SVT-VR hum question

[km6xz]

I have been presented with the SVT VR Ampeg to repair and found low power due to fried screen resistors and 3 bad tubes. I got a deal on some genuine Svetlana 6550's so replaced all of them. Everything is fine except for preamp hum. It is obviously a ground current loop, the power amp hum is 68db down which is very good for such an amp but the 100hz hum from the preamp section is not sensitive to magnetic fields(did not guess that it would since it was twice the mains freq), b+ and heaters are clean, and hum is nulled out at a gain control position of about 40 degrees. Also the idle current in the output directly impacts the hum amplitude. All this points to ground currents from multiple ground points. These are usually a design issue, pc board layout. I have been out of real bench work for a few years so have not kept up with details of various models. Does this model have a factory mod or advisory to correct the grounding problem?

I was also brought, by the same band, a beautiful HiWatt UK built Custom 50 that looks factory new inside. Low output, all the usual suspects in output feeding conditions(predicted no signal cathode current, bias spot on. Measured the transformer Z and found something terribly wrong, a calculated primary Z of 160 ohms. Has the reissue Partridge transformers lost their reliability reputation or has this been a non-issue with new models. Any guess where a transformer can be found that will ship to Russia by mail? Loud will not cooperate of course.

I've got my little one man shop going as a part time gig alternative to my day business. Suddenly my name has gotten around and I am getting 2-4 amps and mixers a day in after two years of just doing a couple of repairs a week or less mostly for hobby. In late December I decided to start charging and, like the proverbial light switch, it is generating decent income despite my low rates with lots of people calling for the first time.

[Enzo]

I'll go look.

[g1]

For the SVT, I assume you have everything bolted back together? Not sure about the re-issues, but the originals could have hum problems till complete re-assembly (especially preamp).
As far as the Hiwatt, I didn't think Loud had anything to do with them? The US version of the company has ceased operations (at least temporarily): Official HIWATT Amplification website
but the UK Hiwatt is still running: HiWatt amplification, classic British guitar and bass amplification have you tried to contact them?
Edit: They even have several dealers listed for Russia, including St. Petersburg : HiWatt amplification, distributors and dealers although from some of your posts I gather that Russian distributors and dealers would prefer to just sell new ones rather than supply repair parts .

[km6xz]

Thanks for the suggestions. Unfortunately I contacted both the manufacturer and Russian distributor and both say only the distributor can sell parts and the distributor refuses to even discuss parts. They said this one was not sold by them so they will confiscate if they see it, and would not repair it even if they did sell it originally because I opened it. I thought that was pretty rude since I never gave them the serial number so they would have had no idea if they sold it or not. Needless to say, the owner won't given them any more business.

Yes, my shop did a lot of SVTs over the years, hundreds in fact. The most at one time was 24 in one day when a group that used them for their entire backline trucked them to us and wanted them all done in 24 hours. Luckily only 8 had real failure modes going on. But they all got full p.m., tubes and burn in.
This SVT VR had another normal problem which was resolved but the owner said it hummed even in the store when he bought it but was told by the salesman that it would go away. He asked them to repair it when the hum did not go away and was told it was normal and that the other one in the store did it also. Even if it could be done under warranty it would require crating it for shipping some distance with a risk of never seeing it again. Customer service has come a long ways here in most fields but the 2 main distributors of all the pro audio and musician gear treat customers very badly.
I found that a suitable copy is available from MoJo in the US, with $72 USPS shipping.

[MarkusBass]

Here is a topic regarding ground loop in SVT-VR: http://music-electronics-forum.com/t15670/
I fixed several SVT-Classic and haven't noticed this problem. In all cases that I remember, the hum was caused either by the input jack not contacting to the ground, or by burnt ground track in the preamp (1st valve). I haven't seen SVT-VR schematic. It may be different from SVT-Classic.
With the other amp - HiWatt, I would try (at least temporarily) a transformer from another amp e.g. Fender. If you confirm that the problem is with the transformer, you can get the old one rewind. I think that there are some methods to measure the transformer if you remove it from the amp.

Mark

[km6xz]

Hi Ma
Apparently the VR model had a problem initially based on Google searches on it and after production ramped up, some change was made to address the hum problem, or at least this is the conclusion I came to after reading many posts and complaints about the first units.
This is a pure 100hz tone, very little noise or harmonics, not the typical violated star ground or bus ground scheme problem. I would like to make it sound better with the gain down although it has very little hum when the gain control is advanced mid way.

On the HiWatt, I calculated the Z based on measurements initially which showed an abnormally low Z at the my test signal of 500 Hz but since I hate to replace transformers needlessly(I doubt I have replaced more than a dozen output transformers in 40 years, they are much more reliable than beginning techs think so many get changed by error). I set up an impedance bridge to confirm this morning and sure enough the same value of 160 ohms was seen at 500 hz. I dug out my vector voltmeters to measure phase angles and all 3 methods confirmed the very low Z in the primary. I did a sweep of 50 to 15,000 hz to check self resonance and nothing this transformer does is normal, there is little reactive component to the Z indicating a good portion of the winding is bypassed.

[MarkusBass]

So you can either buy a new transformer (this may be difficult), use a different one, or fix the one you have. There are companies that do it and this may be an easiest solution.

Mark