?

Any help would be appreciated. I'm stiil getting aquainted with this amp so please have patience.

[Jazz P Bass]

Can you post a schematic?

[Trey]

Here is the 6146B s Svt
The Free Information Society - Ampeg SVT 6146B Electronic Circuit Schematic

Thanks

[Billy R aka DynaFreak]

Slow and easy is a good motto to use when working on old SVT's with unknown history. A good approach is to pull the power tubes, maybe even all the tubes, and use a variac to feed it low voltage whilst testing to make sure all tubes are getting plate, screen, and bias voltages, that all diodes are ok, etc. SVT's of that era will inevitably have bad solder joints in various places, particularly where the flying leads tie into the PCB. After the low voltage check, I would give it a good going over under magnification and good lite, and touch up anything suspect. In some cases you may need to pull the wire, clean it up, remove old solder and then resolder with fresh. I love working on SVT's but you cant be in a rush to get instant fix gratification.

[Jazz P Bass]

SVt 6146B schematic

The 8 ohm cab did not take out those resistors (R32/ R44)
Sounds like those two tubes are bad.
I would test each tube one at a time, while monitoring the line current.

[g1]

You mentioned voltage going into rectifier but we need to know what is coming out of it. Is it around the 660VDC shown on schem.? Screen voltage around 220V?
As JazzP said, the tubes that fried those resistors are suspect, new or not. You asked if the resistors could have been compromised by previous bad tubes. Perhaps if they just opened up, but you said fried (so I'm thinking up in smoke) which would imply shorted tubes.
You asked if the tubes will fry if the 10R resistors open. It is the other way around. The 10R resistors fry if the tube shorts, basically acting as a fuse and allowing the amp to keep working with the remaining tubes.
The 8 ohm load would not have caused the problem, nor tweaking the bias.

[Enzo]

Or if those two tubes lost their bias, that ought to burn up plate resistors.

[Trey]

Thanks Guys,

I will replace the burnt up plate resistors, then remove the power tubes and check the screen and plate voltages to see if there within specs.

2 other things to add. I installed a sextet of supposedly matched Chinese 6146 tubes. Could the 2 have shorted due to poor quality and thus frying the 2plate resistors? Is this likely or not so much? My original problem was finding out that the power transformer primary was not connected to A/C, but upon finding a schematic I hadn't seen yet, I noticed there is an interlock that one end of the primary was tied to. I didn't think it was correct at 1st but now it ended up that the interlock was missing the plug that completes the circuit. It doesn't show it on all the schematics, it appears they discontinued it.

[Amp Kat]

Thanks Guys,

I will replace the burnt up plate resistors, then remove the power tubes and check the screen and plate voltages to see if there within specs.

2 other things to add. I installed a sextet of supposedly matched Chinese 6146 tubes. Could the 2 have shorted due to poor quality and thus frying the 2plate resistors? Is this likely or not so much? My original problem was finding out that the power transformer primary was not connected to A/C, but upon finding a schematic I hadn't seen yet, I noticed there is an interlock that one end of the primary was tied to. I didn't think it was correct at 1st but now it ended up that the interlock was missing the plug that completes the circuit. It doesn't show it on all the schematics, it appears they discontinued it.

The 10 ohm resistors can most definitely fry from bad tubes and you can get bad tubes out of box. I agree with above that you should take them and have them tested for shorts with a tube tester. Not sure on how much you plan on using it and the 6146's are unique in tone but they are also quite expensive I'm sure and not real popular. I would consider converting it to 6550's which wouldn't be really hard in the future.